HELP ! Build of a Vibroverb on steroid

[Tube1]

Hello everybody,

I project to build a Vibroclone which includes major mods to achieve kind of Vibroverb tone on steroids
I plan to use a silverface bandmaster and convert it , using chassis and also the wood from his large cabinet to build an accurate Vibro cabinet period correct.

Here is the heart of project : I would like to achieve with my clone the same sound as we can get with an amp coupled to a Kingtone Blues Power pedal which is made to replicate a blackface pushed hard on tubes , however the tone with this pedal is way more than that and sounds beyond belief , it is a Vibroverb tone but bigger , fatter, juicy , low mids and still with ton of chime and glass. It's like the whole Stevie's stack of amps in just one amp !!

The thing is I don't want to use a pedal because it's just full of print and chips , I am sure it's possible to tweak and elaborate the AB763 circuit to get that fabulous tone naturally or closer .
Here is a link of the tone I am refering and despite the title , it's all from a Blues power pedal
https://www.youtube.com/watch?v=OMxzXkz-5A8
And here the link of the Blues power schematic and components list
https://aionfx.com/app/files/docs/duran ... tation.pdf

I played trough an amp with a B/P pedal and it's absolutely stunning !

The SRV sonic possibilities with that pedal are incredible , you have that specific 1983 ACL concert tone or famous Mocambo tone or even that 1985 soul to soul session tone in a fingers snap , it's just the combination of toggle switches and EQ.

My question is how to get the same result from a tweaked blackface circuit ? What components and values to change ?
Before continue I have to mention I owned in the past a real Vibroverb/Altec 15" modded with some SRV tips and it sounded glorious and more consistent , however when pushed hard , the tone became high mids /muddy and kind of collapse in some way . This is where the B/P pedal is fantastic , you can counteract that to keep a huge GLASSY and bassy tone at high volume, his EQ allow to beef up all frequencies.

I think the leads are basically usual SRV mods (already well documented) and here the main issue, a more elaborate EQ design than the usual blackface Bass/mid/treble option value.
The pedal clearly expore enlarged range of frequencies so the idea is to "open" the range of the original Fender EQ. More bass , lowered mids BUT still keeping the clear and glassy tone.

Here are my questions and what with common sense I think are the direction to follow, but ANY suggestion are welcome
How to implement in the circuit a controle pot to increase the "Glassy" effect in tone ? (Glassy is reffering to a boost of treble in specific range of frequencies)
How to enlarge the Low mids frequencies band ?
The pedal use toggle switces ( 4) to voice it differently , is it pertinent to implement this in the amp circuit ?

I am not qualified enough to redesign an AB763 circuits and I would be grateful to have the help of skilled technician in the forum , it would be so helpful for all of us sought after this tone.

Any constructive suggestion or comment are welcome
Thank you very much

Mark

[stratomaster]

There's a Tone Quest article whereby the author gains access to one of Stevie's Vibroverbs--through less than above-board means. This makes the article hard to find online, but it's out there.

They document Cesar's mad scientist mods to keep the amp tight at high volume. Transformer changes, smaller PI coupling caps, and cathode network changes were done if memory serves. The conclusion of the author was the amp was basically unusable for anything but loud, bright, tight tones that Stevie used it for. Keep in mind he almost never used a single amp, so the Vibroverb is one ingredient in the tone--not necessarily the main ingredient.

All that to say, you'll probably have to go a very different route to what was done to Stevie's amp to end up closer to a tone that you're after. I can see splitting the cathodes of the 2nd stage, increased plate resistance, perhaps converting V1 for use as a cathode follower to drive the tonestack for less signal loss (complete with a lower value cathode resistor to increase compression) to mimic the note decay characteristics of high volume operation. Another option is to revoice the normal channel to complement the tighter modified tone of the vibrato channel, splice them together pre reverb to put them in phase then blend the two to get the effect of two amps being played at once.

[ThePanda]

There's a Tone Quest article whereby the author gains access to one of Stevie's Vibroverbs--through less than above-board means. This makes the article hard to find online, but it's out there.

They document Cesar's mad scientist mods to keep the amp tight at high volume. Transformer changes, smaller PI coupling caps, and cathode network changes were done if memory serves. The conclusion of the author was the amp was basically unusable for anything but loud, bright, tight tones that Stevie used it for. Keep in mind he almost never used a single amp, so the Vibroverb is one ingredient in the tone--not necessarily the main ingredient.

All that to say, you'll probably have to go a very different route to what was done to Stevie's amp to end up closer to a tone that you're after. I can see splitting the cathodes of the 2nd stage, increased plate resistance, perhaps converting V1 for use as a cathode follower to drive the tonestack for less signal loss (complete with a lower value cathode resistor to increase compression) to mimic the note decay characteristics of high volume operation. Another option is to revoice the normal channel to complement the tighter modified tone of the vibrato channel, splice them together pre reverb to put them in phase then blend the two to get the effect of two amps being played at once.

Is this the one, ToneQuest August 2007?
https://www.thegearpage.net/board/index ... ds.557026/

[Tube1]

Hi ,
Thank you very much both for your replies,
I used to modify my Vibroverb as per the tone quest article back then but not all the mods . However I used a master volume to achieve a cranked tone at low volume. Sounded great but nasal and muddy at high volume.
I agree with stratomaster, the way Stevie´s amp was modified was to be a specific piece of his big sound chain.
The Blues power pedal amazingly reproduce the whole sound chain in one amp.
Your proposition is really pertinent however I am afraid not be qualified enough to do this modification or more precisely designing such circuitry.
If I understand , you would tweak the normal channel and add both preamps to beef up the tone of the vibrato channel ? Or rather increase the gain ?
I would like to ask about the modification of the equalization stage .
How to add bands ? I mean something like two or three knobs for each basic tone control. Each knob would be a band frequency. Or why not using the Dumble SSS equalization with his basic tone controls and the famous high/low step filters ?
And how can we alter the circuits to add or boost the « Glass » effect ?
Finally the layout would be according to all ideas an amp based on AB763 circuit, we need more growl and gain or more likely « tube pushed on limit » with that saggy tone
An equalization like in the SSS to increase and enlarge the sonic possibilities and a way to control and increase the natural glassy tone .
Why not using a 1x15 added with 2x10 for treble ?
It would be kind of all in one in the SRV amp tone field with better tone control than the regular Blackface.
What do you think ?

[romberg]

I agree with stratomaster, the way Stevie´s amp was modified was to be a specific piece of his big sound chain.

Nah. SRV's sound was 99.5% in his hands. The rest was in a big pile of random amplifiers all set on 10. See Bruce Egnater's first hand account of working on this pile of amps here:

https://ampgarage.com/forum/viewtopic.php?t=30133

Mike

[Tube1]

Hello ,
Thank you for the interesting link , I agree totally and understand what you mean.
Of course Stevie ‘s hands were the major Parts of tone but tech side we can approach his sound with well prepped amp and having fun. It’s just an interesting quest and passion to try improving an amp to a personal taste.
I think it’s the aim of this forum.
My question is how to tweak and implement in a blackface a more complex equalization to reproduce the sound of the Blues Power pedal engaged and still retaining that specific articulate and chimey blackface tone.

M.

[Roe]

I'd get a twin or a dual showman

[stratomaster]

Hello ,
Thank you for the interesting link , I agree totally and understand what you mean.
Of course Stevie ‘s hands were the major Parts of tone but tech side we can approach his sound with well prepped amp and having fun. It’s just an interesting quest and passion to try improving an amp to a personal taste.
I think it’s the aim of this forum.
My question is how to tweak and implement in a blackface a more complex equalization to reproduce the sound of the Blues Power pedal engaged and still retaining that specific articulate and chimey blackface tone.

M.

If you're getting where you want to go with the pedal, then keep using the pedal.

If I were modifying a Blackface clone to do the Stevie thing I'd probably leave one channel stock, except for maybe a 2.2μF cathode cap in the 2nd stage, and put a basic Dumble clean TMB preamp in the other. No local negative feedback or SSS filters or any of that--hardwired to Rock. I'd use mix resistors to tie them in at the reverb input. Put the Intensity pot on a switch to lift it from ground. Then I'd add a lar mar master volume and shrink the PI input cap to 680pF and the PI couplers to 0.033μF.

You'd probably need to tweak some more, but I think having the ability to blend those two preamps into a minimally modified Fender power amp, crank them, then tame them with the MV would get you very close. And if not, then you have a very versatile amp on your hands and you can plug the pedal back in.

[Tube1]

Thank you very much Stratomaster ! Well appreciated
Will have a try inspired by your advices