Blackstar HT soloist 60 out of bias range

[Mick Bailey]

Just installed new tubes and can't get better than 44mv total with the bias control at maximum. Unfortunately (for me) all my tubes just in are from the same batch and similar current draw.

Does anyone know off-hand what needs to be done to reduce the bias voltage? No schematics available from Blackstar. I was hopefully looking for an easy fix without having to pull the board and trace the unconventional circuit, which switches the bias to cut off the tubes when the input plug is removed.

[audiopete]

Just installed new tubes and can't get better than 44mv total with the bias control at maximum. Unfortunately (for me) all my tubes just in are from the same batch and similar current draw.

Does anyone know off-hand what needs to be done to reduce the bias voltage? No schematics available from Blackstar. I was hopefully looking for an easy fix without having to pull the board and trace the unconventional circuit, which switches the bias to cut off the tubes when the input plug is removed.

I think you will have to trace the resistor in series with the trimpot to ground after the bias diode, and reduce it to reduce bias voltage, but might be easier said than done according to your description. You have to pull the board anyway to change the resistor. Good luck.

So lame that Blackstar won't give out schematics when almost every major amp maker has their schematics available, usually on their website. Hell VHT prints the schematic right in the user guide that comes with the amp. Blackstar is going on the Line 6 list of disposable junk, I think. I have no time for companies that won't do everything they can to accommodate their customers. Who knows where the BS "authorized" service centre is near me. There isn't much near me, which is why I do what I do here.

[Mick Bailey]

I'm going to leave it until the morning and take a fresh look at it. I'm having a run with Blackstar amps at the moment and similar to your own view I class the construction as 'consumer-disposable', except for the much rarer hand-wired stuff. It's a shame, because apart from the total lack of technical documentation the company is pretty good. Spares are fairly priced and the guys there are cheerful to deal with.

[audiopete]

Good plan. I think its the wrong attitude in this day and age of DIY and internet and more independent modders than ever. Problem is, not everyone can work on that smt stuff. The VHT amp I'm working on was designed to be modded, although the dual circuit board sandwich and clamshell chassis make it a pain. In its defense, there are a lot of features packed into a small space. The tagboards are old Fender style but modern phenolic plastic and all the components are axial and right there. They even put the schematic in the user manual. I think they are on to something encouraging DIY or modding. Look how popular Blues JR modding is - although by necessity.

Here's a VHT gutshot I snagged off the net:

[TimmyP1955]

If the resistor needs to be a lower value, there's no reason to remove it - just tag another onto it and off you go.

[Mick Bailey]

That's what I was planning to do, and put a note inside the chassis. That way if someone in the future wants to to reverse the change it's easy to get back to stock. Looks like the board will need to come out anyhow to trace the circuit.

[audiopete]

If the resistor needs to be a lower value, there's no reason to remove it - just tag another onto it and off you go.

Yes you can do that, but personally I would remove the board and put in the proper value resistor. I don't like "tacking things in". I spend a fair amount of time undoing work like that.

[Mick Bailey]

I used to work as a field service engineer for broad range of industrial electronics companies. We'd get revision kits in for equipment with precise instructions how to carry out the work. The rule in every case was to carry out the minimum amount of invasion in order to effect the changes. So, if a component needed changing it sometimes meant using the hook technique to secure a parallel component - sometimes a cap across a resistor, parallel resistor, flying lead or whatever. Or clipping a component and soldering on to the remaining leads (as recommended by Ampex in their service manual). If there was a way to modify a board without having to remove it, that's how the manufacturer would spec up the rework. Sometimes with wire-wrapped boards you could save days of heartache by leaving the board in place and not having to re-wrap and troubleshoot dozens of joints, just to change a resistor.

There's a difference between a bodge/kludge, and a correctly executed modification. I see a lot of pretty crap work, but also a lot of modifications done well - particularly studio equipment that's been back to the manufacturer for interim board revisions.

With the Blackstar, a resistor change to bring the bias into line would be a temporary fix - possibly even just for the life of the tube set. Those tiny pads can lift easily and there's only so many heat-reheat cycles they'll take before they break. Making reversal easier for the next guy and reducing the risk of him/her having to repair a DS board, or re-stressing those pots when re-tightening and having to replace those as well would be my intention. More usually for a permanent change - re-cap, repair or whatever - I do want to desolder the component and replace it. That way I can also see if there's any damage to the underside of the board. But I always look at the risk of pulling the board and see what else could go wrong as a consequence.

As it turns out I tried a number of tube sets and finally got a set that will bias correctly. The board is currently pulled because of other problems with channel switching. While it's out I'll trace the bias circuit if I have time. From a superficial look it seems that paralleling a resistor would be a bad idea anyhow without properly understanding the circuit.

[audiopete]

That makes total sense. I never really thought about it like that I guess, but now that I have, I have seen a lot of clean factory mods done properly. I realize desoldering can be a damaging event, and of course not desoldering does less, so I understand your point. The Hakko does some quick clean work fortunately.....but I hear you. Thanks for your insight.

[g1]

Agree with Mick and to add a few points, this is more of an adjustment than a repair. Tacking the resistor in parallel with the existing one will hopefully show the next tech "hey look at me, I'm the bias adjust resistor" .
I would never tack a resistor across a burnt one and clip out the bad part, but I think this is a very legitimate occasion for a "custom" resistor. Lot's of stuff comes from the factory like this, sometimes it will be marked on the schematic as "factory selected part".

[Mick Bailey]

I suppose the sensible thing for me to do is not to repair Blackstar amps and redirect customers to their approved service centres - even for a re-tube, or minor/obvious component repair. I'm sure the pro or semi/pro players would appreciate being able to take an amp at zero notice and get it fixed. Weekends/evenings no problem drop it off at 8pm and pick it up 8am the next day. No need to travel - right on everyone's doorstep

I've been on the other side - running workshops providing warranty and out-of-warranty repairs. NDAs, contracts, fixed price payment for each repair. Usually fixed hours and closed on weekends and public holidays. I've never seen small-scale outfits as a threat to that core business, and the enhanced support provided by small repair shops helps support the brand. There are lots of benefits in having an approved repair network; usually good stocks of spares, familiarity with the equipment due to working on it regularly, direct access to factory technicians etc.

But, convenience isn't usually high on the list - especially for the guy with a broken amp today, with a gig tomorrow.

[Enzo]

I suppose the sensible thing for me to do is not to repair Blackstar amps and redirect customers to their approved service centres - even for a re-tube, or minor/obvious component repair.

Oh, why make it an us versus them thing? It is one thing to decline a repair because we lack the roadmap, but what is the point of turning away tube and obvious minor repair jobs? The customer needs to be inconvenienced further just so we can make some kind of statement?

I run a pro shop, and I appreciate that we all want everything we want. It irritates me when I call NAdy for some stupid wireless issue, and am told that they will not hand out drawings, and in fact they do not have authorized repair centers - all work is done at Nady. I love Peavey because they bend over backwards to help everyone. But I understand how companies get to such policies. it sounds great to have service shops scattered hither and yon, but that also costs them. Someone has to administer service center systems, accounts have to be managed, invoicing has to be done. Service reps have to be provided. Plus we cannot assume technicians are generic. Who among us has not followed some hack into an amp, "Who the hell did THIS?" If someone takes their amp to some AmpGuru and he destroys their XYZ amp, the customer is not going to think, gee that technician was an exception. The customer is going to think, "Wow, XYZ amps suck." By controlling who does the service work, brands are trying to avoid this sort of thing.

[Justin Thomas]

Or, AmpGuru destroys XYZ amp, and then all AmpGurus are a-holes who don't know what they're doing.

Justin

[audiopete]

I too really appreciate companies like Peavey and Yamaha, to name a couple. It also helps considering the sheer volume of gear that they have out there. I take things case by case. Regardless of brand or lack of schematics, a lot of the time repairs are simple things like blown voltage regulators or PSU caps, shorted decoupling caps on opamps and other things that can be diagnosed without schems. The guitar amp stuff is generally even simpler. I must admit though, I do love a schematic to get the big picture on the entire circuit.

[Mick Bailey]

Oh, why make it an us versus them thing? It is one thing to decline a repair because we lack the roadmap, but what is the point of turning away tube and obvious minor repair jobs? The customer needs to be inconvenienced further just so we can make some kind of statement?

The us versus them is already defined - that's how the manufacturer has set out the business model. But sometimes the simple repair gets more more involved and then that's where the independent repairman gets hung out to dry. I have yet another Blackstar in for repair (three right now). Simple job - loose input socket and all the nuts had come well-loose on the pots. But when I came to test the amp there's a very slight distortion on the clean channel at low volume - like crossover distortion. Scoping the output shows a varying staircase waveform riding on the 1khz test signal that disappears with no signal present. So I'm thinking some digital noise leaking through or a digital/signal ground issue. Now that's going to take much longer to fix than if I'd got a schematic and at least could visualise the signal paths.

So, would it have been better to have turned the repair down and redirected the guy to a service centre, on the face of it just for some loose pots and a socket (however inconvenient)? In this case, for me, yes. I could probably get 3 or 4 Devilles turned around in the same time. But then again, the service centre would probably look at the repair/replace balance and possibly just replace the entire board because it's cheaper than spending time on a repair. I still do that calculation for brands that I'm approved for.