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Old 13th Aug 2019, 10:43 am   #21
Paul JD
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Default Re: Convert Vintage Valve Radio to Guitar Amp

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Originally Posted by crackle View Post
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.
Mike
I beg to differ! There are lots of advantages to having valve practice amps. There is a reason why small vintage guitar amps such as the Fender Champ are so desirable and many big name guitarists often use small valve practice amps for recording studio use. In my experience a cheap solid state practice amp will always sound like a cheap practice amp but a good small valve amp can sound infinitely better.

As for damaging the speaker in a radio that is unlikely. The volume level when plugging a guitar into the gram input will be quite low, when I have done this in the past it was necessary to have the radio volume turned up fairly high to get a reasonable volume level and even with the volume on maximum there was no evidence that the speaker was distorting or under any stress. If the speaker is able to cope with the radio output then it will almost certainly have no problem with a guitar through the gram input as the volume level will be much less.
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Old 13th Aug 2019, 10:53 am   #22
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Default Re: Convert Vintage Valve Radio to Guitar Amp

(Trader sheet refs.)

The input impedance of the KB LR10 when switched to gram input is approx. 500kohm (R9) The gain can be increased by fitting a suitable capacitor across the audio output valves cathode bias resistor (R15) The bass response can be reduced by reducing the value of the audio output valves control grid coupling capacitor (C19)

C19 should be replaced anyways to prevent possible damage to the output valve and transformer etc due to electrical leakage of the said.

Lawrence.

Last edited by ms660; 13th Aug 2019 at 11:04 am. Reason: Advisory addition.
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Old 13th Aug 2019, 11:53 am   #23
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Default Re: Convert Vintage Valve Radio to Guitar Amp

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I beg to differ! There are lots of advantages to having valve practice amps. There is a reason why small vintage guitar amps such as the Fender Champ are so desirable and many big name guitarists often use small valve practice amps for recording studio use. In my experience a cheap solid state practice amp will always sound like a cheap practice amp but a good small valve amp can sound infinitely better.
In my experience if you want something which sounds like a Fender Champ then you want to buy a Fender champ.
What the OP is going to end up with here is a cheap sounding valve amp, which is likely to be no better than a second hand cheap transistor amp designed for the job.
What did Brian May use, wasn't it a one watt transistor amp?

Mike
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Old 13th Aug 2019, 12:25 pm   #24
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Default Re: Convert Vintage Valve Radio to Guitar Amp

Those little transistor amps have a very special sound - here's a quick Wiki to bring you up to speed:
https://en.wikipedia.org/wiki/Deacy_Amp

In my experience many of the 'less expensive' practice amps out there sound awful. The amp is part of the instrument; a guitar is about having fun and I fully understand why the OP would want to use an old radio... It's fun!

The high impedance input will be OK and guitars have more output level than you might expect.

Here's a handy little piece of research giving guitar output levels and impedance info that should help you.

http://tomsguitarprojects.blogspot.c...ge-levels.html

Get in there and create something! Make your own sound like Brian May and John Deacon did

Cheers,
Steve.
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Old 13th Aug 2019, 12:42 pm   #25
Paul JD
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Default Re: Convert Vintage Valve Radio to Guitar Amp

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Originally Posted by crackle View Post
In my experience if you want something which sounds like a Fender Champ then you want to buy a Fender champ.
What the OP is going to end up with here is a cheap sounding valve amp, which is likely to be no better than a second hand cheap transistor amp designed for the job.
What did Brian May use, wasn't it a one watt transistor amp?

Mike
I am not suggesting that a valve radio is going to make the perfect guitar amp but not quite sure what you mean when you say "cheap sounding valve amp". Most of the old valve guitar amps that are so desirable now, Fender Champ included, were built as cheaply as possible at the time and often used under-specced components and used the same components as valve radios of the time (they were readily available and cheap). As I said in an earlier post I have tried playing guitar through the gram inputs of several valve radios in the past and the results have been quite good. The gain will be much less than a typical guitar amp circuit so not much in the way of distortion but it was a nice "warm valve sound" and to my my ears better than many of the cheap transistor practice amps I have heard. If the OP is looking for a dedicated guitar amp with built in effects etc then yes, that is what he should buy (but please avoid the very cheap things - they really do sound awful). However if the OP wants something a bit different and quirky then there is no reason why a valve radio could not be easily re-purposed into a good sounding guitar amp.

As for the amp that Brian May used that was built by John Deacon from a transistor radio circuit board so apparently radios can make good guitar amps!
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Old 13th Aug 2019, 4:35 pm   #26
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Hi all,

Thanks so much for the technical information and I have really enjoyed the discussion that has gone along with it.

The characteristics of a great guitar amp are not the same as a great hifi amp (or similar). In the guitar world the valve amp is highly sought after because the characteristics of the valves, when driven hard, create a beautiful rich breakup, which can't easily be reproduced by solid state systems. This is the experiment I want to try with the vintage valve radio.

My son, although very young, isn't a beginner, he is around the grade 7 level, so we are looking for some fun tones to play around with. Kinda like when guitarists used to cut slashes in their speakers to make a dirty fuzz sound. I am calling it a "practice amp" because it will be a valve amp, which is quiet enough for him to play in the house without annoying the whole street, whilst still driving the valves into a bit breakup. He already has a 100W solid state Marshall amp, but like most solid state amps, it sounds a bit like a wasp in a jar and doesn't have a lot of character.

In terms of the speaker driver, I plan to buy something like this: https://www.gear4music.com/Guitar-an...m-Speaker/2JPG. But I will hold-off getting it until I have proven the amp section of the radio is working.

I shall keep you all updated with progress, but in the meantime, please keep the discussion and tips coming.

@ms660

In terms of replacing C19 - I presume something like this would be suitable (400V DC rated 0.1 uf/100nf).

Interestingly, R15 doesn't show on one of the versions of the schematic I have found, and it is listed as R13. But it looks like 270 ohm 1W. When you say "a suitably rated capacitor" - what would you advise. I have looked at some calculators, but am a little confused (including what voltage rating it should be).

Finally, in the event that I do create a buffer circuit to increase the input impedance, I was thinking of powering it using the lamp circuit, which is 6.5V. I need to see if I can find a suitable opamp that will work at this power (possibly the LM741?). Does this seem a sensible approach?
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Old 13th Aug 2019, 8:47 pm   #27
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Default Re: Convert Vintage Valve Radio to Guitar Amp

I agree broadly with Paul JD eg post 25*. Clearly there have been very different experiences [post 12*] and these were only practise rigs for use in the front or box room. The OP said he wanted to demonstrate valve gear. We youngsters were aware, back then, that "real" guitar speakers seemed more substantial so substituting another seemed a good idea. I didn't want to risk damaging the small in-built spkr in the tape recorder or damaging my friend's parent's radio. Different times-less money-more accountability-more caution!

I had some sort of home built guitar and was also given a single [hum bucking?] pickup that I mounted on this. Surprisingly, for some reason, it produced much more output than other commercial guitars, overloading the amp and producing Hendrix type effects without any electronic "add ons" but also without any similar playing ability

Dave W

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Old 13th Aug 2019, 9:19 pm   #28
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Default Re: Convert Vintage Valve Radio to Guitar Amp

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@ms660

In terms of replacing C19 - I presume something like this would be suitable (400V DC rated 0.1 uf/100nf).

Interestingly, R15 doesn't show on one of the versions of the schematic I have found, and it is listed as R13. But it looks like 270 ohm 1W. When you say "a suitably rated capacitor" - what would you advise. I have looked at some calculators, but am a little confused (including what voltage rating it should be).

Finally, in the event that I do create a buffer circuit to increase the input impedance, I was thinking of powering it using the lamp circuit, which is 6.5V. I need to see if I can find a suitable opamp that will work at this power (possibly the LM741?). Does this seem a sensible approach?
The original value for C19 is 0.02uF (0.022uF/22nF in todays money) If you want to reduce the value try 0.01uF or 0.0047uF or even lower, it's a case of what suits for the guitar sound.....400 VDC rated is ok.

The cathode bias resistor for the audio output valve is circuit ref R15 in the Trader sheet and R13 in the manufactures manual...270 Ohms in both cases, try a 22uF@50VDC for a cathode bypass capacitor, that will be an electrolytic capacitor and will be polarized, therefore it's -ve lead should connect to the chassis and it's +ve lead should connect to the cathode, pin 2 of V4, the audio output valve.

I'm not really qualified to comment on the suitability of any op amp (someone else will be) but if you do decide to power a separate pre-amp from the heater supply you need to be aware that, in this receiver, one side of the heater supply is connected to chassis which means you can't use a bridge rectifier for the supply with the heater supply as configured, if you do, something will go splat, so it will need to be half wave rectification only from the heater supply.

https://www.cricklewoodelectronics.com/

(Other component suppliers are available)

Lawrence.
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Old 13th Aug 2019, 10:27 pm   #29
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Default Re: Convert Vintage Valve Radio to Guitar Amp

The 741could be used but wouldn't be ideal, especially if you're looking for high input impedance. A FET input type like TL071 would be better.

A simple bipolar supply for the opamp can be made with two diodes and two capacitors two provide + and - half wave rectified supplies of about 7V each. 1N4148 and 100uF should do.

You could get cunning and rewire the old chassis to use the IF valve, triode connected, as your preamp/buffer......
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Old 13th Aug 2019, 11:09 pm   #30
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Default Re: Convert Vintage Valve Radio to Guitar Amp

I would be tempted to stick with 0.022uF for the coupling cap, that is a fairly typical value for guitar amps. If you decide that it needs more or less bass response you could tweak it later to suit (higher values = more bass). As stated 22uF would be a good value for the cathode bypass cap (anything from 22uF to 100uF should work fine, the value is not critical).

There is absolutely no need for a buffer on the input, the input impedance is ideal for guitar as it is. If you find that it does not have enough gain the simplest solution would be to use an overdrive / distortion pedal in front of the amp.

The Celestion speaker you linked to is a good choice, I have one of those in a practice amp that I built myself and it sounds good. I don't know what size speaker is in the radio you will be using but I would definitely try that first before buying another one, you might be surprised at the results. Many of the low power valve amps back in the 50's and 60's used standard radio speakers and many of those are considered very desirable amps now!
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Old 14th Aug 2019, 8:32 am   #31
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Default Re: Convert Vintage Valve Radio to Guitar Amp

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Originally Posted by Herald1360 View Post
The 741could be used but wouldn't be ideal, especially if you're looking for high input impedance. A FET input type like TL071 would be better.

A simple bipolar supply for the opamp can be made with two diodes and two capacitors two provide + and - half wave rectified supplies of about 7V each. 1N4148 and 100uF should do.

You could get cunning and rewire the old chassis to use the IF valve, triode connected, as your preamp/buffer......
Thanks - The IF Valve pre-amp sounds like an interesting option, but probably a little bit too advanced for me. I think I will get the system working first and see where the project takes me.

A simple FET system would probably be ideal. All I really need is something to increase the input gain up a little bit, to allow the valve section of the pre-amp to break-up a bit and distort. Again, something that can wait until I have got the basics working.

Quote:
https://www.cricklewoodelectronics.com/

(Other component suppliers are available)
Nice one - I could spend all day buying stuff from your store. I have stuck to the essentials... for now.
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Old 14th Aug 2019, 9:50 am   #32
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Default Re: Convert Vintage Valve Radio to Guitar Amp

I constructed a Velleman 'Universal Mono Pre-Amplifier' Kit K1803 to raise the signal level from an I-phone for the gram input of a Bush AC41 valve radio for my son. It works a treat. Ordinarily, I make little projects such as that myself as I enjoy designing and making little PCB projects, but this kit is so cheap it made no sense to do that.

Might be worth a thought?

Product overview:

Quote:

8-<

This kit has been developed as a pre-module for a number of audio applications, where there is insufficient input signal.
Ideal for use as a microphone pre-amplifier for signal matching of tuner or tape outputs and for level correction.
Adjustable output level.

Power supply: 10-30VDC / 10mA
Output impedance: 1Kohm
Adjustable output level: max. 40dB
Frequency range: 20Hz to 20kHz 3dB
Max. input signal: 40mV

8-<

End quote.

It's available from several suppliers, but ESR Electronics, who sell the full range of Velleman kits, (and lots more stuff) seem to be cheapest.

4.32 + 3.50 P&P + VAT:

https://www.esr.co.uk/velleman/k1803.htm

The construction manual & circuit is here:

https://www.esr.co.uk/velleman/k1803.htm


Hope that's of interest.
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Old 14th Aug 2019, 10:25 am   #33
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Default Re: Convert Vintage Valve Radio to Guitar Amp

I remember I did use an old valve radio back in schooldays. I don't remember any issues but I would now worry a bit about safety. The guitar I used then had low output pickups so it was quiet, if I remember rightly.

Guitar pickups ar a bit variable as (although probably more a relative thing) some are designed for lower impedance hence less prone to hum. Some are much higher output too. In general terms they are low impedance as compared to valve inputs.

I have a Vox practice amp which I have had for some years. It's circuitry is IC based (TBA810S according to the diagram I have) with input transistor. Extremely simple, but I think it's good sound is due to the well constructed cabinet and perhaps the speaker used (it looks a bit like a miniature AC30 - traditional Vox).

My point here is that a radio can be used but for more serious use in the longer term a well designed unit for the job is better. I do wonder how well a valve radio (specifically speaker) would cope with a heavily overdriven input via various pedals?
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Old 14th Aug 2019, 10:57 am   #34
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Default Re: Convert Vintage Valve Radio to Guitar Amp

The Vellemann kit as is has a low (2k2) input resistance. However, with a few changes in resistor values and a TL071 substituted for the 741 the PCB could be just the job.


The PSU for it would need to be a common terminal voltage doubler rather than the bipolar version I suggested before. Could use the same Ds and Cs, though.
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Old 14th Aug 2019, 12:13 pm   #35
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Default Re: Convert Vintage Valve Radio to Guitar Amp

To give an idea of the sort of results that can be expected I have recorded a couple of short audio clips playing guitar through a valve radio. The radio is a Bush VHF61 and the guitar is plugged into the gram input. The audio output stage of the Bush is the typical single triode gain stage into a pentode output valve so is a similar topology to the radio that the OP is looking at using. The speaker in the Bush is a relatively small 6x4 elliptical speaker so not ideal but even so I don't think it sounds too bad.

Clip 1 is just the guitar straight into the radio, clip 2 is with an overdrive pedal (Ibanez Tube Screamer) set to boost the signal and overdrive the valves.

http://www.pjdamplifiers.co.uk/media/radio-amp.html
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Old 14th Aug 2019, 12:27 pm   #36
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Default Re: Convert Vintage Valve Radio to Guitar Amp

Nice playin'...Sounds ok to me.

Lawrence.
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Old 14th Aug 2019, 12:35 pm   #37
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Default Re: Convert Vintage Valve Radio to Guitar Amp

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To give an idea of the sort of results that can be expected I have recorded a couple of short audio clips playing guitar through a valve radio. The radio is a Bush VHF61 and the guitar is plugged into the gram input. The audio output stage of the Bush is the typical single triode gain stage into a pentode output valve so is a similar topology to the radio that the OP is looking at using. The speaker in the Bush is a relatively small 6x4 elliptical speaker so not ideal but even so I don't think it sounds too bad.

Clip 1 is just the guitar straight into the radio, clip 2 is with an overdrive pedal (Ibanez Tube Screamer) set to boost the signal and overdrive the valves.

http://www.pjdamplifiers.co.uk/media/radio-amp.html
Awesome! That is what we are looking for!

I can't wait to get this project going!
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Old 14th Aug 2019, 2:54 pm   #38
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Default Re: Convert Vintage Valve Radio to Guitar Amp

Have the collective on here ever thought that the OP might, purely and simply, want to play his guitar through a vintage radio? Yes, there are (maybe?) a thousand and one reasons not to do that, but if he wants to do it, that's what he wants! I sometimes think in our quest to be helpful, a lot of threads become skewed by well meaning attempts for the OP to do it 'another way'. An OP can sometimes be made to feel even foolish, such is the backlash of "What do you want to do that for?" or "Why not do this instead?". I personally find that frustrating sometimes when I have a clear view of what I need (and it's viable), I just need some specific information along the way, not a 360 degrees change of direction. Nonetheless, the upside of this situation is the 'side' information that comes out of the thread. And I guess that's what a forum is all about, the sharing of ideas and information.
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Old 14th Aug 2019, 5:26 pm   #39
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Default Re: Convert Vintage Valve Radio to Guitar Amp

Isn't that half the fun of forums

From my perspective (as the OP), I did wonder what response this thread would get, fully expecting a range of responses from:
  • You monster, you can't destroy a beautiful vintage radio and you will electrocute yourself; through to
  • That is a cool project, you might get some cool sounds!

I figure that the rationale for doing such a project involves some specific knowledge that non-guitar players wouldn't know about. So I probably should have added a little more detail in the first post, to make it clear why it is a good project in the first place.

From a new forum user perspective, I am extremely delighted with all the post responses. It is great to find such an active forum, with such knowledgeable and helpful people.

Thanks for everyone's input and comments so far.
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Old 15th Aug 2019, 6:04 pm   #40
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Default Re: Convert Vintage Valve Radio to Guitar Amp

So, unfortunately, the person I bought the radio from, thought it was completely fine to ship it without any bubble wrap.

When it arrived, the whole wooden case was broken and fell apart in my hands and the tuning glass completely smashed.

The chassis seemed OK, so I fitted the new flex and That Capacitor, stood well back and turned it on.

I turned it on, and nothing happened, and as I turned the on/off dial back to off, the house RCD tripped.

I turned it all back on again and the audio transformer got very hot and some wax dripped out of it.



Thoughts regarding next steps - if any?
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