Gretsch 6161 Electromatic Amplifier

The amp in this combo was originally manufactured around 1961, but oddly it never played a note until 2010!

Although it is branded Gretsch, it was actually built by the Valco company of Chicago, but seems never to have been completed, and was eventually sold off this way in the Valco closedown auction sometime around 1970. ┬áThe amp didn’t ever get installed in to a cabinet, and its power cord was still neatly coiled up as for shipping; it may never even have been powered up. This is what a finished one would have looked like, with its interesting trademark Gretsch cab, and yes it did use two automobile speakers plus a tweeter:

Over the years it had been somewhat cannibalised for parts (fuseholder, power switch, cathode bias resistor, etc), but fortunately the original paper wound output transformer (a big part of the grind of these Valco amps) was still present. The canister multisection power supply filter cap was broken off at its mounting, and most of the power supply wiring was missing. There were no valves, and sadly no original Gretsch ‘arrowed G’ control knobs:

When I got the amp chassis, I verified all of the point to point wiring against the circuit schematic, and reflowed a few suspect solder joints. Checked the more critical component values, no drift or leakage problems evident. Rebuilt the power supply where components were missing (the power on-off switch has an unusual ‘standby’ position which mutes the amp by connecting together the power valve grids, with the HT actually remaining powered up). Replaced the two pin power cord with a properly earthed one (being US the amp is 110V mains only). Fixed the filter cap mounting, and used a reforming process to see if this 50 year old cap of three electrolytic sections was viable, which it seemed to be. The amp uses 6973 power valves which are now quite rare and expensive, but much cheaper 6CZ5 type can be substituted in to low voltage cathode biased amps ok, so I got some of these and rewired the sockets to suit (it will still take the 6973s), and added a bias resistor, put in a 5Y3GT rectifier valve, and three 12AX7s for the preamp and trem.

Powered up, voltages seemed ballpark ok. Nothing got too hot. Connected a speaker, and a guitar. Strummed a chord. Wow.

I had a surprisingly well constructed and substantial empty cab from an Epiphone Valve Special amp which had bitten the dust (aka pcb was modded to death), with a Celestion G12 Alnico Blue (Ipswich-made, not China) installed which I thought would suit this amp (cosmetically too). Boy did it. That 15W rated speaker in a fully floating baffle with the 17 watts this amp makes sounds like a mini flat out Vox AC30 when the volume is anywhere past about 10 o’clock with humbuckers; with single coils it still has lots of that Voxy sparkle even when maxed. It has a nice tremolo too, and the single tone control is effective. Because the ‘Blue’ is an efficient speaker the amp is actually pretty loud for 17 watts. It was definitely worth bringing back to life, although this really shouldn’t have been a surprise; the circuit is virtually identical to the also Valco-made Supro S6424 reckoned by many to be all over the first Zeppelin album.

Here it is with the back off:

There is a liberal sprinkling of ceramic caps (usually avoided in combo amps because of their reputation for piezo-microphonics). I ended up paralleling in some extra power supply filter caps to help out the venerable original multistage electrolytic. It’s pretty quiet overall, despite the chassis being used as a ground for everything, and considering it’s full of 50 year old carbon composition resistors it’s not hissy. The fact that nothing has ever had any voltage across it during all that time probably helps. It does crackle occasionally once warmed up, as the valve sockets are low grade with little tension possible for the pins, and will probably have to be replaced. These were mid to budget priced amps for the time, and parts and build quality shows it in places. However, much of that is what makes for the sound of these Valco-made amps, so you really don’t want to get in to ‘blueprinting’ them.

As for the Epi Valve Special, that came back to life as my test bed amp, with a two channel AX84 High Octane circuit taking either a 9-pin or an octal power valve for single ended output. I use it to evaluate different valves, caps etc. It has two HT settings for running at either 250 or 300V, and a 25W rheostat that goes from 0 to 1k ohms for varying the power valve cathode bias, and various output impedance matching options. With two cascaded gain controls per channel plus a master volume (so basically a hotrodded Marshall JCM800 preamp topology), and only breadboard quality lead dress and ac heaters, unsurprisingly it starts to oscillate when everything is up on ’10’…