Blackstar ht40 No Sound

[Perkinsman]

Thanks guys, good to know. I was afraid that the amp still wasn't right. All my amps are just one volume control Fenders...so would it be accurate to
say that the preamp tube volumes are controlled by the Volume control & the power tubes by the Master control in this amp.

[Jazz P Bass]

"so would it be accurate to say that the preamp tube volumes are controlled by the Volume control & the power tubes by the Master control in this amp/"

Yes. That is correct.
In a very simplified manner.

For the nuts & bolts of it look at the schematic.

[LarsonLE]

Hey guys, I hate to revive a 3 year old thread, however my friend just gave me his Blackstar HT Club 40 amp to look at. I have built a few tube amps from kits but I definitely wouldn't consier myself a technician. Anyhow, the first thing I thought the issue was were the output tubes because the volume on both channels are dead quiet. You can just barely hear the guitar with volume and master on 10/10. I checked all of the tubes and they were still good. Nothing wrong there. I am now taking some readings with my multimeter and I was taking the readings at the MOSFET. The gate was at 0V. I noticed that the drain on the MOSFET was reading 258 volts, and the source was reading -3V. The drain and source readings are way off. Is this my culprit? I was reading this thread all of the way through and I saw that Mick Bailey had said that this issue could happen with the phase inverter MOSFET and it would sound like as if the output tubes were dead and I'm almost positive that this is my issue at hand. Can I just replace these MOSFET parts and go on my way or is the amp fried?

Also, I was just looking at my amp compared to someone elses, can you guys tell me if this looks right? This is the amp I am working on : https://i.imgur.com/ab2c2H7.jpg

Do you see the red arrow pointing to FET19? Well mine doesn't look at all like this picture I just saw: https://music-electronics-forum.com/...1&d=1405538907

One last thing while I am thinking of it. What should pin 4 of the EL34 measure? I think I remember it being higher than normal.

[Mick Bailey]

The difference in FET19 is down to package type - they're the same transistor but the SMD one is on a later revision board.

Voltage leakage affecting the PI MOSFETS usually significantly raises the gate voltages at idle, but that's only on the amps I've worked on. There may be others where the gates are unaffected. What's the voltage on both MOSFETS? If the board is leaking (a really common fault with these amps) you can't just replace the MOSFETS - you have to modify the board beforehand.

EL34 pin 4 measures around 380v or slightly higher - depends on how the bias is set (more idle current lowers B+ and hence the screen voltage) and your mains voltage. What is your higher than normal measurement?