MK3s new build issue

Hi everyone,

I just finished building a pair of MK3s from scratch using the M125 driver boards. Both builds went well but a couple of things happened when installing the amps on my system:

-Phase: I’ve check the polarity of everything (from the PT to my speakers wiring) and there seem to be some kind of bass shift in the phase. The sound is quite lean and a bit hollow. One side sounds noticeably louder than the other as well. The soundstage is off. So far my system had a single Micromega PW400 Power amp (SS) and the sound was very 3D, great separation, etc… Now the image is quite smeared which is not surprising with the loudness difference and phase incoherence.
To give you an idea of the volume difference, with the louder amp switched to Triode, the balance is about right but I can still hear a phase problem (I’ve double checked the triode/pentode switch wiring, that’s not it).

-Depending on the tubes I use on the driver board of the louder amp, I get a very low end rumble (I’m talking 30hz or below) which affect my speaker woofer (and shakes the walls) as soon as power hits the power tubes. It happens with 2 different pairs of NOS E80CC. Everything is normal with a pair of current EHX 12BH7. If I use the E80CCs on the other amp, no rumble.

First time building power amps so I need some help on what to look for.

Here are the specs:

PT Hammond 278X (800Vct 230ma, 5V 3A, 6.3V 6A using a .022 cap on the CT)
OT Electra Print Audio 4K2 primary, 40% UL, 6 OHMS secondary, 70W
Genalex GZ34 (Russian)
TAD 6550 black plates version
B+ 515VDC

When listening to the amps individually, I set the bias to .65ma which sounded better to me than .5ma.

I’m looking forward to your suggestions!

Thanks.

I also noticed the T/P switch has much less volume drop on the quieter amp. Can this be a faulty OT?

IME some M125 driver boards struggle to maintain bias steady. Have you confirmed yours are holding steady?

Yes, very steady.

I just made a bunch of measurements and both amps are identical (+/- 2V).

B+ 526
B+ post choke 517
B+ to board 392
6550s pin3 513  pin4 515  pin5 -59.5
Board (12BH7s):
V1 pin1&6 121, pin3&8 5.3
V2 pin1 243, pin3 12, pin6 237, Pin8 12

I still believe it is some kind of phase issue. Listening to my system, it’s like the centre disappeared… Kick, snare, vocals which are usually in the middle have mostly shifted to one side.

it doesn't look like this thread got updated, I got an email from the builder who wound up reversing the primary polarity of one of the OPTs,
and that solved the problem and all is well and good now. A common problem and solution when using new/unknown transformers.

Hopefully the OP will come on here and tell us how his amps are sounding now, a tube amp with no depth, soundstage, or placement sounds solid state!!!