Monkeymatic Acetone Princeton

[xtian]

I've used febder hotrod deluxe OT with 6v6's with good results.It;s a beefy tranfo for 2 6v6's.

Excellent idea, and just in time. I had my CEDIST cart loaded with the Hammond 1750e. No, I don't care if it sounds like a Princeton...I want it to sound good! Same price for 40 watts sounds good.

[xtian]

I've used febder hotrod deluxe OT with 6v6's with good results.It;s a beefy tranfo for 2 6v6's.

Excellent idea, and just in time. I had my CEDIST cart loaded with the Hammond 1750e. No, I don't care if it sounds like a Princeton...I want it to sound good! Same price for 40 watts sounds good.

[JazzGuitarGimp]

It's my understanding that bringing the grid resistors right to the grid not only helps with radio interference, but it also helps with high frequency parasitic oscillation.

After rereading the thread, I see your bias voltage is -40.7V. You're going to need 81.4V p-p at the grids of the 6V6's just to get up to full CLEAN output. You'll need more than that to overdrive output stage.

[xtian]

After rereading the thread, I see your bias voltage is -40.7V. You're going to need 81.4V p-p at the grids of the 6V6's just to get up to full CLEAN output. You'll need more than that to overdrive output stage.

Thanks, Lou, that's important info I was looking for.

[JazzGuitarGimp]

-40V seems cold for 6V6's. What is your idle dissipation? -40V is closer to where I'd expect to see 6L6's biased.

[xtian]

AT idle I see 440v on plates and 0.024v on 1R cathode resistors (per tube).

[JazzGuitarGimp]

AT idle I see 440v on plates and 0.024v on 1R cathode resistors (per tube).

That puts you at about 75% idle dissipation, which includes the screen dissipation since the 1R current sense resistor is in the cathode circuit. This seems reasonable.

The Deluxe Reverb, which has essentially the same power stage, yields 22 watts of clean output power. So that is what I would expect from your build.

Since this is a push-pull design, we really only care about the positive peaks from the PI. If your PI voltages are similar to the AA964 schematic, the anode output of the PI can swing up to a peak of 70V from its resting point of +220V (290-220). This is more than the 40.7V you need to get full clean power output. The cathode output of the PI starts at 65V. If the tube were capable of conducting down to 0 ohms from anode to cathode (I'm pretty sure it can't), then you'd have 146V at the cathode, which is 81 volts of swing since the resting point for the cathode is 65V. Again, I don't expect the tube is able to conduct that much, but 81 volts is twice what you actually need.

Have you measured the p-p voltage coming out of the PI?

Also, since Tillydog confirmed my suspicion that there is no current flowing in the grid of the PI, I think the grid stopper is better placed between the 1M resistor and the grid.

[Tillydog]

I agree that if the bias is ~-40V then you should be looking for ~80V P-P out of the PI for max clean power.

....I put 1v p-p sine wave on the input, and turned up the volume until just before output distorts. I get 65v p-p going into the grids of the 6V6's...

Is this still the case?

Can you tell which stage is distorting first? (scope?)

[xtian]

Received a replacement OT. Measured the primary: red to blue: 108.7ohms. Red to brown: 83.7ohms. Whoah. Never met a push-pull OT that unbalanced.

Would you call this OT bad, based on this measurement? Help me decide whether to ask for a replacement before I clip the leads.

[cbass]

Did you get the HRD OT?
I think I have the ohms written down in the chassis for bias measurment.I write the OT Measurment and what the voltage drop should be between CT and primaries in my amps.If that makes sense.

I could open one and look.I don' t think mine were more than 5 or 10 ohms apart.I'd have to look to be sure.

[xtian]

Yes, this: http://www.cedist.com/products/P-TF50438

I measured two other loose OTs, and see that both primary legs are much closer balanced. This has been my experience. Never seen one that was 25% different!

[cbass]

I'll look In the morning.
Hope I didn't steer you wrong.
Serves you right for listening to me

Wow that was quick my orders from CE take forever.I tracked my last package and it went from AZ to seattle before coming to Missouri

[cbass]

I had 95 and 82 written in the chassis

But when I measured again I got aprox 100 and 80 .I don't know if thats a sign of a crappy OT or not.I think mine sounds good

[xtian]

OK! Nearly the same as mine. I'm a-gonna throw it in and see what happens.

[cbass]

Let us know .How it sounds and don't forget its 8 and 16 ohm taps now with the 6v6's