SRV's Vibroverb dissected surprising mods.

[soldersucker]

Read an interesting(to me) article in Tonequest magazine about one of his Vibroverbs.They waste alot of type describing the Meth-heads that stole it and the deal that brought it to another guy's possesion.

When they do get down to the nitty gritty details some unexpected(to me)things were done by Mr.Dias(RIP)

• -Orange Drops with .033's in tonestack
• -A 2 ohm Output tranny feeding an 8 ohm EV
• -Groove tubes SS pop in rectifier
• -33k vs 1 meg BF values in the phase inverter
• 220mf main filter's

There is more if you read the full story http://www.cpthorntonguitars.com/TQRAug07.pdf.Funny thing is they claim the amp would be useless to 99.9% of us normal folk.Three years ago i tripped over a very cheap and severely cosmetically brutal Super Reverb and like many made myself a Vibroverb.Not to sound like SRV mind you but to have a different flavour as i already own too many Supers.

The way his was dialed obviously suited him.Loud Clean and roadworthy....YMMV

[Timbre Wolf]

The tube -> s.s. rectifier change makes sense to me, for the "loud, clean and roadworthy" mods. But, could you please briefly explain what the other several mods would each typically do to the performance of the amp? Thanks.

- Thom

[TweeDLX]

This is my take on it:
.033's in the tonestack=more low mids, more bass frequency drop-off in the lower registers - less mud
2 ohm OT would have to work a little harder to drive the 8 ohm speaker, maybe get a little closer to output tube distortion.
SS rectifier seems to go with the 220uF filter caps = really tight bass (one of SRV's trademarks)
Don't know enough about PI's to conjecture what the 33K's added.
OK, tear me a new one...

Mike

[wichita]

Stevie had a little to do with it guys. I heard him through tons of gear. He had that in his hands, Diaz helped. Stevie was a good man with great tone through anything....

[Timbre Wolf]

Originally Posted by TweeDLX

OK, tear me a new one...

Or not.

Satisfied my curiosity. Thanks Mike!

- T

[soldersucker]

Originally Posted by wichita;6183677[U

]Stevie had a little to do with it guys[/U]. I heard him through tons of gear. He had that in his hands, Diaz helped. Stevie was a good man with great tone through anything....

We all know that but you must also realise he wanted to hear some thing's out of his amps to let us enjoy those wonderfully talented hands.From interview's Ceasar gave Stevie wasn't a passive spectator when it came to his tone hence the mod's which may be considered pretty major when done on a pre-CBS Vibroverb.Now back to your Peavey Bandit's

[hasserl]

Originally Posted by TweeDLX

This is my take on it:
.033's in the tonestack=more low mids, more bass frequency drop-off in the lower registers - less mud
2 ohm OT would have to work a little harder to drive the 8 ohm speaker, maybe get a little closer to output tube distortion.
SS rectifier seems to go with the 220uF filter caps = really tight bass (one of SRV's trademarks)
Don't know enough about PI's to conjecture what the 33K's added.
OK, tear me a new one...

Mike

Re 2 ohm OT into 8 ohm speaker: the OT doesn't drive the speaker. The OT reflects the impedance of the speaker back to the tubes, it transforms the low impedance (8 ohms) to a high impedance (~16k in this instance). In this case the tubes see a reflected load ~4x greater than optimal for best power production and bandwidth. Power production is limited and bandwidth narrowed.

On the 33k PI resistors, I recall an experiment one of the guys on Ampage, John Kelly Brown, conducted with these resistors connected to a rotary switch with several different values, and he could not detect any audible difference at all when changing between values, though he didn't go as low as 33k. I'm not sure what the effect would be, but for an interesting read (IMO) here's what I have saved from that thread:

The grid resistors.
Jeeeze. Can someone honestly tell me they hear something here? Here's where I embarrass myself with something probably everybody can hear, but I have to give up because I'm just not getting it. I have it set up so I can select between 680K, 820K, 1M, and 1.2M, I wasn't hearing the difference. Now, sometimes I don't hear the difference with something subtile but eventualy learn that I was listening for the wrong thing, then it slowly becomes more apparent, like the PI cathode resistor value. So I listen closer, then I listen even closer, then I give a determined listen even closer yet still, I listen so hard my eyes cross, sweat is running down my face and all I can see is that forsaken rotary switch for changing these damn values! Ok, back off, relax, that's it, repeat the mantra, "you are a tone god, you can hear all, you are a tone god, you can hear all...". Then I calmly turn the volume knob on the Strat off, move it to the other side of the room, crank the amp up as Nigel taught us so there's enough white noise, then go to that damn rotary switch and listen the way I would if I were a tone god and could hear all. Oh I'm hearing the white noise alright, and the sound of a rotary switch going through it's rotation, BUT NO FRIGGING CHANGE IN TONE WHATSOEVER!!! Lotta good that mantra did me! So you guys who can hear the difference, can you really hear the difference, even in a double blind test would you hear the difference? This isn't even a blind test, I'm looking at the matriarcal forenacatin' switch move and I'm hearing the imperfections in the switch as it goes through it's rotatin' hitting all the four positions, but the amp tone just doesn't change! Ok, if I can't hear it, and it's my amp, who cares, I'll divine one of the four choices by pulling a number out of a box. And it's not like it's dependant on the other values in the PI, I went through this agonizing process in association with varying the values of all the other resistors in the circuit.

[Timbre Wolf]

Originally Posted by hasserl

he could not detect any audible difference at all when changing between values, though he didn't go as low as 33k.

I wonder if there may have been changes in feel, rather than actual "tone" changes. If that were the case, a closer listen would not be helpful. Rectifier tube swaps, for instance, are more detectable as a change in the touch response of the amp, rather than frequency range amplitude increase/decrease.

Just theorizin'...

- T

[jellewelagen]

Well, the article says that 'the 1M grid resistors in the PI are changed to a later 70's value that gave the amp more clean headroom.'

If you put 330K there, the PI will not break up as much.

Does anybody know what type of orange drops were used by Diaz?

Jelle Welagen

[itkindaworks]

Cool find.

[strat a various]

This amp changed hands a couple times. Speaker could have been changed, as in a blown EV replaced with a similar speaker, different impedance. Also, maybe SRV always used an outboard cab or cabs with the internal speaker ... extension cab could have held speakers totaling a sensible load to pair up with an 8 ohm speaker.

[Vibrolucky]

There is no rhyme or reason to alot of amp used back in the day....people did not have these rules that the "guitar police" enforce....Alot of guys just made do with what they had and some liked the results. For several years, I played a Bandmaster Head with 2x12 Cabinet that was wired backwards. I had two Celestions G12-75's (Guitar Police say wrong for this amp) that were 8 ohms each, they should have been wired to achive 4 ohms.

I screwed up not knowing how to do it and wired them for a 16 ohm load, which BTW I played with an instument cable instead of a speaker cable.

All these things seem "wrong" now, but when I played the amp it sounded fine to me.....Hell, I even remember getting a couple of compliments on the tone in a couple of bars.

Point is, with an old amp there is no telling what anybody was thinking....could have been an emergency repair that was never changed or just an accident.

Seems like a few things cancel each other out, i.e., 2 ohm transformer to get more overdrive - yet it has a solid state rectifier. Alot of the mods done by Cesear Diaz were to make the amps more Roadworthy and last longer.....Especially since SRV liked (alledgedly) liked his amps biased on the Hot side beyong recommended settings for tone purposes....

Who knows really?

[jay42]

Originally Posted by Vibrolucky

....could have been an emergency repair that was never changed or just an accident.

Yup...sounds like an attempt to create a loud/clean amp for his multi-amp setup. They would have been better off starting with a twin reverb imo. Probably didn't have the time/money/inclination. Since it didn't stay in his estate, I'd guess it was deemed 'a nice try, but.'

[Bobby D]

yep...that's DIAZ.....RIP brother Cesar, you were well loved.

[tcmono]

50% chance that this was the Vibroverb that drove the Vibratone speaker.