1967 Bassman build

[Randall]

Coming into the home stretch now. Tonight I installed the board, input jacks, Mid controls on back panel, 1 ohm bias resistors, and miscellany. Next is transformers, and I am so glad I waited to install them last this time, so much easier. I am only focusing on the Normal channel preamp at this point to get it situated so I can set about trying different applications with the Bass channel, namely a 5F6A preamp. As always, this is fun!

[mikepukmel]

Nice clean build. If you don't mind a few more questions...

When you use the thin fiber board, does something go underneath it to prevent shorts to the chassis?

What kind of caps are the white ones?
What are the 2 pots on the back panel for?

Thanks!
Mike

[Jazz P Bass]

When you use the thin fiber board, does something go underneath it to prevent shorts to the chassis?

Fender used a fiber board 2 layer sandwich.

[Justin Thomas]

What kind of caps are the white ones?

Those are current-manufacture Mallory 150s. They used to be yellow. I prefer them vastly over Spragues because they take up half the space or less of an equal-value Orange Drop. And I don't use boo-teeky capacitors at all.

Justin

[Leo_Gnardo]

When you use the thin fiber board, does something go underneath it to prevent shorts to the chassis?

It's always a good idea to have an insulating backing board, yes.

[mikepukmel]

It's always a good idea to have an insulating backing board, yes.

Is it the same stuff, just without holes? I like the way the boards mount in Randall's build, since the boards sit close to the chassis. I had more of a fight with wiring since the offsets/standoffs I got for my build put the board way up high, taking away valuable finger room. I ended up cutting the rod nuts in half with a hack saw to get the board down lower but its still up pretty high.

[Justin Thomas]

Fender would use a same-sized blank piece of fiber board under the actual circuit board. The mounting holes for the circuit board were drilled in the same locations as the insulating board, then the mounting screws go through both boards and keep the lower board aligned.

Also - the same principle with the little bias board AND the filter cap boards.

All that said, if you're using the fiberglass G10 stuff, that's so thick and stiff that you don't NEED to use a bottom board IF you mount it high enough. I mount mine fairly high because I like to run wires underneath. Because you can do that with turrets too. Of course, I also use those Mallory 150 caps, and I can mount the board higher cuz they're so much tinier than Orange Drops...

Justin

[Randall]

Nice clean build. If you don't mind a few more questions...

What are the 2 pots on the back panel for?

Thanks!
Mike

There were pots in those drilled holes when I got it. Since the holes are there, I will install Mid controls for each channel, which I like better than sticking a pot in one of the input jack holes.

[Randall]

Up and running! Two minor setbacks, a broken wire under the board (hard to find), and a bad preamp tube. Actually two bad pre tubes, they were from my used stash. I put in a set of matched Groove Tubes 6L6B's, which according to the Tube Store are actually Sovtek 5881WXT's. At around 25 watts, they crunch a little earlier. Eventually I may look for a NOS set of American 6L6GC's. Blackface side is rocking and sounds great, real nice break up, and very articulate. I love when you can bend a note against another and count the "third note". I forget what it's called.

Next is the Tweed side. I think I am just going to wire to 5F6A tone stack, but leave it where it is in the signal chain for now. Maybe try to move it up a gain stage later once the rest of the amp is solid.

A strange thing happened with a set of 60s RCA 6L6GC's. I put my Fluke probe on pin 3 of one tube and measured across the 1 ohm cathode resistor with no issue. But when I did the same to the other, the amp hummed loudly and I could only measure a fluctuating voltage of around 100v or so. Swapped tubes and the same result on the same socket and pin with different tube. Take out tubes, measure 50 to 53 ohms from CT to either side. I tried the set of 5881WXTs that I have in it now, and no problem. How does that work? If there is a bad tube it should follow the tube. If it is a problem with the transformer I would expect it to happen with other tubes as well.

[Leo_Gnardo]

A strange thing happened with a set of 60s RCA 6L6GC's. I put my Fluke probe on pin 3 of one tube and measured across the 1 ohm cathode resistor with no issue. But when I did the same to the other, the amp hummed loudly and I could only measure a fluctuating voltage of around 100v or so. Swapped tubes and the same result on the same socket and pin with different tube. Take out tubes, measure 50 to 53 ohms from CT to either side. I tried the set of 5881WXTs that I have in it now, and no problem. How does that work? If there is a bad tube it should follow the tube. If it is a problem with the transformer I would expect it to happen with other tubes as well.

Sometimes placing a probe - whether voltmeter or scope - on an output tube plate will send it into ultrasonic oscillation. I think this is what you're seeing here.

Good on ya' for getting your Bassman bassicaly up 'n running !

[Justin Thomas]

If you're not aware of it, BF & SF Bassmans are not exactly known for the headroom... just an FYI so you don't chalk up the early breakup to the Sovteks...
I think you're gonna love this one once you get the small cooties worked out!
And if you ever get a busted up Bassman 100, add that fourth tube... I'm still debating putting that extra stage on a switch of some sort.

Justin

[Randall]

I know, but explain why it happened on same socket, different tubes?

[Tom Phillips]

I know, but explain why it happened on same socket, different tubes?

Could be the wiring dress associated with and in proximity to that socket that puts that side closer to instability.

[Randall]

Maybe, but wiring dress is my OCD. I invite you to zoom into my photo and tell me what you think of my wiring dress. I adjusted it, it made no difference, and anyway it's a lot neater than the original wiring was.

[Enzo]

Maybe, but wiring dress is my OCD. I invite you to zoom into my photo and tell me what you think of my wiring dress. I adjusted it, it made no difference, and anyway it's a lot neater than the original wiring was.

In terms of stability, lead dress issues is not about how neat it looks. It is about the relationship spatially of certain wires. I am pointing out some items not as criticism, but as the SORT of thing I mean. You have two yellow wires to the grids of the power tubes. The longer one snakes by the plate leads for the power tubes pretty close. Some guys reverse the resistors on the board - put the 220k grid resistors to the rear and the plate resistors up front. That shortens the grid wires by a couple inches. Also, the blue wire to V3 and the yellow wire to V4 run along each other a moment.

Now I don't know that any of those are having even the remotest effect on your amp. I point them out as the KIND of thing we look for in lead dress.