Blackstar HT-5 technical help please!

Fortheloveofguitar

Hi guys My father in law is having trouble with the head version of this amp. He thinks he might have a tube gone??? When he turns onto clean channel from the overdrive, he cant get the volume up whatsoever and can hardly hear the guitar no matter what he fiddles with. Surely this can't be a tube as the OD channel is working fine and both channels share both valves ? Many thanks

richardhomer

@ICBM is you're man for this. IIRC there is a Tube Screamer-like circuit that supplies most of the drive - so it could be the pre-amp valve. On MR ICBM pretty much blew the whistle on Blackstar, from memory.

EDIT: Meant this in a good way about @ICBM - he told the truth about a circuit which Blackstar advertised 'optimistically'.

ICBM

That's a little unfair . I actually tried to point out that they were a clever hybrid design *as well* as pulling the company up for originally advertising them as 'all valve'.

@ecc83 is actually 'the man' on this - he worked at Blackstar in the service department. He'll hopefully pop in, but in the mean time...

It could be the preamp valve (12AX7). If the valve has lost gain, that will have much more of an effect on the clean volume than the overdrive volume. The reason is that although the extra gain in the overdrive channel is indeed supplied by a 'Tube Screamer-like' circuit, it does almost only provide extra *gain*. Almost all the *distortion* comes from the valve, which is thus fairly self-limiting - so if its gain goes down, it will reduce the amount of distortion rather than volume. But on the clean channel, the same valve is being run clean so it will show a much greater drop in volume.

I could be wrong, but I would try changing the 12AX7 before doing anything else.

It's actually a very sensible way to use a mixture of valve and solid-state if you're trying to maximise the effect of the valves in the circuit by using them where they make most difference, while keeping the number to the minimum - and I think it's a shame Blackstar have chosen to hide this. It's really very little different from running a Tube Screamer set for minimum gain and full volume in front of a valve amp, in fact - the distortion still comes from the amp, you're just pushing it harder.

Fortheloveofguitar

Thanks guys. Very much appreciated.

I will tell him to change that valve first and go from there.

Will let you know what happens

musteatbrain

Is it an old one? Does the emulated out still work ok when it is doing this I think there was a known problem associated with something going wrong on standby with some of the first ones. I can't remember what it was now though as it was a while ago. I ended up getting a replacement.

ecc83

Mornin' all!

ICBM is quite correct, the circuit uses ICs for their very low noise (and low cost!) qualities but most of the gain (and dirt!) in the HT-5 pre amp comes from the valve.

I only have the mkll circuit to hand (have to dig deep for the earlier one) but I am sure the routing is much the same and in fact the clean channel is picked off from the first valve section so if you have the right OD gain the valve MUST be ok!

My guess would be either a FET not switching or not being switched. Should not be hard to trace armed with map and scope. Such faults are extremely rare but have happened, not, IIRC to the 5 but other chassis.

Best of,

Dave.

ICBM

richardhomer said:

EDIT: Meant this in a good way about @ICBM - he told the truth about a circuit which Blackstar advertised 'optimistically'.

I wasn't the first to look into it by a long way - by then, a lot of people had jumped on them saying that the distortion channel was just a Tube Screamer and that the valve was more or less there for show, or at best just to drive the tone stage. I would admit that it does look like it from the schematic (or just a peek inside the amp really), you can clearly see the IC gain stage with 'clipping diodes' in the feedback loop, so it's easy to assume this is where the distortion is generated.

This isn't true either. The bulk of the gain occurs in the valve. The IC stage certainly does add a lot more on the distortion channel, but really it's just pushing the valve harder - the 'clipping' diodes do not in fact start to conduct until the gain is set at least 3/4 of the way up, and at that point the valve is already a long way into distortion and the effect of the diodes is totally masked by it - you can see this clearly on a scope if you disconnect the diodes, the wave shape does not change. I *may* have been the first outside Blackstar to scope the circuit and find out exactly what's going on, but I still wouldn't take much credit for that!

The purpose of the diodes is actually to limit the signal level and prevent the IC driving the valve so hard it goes into blocking distortion, which doesn't sound at all nice. This was very obvious with a guitar going into it and the diodes disconnected.

To me, the problem was Blackstar originally advertising the HT series as "ALL valve" - which they did, even though all the copy has now been altered to read just "valve". Unfortunately by then it had got carried over into various reviews. To me this was not only dishonest, it was counterproductive - someone was bound to find out eventually. In fact, I think it's a great pity that Blackstar have caught themselves out - the circuit is truly "innovative" as they claim. It uses a mix of both valve and solid-state to give better performance and more of a 'big amp' sound from a pretty low-powered output stage, with very low background noise - it's not just the preamp, the HT5 is actually a miniature push-pull using the two halves of a double triode with a solid-state phase inverter (again not contributing to the distortion), which I think was the first time this was done.

Perhaps they just felt that they couldn't actually say this because there's such a strong market resistance to anything 'solid state' or 'hybrid' - ridiculous, when some of the greatest rock guitar sounds ever recorded have been done with hybrid signal paths.

thermionic

ICBM said:

it's not just the preamp, the HT5 is actually a miniature push-pull using the two halves of a double triode with a solid-state phase inverter (again not contributing to the distortion), which I think was the first time this was done.

 

The Vox Valvetronix claim to run a push-pull output stage using an ECC83, and my ancient desktop Tonelab even says A or AB depending on what type of output stage it's modelling. The Class A or AB thing is obviously BS though.

ICBM

thermionic said:

The Vox Valvetronix claim to run a push-pull output stage using an ECC83, and my ancient desktop Tonelab even says A or AB depending on what type of output stage it's modelling. The Class A or AB thing is obviously BS though.

Ah! I'd forgotten about those. But actually, Vox's description of the valve operating as a "power stage" is BS, regardless of Class!

This may be of interest…

http://www.everythingsg.com/forum/amps-cabs/5844-why-valvetronix-like-platypus.html

thermionic

Interesting link @ICBM (speaking as an ex-electronic engineer...) I don't quite follow what's going on there, whatever it is, it's unconventional. I swapped some ECC83s in my Tonelab, some gave a 12dB volume drop (roughly measured on a mixer LED meter). I think I have downloaded a Tonelab service manual somewhere, there's a section on adjusting parameters of the valve section. 

ThePrettyDamned

thermionic;375021" said:

Interesting link @ICBM (speaking as an ex-electronic engineer...) I don't quite follow what's going on there, whatever it is, it's unconventional. I swapped some ECC83s in my Tonelab, some gave a 12dB volume drop (roughly measured on a mixer LED meter). I think I have downloaded a Tonelab service manual somewhere, there's a section on adjusting parameters of the valve section. 

How is the tone lab for recording? I've used it into an amp before and was quite impressed, and I like the old valvestate amps

thermionic

It's brilliant! I probably play it through studio monitors more often than I power up one of my real amps. The Fender Twin model sounds a bit harsh/brittle to me, not like the Silverface Twins I remember playing through. The Marshalls, Voxes and Tweed Fenders are great. 

I don't understand why Vox discontinued this - perfect use of modelling technology imo, leave it on your desktop and dial up any classic sound in a second. In fact, I've almost bought another as a spare, as the pots on mine are a bit scratchy.

ecc83

"it's not just the preamp, the HT5 is actually a miniature push-pull using the two halves of a double triode with a solid-state phase inverter (again not contributing to the distortion), which I think was the first time this was done."

Yes and the HT is very thought provoking 400V+! Just because it is a wee 5watter does not mean it can't kill!

The OP stage is also fixed biased (and has a balance control so PLEASE get it set properly if swapping the 12BH7) . This means that you can push the stage into higher power (with more distortion) and in fact the amp will kick out nearly 10watts if hardly pressed. It won't be clean but it will be impressively loud thru 4x V30.

OP stage configuration is done by listening tests. The HT-5 is "all" FB. The Artisans  (15/30are CB.

The HT-1 is cathode biased push-pull using an ECC82 and the HT-20 uses CB and adjustable FB!

The PI as ICBM  states is in most designs a pair of power MOSFETS and is capable of  very big, very clean signal output. I was very miffed to read in an American forum some time ago a criticism of the stage as "Bipolar transistors well known for bad sounding distortion". Well, it ain't and it don't!

Dave.

Bod

All interesting stuff, and it's great to have such a great wealth of knowledge on the forum.  Forgive me for my ignorance, but what is FB/CB/PI etc?

I've got a first gen HT5 and I love the OD channel, but the clean channel is just awful.  Is the HT20 different in any way that would improve the cleans?

ICBM

Bod said:

All interesting stuff, and it's great to have such a great wealth of knowledge on the forum.  Forgive me for my ignorance, but what is FB/CB/PI etc?

FB - Fixed Bias. This doesn't actually mean the bias is not adjustable! It describes it being 'fixed' by a separate control circuit, and are generally the only type which *is* adjustable. It tends to produce a bolder, less compressed sound.

CB - Cathode Bias. (Sometimes called self bias.) This means the bias is generated by the current flowing via the cathode of the valve, and is therefore self-regulating. It tends to produce a more compressed, bouncy sound.

PI - Phase Inverter. The part of the circuit which splits the signal into equal and opposite halves to drive a 'push-pull' output stage.

Bod said:

I've got a first gen HT5 and I love the OD channel, but the clean channel is just awful.  Is the HT20 different in any way that would improve the cleans?

Possibly. The HT-20 has the same arrangement as the later HT-5R, with a separate clean channel with its own single tone control. Even though I normally find that a restriction compared to a 3-band EQ, I far prefer the sound of the HT-5R and HT-20 to the HT-5.

It's not going to give you deep, clear Fender Blackface cleans though, if that's your reference for a great clean sound.

ecc83

Hi Bod,

"I've got a first gen HT5 and I love the OD channel, but the clean channel is just awful. "

I touched on getting the OP valve properly biased and balance?

Remember, we are talking just FIVE watts and if you have the Mk 1 combo that will be into a not top sensitivity ten incher. I won't get technical but a push-pull stage need to have the currents through the two valves |(or two sections here) the same otherwise distortion increases and it does not help the very wee output transformer none either!

But essentially if you want even moderately loud cleans you need another 15 watts at least!

The OD sound of the HT series is also a matter of some "Marmite" debate. My son has tried the 5 and I bought him  a 20. He does not like the OD sound of either AT ALL! Oddly though he quite likes the OD sound of the HT Dual pedal! Oddly because the ECC83 is essentially doing the same job in all three circuits! What is different is the "voicing" and this is what makes valve guitar amps such fun and infuriating! How much and at what frequencies do you "distort"?  

"Custom cannot stale their infinite variety!"

Dave.

Danny1969

A friend of mine just brought the 5 watt combo and I have to say it's no where near as nice sounding as my HT5 head into my large ish  1 x 12" cab. The 10" combo is boxy sounding and nowhere near as loud as mine. I can get a loud enough clean sound to gig stuff like Sultans of Swing and Under the Bridge. 

Running a multi effects like my GT10 helps as well as you can sweeten the HT5's pretty bland clean channel with a touch of compression and EQ, you can switch the HT5 with the GT10 so when you step on your clean patch it switchs the amp channel as well as adding in the comp and EQ

tassos

Greetings. I had that bad experience with a dramatic volume drop, during a gig... I thought that the amp was dead. When i return home, i plug it and i realise that there is a sound coming out from the amp but it's, let's say a 0.1 watt with gain and volume at maxium! At the first 5 seconds the volume is ok, but after that, i can only hear that 0.1 watt that i said before. Every other function in the amp is working ok, like equalizer, channel switch, isf, but all these in very very low volume and with the knobs cranked.Only the emulated working properly.
So, i replace tubes.Nothing. I replace the output transformer, nothing again, the two PI driver MOSFETs and still nothing...... Is the HT5 combo with no reverb from 2009. Anyone, please? I appreciate anything that might help.

ecc83

Tassos, PM me.

Dave.

tassos

ecc83 said:

Tassos, PM me.

Dave.

 

ecc83

Did not get any notification of the PM . Mods, have I got a setting wrong somewhere?

Dave.