Another Bassman Build - Tweed 5F6A : New Digs

[ToneMerc]

When I finished up my 2nd 6G6B Bassman build last fall I decided to do a tweed 4x10 Bassman and have been collecting parts when I run across them.

My only real hard requirements was era correct PT (nearly every off the shelf tweed Bassman PT design is wrong, some are plain dead wrong), vintage voltages at 120V+ mains and era correct tubes.

Also, it will have the correct presence circuit found on actual vintage amps, not the one seen on the Fender layout and adjustable bias.

NOS and ANOS Tubes:
V1: 11/56 GE rebranded RCA 12AY7 D getter
V2: 12/60 RCA 12AX7A short gray plate
V3: 5/59 GE 12AX7 long plate horseshoe getter
V4 & V5: 1/59 Tung-Sol 5881 matched pair
Rectifier: GE copper post 5AR4

Iron:
Mercury Tweed Bassman clone OT, 2/4 outputs
Mercury Tweed Bassman clone choke
My Triad 8087 clone wound by Heyboer, setup for 123V mains and includes an extra low current 6.3V tap

Components:
Mojo tweed Bassman chassis
Jupiter yellow coupling caps
Piher carbon films
Sprague TE lytics on main board
F&T lytics on power board
CTS new and vintage pots

TM

Speakers: TBD
Cabinet: TBD

[mr_hankey]

What is the correct presence circuit? More like the 6G6?

[steveneddy]

When I do a Bassman 5F6A I like to use the presence circuit that doesn't put DC in the pot so it is quiet when turning the knob.

Sound guys hate noisy pots - especially when you turn them while playing.

[martin manning]

The Fender 5F6-A schematic and layout (dated 1957) shows the 5k linear presence pot element making up the last 5k of the PI tail, and the 0.1uF presence cap going from its wiper to ground. This is the circuit Marshall copied for the JTM 45, and the DC on the pot makes it slightly noisy when the pot is turned. The "improved" circuit, as seen on later production 5F6-A's and Marshalls, uses a 4k7 resistor to ground and a series combination of a 25k linear pot and 0.1uF cap in parallel with the 4k7. This eliminates the noise, but at the expense of concentrating the presence effect at the top end of the pot rotation and the inability to turn the presence completely "off." Some people, me included, prefer the old-style.

[ToneMerc]

Here's the main board populated. I decided not to burn up 68K Pihers so I used 56K as the grid stopper value.

TM

[Blackburn]

Nice job, TM. I just built a Bassman/JTM45 with a much cooler B+ (400) and have been fiddling with it for a couple weeks. Been having some issues with brightness. Might just be the room. I used Sozo mustards in mine, but dropped values somewhat. I see that Chris over at Jupiter changed the look of his caps a bit. I ordered from him a long time ago and wanted to do all my stuff with them, but decided to go with Sozo instead. How do you feel about the Jupiter caps in these kinds of circuits?

[ToneMerc]

Nice job, TM. I just built a Bassman/JTM45 with a much cooler B+ (400) and have been fiddling with it for a couple weeks. Been having some issues with brightness. Might just be the room. I used Sozo mustards in mine, but dropped values somewhat. I see that Chris over at Jupiter changed the look of his caps a bit. I ordered from him a long time ago and wanted to do all my stuff with them, but decided to go with Sozo instead. How do you feel about the Jupiter caps in these kinds of circuits?

Thanks BB, this build should be around 430V on the plates at 123.5V with a GZ34. Honestly, this is only my second time using Jupiter caps and the first time using the yellow ones so I can't get you an evaulation. I had been using both the blue and yellow Sozos in the past, but decided to try something different this time.

TM

[Blackburn]

Nice job, TM. I just built a Bassman/JTM45 with a much cooler B+ (400) and have been fiddling with it for a couple weeks. Been having some issues with brightness. Might just be the room. I used Sozo mustards in mine, but dropped values somewhat. I see that Chris over at Jupiter changed the look of his caps a bit. I ordered from him a long time ago and wanted to do all my stuff with them, but decided to go with Sozo instead. How do you feel about the Jupiter caps in these kinds of circuits?

Thanks BB, this build should be around 430V on the plates at 123.5V with a GZ34. Honestly, this is only my second time using Jupiter caps and the first time using the yellow ones so I can't get you an evaulation. I had been using both the blue and yellow Sozos in the past, but decided to try something different this time.

TM

Cool, man. Do keep us posted on your results with them. I think the red ones he makes are really sexy, but the cost just didn't jive and after I saw makers like Metro and Morgan using Sozos, I had to as well. I've only ever used them in my builds, with the blues in four of them. Two 5E3s, a 5C3 and a 5B6. May the 5B6 rest in peace.

But like a phoenix, those trannies and chassis will become something new.

[ToneMerc]

May the 5B6 rest in peace.

Yeah same here, mine while kind of cool, didn't really trip my trigger. I may revisit it one day.

TM

[Blackburn]

Yeah me too. I really dug it, but ironically, I wired the pre with some cotton jacketed wire from Jupiter and later I realized that it's that high purity crystal stuff that tends to be faster and inherently brighter sounding and I felt the need to rebuild because of the harshness. Even at the most clean setting it could be very brittle sounding, but some days it was quite nice especially with my R8's neck pup! Super jazz tone!

[ToneMerc]

Finished the chassis prep, wired the filament string and mounted the L shaped chassis support bracket.

TM

[jelle]

Very nice! .1uf bass cap?

[ToneMerc]

Very nice! .1uf bass cap?

Thanks, yes .1

TM

[ToneMerc]

Finished my pig finally, I'll power it up in the next day or so.

TM

[Colossal]

ironically, I wired the pre with some cotton jacketed wire from Jupiter and later I realized that it's that high purity crystal stuff that tends to be faster and inherently brighter sounding and I felt the need to rebuild because of the harshness. Even at the most clean setting it could be very brittle sounding, but some days it was quite nice especially with my R8's neck pup! Super jazz tone!

DB, when you say high purity crystal stuff, was that wire silver plating over ultra high purity copper or just straight cotton cloth over non-plated high purity copper. Just trying to get a feel for where that brightness might have come from.

Thanks man.

Dave