Blackstar HT60 bias circuit

[drewl]

Pretty sure I posted the same questions once and someone told there needs to be something plugged into the input jack to fire up the output circuit.

[Mick Bailey]

Yes, you have to be plugged in - the input jack mutes the output section. The measurements referred to are with the amp operational. With the amp muted it looks like it's faulty.

[tomobedlam]

I bought an HT60 and needed to get info on biasing this amp after I replace the junk Groove Tubes that were in the amp. I've never owned a tube amp, so obviously I know nothing about how to re=bias an amp, but am electronically and mechanically inclined, so I wanted to know how to do this. Blackstar is no help. They didn't even bother to inform the consumer, and obviously their dealers that they moved production out of Korea to China. And now Korg is their distriibutor. Anyway, sorry about the rant, but this is what I have found on YouTube. To re-bias. Warm up the tubes. You must have a cable plugged into the input. power on and standby off The guy had a pedal plugged in. Ground the negative probe to the chassis. Easier done with a gator clip, of course. Set volt meter to milivolts. The positive probe to D27 on the circuit board. PR2 is the bias pot. PR1 is the balance pot. The positive probe touching the left side of the D27 (same side where it says D27. Supposedly the reading should be between 50 and 120mv. They say Blackstar recommends the bias setting at 50mv. Bear in mind that 72 would be equivalent to 36mv. I have no idea what the optimum bias setting should be, since some tubes are hotter than others, and vice versa, but from what I've read on the forums, somewhere betwwen 25 and 30 should be spot on. The plate voltage is at Pin 3.

Arch. I must be blind. I see d26, d28, d-everything-but-27

[Jazz P Bass]

Here is a board pick from the Blackstar Forum.



Take note of the crappy solder joint on R207.

[Chuck H]

Here is a board pick from the Blackstar Forum.

[ATTACH=CONFIG]32988[/ATTACH]

Take note of the crappy solder joint on R207.

I count FIVE crappy solder joints in that pic! Three look grainy, two have gapping. There may be one other bad-ish joint but the pic isn't clear enough to tell. Blackstar is probably using lead free solder for compliance. I don't envy them for that but 5/16 marginal solder joints in that pic is pretty poor.

[tomobedlam]

Here is a board pick from the Blackstar Forum.

[ATTACH=CONFIG]32988[/ATTACH]

Take note of the crappy solder joint on R207.

Thanks, I'll take a look tomorrow.

[g1]

As far as the solder goes, those will be plated-through holes, and the solder will be done on the other side. So they won't care about how the solder looks on the component side.

[tomobedlam]

I bought an HT60 and needed to get info on biasing this amp after I replace the junk Groove Tubes that were in the amp. I've never owned a tube amp, so obviously I know nothing about how to re=bias an amp, but am electronically and mechanically inclined, so I wanted to know how to do this. Blackstar is no help. They didn't even bother to inform the consumer, and obviously their dealers that they moved production out of Korea to China. And now Korg is their distriibutor. Anyway, sorry about the rant, but this is what I have found on YouTube. To re-bias. Warm up the tubes. You must have a cable plugged into the input. power on and standby off The guy had a pedal plugged in. Ground the negative probe to the chassis. Easier done with a gator clip, of course. Set volt meter to milivolts. The positive probe to D27 on the circuit board. PR2 is the bias pot. PR1 is the balance pot. The positive probe touching the left side of the D27 (same side where it says D27. Supposedly the reading should be between 50 and 120mv. They say Blackstar recommends the bias setting at 50mv. Bear in mind that 72 would be equivalent to 36mv. I have no idea what the optimum bias setting should be, since some tubes are hotter than others, and vice versa, but from what I've read on the forums, somewhere betwwen 25 and 30 should be spot on. The plate voltage is at Pin 3.

Hmm..Mine is a Soloist 60.1x12 I'm guessing the circuit is different?

[The Dude]

Resurrecting this thread in the hope that someone may benefit from my experience.

I have a Blackstar HT Stage 100 on the bench and was running into the same headscratching thing about the bias, -95 on the grids and no current. After reading this thread I decided to plug my signal generator in.

Suddenly, I got current draw and the bias pots started working as advertised, with about -40V on the grids.

Apparently this thing has a switch in the input jack that puts it into some kind of standby mode when there is nothing plugged in! So unless you have a plug in the input jack, you'll get these weird readings.

Old thread, but I have to thank Euthymia for the tip. I was scratching my head trying to get the bias set on this thing. I would have never thought to plug something into the input to adjust bias.

[g1]

Similar scenario to the Marshal VS amps that need something in the input jack in order for the "power amp in" jack to function.
Can really mess you up and waste a lot of time if you don't have the "secret info".

[The Dude]

Yep, I chased what I thought was a bias problem around for a good long time (without a schematic) before I found this thread. I guess I should have searched "the database" sooner.

[Chuck H]

Yep, I chased what I thought was a bias problem around for a good long time...

And I never feel ok about charging for any "I'm a need some ejumacashun" time I don't officially do repairs. Because of that, when I do I always separate lernin' time from repair time. Not diagnostic time, but the time I spend developing the depth of knowledge I should have had before taking on the repair.

This situation is definitely gray area though. You would have needed a schematic or the time to realize you wanted to check the forum. Is a schematic available? Is the amp out of warranty? Variables that might make me charge a little time for circumstances out of my control.

[The Dude]

And I never feel ok about charging for any "I'm a need some ejumacashun" time I don't officially do repairs. Because of that, when I do I always separate lernin' time from repair time. Not diagnostic time, but the time I spend developing the depth of knowledge I should have had before taking on the repair.

Good, bad, or otherwise, I kind of charge by the "job" anyway. Of course time is factored into the equation, but I also look at the value of the piece, what is "typically" charged for a similar repair, etc., etc. I probably overthink it, but it can be a balancing act sometimes- making a living vs. ripping people off. Sometimes you bite the bullet on a repair, but you'll make it up next time when you know exactly where to look. On this particular repair, I'll come out ok because there was enough other work done.

[g1]

My favourite boss used to tell me to charge the amount of time it would take to do the job again, knowing what you now know.
The extra time for education (first time) is just something you just have to eat. OJT is not free.

[Chuck H]

My favourite boss used to tell me to charge the amount of time it would take to do the job again, knowing what you now know.

For a "boss" to take that position with employees is about as rare as hen's teeth. But it explains my personal philosophy pretty well. Someone's got to be your favorite boss though