I'd love your input on this design idea. Dumb-Lux

[markmalin]

Hi, all

I'm trying to lay out a 2 channel amp based on an AB763, but with the following:
- No Vibrato
- 2 channels
- one channel based on a Vibrolux, the other a Dumble #185 (w/out OD)
- both channels will have Reverb

Please have a look at this schematic.  There are some specific things I'm uncertain about:
- placement of the choke
- mixing the 2 channels into the reverb.  Basically I have them both feeding into the gain stage that typically is only used by the Reverb channel on an AB763
- Do I need another section of the filter supply (an E section), or am I OK with just B, C and C?
- Placement of the Master Volume.  The 220k/47k voltage divider in front of the Master is somewhat left over from the original design.  Is this correct?

I really value you'all's expertise and input.  Again, the goal is a 2 channel amp, Dumble (#183 with no OD) in one channel, Vibrolux Normal in the other.  Reverb in both channels, Reverb, PI, power section based on the AB763.


humbly,
Mark

[SEL49]

Please have a look at this schematic.  There are some specific things I'm uncertain about:
- placement of the choke

Choke is fine.

- mixing the 2 channels into the reverb.  Basically I have them both feeding into the gain stage that typically is only used by the Reverb channel on an AB763

That's fine. A simpler way is to just connect V1B plate directly to V2B plate, thus eliminating the .05 cap on V1B plate. (See Robinette's drawing)

- Do I need another section of the filter supply (an E section), or am I OK with just B, C and C?

Fender always used one filter node to supply 6 triodes same as your schematic. Many people, including me, think that's not good enough. I would use a node E to supply V1 and V2, leaving node D to supply only V4.

- Placement of the Master Volume.  The 220k/47k voltage divider in front of the Master is somewhat left over from the original design.  Is this correct?

You don't need the 220K. I suggest using a 100K master volume pot and eliminate the 220K and the 47K.

[markmalin]

Thanks for the suggestions.  I've modified my schematic (attached) to include:
- another supply node, E
- removal of post recovery triode 220k/47k divider and 100K master instead of 1M
- changes to the mixing of the channels for reverb.
For the reverb channel mixing I read both of Robenette's mods and he suggested the 220K mixing resistors to keep the channels from interacting with each other and allowing them to be bridged...so it's slightly different from your suggestion.

[dogburn]

Minor thing on the schematic: you've got a couple of mixed up labels on the tubes - I believe the Reverb Recovery should be labelled V4a and the second PI triode should be V5b rather than V3b.

[markmalin]

Thanks!  I'll fix that :)
Mark

[acheld]

Complex amp!   

There is a 10pF cap coupling your two pre-amp circuits with the output driver.  Most Dumble style circuits I'm aware of will use a 0.005uF cap in that position.    The larger cap will pass more low frequencies, but how much difference in your tone it makes -- I'm not sure.  In my own amps, it sometimes will make a big difference, but not always . . .   

[markmalin]

Complex amp!   

There is a 10pF cap coupling your two pre-amp circuits with the output driver.  Most Dumble style circuits I'm aware of will use a 0.005uF cap in that position.    The larger cap will pass more low frequencies, but how much difference in your tone it makes -- I'm not sure.  In my own amps, it sometimes will make a big difference, but not always . . .

I see the 0.005uf cap at the output of the OD circuit on the Dumble ODS (#185).  But without the OD, there is a 0.05uf cap coupling the plate of V1b (the output of the clean section of the Dumble) to the Master and input to the PI.  I'll have to keep these in mind when I get the amp up and running.  Thanks for the info.

[tubenit]

Mark,   I always appreciate your amp building handiwork and that you share it!  Grateful. I find it consistently of interest to me.


I borrowed some from the Dumblish preamp and built that into a cathode biased kinda sorta approach to a DR. There may not be anything useful in this schematic, but I'll share it incase there is to someone else.


I will say, I found a need to adjust (reduce value) some of the coupling caps to get the tone I wanted.  I also found the modification I'm showing on the PAB to be of help to me also tonewise.


I'm getting some of the best clean tones I've ever gotten from any of my builds. Very very happy with it.  The PT is 300-0-300 .............. and I've used 6K6's with a 5Y3GT,  6V6's and 5881's with GZ34.  Since it's cathode biased, I've not need to rebias the tube changes.


I used a Digitech RV-7 and a delay in the psuedo pretend active FX loop. 


EDIT/CORRECTION:  The shared bias resistor for the power tubes is 300ohm not 360ohm.  The PT and power rail are on following pages of the SCH schematic along with the relay switching info.


With respect,  Tubenit

[markmalin]

Mark,   I always appreciate your amp building handiwork and that you share it!  Grateful. I find it consistently of interest to me.

I borrowed some from the Dumblish preamp and built that into a cathode biased kinda sorta approach to a DR. There may not be anything useful in this schematic, but I'll share it incase there is to someone else.

I will say, I found a need to adjust (reduce value) some of the coupling caps to get the tone I wanted.  I also found the modification I'm showing on the PAB to be of help to me also tonewise.

I'm getting some of the best clean tones I've ever gotten from any of my builds. Very very happy with it.  The PT is 300-0-300 .............. and I've used 6K6's with a 5Y3GT,  6V6's and 5881's with GZ34.  Since it's cathode biased, I've not need to rebias the tube changes.

With respect,  Tubenit

Thanks for the kind words, and for your help and input.  I will take a closer look at your schematic.  I'm intrigued by the idea of changing the coupling caps to change the tone. 


I'm curious about the D-style preamp going into alternate power sections.  I tried to do a Dumble preamp into a DR power section, but it just had no life.  Long story short, the chassis was dismantled and I took the parts and made one of my favorite gigging amps so far...but I digress...


That cabinet is beautiful!  I need to learn how to do that - I have this Filmosound I just finished and it's dying for a nice, hardwood head cab.


Mark

[acheld]

I'm curious about the D-style preamp going into alternate power sections.

I've built several D-style amps, and have not cracked the code on "knowing" what will work and what won't in the schematic stage of the build.  I've always had to adjust various component values  to make sure there is adequate signal feeding the power amp, but have not seen a pattern of what works other than that.

Been meaning to build a Dumble pre-amp into a 15 watt (or so) power amp . . . not sure when I'll do that, but that should be interesting.