Dukane 1F460 project

[Phil_S]

You may recall, someone gave me two amps to do with as I please. We've made good progress on the RCA, settling on a Matchless variant with MV, Vox tone stack, and 6SJ7 on a switch for input location.

This is the other one. I realized, if I am going to order from Mouser, I really ought to get it all in one order. Maybe you guys are up for project #2 before we build #1?

The Dukane is the nicer of the two, IMO. It's got a buttload of tube sockets. I think you can see on the left 4 octal + 4 noval from top to bottom. Then going across in the middle are 3 noval. Below that is on octal and one noval. At the extreme right are 3 octal for the 5881's and 5U4. There are 3 can caps and then lots of real estate in the southeast quadrant. So, that's 8 octal sockets and 8 noval sockets! Five of the octal sockets functioned as plugs, so don't get too happy about the amount of filament current. The novals were all 12A?7 so I get 6.3V @ 4.2A which I think is a fairly generous amount of current.

There are 5 mic inputs, which explains 5 octal-noval pairs. My friend who gave me this raided it for the mic transformers (or some such thing) that I wouldn't have cared about. I wonder if the mic inputs have any eBay value? Also, this will give me up to 5 good holes on the back for whatever doesn't fit on the front.

There are (I think) 11x .047u 160P black beauties. I am inclined to extract these for sale on eBait to fund both projects. There are also a couple of bumblebees on the right side.

There are 7 pots and 6 knobs. I think I can get a replacement knob or something darn close. These have round shafts with set screws in the knobs. They look common enough.

The good news on the power section is that it's marked 5881/6L6GC, which I assume means we're good to go with 5881's or 6L6GB's and maybe even any 6L6. Rectifier was 5U4GB. I'm anticipating B+ will be considerably below the 450 I found in the RCA, and this is a good thing. It will be some time until I have the power supply isolated and safe to test.

Meanwhile, though, I think we can make some reasonable assumptions about B+ (400-ish?) and go from there (unless someone has a schematic or service manual.)

I'm thinking this one can be a real monster with at least two distinct preamp channels. Once again, though, I've got no idea where to take this one and I'd be grateful for any suggestions. I understand repurposing is not everyone's thing. I hope you don't mind my asking here.

Let's have some fun with this!

Thanks.

Phil

[martin manning]

I vote Fender 6G4-A Super with harmonic vibrato.

[Phil_S]

I vote Fender 6G4-A Super with harmonic vibrato.

Yeah! The vib channel just might fit like a custom made glove. But I think maybe the Normal channel won't fit
Is there a source for a 4M-RA (speed) pot? I suppose something makeshift can be done. It's wired as a variable resistor.

[martin manning]

The LFO part of the circuit could easily be replaced with the later Fender type, and use the standard 3M RA pot for speed adjust. I was thinking vib channel only too. Maybe do some kind of a bypass and mixer to retain "normal" function? This would be something novel and different for sure.

[Structo]

Is that item I circled in green a Selenium rectifier?

[martin manning]

There is an unused triode in the three-tube harmonic vib. That could be used to invert the output of the vib channel preamp so it would be in phase with the output of the vibrato.

[Structo]

Is that item I circled in green a Selenium rectifier?

[martin manning]

Is that item I circled in green a Selenium rectifier?

Yes, for sure. It has a transformer lead going to it, electrolytic cap with positive end to ground afterward followed by a voltage divider, output lead (white) going to the junction of a pair of 68k, the other ends of which go to power tube pin 5's (grids)... looks like the bias supply.

[Phil_S]

Is that item I circled in green a Selenium rectifier?

I'm thinking that it is. There is a white supply wire from the PT (bias supply?), two 20u 150V caps, a couple of resistors, and a thin white wire to pin 6 of the 6L6's. There is a 68K (blue-grey-orange) between 6 and 5. There is a large black Aerovox cap attached at pin 5, as well. I think 68K is strange for a grid stopper. I guess the Aerovox could be coupling cap even though it is so large. I've got to free up that one orange paper cap next to it to have a good look. There is an open hole in the middle of it (dental mirror). I think I'll save it to sprinkle on my breakfast cereal...yes, I know what not to do with those. Sharp eye there, Tom!

[martin manning]

The 68k's are the bias feed resistors.

[Phil_S]

The 68k's are the bias feed resistors.

D'oh!

There are other oddball parts....00025 cap I suppose can be had easy enough, close enough.

What about the 350K 70% tap indication on the treble pot? Use a 500K with a 1K2 across the outer lugs? A or B type? Or can this type of pot be found?

About that unused triode and in/out of phase, I'm having trouble grasping this. The vibrato circuit consists of the first two triodes as a typical oscillator. The single 12AX7 is a split load inverter. The 7025 looks a lot like a LTP inverter without the tail, I guess that's a mixer? The normal channel is inserted after that, so we ignore it completely if not being built. I think I'm going to need a drawing a direction the insertion points please.

[matt h]

(deleted)

[martin manning]

The 68k's are the bias feed resistors.

D'oh!

There are other oddball parts....00025 cap I suppose can be had easy enough, close enough.

What about the 350K 70% tap indication on the treble pot? Use a 500K with a 1K2 across the outer lugs? A or B type? Or can this type of pot be found?

About that unused triode and in/out of phase, I'm having trouble grasping this. The vibrato circuit consists of the first two triodes as a typical oscillator. The single 12AX7 is a split load inverter. The 7025 looks a lot like a LTP inverter without the tail, I guess that's a mixer? The normal channel is inserted after that, so we ignore it completely if not being built. I think I'm going to need a drawing a direction the insertion points please.

OK...

0.00025uF is 250pF

You can get a 350k/70k tapped pot from Weber, or you can do as shown below using a 500k pot and a couple of resistors.

The intensity pot is a 10M RA, which is also carried by Weber.

The harmonic vibrato consists of a LFO connected to a cathode follower stage that drives a cathodyne inverter, whose outputs are sine waves 180 degrees out-of-phase. The preamp output comes in through two filters, a HP and a LP, each feeding separate gain stages. Those gain stages both have their bias' modulated, out of phase, by the inverter outputs. The HF/LF outputs from the gain stages are mixed together by the two 470k resistors, and the sum is passed on to the PI.

You could just set up a switch to bypass the vibrato, or split-off signal from the preamp output and run it through a common-cathode stage to invert it, and then mix that with the output of the vibrato going into the PI.

[Phil_S]

... or split-off signal from the preamp output and run it through a common-cathode stage to invert it, and then mix that with the output of the vibrato going into the PI.

Uh oh, I don't understand this. Wait a minute, this is the first chapter of Merlin's book. It looks like I've got reading to do...OK, I'm back. It looks like I feed the preamp signal from the junction of the .05/220k to both the grid and the cathode of the triode and take the output from the cathode and the plate. http://www.valvewizard.co.uk/gainstage.html The output gets mixed where...at the spot where the two 470K mix resistors feed the upper half of the LTP inverter? Or into the top of the 470K (of the two 470K mix resistors). ???

You're making me work! (I don't mind.)

[Phil_S]

...would a quad of 6v6's appeal? I like to have a quad of 6v6 OR pair of 6l6 option...

It would be a nice touch. Unfortunately, when you see the picture, you'll understan. The 4 octal sockets are all on the west coast (CA-OR-WA). The two 6L6 sockets and the 5U4 are in the northeast corner (ME-VT-NH). I think it would be a lead dress nightmare, or simply impossible.