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Old 12th Aug 2019, 4:51 pm   #1
robbo100
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Default Convert Vintage Valve Radio to Guitar Amp

Hi there.

New member here.

I am looking to use an old vintage valve radio to make a small guitar practice amplifier as a project with my son. I know the tones won't be perfect, but I am keen to see if you can get some interesting sounds from it (and it will be a fun project).

I am thinking of using a Ferguson 461 (or similar), because I can plug the guitar directly into the Gram socket to get approximately the right input level without a separate pre-amp.

I presume that I would need to modify the radio by adding an isolating transformer to make sure it is safe, and was likely going to remove the "radio" elements and rehouse the amplifier section into a new housing (although I may be tempted to keep it as it is, if I can get one that looks smart enough). I would then drive a larger 8 ohm speaker in a separate cabinet.

Has anyone got any tips which might be useful for such a project.

Is there anything (other than the isolation transformer) which I need to do to ensure things are safe for this sort of setup?

Thanks in advance

robbo100
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Old 12th Aug 2019, 6:56 pm   #2
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Default Re: Convert Vintage Valve Radio to Guitar Amp

The 'Gram' inputs of post war radios were designed to be compatible with high output cartridges and gramophone pickups.

The output from the guitar will probably be much smaller.

You will need a preamplifier between the guitar and the Gram input.
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Old 12th Aug 2019, 7:09 pm   #3
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Default Re: Convert Vintage Valve Radio to Guitar Amp

The Ferguson 461 already has a mains isolating transformer in its standard state. Adding another won't add anything other than another layer of complexity.

I would replace C30 (Trader sheet reference https://www.service-data.com/product...39/2541/t12039 ) with a suitable Y-class capacitor or just remove it altogether as a matter of priority and fit a 3-core mains lead with earth to chassis. It wouldn't hurt to add a dedicated RCD mains plug too.

Otherwise, just replace the usual leaky paper capacitors with modern film types and ditto any seriously out of spec resistors. Removing all the radio bits is up to you, but pulling the RF valves will reduce the load on the mains transformer a bit.

"Silicon's" comment about signal level and preamps may point the way out of disappointing results......
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Old 12th Aug 2019, 7:12 pm   #4
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Default Re: Convert Vintage Valve Radio to Guitar Amp

I had a badly modified sobell 439 too far gone to restore and have converted it into little practice amp using parts from a 50s hmv ac only radio. I used a modern car radio speaker spaced so the cone doesn't hit the cabinet
It sounds pretty good.

I love old Radios but sometimes you have to make use of what otherwise would be binned.

Obviously plugging a guitar into something like this safety is essential.

Cheers

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Old 12th Aug 2019, 7:21 pm   #5
Mike. Watterson
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Default Re: Convert Vintage Valve Radio to Guitar Amp

You'll need a pre-amp with low impedance for ANY valve radio.
The Gram/PU sockets are for hugely higher signal and x10 or more impedance.

Also most don't have a large enough speaker or very much power. So to make a PRACTICE amp you need a pre-amp.

No modification needed. Just a preamp. Also ANY radio, unless restored by someone expert, is going to fail in a few hours due to leaky paper capacitor.

Even the 1960s valve radios with a DIN socket are no use for a guitar without a preamp. The line level from a reel to reel tape recorder is similar to the crystal cartridge and even an phone/MP3 earphone is often a lower level. It's just the record player cartridge or tape line out can't drive a low impedance like modern earphones or a speaker directly.

Even a DJ mix console with magnetic phono / line in switching is no use for an Electric guitar, you need a proper mixing desk or proper Guitar Pre-Amp.
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Old 12th Aug 2019, 7:45 pm   #6
60 oldjohn
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Default Re: Convert Vintage Valve Radio to Guitar Amp

HI Robbo, Welcome to the forum, Please be aware the Ferguson 461 comes in three versions 461A, 461RG, 461U. You need the 461A, the 461RG Radiogram would be rather large. The 461U would be lethal for your aplication and should be avoided at all costs, it is a live chassis design and cannot be earthed.

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Old 12th Aug 2019, 7:49 pm   #7
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Default Re: Convert Vintage Valve Radio to Guitar Amp

This takes me back 60 years, my dad added a valve pre amp to an old radio for my electric guitar. No idea where the circuit for the preamp came from but it worked really well. He took care of the safety side, so that was ok.
Unfortunately I cannot remember what the radio was either but there was enough room on the chassis for an extra valve holder, an octal one at that.
Has Mike states it was a practice amp not something to use in a hall, my playing wasn’t good enough for that.
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Old 12th Aug 2019, 7:54 pm   #8
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Default Re: Convert Vintage Valve Radio to Guitar Amp

Hi all,

Thanks for the very fast and detailed replies.

As it turns out, I have just won an ebay bid for a KB LR10, so it looks like that is my weapon of choice, rather than the 416.

Looking at the wiring diagram it looks like this one also already has an isolating transformer fitted, so that one will be safe too. I would be grateful if someone could confirm my reading of the circuit diagram is correct though, so I don't fry myself.

I will probably just make one of these as a simple pre-amp (powered by a 9V battery). I have made them before and they work really well. That way I will be able to have some gain control going into the valve power amp stage. Amusingly opposite to most modern amps which have a valve pre-amp and SS power amp, this will have a SS pre-amp and Valve power amp

I have no idea if this amp is working or not, but I will fit a 3 core cable, earthed to the chassis (assuming you guys confirm it has an isolating power supply), before I turn it on for the first time.

Reading some of the other posts, I suspect that replacing some of the caps would be very wise before even switching it on for the first time.

Does all this make sense??

Thanks again

Last edited by AC/HL; 12th Aug 2019 at 8:02 pm. Reason: Rule B8
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Old 12th Aug 2019, 8:06 pm   #9
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Default Re: Convert Vintage Valve Radio to Guitar Amp

You're quite right, the set is isolated from the mains, and may be safely earthed.
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Old 12th Aug 2019, 8:40 pm   #10
Mike. Watterson
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Default Re: Convert Vintage Valve Radio to Guitar Amp

Quote:
Originally Posted by robbo100;1167772
I will probably just make [URL="http://runoffgroove.com/ruby.html"
one of these[/URL] as a simple pre-amp (powered by a 9V battery). I have made them before and they work really well.
Not optimal for a guitar pre-amp:
1) It's a small complete amp to drive a speaker
2) FET buffer for a high impedance input, bad match for guitar and prone to pickup hum.
3) It's more for a ceramic microphone or ceramic pickup cartridge to drive a small speaker.

A simple op-amp on it's own or a couple of npn transistors will work far better, and is simpler to build, less hum pickup and cheaper.
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Old 12th Aug 2019, 8:47 pm   #11
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Default Re: Convert Vintage Valve Radio to Guitar Amp

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Originally Posted by Herald1360 View Post
It wouldn't hurt to add a dedicated RCD mains plug too.
No point if the house is already RCD protected and probably not much point anyway if the chassis is earthed.
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Old 12th Aug 2019, 8:47 pm   #12
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Default Re: Convert Vintage Valve Radio to Guitar Amp

It's correct that a pre-amp would help you greatly but as a teenagers in the sixties we often used domestic radios [being unable to afford amps] with no mods or "add ons". The volume could be low but I was sufficiently concerned [with one example] that I unplugged the speaker in the set, replacing it with a "substitute" [not a Who joke] one in a separate cabinet I'd built from a wind up gramophone case. The wireless could then return to domestic use unharmed! I think the output varied quite a bit with different guitars. I had a Tape Recorder that could be switched to PA and protected the internal spkr in much the same way.

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Old 12th Aug 2019, 10:04 pm   #13
robbo100
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Default Re: Convert Vintage Valve Radio to Guitar Amp

Thanks all,

Are there any components which should be replaced to ensure suitable safety on an KB LR10?
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Old 12th Aug 2019, 10:08 pm   #14
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Default Re: Convert Vintage Valve Radio to Guitar Amp

Quote:
Originally Posted by winston_1 View Post
No point if the house is already RCD protected and probably not much point anyway if the chassis is earthed.
Yes, but not all are. Earthing should be sufficient but similarly not all installations are up to spec. And old mains transformers are not perfect either.

The certain extra few £££ vs your kid's possible wellbeing?
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Old 12th Aug 2019, 10:24 pm   #15
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Default Re: Convert Vintage Valve Radio to Guitar Amp

I know people who just plug their guitar straight into the gram inputs of valve radios, no pre amp necessary, they work as is.
Always worth earthing the chassis mind you, just in case. Those old mains transformers are insulated with paper and wax!
Shame to ruin that Ferguson though, nice radio
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Old 12th Aug 2019, 10:36 pm   #16
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Default Re: Convert Vintage Valve Radio to Guitar Amp

I'm not too sure why you didn't buy a practice guitar amplifier. The dynamic range of a guitar being plucked is quite something and the 1/4" throw of the voice coil from a domestic radio will cause it to bottom on the most simple of chords.

A suitable guitar amp can be found for around £20 and this will have both pre amp with tone controls, sizeable amp - 10 watts or so, 1/4" jack socket and decent loudspeaker.
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Old 12th Aug 2019, 11:18 pm   #17
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Default Re: Convert Vintage Valve Radio to Guitar Amp

That's a good point. I appreciate the tutorial aim of the OP but Guitar amps have never been more [relatively] cheap new or otherwise [like cars in fact].
In the sixties an amp chassis would be maybe £6-£15 [broadly speaking]. Then £6 [or much less] might be a good weeks wage. Beer [10p a pint] Spirits 25p a shot [rare] fish and chips not a luxury-a basic but all the same, electronic gear was expensive-hence hire purchase! Not just swipe and buy as now

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Old 13th Aug 2019, 12:21 am   #18
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Default Re: Convert Vintage Valve Radio to Guitar Amp

Quote:
Originally Posted by Mike. Watterson View Post
You'll need a pre-amp with low impedance for ANY valve radio
Electric guitars need a high impedance input. The input impedance of a guitar amplifier is typically 1M ohm.

It is perfectly feasible to plug an electric guitar directly into the gram input of a valve radio, the only caveats being that it must have an isolating mains transformer and a 3 core mains cable with an earthed chassis. The results may not be perfect, the volume level will be fairly low and there will not be very much in the way of distortion but for a practice amp it can be quite useable and produce some pleasing sounds. I have tried it myself with several radios out of curiosity and been pleasantly surprised at how good it can sound.

Most small valve guitar amps will consist of 2 triode gain stages followed by a single ended output stage. The gram input of a radio will typically have a single triode gain stage followed by a single ended output stage. It will make a fairly "low gain" guitar amp but certainly useable.
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Old 13th Aug 2019, 9:24 am   #19
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Default Re: Convert Vintage Valve Radio to Guitar Amp

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I'm not too sure why you didn't buy a practice guitar amplifier. The dynamic range of a guitar being plucked is quite something and the 1/4" throw of the voice coil from a domestic radio will cause it to bottom on the most simple of chords.

A suitable guitar amp can be found for around £20 and this will have both pre amp with tone controls, sizeable amp - 10 watts or so, 1/4" jack socket and decent loudspeaker.
I agree, I see loads of practice amps of all makes around in the boot sales. Even buying a new one they are quite cheap, there is no advantage in having a valve amp for something like a practice amp.
Vintage speakers in old radios have quite thin paper/card diaphragms and go quite brittle with age, the movement is also very limited.The speaker in the KB LR10 will probably split its diaphragm on the first chord struck and will sound horrible.

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Old 13th Aug 2019, 9:39 am   #20
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Default Re: Convert Vintage Valve Radio to Guitar Amp

Yep I don't see why you'd want to convert an old radio, well I do but there are much better options out there. You can pick up a Fender Mustang amp in one of its guises for £30 to £40. On board effects, that all important for Mums and Dads headphone socket and you can stick an MP3 player in it as well to play along to. I'd have loved one when I was learning to play.

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