Gibson GA-95 RVT

[Travst]

265v at C29A. The 12ax7 was known good. I just stuck a new 12ax7 in V4 and got very near the same voltages as before.

It seems like we've gained a bit more volume now.

[Phil_S]

Simple but not so simple test for the i/s tranny is done like this. Inject low a/c voltage into the secondary and see what comes out the primary. How's it done? If you have a low voltage a/c supply it's easier. If not use the 6.3vac filament supply. I'll assume you need to use the filament supply.

1. Pull all the tubes.
2. Discharge caps.
3. Disconnect the center tap from the bias supply (lift at R53).
4. Disconnect the primary from the B+ rail - this should be a red wire, probably at cup #17 - trace it back to the tranny on the outside of the chassis, opposite the middle of the board.
5. The power tubes are in pairs V6+V7, V8+v9. Any reference to one member of the pair is a reference to either -- use whichever socket of the pair that is most convenient.
6. Use two jumpers. To assure that you get a jumper from each leg of the 6.3v supply, I'd tap that on only one tube socket (pins 2 and 7) -- this will prevent a mistake. Run one jumper to pin 4 of V6/7 and the other to pin V8/9.
7. Clip your volt meter onto V5 pin 1 and the loose red wire to the primary of the tranny (make sure it is in the air or insulated).
8. Turn the power on. 6.3v into the secondary at 2.5:1 turns should give you about 15VAC at the meter.

[Travst]

I'm not sure how to run the test correctly. I have the meter set up, no problem. The bias supply from the PT is disconnected and so is the red wire for T3. I have a 6v transformer. I ran one wire to pin 4 of V6 and one to pin 4 of V8, but I'm not getting a reading.

[Phil_S]

See if you can find the secondary leads of the i/s transformer. If yours is same as mine, they are green, green/white, and yellow. I don't recall which is the CT. Anyway, trace them back to the transformer. Wherever they terminate inside the chassis is the place to clip onto. I may have given bad directions. The 6L6's have 1.5K grid stoppers connected to pin 5 - I forgot about these (sorry). You need to feed voltage on the transformer side of the 1.5K resistors.

If I told you pin 4, that was bad info. Sorry, I wasn't paying close enough attention. Pin numbers are OK, but the tube part is a more reliable reference. We are looking for the grid (pin 5). Pin 4 is the screen. 6v there shouldn't be any sort of issue.

Your voltage source must be AC, not DC. Common wall warts are usually DC if that's what you might be using. It need not be 6v. It's just that you don't want voltage that's too high. 5-20vac range should be OK.

[Travst]

I traced the wire from R43 to cup #17, where the red wire from T3 is located. I fed voltage in there, as well as to the black wire that runs from T5 to pin 1 on V5. The termination points of the green and green/yellow wires are pin 1 on V7 and V9 (transformer side of the resistor spanning to pin5 on each tube.

I'm reading 8.26v at each pin 1 and feeding in 5.8v from one lead and 4.8v from the other lead. Supposedly, this little transformer is supposed to put out 6v using both those leads. I measured to voltage on each lead by grounding my black MM lead and clipping the red MM lead to each lead from the transformer.

This is my transformer:

http://www.radioshack.com/product/index ... Id=2103732

[Firestorm]

We're doing a self-contained AC test here, so you shouldn't be grounding any lead. How are you supplying 120VAC to the transformer?

It seems you're using the center tap of the RS transformer secondary and one other secondary lead. Keep the remaining secondary lead away from everything. Wrap a little black tape on it if you want. Those two leads will go to the connection points of the green and green-white wires on the 6L6 sockets you've identified. There is a yellow cloth-covered wire coming up through the chassis to cup #18. That wire needs to be unsoldered. Maybe throw a little tape on it too. The red cloth-covered wire coming through the chassis to cup #17 has been unsoldered right?

Set your meter to at least 6 volts AC and connect its leads to the same places where you connected your RS transformer leads. Plug in the transformer and note the AC voltage. No need to reference it to ground; it's just one side against the other.

Turn off your RS transformer and move your meter leads to the red cloth-covered wire you unsoldered from cup#17 and to V5 pin 1 where the black cloth-covered wire is attached. Set the meter for at least 15VAC. Plug in the RS transformer and make a note of how much AC you measure on that side of T3. Then let's see what we have.

[Travst]

Ok, that cleared it up. I fed 6.9v into Pin1, V5 and the red wire from T3 that I disconnected from cup #17. I measured the output at 16.7v with my MM leads at pin 1s of V7 and V9.

[Firestorm]

Okay. 1:2.4 is what Phil said his was, so good. As long as everything is disconnected, You can perform the same test again, putting 6.9VAC into the primary side (black and red wires) but this time connect one meter lead to the center tap (the yellow wire that used to go to cup #18) and then compare the reading you get with the other lead connected first to the green wire and then to the green-white wire. Ideally, both will read around 8.28VAC.

[Travst]

8.26v exactly.

[Firestorm]

Okay. V1 seems to work; V4 seems to work with a good tube in it. We're not sure of the health V5. That is supposed to be another 12AU7. Again, you should be able to sub 12AT7 in a pinch. Interstage transformer seems working.

A while back, Phil suggested you run the OT test in reverse of what you did the first time (6.9VAC in on the secondary - speaker side - and measure AC on the primary - tube side.) Ever do that? Volts should be over 200VAC so your meter readings will be more precise. It's best to disconnect the two transformer leads from the speaker jacks to avoid disasters with the shorting tab. Test the other side using one plate connection from each tube pair. We ignore the center tap completely (well, drain the caps first to avoid getting zapped). Should also measure resistance from center tap to each primary end to compare.

This would also be a good time to examine and clean up the speaker jacks. Final link to the speakers. The schematic shows a cap connected to the speaker jack via a switch on the reverb channel's treble pot. How is that labelled? Another thing to test since it's tied to the output.

[Phil_S]

cup #18

Good news on that transformer. What is it with the numeral eight? It's driving me nuts!

[Travst]

Thanks guys, I very much appreciate all the help and I'm learning a ton of practical things. I'll get back on it tonight after work to perform the next steps, including testing the OT properly. I understand how to do that now, and I'll definitely disconnect the speakers.

[Travst]

I measure 257mv at the two OT secondaries after applying 6.9v at pin 3 of V7 and V8.

I cleaned the jacks as well. Not much has changed as far as volume goes.

[Phil_S]

This test is only partially conclusive because of the low output and concern about the accuracy of a measurement of 1/4V. It suggests the OT is OK. The math says you have a primary impedance of ~2.9K which I'd guess is within reason.

I suggest you reverse the test. Input 6.9V to the secondary and you should get ~185V on the primary.

Let's ask a related question. The two 8 ohm speakers should be wired in parallel for a total load of 4 ohms. If they are in series, that's all wrong and should be fixed. It would affect volume, but I'm not sure by how much, and I don't think the degree would be what you describe.

[martin manning]

Down at 60Hz the response of the OT may be falling off a bit, which would make the impedance ratio appear high.