tweed bassman build

[CHIP]

Great looking build.

[LeftyStrat]

I've been wanting to build one ever since I saw this on youtube:

http://www.youtube.com/watch?v=Y0r1kVXSX3Q

He stops talking and starts playing around the 1:06 mark.

The tone is just huge.

[Firestorm]

I've had few of those in my shop over the years and they are just crazy. In fact there was once a basket case '55 in here where I redid the circuit to 5F6A (it had already been parted out by somebody so no offense to the amp gods) and that one sounded crazy too.

[Structo]

I've been wanting to build one ever since I saw this on youtube:

http://www.youtube.com/watch?v=Y0r1kVXSX3Q

He stops talking and starts playing around the 1:06 mark.

The tone is just huge.

Those old amps sound great!

They kind of have a swirly/ phasing thing going on.
That one even sounds like it has a low level tremolo going on.

[Cliff Schecht]

TBH my 5F6A clone sounds just like this one (in the video). I heard this video as well before building mine and am always surprised by how loud and proud the amp is before it distorts. With AX7's it's a distortion monster but I prefer the stock circuit for my needs.

I used some nice old 6L6WGB's in the power amp and RCA 12AX7/12AY7's in their proper spots in the preamp. Also got my voltages right on (although I think this amp got a 5U4 instead of a GZ34) and the bias conditions right which IME is a big part of getting that old Fender sound (much more important that CC vs. CF resistors or coupling cap choice, really).

[martin manning]

...Also got my voltages right on (although I think this amp got a 5U4 instead of a GZ34) and the bias conditions right which IME is a big part of getting that old Fender sound (much more important that CC vs. CF resistors or coupling cap choice, really).

Interesting comment, Cliff. I wonder how much of the sound is due to the lack of grid stoppers on the power tubes? On the reissue '59 Fender added them but they are tiny- 47ohms.

[Colossal]

Cliff et al,

Clarify something for me: My understanding of the very cold Fender bias condition is that (at least) on the Fender circuits with bias tremolo, you would need to set the bias sufficiently low so that you don't continuously exceed the screen current limit when the bias spikes on each oscillation. I've monitored the current on a Princeton when the trem is chopping away. Interesting to watch it bounce and you can see the peak relative to the bias point.

Also the cold bias is just part of the 7ender sound though right?

[Firestorm]

Cliff et al,

Clarify something for me: My understanding of the very cold Fender bias condition is that (at least) on the Fender circuits with bias tremolo, you would need to set the bias sufficiently low so that you don't continuously exceed the screen current limit when the bias spikes on each oscillation. I've monitored the current on a Princeton when the trem is chopping away. Interesting to watch it bounce and you can see the peak relative to the bias point.

Also the cold bias is just part of the 7ender sound though right?

The Fender bias trem pulls the negative bias MORE negative (shutting off the tubes). The cold bias overall, IMO, is because Leo wanted to squeeze every possible watt out of a configuration. The more you bias toward Class A, the more power you lose. Fender circuits weren't designed for AB2 so you run them AB1 and get the biggest bang for the buck.

[martin manning]

The bias vary tremolo signal is sinusoidal, so it shifts the bias both ways. But, it's a moot point for this discussion since the 5F6-A has no tremolo :^)

[tubeswell]

Very tidy! and nice dovetails too! Kudos to you!

[LeftyStrat]

I built a JTM45, that never really floated my boat. I tried 6L6's and it still didn't. Once I made it a bit closer to a JMP50 (or more like he Germino Club 40, with tube rectification), I liked it.

But it never sounded anywhere close to the video I posted.

I'm assuming the magic lies not only in the circuit, but the open-backed combo cab and the quad of 10" speakers.

[Firestorm]

The bias vary tremolo signal is sinusoidal, so it shifts the bias both ways. But, it's a moot point for this discussion since the 5F6-A has no tremolo :^)

Well, yes I guess; but there are phase cancellation effects, too. Depending on signal strength, the LFO signal is being applied to opposing grids so the negative-going cycle can pull both tubes into cutoff, while the positive-going cycle is such that one tube (already in cutoff) never comes out of cutoff, and the other tube (already conducting) will not really see the difference in bias condition (it may get closer to saturation, but I doubt you could hear it). But, you're right, moot for a 5F6A discussion.

[Cliff Schecht]

...Also got my voltages right on (although I think this amp got a 5U4 instead of a GZ34) and the bias conditions right which IME is a big part of getting that old Fender sound (much more important that CC vs. CF resistors or coupling cap choice, really).

Interesting comment, Cliff. I wonder how much of the sound is due to the lack of grid stoppers on the power tubes? On the reissue '59 Fender added them but they are tiny- 47ohms.

I don't think this is a big part of the sound but there is merit to the notion that a lack of grid stoppers can change the way the power stage distorts. Perhaps as the signal swings around, there is a change in stability that adds some distortion to only parts the waveform. I did probe my 5F6A looking for HF oscillation and didn't see anything, but that was in the quiescent state just making sure it was stable with no input. In my experience the 6L6 is a pretty dang stable tube anyways (lots of inter-electrode capacitance), it's those pesky 807's that'll oscillate to hell and back if you don't take certain precautions.

[Richie]

I built a JTM45, that never really floated my boat. I tried 6L6's and it still didn't. Once I made it a bit closer to a JMP50 (or more like he Germino Club 40, with tube rectification), I liked it.

But it never sounded anywhere close to the video I posted.

I'm assuming the magic lies not only in the circuit, but the open-backed combo cab and the quad of 10" speakers.

Yep that has a bit to do with it. And 4 sounds different than 2.
Same with the supers,concerts.

[Diablo1]

Beautiful build!

I'm with Firestorm, do try flipping the phase to try it out. When I first heard of this I didn't believe it, but I tried it in a small combo and it was noticeable.

I actually measured this in a very unscientific way: the fixed field behind an alnico extends straight back and can deflect a compass needle 17 inches away. That field is modulated by the signal, so you gotta believe a tube sitting in that field is affected somehow. Probably not the electrons travelling to the plate 'cause they get up to about 70km/sec. by the time they hit the plate, but the electrostatic fields in the tube have to be affected: notably the virtual cathode, which we were on the verge of understanding when all the smart guys transitioned to solid-state. Just a recurring interest of mine, arcane though it may be.

The magnetic field behind a speaker is fixed because it's a permanent magnet. The field coil gets the AC signal from your output tube. Changing the speaker polarity does not change the strength or orienttion of the field from the permanent magnet. I doubt a tube is influenced from the magnetic field from the speaker. Stick a strong neodymium magnet up close to one of your output tubes and see if you can hear a difference while playing.