1940's Silvertone phono conversion?

[daveD]

I recently found an old Silvertone phonograph and have become intrigued by the possibility of converting its tiny tube amp into a guitar amp. Compared to those who frequent this forum, however, I am a total beginner. I once followed step-by-step instructions to replace the OT and caps on my Kalamazoo, but I probably have just enough knowledge to be dangerous…

The only transformer in sight is attached to the speaker itself. Its tubes include 35W4, 50C5, and 12AV6. It powers up – all 3 tubes glow – but generates a loud hum that is unaffected by the volume control. When I tested the input with an ipod, I could faintly hear the distorted signal underneath the hum. Questions:

1) Is this thing worth messing with, and do I stand any chance of fixing it without someone having to spoon feed me every step of the way?

2) If the answer to #1 is affirmative, would replacing that can capacitor with individual caps be a good place to start?

3) What’s the point of the transformer being attached to the speaker? Does it have to be located in immediate proximity to the speaker like this, or can I remove it from that stock speaker and incorporate it into an amp enclosure so I can use this amp with another speaker?

This seems like an appealing project, but I want to be realistic.
Any suggestions/advice would be most appreciated...

[Chuck H]

What you have is an amp that doesn't use a power transformer. The first two digits in the tube designation numbers are the voltage requirements for the filaments. 35+50+12=97. The extra 13 volts (this amp was probably intended for 110V) is probably dropped by a big honkin resistor. This kind of setup doesn't usually make for a very satisfying guitar amp, and the odd tubes are cumbersom to aquire. The gain is probably pretty low so adding another 12V tube would be a good idea. You would still need the big resistor though because todays wall voltages are around 120V. Adding the extra gain would increase the inhearant hum in this kind of design which is another drawback. This kind of amp was considered cheap when it was made and IMHO is not any better today. There is also an added element of danger due to electric shock because there is no isolation from the wall AC via a power transformer. The transformer on the speaker is probably the output transformer and does not need to be on the speaker. The output transformer, as well as the speaker and perhaps many other componants, could be salvaged and used in a different project though with much more satisfactory results.

JM2C

Chuck

[stingray_65]

greetings Dave,
The amp you are discribing uses the 120v house current to run your tubes. it is very much like the old AA5 (all american 5) radios.
answer to question 1. Maybe . I've made a few small amps using the same tube compliment you discribed and i love the sound of a 12AX7 driven 50C5 amp. there will be costs involved to make this a safe amp. it will not be loud by any means, but it should have a very rich sound (IMHO)
2 YES! this will in most circumstances bring that type of amp back to life. because this is not a high voltage amp, the capacitors (ALL OF THEM) need not be rated at more than 200v and should cost way less than $20 to replace them
3 the point of the transformer is to match output imedance between the output tube (50c5) to the speaker.

to make this syle of amp safe to operate, even as a phono amp, you will need to make some mods. the first is to use an isolation transformer at the mains (120v)
the reason being that the plug that is there has no polarity meaning that depending on how you plug in to the wall you could make the HOT from the mains the chassis ground and incedentaly all metal items on your guitar that are grounde will be as if they are plugged in directly to your wall. SO if you are touching the guitar strings and some other part of you touches a properly grounded source (damn near anything) you will get a severe shock.

some say you can just use a 3 prong plug and it will work, but it relies on every plug that you plug into to be wired correctly. ask ANYONE who has gigged how many times they have run into miswired outlets.

I fell these sort of projects are simple, affordable, fun and educational. The people here would be more than willing to help you along every step.

BEFORE YOU START POKING AROUND ANY WHERE IN THIS OR ANY OTHER AMP FAMILIARIZE YOURSELF WITH ALL THE SAFTEY INFORMATION AVAILABLE IN THIS SITE. TUBE AMPS HAVE LETHAL VOLTAGES ALL THROUGH THEM.

THIS STYLE OF AMPLIFIER HAS KILLED PEOPLE.

OH!
see if you can find a schematic and post it. it will make it much easier to form a game plan and help you along on your journey much easier.

Ray

[daveD]

Thank you for the very informative responses! Although it's cute, it's sounding like this may not be the project for me at this point. I think I'd prefer to cut my teeth on something that has "better bones" in the first place, and perhaps save this for when I have a little more experience under my belt...

[stingray_65]

Dave,

I sincerely did not want to discourage you in any way. Amp building has become a VERY rewarding hobby for me and I encourage any one with even a slight inkling to give it a go.

I guess the "difficulty" of any project like this, be it a conversion, repair or full blown build from the ground up, depends on your skill levels.

The three main skill areas that are needed to build amps are:

1) Electronic / electrical skills.

2) Woodworking / carpenry skills.

3) Sheet metal / fabrication skills.

The most basic and perhaps the most critical skill is the ability to KNOW and follow ALL saftey procedures when working with any aspect of this hobby.

Basic electronic and electrical skills would include component identification, schematic reading, soldering/desoldering and meter reading.

Basic wood working skills would be enough knowledge to be able to construct a simple box of pine or plywood and how to safely use the tools to do that.

The metal fabrication skills to do basic modifications like being able to layout and drill holes to mount various parts on to a metal chassis.

In a nutshell what it would take to convert your amp would be:

1) Add an isolation transformer where the plug is.
2) Add a 3 prong cord to the isolation transformer.
3) Remove and replace ALL the capacitors in the amp.
4) Change out the the phono input to a 1/4" jack.
5) Depending on how the chassis is mounted currently you might need to construct a suitable cabinet to house the amp and speaker.

I do not wish to be contrary to Chuck, but finding those tubes are not difficult, they are inexpensive compared to the "common" audio tubes, and I really do think they sound good albeit different than typical Fender, Marshall, Vox amps.

Ray