Convert Vintage Valve Radio to Guitar Amp

[DigitalNoMore]

I have a masters degree in electronic engineering and before I retired worked as a principal design engineer and design team leader for a large telecommunications company so can't possibly know what I'm talking about of course.

All I'm interested in is passing on advice to people if I can. Not trying to play the smart a**e like some people on here it seems.

Regarding guitar pickups, the output voltage can vary dramatically.
Single coil pickups such as on my Stratocaster typically output well under 100mV and so will be unable to directly drive a valve radio PU input sufficiently to get much usable volume. I know, I've tried it.
Humbucker pickups such as on my Les Paul typically output at least twice as much as single coil and are more successful but still less than ideal.

Just for completeness, early crystal phono cartridges, which a valve radio PU input would be designed to accept, would typically output as much as 1-2V.

[Paul JD]

I wasn't aware that telecommunications companies were in the business of designing valve guitar amplifiers but maybe I am wrong.

I am not being a smart arse and unlike some people I am not calling people names. I am simply using my experience as a designer and builder of guitar amplifiers to try to help the OP with his project. Several people have tried to dissuade him from his project but many of the reasons given have been invalid, I'm sure they were only trying to be helpful but it is clear that some of the people do not have any great experience with guitar amplifiers and they were trying to dissuade the OP from what is a perfectly viable project that could be a lot of fun.

As for single coil pickups not being able to drive it I would refer you post #35 of this thread where I posted a link to some audio recordings of a guitar being played through a radio output stage, the first of the clips was recorded with a single coil Fender Stratocaster and the volume level was more than adequate (in fact I have just measured the SPL at around 85dB 1M from the speaker). Incidently the amplifier circuit that you suggested as an alternative is the audio output stage of a typical 1960's television, it is essentially the same thing as the audio output stage of a typical valve radio and no more or less suitable as a guitar amp. I assume that works with your Stratocaster.

I have nothing more to add to this discussion.

[Radio Wrangler]

Arguments are liable to get a thread closed, and that would be rather unfair to the original poster.

Sensitivities of PU inputs vary. Some were intended for crystal cartridges, some were intended for old magnetic heads on 78 players. The impedance presented to the guitar ought to be high enough.

Voltages from guitar pickups also vary.

Connecting a random guitar to a random radio might be fine, or might not meet the user's expectations. Expectations vary, too.

It's worth trying. If necessary, a small transistor preamp can always be added.

David

[stevehertz]

That's my view too David. ie in general, an electric guitar plugged into the PU input of a valve radio should, normally, be ok. But there will be instances, for example, a low output guitar pickup into a 'lower' impedance input that is not very sensitive will not work too well. In other words, there's no standard output from an electric guitar and there's no standard spec PU input on vintage radios. = won't always work, but in general it does.

[DigitalNoMore]

I'm fully aware of the fact that the amplifier I built is the audio output stage of a typical 1960s television. As a result the PCL82 triode-pentode it uses can be picked up for not much money and I wanted to see how cheaply I could build a usable valve guitar amplifier for home practice. I used an old wall wart isolated supply for the valve heater and then stepped this up to the HT supply using a DC-DC step up converter bought for £5 from China via Ebay.
As I stated in my original post, I used a single transistor treble booster in front of the amplifier I built because it wasn't really sensitive enough without it using either my Fender Stratocaster or my Antoria Les Paul (fitted with high output DiMarzio pickups). The sound was also very flat and muddy without the treble booster.

I've no interest in arguing. Just passing on the experience I had in case anyone else decides to build it or use an old radio with a similar output stage.

[AC/HL]

Originally Posted by Radio Wrangler

Arguments are liable to get a thread closed, and that would be rather unfair to the original poster.

Quite.

We're all here to help. We don't know each other's interests, experience, qualifications, whatever. Without physical or visual feedback we can only take replies at face value, without reading anything extra into them. If it's not in writing it isn't there as far as the thread is concerned.

[G8UWM-MildMartin]

It would be interesting to know which version of the 501AC it turns out to be.
Two versions had a 6V6GT octal-based output valve as used in many guitar amps. (and still in production), one had the loktal/B8G electrical equivalent, the 7C5, and the other had a B8A(rimlock)-based 67PT (=EL41).
All had a decent 8-inch loudspeaker.
Two versions had a 7Y4 rectifier which doesn't seem to wear as well as the other valves, but can be replaced by a couple of cheap modern diodes and a resistor or two.
It should be easy to add a switch to cut off the HT or heater supply to the unused stages, a 1/4" jack socket in parallel with the PU sockets, restore the set as a working radio as well as a guitar amp. and enjoy using it for at least a further 69 years!

[Sean Williams]

Hi Robbo,

I've got a Pye Woodie lurking here - it's not immaculate, but it has a transformer, so is not live chassis, and would lend itself to conversion to a practice amp.

I can drag it out of storage this weekend, and even give it a look over to make sure all the important bits are working.

From memory, it's one of these - http://www.richardsradios.co.uk/pyet19.html

Any good to you?

[robbo100]

Morning all,

A bit of a project update for you all.

I now have the radio (the 501 AC) and it works really well. I have taken it apart and given it a good clean, as it had 50+ years of dust inside (as expected).

It is surprisingly loud when tuned into some radio stations, which is a great sign.

I tuned the radio away from a station, to science the radio sound, and connected a guitar to the gram input and it worked! However, as many have predicted, it was very quiet.

I have therefore built one of these (to run off a 9V battery) to give some pre-preamp boost.

Unfortunately, so far I have been unable to get it to work with the radio. It may be an error in my build process, so I will check it with a normal guitar amp at some stage (when my wife isn't telling me off for not decorating the lounge).

Thanks all.

[Herald1360]

That preamp circuit looks just fine. Is there a possibilty you may have reversed one of the electrolytics? That could upset the dc bias conditions causing the opamp to slew hard to one rail and do nothing useful. Check the voltage on the opamp output pin, it should be fairly close to mid rail (about 4.5V).

You could replace both electrolytics by 1u non polarised film caps if you leave off the 10k pot (or change it to 1 meg).

[robbo100]

Hi all

Just to let you all know, I got this working today and it sounds sweet!

I still need to get the replacement speaker and finish modifying the original enclosure, but when I have done it all, I will post a video.

The problem I was having with the home made guitar gain boost was that I was stupidly using a TL072 Op Amp chip, but thought it was an TL071. It would have been fine, but the pinout for the 072 is different to the 071, so it didn't work.

[robbo100]

Quick question:

The Melody Maker 501 AC has an input for an external speaker.

Is the system designed to drive two speakers, in parallel configuration, or do you need to disconnect the internal speaker in order to connect an external speaker?

My thoughts would be that connecting both speakers would give the power stage amp too low an impedance and could risk driving it too hard, but I wanted to check this assumption.

If the amp is designed to take both speakers in parallel, then it would double the power output of the amp, so could be quite an interesting addition.

Thanks in advance.

[Edward Huggins]

I think you mean OUTPUT for an external speaker.
Yes, this will drive 2 speakers, but the impedance will be lower as they will be run in parallel.
Running 2 speakers does not mean it will double the power output. What makes you think this?
It just gives you the option to use anther speaker and hopefully one with a larger drive unit and enclosure.

[paulsherwin]

The OP is probably thinking of modern transformerless transistor amps, where the output power will indeed double if the load impedence is halved (if the PSU and other components can cope of course). With a class A valve amp you just get an incorrect anode load so the output power actually goes down, though the effect isn't obvious.

[Refugee]

Varying the speaker impedance may add some effects that are either wanted or not for guitar use.
It would depend on how you want distortion to set in at higher powers what speaker combination you choose.
You will need to experiment.

[robbo100]

Thanks all,

So if I understand the responses, it will not damage the amp to run two 4 ohm speakers in parallel and it might be a good experiment to see if it produces some good sounds.

I may well give it a go.

It is sounding really really amazing with the new speaker. I should get it all finished tomorrow and will post some photos and videos.

[Radio Wrangler]

In general:

Paralleling up more speakers makes a lower load impedance on the amplifier. This can damage transistor amplifiers if you go too far. Valve amplifiers tend to survive, giving reduced total power.

Running without a speaker is normally fine with transistor amps, but risks damaging valve ones... voltage swings can get too large, and something gotta give.

David

[robbo100]

Thanks David

I just realised that I haven't changed "That Capacitor" on this radio. Can anyone help by pointing out which specific capacitor I need to change.

Thanks again.

[ms660]

Originally Posted by robbo100

I just realised that I haven't changed "That Capacitor" on this radio. Can anyone help by pointing out which specific capacitor I need to change.

C25 on the BRTR service sheet.

Lawrence.

[robbo100]

The project is now complete!

Thanks for everyone who helped make this happen, I would go as far at to say, this could be one of my most successful projects ever.

Here is a video of the end result. >>VIDEO<<

The volume and tone knobs remain the same. The dial lamp has been used as a power light in place of the band selector knob. The tuning knob has been replaced with a gain knob.

I ended up with a pretty huge 30dB pre-preamp gain stage using a TL701, but it needed that to allow some overdrive. This circuit is powered by an old camcorder charger connected via the radio's on/off rotary switch

Sounds far better than I ever imagined and looks really cool!

(The video is of my young son playing, and me drinking the beer at the end )