new build choice, 2nd gen 50w

[talbany]

Not from an actual Dumble.

unless there is another 33 without a loop
Posted by Just the facts MAX

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."

[erwin_ve]

That Od entrance on the #33 schematic is looking different. Is that correct or a drawing error, Tony?

[erwin_ve]

Marcus, the PI tube: did you use balanced ones? It can make a huge difference.

[norburybrook]

Marcus, the PI tube: did you use balanced ones? It can make a huge difference.

No I didn't Erwin, neither the AX or AT I used were balanced. I had a mix to finish yesterday so didn't get chance to try my NOS GE AT that arrived in the post.

I've a day off today so I might even make a start on the cabinet as the sun has just come out...the first time since last September!!!!!

Oncew the cab is done I can start using the amp properly and I'll do some serious testing.

As it's a combo I'm seriously thinking about adding either a buffered loop or spring reverb. My main concern at this stage of the proceedings though would be drilling out the hole for the extra valve. I'd be worried about swarf getting into places it shouldnt.

I could make a small dumbleator and put that in the bottom of the combo with vecro. Or my other thought is to build a reveibe, as I love trem and spring verb, it's like carrying another amp though.....so many choices....so little time.


M

[talbany]

That Od entrance on the #33 schematic is looking different. Is that correct or a drawing error, Tony?

Erwin
The standard "funky" 3 resistor network (220k/33k/1M2) is all there. The bypass cap across the 220k is lower as this is to prevent oscillations at high volumes and could be Dumble tweak at the time of the build. I hear some of these amps have different values here. He did forget to draw the ground reference for the network (@ 220/33k junction)as well as what looks like a 9M grid to ground resistor.Not sure what that high M resistor would do if anything there.That is different!
Looking over the schematic real quick it does look like most of the signature design and parts values are all there as per 2nd gen as well as the design and implementation of the parallel loop, everything should work.The only other thing I see that is different is the 2M master?.Maybe he prefers using that value to help with the load on the PI with the loop implimented so maybe it's right? don't know,I've used 2M's for masters before
BTW.The more I stare at the loop the more I dig it.If you jump it out what you get is a parallel gain stage that you can control with the return knob to add some extra gain to the whole amp (or put it on a relay for an overall boost or turn on off the verb) Don't know how it would sound but looks pretty cool on paper and may be worth a try

Tony

[erwin_ve]

That Od entrance on the #33 schematic is looking different. Is that correct or a drawing error, Tony?

The standard "funky" 3 resistor network (220k/33k/1M2) is all there...He did forget to draw the ground reference for the network (@ 220/33k junction)as well as what looks like a 9M grid to ground resistor.Not sure what that high M resistor would do if anything there.That is different!

It looks like a funky od entrance without the ground reference between the 220k/33k junction. If this truly is whats in #33 the 9M resistor would make sense.
If the ground reference between is forgotten the 9M resistor doesn't make any sense IMO.
So are we looking at a something completely else?

[norburybrook]

well, just spent an hour with the amp and the NOS AT7, it's better thank the JJ but I still prefer my random Chinese AX7. It's like the difference between a 50w and 100w amp. The AX just seems to give it a bit more...of everything....I didn't feel the AT added anything to the sound in the way Tony described, it just seemed slightly less full, in the same way a 50w amp does compared to the 100w version.



I will keep trying though.


I will also say, it's a great sounding amp through the EVM, I'm hoping I can make it work as a combo. It also sounded great through my alnico 2x12 cab I use for my rocket.

onwards.

M

[talbany]

The standard "funky" 3 resistor network (220k/33k/1M2) is all there...He did forget to draw the ground reference for the network (@ 220/33k junction)as well as what looks like a 9M grid to ground resistor.Not sure what that high M resistor would do if anything there.That is different!

It looks like a funky od entrance without the ground reference between the 220k/33k junction. If this truly is whats in #33 the 9M resistor would make sense.
If the ground reference between is forgotten the 9M resistor doesn't make any sense IMO.
So are we looking at a something completely else?..

Erwin.Hard to say!
The ground still needs to be there to set the frequency response on the network to function as intended! (220K 20pf).After looking at it further the 9M and 4M on the previous stages looks to be grid biased (old tweed) Something else!
Personally, I think he just missed the ground connection because it's under the board and if he did not know it was a ground he never would have found it!
In the Shot of 33 we clearly see a grid resistor on the socket that also looks to be omitted from the schemo.I would just ignore this and just do it the way Dumble intended axe that resistor and ground the 33k and use 56K grid blocker!.Not sure why you would grid leak an amp with overdrive since they are prone to noise, effect headroom and overloading.

Have a nice day!
Tony

[erwin_ve]

I agree Tony, but you have to know your stuff with these errors in the schematic. Interesting it is for sure with the non buffered loop.

[erwin_ve]

well, just spent an hour with the amp and the NOS AT7, it's better thank the JJ but I still prefer my random Chinese AX7. It's like the difference between a 50w and 100w amp. The AX just seems to give it a bit more...of everything....I didn't feel the AT added anything to the sound in the way Tony described, it just seemed slightly less full, in the same way a 50w amp does compared to the 100w version.



I will keep trying though.


I will also say, it's a great sounding amp through the EVM, I'm hoping I can make it work as a combo. It also sounded great through my alnico 2x12 cab I use for my rocket.

onwards.

M

Maybe you end up with a chinese 12ax7 as the "magic part of this build

I had great results with a Fane Studio 12L speaker, it's no longer in production but maybe in the uk they pop up more frequent on ebay etc? It has slightly less mids but a nice bottom end for having it in a combo cab.

[talbany]

I agree Tony, but you have to know your stuff with these errors in the schematic. Interesting it is for sure with the non buffered loop.

Cool! keep in mind the only reason why I posted the schematic was to point out the pot values on the stack of a typical 2nd generation amp (not for a clean build) this was the 1st schemo I pulled from my 2nd gen folder.
I hope anyone who builds these amps would cross any schemo or layout before building any amp to check for errors!

This is a common complaint with the 2nd gen's lack of range of the tone controls!..Even people who own an original told me the same. This could be due to the higher valued Mid control and higher 1M bass (see schematic attached)

Tony

[mojotom]

You were right Tony, I found the thread about that 33 schem and it was given by a Japanese builder to Andy Fuchs mentioning he did clone the amp.
Some values seemed strange to me back then.

[talbany]

You were right Tony, I found the thread about that 33 schem and it was given by a Japanese builder to Andy Fuchs mentioning he did clone the amp.
Some values seemed strange to me back then.

Tom
Some of the values are still strange today.
Most, if not all the hand-drawn overseas schematics done a while back, contain errors (#002/33/SSS)
Erwin is right you have to know your stuff or have a cross-reference or modify some circuits to better suit the sound you are going after.
Ambiguity obviously still surrounds the evolution of ODS #033 and when the schematic was drawn?

P.S I did not want to hijack Marcus's thread with 33,so if someone wants to start a new thread build a 033 clone, or discuss the amp further I'll be happy to contribute what I can

Tony

[norburybrook]

OK, back on topic.

I'm home for a couple of days and it's a fine day.

Got the cabinet work done in my makeshift way.

DSC_1687.JPG

DSC_1688.JPG

Question: as I'm doing a light colour(sand) suede should I still paint the inside of the combo black? Tomorrow morning I'll paint it so I can do the covering later in the day so just wondering if I should go ahead and paint it black inside.

M

[martin manning]

Question: as I'm doing a light colour(sand) suede should I still paint the inside of the combo black? Tomorrow morning I'll paint it so I can do the covering later in the day so just wondering if I should go ahead and paint it black inside.

Yes.