Blackstar HT-5R Excessive distortion on overdrive channel

MikeE7

Member
Messages
5
Got hold of a Blackstar HT-5R Mk 2 (the one with the tone control on the clean channel) and happy with it's size, weight and clean sound. Not happy with overdrive channel. It's A, Too much distortion for an "overdrive" channel, B, Too fizzy on distortion, and C, Too much distortion for an overdrive channel ( I know I've repeated myself but I'm trying to make a point ). Anyway, I've got rid of about 80% of the fizz by replacing the 12" speaker for an old 25 watt Celestion G12M that I had lying around from the 70's. But the excess distortion seems to be most people's gripe when checking on the net. Most advice from the net is to change the 12ax7 for a 12au7 which lowers the gain. Blackstar recommend swopping it for a 12at7 which is still a lower gain tube than the 12ax7. But this is not the way! Lowering the gain of the 12ax7 tube lowers the overall volume of the amp which is not what is required. It's the gain of the distortion that needs reducing not the volume of the amp! The clean channel is fine with the 12ax7 so that is not the problem. The problem as I see it, is in the overdrive/distortion circuitry around IC 3A which is a TL 072 op-amp. I have a schematic for the previous HT-5 Mk1, but I think it's been modified on the Mk2 - the difference is a tone control on the clean channel and reverb (electronic not spring). So the circuitry must have been modified if only to accommodate these 2 additions. So what I really need is a schematic of the pre-amp of the Mk2 combo/head so I can start to modify the circuit. Was hoping to fit a switch to allow the distortion as is now and the proposed reduced distortion that I hope to achieve. This in my opinion is the logical way to go with this amp, but if anyone can offer another way of dealing with the excessive distortion problem, please let me know. Blackstar don't seem to want to hand out schematics.
 

Kyle B

Member
Messages
5,074
Is this the schematic you're talking about?
https://vianegativa.files.wordpress.com/2012/07/blackstar-ht5-schematic.pdf

I see there's an 'OD Gain' pot... when that's turned to '0' it should turn IC3A into a unity-gain opamp (i.e. neither adding to the signal nor detracting from it.... what goes in comes out). Is that how yours works? You turn the OD gain to zero and the distortion vanishes? Or no?

You're (probably) right that the distortion comes from IC3A. But the fact you can't 'turn off the distortion' suggests that indeed the MK ii is modified from the previous version. I'd look very closely at the layout on the PCB (you're right that you'll never get a schematic from Blackstar). Check out a tubescreamer schematic (http://www.geofex.com/Article_Folders/TStech/tsxfram.htm) and compare it. Did you ever note how you really can't quite completely 'turn off' the distortion in a tubescreamer either? I believe that's because of the 51k resistor in series with the drive control pot. It can never be dialed down to 'unity gain' state. If you can get that feedback loop down to zero resistance, the clipping diodes would be short-circuited and thus never be able to have an effect on the signal. Note the circuits are NOT exactly alike --- the signal goes to the non-inverting input in a tube screamer, but I believe the overall principal is the same regardless.

This implies that if you COULD turn it down to unity gain, you could also dial out the clipping distortion. Take a good look at the PCB layout - Is there a comparable resistor to the 51k resistor in the opamp feedback circuit??? If so - try short-circuiting it! That might be all it takes.

Alternately --- There's a 100-ohm resistor in series with the diodes in your amp (presumably) - You could try INCREASING that. Or, try changing the diodes to some with a higher forward voltage eg. red LED's. Doing either of those things should increase the threshold of the clipping distortion.

If this works, you MUST return this favor by posting back here with the results! Don't leave me & the rest of the internet hanging ;)

I'm assuming you know the standard rules about discharging the power supply capacitors before you touch anything cuz they really CAN kill you. If you have no idea what I'm talking about --- Just sell the amp and get one you like out of the box....
 
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MikeE7

Member
Messages
5
Hi, thanks for reply.
When the overdrive gain is set to 0 there is no signal, even with the level control turned to max. I have to turn the gain to about just under 1 before any signal comes thru. Then I have to turn the gain up just nearly one notch (that's where it sounds something like overdrive) then from that position upwards it's just distortion.
It's a weird sort of distortion, hard to describe, it's like there's a basic type distortion at low level of the gain control, then when the gain is increased it's like a secondary distortion is heard alongside the basic one, which just gets fuller with the turning of the gain control. But in my opinion, and seems like many others in forums on the net, it's not a nice distortion. It's just too much and quite fizzy (prior to changing the speaker). The reason that others change the tube to a lower gain version is that the distortion then happens further along the gain control travel instead of almost instantly. This makes the overdrive channel much more usable. But why lower the overall volume of the amp when possible modification of the gain circuitry would be a better option?
I've thought of increasing the 100R resistor, my electronic education from years ago tells me that changing the resistor in the feedback loop would change the gain of an op-amp. I'll probably try this mod in a few days time and I'll post the result.
And thanks for the safety warning, but I've worked on tube amps off and on for a number of years - it's the solid state amps I'm not so familiar with.
Mike
 

teemuk

Member
Messages
3,180
AFAIK, the gain control stage - albeit featuring clipping diodes - does not contribute very much to overall clipping distortion. At around 400 mV threshold or so the diodes will start to add a slight bit of very soft symmetric and soft clipping. First tube gain stage isn't even approaching clipping but at that point the grid of the 2nd tube gain stage is already driven to conduction, clipping one half wave, and at the plate running to B+ limit has clipped the other half. Both are "harder" in terms of clipping than slight harmonic distortion introduced by those diodes.

The clean channel sounds fine because its signal output is so weak that it doesn't even overdrive the tube gain stages.

Your problem is likely just too excessive range of gain and distortion. You can reduce amplitude of the signal send to the tube clipping stages by reducing the opamp stage gain. I would try something like replacing the 100K potentiometer with a 50K. Or if there's a LIN taper potentiometer try replacing it with LOG tapered one.
 

Organtis

Member
Messages
593
I'm putting my money on this being the wrong amp. Just because you call channel 2 overdrive doesn't mean it's designed to play hotel California.

This is supposed to be a clean and Distortion channel...... Or try rolling off the volume on your guitar.
 

Kyle B

Member
Messages
5,074
Your problem is likely just too excessive range of gain and distortion. You can reduce amplitude of the signal send to the tube clipping stages by reducing the opamp stage gain. I would try something like replacing the 100K potentiometer with a 50K. Or if there's a LIN taper potentiometer try replacing it with LOG tapered one.
There might be some validity to this also. Rather than chase new pots, you could reduce the range (make it less 'touchy') by soldering a resistor across the two outside lugs of the pot 'just to experiment'...something like 20-50k would do the trick. That turns audio taper into linear taper, so you'd probably not want to leave it like that...just see if it works. Then source a pot with audio taper and a similar resistance.
 

MikeE7

Member
Messages
5
Thanks for your replies. I think I'll try the resistor across the pot first, it'll save me un-soldering the pot and re-soldering a new one. If the resistor across the pot works, I could connect it to a switch so I'd have the option of both sounds. Worth a try I think.
 




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