Gibson GA20 (old, octal preamp)

[EFK]

Anyone know where I can find some good info on this amp? I found a schematic, but what I'd really like is a blueprint of the expected voltages. I use a bucking transformer to keep the incoming AC at about 115-116V, and while this keeps the filaments happy, the plate voltages on the 6V6 tubes (cathode bias w/ 250 ohm resistor) are only 286V - this is w. the stock 5Y3, 8 watts dissipation per tube. If I pop in a GZ34, I can get them up to 326 and 10w dissipation. I have no way of knowing if the amp is supposed to run this mild because I can't find a voltage chart anywhere. And I have NO idea what is going on with the octal preamp tubes - those voltages don't even make sense to me, coming from experience w. 12ax7 tubes.

Any thoughts, anyone?

[Chuck H]

1951 Gibson GA-20 help - The Gear Page
The Amp Garage :: View topic - Gibson GA-20 circuit?
Need help with Gibson GA20 rewire - AMPAGE Archive

A quick Google search garnered this info.

Chuck

[EFK]

Hi Chuck - shortly after posting that I did manage to find some of those links and a few more, although I guess there is no such thing as an intended voltage chart. This amp is in beautiful shape and was largely left alone; it still had the monster 20/10/10 wax coated filter cap and all the big honking wax grey tigers. Changed out the filters, and 4 of the grey tigers were bad so they had to go; all I had in .05 were orange drop ps caps. I did manage to save 4 of the big wax jobbies in there, although they're taking up a lot of real estate. I'm sure a good amp tech would probably raise an eyebrow at leaving nearly 60 year old wax caps in there. Also had to change some of the resistors, I had some of the values in carbon comps but others had to use carbon film. Converted to three prong, and then I cried a little because it now looks like rainbow confetti exploded in there. I like to try to preserve the original look, but this thing is a rats nest w/ no board and I just didn't feel like spending all the extra time to try to restuff the wax caps, order and wait for more carbon comps etc. I made two changes which I have read about: I changed the main power supply input resistor (no choke in this) from 10K to about 2K, and I changed the 470K plate resistors on the 6sj7 preamp cans to 220K. Haven't changed anything else yet - pots are still good, remarkably.

A couple of questions: I have never played with octal preamp tubes before. V2, which is a 6sj7, seems to be microphonic. If I crank up the volume, it starts ringing on me unless I touch it with a stick and hold it there. Is this typical of these "tubes" or should I try another? Also: what's w/ the high value input resistors (100K) - a lot of the later Gibsons (cathode bias, not grid leak, I know) dropped back to 47K. I don;t know anything about grid leak biasing so I'm wondering if those 100K resistors have something to do with that.

Also, the heater CT is grounded to the positive side of the power tube cathode bias resistor. Can this be problematic? The amp is quiet per se, no hiss or scratchiness or anything, but it has a respectable hum that no amount of chopsticking can change. Since the heaters are floating on the cathode bias DC, I would expect it to be quieter. The amp is kind of star grounded in that all the grounds - and there don't seem to be many - run back to one point.

Also was wondering if adding a choke could do anything?

If I run it with the stock 5Y3, I can only get about 8 watts or so of dissipation out of the tubes at idle w/ the stock 250 ohm bias resistor. If I use a gz34 however, I can get them up to almost 12 watts at idle. Is there any problem with doing this? I'm going to try the ear test with both, of course, but just curious if there could be any problems w the higher voltage. B+ at the plates w. gz34 is only about 320-326 and the screens are around 313.

I much prefer an amp with a turret or eyelet board!!!!!!!!

[Chuck H]

I don't know that the 6sj7 has a rep for being microphonic. Not compared to the EF86 anyway. A lot of what you may find is not going to be the cream of the crop since this is an NOS only tube or a tested good pull.

I've read that the heater wiring on these amp could use improvement. The schem doesn't show. Also, with a can type cap there's bound to be grounding issues with respect to where the preamp filters are grounded. Addressing these two areas may help with hum.

People do rave about the tone of these amps, but they're one trick ponies. Crank 'em up and they're "creamy". That's the standard adjective. I can personally attest to it's accuracy having played through a nice example of that amp, but it did hum. Surely it's primarily a grounding issue. Try modding the grounds so that the preamp and it's filters are grounded at a seperate star point near the input.

From what I've read a lot of guys run a GZ34 in that amp. I have a PP 6v6 amp with 300Vp and I actually like the tone of these tubes at lower voltage. Kind of fat and squishy with a bristly splash on top and a nice mid scoop. Not the ideal tone for everything for sure, but fun anyway.

Chuck

[EFK]

I think you're absolutely right because all of the grounds are willy-nilly going to one chassis ground and that's where the AC input cord is grounded also - at least, that's what I can see so far with this mess of wiring!

The heater wiring is very simple - no twisting, they just ran the lines off the output tubes down the edge of the chassis to the preamp tubes. Moving them around didn't make any difference, though, so I'm not sure the lack of twisting is an issue here.

I got a great deal on it, and the exterior looks so new you'd think it was only about 10 years old, so the one trick pony nature is not a problem. That's a great excuse to "need" more amps! I love such single-purpose type amps because frankly I can;t stand a lot of knobs and switches - my simple brain finds it easier to switch amps, and I'm kind of one-trick player anyway.

[Chuck H]

Well, with time I suppose it's possible to rewire the whole thing with more ideal layout and practices. The one I played through sounded very nice, and unique. Those are qualities usually exclusive to more pricey amps. I've thought about trying to find or build one for myself but even my back burner is full of other projects

Chuck

[EFK]

Hmmm. The preamp tubes are grounded to the PI and the negative side of the cathode bias resistor, all in one spaghetti soup, and this mess of grounds is NOT going back to where I thought it was; it is tied to the input jacks and the pots!!!!!

[Chuck H]

In a recent post I read (if I understood it correctly) that Merlins book suggest the ideal ground scheme does exactly that. Everything to the input jack. I've never done it that way but others have and report that it worked for them. I've always used a three star system. All grounds run individually with no daisy chains. Point one is near the AC cord ground for the main filter, screen filter, PT ctr, filament ctr and OT 0V. Point two is in the middle of the amp and is for the NFB (or presence control), other filters, PI 0V and any reverb or trem grounds. Point three in near the input and is for preamp signal grounds and preamp cathode 0V. This seems to work for me and I have next to no hum in very high gain circuits. It is important to twist the filament leads, You wouldn't really be able to tell if that would help by pushing leads around. The twisted pair filament arrangement helps to cancel hum with inductance and you can only get a fraction of that effect by pushing leads together. Any hum that remains in the filament circuit rides above ground by whatever resistance is in the ground lead, buss and chassis. With a very sensitive input it can be enough to get picked up and amplified. Thats why, to me anyway, it's best to keep at least filament ctr, PT ctr and main filter grounds on a seperate point that isn't as close to the input as the preamp grounds. I think a proper twisted pair filament arrangement and a ground scheme modification could help you, but it's also possible that these amps hum due to some other cause like proximity of the transformers.

Chuck

[EFK]

That's very interesting. The hum is definitely coming through the speaker. The transformers are a good distance apart, the PT is up in the upper corner of the amp while the OT is mounted down on the speaker frame. Never had one on the speaker before, maybe those long wires are influencing something. As far as the filaments, it's easy enough to unsolder the two really long wires that run between the output section and the preamp and twist them a bit - might give that a try.

[Chuck H]

The hum is definitely coming through the speaker.

I wasn't suggesting that the transformers proximity would cause them to physically vibrate (though transformers really do physically vibrate, quite a lot). While it doesn't sound likely in your case sometimes if the transformers are too close or oriented poorly the PT can induce EMF onto the OT which in turn comes through the speaker.

the OT is mounted down on the speaker frame. Never had one on the speaker before, maybe those long wires are influencing something.

If those leads were cross talking with other amp circuits I would expect oscillations or maybe buzz, but not hum.

As far as the filaments, it's easy enough to unsolder the two really long wires that run between the output section and the preamp and twist them a bit - might give that a try.

Doing anything half way may not even give you half the results. Twist all the filament leads. Not just a bit, but tightly. Be sure the PT ctr, main and screen filters, cathode resistor and cap and the OT secondary 0V for the output jacks are NOT grounded with the preamp grounds. I'll bet 6 beer (German, Canadian or American) that your hum will go away.

Chuck

[EFK]

Well I messed around with the grounding a bit but nothing made any difference. I do have the preamp separated from the output section now. I also took the OT off the speaker and moved it around to different places but it made no difference. Have not went at the filament wiring yet, but doesn't that usually yield 60 hz hum? This is definitely 120. The amp sounds GREAT and it's one of those things where you have to debate - maybe just play it? Certainly can't hear it when you play it. Even with both volumes (mic and instruments) on 0, there is still that hum coming through the speaker. If I pull the PI which is a 6SL7, it mostly goes away. I wonder if this is some kind of filter issue? The filter caps are new 500V sprague atoms, 20 and (2) 10s. As I mentioned, there is no choke. I'm not sure if a choke would make any difference anyway, I only mention it because I've never had an amp with no choke.

There is no 0V OT ground: the OT is on the speaker, CT and 2 plate leads up into the chassis to the tubes and two little short (@ 1.5") output leads soldered directly to the speaker tabs, no neutral at all. Maybe THAT is an issue? I do think the hum might be coming from the OT as I can feel it vibrating pretty noticeably.

On a different note: those 100K input resistors MUST be excessive. The mic input has no resistor at all, and plugging into that is spectacular with no additional noise that I can hear. The instrument inputs are substantially padded down compared to that, literally half the volume and much less treble. I bet dropping them down to 47K like most later Gibsons would be a good improvement.

[Chuck H]

A lot of guys change those resistors. But really, the difference between 47k and 100k in that position is small. But subtle changes are acceptible too as long as you like it.

I didn't realize there was no OT ground (just hadn't pictured it fully in my head yet). Try grounding the OT 0V/- speaker lead to the chassis. It can't hurt to try.

Chuck

ps... Just found this http://music-electronics-forum.com/t3560/

look at post #10

[EFK]

I found that thread before I messed with the grounding and I tried that - the PT ground was to one of the tubes so I moved it to the chassis ground. No difference.

What exactly do you mean: "Try grounding the OT 0V/- speaker lead to the chassis. It can't hurt to try." Do you mean run a ground from the negative speaker terminal to the chassis ground? Hadn't thought about that but I guess in practice that's how most OTs with a neutral are wired aren't they? This might be interesting.

[EFK]

Made no difference to ground the OT/speaker negative to the chassis. Also just for giggles, I lifted the filament ct from the positive side of the cathode and grounded it to chassis - no difference there either.....