Hi, you may have read from my other postings that I’m renovating a lot of guitar gear that’s been in storage for a good number of years.
Well, one of the items I’m sorting out at the moment happens to be an original Vox AC15 from either 1959 or 1960. As I’m proceeding with the refurbishment I’m taking photos along the way as a record of the procedure.
Now, a strange thing (a conundrum I suppose really) with this amp is that, although I’ve got copies of the original circuit diagrams (schematics) – when I check out the original wiring I find that it was actually wired incorrectly when it was built fifty years ago! If this wiring error proves to be similar for all the production run in those early days – could it perhaps explain why HBM got the sound he did on all those ‘60’s recordings?
The amp obviously would still have worked reasonably well, but the resulting sound would have been just that little bit different to what its designer originally intended. Could this be the key to that elusive Hank sound we all now search for? I wonder!
Anyway, to illustrate what I mean, I’m including a couple of photos of the circuitry around the two EL84 output stage.
I’m afraid that you’ll now have to excuse me for being a bit technical, but it’s the only way I can describe the wiring error correctly.
What has happened is that a 100 ohm resistor (it’s the brown / black / brown / silver banded resistor) that is supposed to feed buffered HT to the screen grid (G2) of each EL84 (each valve has one of these resistors) has incorrectly been wired in series with the output transformer primary feeds to the EL84 anodes. The screen grids (G2) have then been fed directly with raw HT from the main smoothing block instead of from HT supplied through the 100 ohm resistors. A thought then occurred to me that, although these are output pentodes and have the extra grid to stop this sort of thing, I wonder if the DC conditions could induce a sort of Miller effect that would bounce back a larger percentage of electron flow from G2 to G1 than was originally intended? Alright, I know the Miller effect is more or less a sort of internal capacitance problem between the valves’ electrodes - and pentodes are intended to prevent this problem – but it’s this sort of technical equation that could inadvertently give rise to some interesting sounds in the final mix.
If you look at the photos below and would like to check it out for yourself, the pin numbers of the EL84 output valves counting clockwise are: pin 2 = input grid, pin 3 = common cathode, pins 4 & 5 = heaters, pin 7 = anode, pin 9 = G2 / screen grid. – Pins 1, 5, 8 are not connected.
This now presents me with yet another dilemma – do I change the circuitry to correspond with the original circuit diagrams – or again, do I leave it “as is†and get the “technically incorrect†Hank sound (assuming Hank’s early AC15 was also wired the same) – what the heck do I do?
Once again, any advice and guidance would be very, very gratefully received – I just don’t know what to do for the best!
Cheers,
Alan.