1/2 a Dumble?

[rfgordon]

I have a request from a guy to build "the ultimate 3 knob amp." He wants a Tweed Deluxe-style output section, but wants basically gain-tone-volume for the controls, maybe with a bright switch.

I haven't yet built a D, but from the clips I've listened to it seems like the clean side of the preamp would be a great, very dynamic basis for this project. So I'm thinking V1a as normal D, into a gain pot, then V1b with normal D values, into a vol-tone combo, and on its merry way to the PI.

What's the wisdom of the collective have to say? Would y'all stick with the conventional values on the plates, cathodes and coupling caps?

[dobbhill]

Just my experience:
100k plate / 1.5k-5uf cathode with a skyline tone stack into a Blackface PI and output section sounds very good. I did this to/for a friends bassman and he loves it. I put a switchable feedback loop on V1b, also.
Hope this helps.
D

[rfgordon]

D-
For the feedback circuit, did you configure it so it switched in/out, or did you have the switch change the feedback resistor values? I reckon the feedback helps generate the Dumble's smoothness, so I think it'd be a good feature to put in an amp.

So, D, would you recommend the following:
V1a: Ra 100K, Rk 1k5, Ck 5mF
V1b: Ra 100K, Rk 2k2, Ck 5mF w/ feedback of 22M Res. w/ .047 pF cap (with second 22M resistor switched in/out of series for 44M)

I see most of the schematics show V1A as Ra 220k, Rk 3k3, Ck 5mf and V1b as Ra 150k, Rk 2k2, Ck 1mF or 5mF.

A Ck of 1k5 is a more traditional Fender value, and it'll pull the plate voltage down a bit and give more gain/less headroom. The combo of a 220k Ra and 3k3 Rk would yield lots of headroom and a wide voltage swing at the plate, which, I suppose, translates into good dynamics and touch. Then again, a TW runs 1k5/100k V1a and 2k2/100k V1b and it's got loads of touch. Hmmm......choices, choices.

[fabiomayo]

Sounds like an interesting project. I'm afraid I can't offer any help but
I sure will keep an eye on this topic.

Best of luck with your project.

[Bob-I]

Very cool idea. I might want to make it volume and tone and a post PI master volume though. With only 2 gain stages a standard master volume type may not be too effective.

I used this songle knob tone control on an amp I built a few years ago. It's very effective and sounds great.

[rfgordon]

Thanks for the tone network schematic! The first cap is, I would think, 220 pF, not 220 Farads, right?

[Bob-I]

Thanks for the tone network schematic! The first cap is, I would think, 220 pF, not 220 Farads, right?

LOL.... 220F would be about the size of a house. You're right, it's pF, my bad.

[rfgordon]

Yeah, 220 F would be perfect for those tactical laser applications! Back in 1985, when I was in govt electronics school, we had a laser that would punch a hole in a razor blade. That is, until the day we pushed the fire button and the current in the power supply cable, which was as thick as a cigar, exited thru a nick in the insulation and arced to the steel desk that was bolted to the floor. Very, VERY loud crack. Kinda like standing next to a small bolt of lightning. Fortunately, the discharge was under the table, so the spatter of molten copper from the cable only went up to our waists. I don't recall what the cap values were in the PS for that thing, but they were about the size of a 2 liter soda bottle!

[keithrick]

Yeah, 220 F would be perfect for those tactical laser applications! Back in 1985, when I was in govt electronics school, we had a laser that would punch a hole in a razor blade. That is, until the day we pushed the fire button and the current in the power supply cable, which was as thick as a cigar, exited thru a nick in the insulation and arced to the steel desk that was bolted to the floor. Very, VERY loud crack. Kinda like standing next to a small bolt of lightning. Fortunately, the discharge was under the table, so the spatter of molten copper from the cable only went up to our waists. I don't recall what the cap values were in the PS for that thing, but they were about the size of a 2 liter soda bottle!

Holy Shit! Sounds like you have/had a killer vocation! No burns on the legs I hope!

[dobbhill]

So, D, would you recommend the following:
V1a: Ra 100K, Rk 1k5, Ck 5mF
V1b: Ra 100K, Rk 2k2, Ck 5mF w/ feedback of 22M Res. w/ .047 pF cap (with second 22M resistor switched in/out of series for 44M)

That's what I used and it worked well. I used a on/off/on switch for the FB loop to select 22meg/no fb/44meg.
HTH,
D