Heidkamp Blues Special--Input Jacks NOT Muting

[nevetslab]

The General Manager here at CenterStaging, LLC in Burbank, CA brought me his Heidkamp Blues Special amp head....one of 35 hand-built & designed by a former NASA Engineer from the Gemini Program. There was no output from the amp.

First one of these I've ever seen. Beautifully built on the outside, and I'd guess likewise on the inside.





Output tubes are EL34's from Valve Art...large bottles. Everything is definitely well built inside. I have no documentation for it. When I powered it up, beginning with self-noise, I did see it was alive, but, once I plugged my pink noise source into either input, I didn't get any output from the amp/speaker. There's a Preamp Out/Power Amp Input jack on the rear, and patching into that, I did get output, so I unplugged the first tube on the far right, assuming that was the input tube. Good call, as replacing it (a J/J ECC83S) with a fresh one yielded a working amp.

But, when I unplugged my source cable, having dialed up decent-sounding settings, I got hum, like you'd get with an amp that doesn't have MUTING jacks if nothing is plugged in. After I took photos of the insides, I blew up the best views I had of the input jacks. The Hi Gain jack has some insulated switching jack function which has a normal contact that is grounded, but....the Tip contact does NOT have a normal, so, no mute function. No idea how those jacks are wired. The jack below it...it's Tip terminal IS getting grounded, but, it isn't switched to accomplish Muting with nothing plugged in either.



I did also find it had a Death Cap installed, which surprised me. I removed it. 600V 47nF Sprague Polypropylene 715P series cap.

Sure are a lot of trim pots in this amp. I don't know if Jimbo has the Foot Pedal for it. So, this is my current project on the bench, to see what I can figure out. I did at least get the amp working. Not sure if I can come up with an Input Jack muting function without defeating how it's jacks are wired.

[Enzo]

The amp COULD still have muting, without a tip short. Like some Marshall and other amps, that extra set of contacts might ground a grid a stage or two inland. Maybe?

[Jon Snell]

It's not rocket science to reverse engineer the input sockets. (Pun intended). I suspect it will have the standard short to ground on the input conductor.

[pdf64]

Yes, don’t need to use a shorting contact on the input per se, eg the switching contacts on Fender 75 mute the signal at the volume control wiper.
https://el34world.com/charts/Schemat.../Fender_75.pdf

[J M Fahey]

FWIW it stinks of Dumble Overdrive Special genes, I´d compare that schematic and check similarities.
Expect more than one.

Of course, this one will not sound that good because it lacks the patented Formica eyeletted board and the toilet pipe repair blue paste covering mystery/secret_sauce/fairy _dust parts.



If your Customer paid less than U$50000 for it he´s already ahead in the game, now he needs a Gibson 335 .

[Helmholtz]

Yes, looks like a Dumble clone. Even has the extra input Fet board at the side.

There are layouts for some models on the Ceriatone website, e.g.:http://www.ceriatone.com/ceriatone/w...rtoneSM100.jpg

Maybe this helps.

[nevetslab]

I looked at the Ceriatone layout, though finding it had Cliff Jacks, it didn't answer the wiring issue I had here. I did look at Switchcraft's Phone Jack circuits, and while their Schematic XI would be the variation of the 13E jack that was used, THAT XI circuit isn't available in any of their production phone jacks. I did verify the shielded cables from that Hi Gain jack type 13E went to the FET input board. The lower NORMAL input jack went to Pin 2 of the Input tube that I had replaced. I also had a wire break off, or discovered it had no longer been making contact, and after stripping the insulation off for fresh wire, I soldered that back into place. Then, after verifying I had all back into place, I powered up the amp, and now have a quiet amp without anything plugged in while the Gain and Master controls are turned up.

Checked plate current, finding in the four tubes, one pair was reading 14mA and 21mA, while the other pair was reading 21mA and 15mA, so they had them set to balance out when all four tubes were in circuit, the 50W mode turns off the inside pair of power tubes, so there is that imbalance in plate current.

I also found one of the alighment pegs on one of the EL34 tubes had broken off, it was at least captured, having fallen out of the socket. It's now glued back into place and curing. I'll check this all out in the morning, now that I've moved the chassis back into the cabinet and set aside.