Blackstar HT60 bias circuit

[Roberto Lasco]

Hi guys,

Got my first one on the bench with a shot (chinese) EL34. Replaced with a pair of JJs; plate current measurement 0mA...Vg1 is -95V (Vp 510V) and bias pot does zilch...
Funny stuff is that amp does not sound overbiased, nor shows some horrible crossover distorton on the scope as expected; nevertheless I can't think this is normal behaviour (Class B amp my ass); besides bias pot don't change any bias voltage.

I can't find any schematic on the net; tried to contact Blackstar, but they BS me that documentation is just supplied to factory authorized techs (i think independent techs makes them a favor trying to keep those crap amps running). Anyway, couldn't figure bias circuit either (double layer/SMD populated board); no obvious tap/reverse diode/cap/voltage divider schem...
I find that -95V voltage in a dozen places around bias pot (middle of the board). On the left part of board, near HV rectifiers, there's what I'm supposing an SMD bridge rectifier (actually bigger than usual SMD, like a 4558 in size), silked BR2 on board, 4 terminals. Two legs shows -42.5V, one leg -95V, one leg 0V (but not grounded, 98ohm from ground). It didn't test like a bridge rect either: diode test shows short between the "-42.5V" legs, and no continuity (or voltage drop) with any other combination. Rang any bell ?

As I said, amp is running and sounds ok, but I'd like to figure out what's really going on...

thanks for any insights,
roberto

[Prairie Dawg]

The first thing that occurs to me is that if it sounds OK it's gotta have plate current. Thanks for the tip about two sided boards with SMDs. Thanks but no thanks.

[teemuk]

If it's anything like the other HT-series amps then there should be one main bias voltage set trimmer in the bias supply circuit and two bias "balance" trimmers near the PI MOSFETs. The cathode circuit of the output tubes may have a diode that limits the maximum cathode voltage to about 600 mV.

[teemuk]

Sorry, meant one bias voltage trimmer and one bias balance trimmer. Both usually located close to each other and the PI MOSFETs.

[Roberto Lasco]

PD (Robert ?), unless B+ is not directed hooked to OT CT (and tranny shunt method don't work for some reason), current is near zero. Not to be surprised, with almost hundred volts on the control grid...
Teemuk, you're right, two trimmers (one balance), both do nothing...It sure seems there's a diode on the cathode, there's a voltage drop of .1V (amp turned off) between pin 8 and gnd.
What a piece of crap...On the other way, a (chinese) Jet City 20 has a pcboard same grade as (US) Soldanos, big trannies and sounds like a low volume plexi...got one myself.

[teemuk]

I may be totally off base here but isn't a bridge rectifer hooked to a "floating" single-ended winding actually supposed to measure about half of the "output" voltage at each AC leg in the first place?

The diode test...? I suppose you didn't do it off-circuit. There could be several reasons why the test doesn't show right results while the bridge rectifier is in circuit.

[Roberto Lasco]

Actually there are two bridge rects I can see on the board. One near the stdby switch, which I suppose control some low voltage supply (there's 2 4700uF/35V caps near it).
The other, the "SMD" bridge rect is hooked to a 70VAC voltage, and sure seems related to bias voltage. Odd thing is, after this bias voltage is generated (-95V), I can measure it in many places (resistors) till the bias pot, without a single drop on it...the trimmer seems to vary resistance ok also, but no change in voltage either. And why in the first place the negative voltage is generated in that unusable range ?

Owner will need to use the amp today, so I'm assembling to return it, but I'm sure would like to take a look on a schematic some day...

thanks for the replies,
roberto

[teemuk]

Well, based on your description, one thing I would try to verify is the continuity of the common reference points of the bias circuit. For example, if the bias trimmer's connection to common becomes open circuit then that trimmer is no longer an effective voltage divider. Same thing with every other voltage dropping resistors that you could assume a bias circuit that brings the voltage down to a proper level from a somewhat high-ish inital voltage would have.

The SMT stuff on dual layers can be a bitch if you want to trace and sketch out the circuit but I trust there's no surprising proprietary stuff there at all. Aside the "balance" pot and the diode at cathodes in parallel with low-ohm resistor it should be a fairly basic bias circuit.

[Roberto Lasco]

I don't have it in my possession anymore. Good point on a faulty ground.
Anyway - unless this thing is Class B - never heard an amp that overbiased that sounded ok/good as this...really strange.

[pecorporation]

I don't have it in my possession anymore. Good point on a faulty ground.
Anyway - unless this thing is Class B - never heard an amp that overbiased that sounded ok/good as this...really strange.

Isn't that the truth. Schematics say set bias by achieving 50mV across D27, which makes crossover notch on the output wave look awful. I set the bias to achieve zero crossover notch (which put the reading at D27 close to 175mV!), had a good looking wave but within minutes of guitar playing test, the output tubes got that rumbling noise effect.

After killing a second set of EL34, I left the bias set to read 50mV across D27 and left it. Much to my surprise, it didn't sound crappy. It sounded good.

[Euthymia]

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.

[lalunatech]

-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.

I have just been through the same head scratching process. You are right, the control grids on the output tubes are dropped to -96 volts on the HT60 I have when there is no guitar jack in the input. I rang my customer back and told him the faulty EL34 (heavy grid leakage) had damaged the bias circuit and I would need to charge more to fix that too. Now I realise it was not faulty at all. Turning off the standby switch does the same so BE CAREFUL, THERE IS STILL 540 VOLTS ON THE PLATES WHEN STANDBY IS OFF. They carry no current but still have HT on them.

[darrellk1960]

How to Bias Blackstar HT Stage60

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.

[darrellk1960]

If I'm wrong, or left out something that anyone can share regarding biasing a Blackstar HT Stage 60. please comment. Thanks!

[Mick Bailey]

The Blackstar setting of 50mV is on the cool side, but I tend to leave it at that and have had no problems with any noticeable crossover distortion during listening tests. But these are customers amps and where possible I stick to manufacturer's specs unless there's a really good reason not to.

As pecorporation points out - they actually sound pretty good like this.