Blackface Bassman Build

[Phrygian77]

Well, a blackface Bassman isn't exactly what I was thinking that I was going to do for my next build, but it's an amp that I've always wanted to have and use as pedal platform for traditional and modern country. I'm probably going to build it as a 1x12 combo. I know vintage AB165s are relatively inexpensive, but this way I get what I want out of it and I'm not modding a vintage amp.

I'm leaning towards using a ClassicTone 40-18029 PT and using the 610v taps SS rectified with 5881s. The real question is what to do with the bass channel... Liverpool, Plexi, 5F6A?

 

[Colors]

I'm partial to the plexi mod myself.

 

[zook]

I'd go with a Cathode follower tone stack using the other 1/2 of the unused 12AX7.

 

[Phrygian77]

I'm leaning towards doing a JCM800 2203/4 master volume preamp. I was a little concerned about putting a .1uf cap between the 1M master volume pot and the 220k mixing resistor going into the PI. With the wiper at ground, that creates a voltage divider with the clean channel mixing resistor. I thought it would probably cause a significant change in frequency response, but when I tried it out in LTSpice, it doesn't seem to have much effect at all, other than -6 dB of overall attenuation.

 

[Phrygian77]

Well this is a bit of a problem...

At least the cap pan matches up.

 

[Bill Moore]

If you flip the cap board will the through holes line up with the holes for the grommets?

 

[Phrygian77]

If you flip the cap board will the through holes line up with the holes for the grommets?

Eyelets would be on the wrong side, but yes, it would line up. The screw holes are also slightly off. I could drill the fiberboard, but I'm thinking about just making one, which is really what I wanted to do to begin with. I think I'll throw the fiberboards into the scanner to use as the basis for the layout.

 

[Phrygian77]

The AA/AB165 cap board should be longer to accommodate the extra cap. I don't understand why Mojotone based their main blackface Bassman fiberboard off of an AA165, but their large cap board doesn't work for the layout. I'm not sure what they were thinking. There's a lot of room under that cap pan. If they're only going to have one version of the large cap board, why not add the extra length and eyelets.

 

[Phrygian77]

I decided to go with a Trainwreck preamp in the bass channel. I couldn't find a good example of a schematic, so I decided to make one. This is based on the Ceriatone 'Liverpull'. The only differences are the fixed mid resistor and the value of the grid leak resistor for the last gain stage.

 

[Phrygian77]

Forgot to attach the schematic...

 

[D'tar]

Phrygian77, have you been to ampgarage.com? there's a lot of info available in the trainwreck files. You will have to register and log in to have access to.

 

[Phrygian77]

My copy of the original is right on the money, so my 6 cap 6.375" board layout should be good also. I ordered a 12"x12" sheet of phenolic today.

 

[Phrygian77]

 

[Phrygian77]

Got my phenolic board from McMaster Carr yesterday. I also ordered a bunch of stuff from Hoffman. I went crazy and decided to go for Jupiter caps for this build!

 

[keithb7]

Dang. There was a 1967 original Bassman PT tranny on Ebay. Ended today. I was watching it. 2 hours to go was $10. Would have been a perfect fit here.

 

[Phrygian77]

Dang. There was a 1967 original Bassman PT tranny on Ebay. Ended today. I was watching it. 2 hours to go was $10. Would have been a perfect fit here.

I haven't really thought about using vintage iron. A 67 PT would probably yeld a much higher B+ than I want. I'd like to get close to 6g6-b voltages.

Phrygian77, have you been to ampgarage.com? there's a lot of info available in the trainwreck files. You will have to register and log in to have access to.

I'm aware of that site. For a long time, it was flagged for malicious software. That seems to have changed fairly recently.

 

[Phrygian77]

Well, in addition to the Mojotone filter cap board being upside down, some of the chassis holes for the cap board are also off. Specifically, the mounting screw hole that is on the front side of the chassis and the through holes for the wires are off by 1/8" towards the rear of the chassis. Pictures here are worth a 1000 words... don't get distracted by the colorful blanket underneath. [emoji14]
Here are the Mojotone boards (flipped over) with the rear side mounting screw in place. The cap pan almost fits like this (the board needs angled corners like the vintage ones), but the wire holes don't line up. Note the position of the mounting holes for the the cap pan.

Now, this is with the font side mounting screw in place. The wire holes line up perfectly, but the whole board is offset by 1/8" relative to the cap pan, so there's no way it's going to fit.

It's not that big of a deal since I'm making my own board. Although, I think I'm going to contact Zachmdhunter about the chassis and ask him WTF is going on here. On the layout for board that I'm going to build, I guess I'll just move those holes by 1/8" ... maybe even the eyelets, and shorten the width of whole board by 1/8".

 

[Bill Moore]

I've done a couple of builds using Weber chassis, and there is plenty of room to run the wires through the grommets, even if they are off a little. (Both were!)
Edit:
I remembered I have a 79 Twin chassis exposed, the fish paper to grommet holes don't line up perfectly either!

 

[Phrygian77]

Wow, got a quick response from the chassis maker!

"Thanks and will recheck that against our original!! We'll note your comment on the "facts" part of the presentation as well. Carl and Zach"

 

[Phrygian77]