Tube Radio Guitar Amp: Part 3 Making Plans – Vox Pacemaker 1965

Yes, there is a very nice and decent guitar amp hidden inside this scruffy old Philips radio, all the components except a suitable output transformer and speaker are present to solder up this nice gem:

Huh? Why? How?

Power rectifier EZ81 — If you are in Europe and hunting for tube radio with any value in the guitar amp field, check for this rectifier tube. If you see it, buy the radio!  This tube is used is the early Vox amp, up to the AC15 as power rectifier.  Most vintage American amp you will be looking at require at least 3 different voltage from the transformers: a high voltage, 6.3V for the tube filaments and 5V for the rectifier. The EZ81 does not require the special 5V, just feeds off the high voltage.

phillipspowersectionIn this way, it is just like a Vox Pacemaker — although below schematic is from a Cambridge Reverb (the same amp + reverb):cambridgereverbpower

Or an AC-15, also using an EZ81:

ac15_detail

And all of these are unlike any 50’s Fender amp, which require three voltages on the transformer 6.3V, HV and 5V especially for the rectifier:

5e3_rectifier
Transformer and rectifier tube in a Fender 5E3 Tweed Deluxe amplifier.

2 Power tubes EL84 & 1 Preamp tube ECC83 — if the transformer can power this in our radio, we need only one extra ECC83 tube to make a Vox Pacemaker. It uses 2 x EL84, 2 x ECC83 and an EZ81.  The Philips radio powers and additional 5 tubes for RF purposes, so it is certain that this transformer from a piece of €10 junk can function as the heart of a Vox Pacemaker, Vox AC-15 or Marshall 18W…. value: $75. My calculation seems to indicate this transformer could pull 5.4A on the high voltage.  I don’t believe you could find one that would feed anything more than a 25W amp.

Voxpacemaker_corrected

The only thing that is missing, is the output transformer, which will cost around €50. Chassis, resistors (after measuring), 6.3V pilot light and mounting screw can all be salvage from the zombie corpse.

I really think you could use a lot of the power transformers in old radios, but the output transformers from household radio sets will never be strong enough.

2 thoughts on “Tube Radio Guitar Amp: Part 3 Making Plans – Vox Pacemaker 1965”

  1. Hi Zjokka,

    There is a flaw in your argument. Guitar amp tubes are working on higher voltages than radio tubes. Have a look at the Cambridge reverb. There’s 260V AC on the secondary windings and that could go up to 300V in other amplifiers using different tubes.

    Tube radios commonly have a single ended output stage using EL84 or 6V6 tubes. They operate around 180-200 plate voltage. Their transformers are built for that maximum voltage and put out 180-190V AC on the secondary windings. It will work, but you’ll have very little headroom. Try using diodes for the rectification. That should bump up the B+ with 10 to 20V.

    I’m working on something similar using a Philips B3W22a. Let me know how yours works out.

    Kind Regards,

    Koen

    1. Hi Koen,

      Thanks so much for your comment and putting me out of my misery. I just put it out there in the hope of somebody reacting… you did! As a matter of fact, I’ve given up on this project. I was warming up for a Tweed Deluxe which is now completed. In the meanwhile I discovered Geloso amps… which for some strange reason can be had for halve of the price they go for on Ebay Italy. They make terrific amp platform and can be had had for the price of a decent aftermarket tube amp transformer.

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