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General Vintage Technology Discussions For general discussions about vintage radio and other vintage electronics etc. |
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27th Jun 2020, 5:54 pm | #1 |
Diode
Join Date: Jun 2020
Location: Tunbridge Wells, Kent, UK.
Posts: 6
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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 |
27th Jun 2020, 6:41 pm | #2 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,309
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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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27th Jun 2020, 6:51 pm | #3 |
Heptode
Join Date: Jan 2008
Location: Sheffield, South Yorkshire, UK.
Posts: 652
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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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27th Jun 2020, 6:58 pm | #4 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,309
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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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27th Jun 2020, 7:00 pm | #5 |
Heptode
Join Date: Dec 2014
Location: Lincoln, Lincolnshire, UK.
Posts: 583
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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. |
27th Jun 2020, 8:46 pm | #6 |
Hexode
Join Date: Mar 2012
Location: East Yorkshire, UK.
Posts: 346
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Re: Help / advice needed - converting my Murphy 146 into a guitar amp?
It’s 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 wouldn’t fancy using it without isolation. I’m 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 don’t think the speaker would be up to it. Just my opinion of course!
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Howard |
27th Jun 2020, 9:20 pm | #7 |
Dekatron
Join Date: Nov 2003
Location: Heckmondwike, West Yorkshire, UK.
Posts: 9,636
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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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27th Jun 2020, 10:13 pm | #8 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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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. |
28th Jun 2020, 1:12 am | #9 |
Dekatron
Join Date: Nov 2003
Location: Heckmondwike, West Yorkshire, UK.
Posts: 9,636
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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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30th Jun 2020, 2:16 pm | #10 |
Heptode
Join Date: Jul 2010
Location: Wellington, New Zealand.
Posts: 653
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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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1st Jul 2020, 10:07 am | #11 |
Octode
Join Date: Mar 2016
Location: London, UK.
Posts: 1,040
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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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1st Jul 2020, 11:16 am | #12 | |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,515
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Re: Help / advice needed - converting my Murphy 146 into a guitar amp?
Quote:
https://www.digikey.co.uk/product-de...LA116-ND/96654
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