Which Vibroverb?

[martin manning]

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.

I think the Lar-Mar would work fine. It takes the signal to Vbias at a zero volume setting. Now, Vbias might be varying at 5Hz or so, but that shouldn't hurt anything. Another way to get some distortion into it would be to hit the input with a clean boost...

[Cliff Schecht]

Does the DC from the bias voltage being on one of the lugs cause scratching in the pot?

I've never tried a PPIMV on a fixed bias amp. I assumed it could be done but I never tried it. I probably won't put one on the Vibroverb, I don't think it needs one, but I have other amps that would benefit from this addition. How does it sound on an Express?

Also doesn't this effect the negative feedback? There isn't a presence control but you are still playing with the PI/output stage gain which mucks with the feedback ratio..

[martin manning]

Does the DC from the bias voltage being on one of the lugs cause scratching in the pot?

I've never tried a PPIMV on a fixed bias amp. I assumed it could be done but I never tried it. I probably won't put one on the Vibroverb, I don't think it needs one, but I have other amps that would benefit from this addition. How does it sound on an Express?

Also doesn't this effect the negative feedback? There isn't a presence control but you are still playing with the PI/output stage gain which mucks with the feedback ratio..

Vbias functions like ground, so there is no scratchiness AFAIK. The only DC would be grid current, I think. Folks here have put Lar-Mar's (or "Modified Ken Fisher Type 2," if you prefer) on Expresses. Seems like it is a toss as to whether VVR or Lar-Mar is better. Yes, the presence control (if you have one) loses its effect at reduced volumes because of the change in NFB. These are known to work extremely well on JTM45's, etc. though.

[PaisleyTube]

I've used post pi mv's in reï brown Vibroverbs for customers without any problems; crancked sounds at lower volumes and the wobble-trem still working.

Weber does offer (small sized) tapped treble pots.

(Just finished working on a '65 VV clone, now doubting about a '65 VibroluxReverb or a '63 Vibroverb for personal use.....)

[Cliff Schecht]

The only thing that bugs me about the VV (and most all of the Brown/Blackface circuits) is the not so judicious use of preamp tubes. Most of the designs have 6 preamp novals which on one hand is fun to swap around tubes in but it feels like such a waste of my stash of good US made AX7's. I find myself getting more anal about only running these amps on 117V (to keep the heater voltages in check) with a slightly cold bias to try to preserve the nice sets of 6L6's I have.

[PaisleyTube]

use the good tubes for V2,3,4 and 6, cheaper/more common ones for the other positions, most guys only use the reverb-channel

[vibratoking]

...with a slightly cold bias to try to preserve the nice sets of 6L6's I have.

I've tried that in the past and came to the conclusion that I'd rather have new tubes in there with slightly hot bias. Cold biasing a NOS tube equals long life with less than stellar tone to me. Why use a stellar tube and cripple it?

[PaisleyTube]

Did play a real '63 VV some days ago at very low volume and it was still very, very nice. Warm, full bodied sound but still clear enough when the treble-control was turned down. Reverb differs from blackface-amps, a bit "smaller".

Couldn't dime the amp but it was great to play such very rare old Fender.

The cabinet-size differs from a bf/sf VibroluxReverb. I got the impression the VibroluxReverb cuts aesier through the mix at gigs used clean or softly overdriven.

[Cliff Schecht]

...with a slightly cold bias to try to preserve the nice sets of 6L6's I have.

I've tried that in the past and came to the conclusion that I'd rather have new tubes in there with slightly hot bias. Cold biasing a NOS tube equals long life with less than stellar tone to me. Why use a stellar tube and cripple it?

Looking at my measured voltage chart (I actually took time to map this amp out completely) shows I am actually biased maybe a volt or two too hot. I haven't checked the bias on tubes in this amp yet, there are other problems to troubleshoot, but I think this bias is fine and just need to find which tubes I like in here. I found a few sets of 6L6GB's I bought and forgot about over the summer so I'm going to roll those through the amp once I figure out why my V1A plate voltage is too low.

All of the typically problematic features in this amp (trem, vibrato) work fine in this amp, it's the normal channel that is acting up for some reason. I think it's a bad solder joint, I'm about to chopstick the amp and find out.

[mmmoser]

Hi- I have attached a few pics of the Brownface VV I build several years ago. Its been rock solid all this time. When I first built it I found I really had to run it with a cool bias to get the tremolo sounding right. I built it with a chassis and faceplate and cabinet from Marsh amps. I build my own eyelet board. I used Allen amps transformers and choke for this amp. Good Iron from Heyboer I believe.

[Cliff Schecht]

I figured out a few of the issues with my build tonight. The biggest problem in the bright channel was the lack of volume. As I said before it sounded great, there just wasn't enough. I accidentally flipped the 82k and 22k resistors on the bright channels second triode which meant I had 22k as the shunt instead of 82k. This was dropping about 4x more signal than it should. Now I can crank the volume and it's nice and loud.

I also chased down a COUPLE of leaky caps in this build, or at least some suspects. One in the phase inverter, one in the send to the reverb volume and one on the normal channels tonestack. Two of these were causing scratchy pots and the one in the phase inverter looked kinda pathetic to begin with, I should have used a known good part there. I think this and replacing the ancient 1 Meg pot in the normal channel should fix things up on that channel.


My last problem is chasing down the hum in the reverb circuit. I used a bus-bar style grounding scheme but grounded the RCA jacks to the chassis (the side of the chassis that isn't close to the buss bar). I'm thinking this was a mistake as the return ground for the reverb has to go through a lot of chassis before it sees the filter cap ground. I'm not sure if isolating the jacks will help but I do need to get an appropriate tank before I do too much debugging here. The one I have is out of a Wurlitzer and the impedances aren't anywhere near right. The reverb works but sounds lame, not Fenderish at all.

I also mentioned in another thread that Martin's pseudo tapped pots do work but the bass control doesn't seem to have a lot of effect. I can hear it doing something when I sweep the pot but it's subtle at best. I'm not sure what the frequency response of this setup is supposed to look like though so maybe I should double check my wiring and run some sims before I talk too much smack about this circuit!

[martin manning]

I also mentioned in another thread that Martin's pseudo tapped pots do work but the bass control doesn't seem to have a lot of effect. I can hear it doing something when I sweep the pot but it's subtle at best. I'm not sure what the frequency response of this setup is supposed to look like though so maybe I should double check my wiring and run some sims before I talk too much smack about this circuit!

Cliff, good to hear you are getting it sorted out. The response on the bass side using the pseudo tapped pot is bang-on the original. That tone stack just doesn't have much muscle there.

[Cliff Schecht]

Martin thanks for looking into this. I will probably tweak on this a bit after I break in the amp for a while. It seems like the treble control dominates the bass response as well as the treble. I can get some great cleans out of it on the bright channel, I just find it hard to get the knob to a good balance between not bassy enough and too bassy/muffled. The sound is in there, the control is just a bit touchy. I'm not sure if the pot is linear or log but either way, as long as I can find the sound I'm after I don't care.

Also I chased down the reverb hum after reading a great post by Bruce of Mission amps. He pointed out that the original Fender grounding scheme worked for many many years so why muck with success. Since there is no way I'm getting a proper brass grounding plate in there, I added in one of those slip on metal washers with a long extension that allows you to bring the chassis ground around the side of the pot. I added one of these just on the reverb pot which I've read is susceptible to hum. Grounding to pot body helps the cover act like a shield, not an antenna. This plus adding in a ground run from the RCA jacks to the preamp right by where the reverb is grounded seems to have cleared the reverb hum right up. There is still a slight hum but it's nowhere near worth chasing down now. I'll bet some of that is just rolling tubes to find some low noise AX7's.

So the only real problem left is the normal channel is still scratching like a mofo. I've tried all of the obvious fixes (new tubes, new coupling caps, new volume pot) and still can't get the normal channels volume pot to not scratch badly. I'm not sure why it's still scratching but for now I'm just not going to dick with it, the bright channel sounds good and dandy as is.

[martin manning]

It seems like the treble control dominates the bass response as well as the treble. I can get some great cleans out of it on the bright channel, I just find it hard to get the knob to a good balance between not bassy enough and too bassy/muffled. The sound is in there, the control is just a bit touchy. I'm not sure if the pot is linear or log but either way, as long as I can find the sound I'm after I don't care.

The taper of the pot doesn't screw up anything in the frequency response using the pseudo tapped pot, just changes the setting needed to get it- I tried a linear taper in the sim to confirm, but the treble does come up quick. If it's touchy then maybe it is a linear pot you've got in there. If you were to swap in a tapped pot at some point, then we'd know for sure if the circuit I suggested really sounds the same or not... :^)

[Phazor]

I have a brown and a Black vibroverb. They both had no cabinets but are oringinal 1963 and 1964. The Black I've had for 40 years and its been worked on many times but I always kept original parts and have returned it to original.

The Brown Amp had a burnt up Power transformer and missing the output tranny. I ordered Mercury Transformers and put it back together. All the parts on the eyelet board are the original.

I love both these amps but the Brown is my fav. The change to 12AX7 do give more gain and hence the 82K/22K on the plate in the bright channel (same as ken fisher komet values). There is something about the tone with a Tonebender that is great on the brown for Hendrix stuff. Then you bypass the bender and you have a great fender tone.

One thing since both these amps are in head cabinets, The brown does not like the Reverb Pan laying next to the chassis in a head cabinet. The black tolerates that a lot better. I don't know if that has to do with the 12AX7 in the reverb drive or the PI 12ax7 or what but the brown is far more sensitive to hum in the reverb return than the Black.

If your going to build a cabinet head for the brown, I would suggest you think about building it tall to get the pan in the bottom as far away as possible. This also suggest you might just build a 2x10 combo and use some alnico to keep it light while setting on your main cabinet.

I want to build a combo cab for the brown vibro like the one that appeared earlier.

mmmoser, Great Job on the new build. Great Cab.

The vibrato circuit is much better sounding in the brown but as mentioned already, you have to bias colder for it to have intensity and that takes away from the all out sound of the amp. I have always preferred a hot biased amp for tone. It a trade off. I added a trimmer in the brown so it could be biased because the original never had an adjustment. I suppose a 2 stage bias with a relay could be added to change the bias for tremolo settings but you would want to make sure its safe so as not to red plate the power tubes if something screws up in the circuit.

All that said I have done more recording in playing with my black Vibro and I dont think I could ever sell that amp. I play it thru a 2x12 cabinet with H30 celestions from 1970 the same time I found that amp head in pieces and they have both held up well. Except for a few nights on stage something went south. I have never had to replace transformers on the black. When I want a bigger amp sound I use my 50watt marshall. It gets used less and less but its been a great amp as well.