Bluesmaster HRM question:

[Raoul Duke]

Thanks rootz, I appreciate the info!

Interesting that the consensus is that the value should stay under 100k with both your experience, the mdroberts schematic, and Martin’s advice.

Does this mean the full 22M has a negative effect or that it’s just one of many “flavors” for the lift? Still figuring it out.

[rootz]

Definitely a flavour. One I don’t like, but it is easy to try it out for yourself. Hang the 22meg resistor from mid pot to bus bar and it is easily replaced by a pot or smaller resistor. Dumble tweaked, revised, refurbished. We should do to. There’s a lot to be learned from these little experiments.

[Dr d]

I ended up installing a 100k pot instead of the 22meg resistor. Dumble did that too and just hang the pot between the bus bar and the mid pot.

Did Dumble actually do this or is it just another TAG tweak?

[martin manning]

Did Dumble actually do this or is it just another TAG tweak?

Excuse me, just another TAG tweak? ;^)
I seem to recall seeing a picture of this. It may have been in a HAD amp or maybe it was a mod by one of the commercial D clone builders.

Does this mean the full 22M has a negative effect or that it’s just one of many “flavors” for the lift? Still figuring it out.

All it means is that for a partial boost the difference between ~100k and the full lift (22M) isn't very significant. The 22M is really there to eliminate the popping that would result if the mid pot were just disconnected from ground. A 1M lift would be indistinguishable from 22M.

[erwin_ve]

The one Rootz probably is talking about; #189 with a trimmer on the mid pot. The amp is unverified as a Ripper model.
See pic.

[martin manning]

Yes! Thanks Erwin. Clever way to add that feature.

[Raoul Duke]

If it’s that simple, I just might try it.

Just so I’m understanding correctly, eliminate the 22M from Com1 to NC1 and install the trimmer as per the photo; but keep the mid pot wired as the layout shows?

A thumb wheel trimmer seems like the best way to go. I’m thinking 1M would be the best value or should I go lower?

That 189 pic is the one I was referring to a few posts back; thanks Erwin!

[Dr d]

Did Dumble actually do this or is it just another TAG tweak?

Excuse me, just another TAG tweak? ;^)

Forgive me Martin! My comment was perhaps a little clumsy. Perhaps I am behind the curve, but as far as I know, Talbanys' Bluesmaster layout is the only reasonably accurate resource we have for this build and it calls for a total lift of the tone stack for the PAB. This makes perfect sense to me as it disables the preamp tone stack when in OD mode and allows one to tune the eq of the OD (via the HRM of course) separately to the eq of the clean mode. Added to to this, I have no doubt that HAD was intending the OD to be compressed in its character. The biggest mistake I made when I first started building these amps however many years ago was to try to outwit Dumble with "tweaks" instead of trying to understand his original design intention. Its an urge that I have difficulty with to this day.....but I am working hard not to succumb! To conclude, I think one should build these amps as accurately as possible to the original design and then live with it for a reasonable amount of time before "tweaking". Fight the urge for as long as possible because he knew what he was doing!

[martin manning]

The biggest mistake I made when I first started building these amps however many years ago was to try to outwit Dumble with "tweaks" instead of trying to understand his original design intention. It's an urge that I have difficulty with to this day.....but I am working hard not to succumb! To conclude, I think one should build these amps as accurately as possible to the original design and then live with it for a reasonable amount of time before "tweaking". Fight the urge for as long as possible because he knew what he was doing!

Sound advice, but I think it would be fine to go ahead with this one. Many people have tried the partial lift and liked it, and with a trimmer pot installed it can be dialed out. As far as how to do it, I think I would replace the 22M on the relay board with a trimmer rather than hang the trimmer from the ground bus. That actually creates a ground loop.

[Raoul Duke]

Martin,
Does that have something to do with the third leg of the pot and where it would be connected? I was kind of wondering about how that would work.

So put the trimmer connecting Com1 and NC1 - one side and wiper with the other side?

[martin manning]

So put the trimmer connecting Com1 and NC1 - one side and wiper?

Yes. You wire the pot as a variable resistor, and when the relay contacts are open it becomes the resistance to ground. If the trimmer is on the bus then there are two ground paths.

[Raoul Duke]

Thanks Martin!
This might be an obvious question, but I don’t understand why it wouldn’t be on Com2 to NC2 since NC2 goes to ground - thereby adjusting how much of the mid pot goes to ground? At least that’s how it looks to my untrained eye.

I must be missing something, right?

[martin manning]

If you are looking at my boards .pdf, you're correct, it uses Com2-NC2 as the path to ground.

The layout shows the other set of contacts being used to open the mid pot wiper-CW short, which will have no effect on the PAB if the ground is lifted 22M. Interestingly, if a partial lift is used, then setting the mid pot to its full value makes the PAB independent of where the mid pot is set. Maybe the BM that the layout was drawn from had, or was prepared for, a partial lift.

[Raoul Duke]

Thanks again Martin!

Yes, I was looking at your boards - which are my guidelines for sure.

Any recommendation on that pot value? After thinking about it - maybe 1M is too much? All the modded relays seem to be under 100k; but maybe there’s more to discover past 100k?

[martin manning]

If you are likely to be around 50-100k, I would say go for a 500k trimmer. 10-20% of rotation for the partial boost, and 500k is only about 1 dB down from full lift.