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Dumble Build - Any schematic errors?

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7/13/1999 1:46 PM

Dumble Build - Any schematic errors?

I am about to start collecting the parts to build a Dumble clone using the Overdrive Special schematic from Steve's Blue Guitar website. Does anyone know of any errors in the schematic? I'm just asking the "call before you dig" question.

Also, the schematic shows a 22M resistor in series with a .05 cap from the anode of the 2nd tube stage. does anyone know if this value (22M) is correct? It seems really high.

Thanks,

Bob

7/13/1999 2:55 PM

I too am planning to build this circuit soon.

Looking at the circuit and component values, 22M sounds like a reasonable value for the feedback resistor on the 2nd gain stage. Assuming the jazz/rock switch is in the "rock" position and taking into account the 1M volume control and the 220K grid resistor between its wiper and the 2nd stage grid, I see that the resistance to ground from the bottom of the 22M resistor varies between 220K (min. volume) and 1220K (max. volume). Doing some basic voltage-divider calculations tells me that the signal level at the junction of the 22M and 220K resistors will vary between 1% to 5% of the signal coming off the .05u cap. Actually, this seems to me like a significant amount of negative feedback. Or am I missing something here?

7/13/1999 10:45 PM
BWilliamson	From the couple of folks I talked to, the 22M is correct. On my build I followed a Bassman power, filtering schem. But I didn't find any problems with the preamp, driver or output sections of the schem. enjoy, that amp sounds great. bw

7/14/1999 3:06 PM
bobCT	Has anyone found a source for the 5mfd caps referenced in the schematic? I have only been able to find 4.7mfd electrolytics (Allied/Mouser) and it appears that the Dumble does not use electrolytics in the design. (O'Conner doesn't use electolytics....except in the power section...of his amps either according to the information on his website.) bobCT

7/14/1999 4:41 PM

Bob,

I doubt that a minor variance from 5uf on those bypass caps would be noticable.

As for non-electrolytic choices, Solen/Angela

polyprop caps can be used (4.7 or 5.1 uf) at about $2.00 a piece.

If it were me (and it almost is since I'm just "cuttin' metal" on my own Dumble project), I'd start with garden variety 4.7 uf electros and upgrade later.

Of course, I'm always interested in other opinions/corrections too...

... Whit

7/14/1999 4:57 PM
Jim S.	I'm pretty sure I saw cathode-bypass electrolytics in a picture I've seen of the inside of a 70's era Overdrive Special. I know Mouser sells Sprague Atom 5uF/50v caps, so you can try ordering these. I also doubt you'd hear much difference, if at all, between these and any garden-variety 4.7uF cap. If you're in an experimental mood, you could als try 4.7uF axial tantalum caps.

7/14/1999 5:18 PM
Gus	I would try a 1uf pan V series cap(polyester).(digikey)If I you go with an electro I would use 10uf on up.

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