Thursday, 1 February 2018

EBS ValveDrive

This is another 2017 leftover, I want to write it down before I forget it.

EBS's ValveDrive is a valve overdrive/preamp that uses a single 12AX7 dual triode. This is the original version, there is a newer model that adds a balanced XLR output and has an internal switching power supply.
 The "vintage" circuit is similar to various Fender Baseman preamps, the "modern" setting adds some diode clipping - the original schematic is available at It uses a 12 VAC power supply and has an internal 12 VAC transformer wired "backwards" to supply the high voltage for 12AX7 plate. This used to be a very popular pedal with bass players, especially as the high voltage supply means it can get really loud and will happily drive a standalone power amp. Maybe it is not as fashionable right now, I don't see them as often as ~5 years ago.

Like nearly everything that comes across my bench, it doesn't work.

After verifying that my 12 VAC supply was working, I removed the top shield and looked at the valve. No wonder it does nothing, the vacuum has escaped from this one.

I put in a Jet City RetroValve that I keep around for testing, and still nothing. After pulling the valve and probing the socket I measured 0V at the plates - something wrong with the high voltage supply. The heaters are getting 12VDC, so low voltage power should be OK.

A leak has developed somewhere...

The build quality is very good, a nice folded steel enclosure and multiple PCBs connected together with ribbon cable (switching signals) and runs of coax for the audio signals. This was probably expensive to manufacture but it's hard to kill and convenient to work on.

The high voltage transformer is on the lower left of the topside of the PCB. The surface mount fuse on the backside is not blown, but the transformer looks weird, as if it had been re-soldered or replaced. The output winding only measured around 0.5 VAC with the power supply connected.

Transformer pins are not coming through the plated holes...

Additional holes drilled in PCB...
Once it was desoldered, it was obvious why things looked strange. The transformer pins were not going through the plated through-holes, instead someone had drilled holes in the PCB (!) and bridged over to the pads with some bus-bar to make the transformer fit. The low voltage measured at the transformer made sense now, scaling 12 VAC to 230 VAC is a roughly 19.16 ratio. If the transformer was scaling 12VAC down by the same ratio I should get 0.6 V AC, which is pretty much what I had measured.

I reinstalled the transformer the other way round and now measured 230V AC, and 400+V DC at the valve plate with no valve installed. With a new valve installed, this dropped to around 350V, and everything worked and sounded great. The original filter capacitors were only rated to 400V, so I replaced this with 450V rated Nichicons in case this was ever left running with a dead or missing valve.

The part number on this transformer is slightly different from the schematic's part list, and obviously the footprint is different as well. My guess is that someone attempted to replace the transformer, installed it backwards, and then gave up and sold the pedal on. The slightly higher plate voltage(~350V vs 300V) is probably due to small differences between transformers.

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