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Thread: Converting an old tube radio into a guitar amp???

  1. #1
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    Converting an old tube radio into a guitar amp???

    I am thinking of doing this but I'm not sure if it can be done. The plan is to use an old tube radio, deactivate the radio part and ad an input jack in order to make it useable for guitar. And that with a limited budget. The only thing it's supposed to do is to make a sound when a guitar is plugged in ... it's not supposed to sound great, be loud or have any bells and whistles. It's just a gimmick. I do not even have the radio yet ... I need to find one at a garage sale or so

    Is there anything I have to look for when chosing a radio (except for the fact that it's working)? Is it just adding an input jack in the right location or would I need to make bigger modifications? Again ... it's just a fun project and doesn't have to sound great ... if it's fuzzy distorted and muddy, that's just fine

    thanks!

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    Are you experienced?
    If not I'd suggest a 5F1 kit build first.
    Educate yourself and be aware that the safety warnings are no joke!

    If you have some experience then I'll give advice for what it's worth (free).
    Radios in general are not good donors for guitar amps.
    Many you will find will be field coil variety leaving you to purchase an output transformer and the related
    circuit changes to make that part work.
    The preamp/output stages are extremely minimal value on radios also.
    Nearly every tube you'll find in them are dedicated to...radio stuff.
    Old car radios are actually better if you are going that route, often a 12AX7 and 6V6's in those.

    With that info the best bang for the restoring buck is old PA amps or discarded organ amps.
    Do not tear up good organs though!
    I've seen organs still going from free to 100 dollars that might be loaded with 12AX, 12AU and AT7's...a couple
    of working amps inside (frequently separate tone and reverb amps).
    I've made Spitfires from just the reverb side of some Baldwin organs for example, only things to buy were a suitable
    power transformer and a blank chassis.

    Take a look around this site, in this area.
    You'll see some of the work done, great lessons in here.

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    Thanks,

    Yes, I have built, repaired and modified lots of guitar and harp amps so far but my little knowledge about electronics is strictly limited to these. There are many components and circuits in a radio that don't make sense to me.

    The idea is not to find something that can be converted to a nice and useable guitar amp. It's got to be an old tube radio and it's not going to be more than a low budget fun gift for somebody who already has plenty of nice amps. The only thing it's expected to do is to produce some kind of audible sound ... volume, tone and background noise are not a factor.

    27 Years ago I got my first electric guitar for christmas but no amp so I took an old turntable with integrated speaker, cut off the cartridge and put an unput jack in it's place ... that was my first amp That's the route I would like to go here as well. I just don't know enough about radios to do it. Of course I could simply build a single gain stage, single output tube design like a 5F1 or 5F2 into an old radio cab but that would blow the budget ...

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    Supporting Member loudthud's Avatar
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    Many of the tube radios you are likely to find in the USA (your profile does not say what country you are in) will be of the AC/DC type which do not have a power transformer. This makes them very dangerous to operate as guitar amps because essentially the guitar strings will be connected to one side of the power line. Very dangerous! To fix this would require an isolation transformer that would ruin your budget. Many old tube phonographs and TV sets have the same problem. Better to find an old Public Address amp.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

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    Senior Member JHow's Avatar
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    I know you are against it, but....


    ...why not build a 5f1 style amp in an aluminum chassis and mount it in a AC-radio set box (like a Philco 1940s)? You can have your input on the back. You can use any small speaker (5-inch), or possibly the orginal speaker, and you have two controls (tone and volume, but orginally tuner and volume) up front? It will look like a radio, but work like an amp. You can also run an output jack and send it to a speaker cabinet. Your basic bayonet-style dial light will become your AC pilot light. Or you could use a funky blue LED to give the set a more "other world" feel. The AC set's orginal circuits are comepletely useless ("loctal" tubes, no transformer) for guitar amp purposes, but they come in stylish art-deco boxes and they are cheaper than the more collectible radio sets from the 30's. You can find them in wood or bakelight versions. Look for something like Philco "Transitone" etc. That should accomplish the goal of being an amp and looking like an old radio. About the only thing that would be tricky would be speaker clearance and you would need to use pots with long shafts and you could file/mill them to fit the "D" shape of the orginal knobs, thus retaining the authentic look. You could disconnect the tuner indicator and leave in place, or if you are mechanically minded, you might be able to get it to turn in synch with the tuner or volume.

    Otherwise, if you are thinking only about conversion candidates, I would look for transformer-powered radio set with regular old octal tubes - for your purposes, bigger is probably better. Or, look for a set with a phono input - with is essentially a ready made audio input. Again, this will mean a larger set.


    Here is a bad example (way over-priced and cabinet is broken), but with good pictures of what I am taking about. Note the funky loctal tubes, the lack of xformer, but also note the layout of the speaker (center) and the knobs (switched volume and tuner). This one is bakelight but the early forties ones are wooden.

    Vtg PHILCO Transitone 49-504 Tube Radio Parts or Repair | eBay

    Food for additional thought. In many ways bigger is better - more room to manuever, transformer-power and larger AF amp section.

    http://www.ebay.com/itm/Deco-antique...item43a9b6c0d1
    Last edited by JHow; 09-27-2011 at 07:00 PM.

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    thanks, that was important info!!!

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    Thanks ... I already suggested fitting a simple amp design into an old radio cab or even building a little custom amp that looks like a radio but I'm neiter the one who came up with the idea nor the one who's paying for it. It was just an idea and I think I know now what I need to know. If it could have been done with an input jack, a couple of wires and a little soldering it would have been fine but since it was never intended as a serious project we'll think of something else. Thanks for the help and input!!!

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    On this forum there is a design for a radio conversion - it's called the AA4. I'm sure you'll find it easily enough. I recently bought a Kriesler ((Australian) mantle radio with the same aim as your idea. I can't remember the valves inside - a 7 pin equivalent of the 6v6 and a triode pre amp (not a 12a*7). It has an isolation transformer etc. I'll just be adding a 3 wire cord and input jack and see what happens!

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    URL for schematic of AA4 radio conversion


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    So I got to work at the weekend. First up, three prong cord and fuse. Second, remove the valves from the tuner section. Third, connect a jack to the volume control and hence grid of the triode (6BD7). Fourth, plug in guitar and play. Next time I do this it will take about 15 minutes. I had to understand what was what on this first. I get a decent bedroom volume from this, albeit a bit clean. However, to convert back to original state of a working radio all I have to do is remove the jack and pop the valves back in. As if. Dead easy and dead cheap. Cost me 15AUD for the radio and a buck for the jack. I love building amps from junk. You should give it a try.

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    Quote Originally Posted by Bluefinger View Post
    I am thinking of doing this but I'm not sure if it can be done.....
    Is there anything I have to look for when chosing a radio (except for the fact that it's working)?... if it's fuzzy distorted and muddy, that's just fine
    thanks!
    I just got a project like this up and running. My problem is, it's not distorted ENOUGH!

    A friend found a perfect candidate - a tube AM/FM radio with a single-ended 6V6 output and a 5Y3 rectifier - total Champ starter package. I made it into a Gretsch 6151 (Supro/Valco/etc.) with one 12AX7 for gain/volume/tone and 1 12AX7 for tremolo oscillator/driver.

    The trem works, is the best thing I can say about it.

    I used a "Universal" PP output transformer which I restacked with a small air gap. I think I have too big a step-down ratio - it's too quiet and too clean. I'll try another set of winding taps and see what happens. If all else fails I'll probably put another gain stage into it - it has plenty of heater current left, the original piece had a few more tubes in it.

    -j

  12. #12
    Supporting Member loudthud's Avatar
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    Post the tube pin voltages. If the amp is working properly, it should have plenty of gain. Also, think about getting a proper single ended output transformer.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

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    Will do - but in the meantime I have worked it out at least a couple steps better than it was when I posted before.

    I have excess heater voltage - the original AM/FM radio had a few more tubes. This ironically did me a favor, taking out a weak 12AX7 I was using in the audio gain position. A better 12AX7 and a change of transformer taps did a lot to help things out. I brought it over to a friend's shop and plugged it into a 2x15 Bassman bottom, and heard some actual players use it with some good guitars, including a sweet Les Paul Jr. (I'm kind of a hack as far as the actual playing goes.) They liked it and the tremolo sounded really wonderful. I found the tone control pretty well shaped, ranging from a full, clear sound at 10, to a definitely tilted, rolled-off sound at 1, with a useful compromise area at about 7 or so. Trem depth ranged from chopped-off to gently wavy and the speed variation was also pretty good to my ears and others.

    The speculation was that a less robust speaker cab (single 8 combo for example, like the original application) might raunch up the sound a little further. The swap of transformer taps did loosen things up a bit - but it was not farty/flabby at the bottom either. I think the hacked OPT is holding its own better than you might expect. I changed the interleaving when I restacked & gapped it - instead of 1x1 interleaving of the laminations,, I did groups of 5, 11, 11, 5 or something like to that. It may well be a "proper SE OPT" at this point, or at least pretty close. I think I have a Hammond 125ESE I can try. And get this: the original radio had TWO tiny SE OPTS in series, one driving a "woofer," one driving a "tweeter" - I am of course dying to try that setup too and I will swap those in soon.

    The schematic I posted has been altered now: I put .01 in parallel (so .02uF now) caps in both the two phase shift sections where there were .01 on the tremolo oscillator, it's a fair bit slower now, Enzo will be happy. Contrary to his opinion elsewhere, I can imagine a switchable option on it, the fast end could be faster. And maybe I can slow down the slow end even a little more and still get it to start, we'll see. I have plenty of .01's I can tack on there to try.

    I'd better dump some of that excess heater voltage first before I kill any more tubes.

    Thanks for the input, this is a very fun project, I may even let it have a "real" chassis if it continues to behave nicely!

    -j

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    Supro 1616T/Gretsch 6151 Voltage Info:


    B+1 (OPT) 370
    B+2 (Screen) 329
    B+3 (Small signal tubes) 292

    6V6 (Power):
    Plate 345
    Screen 329
    Cathode 16.1
    Grid 0
    DCR of OPT primary 491

    Calculations:
    Ik= 16.1/300 = 54 mA
    Ip = (370-345)/491 = 50.1 mA
    Is = Ik - Ip = 3.9 mA

    V1A (audio stage 1):
    Plate 233
    Grid 0
    Cathode 2.36

    NOTE: THIS CATHODE RESISTOR IS SHARED WITH V2B

    Calculations:
    IV1A + IV2B = 2.36/1500 = 1.57 mA
    IV1A =( 292-233)/270000 = .22 mA
    IV2B = 1.57 = .22 = 1.35 mA


    V1B (audio stage 2):
    Plate 179
    Grid 0
    Cathode 1.53

    Calculations:
    Ip = (292-179)/270000 = .42 mA
    Ik = 1.53/3900 = .39 mA *** a little grid current, or a little parts inaccuracy, pretty close agreement ***

    V2A (oscillator): Hard to measure the oscillator with a voltmeter! It's going at all times, whether the intensity is run up or not.

    I was able to measure the lowest and highest frequencies, though: flow = 3.11 Hz, fhi = 5.8 Hz.

    V2B (Oscillator buffer)
    Plate 289
    Grid 0
    Cathode 2.36

    Calculations: none. From previous section, Ik ~= 1.35 mA

  15. #15
    Supporting Member loudthud's Avatar
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    I'm looking at a Gretch G6151 schematic. There are no voltages listed so there is nothing to compare to, however your voltages look like the amp should function. The one voltage that kind of bothers me is the plate voltage on the first 12AX7. I'd lke it a little lower. How does the amp sound if you remove the Tremolo tube? If it gets louder, try changing the 1.5K to 1.2K or 1K.

    One calculation you didn't do was the plate dissipation of the 6V6. It comes in about 16 Watts. A little on the high side. I would increase the 6V6 cathode resistor to 390 ohms and bypass it with 50uF or more. Adding a bypass (10uF) to the bathode of the second 12AX7 stage will give a small increase in gain also.

    It's my understanding that the air gap in a single ended transformer doesn't have to be that much. Maybe only the thickness of a couple of sheets of paper. Did you get a chance to try the other Hammond OT?
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

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    Quote Originally Posted by loudthud View Post
    I'm looking at a Gretch G6151 schematic. There are no voltages listed so there is nothing to compare to, however your voltages look like the amp should function. The one voltage that kind of bothers me is the plate voltage on the first 12AX7. I'd like it a little lower. How does the amp sound if you remove the Tremolo tube? If it gets louder, try changing the 1.5K to 1.2K or 1K.
    You're saying that because it appears to be biased pretty "cold," right? I thought so too. I should have more than just a fraction of a mA, right?

    I tried something today that didn't really work out but it maybe speaks to your question.

    Looking at the Aiken Amps article on designing phase shift oscillators I decided to try bumping up the caps x10 so I could use a dual 100K pot (which is the only dual I have on hand) and see if that was more effective in getting a wider spread between low and high trem frequency. Well, those resistor values seem to be loading down the oscillator such that it doesn't really drive the audio stage very well. The oscillator is definitely starting, I can hear trem, but it's not doing much. As a result, yes, louder audio. But weak trem.

    So this version has .33, .22, .22 caps (reading away from the plate) and 22K feeding 100K variable, 22K feeding 100K variable, and 100K shunts. That's a load of, best case with the pots all the way up, 100K||122K||122k, right? Which is like 30K or so. The caps add some impedance to that but then the pots aren't always all the way up, either. So a pretty bad load.

    I tried shorting the 2M2, and also both the 2M2 and the 1M feeding the grid of the cathode follower, to try and get some more signal into the cathode follower, but that didn't work much better. Losing too much of the oscillator before that grid network, in the phase shift sections, I think?

    I will try bypassing the cathode of the oscillator and see if that helps any with the intensity. Also, as you mentioned, I will try pulling that tube out entirely and see what happens. But I don't dare do that until I bring down the heater voltage to something saner though, it's like 7V now.

    Quote Originally Posted by loudthud View Post
    One calculation you didn't do was the plate dissipation of the 6V6. It comes in about 16 Watts. A little on the high side. I would increase the 6V6 cathode resistor to 390 ohms and bypass it with 50uF or more.
    Thanks for doing that - (Vp - Vk) * Ip, right? I get 16.45 W, hmm, 14 on the data sheet, huh? OK, that needs attention too.

    And if you noticed in the calc, I am only using 300 in the cathode so I went the wrong way there - the schematic calls for 330 - so that's at least part of the reason. I don't have anything between 300 and 500 or so in any decent wattage on hand but I can put 68 or something like that in series with my 300, I have that value in a reasonable wattage rating here.

    Quote Originally Posted by loudthud View Post
    Adding a bypass (10uF) to the cathode of the second 12AX7 stage will give a small increase in gain also.
    OK, worth trying for sure.

    Quote Originally Posted by loudthud View Post
    It's my understanding that the air gap in a single ended transformer doesn't have to be that much. Maybe only the thickness of a couple of sheets of paper. Did you get a chance to try the other Hammond OT?
    That's what I understand too - I have only one Post-It note in there now. Did not swap transformers yet.

    Thanks for your suggestions, helpful stuff and I will continue to post my progress.

    Here is what I posted to the Joenet about this thing BTW, it has some entertainment value if you read to the end:

    I got the little guitar amp project up and running and I was not completely satisfied with it at first, so I thought I give a little report on the process I've been through.

    Schematic
    here, for those who are playing along at home.

    At first , this was kinda weak and clean-sounding, which was totally not the point. Fortunately, my conversion omits the two dial lamps and several tubes from the original, giving me about 7V on the filaments. This was helpful because I blew up the weak 12AX7 in the input position that was causing part of the problem. I'll burn off some of the excess filament voltage with a lamp and some load resistors before I kill any more tubes, I promise! A different 12AX7 gave better results.

    The OPT is a "Universal" P-P type which I restacked and gapped, changing the interleaving from 1x1 to 5,11,11,5. Not quite butt-gapped, but almost. The first time I tried it, I used secondary taps 1 & 6 and that was too much stepdown and too clean. I moved over to 1 & 4 and the results are better, it loosened up the sound without making it flabby.

    Another adventure: I kept letting some magic smoke out at first. Crappy original socket on the 6V6 had an invisible arc-over between plate and screen grid, found it and swapped sockets with a decent ceramic socket, no more smoke.

    Tremolo worked really well right from the get go, but I wanted to slow it down a little, so added some extra caps in parallel with what was there already. The two .01's in the phase shift sections of the oscillator are now .02, I may go even higher if it doesn't keep it from starting. I may put in a switch for Fast Range/Slow Range too, to keep all the original sounds available. The trem is VERY good-sounding, I like it a lot.

    At this point I wanted to visit my pal who deals in vintage gear - I wanted to hear a REAL player using a REAL guitar on it, and perhaps a decent speaker too.

    We hooked up to a 2 x 15 Bassman cabinet which sounded lovely although the amp could not overdrive those of course. One of the crew plugged in a Les Paul Jr, and had a great time playing tremolo'ed riffs with it. We also tried it with the shop's latest prize, a fully restored '72 Mellotron, which of course was drippy and trippy as all get-out. That was pretty cool indeed, and the mechanism and conception of the Mellotron is absolutely fascinating.

    Mellotrons appear to use 3/8" audio tape, recorded with 3 tracks. The tape heads appear to be 1/2" though. Is there any other machine that uses 3/8" format? Where the hell did it come from? I doubt you could even get the tape to make the "samples" today. Have any Joes ever seen that format before?

    Then Jack White walked in with a few of his pals to check up on a repair and to ask some questions about an unusual guitar he brought with him, an old small oak-bodied acoustic. Best guess was Lyon & Healy-manufactured, circa 1890's. Very sweet-toned guitar. Yes, he was wearing the hat. I think maybe he always wears the hat.

    As you can probably tell, that shop is a fun place to hang out!

    -j

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    Quote Originally Posted by tubegeek View Post
    You're saying that because it appears to be biased pretty "cold," right? I thought so too. I should have more than just a fraction of a mA, right?
    I drew a load line on the plate curves. Because of the 270K plate resistor, the load line is lower than the normal 100K. The load line crosses the 0V bias line about 50V so for headroom, the idle plate voltage should be roughly half way between 292V and 50V or about 170V. The gain should be a little higher there too.

    For the trem oscillator I would try a 1meg pot, or possibly a dual. Just keep increasing the caps. When bypassing the cathode, use a large cap, 100uF or more.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

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    OK, here's today's action:

    1. I put a 200 ohm resistor across the heater circuit, the heaters read 6.2VAC now, so that's just fine.

    2. I tried your suggestion to check what happens with the tremolo tube pulled; the result is only slightly more gain (by listening) than with the trem turned off.

    (I moved the trem on/off switch from the trem SPEED control to the trem INTENSITY control, by the way, it made more sense to me. The donor AM/FM radio even had the 500K pot-with-switch I needed - I really got lucky with that trash-pick from my friend!)

    I think the gain is OK now - I think the biggest problem I had originally was the weak 12AX7 in the first position which I guess got killed by overvoltage on the heaters anyway.

    3. I found a dual 1M pot in the basement! So I put the trem circuit back more like the way I had it a couple of days ago: the three phase shift sections are now like this, starting closest to the plate:

    .033, .022, .022 caps;

    220K feeding 1M variable, 220K feeding 1M variable, 1M5 resistor shunts.

    Well, hey now! Slow, nice wide adjustment range, plenty of depth. Very, very nice.

    Of course the dual pot I had was audio taper so I installed it backwards as a "SLOTH" control (fast on the left, slow on the right.) This turned out to be interesting - the log taper really does make the control's action seem much more "right." I didn't expect it to make much of a difference but I found I do like it better this way.

    Based on what I read in the Aiken article I linked above, I think I'm going to leave the bypass cap on the trem oscillator off. More gain will just add distortion and I don't need more swing anymore - it was a loading problem, not a gain problem. I will try bypassing the second audio triode at some point, see if it'll scream a little more.

    4. Changed the cathode resistor on the 6V6 - I used 200 + 125 in series.

    6V6 (Power):
    Plate 347
    Screen 327
    Cathode 16.5
    Grid 0
    DCR of OPT primary 491

    Calculations:
    Ik= 16.5/325 = 50.8 mA
    Ip = (370-347)/491 = 46.8 mA
    Is = Ik - Ip = 3.9 mA

    Wp = (347-16.5)*.0468 = 15.47 Watts

    So the plate dissipation is a little bit better. I'll up the resistance another 30 - 50 ohms and see how it looks.

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