Randall 6/13/2018 8:17 PM
Damn you Blackstar!
Discovered the long and hard way that with the Blackstar HT Stage 60/100, you can't get a bias measurement unless there is a plug in the input jack. Don't ask me how I know, but after 90 mins of questioning my own sanity I discovered it. That and the standby switch does not turn off the HT, so I wonder if they are the same circuit, stopping the tubes from conducting by sending a large neg voltage to the grids? I don't know how or why, but it's there.

If I didn't already have so much time in a 1 hr job, and it wasn't already buttoned up, I'd take some measurements and find out. I just hope this prevents others from wasting as much time as I did.
 
Enzo 6/13/2018 8:29 PM
Well there is this:
 
Randall 6/13/2018 9:58 PM
Now you tell me!
 
Jazz P Bass 6/13/2018 10:41 PM
And be aware of that screwy mosfet phase inverter circuit.

The tube is simply used as a voltage doubler.

The mosfet pair is the actual LTP phase inverter.

The opamp drives one side and the feedback from the output drives the other side.

This circuit is real picky about a good set of matched tubes.
 
Randall 6/13/2018 11:52 PM
I didn't even look at that. It came in for a simple install customer supplied tubes and bias because it "didn't sound right". The old tubes were running at 22mA and 48mA, so I bet it didn't. Sent it out balanced to within 1 mA, didn't do the symmetrical clipping thing. Hope I never see it again. Plus all those connectors.
 
Zozobra 6/14/2018 4:58 AM
While I only poke in amps for my own amusement, I've had the misfortune of having a few budget blackstar amps land on my desk and every one of them has been total trash. Most memorable was the model below the HT series which had the smallest OT I've seen in a 50W amp. It also has a very odd modular chassis that made replacing the preamp tubes unnecessarily difficult as only the top 1/3rd or so pokes out. Can't imagine many of them lasting more than 10 years. Say what you will about Behringer but I'd take a bugera over one of these any day of the week.
 
drewl 6/14/2018 6:02 AM
That got me on the first one I did.
Not for 90 minutes , but for a few.
 
glebert 6/14/2018 7:52 AM
If it makes you feel any better, I spent 30 minutes measuring voltages yesterday, and then 45 minutes thinking the power supply was bad, only to find that the chassis paint was interfering with my meter ground.
 
J M Fahey 6/14/2018 8:15 AM
Quote Originally Posted by Randall View Post
Now you tell me!
Only now you asked !!!

Hey, Blackstar is a ***modern*** design , not a re re re re re re re refried 60 y.o. Fender design (which in due time was pulled from an 80 y.o. RCA Tube Manual)

Now that you mention it I like the idea of "automatic Standby" when nothing is plugged in, presumably by applying full raw negative bias voltage to Power Tubes, turning them OFF for good (and keeping amp cooler if unused).

I bet itīs mentioned in the Manual.

Now WHO reads them?
 
Jazz P Bass 6/14/2018 8:47 AM
The same idea is used in the speaker jack.

No speaker/ bias cold.
 
nickb 6/14/2018 10:54 AM
If it's any consolation I heard that Blackstar is heading for trouble. At the end of 2017 their debts exceeded assets by about $200K. A bit like the US I guess
 
Randall 6/14/2018 11:29 AM
OK, I confirmed it. Standby, input jack and speaker jack all switch -100v to the grids, shutting down power tube conduction. Now we know.