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Old 27th Jun 2020, 5:54 pm   #1
earlybay
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Default Help / advice needed - converting my Murphy 146 into a guitar amp?

Recently I acquired one of the Murphy 146 'batwing' radios as a restoration project.

However, given my love of old vintage guitars and amps I am now wondering how feasible it would be to convert it to an amp ?

I have a pretty good knowledge of electronics, but I have never worked with valve based circuits before .. (with all the talk of very high voltages etc..)

As I look inside the Murphy it looks in simple terms to be the tuner circuit in the top of the cabinet, and the amp on the chassis in the bottom ?

Any advice on whether this is a silly idea, or indeed what would be involved ?

Thanks,

Mike
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Old 27th Jun 2020, 6:41 pm   #2
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Default Re: Help / advice needed - converting my Murphy 146 into a guitar amp?

It looks like it's already got a 'gram' input. That would be the way to get external audio in. I don't know either the guitar's output or the radio's sensitivity well enough to know whether any additional amplification or attenuation would be needed though. And given the tragic consequences of a fault you would want to be very sure indeed that there was no chance of the mains getting back to your guitar.

Cheers,

GJ
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Old 27th Jun 2020, 6:51 pm   #3
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Default Re: Help / advice needed - converting my Murphy 146 into a guitar amp?

I beleive the Murphy 146 uses an autotransformer so is not isolated from the mains. I'm not sure whether it has a gram input, but I wouldn't recomend plugging a guitar into it as it stands!
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Old 27th Jun 2020, 6:58 pm   #4
GrimJosef
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Default Re: Help / advice needed - converting my Murphy 146 into a guitar amp?

I was working from this pic. But if it's not properly mains isolated, and reliably earthed, then I absolutely agree. I'd want a reliable isolation transformer, with safety earthing on the guitar side, between me and it.

Cheers,

GJ
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Old 27th Jun 2020, 7:00 pm   #5
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Default Re: Help / advice needed - converting my Murphy 146 into a guitar amp?

I am not familiar with these radios but on the one hand - I think it better to keep a good old radio as original as possible and on the other - It is better for a first project to build a valve based amp from a valve based amp design rather than to jump a few stages and end up in a mess. My suggestion would be to search out an old guitar amp for refurb as they would likely be more of a match for what you need. A radio circuit is not really designed for that purpose and would be lacking. Yes - Do what you wish but at least put together a few test circuits first to learn.

Others here will know more about the radio but, apart from the distinct possibility or the demise of yet another product of yesteryear, I feel a converted radio is a poor compromise when a bit of research (and obviously a bit of expense) could result in a unit you are proud of.

Yes. I know it is nice to make use of what you have but there is a world of difference between a valve radio and a guitar amp. Not insurmountable, and to some extent many of us have been there but I feel a simple valve amp design would be a better direction. Have you tried looking up circuitry for old vintage guitar amps (or even the availability or those for refurb). In my view the sound quality of a radio audio circuit would not be ideal for your needs and really not show an old guitar to it's best.
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Old 27th Jun 2020, 8:46 pm   #6
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Default Re: Help / advice needed - converting my Murphy 146 into a guitar amp?

Itís push pull output and a decent sized speaker so it should give very good audio. However it does have an auto transformer as has been mentioned so I also wouldnít fancy using it without isolation. Iím not sure of the output from a guitar but I suspect some pre amplification would be needed.

My main concern would be the speaker. A power chord on a guitar I suspect could damage a speaker designed for mellow radio reproduction. I think guitar speakers used to be rated at twice the amp output for this reason. I honestly donít think the speaker would be up to it. Just my opinion of course!
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Old 27th Jun 2020, 9:20 pm   #7
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Default Re: Help / advice needed - converting my Murphy 146 into a guitar amp?

One of those often repeated late night 60s music programmes features one of these from memory. Quiet strumming, not heavy metal though. You will need a mains isolation transformer to use it safely as mentioned above.
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Old 27th Jun 2020, 10:13 pm   #8
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Default Re: Help / advice needed - converting my Murphy 146 into a guitar amp?

Mo Foster with his bass guitar played through a Murphy A188c......50's Britannia

Lawrence.
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Old 28th Jun 2020, 1:12 am   #9
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Default Re: Help / advice needed - converting my Murphy 146 into a guitar amp?

That'll be the one, perhaps not quiet but not enough to disintegrate the cone!
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Old 30th Jun 2020, 2:16 pm   #10
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Default Re: Help / advice needed - converting my Murphy 146 into a guitar amp?

Use to have an old Phillips valve radio with a gram input and fed the guitar through that in my younger days - even though it had a proper transformer the grid cap was a bit leaky so after an hour or so you got a slight tickle off the strings!! Would not reccomend it BUT one possiblity is a Practical Electronics circuit from years ago that used optical isolation - the receiving side was an ORP 12 and preamp connected only to the main amp while the transmitting side used a small lamp(? might have been an led) and self contained powered amp circuit running off a PP3. The electrical isolation would be complete that way.
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Old 1st Jul 2020, 10:07 am   #11
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Default Re: Help / advice needed - converting my Murphy 146 into a guitar amp?

The pickups will be looking for around a 200K termination resistance. Assuming the Murphy is designed for a ceramic cart, the Gram IP will be a lot lower than this, which will create a filtered effect. A transformer which steps down in ratio will isolate the guitar for safety, but will also give better frequency response, as it will decrease the output Z at the same time. Assuming the Gram IP works ok as it is, you could try a transformer or two right now, and you will know whether the idea is a goer within a few minutes of experimentation.
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Old 1st Jul 2020, 11:16 am   #12
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Default Re: Help / advice needed - converting my Murphy 146 into a guitar amp?

Quote:
Originally Posted by avocollector View Post
Use to have an old Phillips valve radio with a gram input and fed the guitar through that in my younger days - even though it had a proper transformer the grid cap was a bit leaky so after an hour or so you got a slight tickle off the strings!! Would not reccomend it BUT one possiblity is a Practical Electronics circuit from years ago that used optical isolation - the receiving side was an ORP 12 and preamp connected only to the main amp while the transmitting side used a small lamp(? might have been an led) and self contained powered amp circuit running off a PP3. The electrical isolation would be complete that way.
A modern device to do the same job, complete with decent safety isolation specs (provided the build standard doesn't compromise them!).

https://www.digikey.co.uk/product-de...LA116-ND/96654
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