Vox AC30 Birch Stolec 1970 – Valve Tube Guitar Amps

Vox AC30 Birch Stolec 1970

Vox AC30 Birch Stolec 1970

Vox AC30  Birch Stolec 1970

Here is a summary of the work done to restore another Vox AC30 Another Birch Stolec 1970.

Background:

When Jennings Musical Instruments ceased operations, the assets were bought by an organization called Birch Stolec and they created the company Vox Sound LTD.
This is a rare model.  It was only manufactured for a number of months using the Jennings Musical Instruments design.  It was replaced with a PCB version which was extremely unreliable and awful to repair.
It is voiced to suit the early sounds of rock. You can read more about Birch Stolec Vox AC30’s on this link
Our customer was lucky enough to have a “pre-PCB” version. Our one looks identical to the top chassis in the picture.
This amp has serial number 26566 ( difficult to read) and it is the more desirable Top Boost version.  The old electrolytic capacitors were dated October 1970 so we know it was made around this date.

Summary of Issues

  • Very weak sounding with lots of mains hum. Amp had no punch and did not sound like an AC30 at all.
  • Speakers were wired in parallel for 4 Ohms and connected to the 8 ohm output. If left, the output transformer would have been destroyed
  • The switching system could have resulted in the output transformer being left open circuit which would have destroyed the output transformer
  • The HT volts on the B+ supply was only around 220 volts rather than the expected 340volts. This then resulted in the preamp vales being starved of volts. This partially explained the anemic sound but it did save the output transformer due to the low power output
  • The EL84 output valves were totally worn which also explained the weak output and lack of tone. These were replaced.
  • At this point the pre-amp  valve tested as a bit worn but still serviceable
  • The amp had seen some work in the past so it was not a green field site

The power supply volts drop was due to half century old electrolytic capacitors having current leakage and pulling down the volts
Main power supply and Output Stage

blankVox AC30 Birc Stolec PSU Output

  • The above two pictures show the restoration of this section and the replacement of the capacitors plus some resistors with modern but more vintage looking resistors. The capacitor which is seen on the chassis beside the valves has no modern equivalent, so the capacitor was replaced by two underneath the chassis. They are on an added section of tagboard and the original capacitor left in place for cosmetic purposes.  It is not seen in the picture but the silver can capacitor on the left of the picture was also replaced.

Preamp Section

Vox AC30 Birch Stolec Preamp section

  • All electrolytic capacitors were replaced. When the amp was fired up, the supply voltages were all in the correct range.
  • The original GZ34 rectifier could not withstand the correct increased voltages and flashed over. It was replaced.
  • The amp now started operating, however the pre-amp valves started failing one by one. The proper volts were just too much for them.  After two failed and one extremely microphonic, it was decided to replace them all, otherwise, I was spending more time diagnosing phantom faults caused by the valves.
  • Some resistors had drifted off causing issues and at least one signal capacitor was leaking current. These were replaced.
  • The pots were noisy, but they were cleaned and are still serviceable.
  • It is worth noting that the output transformer has possibly been replaced at some point

Speaker Wiring

  • The switching system was removed. A speaker switching system had been added and this could result in the amp running with no speaker connected Mods such as  this , ruin the market value of a vintage amp and the fail to danger was not good at all. No point is running the risk of a blown output transformer
  • The wiring was corrected to run on 16 ohms with the two speakers running in series. The connector block was broken and this was glued back together ensuring that the 8 ohm transformer connection is on the weaker part of the block. The 16 ohm impedance option is used. This keeps the appearance as original as possible.
  • The amp was re-assembled and soak tested for the final time. In previous soak tests various failures appeared after about and hour. The components were seeing the correct volts and this is what caused the failures. The amp has now been successfully ran for a number of hours.

Valves Used
We used the JJ ECC82, ECC83S and GZ34SFour Sovtek EL84 output valves were used as these tend to last better especially in a rock voiced amp
We also restored a pcb version of the AC30 Birch Stolec and this one was a big job as it had pcb’s that had been cooked over the decades. You can read about it here.
Overall a great sounding Vox AC30 Birch Stolec  ready to sing again!

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