(Archive) REVIBE

[beatroot]

Hi all, I have been putting off finishing my revibe (hoffman style) because a lot of people don't seem satisfied with the hum and ground loop issues, and so i have tried to find out ways to overcome this and waiting for people who have already built the revibe to tell us their solutions. I came across a way of fixing the ground loop issues but wanted to see what you guys thought. I think the idea came from rikstone amps ( am pretty sure he contributes to this forum but not sure of real name) who puts two 5a silicon diodes in opposite polarity in front of the chassis ground, the idea being the threshold voltage of the diodes (~.6v) is much higher than the ground loop offset which is only a few mv, so loop is broken. Does this make sense?

Is it really this simple???

Rikstone, are you out there???

[triode]

I have found that the most effective way to kill the noise in the Revibe is to use Jeffs new design.
He designed it, but he went back and changed the front end of the vibrato section to increase
the signal to noise ratio... basically you need to mod it a little, but not much. I did this to one of mine,
and I can say that after this mod, you do not even have to mess with all of the "ground workarounds"
floating about on the net and in forums. The one I was experimenting with went from "barely acceptible
level of hum" to "is this thing on??" in terms of background noise and hum.

Note the changes to V1B in Jeffs schematic here:
http://www.webervst.com/revibe/5h15_schem.jpg
Compare that to the old one here:
http://www.hoffmanamps.com/images/revibeschematic.gif

Oh, almost forgot, it also helps if you use a bigger choke... like a 125C1A or a
bigger one (hammond 10H)... it does not use much current, so you do not need
to get a huge rating... just bigger than stock, this is noted on the new schematic.

Hope that helps some.


Almost forgot: all of the ones I have done after this mod seem to no longer have any
"ground loop" issued when plugged into any amp. Seriously, I am not sure why but I have
not had a lot of time to think about it either. After this mod, I have never run across an amp
that "looped up" when I plugged it in. So that is even more icing on the cake...

[beatroot]

Hi Triode, thanks for the reply. It was the assurance i was looking for. :)
Time to put it to practice.....

[john_t]

Triode, how hard would it be to make this revision on my completed Hoffman style revibe. I have already isolated the jacks and reverb input outputs. My revibe is pretty queit but not humless. Also will this mod affect the sound.

Thanks john T

[Tiny_Daddy]

If adding a larger choke helps, does this mean there should be an extra stage of power supply filtering? And the Weber schematic has that voltage divider of R50 and R24 which is going to cut the tremolo signal way down. Is this voltage divider a the reason there is less hum? Would anyone be willing to ship me their noisy unit so I can investigate?

[john_t]

Tiny daddy, I wish you were local I would have you look at my revibe in a heartbeat. I have my revibe fairly queit right now. I isolated the grounds and a local amp tech added so filters that made a big difference from the original. So at this point the only thing I have not done is this latest mode. I see the changes on the schematic and on the layout on the weber site. As mine is the Hoffman layout Iam not completely sure about the mods on the Hoffman style board. Maybe someone could show what the mods would look like on the Hoffman layout. I can follow the layout a little better then the schmatic. If I cantI will keep your offer in mine though.

Thanks John T.  Ps Greg at Vintage amp works made my narrow panel tweed reverb cab. That Guy is the best. Killer looking cabinet.  

[GroundhogKen]

Interesting.

The tremolo input stage was changed from a cathode follower with an AC-coupled input to a regular inverting gain stage.

And the power rail for the tremolo tubes was moved to the output side of the choke.


Ken

[GroundhogKen]

John,

Here's something to try with the Hoffman layout of the old Gehring schematic.

At pin 7 of tube V1 there are two 2.2 Meg resistors and one .047uF capacitor.

Change both resistors to 1 meg and change the cap to .01uF or .0047uF.

This will roll off the low end response (hopefully not too much) and make the input more immune to 60 cycle noise.


Ken

[john_t]

Ken thanks for the reply. That sounds easy enough. Also should I leave everything else as is. I wont be able to work on it for a few weeks. I will be in Montour falls for a class Iam taking all next  week. From there Iam going to Boston for a long Weekend.  ( MY Birthday) I will post Once completed.
Thanks much for the help.

John T

[GroundhogKen]

Hi John.

I would just try my little cap/resistor mod first to see if it helps.  Leave everything else they way it is.

If it doesn't work, it's easy to put it back to original.

Maybe I'll think of something else by the time you get back.


Ken

[tore_t]

Hi there !!

First a couple of word about myself, as I'm new to this forum.
I'm a norwegian mech. engineer and guitar-player, running my own tube amp repair shop on hobby basis.

I've been posting on the Weber amp building forum for some time now, and I'm  responsible for the last ReVibe revision... (Called "the Tore T mod" on the Weber forum)   8-)  

Let me explain how it works:

Changing V1B from a cathode follower to a "normal tube gain stage" brings up the signal through the vibrato cirquit  to a level  35-40 times higher than in the original ReVibe design... Bettering the signal-to-noise ratio by the same amount... And bringing the signal level in the vib circuit closer to the levels found in the original brown Fender amps.

(I realized this after trying to understand why my '62 Pro was totally noise free without any shielded cables or magic grounding etc, while the ReVibe seemed to pick up tons of noise, even after lots of shielding and trying different smart grounding schemes...  The signal level in the original revibe was BELOW the guitar output level, where it is amplified 50-100 times before the vib cirquit in the Pro...)

The R50 / R24 voltage divider brings the "vibrated signal" down to "normal guitar level" again, but it DOES NOT CUT THE VIBRATO LEVEL....! It even adds the possibility to tweak the overall gain of the unit, by playing with these resistors.

It's no magic at all, it's just bringing the operation a little closer to Leo Fender's original cirquit...!

It should be no problems to mod the Hoffman Layout to the modded schematic either....

The revibe still needs close attention to the wiring layout in the vibrato oscillator cirquit even with my mod, to prevent ticking, oscillation and "putt-putt"-noises... But hum and hiss is guaranteed to disappear with the mod...!

Good luck and best wishes from Norway,

Tore T

Here's the link to the modded schematic once more:
http://www.webervst.com/revibe/5h15_schem.jpg

[GroundhogKen]

Thanks Tore.

It's nice to have your insight about this issue.  I noticed your name in the revision notes of the updated schematic.


Ken

[john_t]

Tore T ,Welcome to the Hoffman Forum. This a great site. Everyone here has been a great help to me as I have limited skills in this field. I do however enjoy it a lot. Iam away at school this week so I cant fool with my revibe. Lucky for me they have internet access. I have read your post,s on the Weber forum. I going to try Groundhogs ken,s suggesstion. If I do the voltage divider r50/r24  What would that look like on the Hoffman layout. Could you post that mod. I have the schematics with the new change.

Thanks John t

[tore_t]

Hi again !

Here's a suggestion for my mod on the Hoffman layout...!

http://www.triple-t.no/tore/gmf/tore-t-hoffman-revibe.jpg

Please note that I have not changed anything in the power sypply, but I think that my mod alone
should be enough to tame hum and hiss in the vib. cirquit..!

Good luck !

[EL34]

Thanks Tore,
If someone here does this to a hoffman re-vibe and it does the trick, I'll change the re-vibe layout on my web site.

I may test it on mine if I get time.

[GroundhogKen]

Should we archive this one, too?

Ken

[EL34]

The end story on my re-vibe is that it is dead quiet after doing Tore's mod.
see the library page for info on the mod.