Jennings Organ Company VOX AC15 - 2 from 1959 Rebuilding the impossible

[Jon Snell]

I have just completed repairs to one of the only AC15 - 2s left in the world.
apart from a few components, transformers and pots etc the tremolo was not working.
Replacement components were not an option so reverse engineering was the order of the day.

Usual dirty valve bases and I replaced the EL84s as there was more of the cathode vapour sprayed on the inside of the glass than left on the cathodes. The EZ81 was just as bad, so replaced that as well. That brought it back to life.
I spent lots of time drawing a schematic for my reference.
After the amplifier was sorted then came the VibroVox controls.

potted in circa 1959 gunge, the rework began.
After some deliberation and figuring out how it works, this is what I came up with.
And after a few cables were attached after removing the B9A free plug from the old cable and fixing it to the new cable. I employed the standard colour code for pin numbers.

Plugged it in and tested all working fine. So potting was next


Wait 24hours for the epoxy to set, the epoxy makes it gig proof! The end result is ...


These amplifiers use an unusual valve to oscillate and produce the VibroVox sound; ECF82, not found on much outside of VHF tuners from the 50s.

Around 1961 The Jennings Organ Company became VOX Amplification.

I hope you found my account interesting.

[Jon Snell]

My schematic.

[Mick Bailey]

Were you able to establish exactly what's inside the potted module?

[Jon Snell]

Yes, after looking around and deliberating with the knowledge of how it works with the C R timing to achieve the correct frequencies, I found a schematic from my collection dated 1960 and luckily it was included. The components turned out to be not a problem, just sorting out where the wires went, that was fun, all seven of them, many combinations if you don't know how it works! The odd ECF82 valve threw me for a short while until I found the correct schematic.
I do like a challenge. As my web site proclaims, "Repairs, Restorations and Refurbishing … miracles take a bit longer".
https://jonsnell.co.uk

[Mick Bailey]

A challenge is nice - I've had a few though that can take a very long time to sort out and one that I'm completely stuck with for the time being - a very early and one-off Bell studio flanger.

On your site is that an amp running 807s?

[Jon Snell]

A challenge is nice - I've had a few though that can take a very long time to sort out and one that I'm completely stuck with for the time being - a very early and one-off Bell studio flanger.

On your site is that an amp running 807s?

Yes it is my HiFi amplifier that I made, number two. Number one was sold.
807s are high voltage 6L6s and have a higher VHF cut off point as they are designed for AF/VHF modulators in transmitters running 813s.
The Snell 807 was a bespoke order, one of many.
Guitar wise the last one I designed and built from scratch, even the chassis and case, is the Type 66 All Valve that I finished production this January. Used and tested by John Illsley from Dire Straights, Eric Clapton and many more. At present one of them is in the studio Noel from Oasis fame, recording with it.
John said, this has exactly the sound I want and remember from the 60s when at the O2. https://jonsnell.co.uk/page-3/
Fame at long last maybe!

[drewl]

Wow, that's really great.

Around here I mostly get old Ampegs that look like they've been in a barn for fifty years.

Oh, do have a cool old Danelectro twin 12, true stereo guitar amp a guy dropped off yesterday in incredibly good shape.

[Mick Bailey]

Most of my work on older amps is on barn finds (or more likely knackered shed finds) but occasionally a nice early British amp will turn up or more rarely an old Ampeg - these weren't so popular over here due to import restrictions in the late 50s/60s.

I have an 807 hifi amp running 1000v on the plates. It was 1200v but the tubes didn't last very long. I still have a box of NOS 807s and some of those ceramic wafer transmitting tube sockets that i was intending to build a guitar amp with.

[Jon Snell]

Most of my work on older amps is on barn finds (or more likely knackered shed finds) but occasionally a nice early British amp will turn up or more rarely an old Ampeg - these weren't so popular over here due to import restrictions in the late 50s/60s.

I have an 807 hifi amp running 1000v on the plates. It was 1200v but the tubes didn't last very long. I still have a box of NOS 807s and some of those ceramic wafer transmitting tube sockets that i was intending to build a guitar amp with.

The 807 is an excellent choice as it is a beam tetrode and very rugged, I based my HiFi amp on the Williamson from 1949, the 66 is based on the Hi Watt front end and large iron output transformer made by a local UK company in Poole, Dorset and it copes with 22hZ with less than 3dB loss at full power. It will stand all sorts of abuse. The 6L6 can't stand the high voltages like the 807 so I limit the HT to 550volts.
I have another VOX AC30 from 1963 coming in soon and a Murphy A146C wireless set from 1949 to refurbish so will post the journey through my workshop as and when. Yes I still have a couple of NOS PEN45s!