Tray or YBA-1A Very Low Output - Please Help

[Coop]

Hello! I just picked up a Traynor YBA-1A tube amp. The output is very low and distorted. Tried the external speaker jack, it was even lower output,. When I got it, it was missing the AC mains fuse. I tested both 6CA7 output tubes, and one was blown, and the other very weak, hence the missing (probably blown) mains fuse. I installed a good matched pair of JJ EL34’s. Tested the 3 12ax7 tubes, they’re all good. Powered it up slowly with my Variac to 110 VAC, took some voltage readings, all were within spec, per the schematic pasted on the top cover. The issue is very low output. I’ve checked all caps minus the two can caps, and resistors, and control pots. Removed the brown box Surge Arrestor too. Not sure where to go from here. Even tried different tubes throughout. Anyone on you awesome tech’s want to give me some advice here? Thanks in advance
Coop

[Helmholtz]

Schematic?

[GoldtoneWorkshop]

Here's what I have

[g1]

Removed the brown box Surge Arrestor too.

Was there ust one, or two? (they are shown on schematic across OT primary)

[mozz]

Mine may have had those flyback diodes, forget it's been years. What about those big brown electrolytics?

[Helmholtz]

As the amp came with a schematic the OP should tell us which one to use here.

[loudthud]

Check resistance of the 470 Ohm 10W resistors in the screen circuit. Also check that you have B+ in the preamp.

[Coop]

Goldstone, it’s the second pdf you posted I do believe. Here’s my schematic that’s pasted on the cover. Mine only had one Flyback diode/Surge Arrestor ( big brown square box)

I’ll double check the 470 ohm/10 watt resistors, and voltages again, first chance I get.

[Helmholtz]

As the "surge arrrestor" seems to be bidirectional it's likely a varistor (MOV).
What happens if you disconnect it?
It's only needed during heavy abuse/clippling or löad-dump.

Do you know how to measure idle current?

If the shared screen resistor is good and screen voltage of both tubes is in the ballpark, unplug the amp, discharge caps and measure resistance from plate pins to the OT CT.

Do you have a scope?

[The Dude]

Those 6RS20SP20-B20 devices are old selenium voltage clamps- the "ancient" version of an MOV. I'd definitely remove them. If they ever go bad you will smell it for years.

[g1]

If you can post some photos of inside the chassis that would help. A lot of these have been modified and not always for the best.

[Coop]

I have good plate voltage on all 12ax7's, and both EL34's

V1 - 12ax7: Pin 1= 200vdc. Pin 6= 198vdc
V2 - 12ax7: Pin 1= 289vdc. Pin 6= 166vdc
V3 - 12ax7: Pin 1= 241vdc. Pin 6= 265 vdc

Surge arrestor has been removed. My amp only had one.

As Helmholtz suggested, I checked resistance between the OT CT, and the plates of each tube socket

V1- 12ax7 - Pin 1= 117 kOhms. Pin 6= 117 kOhms.
V2 - 12ax7 - Pin 1= 23 kOhms, Pin 6= 119 kOhms.
V3 - 12ax7 - Pin 1= 103 kOhms. Pin 6= 94 kOhms.

V4 - EL34 - Pin 3= 0.9 ohms
V5 - EL34 - Pin 3= 53 ohms.
Ckecked both 470 ohm-10 watt resistors, Both good

I do have a scope, but not real familiar with using it. If someone could give me some advise, I'd give it a shot

[Helmholtz]

I don't have B+ voltage on V3 - Pin 1 (plate) 12ax7 phase inverter tube.

V3 - 12ax7: Pin 1= 0.302vdc. Pin 6= 242vdc
V3 - 12ax7 - Pin 1= No resistance. Pin 6= 94 kOhms.

What is no resistance? Do you mean infinite resistance (meter reading O.L.)?
If so the plate resistor is open (R22: 100k).

As Helmholtz suggested, I checked resistance between the OT CT, and the plates of each tube socket

V4 - EL34 - Pin 3= 0.9 ohms

(I actually only meant the power tube plates).

Low resistance at V4 plate means a shorted OT primary.

[Coop]

After doing the resistance check, I noticed one end of R22 was not connected. I since edited my post. I now have B+ on all tube plates

[mozz]

I have good plate voltage on all 12ax7's, and both EL34's
Well which is it? Got plate v or not? Those EL34s should have over 600v on them.