Amperial Valve Amplifier. British Ceramic Research Association Asset AM4
I found this in a junk room in the cellar adjoining the 'calibration room' of the BCRA Building Materials site, back in about 1993! It used to be used to power the site's call system PA, fed from a mic in Reception.
As it hadn't been used for decades, and the room needed a clear out, I took it home!
It had an excellent pair of RCA branded Mullard XF4 EL34, which got swapped into my Sound City 50 Plus guitar amp and gave excellent service through many gigs.
As a mono PA, the amp itself seemed a bit useless for my purposes; The front panel was very nice but had no room for an input jack, so difficult to repurpose as a guitar amp. The build quality inside seemed low and had some bodged repairs / mods - no use as a hifi or anything really.
So it got put into the holding tank of 'projects I'll progress one day', ie in the garage, where it rusted away till I thought to re-appraise it.
In 2016 I powered it up slowly to reform the ecaps and after checking that it all seemed in order, put some valves in. The HT was ridiculously high, over 550V, and with the EL34 in cathode bias they were hopelessly overdissipating. Someone had tacked in a transistor regulator to bring the HT down but as it wasn't heatsinked it had overheated, shorted and wasn't doing anything.
The EL34 were arranged in UL, screens fed from taps on the OT primary. On examining the OT more closely, the layout of the primary and secondary terminals rang a bell with me, so I took it out and beheld an RS DeLuxe OT, that had been in my possession all this time.
So I got around to putting together the bits for a JTM45 type build.
As for the rest of the chassis, I'm thinking of turning it into a test bed amp; the HT is from a 200Vac winding and a voltage doubler, about 600Vdc unloaded. Replacing that with a full wave bridge should get a HT in the low 300s, which could be used to power a pair of EL34 / 6L6 in class A. If in fixed bias, that will allow me to tinker with the effect of how changing the bias from A to AB affects an amp's performance / operation.
Here's the front panel, cleaned up a bit -
As it hadn't been used for decades, and the room needed a clear out, I took it home!
It had an excellent pair of RCA branded Mullard XF4 EL34, which got swapped into my Sound City 50 Plus guitar amp and gave excellent service through many gigs.
As a mono PA, the amp itself seemed a bit useless for my purposes; The front panel was very nice but had no room for an input jack, so difficult to repurpose as a guitar amp. The build quality inside seemed low and had some bodged repairs / mods - no use as a hifi or anything really.
So it got put into the holding tank of 'projects I'll progress one day', ie in the garage, where it rusted away till I thought to re-appraise it.
In 2016 I powered it up slowly to reform the ecaps and after checking that it all seemed in order, put some valves in. The HT was ridiculously high, over 550V, and with the EL34 in cathode bias they were hopelessly overdissipating. Someone had tacked in a transistor regulator to bring the HT down but as it wasn't heatsinked it had overheated, shorted and wasn't doing anything.
The EL34 were arranged in UL, screens fed from taps on the OT primary. On examining the OT more closely, the layout of the primary and secondary terminals rang a bell with me, so I took it out and beheld an RS DeLuxe OT, that had been in my possession all this time.
So I got around to putting together the bits for a JTM45 type build.
As for the rest of the chassis, I'm thinking of turning it into a test bed amp; the HT is from a 200Vac winding and a voltage doubler, about 600Vdc unloaded. Replacing that with a full wave bridge should get a HT in the low 300s, which could be used to power a pair of EL34 / 6L6 in class A. If in fixed bias, that will allow me to tinker with the effect of how changing the bias from A to AB affects an amp's performance / operation.
Here's the front panel, cleaned up a bit -
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