Blackstar HT5-R

[Mick Bailey]

Does anyone have the schematic for the Blackstar HT5-R? I have one for the non-R model but there are too many differences and the component designations are different.

[Drewline]

I think one of these may be what you are looking for:
HT-5 schematic earlier.pdf
HT-5 schematic later.pdf

[Mick Bailey]

Thanks for the schematics, but the R version has separate EQ for clean and drive channels, with a single tone control for clean. It also doesn't mute off the headphone socket - you have to put it in standby, which kills the signal to the PI and removes power to the output tube.

I did find the problem with the amp - eventually. So, scoping the output with a sine wave at the input showed a clean symmetrical sine wave on the output. Turn up the amp until it clipped, and the top side started to flatten and the bottom side had a slightly scooped point. The customer complained of low output and sure enough there wasn't much at all coming out of it. I measured the AC on each half or the 12BH7 and was getting 90v RMS on each plate (460v plate voltage). So, balanced input from the PI, I though, but not enough signal going into the PI.

Tracing back through the signal path didn't turn up anything, and there was good output on the headphone out. It looked like everything was working fine. So, I subbed in a spare transformer - no change.

I decided to do a quick ESR check on the caps and the first two I tried were the 1uF PI coupling caps. I got a high reading, then measured the voltage on the grids of the output tube and discovered that the A half was getting about 12V max, and the B half zero - no output whatsoever from one half of the PI.

So how does that work? How do you get a symmetrical 90v on each triode half, when one grid is disconnected? For some reason it behaves like a self-split output.
Anyhow, subbing in a couple of film caps fixed it while I'm waiting for the right caps to arrive.

The PI stage is identical in layout to the earlier amps, with just some minor resistor value changes. The designators are also the same, though only for the PI.

Still would like to get the right schematic - these amps aren't easy to read.