Gibson Ranger GA20-T Rebuild

[guitician]

OK then, it seems that both inputs are coupling signals. How well do those shorting jacks work? Check to see if they are really shorting to near zero ohms. Another thing you can check is the lead dress of the wires for the separate channels and keep them apart. With that type of circuit there will be interaction between volume controls since they a tied to the same point.

[TomCarlos]

My plan for the next week or so is to test each channel separately. I am going to remove tubes not needed, remove connections to the Phase Inverter for the unused channel, and look at wiring and lead dress. I then reverse the process for the other channel. Once i know that each channel is ok, i will then reconnect both channels and see where I stand.

And while doing this, I'm going to look at other versions of the Gibson GA-20 and 20T. I see differences in how the channels are combined and differences in the Voicing control. I wonder if Gibson realized some difficiencies in the first design and then made some changes. I believe it is the Ranger model that has the changes. I think our goal is to keep the amp as original as possible. But if this problem persists, maybe that will be a deciding factor.

Thanks for the input!

[TomCarlos]

A quick note on the inputs....

Channel 1 jacks seem to be ok, near "0" ohms.

Channel 2 does not use jacks that short to ground. There is no cap on the inputs. Each input has an attenuator of some sort - a 220K to ground and a 100K to the grid. So when I measure at the grid, I get 168K ohms (two 320K ohms in parallel). I guess that is the design for this channel? Seems odd you wouldn't be grounding the inputs somehow.

I found a couple interesting articles on this amp and the Fender 5E3. This one comes from "The Gear Page.net." Besides grounding issues, the post also has some other tips. Anyway, the issue is Grounding. I asked about this before but our friends here on MEF advised against changing any grounding connections. But look closely, you will see a grounding issue with the 6SQ7- It has two paths to ground. The same can be said for the 12AX7, 5879, and 12Ay7 tubes.

The other flaw is that the Power Transformer Center taps, the Power Supply Caps, the 6V6 Cathode cap, all these are connected at the 6V6 Pin 1 terminal, then go through a very thin wire to the preamp section, and you have daisy chaining to multiple ground connections. I'd think you would want the "hi side ground" going to the chassis on it's own.

Now then, this amp might be working to some degree but I bet you all wouldn't design it like this if you had the chance!!

[guitician]

Having the power ground all on one point is good, but using the chassis as a signal ground is bad. A thin wire to the preamp section is not bad, but having another path could cause problems. It's best to just use the wire as the ground, maybe thicker would help. That lead dress doesn't look too good, were they all neatly tucked along the chassis at one point?
The 5879's 168K impedance will make that tube more sensitive to picking up stray signals.

[guitician]

Having the power ground all on one point is good, but using the chassis as a signal ground is bad. A thin wire to the preamp section is not bad, but having another path could cause problems. It's best to just use the wire as the ground, maybe thicker would help. That lead dress doesn't look too good, were they all neatly tucked along the chassis at one point?
The 5879's 168K impedance will make that tube more sensitive to picking up stray signals.

[TomCarlos]

I will clean up the lead dress. Some of that is temp wiring so that I do not lose track of connections.

I guess the inputs to the 5879s stays as-is. I don't how much modification the amp owner will accept.

As for the grounding, I guess that is trial and error. This amp isn't using any Star Grounding techniques. Looking at how then preamp section is grounded, and wiring on the tubes, i am hesitant to touch that. But I might try cutting off anything that is a second path to ground.

Thanks!

[soundmasterg]

The one that I fixed for my uncle is a GA20T, we figured it was 1958 or 1959. The point is, when I got the amp it didn't work as one of the electrolytics had physically leaked goo in the chassis. Once the electrolytics were replaced I was able to check the amp function out more fully and realized it needed an overhaul. Once I did the overhaul the amp worked flawlessly with the original circuit and grounding. This amp should also be able to work correctly once the likely out of spec parts are sorted.

Did you replace the coupling caps in the amp? Or test them for leakage at least? I realize it is a pain because they are under the board, but check that before you check anything else. Having leaking coupling caps throws off the bias for the stages that follow them and can drive something that would normally be stable into oscillation. Every single coupling cap in my uncle's amp leaked too much DC and had to be replaced. Likely you will have the same condition in that amp. Those old Astron paper caps and the bumblebee paper and paper in oil caps that came in that amp are notorious for leaking DC.


The 5879, being a pentode, can amplify at higher frequencies quite well, and as such is more susceptible to oscillation that a triode. Also, I haven't looked closely at the schematic in awhile and don't have time at the moment but the 5E3 has interactive channels with the same behavior that you describe about the volume going up and down depending on the position of the unused channel's volume control. On a 5E3, if you turn the unused channel up half way on the volume, and then adjust the channel in use to whatever volume desired, this results in the loudest volume, and of course having the other channel operating changes the tone slightly also. I would guess that the GA20T is much the same as far as having interactive channels. If you really need to know about it, I can have my brother check the amp as he has purchased it from my uncle.

Greg

[TomCarlos]

Hi Greg....

Yes, I replaced ALL caps. Once I had that PC board out, I wasn't going to put it back in and solder up connecting wires without replacing the caps. As I saw in a restoration video, just do it!

Now that the board is back in place, I am going to shorten all connecting wires between the pots, tubes, and board. Hopefully, this will solve some of the problems. I agree with you on the grounding- the amp should work. But optimally, this isn't how we would ground various points in the circuit.

Maybe the happy point here is to get the Tremolo channel working and we leave the 12AY7 out. I think the owner really cares about Channel 2. Beyond that, I can only see myself spending tons of time on this project.

As an FYI, I found another blog for this amp and the problem. So let me post it here just in case anyone else comes upon this thread.

The Gear Page.net - High Pitched Squeal

I'll report back my Monday night.

Thanks Greg and Jeff!

[guitician]

Yeah, grounding is most likely the issue. I don't think grounding the speaker frame would fix it by itself, but if you have a clip lead, it can't hurt to try.

[TomCarlos]

So.... I cleaned up the leads, shorten wires, fresh solder, all that didn't solve the problem. But I found this link and suggestion of running a wire from the speaker negative terminal to the chassis. It's the equivalent to grounding the output jack.

While I was at it, I used some Star Grounding hints as shown here on the Aiken Amps Web site.

Well, for now, no squeal.

Here was another post with a similar problem: The Gear Page - Ground Issues in GA-20T.

I looked at everything else, not wanting to change the circuit components or add anything new. The same article suggests "grid stoppers." I'll see if I can understand what that does.

So for now... I guess I can claim victory.

And a footnote -the Celestion G12-M is too big for this cabinet (the magnet is too big). The chassis has no room to be mounted. I believe the original speaker for this amp is the Jensen P12R.

Thanks everyone who helped me along the way!!

[guitician]

That's good to hear Tom. The speaker is a personal preference thing that is best suited to what the player wants out of the amp. Yes, the P12R was probably the orig. model and is a real good performer, but since the owner can put anything that will fit in it, they should find their ideal speaker if it's to be their "sound".

[TomCarlos]

Gents.... A follow up to this thread with a question about tube sockets.

My friend has been using this amp during the last year. He had someone else swap out the rectifier and output tubes. The amp is plenty loud - EXCEPT that is drops in volume for short spurts then goes back up to full power. That is the problem - it is a quick drop in volume then back to full power. So it is hard to get voltage measurements or to see this on the scope. I'll try to use a sine wave to see if this will happen again and if I can see it on the scope.

I noticed this problem with BOTH channels.

My initial testing included -

(1) Swap out the Rectifier tube - No difference
(2) Swap out the 6V6 Power tubes - No difference.
The tubes mentioned above are tight in their sockets and no visible corrosion.


So... knowing that this happens going through both channels, that leads me to the Phase Inverter and beyond.

I noticed that all the 9 pin preamp tubes are very loose, including the Phase Inverter. So I want to swap out these sockets. I tried to retension the sockets but I think they are beyond help. I see that Antique Radio Supply sells shield base tube sockets - some have the Center Shield, others do not. I don't know if there is an advantage to having the center shield. Anyway, this is something that needs to be done and at least I will be able to rule out a loose tube connection.

After that, if the problem continues, I'll dive into the right side of the circuit.

[g1]

How abrupt is the drop & recovery? If it's heater related it takes a second or two. Watching the heaters in a dark area may identify an issue if you can get it to exhibit the fault while you are watching.
If it's a more abrupt issue, have you tried prodding around with chopstick or similar?

[TomCarlos]

Hi Greg....

Once the amp warms up, my bud will begin playing at a medium volume level. Maybe around the 4 or 5 minute mark, the volume drops suddenly. Then, maybe 10 or 15 seconds later, it shoots back up to a normal level.

This amp has a new power transformer and all caps have been replaced. I think we'll start by replacing the tube sockets (9 pins) because it needs it anyway.

Yes, I poked around and couldn't nail down anything that would cause the volume drop. I'll try to get a scope on this thing. I wish I had a generator with pulsing!!

The only remaining original component is the Output Transformer. But I am thinking if it were bad, it would be bad all the time... not just intermittently.

Tom

[Chuck H]

Poking around should probably also include thumping on the tubes, wiggling the pots at the knobs and giving the amp the ol' Enzo whack.

Also, has the guy been using any other guitars, amps or cables with the amp in question being the only common piece of gear in association with the problem? If not then the problem can't be isolated to the amp yet.