1957 gibson ga-20

[bruno]

Trying to restore an old 57 gibson ga-20, with a paraphase phase inverter... I got it working but somehow, the sound is really low and somewhat distorted. I got 360 volts on the 6v6 plates, 208v on the pi plates, and around 150v on the pre amp plates. The speaker is an old jensen and although really old and rusty, seems to work with other small amps I have in the house. So everything looks healthy except for the plate voltages on the phase inverter and pre amp stages, which I guess could me higher.

Is this where the I should look?

thanks

[Platefire]

Some things to try:

1-Clean all tube socket and tube pins with switch contact cleaner. I usually do this by pulling the tube and spraying down in the socket holes, on the tube pins and then work the tube pins up and down in the tube socket before pushing them down for the final permanent seating.

2-I would also spray the cleaner in the pots and twist the knob quickly back and forth for a while.

3-If cleaning these items don't make any difference, try probing the exposed circuit while hot(be careful!)  with a wood chop stick to see if you can get any change by toutching the components for possibly a bad connection--if the amp picks up when you totch a certain thing take note of that connection for possible bad solder joint or bad component.

4-Be sure all your tubes are good? Try good replacements to see if it makes a difference.

5-Check your ground connections for continuity to chassis.

[HotBluePlates]

We need more info on your supply voltages, not just plate voltages.

360v - 208v = 152v. I see 10k and 47k dropping resistors between these points on the schematic in the library here (don't know how exactly it matches your amp). 152v / 57k = 2.67mA, which is a believable amount of current, even though that doesn't account for the plate load resistors in the phase inverter. You can figure more precisely by looking at supply node voltages, as well as plate voltages.

So, the voltages seem reasonable to me. Yes, they are low compared to some amps. 50's Gibson amps didn't always optimize for huge gain or most output power, and this amp is a pair of cathode biased 6V6's.

I'd look hard at the input (grid-leak) coupling caps. If these leak much, it will prevent any big signal from being applied to the grid. If these are leaky, there's probably a good chance the input stage screen bypass caps leak too. If those leak, you input stages might as well be low-mu triodes.

I'd check for leaky signal caps first. A substitution is the fastest way to check these.

[bruno]

Hey guys thanks for the answers...

here is the schematic I posted of the amplifier, it's the sch file:

http://www.el34world.com/Forum/index.php?topic=605.msg12767#msg12767

I also have a scan of the schematic I drew:



My amp, as 10k resistors only in the power rail... The first supply node (to the OT) has 370V, the second supply node (after first 10k resistor) has 278V, and the third and last node which feeds the 220k resistors and all of the preamp plates has 254V.

I removed the grid leak bias schematic, since the owner of the amp will be using it with PAF style humbuckers mainly, and pretty much wired the inputs like a regular tweed deluxe up to the first gain stage.

I've changed pretty much every cap with new ones... and cap the old ones just to make sure.

Any other info I can supply that might help? I'm a bit clueless at this point, I've even tried a new output transformer, still no luck.

At this point I'm a little unsure, should the amp behave this way, it doesn't sound louder than a 5w champ, I actually think it is quieter.

[bruno]

problem solved... thanks guys again, you were great, thanks for all the quick replies.

The problem turned out to be that I misreplaced a 1K resistor in the cathode of the inverter with a 1M resistor.

[eleventeen]

"The problem turned out to be that I misreplaced a 1K resistor in the cathode of the inverter with a 1M resistor."

Hey, what's a few orders of magnitude among friends? LOL.

[Willabe]

bruno, hows that amp sound?  Good, ehh, middle of the road, great?


     Thanks,    Brad     

[bluesbear]

If it's like most older Gibson amps, it sounds great but is a real pain to work on and get tubes for. I particularly enjoy working on a fairly complicated amp with components on BOTH sides of the circuit board. Actually, this one looks fairly simple. Some of them look like it was a contest to see who could design the most complicated and bizarre circuit. They also liked to use whatever tube was being discontinued so they could get a truckload cheap. That being said, I have two, a GA-8t and a GA-40 Les Paul.
Dave

[Willabe]

Hi bluesbear,

I have a tranny set (MM) sittin here for 3 to 4 years now for a GA-40 Les Paul. They are some very big transformers for a small amp.

I read about the amp in Tone Quest Mag. They loved it. I've also read good comments from the late Ken Fisher and G. Weber on this Gibson amp. Victoria amps make one and he, Mark Bair, loves it too.

So I thought I'd like to try to build one. I also wanted to try a 5879 tube.

I'm wondering, hoping it will be good/great for slide mainly.

One of these days I'll get to it.    


      Thanks,   Brad      

[bmack]

I had one of these GA-20's in for repair recently.  The only schematics I could find were for the version with the octal pre-amp tubes.  Thanks for drawing out the schematic, it's going into my library.

The owner complained the amp sounded too dark, didn't have much gain and little or no headroom before breaking up.  I couldn't find anything wrong with the amp, all the voltages looked good and it had been recapped recently.  After much research I found that everyone was saying this is just how this amp sounds.  I think a big part of the sound (or lack there of)  comes from the grid leak bias in the pre amp.  I switched the first preamp tube to a cathode bias and it definitely made the amp much louder and peppier.  

In the end, I built this client a tweed deluxe and kept the GA-20.  I'm going to go back and play with pre amp some more and see what I can get out of it.  

Bruno, did the amp sound much better after re-building the pre amp?   

[HotBluePlates]

Grid leak biasing has trouble accepting large input signals. I once had a mid-50's tweed Princeton with a grid leak biased input stage. I never had any problems, because I only played a strat through it and didn't use pedals to boost the signal prior to the amp. I also wanted low output power and high distortion, because it was basically a Champ with a tone control.

Cathode biasing probably works best for the majority of situations, as you found.

[bluesbear]

Willabe,
A GA-40 (or 20, for that matter) would be a great slide guitar amp, really warm a lots of smooth overdrive. Duane territory.
Dave

[Platefire]

I had a 1961 GA-40 Les Paul a couple of years ago--It had very smooth overdrive, nice gain when overdriven and even though it didn't have much clean headroom it had a pretty good clean tone with the guitar volume down. A great amp to do the old blues thing with a cranked amp and controling the sound(dirt/clean) with your guitar volume.

BTW-Willabe--how is your GA-20 sounding now you got it working pretty good?

[Willabe]

Hi Platefire,

Good to hear, another good review of a GA-40!

BTW-Willabe--how is your GA-20 sounding now you got it working pretty good?

I don't have a GA-20, I think you mean one of the other guys earlier in the topic. Wish I did.    


        Brad    

[Platefire]

Ok, I'm sorry! It was bruno---same question bruno--how does it sound to you now?

I would still have my GA-40 but I gave it to my Son to make up for a old 65 strat that I sold and latter found out he wanted for himself(long story!). I think Doug favors a GA-5 over a Champ if I recall correctly. So Gibson did do some good things ampwise even though some of their old stuff is very strange compared to fender to me. They definitly were not copying fender(in most cases) and was following their own drummer so to speak. I never did have to work on my GA-40 while I had it because it worked perfect but did acknowlege components on both sides of the board that I'm glad I never had to fool with. I did re-tweed the cab, built a missing backplate to match the original, installed a three conducter power cord and installed a standby switch in place of the old ground switch. I had a fairly modern Celestion Greenback in it. The old "Gibson" Logo was broken on the speaker grill and I went to a lot of effort to re-create the logo in the original text style, cut it out of vinyl base board and attached it at the same angle in the same old screw holes the original was attached. I took a lot of time to restore it as close to original as possible with the limited amp skills I has at the time. If was a very cool looking/sounding amp. A Classic in amp history for sure! Platefire

[Willabe]

Platefire, do you remember if the transformer set seemed kinda large for a low powered amp?

The set I got from MM are a lot bigger than what I would expect for this amp.


           Thanks,     Brad      

[bluesbear]

The transformers on mine don't seem particularly large. Mine's a ''59, BTW. It's the only '59 Les Paul I'll ever be able to own!
On another note, My GA-8t now has a GA-8 power section (EL-84's) and a GA-5 preamp (12AX7's instead of 6EU7's) with a tweed Princeton tone control. No trem. I added a master for home practice but it's really unnecessary. It has a Weber Signature 12" alnico, 25 watt speaker ($45 new!!!). I used it in a small bar last week. It was just loud enough. Leads sound more like a violin than a guitar, my favorite older Gibson amp trait.
Dave

[Platefire]

I can't remember the transformer size--as I recall it seemed maybe a little bigger than a normal 5E3 transformer. Here is the only pix I could find on my GA-40. Got a bunch of project pictures on old
3 1/2" Disk but my new computer only takes CD's!! Seems I heavly document every project I do. Platefire

[Willabe]

Thanks Dave and Platefire.

The PT I have from MM is 3-3/4" tall, 3-1/8" wide, and bell cover to bell cover is 3-1/2". Core is 1-5/8" thick. Just seems a little big for a 2X6V6 cathode bias amp with only a 1A, 5v fill. wind? Not that thats a bad thing.

OT is 2-3/4" tall, 2-1/4" wide, and bell to bell is 2-1/4". Core is 1" thick.


       Thanks, Brad