I don't get the expected power output.

[Alan0354]

Hi, I got my amp with unbalanced PT all working and I am happy with the sound. So I did the final test to find the output power. This is a 6V6 running at about 26mA each tube.

I only get 14W!!!! That's low. This is how I test it: I measure the voltage swing using my 8 ohm THD HOtplate. I measure the signal at full power using my guitar and monitor the peak to peak of the clipping signal. The voltage is 30Vpp clipping to the rail. So I back calculate the RMS voltage is (30V/2) X 0.707= 10.6Vrms. Then I calculate W =(V^2)/R= 10.6^2/8=14W.

That is much lower than I expected. This is the condition of my amp:

1) The +B =400VDC at idle, it sag to 350VDC when hit hard with guitar. That seems to be a lot.

2) I use the output transformer for Fender Vibrolux which is I believe 4K. The way I cheat is I hook the 8 ohm load ( speaker or THD) to the 4 ohm tap of the OT to get back the correct load.

3) Because the need to balance the winding of the PT, I use a 75 ohm resistor on each leg of the 2 diode rectifier to take up a few volts to help balancing the two sides. That might cause the extra sag. When I shorted out the two 75 ohm, I still have maybe a little less sag. I measured about 16.7W to 17W output. This is still low.

Please advice what I can do. Do you think getting the correct PT will fix the problem? It is supposed to arrive today, but I am still waiting. With that, if it test ok, I can use that and remove the two 75 ohm resistor and gain back a few volts.....and more importantly, holding the +B more constant when hit hard.

Thanks

[nickb]

Your 4k impedance is too low. 8k would be a better match. Check the plate voltage swing, it's probably only going down to 160V or so. With 8k load you'd get nearer to 50.

[Alan0354]

Your 4k impedance is too low. 8k would be a better match. Check the plate voltage swing, it probably only going down to 160V or so. With 8k load you's get nearer to 50.

Yes, but I intentionally put the 8 ohm load onto the 4 ohm tap of the OT, that will reflect back to the primary as 8K load onto the 6V6. I'll look at the plate voltage and see how low it goes down.

Thanks

[Leo_Gnardo]

I see Princetons all the time with similar supply characteristics, and depending on output tubes deliver 11 to 17 watts at clip. I'd say 14 is what you should expect, and lucky to get that much with your unbalanced PT and all the folderol you have stuck in there to panel-beat it into being somewhat more balanced.

If it sounds good, it IS good.

Stop worrying, play the amp & smile, you've done it!

And yes, you'll likely gain a couple watts with a balanced PT. Bet your ears will hardly be able to tell the difference. But you will have the peace of mind that the power supply is what it's supposed to be.

[Alan0354]

I finally received the new transformer, and it is balance!!!!

I am at the process of putting it in. It'll take me a little time to do that. Now that I have balanced voltage, I am planning to use the 680V taps to get 480VDC unloaded. I am setting up the MOSFET source follower in the power scaling to drop about 10 volt and with loading, it should drop to 450VDC. I am going to use the JJ 6V6S instead. The JJ can stand higher voltage. I should get about 25W with 25mA of tail current. I am crossing my fingers.

[jazbo8]

This is how I test it: I measure the voltage swing using my 8 ohm THD HOtplate. I measure the signal at full power using my guitar and monitor the peak to peak of the clipping signal.

The Hotplate is not a purely resistive load, and depending on the setting, it has a "frequency compensation" feature, both would mess with your reading. Also, the ears can tolerate quite a bit of distortion, so what you see on the scope as clipped may in fact still be "clean" with relatively low distortion, i.e., the actual full output at 10% distortion might be quite a bit higher than the 17W you calculated.

[Alan0354]

The Hotplate is not a purely resistive load, and depending on the setting, it has a "frequency compensation" feature, both would mess with your reading. Also, the ears can tolerate quite a bit of distortion, so what you see on the scope as clipped may in fact still be "clean" with relatively low distortion, i.e., the actual full output at 10% distortion might be quite a bit higher than the 17W you calculated.

So this mean I have to use a resistor to test?

[jazbo8]

Yes, usually the output power ratings are done with purely resistive load and sine wave input, while the guitar signal is close, it's hardly a pure sine wave. All of that would affect the reading you get at the output.

[Alan0354]

Another very funny thing. I put in the new PT, adjust +B to 420V, bias at 25mA each tube. I still get about 15W only. Then I plug the 8 ohm load to 8 ohm tap ( originally at 4 ohm tap for the 6V6). I got very close to 20W!!!

Doing this, I actually mismatched the tube as the OT is a 4K primary for two 6L6. If I put 8 ohm load on 8 ohm tap, the power tubes only see 4K instead of 8K. Would that do damage to the tubes or the OT? My guess is OK, but just want to run it by you guys.

[jazbo8]

Yes, lowering the load increases the maximum plate current thus the output power, you could damage the tubes and/or shorten their lives, not recommended unless you can't live without the tone that it produces...

[Alan0354]

I found it. Nothing make sense, so I started to think maybe the preamp tubes are too old and it clipped, not the power amp. I changed all the preamp tubes to brand new JJ 12AX7, WALLA!!! With 412V +B, I got 20.1W!!!

When I start powering up the amp, I just stuffed all the old tubes lying around in the box, those are really old tubes. After I tested and massaged the amp, I forgot to switch to the new tubes all together. Now I have all new tubes in the amp.

Tomorrow, I am going to test the sound as all tubes are new. Hopefully the sound is still good.

Thanks for all your help.

[jazbo8]

Great, glad you sorted it out. Now crank it up and wake up the whole neighborhood.

[Alan0354]

What prompted me to look at the preamp tubes was because both channels have the tone stack at the end before driving the power amp, just like the Marshall. The tone stack attenuates the signal quite a bit and if the tube is too old and does not swing large enough, there won't be enough signal to drive the power amp into saturation. That's the reason I experimented changing the preamp tubes.

I think I figure out why I got more signal with the 8 ohm tap. I have the feedback from the 4 ohm tap, the 8 ohm tap by default has more swing ONLY because it's not the feedback tap.

[frus]

how much signal have you got now on 8 ohm tap?

[g1]

Yes, lowering the load increases the maximum plate current thus the output power, you could damage the tubes and/or shorten their lives, not recommended unless you can't live without the tone that it produces...

Agreed about the increased current and shorter tube life, but I have never seen a tube amp that has increased output power when running into a mismatched load. Whichever setting gives the highest power output is the correct match.