Vintage radio amp conversion

[Ol'bob]

Hi everyone, I have an antique console mono record player that I want to convert into a combo amp or head. It is a 60w Hoffman from the late 50's. The electronics still work and all the tubes are sound. Does anyone have experience with this type of conversion? like how to test the output to match up with a speaker combo with the proper impedance? I have to surplus 12" 8ohm speakers I want to incorporate if it can work. Any help would be great! Thanks!

[WRGKMC]

I used to do that to Hi Fi Gear many years ago. Most sounded pathetically weak for instruments.

 

The two main issues are the inputs are line level and the amp is high fidelity, not guitar fidelity.

You also have to re-voice the tone stack to guitar frequencies which will reduce the gain quite a bit.

The 4 connector speaker connection usually indicates a 70V output which requires an external output transformer. Speakers on old Hi Fi gear often had the output transformers mounted on the speaker frames.

 

If the chassis does have an output transformer the simplest and least expensive way to use this is to do nothing. Get a preamp booster pedal that's capable of boosting your guitar signal up 10~20dB and plug into the Aux input and just run it that way.

 

If you want to go any farther you need a schematic which you'd need for anyone to advise you on modding the thing. There aren't many techs left that know how to repair these old 1950's no less mod them. If you were an electronic tech you could draw a schematic up yourself A tube chart alone doesn't tell you much. Its got EL34 tubes which is a good sign but that alone is no indication of how much work is involved. It looks like it has a hum balance pot so you'd likely have to add individual bias pots for the tubes to get enough gain out of them for an instrument. The one preamp tube is noted for being highly microphonic so the best bet is to modify that socket for a 12AX7

 

Maybe with some detailed chassis shots I could tell you a little more. The demands of an old Hi Fi amp is not the same as a musical instrument amp. If I saw the caps and transformers I might be able to tell if they would be hefty enough for an instrument.

 

You may be able to modify the phono input to guitar level as a high gain channel or change the impedance of one of the other inputs to instrument level. You need to revoice the tone stack for guitar too. Then there's a question about the tubes gain staging and whether the output transformer can handle instrument levels. Then recapping everything is most likely needed on an amp that old, adding a grounded plug to prevent it from being a death trap.

 

I used to repair that old gear back in the day so much of its familiar to me. Advising someone with no experience else via the internet isn't going to happen.

 

Its like some kid finding an old jalopy in a junk yard and wants to convert it into a race car. They have no idea of the work and cost involved. You need an old hand at such projects to mentor you step by step. If you had an electronics degree you may be able to get enough info to start a project but by the end you'll wind spending way too much you still wont have anything but an inadequate piece of repurposed junk.

 

 

If the amp design was closer to a guitar amp build I'd say go ahead. Allot of old Bogen PA heads have been repurposed so you could find some mod procedures to guide you. Just a guess would cost you a hundred or more in parts to mod this to take a guitar signal and given its age there's no guarantee it would hold up. This is a matter of whether you're doing this as an educational project or for practical purposes. There's no tube industry any more and no need for people educated in that field. The few guys who still know tubes can at best earn a little pocket change on the side but there is no major industry using tubes any more. If its to get a practical amp its a sink hole. I bought a 15W Marshall MG amp the other day for $38. There's no way you could mod that old boat anchor to match it for that money.

[isaac42]

I have a thought or two.

 

WRGKMC left out one other motivation: fun. I'll leave it to you to decide what your motivation is, and whether or not it's worth it to you.

 

The unit uses 60 watts. Amplifier output is more like 12 watts. It doesn't have a guitar preamp. Does it have a preamp at all? Tone controls? Keep in mind that high fidelity preamps and tone controls aren't necessarily well suited for guitar. As WRGKMC said, it's high fidelity, not guitar fidelity.

 

The power amp is also hi-fi. Many guitar amps use undersized transformers so that they saturate early. That's not suitable for a hi-fi amp, but lots of guitarists like what it does to the sound.

 

I did find a scematic for the power amp. Might come in handy, if you decide to go ahead.

 

http://www.angelfire.com/vt/audio/hoffman.gif

[WRGKMC]

If the amp works, you don't need to touch it. All you need is a guitar preamp that will bring the signal up to line level and give you the right EQ, Drive to push high fidelity amp.

 

They make just the right devices for this. I use them for recording direct all the time. Studio monitors are high fidelity amps that have line level inputs just like your amp has.

 

Most multi effects pedals made by Digitec, Vox, Korg Boss all have switchable output levels for direct recording. You can switch the output from instrument level to line level plug right into that Hi Fi and run it like a power amp without having to do any mods. The pedals also have speaker emulated outputs so the high fidelity isn't a problem.

 

The other benefits are the pedals have amp and cab emulation, EQ setting, Drive, gain and effects so you can make that Hi Fi unit sound like any amp you want. They sound fantastic too. You can make a Hi Fi sound as good as any small combo out there. Of course you'd essentially have hybrid amp by doing this, solid state front end and tube power amp but you'd have enough gain to get the tubes to sound good. The cost and effort involved is going to be allot easier for you too.

 

The cheapest would be a sans amp clone like this one. If you were to mod the amp you'd need to build a circuit like this into that amp. it makes no difference if that circuit is built into the chassis or if its left external. http://www.musiciansfriend.com/amplifiers-effects/behringer-gdi21-v-tone-guitar-driver-di-guitar-amp-modeler-direct-recording-preamp-di-box-effects-pedal?cntry=us&source=3WWRWXGP&gclid=COWS27GMv9ECFQ9rfgod9-oExw&kwid=productads-adid^156727059247-device^c-plaid^143209120602-sku^H85679000000000@ADL4MF-adType^PLA This will get your signal up to line level and have enough gain and EQ to sound decent.

 

 

This is what I'd suggest because its got about 20 different amp and cab emulations that sound killer, plus you have the full array of effects. http://www.musiciansfriend.com/amplifiers-effects/vox-stomplab-ig-modeling-guitar-effect-processor?cntry=us&source=3WWRWXGP&gclid=CIGGnuWMv9ECFYpffgodRCIH-w&kwid=productads-adid^156403583515-device^c-plaid^140837540661-sku^H93379000000000@ADL4MF-adType^PLA

 

 

 

One of these will do the job nicely too. http://www.guitarcenter.com/Used/Digitech/RP150-Effect-Processor-112703495.gc?cntry=us&source=4WWRWXGP&gclid=CJrEmJqNv9ECFYRrfgodSSMM2A&kwid=productads-adid^78244844802-device^c-plaid^140859264021-sku^112703495@ADL4GC-adType^PLA

 

There are many others including the larger floor board units. it all comes down to what you want to spend.

 

One other item. Since these kinds of pedals are designed to sound best through a high fidelity amp like studio monitors when running in line level mode, the only thing I'd suggest is adding a high frequency horn with your guitar speaker. That tube amp likely came out of a console that had a woofer and tweeter or a full ranged speaker.

 

If you only use a woofer it might sound muted without a high frequency driver. You can connect a Piezo horn like this http://www.parts-express.com/goldwood-gt-1016-2-x-5-wide-dispersion-piezo-horn-tweeter--270-041 in parallel to the woofer without any crossover and you'll get the highs you need. They are dirt cheap too. This ones only $2.60. you can buy the small square ones even cheaper and put a pair in the speaker cab. if you find you have too much highs simply put a rheostat in the back of the cab so you can adjust the horns volume.

 

 

 

[Ol'bob]

Thank you for the info! My motivation is primarily for fun (and to find a worthy use for the old thing other than the landfill). I have added a better shot of the chassis. It has tone controls as well. The driver pedal setup sounds like a great option. The unit did come out of a console with a woofer/tweeter setup.

[Ol'bob]

hey all,

I wanted to give an update, I completed the stereo-amp conversion. It turned out far better than I had expected. I swapped out the original two-prong power chord for a grounded three-prong, and removed the "death capacitor". Other than that I left the rest of the chassis pretty much stock, and just built a new cabinet to house it. Thank you all for the great advice. I got a hold of a pre-amp pedal which turned out to be pretty unessesary but it does allow the amp to push a little more volume. I have posted a quick video of the finished product. Thanks again for all the help.

[WRGKMC]

Cool, glad it worked out for you.

 

Back in the 60's when I first started getting into electronics I did a bunch of those. My parents encouraged me in that from the time I was very young and were always giving me old gear to tinker with. The old table top radios had the output transformer mounted to the speaker basket. You could connect another speaker to the line going into that transformer and make an intercom out of it. I'd place a box with a speaker and switch down stairs and my folks could call me over the radio by flipping the switch.

 

Later when I set up a shop in the basement I set up a telephone system. I even had the rotary switch working to ring a bell to pick up the receiver.

Not a great accomplishment by todays standards where you can buy anything you need for this but for an 8 year old kid who had to learn everything from scratch and a couple of books on basic electronics it was quite a thrill. I also built a telegraph which I had connected to a friends house next door, built motors from nothing more them some wire, sewing thimble, pencil and battery. I eventually got into radios starting with making a crystal radio, then learning to wrap my own coils so I could tune in higher short wave stations. Even with guitars I learned to wrap my own guitar pickups by the age of 10. Didn't work half bad either.

 

It all fascinated me and still does 50 years later and 40 years working in the industry. There are still unlimited ways of using electronics and many more that can be highly useful and make people a mint. You simply have to experiment, evaluate and learn where you can improve off of different ideas. Not exactly the cheapest hobby but you can get allot of satisfaction troubleshooting problems and coming up with good fixes.

 

I think that's what gave me a big edge in my field because I was never satisfied with incomplete answers. If you had a component or board fail, its east to find that part or board and simply replace it. I'd always try and find out why it failed and find ways of preventing that failure or improving the lifespan of the replacement part. I eventually worked my way into jobs providing technical support, teaching, and even engineering support where you'd try and cause components to fail and evaluate they're weakness. Engineers can design things that look great on paper but it takes some common sense and hands on experience to refine the design to the point where it was also human proof.

 

Too bad most electronics are disposable now. I used to like doing that kind of work. Today the manufacturers wont even give you a schematic so you can test what's wrong with a board. They protect they're designs and R&D from pirates who simply steal their designs. Even that doesn't help much because you can connect a circuit to a computer and it can tell you what's going on inside.

[isaac42]

Congrats, Ol'bob. Glad it worked out for you.

[AMZ-FX]

hey all,

I wanted to give an update, I completed the stereo-amp conversion. It turned out far better than I had expected.

 

Looks great; nice project!

 

I have an old 1930s radio in a cabinet and can't decide to restore it or convert to a guitar amp. The radio uses non-standard tubes but I could probably find them at Antique Electronic Supply. The tubes are the type that just have numbers as type identifiers, such as 31 for a triode tube.

 

regards, Jack

 

 

[WRGKMC]

I had a whole bunch of those tubes given to me as a kid. I built a few things from them but eventually sold them off.

 

They don't have a key for plugging them in, simply different pin sizes. I saw some being sold on Ebay just a few days ago in fact. The problem is anything you'd buy would be used. You may find some NOS but it would need to be 50+ year old new old stock.

 

There are a bunch of places that restore those old radios and when they are well done can bring in a pretty penny. Unfortunately, not all are so good. Many are purely AM and unless you like talk radio its kind of overkill to just listen to people talk. Some of the old mono FM tube receivers used to sound killer. Problem with broadcast quality isn't as much with the receivers any more, its because stations have switched to digital and play poor quality MP's which simply don't have the fidelity by the time it gets to your radio.

 

 

I cant even stand to listen to many stations any more because of the poor sound quality. When they aired vinyl with all analog you could get those nice tones, but they simply don't exist any more. Satellite radio is the absolute pits. The artifacts are so bad its like fingernails on a chalkboard.