Dukane 1A385

[Phil_S]

This looks somewhat smaller than my Dukane. Mine's a monster, about 8.5" x 18" and also much more stuff to discard. Still, there is a good amount of wide open space in the chassis. I might be tempted to buy an inexpensive generic turret or eyelet board and use that to simplify the build. It will also be easier to mod. You might not get all the components off the tube sockets, but many/most can move to a board. Remember, if you can't fit them on a board, you can always clamp e-caps to the sidewall and/or other open area on the chassis. Radial caps work very well for this.

[angelodp]

Phil, good thoughts. With 4 preamp tubes I could go for a number of differing ideas. What did you do for yours? You suppose I could get a reverb circuit going? Deluxe reverb, no trem. Tweedle Dee w/ verb?

Ange

[Phil_S]

I haven't "done" mine yet! You should do whatever you'd like. I've repurposed several old chassis and each time I let the chassis speak to me about what might fit. Have you figured out where you'd mount a reverb tank so that it works as it should? Do you have the cage? Is there room on the ceiling? Will you need to add a dog house for the reverb tank? Personally, I wouldn't feel too obligated to put a tube in the 4th socket just because it exists. Good luck!

[angelodp]

Well after a false start I now have it going. Thanks Phil for the inspiration. I am using some off brand 12A jacks and it turns out the shunt and tip are in the opposing position to switchcraft. That lead me down the garden path with no signal and wondering about the amp.

Once I realized that the input was not conducting, with a newly built audio signal tracer, I swapped the input to the correct tab. Then I realized that I was using the same jack on the output. Swapped that and now sound, but with hum. Strapped a 22uf to the first B+ and 98% of hum gone. I will recap next. Now I can get on with shaping this amp. It already sounds very good indeed.

Cheers Ange

[Phil_S]

Yeah, simplest explanation is often the right one! Great that you got it going! BTW, I've encountered those knock off jacks. It is too confusing to have a look alike that is wired differently. Sometimes it isn't worth the penny pinching. I don't like them and will never order them again.

[angelodp]

Here is the amp with a proper power section. Nice and quiet. Sounds good. Take a bit of time to evaluate what to do next. NFB pot raises gain as expected. Tone stack needs work.

[angelodp]

Hey guys, been thinking about a master for this little guy. The circuit is kinda funky for the tone controls and it's crazy loud. Maybe even a Tweedle style amp?

[angelodp]

Before I dismember this amp, I though I would try a few things. I removed the RCA input and now have an extra pot to play with.
I decoupled the first stage preamp cathodes and they now each have separate resistor and capacitor. That change did clean things up and allows me to use my radial Big shot into the two mic inputs. I replaced all the electrolytics and also found a few leaky caps that I swapped out. Added a three prong grounded plug, rearranged the switch and fuse. The tone control is ok but not stellar. I can get a nice treble tweak at the end of rotation. I also added a NFB pot at the rear which allows me to dial in the level of defeat.
All in all sounds pretty good and especially at breakup, which is quite loud. I have included a schematic with some of the changes showing and a red circle indicating where I might try a master volume pot. I have read that a PPIMV on a Concertina PI may not be pleasant. Would that red circle area be a good spot to implement the MV.

Found a few ideas.

[angelodp]

Ok I inserted the 2.5M pot just after the Plate of V3 (now a 12ax7). I added a 600v . 022uf cap in series to the input, wiper back to the grid of the concertina. It works. So I have .00025 on one side of the pot and .022 on the other side. DC is fully blocked into the pot. I used the 2.5M pot since it was there as a result of eliminating the RCA input. Output is to ground.


Does this all seem like its not gonna create any issues? Tubes are not redplating.

[angelodp]

Here is the implementation. Sorry for the botched version, this is actually the way its wired with caps on either side of the pot.

[angelodp]

PHIL_S, what do you think.... this an ok method?

[angelodp]

Actually this is what I have in the circuit.

[sluckey]

That's really messed up.

[Phil_S]

That's really messed up.

+1 on that. Not sure what you did. Does it work? Voltage chart? Why retain the concertina? You could easily modify to LTP. BTW, IMO, the 12AU7 is a better choice for a concertina than 12AX7. The concertina is all about current (amperes) not voltage, supplying approximately equal current to both phases. The LTP is more about supplying equal voltage to both phases. Disclaimer, this is not a technical explanation, so please give me a bit of room here.

[angelodp]

I have dyslexia and screwed up the drawing. This is more accurate. I can run the amp with either tube and it sounds good. I will get some voltages asap. I just lifted the connection from the plate to grid and inserted the MV.