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old (very old...) amp


 
6/5/1999 7:26 PM
John C.
old (very old...) amp
I have a very old amp that was part of a big "record player", one of those with a steel nedle, about 40 or 60 years old.  
It has no loudspeaker (someone told me it coul be a field coil speaker)  
The tubes are:  
2--> 648  
2--> A3  
1--> 53  
1--> 27 (this one is protected inside a metal case)  
 
As don't know much about electronics, but before I take it to a technician, I would like to ask:  
Could it be converted to a guitar amp?  
 
The lodspeaker connections are 5:  
One goes to ground.  
Two of them, each one goes to a "certain point" in each of the A3's.  
One goes to one of the 648's.  
The last one, to one end of a huge metal resistor (3.000 ohm) and to a + sign of a 8uF capacitor, then the resistor goes to somewhere in the "metal case protected" 27 tube.  
 
In two of those LSpeaker connections (I guess the ones going to the A3's, I can't remember now) I measured 240 volts, although the amp funtions at 125v.  
 
I know it's not enough, but maybe...  
 
I forgot. It has four transformers. two "small" ones with just two connections one four the other, a bigger one with 8 cables coming out of it, and a much bigger one with a lot of connections and cables.  
 
It looks pretty good, old (very old!) components, but no overheated parts at all.  
 
JohnC
 
6/7/1999 6:08 AM
MKB

Hi, John. Concerning your amp, I would first suggest that you get a schematic for the unit. I have not had much trouble getting old schematics, Antique Electronics Supply can probably provide one for $5 if you can get the mfg and chassis number. If you have HV on your speaker connector the unit is probably looking for a field coil speaker; the field coil can be replaced with a large value resistor that is the approximate value of the coil resistance (maybe on the schematic?).  
 
I'm sure you could convert it to some type of guitar amp, but it may cost big bucks. I haven't heard of many guitar amps that use this tube complement, this seems to predate any Fender amp I have heard of. You may want to check the price of replacement tubes, some of these old radio tubes are completely outrageous in price, although hamfests can be a good place to buy them. I don't recall tube types 648 and A3 (could it be a 43?); the 53 and 27 sound like good numbers. The 27 is a medium mu single triode, one of the early ones with a cathode. On second thought, you may be able to use the transformers in a DIY amp if you can guess the power consumption of the unit and find output tube info.  
 
Oh, and be sure to replace all the electrolytic caps as they are SURE to be bad by now. Judging by some of the tube numbers, this unit was probably built in the 1930's. Hope this helps!
 
6/7/1999 8:02 AM
Graywater

John,  
 
 
MKB hit most of it and I agree that you have an "electrodynamic" loud speaker which uses an electromagnet whose field. current derives from the B+ source (the field also serves as a filter choke for the B+). As to the "A3s" these are bound to be 2A3s - probably the most popular output tube before the 6L6. These are directly heated triodes which have a very high transconductance requiring individual bias adjustments - they also require a fairly high drive signal. They have a 2.5 volt filament and there may be a separate winding for each one if the amp is cathode biased (likely). You also seem to have a couple of chokes (the two lead "transformers") - is there another transformer mounted on the speaker frame? If so, this is your output transformer.  
 
Anyway, I suspect that this amplifier really wouldn't make a very good guitar amp. Triodes are generally regarded as having poor guitar amp tonal signatures and the amp probably has greater worth for its historical significance than worth as a potential guitar amp. If the amp is still in the "big record player" you could probably sell it to hifi buffs. Your 2A3s are valuable, even used, to the hifi nuts, especially if they have the double + + plate structures. Also, the output power is 15 watts or less - not much power for what is probably a really large chassis. I'm never one to discourage a conversion with good learning potential but I believe in this case you would destroy a historic artifact and learn only how to identify antique resistors and capacitors that you may never see again in a more "modern" (post 1955) amp.  
 
GW
 
6/7/1999 6:46 PM
John C.

Allright, MKB, Graywater.  
 
I'll leave it as it is now.  
Maybe I may find a tech who is able  
to retune this thing, in the future.  
 
Some of the caps problably  
electrolytics, are the size of half  
a packet of cigarettes :) and they have a cover of cardboard that folds inside itself like if it was made for containing pins or something like that. The brand of these caps is DUCATI (yes, the same of the italian motorbikes)  
 
It's very nice, and I know it actually works  
except for the louspeaker, and it's a shame I don't have the required knowledge to readjust it properly. A real shame.  
 
I'll leave it as it is untill I find someone who knows what he's doing. (Someone near where I live)  
 
The big peace of furniture, wooden, is in a terrible conditon, and the louspeaker is missing. All the rest is in a great shape.  
 
Thanks a lot.  
Sincerely.
 
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