Blackstar HT-5R Mods to overdrive channel

[MikeHop]

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.

[Chuck H]

Well, sans schematic there isn't much for anyone unfamiliar with the amp to suggest. The hope that someone, anywhere, is already familiar with mods for this model are small. But you don't get anything is you don't try. FWIW the read on the amp makes out like the dirty channel is basically the clean channel with their distortion pedal built in. So yeah, that means the dirt is coming from the chip. Ergo, a modification to reduce distortion would mean less chip dirt and probably not more tube dirt. Bypass the chip and you'll likely just have another clean channel. With only one preamp tube I have to assume there isn't much opportunity to boost tube gain while eliminating the chip distortion. In other words, the amp is what it is short of major circuit reconstruction.

Since the chip, which apparently sounds buzzy and too distorted, is only part of the dirty channel you can eliminate it by staying with the clean channel. Have you tried a clean boost into the clean channel for a more natural sounding overdrive? I sometimes use a BOSS GE7 EQ for this. That is, it doesn't even need to be a special "boost" pedal. Anything that will give your guitar signal a boost will do.

[Jazz P Bass]

That whole front end is opamps Into the 12AX7 tube.

The distortion is 'diode clipping'.
You could maybe try different leds instead of the diodes.

Here is a crop of the Gain Stage of the HT-5.
blackstar HT5 Gain Stage.pdf

[MikeHop]

Hi, thanks for both replies.
Looking at the part schematic of the gain stage from Jazz P Bass, I've got something in the back of my mind where changing the resistor in the feedback loop will also change the gain. Do you think resistor R 57 (100R) could be a candidate for changing? Would a higher value resistor reduce the effect of the diodes on the signal? It's been a good few years since I had my basic understanding of electronics!
Chuck H, thanks for reply. The reason I got the amp was because of the switchable overdrive channel and reverb, I wouldn't have to use any effects pedals for private practice or going round a friend's for a jam.

[Chuck H]

Well... If you don't like the sound of "their" distortion "pedal" (which is simply built into the amp instead of on the carpet) then your only options are to:

1) Live with it.
2) Modify their pedal distortion circuit until you like it (with no specific instructions).
3) Use a distortion or boost pedal you do like into the clean channel.

I don't think making modifications to the tube amp portion of the amp is going to yield any improvement since you stated you are happy with the clean sound as it is. And that's ALL there is as far as the tubes in this model. The distortion is SS. So how can modifications to the part of the circuit you like improve the part that you don't? All you can accomplish that way is to preserve the part you already don't like and potentially detriment the part you do. Like looking for something you lost in the living room, but you're looking in the kitchen because the light is better in there.?.

[Steve A.]

Like looking for something you lost in the living room, but you're looking in the kitchen because the light is better in there.?.

Works for me!

Steve A.

[MikeHop]

Chuck H, with due respect, can I ask where you got the idea that I wanted to modify the tube part of the amp? I've not said that I wanted to do that. In fact, I said that changing the tube is not the way. That was advised by other forums and Blackstar. As I said in my first post, the modifications, I feel, should be in the overdrive/distortion circuitry around IC 3A which is a TL 072 op-amp, which is the solid state part of the amp - nothing to do with the tube. Anyway, haven't you, or do you know of others, who have tried an amp or a pedal in the store and thought it was great, just what you've been looking for, only to find a few days later when you've spent time twiddling this knob and that knob that it really didn't match up to your expectations in one small way? Well that's what happened to me. I like the size of the amp, the weight, the shape, the colour, the 12" speaker (always think a 12" speaker is best for guitar), the option of 2 channels with a footswitch, the reverb and the clean sound. It's just the overdrive/distortion sound that I'm not now happy with. Now 9 out of 10 doesn't seem too bad to me. I'd just like to make it 10 out of 10 and I know it can be done. I just need a little help with undertaking the theory behind the modifications necessary to reduce the initial amount of distortion gain. So I will be looking in the living room with the kitchen door firmly shut.
Mike

[The Dude]

Is it that the amp simply distorts too early on the gain knob? Can you get what you want with the gain turned down?

Or, is it some tonal or response aspect of the gain that you don't like? I guess I'm unclear as to what we are trying to "fix" here. Can you give us a little more detail as to what it is that you don't like?

[MikeHop]

Hi, thanks for reply.
With the overdrive channel gain on 0, no signal comes thru, even with the overdrive channel level at max. When you just crack the gain control up the distortion is too much. It's an overdrive channel, and should behave like one, but it's just behaving like an almost full-on distortion channel. If you look at the gain stage schematic in Jazz P Bass's post above, you can see the distortion circuit around IC3a which is a TL 072 op-amp. In the feedback loop there are 2 diodes in series with a 100R resistor. Can I reduce the amount of distortion produced by the 2 diodes by changing the resistor to a higher value?
If that would work, I could temporarily connect a, say 100k pot across the 100R resistor and adjust till I find the sweet spot. Then I can work out the required resistance by measuring the resistance of the value achieved by the pot including the 100R in parallel. That's how easy I want it to be.....he said wryly.

[g1]

Have you checked out that gain pot? Maybe it's jumping to too high a value when you barely crack it?
Maybe a much lower value there would serve your purpose? Or a low value resistor in series with the pot (which you would leave set at zero) ?

[MikeHop]

Mmm I'll check the pot. And I'll try the resistor in series and see what happens.
Thanks.

[The Dude]

Also check that the proper taper pot is installed.

[MikeHop]

I assume it should be a log pot?

[The Dude]

In the schematic Jazz posted, it shows an audio taper pot. If the schematic is similar enough, yep- log.

[olddawg]

It's very common for players to just use the clean channel of some amps. I would recommend trying a $39 Bad Monkey or other Tube screamer derived boost pedal and a dirt box of your choice/tastes like a Rat. Not super expensive and 2 pedals aren't unwieldy. There are even combined analogue boost/dirt pedals if you just wanted one. If you are playing at a lot of different volumes, you might consider a modeling pedal like a Zoom or something.