Opinions: SE Class A bias in PP amp

[ldatlof]

I have the TAD-60 amp and I have biased it using the following instructions from TAD's website:

*NEW SINGLE ENDED CLASS A BIAS: This amplifier will operate class A for the first 10 watts and class A/B 10-50 watts if final biased this way:for EL-34 & 6550/KT-88: V1, V3 set to 200, V2, V4 set to 500-550. Notice improved smoothness in the midrange and richer bass compared to regular bias mode. Only giving up 10 watts on top (50w instead of 60w) for class-A mode. Single ended - because the hot biased tube is doing most of the amplification 360 degrees up to 10 watts while the cold biased tube is mostly acting as an out of phase damping brake, offering little in phase amplification under 10 watts. Have it all, single ended class A and 50 watts A/B when needed.

I have had the amp biased like this for a month now and there is definetly more bass now from the EL34 tubes. The midrange and highs seemed to be unchanged. I was wondering if anyone has heard of biasing a tube amp like this before? The EL34s are a matched quad and are the "hot" tubes going to wear out sooner because they have more current running through them? Just thought I'd ask for opinions on this type of biasing. Loren

 

[jon_s]

Huh... so they are advocating a huge DC imbalance in the output transformers?

Unless the transformers are air-gapped (maybe they are?), I would expect deleterious effects on sound quality. A 1mA DC imbalance on my amps is enough to yield audibly muddied sound ... but if it works, and you are enjoying it, I wouldn't worry much. Yes, the tubes running more current will wear out sooner, but EL34s aren't too expensive, and if you rotate them side-to-side it shouldn't make a big difference.

The added bass and richer mids are likely due to the static magnetic field offset in the transformer core causing premature saturation and 2nd harmonic distortion.

 

[PakProtector]

Hey!!!,
That is an AB amp. If it runs in AB at less than full power it is an AB amp across the board. It is a politician's way of interpreting the definition of class A to their own advantage.

They so much B+ that they can barely make class B conduction angles. They are not giving all the details and assuming that we'll fill in the blanks between their buzz words to their advantage.

As far as the real hardware goes, I'd only say that if you like it that is fine by me. Just because I don't appreciate the liberties their advertising department took is no reason for you to enjoy it any less.

cheers,
Douglas

 

[NOSValves]

Chances are the increase in perceived bass response is actual increased distortion in the lower frequencies causing erratic woofer movement because the amp ran in this fashion can not control the woofer from lack of clean output. Running a huge imbalance on the PP pair is nothing but a great way to increase distortion. Not saying you will not prefer the sound of your particular amplifier like this or that its wrong to like it but it is what it is.

Craig