Power output is not what is expected

[tubesornothing]

Howdy gang, long time, no chat!  Good to be back.  I have been playing in the machine shop and with CNC for the past couple of years - haven't spent much time with the amps.


I dont do much repair, except for a couple of close friends.  I am too damn busy anyways, so they tend to take it to other guys.  Anyways, one buddy brought me his Dr Z Galaxie saying it does not put out very much.  He had another tech look at it and said it only puts out 22-25W.  It is PP 6L6GC  (Class AB?) with fixed adjustable bias. Sorry - no schematic.


I told him I would take a look.


I measured the plate voltages throughout the amp, and here is what I got (unloaded)


V1 - preamp - 189V
V2 - CF - 289V
V3 - gain after tone stack - 208V
V4 - PI - 278V
Power tubes 402V


Then I ran a 300mV 1Kz sine waver into channel 1.  8.9 ohm dummy load. Scope on the output, turned up volume, just prior to distortion, got 13.49V - that is 20.4Watts


I measured the bias of each power tube and got around 56mA for each tube.  At 402V that is about 22.5W dissipation per tube.  Don't know if that has anything to do with power output getting to the speaker.


Hrrmmm...  I dont really every measure amp power, so I am naive in this area.  The process I used seemed to be pretty common, so I expected to see something in the 40W range.


Suggestions?


thanks!


TON

[HotBluePlates]

What's the actual bias voltage applied to the tubes as you tested it?

[HotBluePlates]

Also, you may want to contact Dr Z and get a schematic. It's a discontinued amp and you're servicing it, so hopefully they won't be too stingy with service data.

[tubesornothing]

Thanks HBP, I measure the bias voltage at 44V

[tubesornothing]

Good ideas.


Triode or Pentode does not make a major difference in the power output calculation.


27mA bias sounds odd to me, but I will give it a go.


Amp output it 8 ohms, and the 12" speaker is 8 as well.

[HotBluePlates]

Amount of bias voltage sounds reasonable for a fixed-bias 6L6. Is the screen voltage reasonably-close to the plate voltage? A low screen voltage would limit peak plate current (as would a very large screen resistance, on the order of 3-4kΩ).

If voltages throughout seem reasonable, it would be very handy to have another OT on hand to be tacked into the circuit to rule out an OT issue.

Then of course you could try signal tracing/listening amp to see if there is a stage which is obviously amplifying weakly.

[tubesornothing]

Thanks for the reply HBP.


On both tubes, screen is 1V diff from plate @ 416V.


Dr Z suggest 27mA bias with the transformer shunt method. So I did a bias current check by measuring the resitance of the two half windings of the OT, then measured the voltage drop (sort of a "safe" transformer shunt methd). 28mA on one side, 34mA on the other side.  So, somewhat close.

I did a "clean" signal analysis through the amp:

V1 in 340mV pk 1kHz sine in
V2 in 93mV pk @ 1/4vol (any more and it start to distort)
V3 in (after tone stack and CF) 700mV pk
Into power tubes 18.3V clean sine

I am thinking it would be a good thing to test the PT.  I will try to get a hold of Dr Z and see if they can help me out over the phone.

thanks for the assist.

[HotBluePlates]

Was there a volume control between V1 & V2?

18v into the power tubes with -44v bias won't cut it in that amp. You'll want to see the phase inverter pushing ~43v peak (~30.41v RMS) into the output tubes at a reasonable volume setting and input jack signal level.

[HotBluePlates]

Come to think of it, if you have a signal generator and a dummy load, try this:

Connect the sig gen to the phase inverter input. Attach your dummy load in place of the speaker(s). Crank the sig gen up until you get the 43v peak/30.41v RMS measuring to a single output tube grid. Now measure the voltage across the dummy load and calculate output power.

[tubesornothing]

Yep - I think you have it got it.  The earlier stages are not putting out enough clean sine to drive the power amp.  I will run the tests you suggest.  I will also look at the preamp, CF and PI stages.