Feel free to ask Myles

[myles_rose]

Originally posted by Chris Hebb:

Myles, should I avoid amps using EF86's like the plague? Although they make 12ax7 adapters, wouldn't that be like dropping a 4-cylinder economy engine into a Ferrari? I can't seem to find any EF86's new and fear that the supply of superior NOS EF86's may dry up someday soon...

Chris -

 

Unfortunately, EF86 front end amps have a certain magic that it seems cannot be gotten in any other way. An EF86 / 6267 is a pentode, not a triode, and has much more gain, and a different distortion character than a triode. In most amp circuits that use these, having a gain area of 400-500 is not uncommon. A 12AX7 on a good day is supposed to have a gain of 100, and most today are in the high 80s and low 90s. You can get a great sound with less noise, as there are no cascading stages of gain needed, one tube does it all.

 

Thankfully, most folks that use the EF86 are amp makers that have their shoes tied, and are darn sharp fellows. You have folks like Dr. Z, Matchless, Bad Cat, and Victoria. These folks hand pick these tubes, so they work pretty darn nicely. NOS EF86's are super, but current ones are not all that great. BUT .... if you hand pick through the new ones, you will find ones every bit as nice as the NOS offerings. You cannot just get these from any old vendor, you need to get them from folks that actually test these, and an EF86 will not even test in a basic way on old tube testers.

 

So ... these tubes are out there, and there are good ones, and they are even back in production from a few folks. It is just a matter of getting good ones, and folks that sell these great amps that use the EF86 should also be able to sell you their own tested replacements, and that may be the best bet of all.

 

These amps are magic, and I would not let less common tube scare you off. If you want the magic of something like a Route 66, Z-28, KT-45, DC-30, or Sovereign etc., then the EF86 is a part of the picture.

 

Now, there are adaptors. They allow a 12AX7 to be used in the place of an EF86. I think these are super if you are out there touring, and don't have spare EF86's... they can get you back up and running in a jam.

 

On the sound of these, they work pretty nicely in the "Matchless" amps, as I would think they would, as Mark Sampson did the design on them. In the Victoria Sovereign, they sounded different than an EF86 in the same amp, enough so, that I did not like these in this amp. In the Matchless / Bad Cat amps, they were different too, but in a less drastic way, and some folks even liked them more than an EF86, so it will boil down to personal taste.

 

I will be playing with the adaptor soon in some Dr. Z. amps as well.

[myles_rose]

Originally posted by daddyelmis:

Myles:

 

Bought a 59 Bassman Reissue and love it. Want to replace the solid state rectifier with tube, and from what I've read you can "go downward" on rectifier tubes . . . GZ34/5AR4 to 5U4 or 5y3. I this correct? My understanding is that the lower your go, the less load put on the power tranny. The Bassman RI is spec'd for 5AR4 or 5U4, but can I go lower (the goal is to reduce the output power a bit for club play). Ultimately, I'll likely work a Hotplate or Weber MASS into this amp, but I want to go to a tube rectifier anyway and thought this might be one step in the right direction.

 

Also, have you ever tried an attenuator on this amp (I note that the speakers are hardwired rather than connected with a 1/4" connector).

 

Thanks.

daddyelmis .........

 

You can use any of the tube rectifiers GZ34 / 5U4 / or even a 5Y3.

 

On attentuators, you will need to use an adaptor, and many do not like 2 ohm loads.

 

Try the 5Y3 first, or a 5U4 or 5V4 with 6L6GE tubes in a #3 rating to give more playing dynamics at a lower level:

 

http://www.groovetubes.com/product.cfm?Product_ID=1713

 

Also, in V1, get rid of the Sovtek 12AX7WA and put in what was used originally, a 12AY7:

 

http://www.groovetubes.com/product.cfm?Product_ID=1779 (wrong picture here, but the text is correct).

 

On my own tweed amps, I use the very low noise and longer lasting 6072M version of this tube:

 

http://www.groovetubes.com/product.cfm?Product_ID=1215

 

Leave the bias (fixed) alone, with the #3 GE's ... it will work great.

[myles_rose]

Originally posted by skip:

Hi Myles

I have a general question about a `cooldown` period for tube gear. Often there`s a situation where you finish playing, you have to move your gear and the tubes are still hot. A bandmate of mine told me that they are easily damaged then because the filaments are still pliable. Is this so, and if so what`s a reasonable time to wait, assuming one has the option, before moving things around?

Skip -

 

Your bandmate is correct. Longer tube life will be a result of waiting five or so minutes after the amp is shut down.

[funky hot grits]

Hi Myles,

 

first of all thanks for your answer regarding replacing the tubes in my blue angel. When I was in my local shop ordering replacements I had a chance to talk to a local repairman, and he told me that a 6L6 can be retrofitted directly into a 6V6 socket with no problems resulting. Do you have any opinions/warnings regarding this practice? -I'm interested in trying it out not only as a possible way to gain extra clean headroom for certain applications, but also to deliver a different tone if required...

[ridger]

Myles,

 

A friend has a Crate VFX5212 with 4 12AX7 preamp tubes and 2 EL34 power tubes. Is it possible to change out the EL34 with 6L6 tubes or would you recommend staying with the same type tube that came in the amp? He is looking for tubes that will distort sooner and have more warmth than the stock tubes. Also, I read in your primer about "rebias" of amps may be required if tube types are changed. Can you point me where I can read more information about "rebias" since I don't know what that means. Thanks.

[meek20]

meek20.......

 

In your amp I prefer EL84S in #6 for the output tubes, and in V1 and V2 I use 12AX7R3 that I pick for specific measurements (SAG stuff).

Thanks for the response!

 

I don't know if you've ever tried the Hughes & Kettner Tube Rotosphere before, but I was considering replacing its original tube before long. The tube is a Ruby ECC83-CZ.

 

If this one was yours, what might be your first choice for a replacement?

 

Thanks!

[myles_rose]

Originally posted by funky hot grits:

Hi Myles,

 

first of all thanks for your answer regarding replacing the tubes in my blue angel. When I was in my local shop ordering replacements I had a chance to talk to a local repairman, and he told me that a 6L6 can be retrofitted directly into a 6V6 socket with no problems resulting. Do you have any opinions/warnings regarding this practice? -I'm interested in trying it out not only as a possible way to gain extra clean headroom for certain applications, but also to deliver a different tone if required...

funky hot grits .........

 

Generally this is fine, using a 6L6 in the place of a 6V6, but the opposite is NOT true. In the Blue Angel, this is fine. You won't get much more power, as the power supply is figuring that there are supposed to be 15 watts worth of class A 6V6s in there, but you will get a bit more power and more clean headroom.

 

Some folks like this change, so this all boils down to personal taste.

[myles_rose]

Originally posted by ridger:

Myles,

 

A friend has a Crate VFX5212 with 4 12AX7 preamp tubes and 2 EL34 power tubes. Is it possible to change out the EL34 with 6L6 tubes or would you recommend staying with the same type tube that came in the amp? He is looking for tubes that will distort sooner and have more warmth than the stock tubes. Also, I read in your primer about "rebias" of amps may be required if tube types are changed. Can you point me where I can read more information about "rebias" since I don't know what that means. Thanks.

ridger..........

 

You can switch to 6L6 tubes, but the bias must be changed. (by the way, the opposite is not true, putting EL-34 tubes in a 6L6 amp, unless designed to do this, as the screen resistors are very different).

 

Ob biasing, there are many articles on the net, just search around a bit.

 

If he wants warmer, and distortion at a lower level, I'd first stick to the tubes that came in the amp, type wise, and just go to a lower rating on the output tubes, say a #3 rating or a #2, and then in V1 and V2 use a 12AX7C if you want things warmer.

[myles_rose]

Originally posted by meek20:

meek20.......

 

In your amp I prefer EL84S in #6 for the output tubes, and in V1 and V2 I use 12AX7R3 that I pick for specific measurements (SAG stuff).

Thanks for the response!

 

I don't know if you've ever tried the Hughes & Kettner Tube Rotosphere before, but I was considering replacing its original tube before long. The tube is a Ruby ECC83-CZ.

 

If this one was yours, what might be your first choice for a replacement?

 

Thanks!

The stock tube, the Ruby, is a JJ ECC83. You may want to try a 12AX7R3 - the Electro Harmonix 12AX7, as it works very nice in this unit.

[Casey4s]

Myles,

 

How's the new bok comming? I am really looking forward to it's release.

 

Thanks...

[johnny5]

hi.

just wondering if it is possible to put 6L6 or KT66 tubes in a Marshall DSL 50 (while aproprietly adjusting the bias)without possibly damaging the amp? i have heard (read) both yes and no on the web. could you confirm this?

 

thanks,

john

[myles_rose]

Originally posted by Casey4s:

Myles,

 

How's the new bok comming? I am really looking forward to it's release.

 

Thanks...

Casey4s ...........

 

Book is coming along actually ON schedule It will be released this summer, hopefully early summer.

 

By the way, I am sending your link to your DIY stuff to a lot of folks.

[myles_rose]

Originally posted by johnny5:

hi.

just wondering if it is possible to put 6L6 or KT66 tubes in a Marshall DSL 50 (while aproprietly adjusting the bias)without possibly damaging the amp? i have heard (read) both yes and no on the web. could you confirm this?

 

thanks,

john

John,

 

You can do this with a rebias.

[Shawners]

Hi Myles ,

I've FOUND THE HOLY GRAIL 12AX7 TUBE...ITS NAME IS MULLARD ! Sorry for the shout , but I tried one of these tubes in v1 this past week-end in my mesa nomad 4x10 combo. I let it warm-up for a while than tried my clean channel..OMG a tube that lives up to its hype. I've never heard such a chimie clean sound before. I could hit harmonics that would of never been heard before that just ring and sustain. In the high gain channels it made my mesa sound like a hot rodded vintage marshal , just with more balls.The sad thing is that after playing for alittle over a hour it started to squeel and make that static fizzle noise , and started to sound flatter.The next day the magic didn't return so i tested the mullard by litely tapping it , and it would break into that ringing sound and that static fizzle noise .I put a new tube in v1 and the problem went away...MAN AM I BUMMED. Before investing in a new Mullard I want to make sure that the harmonic magic and tone I heard wasn't because the tube was on the verge of dying and making sounds that a new mullard wont make? (.. like a light bulb burns britest right before dying??).

I've bought a used Telefunkin 12ax7 and was dissapointed.It really lowered my noise floor like no tube has( even the Mullard), but seemed to choke out higher frequency sound (maybe thats why it was so quit and dark).It is a good tube , but I didn't experience that Magic hype (not that lively of a tube).Thats why I want to make sure before I buy another mullard. I also tried a NOS Bugle Boy labeled for medical use Holand.

That tube wasn't as quit as the Telefunkin , but had more life and high end sound , but still not that magic that the mullard created.

Soo.. If I buy another NOS Mullard , will i get that sound I had with the other , or was that its last dying freak sound never again to be reproduced? If thats the case I'll probably keep the Bugle boy in v1 and reminece of what was that can't be no more.( That would make a good song title.. )

take care Shawners

[myles_rose]

Originally posted by Shawners:

Hi Myles ,

I've FOUND THE HOLY GRAIL 12AX7 TUBE...ITS NAME IS MULLARD ! Sorry for the shout , but I tried one of these tubes in v1 this past week-end in my mesa nomad 4x10 combo. I let it warm-up for a while than tried my clean channel..OMG a tube that lives up to its hype. I've never heard such a chimie clean sound before. I could hit harmonics that would of never been heard before that just ring and sustain. In the high gain channels it made my mesa sound like a hot rodded vintage marshal , just with more balls.The sad thing is that after playing for alittle over a hour it started to squeel and make that static fizzle noise , and started to sound flatter.The next day the magic didn't return so i tested the mullard by litely tapping it , and it would break into that ringing sound and that static fizzle noise .I put a new tube in v1 and the problem went away...MAN AM I BUMMED. Before investing in a new Mullard I want to make sure that the harmonic magic and tone I heard wasn't because the tube was on the verge of dying and making sounds that a new mullard wont make? (.. like a light bulb burns britest right before dying??).

I've bought a used Telefunkin 12ax7 and was dissapointed.It really lowered my noise floor like no tube has( even the Mullard), but seemed to choke out higher frequency sound (maybe thats why it was so quit and dark).It is a good tube , but I didn't experience that Magic hype (not that lively of a tube).Thats why I want to make sure before I buy another mullard. I also tried a NOS Bugle Boy labeled for medical use Holand.

That tube wasn't as quit as the Telefunkin , but had more life and high end sound , but still not that magic that the mullard created.

Soo.. If I buy another NOS Mullard , will i get that sound I had with the other , or was that its last dying freak sound never again to be reproduced? If thats the case I'll probably keep the Bugle boy in v1 and reminece of what was that can't be no more.( That would make a good song title.. )

take care Shawners

Shawners .........

 

There are a lot of "Mullards", including the CV4004, 10, 10M, etc. The box plates are something that would have to be slanted toward personal tastes, as many folks do not care for these all that much. The same is true of any Mullard, or any tube in general.

 

Each amp is different too. A BF Super Reverb will love a long smooth plate 7025, Raytheon, or Tung Sol from most folks point of view than many other tubes. Telefunkens were made in the Ei factory at times, and Mullard was owned by Philips, and at times made in countries other than the UK. It gets pretty darn confusing at times.

 

The bottom line is that it all boils down to personal taste. I know one fellow that is a hard core great jazz player that uses Mullard Box Plates in V1 of a Fender Blues Jr., and folks are always commenting on his super tone.

 

In your Mesa, if you love what you found, that is the end of that part of the tone game, or one leg that you got through where you ended up happy. Very cool.

 

One of the problems you found with your tube going microphonic, is pretty common. Many of these are not actually new, they are pulls. Even if new, they have been bounced around perhaps for decades. The percentage of great NOS tubes out there that are great ones, is less and less every year. These a lot of time were returns, or rejects in the first place, and now the bottom of the bucket is what is more often on the market.

 

In recent "Mullard" tests, going through a LOT of them, I found HUGE ranges:

 

DATE TUBE B+ volts BIAS volts mA actual TC (gm) actual ^ mA spec TC spec gp v Gain

 

12AX7 Tests 250 -2 1.2 1600 100.0

 

2/28/03 1a calibrate 1.1 1360 0.016 85.0

1b calibrate 1.2 1520 0.017 89.4

 

2a Mullard 1.6 1620 0.019 85.3

2b Mullard 1.5 1640 0.019 86.3

 

3a Mullard 0.9 1180 0.014 84.3

3b Mullard 0.8 1010 0.012 84.2

 

4a Mullard 2.2 1940 0.024 80.8

4b Mullard 1.3 1490 0.018 82.8

 

5a Mullard 1.4 1490 0.017 87.6

5b Mullard 1.5 1360 0.016 85.0

 

6a Mullard 1.3 1480 0.017 87.1

6b Mullard 1.4 1630 0.018 90.6

 

7a Mullard 0.9 1210 0.014 86.4

7b Mullard 1.2 1270 0.016 79.4

 

8a Mullard 1.1 1420 0.015 94.7

8b Mullard 1.1 1410 0.015 94.0

 

9a Mullard - marked GE 1.2 1510 0.017 88.8

9b Mullard - marked GE 1.1 1440 0.016 90.0

 

11a Mullard 1.3 1540 0.018 85.6

11b Mullard 1.9 1600 0.02 80.0

 

12a Mullard 1.3 1410 0.017 82.9

12b Mullard 2.8 1790 0.025 71.6

 

14a Mullard 1.4 1460 0.017 85.9

14b Mullard 1.4 1490 0.017 87.6

 

4/24/03 15a Mullard 3.1 1460 0.024 60.8

15b Mullard 0.9 1180 0.015 78.7

 

4/24/03 16a Mullard 0.9 1090 0.013 83.8

16b Mullard 1.4 1650 0.019 86.8

 

4/24/03 17a Mullard 1.1 1360 0.016 85.0

17b Mullard 1.3 1530 0.018 85.0

 

 

Average Mullard 12AX7/ECC83 1.4 1470 0.022 85.6

thru tube 14

......................

 

The formatting probably got messed up here, but the bottom line, is that in a nutshell, a 12AX7C9 or 12AX7R3 has a better chance of meeting specs.

 

The bouncing around of your tube over the decades had it's effects, which you saw the end result.

 

It is important to really know and trust any NOS vendor. Very few of them can afford and know how to use tools such as the George Kaye testers, VTV testers, and Tektronix 570 curve tracers. You need to have a vendor that knows how to test and will test. These tubes are pretty pricy at times, so you should expect a nice battery of tests were done and a nice return policy. New versions of these tubes lasted in excess of 10,000 hours and MUCH more. If a "new" tube goes bonkers in a month, that says something right there about it's prior handling, testing, and that is may have been a reject originally.

On Telefunkens - they are long plates, same situation as the above by the way in regard to quality, and these can be even more microphonic. What makes things worse, as these are VERY easy to counterfeit, and there are fakes all over the place. These are made on Ei smooth plate tooling with ease, as they were an OEM for Telefunken. Most fakes do not have the diamond in the base glass molding in the center of the pins - one tip off.

 

Remember, all these tubes are a crapshoot. Just because you don't care for a specific Bugle Boy, that may just mean that your example is a ODT (old doggie tube). If you are near me, feel free to bring me some of these and I will test and trace them for you and show you how this is done.

 

If you are working with a great tube seller, ask them for the following at 250 plate volts and -2 volts bias:

 

1. Current output in milliamps on each side (Ik)

 

2. Transconductance on both sides (gm)

 

3. Plate resistance on both sides (gp)

 

Then email me this data and I will give you some info that may be of help or that you can use to talk to the seller.

[Shawners]

Thanks again Myles,

I bought these from Doug at Dougs tubes( he is a super guy), but unfortunitly he's out of Mullards.Mine was a Mullard w/ a Heath lable , long plate version 105/100 . Unfortunatly he didn't have the other information listed on the tube. Do you think I really experienced the mullard hype , or was all that nice harmonic tone just because the tube was dying?? The tube still works , but it seems to of lost gain , and sounds kinda flat now along with that staticie fizzle that comes and goes. If you think that majic sound I was hearing was real because its a Mullard , who do you recommend for buying a replacement. I cant thank you enough for all your help , and the offer of visting you ( I wish I could , but I live in Toledo ,OH).

Your friend Shawners

[paranoid]

What do you think about Marshall superbass heads (for guitar use)?

Is it easy to convert them to lead specs?

[myles_rose]

Originally posted by Shawners:

Thanks again Myles,

I bought these from Doug at Dougs tubes( he is a super guy), but unfortunitly he's out of Mullards.Mine was a Mullard w/ a Heath lable , long plate version 105/100 . Unfortunatly he didn't have the other information listed on the tube. Do you think I really experienced the mullard hype , or was all that nice harmonic tone just because the tube was dying?? The tube still works , but it seems to of lost gain , and sounds kinda flat now along with that staticie fizzle that comes and goes. If you think that majic sound I was hearing was real because its a Mullard , who do you recommend for buying a replacement. I cant thank you enough for all your help , and the offer of visting you ( I wish I could , but I live in Toledo ,OH).

Your friend Shawners

Shawners.........

 

Doug Preston is a very cool guy. He is the one I use for most of my client tour support when they are on the east coast.

 

He uses the Vacuum Tube Valley tester, a MUCH more up to date and modern unit than most of the uncalibrated and older B&K and other testers out there from the 50s and 60s. This is a killer unit that tests also for noise and hum, as well as balance statically. Many of the bands I support have one of these in their techs toolboxes.

 

http://www.vacuumtube.com/toppage31.htm

 

If you buy tubes such as NOS ones, and can afford one of these, they will save you a ton of money as well as let you actually KNOW what preamp tubes are actually doing in your amp, and how they are rated. This goes for new or old tubes.

 

The problem you are having since you got this tube from Doug with the ratings off the VTV tester, are probably simply that this tube type is not as happy in your type of amp's front end. Lots of folks fall into the "Mullard mistique", where they may not be the best tube in a particular amp. Don't fret though, you can probably sell the tube on Ebay for a profit, or sell it to some audio hi fi type, where these work really nicely.

 

There may also have been physical damage, or another factor. Remember, on tubes that are 30+ years old, those nice pliable mica spacers deteriorate every single year, in a vacuum or not. Some last for 100 years, some only make it for ten.

 

You might want to ask Doug if you can send the tube back to him and have him retest it. Maybe it has changed. He may charge a few bucks for a retest, but for a pricy tube, this is well worth a few dollars.

 

As far as a replacement, that would be based on my really limited tastes I'd need to know your amp, guitar, music style played, for a start.

[myles_rose]

Originally posted by paranoid:

What do you think about Marshall superbass heads (for guitar use)?

Is it easy to convert them to lead specs?

paranoid ..........

 

These are great for guitar. Before you make any changes at all, play a guitar through the amp first, as many times, name labels were changed and not the amps themselves. These are great, as were the PA amps.

[meek20]

BUMP!

[vengence]

hi i just recently picked up a carvin x-100 head for 25 bucks that just hummed i figured it would be an easy fix but now im stumped i have clean sound if i run just through the power amp but hooked up through the pre-amp it just hums its driving me crazy ive tried 3 sets of preamp tubes checked the filter caps and tested the resistors and caps in the pre amp i was told recently that the guy i got the amp from had it checked out and they said there was a power supply problem the power wasent distributing right through the amp is it possible that the power supply is shot and is there any thing else that could cause this problem thanks Steve

[myles_rose]

Originally posted by vengence:

hi i just recently picked up a carvin x-100 head for 25 bucks that just hummed i figured it would be an easy fix but now im stumped i have clean sound if i run just through the power amp but hooked up through the pre-amp it just hums its driving me crazy ive tried 3 sets of preamp tubes checked the filter caps and tested the resistors and caps in the pre amp i was told recently that the guy i got the amp from had it checked out and they said there was a power supply problem the power wasent distributing right through the amp is it possible that the power supply is shot and is there any thing else that could cause this problem thanks Steve

Steve,

 

Generally the most common issue is a preamp tube in V1 or V2 ....

 

Then, filter caps in the power supply .........

 

Then power transformer, especially the heater windings.

 

I'd need to see the amp on a scope.

[steve f]

Myles,

 

I have a friend with a Dual Rectifier. She plugged it into two 4x12 cabs (Marshall 1960). She went from the two 4ohm connections on the amp to the 4 ohm jacks on the cabs. Of course this resulted in a 2 ohm load, which the amp isn't designed to handle. She said the sound level was very low and "distant". She unplugged after a few minutes and went back to using a single cab (16 ohm to 16 ohm) and the sound was fine.

 

What is the possibility of damage to the amp? She's played the amp for several hours since then (using a single cab with the correct connection), and it seems fine. Is there anything that should be checked?

[vengence]

hey thanks for the help i have another question can i replace a filter cap with one with different values cause im having a hard time finding the right ones for my amp the ones in it now are 40uf/40uf/20uf/20uf-----500vdc what if any other could i replace these with

[Mitch Harris]

Myles,

I recently had my 2001 VibroKing in for service and the amp tech noted that I had changed out the PI 12ax7 for a NOS(balanced) 12at7 I got from Mike at KCA. He's trying to tell me that this is not a good idea as he says the VK is designed for a 12ax7 in this slot. He further says that in his experience sometimes using a 12at7 where a 12ax7 was ment to go can lead to additional noise. This does not make intuitive sense to me. As one of the reasons I have the amp in for service is a loud background hum(which he says is normal and can't be fixed) I am particularly interested on your take on this. I know a 12at7 has much lower gain, so why would it contribute more noise than a 12ax7 which has higher gain? I do have some balanced low gain 12ax7's I got from Mike also, so I could use them instead, but I am under the impression that the typical Fender design uses a 12at7 for this spot.

 

Thanks for your input

Mitch

[myles_rose]

Originally posted by steve f:

Myles,

 

I have a friend with a Dual Rectifier. She plugged it into two 4x12 cabs (Marshall 1960). She went from the two 4ohm connections on the amp to the 4 ohm jacks on the cabs. Of course this resulted in a 2 ohm load, which the amp isn't designed to handle. She said the sound level was very low and "distant". She unplugged after a few minutes and went back to using a single cab (16 ohm to 16 ohm) and the sound was fine.

 

What is the possibility of damage to the amp? She's played the amp for several hours since then (using a single cab with the correct connection), and it seems fine. Is there anything that should be checked?

Steve,

 

She may have been lucky. Output transformer failure is the usual result. Since the amp still works, the OT is probably okay, although may have a bit of life zapped out of it due to possible heat damage.

 

The output tubes probably took a bit of premature wear. I'd check the bias to get the amp back to a proper range.

[myles_rose]

Originally posted by vengence:

hey thanks for the help i have another question can i replace a filter cap with one with different values cause im having a hard time finding the right ones for my amp the ones in it now are 40uf/40uf/20uf/20uf-----500vdc what if any other could i replace these with

vengence.........

 

Try to stay with the same uf rating, but if you have to change voltage, it is generally fine to go higher.

 

Your caps can be found at:

 

www.tubesandmore.com

 

www.eurotubes.com (JJ caps, very nice)

[myles_rose]

Originally posted by Mitch Harris:

Myles,

I recently had my 2001 VibroKing in for service and the amp tech noted that I had changed out the PI 12ax7 for a NOS(balanced) 12at7 I got from Mike at KCA. He's trying to tell me that this is not a good idea as he says the VK is designed for a 12ax7 in this slot. He further says that in his experience sometimes using a 12at7 where a 12ax7 was ment to go can lead to additional noise. This does not make intuitive sense to me. As one of the reasons I have the amp in for service is a loud background hum(which he says is normal and can't be fixed) I am particularly interested on your take on this. I know a 12at7 has much lower gain, so why would it contribute more noise than a 12ax7 which has higher gain? I do have some balanced low gain 12ax7's I got from Mike also, so I could use them instead, but I am under the impression that the typical Fender design uses a 12at7 for this spot.

 

Thanks for your input

Mitch

Mitch,

 

Mike at KCA is right.

 

The reason that in some amps there is more noise with the lower gain 12AT7 is that there are more factors than just gain. The TC of a 12AX7 is about 1600 vs 5500 for an AT7. The plate resistance of the two tubes is different, and the current output is 1.2 milliamps versus 10.0 milliamps.

 

In the bias network for this amp, the resistor and cap for the cathode biased tube, is of a value that is more suited for a 12AX7, or should I say, a Sovtek 12AX7WA tube. This tube does not generally have a gain of 100, it is closer to 83, or closer to a 12AT7. It's TC (transconductance) is also typically around 1278 versus the 1600 expected average of a 12AX7 / ECC83 / 7025.

 

Big tube users such as Fender have a lot of pull with tube makers. They can get them to drop specs until the tube runs cool, making them quiet, have a lower reject rate in a batch of a lot of them, and have long life in an amp with less warranty issues.

 

Some amps can ingest 12AX7 or 12AT7 with no issues at all. Some cannot do this as easily as there may be stability (and noise) issues that surface. Your Vibro King is not based on an older design. It is a new Fender design with a foundation in amps of the past, but is different. Fender chose to use the 12AX7WA, and in this amp, it is a great tube.

 

If you want to get more headroom while using a 12AX7 in this amp, then you need a selected version of a 12AX7 that has been chosen for lower gain. Mike can do this with any of his NOS tubes, as he has a VTV high end test unit that works super for this sort of thing, and it also is a great tester for noise and hum.

 

I steer clear of NOS tubes for phase inverters personally, as the cost is not justified to me. These are current drivers, and need to be picked for their specs, not their type, name, or cost. Save the NOS bucks for the tone and gain stages.

 

Hope this was of some help.

[roy d]

Hey Myles,

 

Could you say something about the differences in sound between pre-amps using 12AX7's, 6SL7's, and EF86? I'm beginning to get gas to spend some bucks on a serious amp and trying to make sense of all of the options out there... Of course, before I make any decisions, I'll try to connect with you to try out some more toys at your house

 

Also, there is a company that builds an adapter to allow the use of the 6SL7 in place of a 12ax7 http://www.electrosonicamplifiers.com/otherproductsservices.htm... any thoughts on how that might affect the sound of any amp/my peavey delta?

 

Thanks Myles.

[myles_rose]

Originally posted by roy d:

Hey Myles,

 

Could you say something about the differences in sound between pre-amps using 12AX7's, 6SL7's, and EF86? I'm beginning to get gas to spend some bucks on a serious amp and trying to make sense of all of the options out there... Of course, before I make any decisions, I'll try to connect with you to try out some more toys at your house

 

Also, there is a company that builds an adapter to allow the use of the 6SL7 in place of a 12ax7 http://www.electrosonicamplifiers.com/otherproductsservices.htm... any thoughts on how that might affect the sound of any amp/my peavey delta?

 

Thanks Myles.

Roy,

 

Basically the octal preamp tubes had about the same specs as the 9 pin tubes used today. The were moved away from, as when higher plate voltages and gain came into play, they were more microphonic. Some classic low front end gain amps are resurrecting these a bit.

 

You can go from octal to nine pin via adaptors, but nobody I know of has adaptors to go the other way.

 

Now, in the case of the EF86, this is a pentode and distorts much differently than a triode. Amps that use these are amps like the Matchless DC-30, some Bad Cat amps, the Victoria Sovereign, an older Vox amp and some Dr. Z amps. The EF86 (6267) is capable of a LOT of gain (400-2000+) depending on the circuit. You can also get this gain in some amps, without a lot of extra noise that you get from cascading 12AX7s to get higher gain. Some folks use this tube for gain, some for tone, some for both. It is a pretty amazing and articulate sound.

 

In Dr. Z amps, he uses them in a way that is quite a bit different than in other applications. This is why the KT-45, Route 66, Z-28 amps that use these in the front end are also quiet and not as picky about the quality of the tube. Dr. Z is one fellow that really did this in a very cool way.

 

In any case, next time you are down, I have some stuff you can play with to compare. I currently have in the place a few amps for you to try - a tweed deluxe (older design with a "low gain" 12AY7 in the front end, but surprises many with the gain it has), a KT-45 and Sovereign (each with EF86 front ends and the same power rating and output tubes, so they are cool to compare), and some other stuff. Anytime, other than the w/e of June 21st, where I will be busy, so just call me at home any evening and leave a message if I am out working.

 

On your Delta, just stay with the 12AX7. I can pick you out a tube with the same specs as an octal based tube after going though about 20 or so.