Feel free to ask Myles

[mightypudge]

Hi Myles,

 

Just yesterday I picked up a used Rivera Knucklehead K100 head. It needed re-tubed so I installed some Rivera EL34's (labeled #10) that I had laying around from a Rivera I previously owned.

 

I wanted to make sure I checked the bias correctly because although the amp sounds OK and the tubes aren't glowing red, I feel a significant amount of heat coming off the amp.

 

I used a DVM set to measure MADC. With the speaker cable unplugged I turned the amp on and let it warm up for a few minutes. I connected the black lead to the chassis and touched the red lead to pin 8 of one of the power tubes. It read somewhere near 25 so I increased the bias to 40. I checked each tube and they were all reading between 40-41.

 

Does it sound like I have it right? If I decide to swap these tubes for Groove Tubes, which rating would get me close without needing to rebias?

[myles_rose]

Originally posted by mightypudge:

Hi Myles,

 

Just yesterday I picked up a used Rivera Knucklehead K100 head. It needed re-tubed so I installed some Rivera EL34's (labeled #10) that I had laying around from a Rivera I previously owned.

 

I wanted to make sure I checked the bias correctly because although the amp sounds OK and the tubes aren't glowing red, I feel a significant amount of heat coming off the amp.

 

I used a DVM set to measure MADC. With the speaker cable unplugged I turned the amp on and let it warm up for a few minutes. I connected the black lead to the chassis and touched the red lead to pin 8 of one of the power tubes. It read somewhere near 25 so I increased the bias to 40. I checked each tube and they were all reading between 40-41.

 

Does it sound like I have it right? If I decide to swap these tubes for Groove Tubes, which rating would get me close without needing to rebias?

Never turn on an amp with the speaker unplugged.

 

Lucky you still have an output transformer.

 

You also need a proper bias probe and set the current at 38mA WITH a current probe and a speaker connected or a load on the jack.

 

The Rivera numbers are 7-13 but these are static ratings, not dynamic GT ratings. There is really no cross reference.

[mightypudge]

Myles,

 

Thanks for the advice. I was informed that Rivera amps were different in that you can unplug the speaker from the jack in order to bias the amp. I believe I learned this on the old R.O.G. and it's also noted on the old Scobro website (now maintained by Richard Johnson).

 

Is this information not correct?

[myles_rose]

Originally posted by mightypudge:

Myles,

 

Thanks for the advice. I was informed that Rivera amps were different in that you can unplug the speaker from the jack in order to bias the amp. I believe I learned this on the old R.O.G. and it's also noted on the old Scobro website (now maintained by Richard Johnson).

 

Is this information not correct?

This is true on the Rivera amps as when the speaker is unplugged the B+ is turned off but it is not a good practice as it is really a very bad habit. On just about any other amp this would be disaster. On top of that issue, with the amp not in true operation mode the voltages are not loaded properly and any bias adjustment will be inaccurate.

[A String]

A question from one of our new members:

 

HI Myles,

I'm new to this forum and my question is short

 

I have a Fender Bandmaster silver face with tremolo (No reverb)and need to know the position of the phase inverter 12AT7. Right now I have it in the slot right next to the 2 6L6s because i saw a list of positions for Fender amps posted on the internet that seemed to have the AT7 always in that slot. But I'm not sure. The other tubes are 12AX7s and the 2 6L6s. Also, do you know if this amp is class A (i.e. does replacing the power tubes require biasing). I would greatly appreciate advice from someone who knows.

Thank You and Happy Birthday,

Rory

[myles_rose]

Originally posted by A String:

A question from one of our new members:

 

HI Myles,

I'm new to this forum and my question is short

 

I have a Fender Bandmaster silver face with tremolo (No reverb)and need to know the position of the phase inverter 12AT7. Right now I have it in the slot right next to the 2 6L6s because i saw a list of positions for Fender amps posted on the internet that seemed to have the AT7 always in that slot. But I'm not sure. The other tubes are 12AX7s and the 2 6L6s. Also, do you know if this amp is class A (i.e. does replacing the power tubes require biasing). I would greatly appreciate advice from someone who knows.

Thank You and Happy Birthday,

Rory

The phase inverter is most close to the output tubes and most far from the input jack.

[cm01]

Hi Myles,

 

I'm new to the whole forum thing, so apologies if I get this wrong!

 

I've got a query regarding a 70's Fender Champ I've recently acquired... I've replaced the valves and stock Fender speaker (with a Jensen P8R) and it sounds great. My problem is that it stays clean all the way up to 10 on the volume pot... every review I've read of Champs says that you get sweet Class A overdrive that kicks in as the volume increases, but as I've said, mine stays clean all the way up to 10.

 

The amp is 110v (US) and I'm using a transformer to run it on UK mains. Any advice you can give is gratefully received,

 

thanks in advance,

 

Chris.

[A String]

Originally posted by myles111:

Originally posted by A String:

A question from one of our new members:

 

HI Myles,

I'm new to this forum and my question is short

 

I have a Fender Bandmaster silver face with tremolo (No reverb)and need to know the position of the phase inverter 12AT7. Right now I have it in the slot right next to the 2 6L6s because i saw a list of positions for Fender amps posted on the internet that seemed to have the AT7 always in that slot. But I'm not sure. The other tubes are 12AX7s and the 2 6L6s. Also, do you know if this amp is class A (i.e. does replacing the power tubes require biasing). I would greatly appreciate advice from someone who knows.

Thank You and Happy Birthday,

Rory

The phase inverter is most close to the output tubes and most far from the input jack.

Thanks Myles! I'll let him know.

[Rock Soldier]

Hi Myles. I've been reading through this thread and learned alot. I have what is probably a dumb question but here it is. Can I safely switch from the internal speaker (by unplugging it) to an external one with my Epiphone Valve Special if I have it in standby, or do I need to power down to unhook the speaker. I know it is a big no no to run a tube amp with no load on the output transformer. I would want to do this with the amp on for A/B purposes in selecting speaker cabs.

[myles_rose]

Originally posted by cm01:

Hi Myles,

 

I'm new to the whole forum thing, so apologies if I get this wrong!

 

I've got a query regarding a 70's Fender Champ I've recently acquired... I've replaced the valves and stock Fender speaker (with a Jensen P8R) and it sounds great. My problem is that it stays clean all the way up to 10 on the volume pot... every review I've read of Champs says that you get sweet Class A overdrive that kicks in as the volume increases, but as I've said, mine stays clean all the way up to 10.

 

The amp is 110v (US) and I'm using a transformer to run it on UK mains. Any advice you can give is gratefully received,

 

thanks in advance,

 

Chris.

Chris,

 

If the amp stays clean to "10" then it should see a tech to check the voltages as something is very wrong.

 

It may be running at a fraction of the output that the amps is designed to run at due to low voltages somewhere.

[myles_rose]

Originally posted by trguitar:

Hi Myles. I've been reading through this thread and learned alot. I have what is probably a dumb question but here it is. Can I safely switch from the internal speaker (by unplugging it) to an external one with my Epiphone Valve Special if I have it in standby, or do I need to power down to unhook the speaker. I know it is a big no no to run a tube amp with no load on the output transformer. I would want to do this with the amp on for A/B purposes in selecting speaker cabs.

You can use standby to do this for a few seconds with no problems.

[Junior 1]

Hi Myles,

Bedroom player here, and I'd like to use a nice sounding 8" combo amp speaker as an extension speaker for my Valve Junior head. Should I disconnect the speaker from the amp and install jacks for both amp and speaker or would it be allright to wire in a jack for the speaker, while leaving it connected to the amp. Of course, while being used as a cab, the amp would not be plugged in. What do you think? Thanks.

[myles_rose]

Originally posted by Junior 1:

Hi Myles,

Bedroom player here, and I'd like to use a nice sounding 8" combo amp speaker as an extension speaker for my Valve Junior head. Should I disconnect the speaker from the amp and install jacks for both amp and speaker or would it be allright to wire in a jack for the speaker, while leaving it connected to the amp. Of course, while being used as a cab, the amp would not be plugged in. What do you think? Thanks.

You can do either but if you use both you have to assure that the impedance of the speakers is what the amp expects to see.

[Junior 1]

Thanks, the only speaker I'll use with the combo is the one in it. That's why I'd like to leave it connected. I use the amp for CD play alongs. With the speaker still connected to the amp and a jack installed, could I plug in another cab while using that amp with a CD player? Would the speakers be in parallel or series? Thanks for your time.

[myles_rose]

Originally posted by Junior 1:

Thanks, the only speaker I'll use with the combo is the one in it. That's why I'd like to leave it connected. I use the amp for CD play alongs. With the speaker still connected to the amp and a jack installed, could I plug in another cab while using that amp with a CD player? Would the speakers be in parallel or series? Thanks for your time.

You can wire it either way but wire it in series. There will be a loss in performance but it will still be safe for the amp. In parallel it would not be safe for the amp.

[Junior 1]

Myles,

When I wire the jack, positive to positive and negative to negative on the speaker and I then plug in a cabinet wired the same way, I assume that's parallel?

[myles_rose]

Originally posted by Junior 1:

Myles,

When I wire the jack, positive to positive and negative to negative on the speaker and I then plug in a cabinet wired the same way, I assume that's parallel?

Look on the net for basic wiring diagrams.

 

You want to be in SERIES! In parallel your impedance will be too low and you will kill your output transformer.

[Junior 1]

Thank you, I found the wiring diagrams on th net and do understand that speakers in parallel halve the impedence and in series it doubles. I'm going to use the 2 jack option Thanks again.

[TieDyedDevil]

Myles --

 

I bought a new Vibro-King in 2003. It was one of the winter NAMM display amps, and arrived with GT6L6B output tubes. After a few months, one of those shorted (taking a screen resistor with it). I replaced the tubes with a pair of Mesa/Boogie 5881 STR-425s that I had on hand -- they have a wafer base and I believe they're made by Sovtek. I've been very happy with the sound and reliability of the M/B tubes.

 

It's time to replenish my stock of spares and I'm considering giving GT another chance, but I have a few questions:

 

1) I like the narrow selection of operating ranges that Mesa/Boogie picks for their tubes; it means that I can swap tubes in an emergency without worrying about having to reset bias. Can I still get away with that if I stick with the same GT rating, e.g 4-7? If not, what if I bought all my replacement tubes such that they had a single GT rating, e.g. 6? (And if that'd work, how do I specify such a choice when I order? The online system doesn't give me that option.)

 

2) Am I correct in assuming that the M/B STR-425 and the GT6L6B share the same origin? If not, what differences should I expect?

 

3) Finally, and I realize that this may be difficult to answer without having heard me play, what would *you* recommend for output tubes in the Vibro-King? I play a guitar with vintage-output humbuckers and really love the sound I'm getting now with the 5881s. I like to dial in my amp for clean piano-like lows with a bit of warmth on the top-end as I dig in.

 

Advance thanks for your help,

Dave

[myles_rose]

Originally posted by TieDyedDevil:

Myles --

 

I bought a new Vibro-King in 2003. It was one of the winter NAMM display amps, and arrived with GT6L6B output tubes. After a few months, one of those shorted (taking a screen resistor with it). I replaced the tubes with a pair of Mesa/Boogie 5881 STR-425s that I had on hand -- they have a wafer base and I believe they're made by Sovtek. I've been very happy with the sound and reliability of the M/B tubes.

 

It's time to replenish my stock of spares and I'm considering giving GT another chance, but I have a few questions:

 

1) I like the narrow selection of operating ranges that Mesa/Boogie picks for their tubes; it means that I can swap tubes in an emergency without worrying about having to reset bias. Can I still get away with that if I stick with the same GT rating, e.g 4-7? If not, what if I bought all my replacement tubes such that they had a single GT rating, e.g. 6? (And if that'd work, how do I specify such a choice when I order? The online system doesn't give me that option.)

 

2) Am I correct in assuming that the M/B STR-425 and the GT6L6B share the same origin? If not, what differences should I expect?

 

3) Finally, and I realize that this may be difficult to answer without having heard me play, what would *you* recommend for output tubes in the Vibro-King? I play a guitar with vintage-output humbuckers and really love the sound I'm getting now with the 5881s. I like to dial in my amp for clean piano-like lows with a bit of warmth on the top-end as I dig in.

 

Advance thanks for your help,

Dave

Dave,

 

Fender ships the VibroKing with the 6L6GE tubes as they do with all their high end amps. They are more articulate than the 6L6B® which is what they use in the other amp lines. Your tube seems to be the 6L6B®. But .... in the end, this is all personal taste. In my own Mesa Mk1 I use the 6L6B®. They tame a bit of the brightness in some Mesa amps. It is sort of a character like using grill cloth made from a heavy flannel shirt but that is what I prefer in the Mark 1.

 

Mesa color codes are in six ratings. These are all very close. They have to be as Mesa has fixed bias in their amps which is non-adjustable. So, all their tubes fit in the GT 4-6 range as follows ...

 

Mesa scale Groove Tubes scale

Red 4

Yellow 4

Green 5

Gray 5

Blue 6

White 6

 

Mesa does a great job of testing and matching. They are also a great value for the testing that is done which is far in excess of many other vendors and tube suppliers.

 

In the end, in my own clients VibroKings I use the GE's in a #6 rating biased at 30mA or the 6L6R2 (real Svetlanas) biased at 32mA in a #5-6 rating. If you have Mesa tubes in there now these would be plug and play for the most part.

 

In the end it is all personal taste.

[TieDyedDevil]

Originally posted by myles111:

Originally posted by TieDyedDevil:

Myles --

 

I bought a new Vibro-King in 2003. It was one of the winter NAMM display amps, and arrived with GT6L6B output tubes. After a few months, one of those shorted (taking a screen resistor with it). I replaced the tubes with a pair of Mesa/Boogie 5881 STR-425s that I had on hand -- they have a wafer base and I believe they're made by Sovtek. I've been very happy with the sound and reliability of the M/B tubes.

 

It's time to replenish my stock of spares and I'm considering giving GT another chance, but I have a few questions:

 

1) I like the narrow selection of operating ranges that Mesa/Boogie picks for their tubes; it means that I can swap tubes in an emergency without worrying about having to reset bias. Can I still get away with that if I stick with the same GT rating, e.g 4-7? If not, what if I bought all my replacement tubes such that they had a single GT rating, e.g. 6? (And if that'd work, how do I specify such a choice when I order? The online system doesn't give me that option.)

 

2) Am I correct in assuming that the M/B STR-425 and the GT6L6B share the same origin? If not, what differences should I expect?

 

3) Finally, and I realize that this may be difficult to answer without having heard me play, what would *you* recommend for output tubes in the Vibro-King? I play a guitar with vintage-output humbuckers and really love the sound I'm getting now with the 5881s. I like to dial in my amp for clean piano-like lows with a bit of warmth on the top-end as I dig in.

 

Advance thanks for your help,

Dave

Dave,

 

Fender ships the VibroKing with the 6L6GE tubes as they do with all their high end amps. They are more articulate than the 6L6B® which is what they use in the other amp lines. Your tube seems to be the 6L6B®. But .... in the end, this is all personal taste. In my own Mesa Mk1 I use the 6L6B®. They tame a bit of the brightness in some Mesa amps. It is sort of a character like using grill cloth made from a heavy flannel shirt but that is what I prefer in the Mark 1.

 

Mesa color codes are in six ratings. These are all very close. They have to be as Mesa has fixed bias in their amps which is non-adjustable. So, all their tubes fit in the GT 4-6 range as follows ...

 

Mesa scale Groove Tubes scale

Red 4

Yellow 4

Green 5

Gray 5

Blue 6

White 6

 

Mesa does a great job of testing and matching. They are also a great value for the testing that is done which is far in excess of many other vendors and tube suppliers.

 

In the end, in my own clients VibroKings I use the GE's in a #6 rating biased at 30mA or the 6L6R2 (real Svetlanas) biased at 32mA in a #5-6 rating. If you have Mesa tubes in there now these would be plug and play for the most part.

 

In the end it is all personal taste.

Thanks for your prompt and informative response, Myles.

[myles_rose]

Thanks for your prompt and informative response, Myles. [/QB]

 

 

You are more than welcome,

[Trucks]

Hi Myles, Ive been looking all over the internet but cant find a straight explaination on what the difference is between lin and log pots. Can you help out at all? I am asking because I have put together a Fuzz Face pedal and have used a Log pot for the Fuzz control, this is all i have that is the right value but I am only getting a sound when the guitar is played extremely loudly with the volume cranked right up... and then the sound swells and dies out very quickly. The instructions say to use a linear pot, could this be the cause of my problem?

 

Im not sure if you can help as I know your an amp expert by trade, but any help would be greatly appreciated.

 

Thanks

 

Lee

[myles_rose]

Originally posted by TheOtherLee:

Hi Myles, Ive been looking all over the internet but cant find a straight explaination on what the difference is between lin and log pots. Can you help out at all? I am asking because I have put together a Fuzz Face pedal and have used a Log pot for the Fuzz control, this is all i have that is the right value but I am only getting a sound when the guitar is played extremely loudly with the volume cranked right up... and then the sound swells and dies out very quickly. The instructions say to use a linear pot, could this be the cause of my problem?

 

Im not sure if you can help as I know your an amp expert by trade, but any help would be greatly appreciated.

 

Thanks

 

Lee

Lee ...

 

There are generally three types of pots ... linear, log, and audio.

 

Think of a linear pot that goes from 1-10 with the numbers being equally spaced so at 1 we are at 10% of the level or effect, 5=50% etc.

 

Audio and log taper pots have their own scales which are not linear ... say 3=20% and 7=40% as an example with most of the change in the 8-10 range. The log pot follows it's own paticular taper.

 

This is precisely your problem.

[A String]

Another question for Rory:

 

Myles,

Thank you for all your sharing. Question: Silver face Bandermaster, Requires biasing or not after replacing all old tubes?

 

Is this a class A trip (In your experience)or not.

Love,

R

[Trucks]

Originally posted by myles111:

Originally posted by TheOtherLee:

Hi Myles, Ive been looking all over the internet but cant find a straight explaination on what the difference is between lin and log pots. Can you help out at all? I am asking because I have put together a Fuzz Face pedal and have used a Log pot for the Fuzz control, this is all i have that is the right value but I am only getting a sound when the guitar is played extremely loudly with the volume cranked right up... and then the sound swells and dies out very quickly. The instructions say to use a linear pot, could this be the cause of my problem?

 

Im not sure if you can help as I know your an amp expert by trade, but any help would be greatly appreciated.

 

Thanks

 

Lee

Lee ...

 

There are generally three types of pots ... linear, log, and audio.

 

Think of a linear pot that goes from 1-10 with the numbers being equally spaced so at 1 we are at 10% of the level or effect, 5=50% etc.

 

Audio and log taper pots have their own scales which are not linear ... say 3=20% and 7=40% as an example with most of the change in the 8-10 range. The log pot follows it's own paticular taper.

 

This is precisely your problem.

Thanks for your help myles... And why couldnt all these sites that state "how pots work" jsut say what you did... You have a good way of explaining I have the right pot on order so hopefully all will be sorted

[myles_rose]

Originally posted by A String:

Another question for Rory:

 

Myles,

Thank you for all your sharing. Question: Silver face Bandermaster, Requires biasing or not after replacing all old tubes?

 

Is this a class A trip (In your experience)or not.

Love,

R

Bandmasters are grid biased - class a/b amps and do require a bias adjustment.

[myles_rose]

Originally posted by TheOtherLee:

Originally posted by myles111:

Originally posted by TheOtherLee:

Hi Myles, Ive been looking all over the internet but cant find a straight explaination on what the difference is between lin and log pots. Can you help out at all? I am asking because I have put together a Fuzz Face pedal and have used a Log pot for the Fuzz control, this is all i have that is the right value but I am only getting a sound when the guitar is played extremely loudly with the volume cranked right up... and then the sound swells and dies out very quickly. The instructions say to use a linear pot, could this be the cause of my problem?

 

Im not sure if you can help as I know your an amp expert by trade, but any help would be greatly appreciated.

 

Thanks

 

Lee

Lee ...

 

There are generally three types of pots ... linear, log, and audio.

 

Think of a linear pot that goes from 1-10 with the numbers being equally spaced so at 1 we are at 10% of the level or effect, 5=50% etc.

 

Audio and log taper pots have their own scales which are not linear ... say 3=20% and 7=40% as an example with most of the change in the 8-10 range. The log pot follows it's own paticular taper.

 

This is precisely your problem.

Thanks for your help myles... And why couldnt all these sites that state "how pots work" jsut say what you did... You have a good way of explaining I have the right pot on order so hopefully all will be sorted

Lee ... it is not a matter of your problem "hopefully" being resolved with the proper pot, it will totally be resolved

[untalented]

Myles -

 

Thanks for your time answering all these questions!

 

Mine is this, I have a Fender Blues DeVille 2x12 and I really like it but it is a bit too loud for my apartment and I've been looking into attenuators.

 

I called the closest place to me to ask about the THD or Air Brake and the guy I spoke with told me that neither was really appropriate for my amp and since it has a master volume he thought I'd probably burn up the amp or wreck it in some way.

 

Do you have any suggestions for attenuation on this amp? I'm not looking for bedroom volumes, just something to keep the police from knocking the door down.

[myles_rose]

Originally posted by untalented:

Myles -

 

Thanks for your time answering all these questions!

 

Mine is this, I have a Fender Blues DeVille 2x12 and I really like it but it is a bit too loud for my apartment and I've been looking into attenuators.

 

I called the closest place to me to ask about the THD or Air Brake and the guy I spoke with told me that neither was really appropriate for my amp and since it has a master volume he thought I'd probably burn up the amp or wreck it in some way.

 

Do you have any suggestions for attenuation on this amp? I'm not looking for bedroom volumes, just something to keep the police from knocking the door down.

Either of the devices you mention work great with your amp ... it has nothing to do with master volume or no master volume.

 

As a side note, in Kenny Loggins amps on stage on the Loggins & Messina tour, there was an airbrake in each Fender Hot Rod Deluxe cabinet.