Bluesmaster HRM question:

[Dr d]

IMO the Mega boost, ie complete lifting of the tone stack, is essential in the blues master. The mega boost is used in conjunction with the OD so that the EQ of the OD is set by the HRM stack, thus following a flat EQ from the clean channel which has no EQ influence on the OD. Hope that makes sense!

[Raoul Duke]

Yes, it does - thanks Doc!

It’s all falling into place as I research this circuit. Trying to be as thorough as possible in my understanding before I commit to the next project.

I appreciate the perspective.

[martin manning]

Are the LM7812CT and L7812CV functionally interchangeable?

Looks that way. One is STMicro and the other is TI. There will be different suffixes for different manufacturers or things like NoPb.

[Raoul Duke]

Thanks Martin!
Still learning about interchangeability and what to look for on data sheets. Seemed like they both did the same thing, but there were small technical variances that I didn’t understand.

Thanks again!

[Raoul Duke]

Moved to tech talk.

[Raoul Duke]

Another new guy question regarding BM/HRM:

I see the 5k1s on the power tubes at grid 1 are soldered directly to the leads heading back to the main board (like an old Marshall or Vox). This looks simple enough, but just wanted to check as to the correct mechanical connection before soldering. Is this a “J” hook situation or is there another more reliable way?

Also - are the caps on the manual/pedal switch understood to be there even though they aren’t on the layout? Can’t seem to find a complete schematic for the BM to try and answer this.

I’m probably going to figure out a parts list and budget for this circuit. I think I’m sold after reading a few build threads and hearing what this amp does.

Still a little confused about transformers though. I plan on building a 50w, so Bassman PT and JTM45 or JCM800 OT sound right?

Any guidance/opinions/experience welcome. Thanks!!

[martin manning]

I see the 5k1s on the power tubes at grid 1 are soldered directly to the leads heading back to the main board (like an old Marshall or Vox). This looks simple enough, but just wanted to check as to the correct mechanical connection before soldering. Is this a “J” hook situation or is there another more reliable way?

Here you can see how that is done: https://ampgarage.com/forum/viewtopic.p ... 37#p145137

Also - are the caps on the manual/pedal switch understood to be there even though they aren’t on the layout? Can’t seem to find a complete schematic for the BM to try and answer this.

I'd include them, at least the ones inside the chassis. MDRoberts schematic in the D Files Bluesmaster sticky is pretty good, but has a few mods re the layout, IIRC.

Still a little confused about transformers though. I plan on building a 50w, so Bassman PT and JTM45 or JCM800 OT sound right?

I'd use the Bassman OT of you are going with 6L6GC

[Raoul Duke]

Thanks Martin,
That’s a heck of a clean build there! Great visual reference.
I’ve been looking at the mdroberts schematic - so I’ll stick with it; just figured I’d mention it in case there was something else I missed. I figured the caps are cheap insurance against any pops so they’ll be included.

My understanding is that these BMs sound better with Marshall type OTs - so I’m digging into those now to see what’s closest to the required specs.

Thanks again!

[martin manning]

Mojo's (likely made by Heyboer) Plexi/800 OT is 3k7 Zpri, and that will make little difference vs. the Bassman OT's 4k. The specific details of its construction (winding pattern) are probably more important. Hammond's JCM800 replacement is 3k2, and I would not go there with 6L6's. A JTM45 OT, even at the lower 6k6 primary impedance would not be appropriate either. I have no direct Bluesmaster experience, so maybe someone who does can chime in.

[Raoul Duke]

Definitely finding that the “mix and match” isn’t the easiest to figure out. I saw the JTM used KT66 and figured that OT might be the answer seeing their similarities with 6L6GC. Researching that now.

Anyone ever done a KT66 BM?

Right now that Mojo is the front runner unless I just go straight Bassman. Probably overthinking this. Just trying to get the text book BM sound.

[Raoul Duke]

All right, I think I have the OT figured out.

Heyboer makes a Marshall type 50w with 3k9 primary, M6 laminate, Dagnall wind pattern. Says “JMP/JCM800 general replacement” designed to work with both 6l6 and EL34 - which sounds about right.

I think this is the winner…

[martin manning]

]Heyboer makes a Marshall type 50w with 3k9 primary, M6 laminate, Dagnall wind pattern. Says “JMP/JCM800 general replacement” designed to work with both 6l6 and EL34 - which sounds about right.

Got a link to that?

[Raoul Duke]

Here it is:

https://shop.amppartsdirect.com/product ... d-40-18025

Almost seems to good to be true, lol.

Thoughts?

[Raoul Duke]

Just found a 59 Bassman reissue PT for $40 locally. Been in a shop for a few years but looks brand new. Pulled for a Mercury upgrade and left at the shop. Guaranteed to work or my money back.

Looked it up and it’s 374-0-374vac with 6.9vac heater wind and 57.6vac bias supply (unloaded voltages). It’s a little higher than the BF spec Bassman, but it’s cheap. Too much voltage or can it be made to work?

Any opinions on this or the 3k9 Drake-style OT are appreciated!

Thanks!

[Raoul Duke]

DISREGARD:
Found the answers - just needed to re-word my question. Wording is everything whether searching TAG or Google, lol. Love it when I discover something new.

Onward…

…noticed the bias sensing resistors are routed through pin 1.

I know, probably basic stuff…

Thanks!