Blackstar Soloist HT 60 bias help

[mikeskory]

This is Mike from Lansing. A Blackstar HT60 SOLOIST is on the bench right now. I have see so few Blackstar amps so I'm unfamiliar with the biasing. The original problem was "dead". We replaced the external fuse and both EL34s. I wanted to rebias because the old tubes had signs of overheating. One was cloudy at the top, the other had a broken center key pin and both appear slightly baked.
I could not find a schematic or anything specific bias instructions for the Soloist. There are plainly 2 test points, TP1 and TP7, about 1/2" apart. It is written between them "58mV". The lowest I can get is 142mV with the bias pot all the way left.
The amp sounds good but with the old ELs so tortured this makes me suspicious. Can anybody offer some experiance or thoughts on the Soloist bias points?

Thanks,

Mike

[alexradium]

https://music-electronics-forum.com/...0&d=1524678556

[Enzo]

Hi Mike

TP1,7 are across a 1 ohm resistor (and D27). On drawing up by the B+ supply.

If the power tubes were fried, do check that resistor for 1 ohm. Could be damaged.

What kind of bias voltage is available at pin 5 of the power tubes? And is it clean?

[mikeskory]

Ok I found the corresponding diode and 1 ohm resistor. They are D36 and R217 on the SOLOIST chassis. They measure 1 ohm and are connected at both ends like your D27 & 207 from the STage schematic. The voltage at the out put tubes is -34 at each pin 5.

[Enzo]

34v of bias sounds low to my mind, perhaps someone knows for sure. I would expect maybe 10 more volts, like -42 or -45. And is that voltage clean DC? Just thinking if the bias lost a filter cap, it would get ripply, and thus measure low.

I guess my next step would be to see the voltage range of the adjustment, and I'd look at the raw voltage the bias starts with to see if it is high enough to adjust from. Basically find why the voltage cannot be raised from -34.

[mikeskory]

Ok...adjusting throughnthe BIAS control I get 33.9 at the left end and 26.1 at the right end. The Balance control allowed me to have the same voltages at both pin 5's. As for the voltage on the scope...the scale illumination does not work. But I see a dirty pattern of about 20mV pp. AT pin 4 I see a nicer sign wave of about .5 pp. The pin 5 voltage comes through the BIAS control circuit. There is a TP13 labeled VBias. The voltage there is only -35.5V. I'll follow it into the power supply now.

[mikeskory]

Now we got something to think about. There appears to be an aftermarket bias control added down near the power supply area. It's a simple pot glued to the circuit board.

It goes from GROUND to TP13. If I turn it all the way left the pin 5 voltage goes to -39v (from -34). Also the bias mV go to 100mv instead of 150. Should I take it out of circuit and check the voltages again?

[Enzo]

That sounds like a good idea.

A few millivolts of ripple on the bias supply doesn't worry me.

[mikeskory]

I think we got it. I un-soldered the add-on bias control and rechecked. Pin 5s are at -43v. The bias points are .58mV. Tested the amp again with a guitar. It sounds good.

[Enzo]

So basically the amp was working, we just had to get rid of someone's mod work?

[The Dude]

...... and no wonder the tubes fried!

[mikeskory]

Yes. Thats was it. The install was neat. The amp would have had quite a growl when cranked up but expensive to be buying tubes all the time. Thanks Enzo and also Alexradium for the schemo.

[francuccino]

I have a HT60 Soloist that was BIAS drifting on V4 to point of red-plating. Turns out one of the 1u/450vdc caps used as couplers was leaking. I replaced both with .68u/630V poly caps and also replaced the 175K bias resistors with 220K.

[francuccino]

Made some progress with a schematic of the Bias circuit

[Jon Snell]

Made some progress with a schematic of the Bias circuit

This will help;
I have it in my archives.