Blackstar HT Stage 60 OT questions

[MervsMods]

Hey Guys (and Gals)

I have a Blackstar HT Stage 60 that has issues:

1. LOW output. There is sound coming out of the speaker, but it is NOWHERE as loud as it should be. With the amp Master on 10, it's slightly distorted (and not in a good way). I'm pretty sure the preamp section is OK, because I used the SEND of the Blackstar to the return of my Crate Stealth, and it sounded great!

2. OT DC resistance readings:
Secondary
Common to 4 ohm tap - 0.2 ohms
Common to 8 ohm tap - 0.2 ohms
Common to 16 ohm Tap - 0.4 ohms
In between (all combos 4, 8, & 16) - 0.2 ohms

Primary
Red - Blk 82 ohms
Red - Grey 52.9 ohms
Blk - Grey 29 ohms

Please forgive my ignorance, but shouldn't there be an increase between the common and different tap settings as you go up (4, 8, 16)? Should the resistance readings be a lot more than 0.2 ohms if I measure between the different tap settings? Shouldn't the DC resistance be the same on the Primary side when measured to the Grey wire (middle)

When I initially inspected this amp (bought it not working from eBay) I found scorch marks right where the OT secondary wires come in to the chassis. Also found scorched wires, but it only went as far as the insulation. (no open or shorted cables)

Tubes I installed are known good.

Could it be the OT, or something between the pre and power amp section that I've yet to find? Everything else on the amp looks good. No burned spots, no exploded parts, etc... OH the power amp board looks very clean as well. Nothing appears to be burned, or broken.

Thanks in advance for your help! I really do appreciate it!

 

[Phrygian77]

Check the bias voltage. Someone brought me one of those with a very similar issue and the first thing I noticed was that the plate voltage was way up at 500+ and the tubes were passing very little current. Then, I checked the bias and the voltage at the grids was -90. Neither of the bias controls did anything. As far as I could tell, part of the bias circuit is relay controlled, and I couldn't immediately figure out what was wrong. Given that there's a lot of SMD on that board, it wasn't worth my time to troubleshoot further. Good luck.

 

[Jon Snell]

The Output Transformers are more reliable than you think.
Unless you use a Low Ohm meter, you will not be able to tell what is what between the speaker windings. Don't get yourself confused!
If you are competent and trained;
Check the driver FETS, they are a known issue in these as they act as the phase splitter as well. If found to be at fault replace them with the exact type otherwise you will have eternal issues!

 

[MervsMods]

@Jon Snell : Great Call! I found one of those FET's to be opened! Just ordered parts (will replace both)

Is there any chance you have a complete schematic for this amp? I find that using FET's as a phase inverter interesting, and want to find out more about it.