Blackstar HT-5 barely any output

[Hallingamp]

Hi everyone and Merry Christmas!

My son's amp suddenly reduced it's output volume to a whisper. I have been messing with it for a couple of hours and I just can't understand why it did. Here are my findings so far:

Preamp works fine through headephones on the speaker emulated output.
12BH7 voltages: Pins 1 and 6: 445V, Pins 2 and 7: -62V, Pins 3 and 8 0V. Pin 4: 0V, Pin 5: 12V and Pin 9: 6V. (Tube in socket)
OT primary measures ca 500k to each side from center tap (No tube in socket)
OT scondary measures 1 ohm on the 8 ohm jack, and 1,4 ohms on the 16 ohm tap.
Swapping the 12BH7 with a brand new 12AU7 made no difference.

With my limited knowledge I think this looks about right.

So any suggestions?

Thanks
Jan

[g1]

Those pin 2 and pin 3 voltages don't sound right. There should be some cathode voltage at pin3, and the negative 62 volts is way too much.
Schematic here may be close enough to help:
http://music-electronics-forum.com/t16338/

[Hallingamp]

Thank you for the reply.

So this is a combination of cathode- and adjustable bias?
Then I should measure some resistance between cathode and ground.
Or what does D20 contribute in that respect?

Jan

[g1]

The cathode resistor is there to provide a measurement point for the bias, the diode protects the resistor if the tube shorts. Although there is an adjustment pot, the circuit is technically referred to as "fixed" type bias.
If that schematic is correct, there is no place for -62V to come from. The bias is derived from the -15V supply. Check fuses shown as F3 and F4 in the drawing. Check that you have + and - 15V supplies.
It is possible that the schematic is wrong, D9 makes no sense.

[g1]

Here are some proper schematics, the output section is not shown but the bias supply is on page 2. It is different from the other drawing but there is still no way you should have -62V.
Check voltages at R4,5, and 6.

Edit: From what I could dig up elsewhere, voltage at pins 2&7 should be in the -20 to -25V range, at pins 3&8 somewhere around .075V (75mV).

[Jazz P Bass]

Blackstar HT5 Schematic

Here is the schematic.

Also, I have included a link to a drawing that fellow member 'Teemuk' so painstakingly drew.

http://music-electronics-forum.com/t16338/

[Hallingamp]

Thank you guys. I will study the schems and poke around some more.

Update on the voltages:

-62 is when the amp is in standby. I wasn't aware of that when I first posted. Also when in standby, the plate volts on the 12BH7 is a whopping 470V! When I flip the switch the plate volts sink to 446V and pins 2 and 7 shows -24V. 55mV on the cathodes. (Having the voltmeter in the 1000V range gave me 0V to begin with.)

So now everything looks right... right?

But still veeery low output.

[teemuk]

Also, I have included a link to a drawing that fellow member 'Teemuk' so painstakingly drew.

Thanks, but there are quite a lot of errors in those, and they are partially incomplete too. Since then Blackstar has released the official schematics.