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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 15th Aug 2019, 7:14 pm   #41
Paul JD
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Default Re: Convert Vintage Valve Radio to Guitar Amp

A shorted filter capacitor (C21 on the Trader Sheet) would be the most likely cause. It looks like C21 and C22 are a dual section can capacitor mounted on the top of the chassis. This could be replaced with a modern dual section cap such as this https://www.cricklewoodelectronics.c...Capacitor.html but you would need to check the size to see if it will fit. Alternatively you could replace it with 2 separate electrolytics such as these https://www.cricklewoodelectronics.com/33A450.html These could be installed under the chassis and just disconnect the existing can capacitor and leave it in place.

With a bit of luck the transformer may still be OK, you could disconnect it and check the DC resistances of the windings to confirm.
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Old 15th Aug 2019, 7:36 pm   #42
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Default Re: Convert Vintage Valve Radio to Guitar Amp

I would ask/demand a full refund on the basis that the packing was wholly inadequate to protect the radio. I wouldn't bother with that set any more, it's too bashed about and the glass is broken and who knows what else has been damaged; valves, broken joints etc etc.

Plenty of vintage wireless sets out there to buy and work on without trying to fix a basket case as a result of someone else's negligence.
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Old 15th Aug 2019, 8:28 pm   #43
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Default Re: Convert Vintage Valve Radio to Guitar Amp

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Originally Posted by Paul JD View Post
A shorted filter capacitor (C21 on the Trader Sheet) would be the most likely cause. It looks like C21 and C22 are a dual section can capacitor mounted on the top of the chassis. This could be replaced with a modern dual section cap such as this
Thanks for the tip. I have put a multimeter across the terminals of the capacitor, and they don't seem to be shorted. However, that cap was bearing the load of some of the transit, and was bent over a bit when I unpacked it.

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I would ask/demand a full refund on the basis that the packing was wholly inadequate to protect the radio. I wouldn't bother with that set any more, it's too bashed about and the glass is broken and who knows what else has been damaged; valves, broken joints etc etc.
I have already asked the seller and will really push for this. Completely crazy to think it could withstand being just put in a cardboard box!

Quote:
Plenty of vintage wireless sets out there to buy and work on without trying to fix a basket case as a result of someone else's negligence.
I have found a KB HR10 locally, which I can pick-up (not sure I want to risk another one through the post). The seller says that it worked last time they switched it on, but can't remember how long ago that was.

The HR10 looks to have an isolating transformer too, which is good (please can someone confirm I am correct on this assumption).

However, it doesn't have a gram input. Can I just sever the radio circuit and hook straight into the pre-amp? If so, where would this be in the circuit and would I need to make any other modifications?

Sorry for all the questions, the radio being smashed up wasn't part of the plan
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Old 15th Aug 2019, 8:38 pm   #44
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Default Re: Convert Vintage Valve Radio to Guitar Amp

The KB HR10 is a mains auto transformer job which means it's a live chassis design (chassis not isolated) I would avoid.

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Last edited by ms660; 15th Aug 2019 at 8:45 pm.
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Old 15th Aug 2019, 9:10 pm   #45
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Default Re: Convert Vintage Valve Radio to Guitar Amp

Thanks.

How do I tell the difference from the circuit diagram?
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Old 15th Aug 2019, 9:12 pm   #46
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Default Re: Convert Vintage Valve Radio to Guitar Amp

Is it because the black (Neutral) line is connected directly to what would be the main power rail, rather than coming from the transformer coil?
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Old 15th Aug 2019, 9:17 pm   #47
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Default Re: Convert Vintage Valve Radio to Guitar Amp

Effectively, yes.

For isolation, the set needs a mains transformer with a mains primary winding which is completely separate from any windings feeding the set itself and no other connection of any sort between either live or neutral and the set's circuits.
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Old 15th Aug 2019, 9:26 pm   #48
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Default Re: Convert Vintage Valve Radio to Guitar Amp

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Is it because the black (Neutral) line is connected directly to what would be the main power rail, rather than coming from the transformer coil?
Yes, that's correct.

A mains isolating transformer will have a primary winding and one or more secondary windings, the primary and secondary have no electrical connection between them, just magnetic coupling.

EDIT: Post crossed.

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Old 15th Aug 2019, 9:54 pm   #49
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Default Re: Convert Vintage Valve Radio to Guitar Amp

Cool. Thanks

So the Cossor Melody maker 501 AC should be fine then from what I can see. It also has a PU input, which makes life easier.
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Old 16th Aug 2019, 9:33 am   #50
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I have put a multimeter across the terminals of the capacitor, and they don't seem to be shorted.
Testing with a multimeter won't really tell you very much, the only way to know for sure is to measure leakage current at the cap's working voltage. Any old electrolytic capacitor that has not been used for some time is almost certain to exhibit significant DC leakage, in some cases they can be reformed by applying voltage through a suitable current limiting resistor for a period of time (Google "reforming capacitors"), in other cases they will need to be replaced.

The Cossor 501 you linked to looks like it could be a suitable candidate, just make sure it is the 501AC model and not the 501U, the 501U is an AC/DC radio with a live chassis.
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Old 16th Aug 2019, 10:06 am   #51
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The Cossor 501 you linked to looks like it could be a suitable candidate, just make sure it is the 501AC model and not the 501U, the 501U is an AC/DC radio with a live chassis.
Blimey, this is a minefield!

I presume the only to tell, is to look inside and make sure it isn't fitted with the octal valve complement (OM10 and OM6), which aren't glass valves?

The back panel says "Cossor Melody Maker Model 501 AC"

Looking at the Radio Museum website the 501U clearly states that it is a 501U on the back panel, and since mine says 501 AC, I think I am safe.

The 501U is a ac->dc model, so the fact that the radio I am looking at says "501 AC" gives me further confidence.

Does all this make sense, or am I talking noob rubbish?
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Old 16th Aug 2019, 10:08 am   #52
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Default Re: Convert Vintage Valve Radio to Guitar Amp

I then need to work out how to earth the radio section, because the 501 doesn't have a specific gram switch to isolate the radio when using the PU.
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Old 16th Aug 2019, 10:28 am   #53
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Default Re: Convert Vintage Valve Radio to Guitar Amp

Minefield is an apt term. You only have to think of the number of professional guitarists electrocuted on stage by commercial, purpose-made amplification equipment.

Yet, people on here routinely fix live chassis radios and TVs. The key is in understanding the risks and taking action against them, in understanding what faults could develop and having precautions in place.

Trying to convert a live chassis set into a guitar amplifier is simply too risky to contemplate. Live chassis sets got used in the homes of non-technical consumers by being totally surrounded by insulation. Even the little grub screws holding knobs on had to have the top part of their hole stopped up with hard wax. But with a guitar or microphone, metal bits touched by the user get connected to the circuitry of the amplifier.

So you want a set with a proper mains transformer.

But even then, you need to take precautions. Don't just go by the model number. On an old set, someone may have lost the back cover it originally had and bodged in one they found... so you can't entirely trust the model number in all circumstances. You need to SEE the mains transformer itself and check the condition of the mains wiring, switch etc. Old insulation sometimes goes crumbly. As said above, those vintage mains transformers have internal insulation made from thin paper impregnated with wax.

It wasn't good enough to trust with people's lives even back then. In addition to the isolation of the transformer, the chassis was connected to the mains earth. The idea was that if insulation failed somewhere, current would flow to earth, popping the fuse and thereby becoming safe. It proved to be safe enough and it took TWO faults to occur before becoming dangerous.... loss of isolation AND loss of earth connection.

Nothing is ever perfectly safe.

You could go into a guitar boutique and buy a second-hand amplifier of a good make, and later found someone had been inside and disconnected the safety earth in an attempt to fix a hum problem.

For minimum safety precautions: Choose a radio with mains isolation. Make damned sure there is a solid earth to the chassis. Make sure a sensible size of fuse is fitted. Make sure power supplies in venues have proper safety earths.

This goes whether you buy an amp, build one or modify a radio.

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Old 16th Aug 2019, 10:42 am   #54
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Default Re: Convert Vintage Valve Radio to Guitar Amp

If it says 501 AC and it has a mains transformer that is fine (sorry I was probably making things more confusing than it needed to be as you did say it was the 501 AC in your post!)

Last edited by Paul JD; 16th Aug 2019 at 10:48 am.
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Old 16th Aug 2019, 11:03 am   #55
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Default Re: Convert Vintage Valve Radio to Guitar Amp

Having used both transistor and valve amps over the years I've found that the best sound, particularly for home use, is to be had using a single-ended low wattage valve amp. You just can't get that creamy overdriven sound with a transistor amp.
There's a very good reason most professional guitarists still stick with valve amps.

But, I wouldn't recommend using an old valve radio for all the reasons already mentioned above eg. potential safety issues, inadequate speaker, low sensitivity, input impedance issues etc.

I built the amp shown here:

http://www.r-type.org/exhib/aai0120.htm

(using an isolated and fully earthed power supply of course and omitting the 3.3M feedback resistor if desired)
and used a couple of old Peerless bass/midrange speaker units connected in parallel built into a simple cabinet.

When driven via a single transistor treble booster I get remarkably good results and a much better sound than the Fender and Vox transistor amps I have.
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Old 16th Aug 2019, 11:49 am   #56
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But, I wouldn't recommend using an old valve radio for all the reasons already mentioned above eg. potential safety issues, inadequate speaker, low sensitivity, input impedance issues etc.
Any potential safety issues are easily addressed with care and are certainly no different to the safety issues involved in building an amplifier from scratch.

The OP has already stated that he plans to use a different speaker.

There are NO ISSUES with sensitivity or input impedance (as I have already pointed out several times).
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Old 16th Aug 2019, 11:49 am   #57
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Default Re: Convert Vintage Valve Radio to Guitar Amp

You could do a Mo Foster and go for a Murphy A188C.

Lawrence.
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Old 16th Aug 2019, 12:15 pm   #58
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Default Re: Convert Vintage Valve Radio to Guitar Amp

This is getting away from Vintage Radio (domestic). Shouldn't the post be moved to General Vintage Technology Discussions, or Components and Circuits, or even Homebrew Equipment.
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Old 16th Aug 2019, 2:06 pm   #59
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Default Re: Convert Vintage Valve Radio to Guitar Amp

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Any potential safety issues are easily addressed with care and are certainly no different to the safety issues involved in building an amplifier from scratch.

The OP has already stated that he plans to use a different speaker.

There are NO ISSUES with sensitivity or input impedance (as I have already pointed out several times).
Paul JD

There could be issues with both input impedance and sensitivity depending on the input circuitry of the radio being used along with the type and sensitivity of the guitar's pickups.

(note the lack of unnecessary uppercase)
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Old 16th Aug 2019, 3:24 pm   #60
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Default Re: Convert Vintage Valve Radio to Guitar Amp

The audio stage of a valve radio is designed to take the output from the preceding stage or the gram/PU input which will be designed to plug a ceramic cartridge into. Both of these require a high impedance input to ensure they are not loaded down therefore it will always be a high impedance input and as such will be suitable for an electric guitar pickup. Almost every valve radio I have ever seen has had either a 500K or 1M pot on the input to the audio stage.

The type and sensitivity of the guitar's pickups are irrelevant as they are all high impedance (unless you have a Gibson Les Paul Recording model which was designed to plug into a mixing desk, not a guitar amplifier). All amplifiers have to accept a wide range of pickup outputs as the designer can not know what type of guitar it will be used with, fortunately the triode gain stage that is used on the input of valve radio audio stages and on almost all valve guitar amplifiers will cope quite happily with that range.

Just for information I design, build and repair valve guitar amplifiers for a living and have been working on valve guitar amplifiers for over 35 years so I do have a fairly good understanding of what will and what won't work as a guitar amplifier.

(My use of uppercase was to reinforce a point that I had already made several times yet people are still bringing up the issue of input impedance as a problem when it is not. If you consider that unnecessary then you are perfectly entitled to that opinion)
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