Blackstar HT5R Bias Test Points

HHwarner

Has anyone had any experience biasing BS HT5R head. My brother wants me to replace the power valve. Found PR1 bias trim and PR2 balance. Anyone know where test points are and mv settings. Finding conflicting info on internet. Cheers Scott

ecc83

D20 mate. With the amp DEAD AND COLD AND UNCONNECTED. clip a DMM across D20 and put the meter on Ohms. Should read close to 5 Ohms (4r7 sense R) .

Switch to DC volts and power up. The voltage should be set to 46mA. The DC balance should also be set but you won't have the kit (such as my Radford noisemeter!) so put Mk1 lug next to a speaker and adjust the trim pot for minimum hum. NB a scope CAN be used but does not correlate well with the audible minimum.

If a very early Mk1 check for a cable tie around the 12BH7 base wires, if present snip it off. You can fit a new looser one if you like.

Any other problems PM me.

Dave.

HHwarner

Thats Great Thanks Dave. Iv just managed to find schematic. I see Pin 3 & 8 going to the 4r7 Resistor so guessing 46mA this is total reading for both sides? I suppose if I measure the Plate voltage I could tweak that to optimum. 

On another note, on my trawl through the forums I have discovered the blowing R38 R39 problems. Iv noticed that the base of R39, at the soulder joint, its black. I think this amp dates to sometime in 2011 so may have an issue there, saying that, no problems so far appart from the Emulated out. The 4x12 Cab emulation when switched, initially sound was bigger but then volume dropped off and now makes no difference if pressed in or not. Tried cleaning the switch but made no difference. 

ecc83

Without finding a map I assume 38/39 are the 22k MOSFET loads?

If so I strongly recommend you ship the whole thing back to B's for a repair. Pretty sure the cost will be very  reasonable and they will probably know what the EM out issue is.

On the bias matter. I would not faff about "optimizing". IMHO it could only increase the clean headroom by a gnat's knacker of a dB. Note of caution to everyone? There is well over 400 V in these wee amps. You would have to be a bit stupid/careless to touch it but it would not half bite!

Dave.

HHwarner

Cheers Dave, Help much appreciated. Ill just set it at 46mA and leave it at that then ;-). Yes I think R38 R39 are the 22k Mosfet loads. 

HHwarner

When listening for hum is that on clean channel with volume up, guitar in or out

DartmoorHedgehog

HHwarner said:

When listening for hum is that on clean channel with volume up, guitar in or out

@ecc83 will know for sure, but from doing my HT20 a couple of years ago I'd say clean channel, normal volume.  And some (most?) Blackstars need a jack in the input to turn the HT circuit on.

HHwarner

I can't hear a change in him when turning the balance pot

ecc83

HHwarner said:

I can't hear a change in him when turning the balance pot

If you have nothing in the input (five's don't have the  bias gate) and all the pots at fully anti clock, the hum level WILL be very low. The HT-5 is probably THE quietest guitar amp on the planet! (I regularly got -80dB s/n below 5 watts) so it could be hard to hear..Or you are going deaf! Heh!

If there is no audible remnant hum the OP stage must be well in balance.

Dave.

HHwarner

Thanks Dave. I just lifted the board to see what was going on under R38. It's been repaired in the past. Holes/slots cut in the board between the legs of tr2 and tr3 and the tracks from the middle legs to R38 and R39 bridged. This amp was sold as ex demo but my guess is returned/repaired. Can't imagine BS doing a repair like that. Easier to replace board I would imagine. Suppose use it as is and keep an eye on it

Take it this isnt BS handy work

ecc83

No, I doubt that was done at the Northampton centre!

And yes, does not look pretty but if it works, it works!

Dave.

HHwarner

Well it seems to work well, Ill tell him to use it as is. If it hasnt cured it (i imagine it has) I guess we will find out soon enough. Been scouring the forums, this is one way to cure the shorts across board between tr2 and tr3. Not that i would like to attempt a fix like that ha ha. Id snap the board knowing my luck

HHwarner

Iv got some odds and ends in my toolbox, if I make a 1 ohm shunt for each cathode in guessing I could set the balance that way?

ecc83

HHwarner said:

Iv got some odds and ends in my toolbox, if I make a 1 ohm shunt for each cathode in guessing I could set the balance that way?

Wha?? If there is no audible hum from the OP stage and the amp seems to work well there is no need to worry about OP stage balance!

It should be understood that the vast majority of guitar amps do not bother about OPS balance (and it is a bit pointless anyway since most of them copy the PI circuit error and the drive is well FUp anyway!)

B's did not do it for the Artisan range but began to do so with the Five. I assume this is because they thought it would maximized the clean output of what is quite a low power valve in the 12BH7? The ACTUAL device used to set balance in the factory is a bespoke AC millivoltmeter reading the hum. I used a very good quality mVm  with a 1mV fsd  but such kit is not usually available to the average gitamp tech, hence the Mk1 lug method. (ICBM stand by for an "ee"!)

It is a nice check to make, makes an already V quiet amp quieter, but in truth is probably lily gilding? Especially in the bigger amps. Do not sweat it!

Dave.

HHwarner

Cool, I'm miss understanding of the whole balance thing. Sorry to sound dim ha ha. I was thinking that although total current = 46mA the balance trim was used to level this out evenly between both sides. Obviously not lol

ecc83

HHwarner said:

Cool, I'm miss understanding of the whole balance thing. Sorry to sound dim ha ha. I was thinking that although total current = 46mA the balance trim was used to level this out evenly between both sides. Obviously not lol

Yes! You are right! It DOES balance the two anode currents. You are also right that this is usually done by a sense R in each cathode but that would have added a bit of cost and complication and it is Just  as easy (if you have the kit!) to set the stage for minimum hum (it is because the then designer was ANAL about noise levels that the amp was push pull! How much simpler to just go for the Bog S EL84 solution? )

Dave.