Radio to guitar amplifier

[TomJS]

So first of all since I'm new to this forum so a quick introduction.

My name is Tom, I'm from the Netherlands and I'm an aspiring sound engineer.

So ever since I got my dad's old tube radio I wanted to turn it into a guitar amplifier and give it a new life. So I asked him and he loved the idea since it was a radio he had had since he was a little kid. Now I've figured out that it is a line voltage amplifier so no power transformer. The way I found out was having my guitar plugged into a jack I added and then while touching the strings touching my mixer... and whoops suddenly I felt something which was a bit worse than just a tingle. By this time I didn't know that it was line powered. So the next thing I tried was attaching a ground cable to it and suddenly all the lights went out... So I had a good start. Turned out it was nothing bad just the ground leak breaker having been flipped. So I started asking on another forum they said I should get a power transformer. I'll just copy how the conversation went.

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My main message:

Philips B3X81U Ground leak and small conversion
Post by Tom-Si on Mon Dec 25, 2017 2:14 pm

I have a B3X81U and I'm trying to get it to work as a guitar amplifier it has a phono input and this input actually gives me great sounding amp. the only thing is when I touch anything else conductive while holding my guitar I get shocked. I have the schematics could it be something what a lot of people seem to call the death cap or is it something else? I can also measure the voltage with my multimeter but it only gives me 0,2 volts ac but its definitely a lot more than that. I hope i can get some help I have the schematics (in pdf but I don't know how to send a pdf but i found them on radiomuseum.org) and if it's the death cap could someone help me identifying the death cap?

Thanks and regards,

Tom

First response:

No power transformer, appears to have one side of the line connected to the chassis. Not something to use as a guitar amplifier without an isolation transformer.

Second Response:

Schematic:

http://www.philipsradios.nl/Schemas/B3X81U.pdf

This is a line powered radio, no power transformer for line isolation.

An isolation transformer between the wall outlet and the radio would provide total protection.

"Death" cap?

It appears that C-65 and C-66 are to be used for phono input. One couples signal and one couples the return to the chassis. Do check the parts list and confirm the voltage value of the those two caps, should be better than 400 volts and a value of about .05 mf. or less. That would pass negligible AC current at that value.

If you still get a "tickle" reduce the mf. value to, say .022 or even .001mf. However expect bass tones to fall off and overall volume reduced.

Measuring leakage voltage with a high impedance meter will often resolve to line voltage. The leakage current must be measured or the leakage voltage across a known load for example 10k ohms then measure the leakage voltage across that resistance.

Check with electrical rules/laws for your jurisdiction in regard to measuring appliance AC leakage.

YMMV

My response:
Hey thanks for the response! Sorry for my very late response. Since I'm still learning A LOT I had a couple of question what is the difference between a output/power/isolation transformer and a choke? Would this amp benefit from adding a ground and do you guys know a place where I can buy the right transformer for this amp in Europe? (a link would be greatly appreciated) How do you find out if a amplifier is a series filament or a paralel? Since I'm still learning a lot do you guys have any tips on things I should read or watch to teach me the basics? I'm watching the guitologist which helps but i sometimes feel like I'm missing some basic knowledge.

Thanks and regards,

Tom

From then on no more response

I send this:
Is this a proper transformer? (sorry for the dutch text) https://www.conrad.nl/nl/block-st-63232 ... 15887.html If this is the proper transformer should ground the chassis?

Thanks and regards,

Tom

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Now my question to you guys can you help me out with this project and answer the questions I asked? Also, do you think it is worth it to make a guitar amplifier out of this thing or is the output power to low to make any noise? Is there any way I can get more gain out of the pre/power tubes? Or should I just buy a different Phillips radio with a power transformer (they are pretty common here in the Netherland) to build a guitar amplifier out of? Getting a cheap guitar tube amp isn't really an option here there not many old tube amp's on the market. I will be building an external speaker cab for the amplifier by the way.

You must now be thinking my god does this kid ever stop with his post? The answer is yes and thanks for reading this far down.

Kind regards,

Tom

[pompeiisneaks]

The first things you want to do are:

1. get an isolation trasnformer in there. there are plenty of 240V to 240V transformers that make it safe to use.
2. Get a 3 prong power cable in there and reference the chassis to earth
3. If there's a death cap remove it.

I'll look at the linked schematic to confirm above.

~Phil

[pompeiisneaks]

Ok looking at the schematic, I think the death cap is there and shoudl be removed or replaced with a high quality X1 rated cap if even that. Then you need to install an isolation transformer, it says 220V not sure if in the NL they have 220 or 240, but I'd find one and put it in.

As for the tone and drive, not sure, your amp has some very unique tubes, you could attempt to bias them a bit hotter, but really you'd only know once you swapped it into a regular audio amplifier. there seem to be two smaller impedance matching transformers at the antenna input near B1/UCC85 seems to be a typical dual triode type preamp tube with simliar pinout to the 12AX7 series, but I think it uses different heater voltages. (26V) I'd try clipping signal in right at that first grid, pin 2. and see how it sounds, take it easy on volume this may be tuned for super weak signal, not sure.

also, though, there are tons of things going on with variable caps etc, in that part of the circuit, so I may not be giving you good advice. I don't know much about radio stuff. Also there looks to be an interstage transformer that's capacitatively coupled to the next stage as well, so yeah, it's pretty complex from my understanding of this stuff hehe.

Maybe someone with more radio experience can tell you more, I should likely step back and learn too .

~Phil

[TomJS]

Ok looking at the schematic, I think the death cap is there and shoudl be removed or replaced with a high quality X1 rated cap if even that. Then you need to install an isolation transformer, it says 220V not sure if in the NL they have 220 or 240, but I'd find one and put it in.

As for the tone and drive, not sure, your amp has some very unique tubes, you could attempt to bias them a bit hotter, but really you'd only know once you swapped it into a regular audio amplifier. there seem to be two smaller impedance matching transformers at the antenna input near B1/UCC85 seems to be a typical dual triode type preamp tube with simliar pinout to the 12AX7 series, but I think it uses different heater voltages. (26V) I'd try clipping signal in right at that first grid, pin 2. and see how it sounds, take it easy on volume this may be tuned for super weak signal, not sure.

also, though, there are tons of things going on with variable caps etc, in that part of the circuit, so I may not be giving you good advice. I don't know much about radio stuff. Also there looks to be an interstage transformer that's capacitatively coupled to the next stage as well, so yeah, it's pretty complex from my understanding of this stuff hehe.

Maybe someone with more radio experience can tell you more, I should likely step back and learn too .

~Phil

Thanks for the help Phil,
You lost me at trying to clip the signal I do know what it means but I have no idea how to do it. Also the voltage standard in the Netherlands is around 230volt ac but when this Radio was made 220volt was still standard so I think all the tubes would already be biased a bit hotter than normal since the amplifier can't be switched higher then 220volt. Would this isolation transformer be any good? https://www.conrad.nl/p/block-st-632323 ... -02-715887 Also any advice on how to tackle replacing capacitors in it since it's quite a messy cabling situation in there and the schematic gives some values of capacitors in K do you know what that translates to in modern values?

Thanks and regards,

Tom

[pompeiisneaks]

I don't know for sure, as I don't read dutch, but you need to make sure the transformer can handle the current the amp pulls. I think it says .27A or 270mA of current, which would be pretty low, as the heaters alone would pull quite a bit. If you have a current clamp meter, you could put it on the input line and see how much it pulls and then maybe add 20-30% on top of that for safety.

If you're unsure how to work on these with the 240 or so volts, I'd be very careful as it can kill. Make sure to discharge the caps, etc.

~Phil

[dotfret]

According to the EU regulations, it is not legal to plug a guitar into any apparatus where there is the chance that the chassis is live. The use of an isolation transformer does not get around the regulations. You break the law if you use such an apparatus in a public place, like playing a bar.

For home use, the easy answer is to get a cheap cordless transmitter and receiver from eBay - that way, you will not be in direct contact with the radio itself.
Because this is an old radio, it is still legal to use it as a radio - but it would not be legal to build a new radio that works this way.

There are surplus valve sellers in the US who will sell you valves at affordable prices for this thing. If you visit vacuumtubes.net you will find that they cross-reference the common European types with US types, so it is easy to get replacement valves.


Lots of people have used the phono socket on an old radio to plug in their first guitar, but you should be aware that the radio will not have a lot of headroom - if you use humbucking pickups and/or pedals that increase the input signal, you are likely to find that the input stage will spontaneously drop dead. I blew the input stage on my tape recorder that way over 40 years ago, and it was painful getting it repaired.

[TomJS]

I don't know for sure, as I don't read dutch, but you need to make sure the transformer can handle the current the amp pulls. I think it says .27A or 270mA of current, which would be pretty low, as the heaters alone would pull quite a bit. If you have a current clamp meter, you could put it on the input line and see how much it pulls and then maybe add 20-30% on top of that for safety.

If you're unsure how to work on these with the 240 or so volts, I'd be very careful as it can kill. Make sure to discharge the caps, etc.

~Phil

The manual says the radio uses 55w on 220 volts so that isolation transformer can do 230x0.27=62.1 Watt's even on 220x0.27=59.4 watts or is that not how that works? Thanks for the word of caution I've been using chopsticks till now to poke around .

Greetings,

Tom

[TomJS]

According to the EU regulations, it is not legal to plug a guitar into any apparatus where there is the chance that the chassis is live. The use of an isolation transformer does not get around the regulations. You break the law if you use such an apparatus in a public place, like playing a bar.

For home use, the easy answer is to get a cheap cordless transmitter and receiver from eBay - that way, you will not be in direct contact with the radio itself.
Because this is an old radio, it is still legal to use it as a radio - but it would not be legal to build a new radio that works this way.

There are surplus valve sellers in the US who will sell you valves at affordable prices for this thing. If you visit vacuumtubes.net you will find that they cross-reference the common European types with US types, so it is easy to get replacement valves.


Lots of people have used the phono socket on an old radio to plug in their first guitar, but you should be aware that the radio will not have a lot of headroom - if you use humbucking pickups and/or pedals that increase the input signal, you are likely to find that the input stage will spontaneously drop dead. I blew the input stage on my tape recorder that way over 40 years ago, and it was painful getting it repaired.

"The use of an isolation transformer does not get around the regulations." wouldn't the isolation transformer do exactly that, make sure the chassis is not live? But what I get from what you're saying is it's pretty much not worthy to invest in the radio and I'm better of getting one with a power transformer included. Is that right? Or should I look for a whole different kind of amplifier that will work for guitar and also pedals without blowing the input stage? If so what should i look for instead of a radio? Since, as I mentioned before there is no market here in the Netherlands even broken guitar tube amps will be at least 200 euro's and that's for unknown brand stuff.

Thanks and regards,

Tom

[dotfret]

The isolation transformer could make it safe to touch exposed parts of the chassis - but only if wired correctly, and you should also consider that you are dealing here with something that is roughly 60 years old. The transformer would not prevent the hazard of a circuit failure that could result in 230V leaking out and hitting the strings - that is why the regulations will not let you plug into it.
Remember that most of the other advice here is from the US, where the mains mostly gets up to 120V, which is rarely fatal.

To cover some more points:

The output will not be more than a couple of watts - not much use for anything but home practice.
You are unlikely to find a radio with a significantly loud output stage that you can modify. Your best opportunities for modification will be with old organs or cine projectors. Cine projectors were often 110v, and supplied with a separate large transformer - you need that to go forward with such a project.

Yes, the Netherlands is an amp-building desert. This is mainly because of those EU regulations, which say that you can't build your own amp and use it in public, or sell it, unless it is CE marked, and it costs around €4,000 to get the testing done by an independent company. On The Other Hand, there are companies that will sell you all you need to build an amp, and if you don't tell anyone I won't ...

[Synchu]

I have a friend of mine that ignited the "scavenge" bit. And with a bit of help from me, he already built few amps and I have a built one recently using the PT and OT ripped directly of an old radio. All of these radios used pretty standard SE EL84 power amps, so quite well suited for guitar amplification purposes. The trafos are not great, but decent.

What I would do is to rip off the internals, get a proper power transformer and rebuild the amp around this (rather rare) UL84 power tube.

Edit: just looked at UL84 datasheet and this is an odd bit, so typical power transformers for tube amps wouldn’t work, looking at custom wound is not a cost effective option.

If you wanna go the old radios route, as in EU organ amps are quite a rarity, I would look for a 2 x EL84 powered radio. Here in Bulgaria they can be found for as low as 50 EUR.
The other option is eBay or UK gear selling Facebook groups. I just scored a Traynor YGL3 for 200 pounds. Just the transformers in it would cost more to get. It is a matter of searching a bit and a bit of patience.


Niki

[TomJS]

I have a friend of mine that ignited the "scavenge" bit. And with a bit of help from me, he already built few amps and I have a built one recently using the PT and OT ripped directly of an old radio. All of these radios used pretty standard SE EL84 power amps, so quite well suited for guitar amplification purposes. The trafos are not great, but decent.

What I would do is to rip off the internals, get a proper power transformer and rebuild the amp around this (rather rare) UL84 power tube.

Edit: just looked at UL84 datasheet and this is an odd bit, so typical power transformers for tube amps wouldn’t work, looking at custom wound is not a cost effective option.

If you wanna go the old radios route, as in EU organ amps are quite a rarity, I would look for a 2 x EL84 powered radio. Here in Bulgaria they can be found for as low as 50 EUR.
The other option is eBay or UK gear selling Facebook groups. I just scored a Traynor YGL3 for 200 pounds. Just the transformers in it would cost more to get. It is a matter of searching a bit and a bit of patience.


Niki

Thanks for the help! I've only been able to find single ended EL84 radios. I'll keep you guys up to date!

Thanks and regards,

Tom

[TomJS]

So I'm kind of looking at one of these: https://www.radiomuseum.org/r/philips_b4x23a74.html it's only single ended but apart from that what do you guys think? I do like the shape since you can make it look like a guitar amplifier head.

Thanks and regards,

Tom

Edit:

I did find out that the speaker output is 5 ohm which seems weird to me but hey maybe. See foto: https://assets.catawiki.nl/assets/2017/ ... 53f8e9.jpg Would this be a problem?

[M Fowler]

I've built several small amps from console record player amps rather then Zenith radio chassis much better, besides they have PT with 6 volt filament audio tubes.

Many are two 6V6 push/pull like the ones I found but the youtube video below has 6BQ5/EL84 tubes.

Mark

https://www.youtube.com/watch?v=juGeOgPowks

[dotfret]

I should point out that the UL84 is not exactly similar to the EL84 - the 6.3V valve that is similar to the UL84 is the EL86, which has a slightly lower power and anode voltage specification.

That Philips radio is a much better choice - 6.3V heater string. The 5 ohm speaker is standard Philips practice, but there is no problem with that because the output transformer is built to match it.


You could use the EL84 and EZ80 from that radio as the basis of a single-ended guitar amp. To go along with established guitar amp designs, you would want a pre-amp valve like a 12AX7 or Russian 6n2p. There is also the possibility of building a Bird Talisman with an EF86 - http://www.vintagehofner.co.uk/britamps ... tali4.html

[TomJS]

I should point out that the UL84 is not exactly similar to the EL84 - the 6.3V valve that is similar to the UL84 is the EL86, which has a slightly lower power and anode voltage specification.

That Philips radio is a much better choice - 6.3V heater string. The 5 ohm speaker is standard Philips practice, but there is no problem with that because the output transformer is built to match it.


You could use the EL84 and EZ80 from that radio as the basis of a single-ended guitar amp. To go along with established guitar amp designs, you would want a pre-amp valve like a 12AX7 or Russian 6n2p. There is also the possibility of building a Bird Talisman with an EF86 - http://www.vintagehofner.co.uk/britamps ... tali4.html

Yeah but wouldn't the 5 ohm speaker output cause a problem if i build my own speaker cabinet with it? Since I don't know if these small speakers will give of enough low end. I would love to be able to switch between de standard 5inch speakers and maybe a 2x 10 cabinet or something.