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.

Tube Radio Guitar Amp: Part 2 Plugging in…

After thoroughly checking the internet ressources and mostly figuring out which buttons do what, it was time to fire it up.  First I had to dig out some speakers to at least have some load connected to the output transformers to avoid them burning up.  IMG_5065

Well, incredible enough: the zombie came alive…. and it’s name is Steve Tyler.

Not all bands are working, but that’s not the part we care about anyway: we want to plug in our guitar.  For that we use the PU/phone/GRAM input on the radio.  In my case is was a sort of three prong plug: left signal, ground, right signal.  I salvage a two prong from another radio, had to snap it in two, then soldered one to the tip, another to the lug of an input jack.  Guitar is mono, this radio is from 1965 and a stereo unit, so we only use either left or right channel. You always have to connect to the middle, that’s ground.

IMG_5066Of course, you will also have to switch the radio to activate the phone channel.

IMG_5072

Well, the sound when playing through a Epiphone Les Paul is mostly a kind of dog fart like fuzz tone that makes the cat up and hit for the garage, the real buy celexa prices lofi dirty gritty overdriven sound. And the volume is also lacking…

Speakers: I think these are 4 ohm speaker, while the output transformers like to see 8 ohm on each speaker. I am overdriving the speaker and overheating the output transfo’s.  I need to address that issue tomorrow.

Settings: The best tone and most volume is with all tone button disengaged. Mono turned on, but even in that mode, the Stereo Exp. function seems to do something (good).

Power Filter capacitors

These suckers are dead as Zed, after 55 years…no doubt about it.  I didn’t have immediate replacements for the big canned 50uF caps, but a third 8uF cap was much more in reach and obviously in need of replacement:

IMG_5076Here it is located in the schematic as C3:

powersectionI rolled up a home made 8µF/900V cap made up of two 4µF 450V’s in parallel:

IMG_5077And mounted them like this:

IMG_5078

This gave me a tad more volume.

Input Resistance

The factory schematic shows 100k resistance in series with the phone jack on both channels. Decreasing the value to 68k (standard input resistance on most Fender amps) also will increase the volume a bit.  You may jumper it altogether, but if you start picking up Russian RF signals, it’s not the radio.

phonoinput

 

 

Tube Radio Guitar Amp: Part 1 Exploring

I bought another tube radio, it’s a Philips B5X42A.  This is getting seriously addictive, but it’s better to be fixed on €10 junk nobody really wants than getting hooked on buying every Vox amp around, even that silly little white one.  So I spotted a guy selling several tube radios on a classifieds website, contacted him and went over there. He opened up his garage and there was a plethora of tube radio and bakelite goodness. I could only be honest:

‘That’s a stunning collection you have there. I am really looking for a tube radio to convert into a guitar amp. So anything you have lying around of which the radio part isn’t properly functioning, but the main amp is, I am interested in. The higher the wattage, the better.’

‘But then you are better off buying a guitar amp, that’s better suited than this. I don’t have any real throwaways, I repair everything myself.’

‘I have several great tube amps at home, a sixties Fender and an original 65 WEM Watkins. I also have two 5W single ended amps and am currently building a 50’s design guitar amp. But have you ever bought a new power transformer for a tube amp? Do you know what they cost?’

‘I just throw them away, had a box of them around here until last week.’

‘Well, if you want to build a guitar amp, the cheapest transformers you can buy are around €50, this is for 5W amps’

‘Whot?’

‘I tell you, if I buy a €15 non working tube radio that isn’t burnt out, I can probably salvage all power transformers (+€50/piece), output transformers (+€30/piece).  I knows it’s junk and the real reason tube technology went down the drain, but if you have to buy them in specialized shops nowadays, you pay for being in a niche market’

‘What kind of tube do these guitar amp generally use?’

‘In European denomination mostly EL84 as power tubes and ECC83 for preamps…’

‘Well, then I think I might have some chassis lying around for you…’

We made a deal for €10 for a Philips B5X42B from about 1964:

philips

 

 

 

 

 

That’s how it looked back in the day, celexa 40 mg what I bought looked more like something the cat dragged in last night, and the reaction of the wife was accordingly.

IMG_5058

Mine doesn’t look at snazzy anymore, the enclosure is not included, but more importantly the looks, is finding the phono input, which may require googling the back panel of the unit in question:

philipsb5x

 

 

 

 

The pickup (PU) jack has three contacts, one for the left mono channel, one for ground and one for the right mono channel… because this is a stereo amp. It has two distinct signal paths: here is only the stereo amp part of the schematic.

deatilEach of the signal paths uses half of a ECC83 (=12AX7) for preamp and one EL84 as power tube. If you consider that the rectifier in this radio is EZ81, it become clear that you have everything to here to build an Vox AC-15 (minus the tremolo).

The reason so many questions about converting radio’s to guitar amps remain unanswered is clear to me now: using it as a guitar amp requires no work at all, just connecting to the phone input. But converting it means getting rid of everything that is in the red square:

schematiccomplete

 

 

 

 

That is like 2/3 of the circuit.  This may seem like a difficult job, but it really isn’t because these parts of the circuit are switched in and out.  In fact, we should also get rid of the double stereo part. In fact, we can make one hell of guitar amp using this chassis, power transformer, tube sockets, even tubes… we may have to buy a new output transformer, BECAUSE: if the power transformer is powering the amp section (= one AC15) plus a whole bunch of other tubes, it may become the heart of a serious amp. I dare not believe what the label promises:

philips

That the cabinet is not included, is a shame, it would need almost no work to be converted into a nice guitar amp head:

b5x

 

Not exactly the same model, only the cabinet is different. Very helpful if you want to know which controls do what.

Stay tuned (haha!) for part 2 where we will be plugging in…