Dumble #33 Schematic

[andyhardy]

Was going thru some files tonite not sure if I got this here Interesting loop

[Max]

Infos and pictures from Rob Livesey's site http://www.roblivesey.com/dumble/

"Dumble Overdrive Special 50W. #0033

This amp is early, has Fender Bassman power and output transformers and a Super Reverb choke. Filtering is as per Blackface Fenders as is biasing. All caps are Sprague Atoms and Orange Drops. The speaker is an EV and the cabinet is hardwood. Construction quality is excellent.
This is one of around 25 Dumble amplifiers that were imported to Germany in the late seventies and early eighties. Dumble had a deal with a shop in Bochum called "Applied Acoustics".

This shop imported the Boogie amplifiers at that time as well. Dumble shipped only the amp chassis, some even without transformers to reduce expensive weight on the transport from Santa Cruz to Germany. So all the cabinets of these 25 amplifiers are not made by Dumble but by "Applied Acoustics" in Germany. The dimensions of the cabinets are the same as the original Dumble cabinets, but the material is hardwood. Dumble himself covered his cabinets with tolex or suede but he never made hardwood cabinets for his amps. Many of these 25 imported amplifiers have been modified by their owners. Effect loops were added, reverbs were added, some of these 50 W amps have even been changed to 100 W and so on.

So, this amp - Serial no: 033

a) An additional tube has been added between the two power tubes for an active effect loop.
b) An additional pot is added on the chassis to adjust the gain of the fet input.
c) The pots for treble, middle and bass are replaced.
d) Some capacitors in the tone circuit are altered.
e.) On the front an additional switch right of the "accent" switch is added."

Cheers,

Max

[talbany]

The loop doesn't really look like an effects loop.. (no buffered send)
Actually looks like a separate OD or very dirty rhythm channel (not exactly sure if it's foot switched in)...I really don't believe it would sound very good with the OD on and send and return jumper-ed..JMO

BTW... The 20P cap across the 220K OD entrance network is much to small to have any real effect on a possible oscillation issue in OD..I would think you would need something in the 200-500 range..

Tony

[ToneMerc]

Isn't this the unverified, possibly bogus #33 schematic that the Rumble? pedal maker was passing around years back.

TM

[talbany]

Isn't this the unverified, possibly bogus #33 schematic that the Rumble? pedal maker was passing around years back.

TM

TM
Yeah I saw those years back but I don't think so..

T

[Rob Livesey]

This amp has been around the block a few times.

The first three pictures on my site, I took myself at my house, I had this amp at home for a while to sell it for the owner (current owner of #155). The rest of the pictures came from somewhere else but it's so long ago I'd have to wade through lots of stuff to find out. I sold the amp to a guy in Japan for about £3.6k, and I shipped it. That's maybe where the other pics came from. I don't recall anyone going through it to draw a schematic whilst it was in the UK, but I can ask the previous owner if he let someone else inside it.

The description posted is mostly information from a guy in Germany who knew the amp as well.

Rob

[andyhardy]

Thanks for the background info on this

After looking at the pictures the schematic matches:
-added volume on the fet,
-extra tube, send and return knobs on the back

-twisted wires on the bottom of the od network going to a switch on front

Maybe this was drawn up by the person that did the mods

[FUCHSAUDIO]

I was given this schematic a number of years ago by the Rumble pedal guy (at a NAMM show), who insisted it was drawn from the actual amp. It's obvious Dumble evolved the circuits over time. The Rumble guy said he had cloned it for himself, and even sent me cut sheets on the different FET's he tried using in that circuit, telling my which was his personal favorite.

The loop is odd, although not phase inverting, which is nice. It would appear it only works on the clean channel. The od is weird as well, as it has no output level control.

[Max]

It's obvious Dumble evolved the circuits over time.

"This is one of around 25 Dumble amplifiers that were imported to Germany in the late seventies and early eighties. Dumble had a deal with a shop in Bochum called "Applied Acoustics".

[...]

Many of these 25 imported amplifiers have been modified by their owners. Effect loops were added, reverbs were added, some of these 50 W amps have even been changed to 100 W and so on.

So, this amp - Serial no: 033

a) An additional tube has been added between the two power tubes for an active effect loop.
b) An additional pot is added on the chassis to adjust the gain of the fet input.
c) The pots for treble, middle and bass are replaced.
d) Some capacitors in the tone circuit are altered.
e.) On the front an additional switch right of the "accent" switch is added."

Source: http://www.roblivesey.com/dumble/

The od is weird as well, as it has no output level control.

2nd generation OD output level control?: https://tubeamparchive.com/viewtopic.ph ... 259#140259

Cheers,

Max

[wicker]

Sorry for digging that old thread, but would this kind of loop (with CF on IRF 8xx) would work with reverb/delay pedal ?

[FUCHSAUDIO]

No reason it wouldn't work.

[wicker]

Ok, bad question, sure it would work, but does anyone have any opinion if it would sound good ?

[Structo]

If you only want to run one reverb pedal on the amp, the passive effects jacks will work pretty decent.
Of course it depends on the pedal and impedance's.

But, I ran a Boss RV-5 for a while in my passive loope before I built a Dumbleator.

[FUCHSAUDIO]

Like Structo said: It depends on the external device more than anything in the stock amp. The impedance and send and return signal levels etc. This loop would work with most external stuff. Remember, the master in most D-styles is 1-Meg, and at low settings that series resistance can really hurt signal levels and sound quality. On the return side, the power amp wants a volt or so to hit clipping, and some effects (especially pedals), may need help (gain).....

With this loop: It's send impedance is higher than a cathode follower, but it has the advantage of having gain if you need it. Usually, you have plenty of signal going out and don't need gain, but the lower impedance is nice, especially for solid state external devices. The return amp is simple, and not too different from the standard D-loops etc. and has gain, which (with pedals for example) is an advantage.

I tell people to try a passive loop first. If you need to add a loop, the sound difference should be obvious. Then you either build or buy one.