Kitchen Sink Trainwreck

[Firestorm]

I got a commission to build a pair of amps in the style of Ken Fischer, from a guy who likes some bells and whistles. (I previously made him a 6L6-powered combo based on Dumble #124, but with a full-blown loop, plus spring reverb, plus a tube rectifier, plus Power Scaling, extra cup-holders, etc., etc.)

Here's how this project turned out (just one of the amps, they’re identical):

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Very traditional except for the overabundance of knobs. The cabinet is Ribbon Mahogany by the awesome Mike Mcauley/ampmike/faithamps

Less traditional inside, though the circuit and layout is about 80% Express (and about 90% ideas I swiped from this site – thanks guys).

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Shoe-horned into the power end is a Power Scaling card and the requisite low-impedance raw bias supply, both from London Power. Output tube bias is individually adjustable.

Scaling just the plates and screens requires some kind of Master Volume to prevent over-over-over-driving the power amp. This one is a dual-gang “bootstrapped” master. I was initially scared of it because there’s so much going signal-wise at that end, but I used a pair of foil-shielded three-conductor cables and it’s perfectly well-behaved (plus the runs are only a couple of inches).

There’s a Cut control in addition to Presence, and again I was anxious that they wouldn’t make good neighbors. I used a 16mm pot for the Cut and a Mallory 150 Presence cap tucked up against the chassis fold and they don’t seem to interact. (BTW, the little green boards are O’Connor adapter things because he ships 16mm PC-mount pots. I repurposed one for the Cut control since I had PC pots lying around for some reason. The Cut control turns out not to be all that necessary, since high-frequency seems well controlled (probably the shielded runs).

I also did the split-plate thing on the tone recovery stage, but put the switch on the other side of Express-appropriate caps (2n and 4n7) with a 2M2 resistor across them so it’s dead quiet. It’s definitely tamer in the lower position, so it might be more pedal- friendly, which was a request.

The trimmer next to that is 50k on top of 100k, so the high-pass filter can be electronically stock, but the signal can be attenuated in case there’s a hot tube in V1. Also, not all that necessary as it turns out.

Output tubes are current production Russian: New Sensor “Mullards” in one amp (they sound pretty good actually) and EH6CA7s in the other amp. The 6CA7s are interesting , still EL34-ish, but fuller.

Preamps, though, are all NOS Euro tubes: Mullard white shield logo with short plates, Telefunken ribbed plates, and Telefunken smooth plates. I was going to put the Mullards in V1 for warmth, and so on, but while they sounded fine musically, they were both prone to hum and spectacularly microphonic. I would have sent them back, but they actually work great in the PI. The rib plate TFs are a bit too brash for V1, but the smooth plate ones are very nice indeed! Quiet and almost as warm as British tubes. These were OEM labelled for “The Fisher” and I’ve yet to run into a bad tube with “Fisher” printed on it.

For anybody still reading, both transformers are Pacific and they sound nice, though the “fit and finish” isn’t as tight as on some Heyboers I’ve had. Not a complaint, just an observation.

[aceofbones]

Nice looking build!

The power supply end looks like it was no fun at all to get everything to fit. Well done!

[Reeltarded]

That thing has more knobs than my repurposed bluesbreakers!

Looks great!

The EH6ca7 is my favorite tube. The bandpass is more like Sylvanias than any similar thing I have ever used.

[MeanMrMustard]

Those are some nice bells and whistles. Nice build too. I imagine they work particularly well with the Express circuit. I’ve been considering options for a customer’s request for serious attenuation of the EF86 AC15 circuit and have been considering power scaling. He wants to retain the clarity and get all the goods at low low volumes. Church guy. Hope he knows that he’s hearing the greenback do its thing when we crank up mine.