Which Vibroverb?

[Cliff Schecht]

Where can I find that 350k/70k tapped pot for a 6G16? Anybody have one for trade?

[martin manning]

Where can I find that 350k/70k tapped pot for a 6G16? Anybody have one for trade?

Marsh sells one, but it isn't full-size. I've had a 6G16 build in a planning state for a few years now. Is the taped pot really necessary? I was thinking why not just do the tone stack from the very similar Vibrolux?

[Cliff Schecht]

Sure it's not absolutely necessary but it's something that I'd like to include. I try to stay as close as possible to the factory schematics before I start doing tweaks or changing the circuit, especially the amps that are legendary in stock form.

The reason I build vintage amps like these is so that I can own something that I can't afford usually as well as so I can have a good collection of amps to start doing characterization work on. It's also nice come studio time when you have a whole room of amps to choose from.

[M Fowler]

Hoffman also sells this pot as well.

http://www.hoffmanamps.com/MyStore/perl ... =209772826

350KA pot with tap point
Used on some brown era fender amps
Four solder tabs.
Body is only 15mm wide.
Mounting hole is .335"
Tap is approx. 70% of full resistance.
Price each = $2.50


Mark

[rp]

I owned a near mint 2x10 brown VB back in the mid 80s. Bought it for $800 before all the hype about it started. Neither I nor the seller knew what it was. I sold it around 2001 to pay 3 months back rent. Hope my landlord bought himself something as nice.

Here's some pics if they're helpful. Sorry for the crappy res, it was the old film days and a scanner way back when.

[Cliff Schecht]

Such a puny output transformer!

Thanks for the sweet pics.

[David Root]

Weber also has them. The ones I bought from them tended to be on the low side of 350K, about 320 +/-. They are also 16mm pots, not full size.

As to whether or not it's a vintage correct necessity, I have heard that the only reason Leo used them in the browns is because he got a price on a Texas S**tload of 'em he couldn't resist. Going back to '62 or so I could believe that. This was not some sophisto hifi CBS engineer decision!

[martin manning]

In recent times Fender thought enough of its effect to include it in the reissue '63 Vibroverb, so maybe there is something to it. Slightly different spec, though, 50k tap on a 250k pot.

[Cliff Schecht]

In recent times Fender thought enough of its effect to include it in the reissue '63 Vibroverb, so maybe there is something to it. Slightly different spec, though, 50k tap on a 250k pot.

50k tap on a 250k resistor is the same ratio (5:1) as a 70k tap on a 350k..

As far as the special Leo may have purchased, would he have purchased them if he didn't need them? Maybe he chose that 5:1 ratio specifically and when seeking out pots he stumbled across a great deal that happened to be a goofy 350k/70k instead of 250k/50k..

Either way, I'd like to build the circuit with this and see how it works. I can deal with a 10% tolerance (320k is in this range) as long as the ratio itself is close.

[Lonely Raven]

Jean, those fake caps are awesome!

[Cliff Schecht]

I got the Vibroverb built. Fired up on the first try and squealed like a mofo. Flipped the OT primaries, tweaked the fixed bias resistors (didn't make room for a pot) and viola!

All of the features (reverb/trem) work and both channels sound great but the normal channel distorts really easily while the bright channel has a hard time even getting into distortion at 10. Both channels sounds amazing, my favorite is the bright channel, but I'm not sure why there is such a discrepancy in volume. The normal channel's 1 Meg volume pot is really scratchy (should have used an AB but I'm ran out!) and is really sudden, I think because it's a linear taper. I can get it to match the bright channel volume wise and it breaks up at anything past this point so I'm not sure why the bright channel doesn't distort.

FWIW new tubes didn't help and I'm pretty sure the wiring is correct. Voltages all looked good too.


Any thoughts? Is this normal for this circuit? I'll try to post pics tomorrow.

[Cliff Schecht]

Oops. Found two solder joints missing in the tonestack. That definitely fixed the super gaininess of the normal channel and both channels seem more even.

I still notice that the bright channel is somewhat weaker than the normal channel. It sounds huge and has a great clean but it sure doesn't want to break up. The normal channel will get some girth going. Perhaps this is attributed to the 220k plate resistor in the normal channel vs the 22k/82k split plate load in the bright channel?

Also FWIW I used Martin Manning's recently posted circuit for replacing the tapped pot with a 500k job and a few resistors. It seemed to have worked like a charm so kudos to Martin for that neat little trick. Definitely sped up my build time dramatically (and there's a lot of friggin parts in this amp!).

[martin manning]

Cliff, glad the tapped pot sub worked out. Here's the link if anybody needs it: https://tubeamparchive.com/viewtopic.ph ... 334#193334

Maybe put a PPI MV on it so you can crank it up enough to distort in the preamp?

[tdvt]

Just to clarify, it was a 6G16 that you built in the end?

I have a Hoffman AB673 about half built that is basically the same as the 6G16 minus the tapped treble & with a few value changes, so I'm interested to hear your thoughts.

[Cliff Schecht]

Martin: How does a PPIMV look like when the amp is fixed bias with tremolo? I've never seen an example of this. Would one of the dual pot MV's work? I've got a bunch of great Allen Bradley 2W stacked pots in 250k, 500k and a few in 1 Meg. I think 250k is most appropriate here..

tdvt: Yes it was a 6G16. I absolutely love it. I would probably always opt to replace the optocoupler trem in the Blackface circuits with the direct coupled trem too. I don't know why they switched but I don't have problems with LFO bleedthrough or anything like that. Heck it's my favorite trem so far, more subtle but very effective.

With that said, I did find some optocouplers from an old pedal project that I might test out in another amp (Deluxe Reverb is on my mind). The nice thing about optocoupler based oscillators is the optocouplers are naturally slow enough (slew rate limited) that they prevent bleedthrough of the LFO. The annoying thing is also the slew rate limiting, it prevents one from getting fast, choppy trem sounds.

Also after a lot of Google-fu I did find other people referencing the Normal channel being louder than the bright channel. I still think it's the difference in plate load resistors in the gainstage following the tonestack on each channel. I might hot-rod the normal channel a bit or figure out a pedal that switches between the two inputs.