Vibrolux Reverb with Dumble mods

[dbeasley]

Greetings everyone, thought I would share a Dumble inspired project I've been putting time in on lately.

This started life as a SF vibrolux reverb that had been what I like to call "Torre-fied" : someone had gone through and done pretty much every godawful Torres mod to this unit, and did the mods poorly. Could have even been Dan Torres himself.

Along the way, someone installed a much larger fender output transformer with 2/4/8 ohm taps. I've never seen a factory trafo (looks bassman sized?) With those multiple taps - but counting the turns ratio got me those impedances for a pair of 6L6.

The original board was beyond hope, so I had a new Formica one made (big shout out to fellow forumite Doxiefan for the boards) and decided to do a bunch of fun D style mods.

Normal channel is essentially high plate skyliner, vibrato is now low plate classic a-la #124. Both have verb and tremolo, which I've converted from the harsh (IMO) opto tremolo to the earlier brownface Tremolux setup - that modulates the bias voltage. Both preamps are wired in rock mode, and no longer share a single filter cap. Couple of small tweaks to the stock fender Verb circuit, and using the Tweed/Bluesmaster PI setup with Dumble style master on the back. Someone had already drilled extra holes in the back for individual channel masters (yuck) so I reused the holes for a midrange control for each channel. I also ditched the standby switch (unnecessary with tube recto IMO) and moved the AC/power switch over to the old ground lift hole to move more of the AC wiring away from the rest of the amp. I also reused the old 3 way ground switch to toggle between 3 nfb settings (3.3K, 56k, no nfb) and reused the pull feature on the old master as a bright control.

I added PAB to the low plate classic via push pull pot on the volume control. The treble control for each preamp is also a push-pull that engages the Mid switch that's common on many Dumbles.

At this point, it's kind of a hybrid of the clean aspect of the Ultraphonix/ODS and Bluesmaster. I tried to keep the layout as clean as possible and didn't want to drill any extra holes. It will all be housed in a nice Vibrolux cab that's had a 1X12 baffle installed. I recently found a really cherry JBL D120F speaker for this project.

At this point I just need to clip in the sub boxes for the dropping string and get my voltages correct/bias.

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[norburybrook]

Fantastic I have a 1970 Bandmaster reverb here that's just itching for this treatment.

It's 100% original and working great so I'm slightly reluctant to start modding it.

Do you have a layout/Schematic?

M

[dbeasley]

Sorry I don't. Replacing preamp values with Dumble values is pretty straightforward however.

The trickiest part was implementing the tremolo. The board was drilled for the opto roach so I just studied the brownface non harmonic tremolo Tremolux schematic and made the layout as I went. It wasn't terribly hard though.

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[fred.violleau]

Great project!
I would be interested on a schematic too!

I have a similar project on mind: I recently bought an old Silverface to mod it so I can get a Tweedle De on channel 1 and a Bluesmater clean on channel 2. I will also try to make these jumpable.

I am juggling with the idea of adding a selector for the tone slope of the Tweedle Dee channel to switch from tweed to more modern values (47k, 68k, 100k and 150k$) or adding the SSS filtering system. Not decided yet.

I though about improving the tremolo as well.

Finally I am hoping to be able to insert a switch that allows me to choose from a diode or a tube rectifier.


Humble and Dumble,

Fred.

[M Fowler]

Nicely done sir.

Those transformers were what I used 2/4/8 before ClassicTone MC transformers became available through dealers in many different options.

Love the boards and wiring job sir.

Mark

[dbeasley]

Nicely done sir.

Those transformers were what I used 2/4/8 before ClassicTone MC transformers became available through dealers in many different options.

Love the boards and wiring job sir.

Mark

Thanks Mark. If it wasn't for people like you enabling me I wouldn't be building stuff like this today!

Were these OTs a generic fender part available to purchase for repair folks? I've never seen one like it.

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[dbeasley]

Regarding a schematic - it's really as striaghtforward as using high plate skyliner clean preamp in the normal channel, low plate classic in the vibrato channel. I used #183 and #124 respectively here.

I mixed the outputs of both channels right before the 500pF cap that splits the wet/dry signal so they both get reverb and tremolo.

For the tremolo, you need a 250k linear pot for the intensity. Really the biggest difference is the trem isn't entering the signal path at the wet/dry mix with a roach, it's using that 2nd tremolo stage to modulate the bias of the power tubes. I just realized that this is also what the Princeton reverb does as well so could be good to look at layout idea for it from there - but easy enough to rearrange a few components on a stock AB763 layout.

As fat as the power supply, I did make a few changes as well. I opted to do away with the pair of 20uF mains filtering in parallel and went with a 47uF 500v F&T cap. This allows me to use the other cap spot to filter the screens, which leaves me an extra spot to give the normal and vibrato channels thier own filter node a la Overdrive Special. I also upped the screen filtering to 47uF as well, considering it's also filtering the tremolo and reverb driver circuit.



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[dbeasley]

Great project!
I would be interested on a schematic too!

I have a similar project on mind: I recently bought an old Silverface to mod it so I can get a Tweedle De on channel 1 and a Bluesmater clean on channel 2. I will also try to make these jumpable.

I am juggling with the idea of adding a selector for the tone slope of the Tweedle Dee channel to switch from tweed to more modern values (47k, 68k, 100k and 150k$) or adding the SSS filtering system. Not decided yet.

I though about improving the tremolo as well.

Finally I am hoping to be able to insert a switch that allows me to choose from a diode or a tube rectifier.


Humble and Dumble,

Fred.

Fred,

Those are a lot of options/switches! Do you have any resistor or cap substitution boxes? You might find it beneficial to experiment with tweaking the slope values that way - it always seems like someone settles on one or maybe 2 different values and just leaves them there most of the time. With these old fender chassis it's pretty easy to fit push pull pots to keep the options you want and still keep a clean look. Good luck with your project!

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[M Fowler]

From my notes I used to buy the OT's from Mojotone so I sure it was a Heyboer unit Mojotone 774 OT 2/4/8

MOJO774 Blackface Bassman Super Reverb Vibroverb Output Transformer.jpg

[dbeasley]

I just wrapped this one up and I have to say I'm very pleased. With the high plate skyliner side cranked up and setting the NFB to 100K at moderate volumes it has very good sustain. With a tube screamer or a Soul Food set clean you can easily push either channel into a very dumble-esque OD tones. The reverb is typically fender, however the audio taper reverb control gives more precision over the overall level of the reverb. The tremolo is smooth and gets slightly gritty when the intensity is cranked up very high. It even sounds pretty cool when both channels are jumpered.



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[dreric]

Impressive work......................

I have a Torres modified Bassman, which did sound good back in the day. Now there's something wrong with it and I can't make any sense of what Torres did so maybe I'll strip it and build it back up.

[hitchcaster]

is there anywhere to get that board material? looks cool... nice build.

[dbeasley]

It's 2 pieces of formica epoxied together. You might check with your local cabinet shop for scraps.

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[Rockwell666]

nice clean work!! almost every amp that comes onto my bench that has had poorly implemented mods or repairs. that's where the faults are.

[dbeasley]

Update - I finally got my cab in a few weeks ago. This is how she looks now

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