Gibson GA-95 RVT

[Phil_S]

Phil,
What do you think of the T3 interstage transformer as a suspect in the low output?
I suppose if an earlier stage is not working right it could be the cause but I think if it were me I would be pulling one or both sides of that transformer and measure the resistance.

Yes, I noted early in the thread that the i/s tx is a good candidate for the weak link and suggested investigation. What gets me, though, is that V4 voltages aren't right when the rest appear to be OK. I do not think V4 voltage is affected by that tx and so needs investigation. This amp likely has more than one problem. The OP has not reported back on the pop test at the finals. Probing the finals with a meter should produce a very audible pop at the speaker, demonstrating the power section and the OT are OK.

[Travst]

There is no reading at all on R44, the 68k resistor. As soon as I drum one up, I can test again.

[Structo]

You're getting closer!

[Travst]

At least I ruled this one out. I couldn't find a 68k resistor, so I put a 10k and 56k in series to test it. I am getting a bit more volume, but not what it should be. I'll keep working my way through the suggestions. As was stated, the amp has more than one problem.

[Phil_S]

Good move on that resistor. Progress is good. Be patient and check your results after each change, one at a time. Did changing the 68K result in a change in the voltage readings at V4?

I would focus on components common to both halves of V4. There isn't much. The 100K plate load resistors R16/R42 form a voltage divider off the same node, so look at those. C11 decouples V4 from the final stage in the preamp. If there is nothing obvious about these, I would replace all of them, one thing at a time, starting with C11. Look to rule out a bad filter cap at C22A; sometimes new ones are bad. Look at R49/4.7K, too.

[Travst]

Good move on that resistor. Progress is good. Be patient and check your results after each change, one at a time. Did changing the 68K result in a change in the voltage readings at V4?

I would focus on components common to both halves of V4. There isn't much. The 100K plate load resistors R16/R42 form a voltage divider off the same node, so look at those. C11 decouples V4 from the final stage in the preamp. If there is nothing obvious about these, I would replace all of them, one thing at a time, starting with C11. Look to rule out a bad filter cap at C22A; sometimes new ones are bad. Look at R49/4.7K, too.

Phil, I should have posted my V4 readings. The voltage did not change significantly and is about double what it should be. I'll try out your other suggestions in the next couple of days.

[Travst]

A bit of progress last night. I replaced two more resistors that were out of spec, including R16. I'm getting a bit more volume, and all the hiss is gone. There's still a bit of hum, but I'll relocate the T3 ground as Phil suggested. I had to stop short when I didn't have a .01 cap to replace C11, but it's moving along. No changes in the V4 voltages yet.

[Phil_S]

Where are you located? If you are in USA, I can probalby find what you need on my bone pile. Leads may need to be extended. If you want, send me an email psymonds at comcast dot net.

Bad plate load resistors wlll hiss. They probably all need to be replaced. Use MF or MO, not carbon comp -- it's a waste of money. There is no tone on the plate load R's.

[Cliff Schecht]

Where are you located? If you are in USA, I can probalby find what you need on my bone pile. Leads may need to be extended. If you want, send me an email psymonds at comcast dot net.

Bad plate load resistors wlll hiss. They probably all need to be replaced. Use MF or MO, not carbon comp -- it's a waste of money. There is no tone on the plate load R's.

Wrong! Carbon comps have a bad voltage coefficient of resistance and actually change slightly in value as you push them near their limits. If you size the resistor correctly, you can safely take advantage of this property and introduce some nice predictable mojo into your amp in the form of extra second harmonic. 1/2W resistors work well for most plate resistors, just make sure that the overall voltage delta is below 350V.

R.G. has an article about it here: http://www.geofex.com/article_folders/c ... oncomp.htm

[Phil_S]

Hi Cliff:
Good article. I have been under the wrong impression. I was told this by an old timer who I had no reason to doubt as he was full of useful and practical information. Thank you for setting things straight!

Phil

[Cliff Schecht]

Don't kid yourself, CC's aren't great plate resistors. They aren't anywhere near as reliable as modern technologies, we just deal with the inadequacies because of that nice distortion they add. For just about any other application I wouldn't go with CC's, just in guitar amps where the benefits are noticeable.

[Travst]

Where are you located? If you are in USA, I can probalby find what you need on my bone pile. Leads may need to be extended. If you want, send me an email psymonds at comcast dot net.

Bad plate load resistors wlll hiss. They probably all need to be replaced. Use MF or MO, not carbon comp -- it's a waste of money. There is no tone on the plate load R's.

I'm in Birmingham, AL. I have access to parts, but don't have everything on hand all the time. There's actually a nice little radio parts store here that carries good stuff, and I picked up the parts I need to move forward last night. Thanks for the offer, when I check T3 I'll let you know what I find!

[Travst]

I’m not really clear how to test T3 with the leads disconnected. However, I measured the voltage on each lead. The two leads on the left side (primaries?) in the schematic read 279v. The three leads on the right (secondaries?) measure at -43v. From what I can tell, that’s right on, but the 279v reading is about 30v high.

I've posted a pic taken when I first got the amp. Note that C28A is in place and there is a lead that's been snipped off. I'm guessing that was C28B. Currently, there is not a cap that replaces C28B. (The amp was cleaned after this photo was taken.) C28A was replaced with a new 20uf cap.

[img:640:480]http://farm5.static.flickr.com/4116/493 ... 66b2_z.jpg[/img]

My guess is that C28B would have connected at the green dot where R43 and C47 join.

[img:640:480]http://farm5.static.flickr.com/4141/493 ... 15f7_z.jpg[/img]

I located three more resistors that were out of spec and replaced them. All the resistors on the board, tubes, and the two terminal strips have been checked.

Tonewise, everything is looking up. The amp is louder and, other than a bit more treble than I'd like, it sounds very good.

There is no audible oscillation, and the Depth/Freq knobs don't do anything. My guess is that the default is "off". I did test the LDR and the two leads on the right of Z1 show 150v. The remaining lead that isn't grounded shows fluctuating current.

Also, the grey IC cap in the picture was removed, as it was an obvious mod to the amp.

[Phil_S]

Tonewise, I would expect almost ice pick treble. It has to do with the notch filters you see at the top row of the schematic -- C5/C6. C12, C22/C23. These are just "wrong". Have you put in or upped the cathode bypass caps to 20u? That's highly recommended. Try increasing C2 and C19 to .015 or anything close to that. Both of these will help counteract the problem created by the notch filters without having to get into a major reworking of the tonestack.

Testing T3. I suggest you disconnect the primary from V5A and the B+ rail (R43? Hard to read). Pull the power tubes, effectively disconnecting the secondary. Lift the secondary CT from the bias circuit. The turns ratio, if the same as mine, should be 1:2.4 (or close...1:3 is certainly reasonable). See if you can find a low voltage AC source to do the test. A filament winding from a spare PT is the thing to use.

That missing cap -- I think you need it. I'm guessing that IC cap was the replacement for it, not a mod. If you haven't replaced that old 3x 20uf cap, IMO it really needs to be done. It is way too old to be relied upon and replacing it will have a marked impact on tone. I used a JJ 20-20-20-40. I just didn't use the 40u. I also put a JJ 32+32 in for the old 20+20 filters for what is your C29 and that really stiffened up the bass, a good thing for this amp.

I'm attaching three pictures and my board layout drawing for the GA20-RVT that might help. I suspect your board is nearly the same.

[Travst]

Thanks for the pictures. Your amp is indeed very similar to mine. I added the missing 20uf cap. I don't have a source of low voltage AC, but I did measure the resistance across T3. I placed a probe on each primary wire in turn and measured between the primaries and secondaries. The readings were all between 52 and 55 kohms.

I did try to run low DC voltage through using an automotive circuit test according to this:

http://www.premierguitar.com/magazine/I ... ester.aspx

However, I don't think the small batteries provide enough juice to make it work.

Also, I replace the 20/20/20 cap when I replaced all the electrolytics. I used the same JJ cap can that you did.