single ended output - output

[prune]

Im trying to troubleshoot a low power, single ended output amplifier (one EL34). The amplifier suddenly lost a lot of volume. I think I have tracked the problem down to the output stage - the plate voltage shows there is almost no gain between the grid and the anode. If I plot the load line, it shows - if i am not mistaken - that the plate should swing hundreds of volts! I have tried replacing the EL34 - no change. I measured the reflected impedance of the transformer and it's around 3.5Kohm and it performs fine when tested with a variac. The components around the EL34 measure fine, although I have not replaced them.

I'm scratching my head and wondering why this might be - can anyone confirm (or disconfirm!) that I should be seeing a much larger voltage on the plate, or provide me with a suggestion of where to look next? Here are some measurements I have made of the EL34 -

all measurements referenced to ground
pin 1/8 = 23V DC 
pin 3 = 346V DC
pin 4 = 327V DC
pin 5 = 0.1V DC

with 9.7V AC on pin 5, I get 12.41V AC on the anode! almost no gain!

[mresistor]

I assume you checked the heater and it's 6.3VAC.

[Willabe]

Your drawing shows 56K for the screen grid R, must be a typo.   

[shooter]

Your DC volts look about right, the 12AC, not so much.  did you check speaker wires, jacks connection?

[eleventeen]

You have that 1K as, in effect, a load resistor for the EL34 output. (Eg; B+ >> 1K >> plate EL34) As if it was "any other tube" in the amp.


Now, we are used to seeing such resistors (2-3 in series) dropping "highest of the high" B+ for the preamp stages and the phase inverter. But generally, the output tube is connected directly to the "highest of the high" B+, straight out of the rectifier section, through the OT primary winding. The way you have it shown, that 1K R should drastically current-limit and thus choke off the EL34 and pretty much foil its ability to swing big plate volts. That would be my take on the theoretical.
 

[Paul1453]

Yeah that diagram has me confused too.

So L2 is your OT???

Instead of a 1K resistor, I would expect to see a 20uF E cap before the OT.

Then maybe a 1K resistor and another E cap for B+1.  With RC filter sections for each voltage.

[PRR]

Smells like a near-shorted output. You have checked the OT, how about the output jack?

The 1K in front of the OT will waste some swing to the load, but you should still get "most" of B+ swing at the plate. Say 320V DC across the tube, peak swing say 80% or 250V peak, 180V RMS, for BIG signal.

Not using some absurd frequency like 2Hz or 123KHz?

> pin 1/8 = 23V DC
> with 9.7V AC on pin 5

You have half-big signal. 23V at K should allow 16Vrms at G. You should expect near 108Vrms at plate. ("Gain of 10 grid-plate" is often a good guide for the usual audio power tubes; bit more for the EL84.)

[eleventeen]

>>EL34<< PRR, not EL84.