Converting PA to guitar amp

[hodad]

I've got this old tube PA amp that looks like an excellent candidate for conversion to guitar amp. Here's a link to the schematic I (hopefully correctly) traced out(I left out the very basic and soon-to be-replaced tone circuit):
http://home.att.net/~giuseppe_poteet/tr_tubeschem.pdf

When I hook a guitar up to it as is, the sound is a bit brittle, for lack of a better term. So I'm looking at possible changes to make it a little more pleasant sounding.

Some questions:
1. The plate voltage on the 6v6's is around 340v--higher than it's specced for. Should I pull it down a bit?

2. I'm considering dumping V1--I don't need the extra gain, & a lot of extraneous noise comes in at this point in the circuit. Shielding helps a lot, but I'm not sure the extra gain is worth the hassle of the extra noise.

3. The .01 uf's are ceramics--is there likely to be sonic improvement replacing these with mica or poly caps?

Any thoughts on schematic flaws, possible circuit improvements, or things to experiment with are very welcome.

Thanks
Tom

 

[mhelin]

V3B cathode circuit looks strange, no ground connection there. How does it work? Shouldn't the PI non-inverting output be taken directly from the V3B cathode, and the cathode resistor an 1M to be connected to ground? Just wondering ..

 

[hodad]

I'm not at home to look at the circuit right now, but I think you're right that the 3K3 & 1M go to ground. Sorry about that.

Tom

 

[Gus]

The schematic looks wrong.

When tracing a circuit it sometimes helps to look at schematics of other amps.

From some of the parts in the schematic look at a fender deluxe schematic with the concerter phase inverter.

Trying to make sense of what you drew. It looks to be a cathode biased output stage 250 ohm resistor and the 47uf cap would go from the connected cathodes of the 6v6s to ground the 47uf would be connected in parallel with the 250 ohm. The 220Ks connected to the 6v6 grids look like they should go to ground

You don't have any B+ dropping resistors or caps in the power supply drawn. 470k on the plate of a 12ax7 is on the high side.

Don't hurt yourself, your schematic makes me think you have not worked with tube circuits.

When building from PA amps what tubes what voltages and the size of the transfomers can give you a hint as to how the parts might sound there best. fender,marshall,vox etc.

That bunch of parts make me think telecaster to fender deluxe type amp, blues amp sounds with just a vol control.

on some cathode bias amps the fill supply is refed to the top of the 250ohm cathode R as a cheap and simple way to reduce hum IIRC.

edit
http://www.kbapps.com/audio/tubeamps/fender5e3.html

 

[hodad]

Gus
You're right that I don't have much experience with tube circuits--part of what compelled me to pursue this was to figure some of this stuff out.
I used a schemo from a Maestro Viscount (model GA-16T) as reference--because I had it at hand & because it's a fairly similar design.

Also, there's a hum balance pot that I think connect to the + side of the 47uf cap--I'll look at it later.

The stuff between V3B & the 6v6's is where the PA amp seemed to diverge the most from the Maestro schematic. I'll take another look at that (& the 470K on the 12AX7) when I get home tonight.

Tom

 

[hodad]

More details:
Aside from the aforementioned missed ground connection off v3b, the only other thing I currently see wrong is that the 2 270K's ahead of the 6V6's (the ones vertically oriented on the schem) both connect to the - side of the 47uF cap--and I left out the power supply caps & the hum balance pot.
The 470K on v3a is correct.
For grins, some measured plate voltages:
V1, 102V V2, 222v V3A, 155V V3B, 204V V4/V5, 340V

Should it concern me that the 6V6's are 25V above their "max" plate voltage? Or is it fairly common to operatet them in that range?

Tom

 

[OldHouseScott]

Fender runs the 6V6's in their Deluxe Reverb at over 400 VDC, so you're in good company. If you're really concerned, get a set of JJ 6V6's. They're reputed to survive in amps rated for 6L6's (over 450 VDC on the B+).

 

[PRR]

> voltage on the 6v6's is around 340v--higher than it's specced for

The Cathode is up around 25V, so the tube just sees 340V-25V= 315V. Which, IIRC, is one of the standard ratings for 6V6. So it is "exactly right". Oh, it may be 20V bias and 320V plate-cathode.... tubes are not precision devices, even for abuse.

And 315V is the Design Center rating, the one you use when you build thousands of amps that will be used all over the land on different utility powers. You can run 350V on 6V6 as long as you know it won't get any worse than that. That suggests a self-biased 6V6 could work with a 380V B+.

Also the old-old 315V rating is for radio sets that must be low-cost, long-life, low-trouble. Plate-stuff is made in different grades at different voltage ratings and different prices; 6V6 is speced so they can use the cheap stuff. But as production matured and declined, it didn't make sense to keep 315V, 360V, 450V, 600V, 800V plate-stuff in stock: the cheap stuff was not much cheaper once the high-spec stuff was in large production. So except cheap replacements, most of the 6V6 you find will probably have better-than-spec plate-stuff.

And guitar amp duty is different again. You may not leave your amp on all day, the way you do with a radio. If you are any darn good, you can get paid to play, so you have cash to replace tubes. And all the paying gigs are across town and upstairs: you want a MAXIMUM power/weight ratio even if it costs you in shorter tube life.

Assume it takes 2 years to forget the last time you replaced tubes. A radio in a store may run 10,000 hours in 2 years. A guitar amp may only run 1,000 hours in 2 years. Since guitar amp power/weight is important, and good guitarists have a little money, it would be acceptable to over-rate the guitar amp and shorten tube life 10X. I'm not sure how many volts that is; I bet Leo Fender found out.

> dumping V1

For guitar you will need two hi-Mu triodes in front of your "power amp", which is V3A V3B and the 6V6. You will also need some tone shaping, which will give some loss.

Give V1 a 2K cathode resistor: guitars need more headroom than mikes. Eventually you may want a 0.1uFd or 10uFd bypass cap there. Re-wire the Mike and Aux gain pots to a Fender tone stack and volume pot. I betcha the 2K7 in V3A cathode is 27K; change to 50K or 100K to get less HiFi and more punch.

 

[clintrubber]

Hi,

Maybe of interest @ the Geofex-site:
Converting Integrated/PA Tube Amp into Guitar Amps

Bye,

Peter

 

[hodad]

I'm starting to work on this thing, & I want to remove the "aux in" volume pot from the circuit, & I'm wondering if I can remove the 27K resistor coming off the pot's wiper as well. I'm thinking it's not needed, but I thought I'd ask before I get carried away.

**And another thing: is there a good strategy for reducing gain in a tube amp like this? I was just playing through it with the volume set to about 4, & it was both very loud & fairly heavily distorted. I'd like to be able to use more than half my volume knob.

Thanks
Tom

 

[CJ]

Rework the grid leak first stage and your noise and distortion will be gone.
Use a 1k, 1.5k, 2.2k or 2.7k bias resistor on the cathode. Bypass it with a 25/25.
Yank the .02 input cap.
Yank the 4.7 meg grid resistor.
Use a 68 k inbetween the input jack and grid.
Use a 1meg from grid to ground.


Then report back.
Immediately.

I run 420 on my 6V6's all day.
Love to smell the paint bake.
Output trans loves the heat. Makes the core saturate.
The guy who uses the most tubes wins.

 

[PRR]

> very loud & fairly heavily distorted.

What CJ and I said. That zero-bias input tube will not eat Fender without fuzzing. Stuff a resistor under its cathode; also change the grid resistor to something more suited to guitar than crystal-mike. You may or may not want a bypass cap on the cathode resistor: depends how much gain you have in the rest of the amp.

The 27K is part of the tone control. If you omit it, the tone will have less action, changing with volume setting. Since you already have too much gain, I say leave it there. A simple high-cut control can be enough for a bright guitar, gives you screaming-lead at 10 and mellow rhythm at 4. Yes, a 2-way or 3-way control is more flexible, but maybe not so essential.

 

[hodad]

[quote author="CJ"]
Then report back.
Immediately.
[/quote]

Okay. I had already installed a 2k & a 15uf bypass on the cathode, so I replace the 4M7 with a 1MEG, removed the .02 cap, added a 68K.

Now the gain is manageable & it sounds like a guitar amp! I haven't pushed it at all since it's well after midnight here, but it sounds good.

The next big issue is the buzz--the kind that gets quieter when you put your hands on the strings? I've got waaay too much of that. Should I be looking at the way the ground wires are run? Shielding? Other issues?

Thanks

Tom

 

[CJ]

Turn the switch off and see if the hum disappears instantly. If not, it's non heater oriented. This means we got trouble. Big trouble. :razz:

 

[hodad]

[quote author="CJ"]Turn the switch off and see if the hum disappears instantly. [/quote]
It disappears instantly, but so does everything else--I'm guessing maybe this is because it's got a solid state rectifier?

I fiddled with shifting wires around a little to try and minimize any noise induced from the heater wires. If it helped, I can't tell. & adjusting the hum balance pot does nothing that I can detect.

I still haven't converted to a 3-prong electrical plug yet--don't know if that might help at all or not.

Tom

 

[OldHouseScott]

You may need to replace the electrolytics in your power supply, if you haven't already, to cure the buzz. If it's dying off instantly, they might be pretty leaky.

 

[hodad]

I replaced the 'lytics first thing.

 

[pstamler]

Do the 3-prong conversion immediately. Otherwise you run a very serious risk of frying yourself.

Peace,
Paul

 

[hodad]

Okay. Did the power cord conversion, which helped reduce the hum as well as the risk of frying myself. Tube shields on the 6AV6's seemed to help with the hum a little too, but that may have been wishful hearing on my part.

Soooooo.... Hum is now at a very adequate level. The thing is still enormously loud and crunchy at a volume of about 4--not that there's anything wrong with that.

One kind of weird thing I've noticed is that if I turn my guitar's volume down, it sounds like I'm turning down the tone knob as well. It's not a subtle difference at all. I'm guessing this is some sort of impedance/loading issue, but beyond that I'm clueless.

Even with these little quirks, it's sounding pretty damn good. Thanks to everyone for all the advice & help.

Tom

 

[pstamler]

[quote author="hodad"]One kind of weird thing I've noticed is that if I turn my guitar's volume down, it sounds like I'm turning down the tone knob as well. It's not a subtle difference at all. I'm guessing this is some sort of impedance/loading issue, but beyond that I'm clueless.[/quote]

When your guitar's volume knob is all the way up, the source impedance is basically that of the pickup. When you turn the knob down, say 6dB, the source impedance is basically that of the volume control, divided by four. So a 1 meg pot would give you an output impedance of 250k.

The guitar is running into two capacitances. The first is the "Miller" capacitance of the input tube, which is more-or-less the plate-to-grid capacitance multiplied by the tube's gain. For a 6AV6 that might be, oh, 60pF. Are you playing through a 20' cable? Assume it's 35pF/ft; that's 700pF. Add some stray capacitance here and there, and you're maybe looking at a load capacitance of 800pF. That forms a low-pass filter at about 796Hz.

Admittedly these are extreme conditions, but you get the idea. The effect will be less with a shorter cable.

Peace,
Paul