Need a little help modifying fender + Dumble OD

[markmalin]

Hi Guys,

I'm building a modified AB763 type Fender amp. Basically it's a one channel Vibrolux Reverb, without the "vibrato" circuit. (see schematic below). What I want to do is add a Dumble O/D circuit to it and this is where I need your help. I'm no electrical engineer. I want to insert the OD gain stage (gain and OD) between the output of the tone stack and the input of the reverb section. (I don't want it after the reverb section because I've found this type of OD doesn't seem to work well with a reverb driving it - but it could be my lack of experience...) In any case, the board is already laid out like so:

Input -> Tone Stack -> O/D option -> Reverb -> PI -> output.

If you guys would, please let me know where best to insert the OD on this schematic and whether I need to add any additional circuitry or not. I would like to switch it like the BrownNote D-Lite non-relay versions (just a switch).

I hope this is enough information - and that my question made sense...

Mark.

[IMG:932:531]http://i110.photobucket.com/albums/n115 ... ibrato.jpg[/img]

[IMG:415:229]http://i110.photobucket.com/albums/n115 ... ttheOD.jpg[/img]

[jaysg]

Place the OD between the .02uF and the 10pF||3.3M.

[Bob-I]

The correct architecture is..

Input > Gain > Tone stack > Gain recovery > OD entrance > OD 1 > Od level > OD 2 > Reverb > PI > output

Good luck with the project. I tried this with a Super Reverb and was less than thrilled but I suspect it had more to do with the speakers and xformers than the architecture.

[markmalin]

The correct architecture is..

Input > Gain > Tone stack > Gain recovery > OD entrance > OD 1 > Od level > OD 2 > Reverb > PI > output

Good luck with the project. I tried this with a Super Reverb and was less than thrilled but I suspect it had more to do with the speakers and xformers than the architecture.

Thanks. Yes, this sounds like what I'm doing. I left out some of the details in my post.

Interesting that it didn't turn out all that well for you on the Super. Hopefully it will sound respectable. I thought if it doesn't sound all that good maybe if I tweak some of the components it will help. Worse case scenario is I just disconnect the D-style OD and have a nice quasi-Vibrolux

I'm curious - what sort of plate voltages did you wind up with? And did you just use the existing Super B+ supply (i.e. the same one used for the other pre-amp tubes) or did you divide out some other voltage special for the OD plates?

Mark.

[ic-racer]

Not an easy problem. I did my Fender Super Twin Reverb similar to the way Bob posted. (Search for Super Twin to see my schematic.) Reverb works OK but it cuts gain to the PI. I put a dpdt switch to totally cut out the reverb circuit, which makes the amp louder.

But, anyway, check my Super Twin thread for ideas. My project sounds great, I'm glad I did it.

[Bob-I]

Interesting that it didn't turn out all that well for you on the Super. Hopefully it will sound respectable. I thought if it doesn't sound all that good maybe if I tweak some of the components it will help. Worse case scenario is I just disconnect the D-style OD and have a nice quasi-Vibrolux

I'm curious - what sort of plate voltages did you wind up with? And did you just use the existing Super B+ supply (i.e. the same one used for the other pre-amp tubes) or did you divide out some other voltage special for the OD plates?

Mark.

Yes, I stuck with the stock B+ rail. The voltages were a tad high for a Dumble, more in the range of 250 on the plates of V1 and V2.

It didn't sound bad, it's just that I have several other Dumble style amps and I prefered this one as a Super. I see no reason that you can't make this a great sounding amp. Stick around here and you'll get tons of help.

[marcos]

IMHO feeding the signal from a Fender tonestack into a Dumble-style overdrive circuit doesn´t work too well.I would at least install a larger mid pot ,100k or 250k,wired the way Dumble does it.More tweaking may be necessary to achieve satisfasctory results.I´ve seen quite a number of amps done this way, and they tend to sound brittle, due to the attenuation of low mids in the Fender tonestack.
I believe that a big part of the Dumble sound is in the tonestack, the fat,singing voice is already there in the clean tone.

[markmalin]

Not an easy problem. I did my Fender Super Twin Reverb similar to the way Bob posted. (Search for Super Twin to see my schematic.) Reverb works OK but it cuts gain to the PI. I put a dpdt switch to totally cut out the reverb circuit, which makes the amp louder.

But, anyway, check my Super Twin thread for ideas. My project sounds great, I'm glad I did it.

Thanks - I will check this thread out. I appreciate everyone's input

Mark.

[markmalin]

Hey guys, just to close the loop here, I finished the amp (chassis) and after getting it up and running, just now connected up the OD. Actually, it sounds pretty nice to my ears. The clean Fender Vibrolux mode is nice and crystal clear - a tad thin for my ears but I can tweak. However, when I switch in the OD it fattens things up and overdrives nicely. Bear in mind I'm really not going for the total Dumble OD tone - I was just hoping for a sweet tube overdrive (ala Dumble) to go with the Vibrolux type build (and have Reverb) So... so far so good.

Now to decide on a pair of 10" speakers and get the tolex on the cabinet!!

FWIW - Here are a couple pictures (for those of you who didn't see it on Hoffman).

Mark.


[IMG468]http://i110.photobucket.com/albums/n115 ... lux006.jpg[/img]

[IMG640]http://i110.photobucket.com/albums/n115 ... lux005.jpg[/img]

[IMG676]http://i110.photobucket.com/albums/n115 ... lux004.jpg[/img]
[IMG667]http://i110.photobucket.com/albums/n115 ... lux003.jpg[/img]

[IMG422]http://i110.photobucket.com/albums/n115 ... lux001.jpg[/img]

[Bob-I]

Nice looking build, congratulations.

I understand you about getting a nice OD witout woorying too much about Dumble tone. There's a lot of focus on the Dumble tone here, but that's why most folks come here. There's tons of other OD tones out there.

[markmalin]

Nice looking build, congratulations.

I understand you about getting a nice OD witout woorying too much about Dumble tone. There's a lot of focus on the Dumble tone here, but that's why most folks come here. There's tons of other OD tones out there.

Thanks, Bob. Yah, I completely appreciate the Dumble tone (especially in the D'lite I built), but for this guy, I'm OK with just a good OD

[markmalin]

Hm... I spoke a little too soon. I just went to try out the amp again (was playing it last night) and noticed the 12AX7 on the return side of the reverb was absolutely blown apart. I mean, the glass was broken at the base and the inside was all white. I assume this happend while running the OD circuit. I put another tube in, turned on the OD and measured voltages and it seems fine. The plates are a tad high, at 209V, the cathode is at 1.5... and it plays just fine. Any thoughts as to what's going on?

Incidentally, I hooked up the OD circuit between the .02uF and the 10pF||3.3M. I used that node, which is to say with the switch on, the circuit is broke there and routed through the OD circuit, then back again. I did not modify the connection to the 500pf cap which feeds the reverb (that's on the same node). Did I do something wrong?

Mark.

[Structo]

I don't really know much about tube reverb circuits but it might just have been a coincidence.
I have had tubes loose their vacuum and do that, turn all white and burn up.
I think most likely there was a hairline crack or the seal around an pin failed.

[Bob-I]

Hm... I spoke a little too soon. I just went to try out the amp again (was playing it last night) and noticed the 12AX7 on the return side of the reverb was absolutely blown apart. I mean, the glass was broken at the base and the inside was all white. I assume this happend while running the OD circuit. I put another tube in, turned on the OD and measured voltages and it seems fine. The plates are a tad high, at 209V, the cathode is at 1.5... and it plays just fine. Any thoughts as to what's going on?

You might have had a hairline crack in the tube that leaked enough oxygen to finall burn the tube up. You're voltages are fine so I'd say run it for awhile with the new tube and watch closley.

Incidentally, I hooked up the OD circuit between the .02uF and the 10pF||3.3M. I used that node, which is to say with the switch on, the circuit is broke there and routed through the OD circuit, then back again. I did not modify the connection to the 500pf cap which feeds the reverb (that's on the same node). Did I do something wrong?

Mark.

No, that sounds right.

[markmalin]

Thanks, guys. I'll run it a while and see how it goes. Fortunately I've got a few spare 12ax7's

Mark.