Gabriel E.

October 13, 2004

Thanks!

October 11, 2004

Myles,

I understand the importance of using a balanced dual triode for a phase inverter if the splitter is of the Shmitt(sp?) type found in most Fender and Marshall-type amps. In these amps, the two halves of the PI basically run in parallel.

However, my Super uses a concertina type where the two halves of the PI run in series. What's to be gained [pun intended ] by using a balanced PI in this situation?

Thanks,

Gabriel

September 21, 2004

Thanks Myles!

September 20, 2004

Hey Miles,

In your primer, when you talk about biasing using the current draw method, you advise against "chasing the bias" (which I've admittedly been doing) as the wrong current draw can damage the tube before you have a chance to set the bias properly. You go on to suggest lowering the current draw as much as possible before installing the tubes and then bringing it back up to the correct setting once the new tubes are in place.

My question is this:

If you drop the bias current way down, aren't you pushing the plate voltage way up? Won't this potentially damage the tube as well.

June 2, 2004

Thanks! I'll let you know how it turns out.

June 1, 2004

Hi Myles,

I am in the process of restoring an old Lowrey Holiday organ from the early 60s that uses 7591 tubes. These tubes are rare and expensive. I've heard bad things about the new Sovtek 7591 tubes (plus they won't fit without poking through the back panel).

THD makes a set of conversion Yellowjackets but they will cause the EL-84s to stick WAY out the back panel. And they cost $$.

I have received a lot of mixed messages regarding whether I can sub 6L6 tubes if I rewire the sockets (easy to do on this unit). Can the 6L6s handle the voltage? Looking at the specs, it looks like KT66s are closer. Can I rewire and use 6V6s?

Thanks in advance for your help.

Gabriel

November 18, 2003

Thanks for the responses guys. Interesting about Curtis Mathis. Never seen one of those TVs around.

Bluestrat, I'll PM you a list this evening or tomorrow.

Gabriel

November 17, 2003

Not a question this time - just bragging.

Bought a box of tubes at a Salvation Army auction the other day for $25. Most were RCA tubes for B&W TVs.

However:

1-Mullard 12AX7

1-Ratheon 5U4GB

1-RCA 12AU7

1-RCA 5R4GYB

1-Amperex EF86

1-Mullard 5AR4/GZ34 with a metal base.

All in unused condition.

Put the RCA 12AU7 and the Mullard 5AR4 in my tweed Super and it sounds incredible. Need to rebias though cuz the plates on the output tubes started glowing.

A bunch of the RCA tubes were rebranded "Curtis Mathers". Know anything about these?

July 16, 2003

Did some work on my Mig-50. Recapped it and increased the power supply filtering (not as easy as it sounds - couldn't find replacements for through-hole Russian can caps so I had to figure out how to stuff in six 350v Sprague Atoms. Ended up buiding a new circuit board and floated it over the PT bell).

Also added a cathode bypass cap to V2.

So now the question is which power tubes to put in there. I have decided to stick with 5881s. The amp puts 520 volts on the plates which is more than most 5881s and 6L6s are rated for. I am looking for easy break-up.

Would JAN Phillips 6L6WGBs be a good choice?

February 7, 2003

Ok. Here's another question:

When played clean, my Sovtek Mig-50 has a much faster response than my Fender 5F4.

I'm assuming this is because of the sag from the 5V4 rectifier in the Fender. Is this the case? If so, what tube(s) would you recommend to speed up the attack? The tube chart specifies a 5U4G but that won't fit since a former owner cut the amp into a head.

Thanks

January 16, 2003

Gabriel,

That is not a big enough difference to matter, the big issue here is making sure the voltage of the cap is equal or greater to the one coming out.

You can also contact Sovtek directly, just go to www.sovtek.com and then get the phone number over there, it is an 800 number I think, call there, ask for Al Otto .... say hello from me (he knows me more from my Blueprinting business and Westwood Music more the from GT I think), and tell him what you need as far as caps, as they have them as they are the amps importer.

There are always a ton of differences in Russian prints and reality

Thanks Myles. Component selection always makes my head spin. New Sensor/Sovtek carry both LCR and JJ caps. Any preference?

Also, Allied has some Mallory CGS computer-grade caps for a reasonable price. Is there a noticable difference between these 3 brands?

On Ampage, someone mentioned increasing the plate resistors on the preamp tubes to 220K for added gain. Will this work? Worth it?

BTW, I switched out the ceramic disks on my Champ 12 for silver micas and replaced the Illinois preamp electrolytics with Black Gates and the thing sings! The tonal improvement is massive.

It never ends!

January 14, 2003

Here's a question re: power supply filter caps.

My Sovtek Mig-50 uses two large can-type 200uf/350v electrolytics. I would like to replace them but am finding lots of 220uf caps and not too many 200uf caps.

If I use 220uf caps, will the extra 20uf make a difference?

BTW, the schematic I found calls for 220uf caps. There are a lot of differences in the pot and cap values between the schem. and the amp.

January 2, 2003

Can I put EL34s in a Sovtek Mig-50 in place of the 5881s? It supposedly puts over 500V on the plates but the tube data sheet I read for the EL34 said it can handle 800V. Is this accurate?

How will the sound differ with EL34s vs. 5881s?

December 7, 2002

The Solens are rated at 400 and 630 volts.

Solen

The restaurant you mentioned is probably Cafe Degas. Just ate there last month - Yum! It's in my neighborhood. As is the place you stay which is right by the Rastafarian church.

December 6, 2002

I'm still a little confused about the Solens. I hear what you're saying about polarized electrolytics normally being used as power supply filter caps (that's what my amps currently have). However some boutique manufacturers such as Carr boast of using Solen Polypropylene filter caps. ??

In the parts list for the single-ended 6v6 project amp at Angela Instruments thay suggest subbing a Polypropolene for a 16uf/475v electrolytic.

By the way, there is, or was, a cool restaurant called Peristyle (I may have spelled it wrong), that is, on a scale of 1-10, about a 25. Try it!

We've been meaning to go there for a while. The owner/executive chef was a cutomer of mine a few years back. It was a big deal when she opened it because she was only 30 at the time. She made it onto the cover of Wine Spectator too.

Next time you're planning on coming to town, let me know. I know some cool places to eat.

December 6, 2002

Not confusing at all Myles. Thanks for the advice (I'm going with the Tung-Sol based on previous help you gave me). I'll go with the 30 watt alnico speaker.

The caps are power-supply filter caps -47uf/400V. Maybe I'll try some Mallorys.I have some paper in oil signal caps in my 5F4 and that amp sounds better with them. The Champ is circuit board construction and is a lot harder to work on so I'm sticking to the basics for now.

While I may be here for the holidays, I wish I was where you are during the summer.

December 5, 2002

Hi Myles:

For the holidays, I am treating myself to a Tung-Sol 5881, Weber Blue Dog speaker and new caps for my Champ 12 (have been planning this for a while).

The Blue Dog is available in 15, 30, and 50 watt models. Since it's a 15 watt amp, I'm assuming the 15 watt speaker will work since I want some breakup. Or is this not a good idea? Should I go with the 30 watt for safety?

I also want to get new caps. Solen fast caps are not electrolytic (they're metallized polypropylene). Can they still be used as filter caps?

Thanks,

Gabriel

October 14, 2002

THX Miles. I appreciate it.

October 14, 2002

Thanks Myles:

After X-Mas, I should have some extra cash so I'll definitely get a Tung-Sol.

I have some questions regarding the Trainwreck Express semi-clone that I'm planning to build and about power transformers in general

Is there such thing as a too large PT? I know how much current I need for the tubes and filaments. I also know I want to put about 410 Volts on the plates. And I will be using a 6K6 plate-plate OT (maybe a Hammond P-T1650H).

1)Will using a larger-than necessary PT affect the tone?

2)How do I fugure out how much voltage I will need for the bias supply?

3)I can't seem to find any well-written information on choosing a PT. Any good books on this?

Thanks in advance, Gabriel

October 9, 2002

Thanks Miles. I appreciate it.

I'll try a Tung-Sol 5881.

While I'm at it, I have another question: It's getting about time to do a cap job. Do you think Solen fast caps would be worth the extra $$? Or should stick to Spragues or Mallorys?

THX

October 9, 2002

Miles:

I've got a Fender "red knob" Champ 12. It's single-ended with cathode bias. Runs two 12AX7s and a 6L6.

My questions:

Can I replace the shitty 6" reverb tank with a full-size accutronics unit? What other changes would I need to make?

Since it's cathode bias, it's self-biasing right? What tubes can I safely swap? I'd like to try a 6V6 (I have a couple NOS JAN Phillips 6V6s) and maybe an EL34. Would I need to install an additional filament tranny?

Thanks,

Gabriel

September 12, 2002

Gabriel ....

All I can say is .... great amp. The Super was perhaps the best of the tweeds from my limited point of view

Myles

Myles:

The really cool thing is that I bought it at a yard sale for $15 two years ago. It had been chopped into a head, covered (badly) in black tolex and had suffered an electrical fire at some point. I had no idea what it was until I got it home. I just figured that anything that said "Fender Electric Instruments" on the tube chart was probably a good deal for $15.00.

After some TLC, it's become my main amp.

Here's a blurry photo.

September 12, 2002

In the case of your Super ... is it the 6L6 version, or one of the very few from 55 that used two 6V6's? The 6V6 ones are quite amazing, and I have seen over 440 B+ volts on these, so be sure to use very strong 6V6's if this is your circuit.

Mine's a 5F4 from '57 so it uses 6L6s. I use the 12AY7 where I'm supposed to but sometimes put a 12AX7 in the PI instead of the 12AU7 - makes it a lot louder and boosts the gain.

Replaced the selenium bias rectifier with a 1N4007 to avoid a smelly disaster.

Thanks for the help.

September 10, 2002

Here's a voltage question.

I recently tested the voltage in my house and it's 125 volts. Will this hurt my 5F4 Super which was designed for 110 volts? Should I use a Variac to bring down the voltage?

Thanks, Gabriel

April 15, 2002

You need a good switch though, that with switch in a clean way without any spikes. Something like what stereo stores use when the demo speakers, switching from one to another using the same amp.

Thanks for the quick reply Myles. Just to be safe - the jump from 4 ohms to 8 ohms doesn't qualify as a spike?

I have a Morley ABY switch. Probably not quiet enough huh?

Thanks again,

Gabriel

Profiles

Forums

Blogs

Events

Articles

Downloads

Gallery

Store

Posts posted by Gabriel E.