freestompboxes.org

*First Post*

Hi Ladies and Gents,

I just scored a massive pile of "garbage" from my school's Physics Department. I work there and as I was sorting out stuff to go to salvage, I set aside some stuff for myself without really looking at it. Turns out I have an old tube amp that is at least 35 years old. (There was a tag on it that was date July 2, 1976.) I am a novice at the whole soldering bit, as I am currently working on my first mod (Boss DS-1), but I've been playing guitar for about a decade now (wow... I feel old...) and I understand most of the basic concepts of the electronic world. So, back to the amp. I want to know if I can bring this puppy back to life and mod it for the guitar. I am but a "poor college student" and am looking to do this as cheaply as possible.

The pics are below. Any help/advice will be greatly appreciated.

Thanks!

-Mike

It looks like you might be able to turn it into a cool little amp, before you fire it up to see if it works give it a good checkout to make sure there's no faulty components, and that the power transformer is good, and that there are no mains shorts to the chassis(which could be very dangerous or even lethal), after that, I'd suggest going through all the forum threads and posts which deal with tube/valve electronics and have a good read of them, hope that helps...

Also, before you power-up the amp, buy a variac and connect it up to the amp and mains and slowly bring up the voltage, this will re-form the filter caps in the amp's power supply, if this isn't done the caps could explode unexpectedly...

Thanks for the reply. Unfortunately-and I probably should have stated this earlier-I do not have the power supply. Given that it is five pronged and requires 50 milliamps @ 300 volts DC and 2.7 amps @ 6.3 volts AC, I am most likely SOL in being able to power this up as is. (I also don't want to burn my dorm down...) So the first step would be to find a way to power it up. Just how...

brokenstring wrote:Thanks for the reply. Unfortunately-and I probably should have stated this earlier-I do not have the power supply. Given that it is five pronged and requires 50 milliamps @ 300 volts DC and 2.7 amps @ 6.3 volts AC, I am most likely SOL in being able to power this up as is. (I also don't want to burn my dorm down...) So the first step would be to find a way to power it up. Just how...

There's a hi-gain tube/valve based pedal project thread which you could have a look at for ideas for a power supply for this, the 6.3V@2.7A might be a bit tricky....

Check out this link:


https://www.freestompboxes.org/viewtopic ... 28&t=15954

checking into it now! thanks again!

brokenstring wrote: I am a novice at the whole soldering bit

brokenstring wrote: but I've been playing guitar for about a decade now (wow... I feel old...)

...So be careful with the HIGH DC voltages that feed the tubes plates. I know this advice won't really bring your amp to life, but it may keep you playing the guitar and getting even older.

I don't see an output transformer, and it says optimum load 600 ohms so you would need to modify it somewhat to use it on an low impedance speaker.

good advice from lolbou

lolbou wrote:So be careful with the HIGH DC voltages that feed the tubes plates. ...

If you do get round to modifying it and building a power supply then check back with the tube guys for some more good advice and pointers on what you need to do.

lolbou wrote:...So be careful with the HIGH DC voltages that feed the tubes plates. I know this advice won't really bring your amp to life

This is exactly why I'm scared to dive into this without knowing exactly what's going on. So research I must. Still, any insight offered is, again, greatly appreciated.

deltafred wrote:I don't see an output transformer, and it says optimum load 600 ohms so you would need to modify it somewhat to use it on an low impedance speaker. If you do get round to modifying it and building a power supply then check back with the tube guys for some more good advice and pointers on what you need to do.

Thanks for that bit of info. And of course I will!

There is a manual for it here http://www.k7jrl.com/pub/manuals/gr/1206b/1206-B.pdf

It would probably make a very nice low power amp but I am not familiar with the 6W6 tube.

Edit
And the power supply here http://www.k7jrl.com/pub/manuals/gr/1203b/

deltafred wrote:There is a manual for it here http://www.k7jrl.com/pub/manuals/gr/1206b/1206-B.pdf

It would probably make a very nice low power amp but I am not familiar with the 6W6 tube.

Edit
And the power supply here http://www.k7jrl.com/pub/manuals/gr/1203b/

Holy crap. I didn't even think that these could be found anymore. God bless the interwebs. Thanks a ton.

also, for those that mentioned it being a low powered amp, that's exactly what I'm looking for.

not to burst your bubble or anything, but I work in a physics department with lots of very similar stuff. What you have there is most likely an RF amplifier and not going to be suitable for guitar on its own. First clue is that in the manaul, the bandwidth runs up to 1Mhz, whereas audio stops at 20khz, and guitar under 10khz.

I double the tubes inside would be good for audio either. The enclosure might be worth salvaging for a project, but not every amplifier is a guitar amplifier.

coldcraft wrote:not to burst your bubble or anything, but I work in a physics department with lots of very similar stuff. What you have there is most likely an RF amplifier and not going to be suitable for guitar on its own. First clue is that in the manaul, the bandwidth runs up to 1Mhz, whereas audio stops at 20khz, and guitar under 10khz.

I double the tubes inside would be good for audio either. The enclosure might be worth salvaging for a project, but not every amplifier is a guitar amplifier.

Looking at the manual it is an amp with 3db bandwidth of 2Hz and 500KHz, put an output transformer in and that will shrink dramatically dependant an the quality of the transformer.

Output tubes are quoted as audio/general purpose - 6W6GT. Phase splitter is standard 12AX7.

Needs a power supply, OP transformer, and gain stage or 2 and tone stack and you are good to go.

I ended up downloading a copy of the amp and power supply manuals too....


Deltafred's right, this amp should easily be able to reproduce audio frequencies judging by the frequency-response graph in the amp manual, and I'm guessing that it should reproduce audio with pretty good fidelity, after all, the amp includes negative feedback and the response is pretty flat....


Note: The manual quotes a distortion figure of 2% maximum at 19kCs, or 19 kHz, 1C=1Hz 1kC=1Khz......

So I have a lead on the power supply, if you can believe it. If I don't end up with it, I'm probably going to gut the amp and rebuild it from the ground up.

brokenstring wrote:So I have a lead on the power supply, if you can believe it. ...

I believe it, there is all sorts of treasure out there waiting to be discovered.

It might be junk to some people but to others, who know what it is and need it, it is treasure.

While I wait until I find out whether or not I'll be getting the power supply, any suggestions as to what schem I should use if I decide to rebuild this hunk o' junk? It seems like most people are building much bigger heads than 3w. I'm looking for a blues-driven sound, in the ballpark of Jimmy Page, etc.

You could try rebuilding the amp as a 3-Watt JTM45, or Plexi, all you'll need is two more 12AX7 tubes and a suitable output transformer and power transformer, that should get you some good blues tones, the bonus is that you'll be able to crank the amp up full and not get the ear splitting volumes, should make a good amp for studio recording, anyway there's some ideas for you....

great, thanks for the suggestions. I'll look into them.

You would probably get a bit more than 3W output it with an output transformer because when directly coupled the impedance matching is not very good (as implied in the manual).

Have a look at the specs for the 6W6GT and 6V6GT here

http://www.duncanamps.com/tdsl/show.php?des=6W6GT

http://www.duncanamps.com/tdsl/show.php?des=6V6GT

If you can get hold of the power supply it would probably be worth adding an output transformer and using it as is. If you need to build a power supply then just break it for parts as it would be better to swap the output tubes for 6V6GTs because the heater current is far less for these.