Heating Up A Blackstar

One of the kids at the shop has a Blackstar HT40C which had a persistent "no volume" problem. and because I'm a nice guy and they do provide a lot of work for me I agreed to tackle it.

It seemed that the power tube filaments were intermittently on the fritz, with the result that no joy was to be had. This was the second go around because the last time I'd thought "What's the issue? It works, dunnit?"

Well, it didn't.

Once the problem was localized to the power tube filament circuit, I measured the raw output from SP14 and SP15 and found that unloaded they were quite different-54 and 64 volts respectively.

Pulling the main board for inspection I desoldered pins 2 and 7, redid them with silver solder  and tightened the connectors. On reassembly with a new set of JJ E34Ls I found that the filament voltage was low-about 5.5 and 6 volts. Scrounging through my stash I found some Valve Art EL34s that yielded 6.0 volts on pins 2 so that was good.

The schematic shows the return path as going straight to ground but as I've illustrated it doesn't. It goes through a 100 ohm resistor and a diode in parallel, and then the entire ground goes through a pair of diodes and another 100 ohm resistor and capacitor in parallel before it reaches home base.through SP3. This goes a long way toward explaining why measuring pin 7 to the chassis is not completely grounded. Also, the ground portion of the bias system which contains a diode and a 1 ohm sampling resistor dumps into the above chassis potential "ground" labyrinth described here.

Because of this quirk I will be adjusting these amps to 50mv across the sampling resistor as is noted on the circuit board and the Blackstar HT Club 40 test specifications rather than using a bias probe as has been my habit.

I think the problem is pretty much resolved with the general cleanup

UPDATE 1: I revised the shop sketch here to show the complete return to ground path which includes the paired diodes D42 and D34, the resistor R223 ,and the capacitor C116.

I've no idea what they're supposed to do but they go a long way toward explaining why ground on the power tube pins 7 isn't really ground although the schematic from the factory says it is. Go figger.