Biasing a Blackstar HT Venue amp

[nocaster]

Following on from my Fender Blues Deluxe/Hot Rod thread here I thought I'd do a wee post on the HT Venue. Again.........

You can die when you play with valve amps. Even if they're switched off. If you aren't confident, stop reading. Don't come crying to me when you're dead.

I only have the HT100 so I can't comment on the other amps, but I believe it's the same across the board.

So, start by taking the screws of the back and bottom. Note that there's 6 on teh bottom unlike other amps that only have 4. Yet another example of Blackstars "belt and braces" approach.




Anyway, once you're done, lift out the chassis by reaching in and holding the 2 transformers on either side. Lay it down BUT DO NOT lean it on the valves. I prop it on a bench edge so the tubes are hanging free. Up to you how you do it - but watch the valves.

Then you need to ensure there's a resistive load on the amp. In other words, plug in your speaker cab.



The way Blackstar have designed the amp means that you have to have a cable plugged in to bias/operate the amp. I use a spare George L patch plug. Nice and neat and doesn't leave a cable on teh floor.



TBC

[nocaster]

So - after you've done that.....power up to stand by and let it sit for a minute.

Now, there's 2 ways to bias the HT and both measure mV between the smae 2 points. Basically you need to know the mV reading between R207 and D27....that then gives you your bias value.

To do this you can literally measure the mV with your multimeter between these components and tweak with the wee white pot labeled PR2. These points are :



And more specifically :



and.....




TBC

[nocaster]

Now, you could just do that and set your amp at 100mV.....then you're done. However, Blackstar have built a wee socket for measuring the biass. This is a LOT safer and a lot quicker. The socket is located on the PCB with the power valves.



And more specifically :



You need a wee plug to go on here. That looks like this :





TBC

[nocaster]

Now take the 2 signals from that wee socket and feed them into your multimeter as per above and bias to 100mV using PR2.



Lovely jubbley.

All done.

Power down and put the amp back together.

K

[jdbwales]

Hi Kev, thanks for this mate, most helpful - however, a few questions - what specifically is the plug you're putting onto the bias prongs and would two crocodile clips into a multimeter do the same thing? Also, is 100mv the optimum bias for these things, ie if I was to change the power tubes would a bias too 100mv sort it?
Cheers!

[longtallronnie]

Nice work Kev.

Now let's see you do one for an AC30!

[nocaster]

Rhys - I'm not sure of the actual plug name. I wouldn't advise using croc clips on the prongs as they're quite close together so theres a chance of shorting. You could bung the croc clips across the diode and resistor though. That would work great.

Ron - An AC-what? Never heard of that.

K

[CubanB]

Thanks for that.

Couple quick questions..

Would you be able to post a pic of the transformers circled so we know what to avoid, so we don't die. (Don't know much about amp electronics) Are they the boxy looking things with like a plastic cube with a metal frame around it? I think they could be transformers. But can't see any of those things in the pics.

You plugged a patch cable into the speaker output as the "resistive load?" Rather then the speaker itself?

What do you set the multimeter to? I can see your pic but a different brand might have different settings. Is it 200m DC? I don't know much about electronics but willing to learn especially if it saves a trip to the amp repairman every 6 or 12 months.

Can you use the little probe things on a multimeter on that socket part, or only on the circuit board? Maybe this is the same question as jdbwales.. oops.

Sorry for all the questions.

edit - Just reread it all. You plugged the speaker in, and the patch cable into the input in the front. Understand that part now.

[nocaster]

The only bit of that post that hasn't been answered is the bit about identifying transformers......and with all due respect if you don't know what amp transformers look like then you should not be going anywhere near the insides of an amp.

K

[longtallronnie]

Ron - An AC-what? Never heard of that.

K

You should check them out, a real man's amp! None of that poncy channel changing midi bollocks!

And I was being facetious, AC30's are cathode biased.