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jule 23rd August 2011 03:30 PM

Approaching the best primary impedance on low or high side?
I have universal PP output transformer. (Hammond 125C - 8W)

My tubes - 6p1p (equiv. to el90 or 6aq5 or 6v6) require 10k load in push pull(not desided about class A or class AB). B+ voltage will be around 200V - 230V.

But my tranny gets only 8.2k or 11.6k. From that side it is the best to approach? low or high for best operation since they are almost equaly off)?

It will be a guitar amp


razorrick1293 23rd August 2011 03:47 PM

i think lower is better, maybe use a slightly lower voltage to make sure nothing ges damaged, bias would have to be adjusted accordingly. ive used 3.5k in place of 4k and 33k instead of 22k, so i dont think 1-2k out will make too much difference. though im willing to be proved wrong if someone with better knowledge can chime in.

hidnplayr 23rd August 2011 05:47 PM

Lower impedance will give more power and more distortion.
Higher impedance will give less power and less distortion.

Thus, for a guitar amp, I would go for a lower impedance.

TheGimp 23rd August 2011 06:07 PM

The 6P1P-EV will take 350V B+ (I actually cranked them to 400 for a short burst). I don't have any 6P1P so I don't know what abuse they will take. Which do you have?

With 200-230V B+ you might as well stick to class A. I was only able to get more power in AB1 when I cranked up the B+ so there is no advantage there for you.

Also, Class A has the advantage of less stringent matching requirement on the tubes.

While theoritically 10K is optimum and lower = more power and slightly higher distortion, I have never been able to hear or measure the distortion difference when using Edcor CXPP class transformers. I can measure the power difference but it is small. I recommend the 8.2K setup.

Although I see no reason for you not to try both configurations and let us know if you can hear any difference.

jule 23rd August 2011 09:04 PM

1 Attachment(s)
I have regular 6P1P. Not matched. Max voltage according to datasheet is 250V. It would probably survive more, but I have power transformer of 200V~ on hand.

Suposedly in class AB I could expect 10W of power(don't know the voltage), but my tranny is only 8W, so I will stick to class A.

I have approx schematic in attachment. It is getting some more controls and one more preamp tube.

TheGimp 23rd August 2011 09:44 PM

I never could get much more than 8W with an 8K p-p transformer and 256V a-k.

Figure you will get a little less than 275 at the first filter cap, 10V through the output transformer and 10V for cathode bias leaves you with around 255 anode to cathode. That is were I ended up with a 200V transformer. I don't think the extra 5V is going to cause you problems.

Since this is a guitar amp, a tonestack and two more stages will be about right.

You might want to decrease C3 to move the -3db point up in frequency. It is currently around 12.9Hz which is fine for audiophile amps but a bit low for guitar amps. 2.2-4.7uF would probably be a better choice.

Lead Guitar? Bass Guitar?

jule 23rd August 2011 10:29 PM

Rhythm and lead guitar. Mostly blues.

Do you recomment 6n2p for phase inverter or should be some of the 12a?7 series?

razorrick1293 23rd August 2011 11:15 PM

12at7 is usually a good PI either that or 12ax7s are common in guitar amps

mashaffer 24th August 2011 01:40 AM

For a guitar amp I would think that putting 10W through an 8W tranny should be no problem. You are not generating any really low frequencies and it is not at all uncommon for guitar amps to use OPTs that would be under rated for hifi use. I say go ahead and try biasing it AB and see how it sounds.

razorrick1293 24th August 2011 01:57 AM

i wouldnt put 10w through an 8w tranny if your going to run it at levels where it will clip unless your going to run it quite cold biased, seeing as guitar amps generally like pushed power tubes, i wouldnt feel too safe with that

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