Tech Questions about Gibson GA19RVT Falcon tube amp

[honkyjonk]

Hey guys

My first post here. I have some questions about a Gibson Falcon amp that I'm working on fixing up.
Anyway, I have a Crest model, and I'm converting it back to a Tweed model as per this forum post:
http://letstalkguild.com/phpBB2/viewtop ... =9&t=12859

Here are schematics for both versions of this amp:

Tweed http://www.freeinfosociety.com/electron ... a19rvt.pdf

Crest
http://www.schematicheaven.com/gibsonamps/ga19rvt.pdf

My questions are:
1. Concerning the resistor changes described in that first post (in the preamp and the phase inverter circuits), do any of them need to be more than 1/4 watt rating? (that's what I have in my drawer)

2. If you click on the Crest schematic and zoom in you can see between V2's pin 9 and ground there is a .1uF cap that is rated at .02uF in the Tweed schematic. I have a .02uF cap, but it's only rated 100V. Should I use a higher voltage rating?

3. Finally, this concerns the Crest schematic, the reverb circuit. The reverb is very faint, and obviously not working right. I'm getting 242V at pin 5 of V3, and 252V on the other primary of T3. Would this possibly be indicative of a bad tube at V3? (This is the reverb circuit tube, the 6C4) Otherwise I can't figure out why the reverb isn't working. I swapped in a different tank, but no difference.

[Structo]

1.) I always use at least 1 watt resistors in a tube amp.
They are easier to work with, have bigger leads, and can take higher voltage.
If you are using carbon comp resistors then the 1/2 watt size are probably OK.
I would not use 1/4 watt resistors unless space requires it, then it would have to be low volts and current there.

2.) The cathodes of the preamp tubes see very little voltage and current so yes, a cap can even be 25v and be OK.

3.) All the voltages will most likely be higher than stated on the schematic because wall voltages are higher nowadays, so the transformer is going to step up that voltage even more.
The 5Y3 rectifier tube drops the most of about any tube I know of so that is about all you can do there.
If you find the voltages overall are too high then you may have to consider dropping it in some other fashion such as with a Zener diode.

What does the ac filament voltage measure at under tube load?

Have you replaced all electrolytic capacitors with new ones?
If not, you should.
Old caps lose their ability to store charge, also can be very leaky letting DC into the circuit where it shouldn't be.
Or they can leak so bad that they short out, causing a short circuit within the amp.


Is this a point to point amp?
Probably a rat's nest inside right?

Good luck on it. I'm sure somebody else will have the answers to your questions.

[honkyjonk]

Yep, it's PTP, and it is a rat's nest. I did luck out in the fact that it has been recapped with Sprague Atoms. It's not the prettiest job, but they're all in the right spot and look reasonably safe.

The amp works really well as it is, just no verb, and it needs a new speaker (I have one on the way)

Any other ideas concerning what might be making the reverb so faint? These are supposed to have enormously lush verb. I've checked continuity through most of the reverb circuit, and can't figure what's wrong. I suppose I could have a bad T3 transformer. That would be a bummer. Is there a way to test for an open coil or something with the transformer. (Is that the right term? I'm not exactly sure what could potentially go bad with transformers)

[Structo]

First suspect in a tube amp is always the tubes.

So you have to do the old, substitute with known good tubes first.

Then, a good way to troubleshoot is to take your DVM and clip the black lead to the chassis and with one hand, probe the amp from the power tubes back towards the front or preamp of the amp.
As you go along, you will hear pops from the speaker.
When you get to a section where the pops don't happen or are very weak, this area should be suspect as it is not amplifying properly.

I don't know much about tube reverbs so somebody else can probably help you with that part.
But I would check out a different tube there.
To check the reverb transformer, you will need to disconnect one end of the primary.
To do this, it appears all you have to do is unplug V3.
Now, make sure all the filter caps are discharged and that you have verified this with your volt meter.
Now switch the meter to read resistance and measure the two wires that go into the reverb transformer. On the primary side of T3.
Now I am not sure how these work but usually a transformer steps the voltage up or down.
It could be a 1:1 ratio but I don't think these are.

For the secondary side, you will need to un-solder something to measure that side of the coil.
The schematic shows a cap and resistor across the secondaries, so you will need to disconnect it to measure it.

I am not sure what the resistance should read on these but you will know whether or not the coils are intact. That is if you get an infinity reading you will know that the coil is open and no good.

I would imagine a low reading is what it will show if it is good.

Hope this helps.
Tom

[honkyjonk]

Hey Tom,

Thanks. I think I probably have a bad reverb transformer. The secondary leads are actually even easier to test because these are wired directly to the reverb tank, so I just disconnected the cable.

Indeed, I get a reading up in the 15 -20M region on my DMM which quickly rises until I get infinity within seconds. I also checked right at the transformer's secondary solder tabs, to rule out the wire to the reverb, and my results were confirmed.

Also, further evidence is the fact (which I initially omitted out of embarrassment) that there was a beetle that had dug down into this transformer.

Bummer. Anybody know how to figure out the specs/replacement for it?
(FWIW, the primary side measured @ about 1.5K)

[chocopower]

Hi,

I have the same amp...with the same problem. Reverb not working.

I tested the reverb transformer and i´m sure is dead.

Can i use a standard Fender reverb transformer? Maybe the one used in the Deluxe Reverb?

thanks in advance!

[Cliff Schecht]

Yeah you can use a Fender unit in there, but you have to drill the original one out (riveted down) or find a new place for the new one. I fixed a GA20RVT recently and ended up redesigning the reverb driver closer to what it was in the schematic (plate follower instead of transformer driven). Mine was a goofy build too because the transformer driven reverb definitely wasn't in the schematic. The cathode driven reverb setup worked out alright but didn't give much in the way of control with the reverb control. I ended up changing the mix resistor as well to give a bit more depth to the reverb.

[Structo]

That is strange that the value would drift up on that reverb tranny.
Acts more like a capacitor does when you put an ohmmeter on it.

I was looking at the two schematics and not sure how much you are doing but you do realize that they utilize two different types of tubes, right?

You have the 6EU7, 7199 and the 6C4.

The 7199 is an odd duck as it has a triode and a pentode in one tube.

[honkyjonk]

Hey Tom,

Actually, I have the Crest model, which has a 6c4 tube in the reverb circuit instead of the 7199. I'm not planning on changing that part of the amp back to Tweed specs, just the input/preamp and phase inverter circuit.

Concerning the reverb transformer, I wonder if my meter just happens to do that when the resistance is on the highest setting. I'll have to try it on something else with infinite resistance and see if it does the same.
Regardless, what I'm seeing would seem to suggest that it's a bad transformer though, right?

[Cliff Schecht]

Yup, if you are reading open like the probes aren't connected to anything then the windings opened up. Make sure you are getting good contact with the tiny gauge transformer windings. I had a problem with this with the Gibson reverb transformer I tried to repair. The fix lasted about 20 minutes and broke quickly. Those transformers are weeaakk and I break often.