Bluesmaster HRM question:

[martin manning]

I wonder how the layout’s dropping string values would interact with a Bassman spec PT? I realize the PI is different and that plays a part.

By the time you get to the preamp nodes it doesn't mater. If you keep the same dropping string, B+3 to 4, and B+4 to 5, the PI will fall into place.

[ijedouglas]

The TF-130 PT was a Fender Twin replacement, so you should shoot for 440-450V B+1.

I have one of these transformers and it runs 460V on the plates. #183 has the same transformer and was documented at 462V on the plates. Slightly higher than a Twin transformer

[Raoul Duke]

Ok, here’s today’s thought exercise. Tell me if this makes sense:

I’m thinking of trying the 290 DX because both the 290 EX and Heyboer 762(I think) Black panel Bassman PTs I previously used started out at ~465+ volts on the plates before fiddling with the dropping strings and tubes. The lower voltage of the DX might get closer to the target initially - thus simplifying the final tuning. It’s lower voltage, but not by much. Might be just right at ~10v difference.

Worth a shot?

[martin manning]

Either one would be fine. I don’t think the power stage voltage is all that critical, and you can adjust the first dropping resistor as needed. Maybe make the call based on source, cost, and shipping charges incurred?

[Raoul Duke]

Of course there’s that too.

Off the bat - I’m noticing that the DX is anywhere from 15 to 30 dollars cheaper than the EX or the “specifically designed and labeled” Bassman models from nearly everyone else. I use the quotes because they’re all just a little different from each other; but they share the same Bassman designation. Marketing I guess?

Probably start drilling some F4 today. Looks like it’ll be warm and sunny.

[martin manning]

290DX has a slightly lower HV current rating at 230 mA, but the tube compliment you have is the same as the Tweed Bassman that it's designed for. It also has a 5V winding, which you could use for relays if you went to a 5V system. That would eliminate the cost, weight, and complexity of the additional small transformer.

[Raoul Duke]

I’ve thought about going to 5v relays in the past. I know many here have done that to eliminate the extra parts and make use of the tube rectifier winding. I see why it makes sense. Maybe I’ll try it in the future, but I’ll stick with the traditional system for now.

Got my boards drilled and eyelets just about finished. After sorting through my parts pile - I had about half the board resistors in 1/2 watt Koa CF, so I ordered the same for the rest. Got the Dales for plate loads and filled in the everything else with a mix of Koa MF/CF just to keep the “theme” consistent. I’ll start building the small boards tomorrow.

I suppose I’ll start a build thread here soon… I guess I’m committed at this point, lol.

[Raoul Duke]

One last technical question; or clarification really.

Trying to work out the 22M “jumpers” on the mid- boost relay board in the layout. It looks like Com1 to NC1, Com2 to NC2 - but it’s hard to be sure - so I looked at the bm_clone schematic and it looks like NO1 to NC1 (with a 72k?) and NO2 to NC2 with the 22M value. I understand that schematic has modifications, so I’m not 100% confident that I’m barking up the right tree.

The question is: which is correct?

My thought process is that this boost should function the same as 102; am I off base here?

Any guidance is greatly appreciated! Thanks!

[martin manning]

Trying to work out the 22M “jumpers” on the mid- boost relay board in the layout. It looks like Com1 to NC1, Com2 to NC2 - but it’s hard to be sure - so I looked at the bm_clone schematic and it looks like NO1 to NC1 (with a 72k?) and NO2 to NC2 with the 22M value. I understand that schematic has modifications, so I’m not 100% confident that I’m barking up the right tree.

The question is: which is correct?

My thought process is that this boost should function the same as 102; am I off base here?

The BM boost is not the same as PAB on the Skyline stack. It lifts the whole tone stack, where the Skyline boost only lifts the treble pot and the bass wiper.
The MDRoberts schematic has a partial lift, with 72k to ground. On the layout, the lift is 22M (full lift), and the other set of contacts are used to disconnect the middle wiper (but I don't see any purpose in it). Several people have done the partial lift and prefer that. I recall someone wiring in a trimmer to adjust it to taste, too.

[Raoul Duke]

Understood regarding the two different approaches, thanks Martin. I saw the mid trimmer in Charlie’s Ripper photos (thanks Charlie, all your photos are an awesome resource and very much appreciated!) and read the discussion regarding it. Interesting.

Did I have the connections correct? That’s where I got crossed-up. So I could follow mdroberts’ schematic and it would work the way most/many prefer?

[martin manning]

Did I have the connections correct? That’s where I got crossed-up. So I could follow mdroberts’ schematic and it would work the way most/many prefer?

You understood the layout connections correctly. They are also shown in my ODS Eyelet Board .pdf. You can use MDRoberts scheme, but you might prefer a different resistance to ground for the partial lift.

[Raoul Duke]

Thanks Martin!
I guess that’s what I was thinking when I said “works the same”; should have said “connects the same”. Words got me again.
I’ve had a couple of TBIs in the past - they must be acting up, lol.

As far as the lift resistance goes - 22M and 72k seem pretty far apart. What’s the best way to figure out where to start that experimentation? Higher value lifts more of the stack?

I appreciate the info; really helps me connect the dots and understand vs just copying the recipe. Trying to understand the whole thing before I turn on the iron this time.

[martin manning]

As far as the lift resistance goes - 22M and 72k seem pretty far apart. What’s the best way to figure out where to start that experimentation? Higher value lifts more of the stack?

22M is the full lift, same as open circuit for all practical purposes. I've seen various values for the partial lift, typically around 50k.

[Raoul Duke]

Thanks Martin! I’ll definitely put some thought and reading into this.

[rootz]

I installed a 1meg pot to do the partial lift and was surprised it had very little useable range. Meaning: I kept it in the first 25% of its rotation, mostly around 100k. Anything above it did not give much of an extra boost, but more compression and saturation.

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.