Tube Amps / Music Electronics
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|6/2/2000 8:20 PM|
I picked up this amp without any tubes installed. I can't find anything about this amp on the web. Zack at Vibroworld doesn't know either, but it was definitely made by magnatone. Anybody have any ideas as to how to figure out what tubes it needs without risking a fire?
|6/2/2000 9:01 PM|
You are going to need a tube manual!
Before going to all that trouble, have you checked the transformers out? Check the PT for correct voltages for filament and expected values for the B+ or AC input to the rectifier tube or diodes. Make sure to bleed the filter caps after checking the PT!!! Check the primary of the OPT. You should get 100-200 ohms from the CT to each end.
If no schematic is around, you have to trace it, which isn't as hard as it might sound. If you follow the wires from the tube sockets to the resitors and/or caps and draw it out you can figure out the pinout of the sockets and then search the tube manual.
-Cathode resistors will be less than 5k and maybe have a bypass cap
-Plate resistors <50K probably and will trace the filter caps
-If it has a rectifier tube, there will be two wires from the PT to the socket at least (the AC for the B+) and likely two wires for the filament, unless it used a 6V rectifier like a 6AX5.
-The filament wires are easy to trace and can be eliminated right away
-Check the filament pinout- 4,5 and 9 is probably a 9A base like 12AX7, etc.
-Maganatone used 6973s for power tubes in some amps, are the power tubes 9 pin?
-If you posted closeup jpgs, people here at Ampage could probably help your detective efforts.
|6/4/2000 1:52 PM|
||HV Testing - Tubes should be IN!|
I just had to barge in here after an experience yesterday with an SVT power chassis. A lot of amps obviously rely on tube current draw thru dropping resistors to achieve various PS voltages, and rate the caps appropriately at the various voltage points. Well, yesterday I put almost 700 volts on an old Sprague 450 volt cap by powering the amp up with the tubes out, something I know not to do (?). I only did it for a few seconds, and the voltage ramp-up was gradual (used a Variac), but the cap made two 'clicking' noises and my fingers will be crossed from now on (the amp works fine now, no problems yet). I consider myself lucky the thing didn't explode, which it might have done had I not used the Variac. It's my own amp, I'll just live with the consequences for now.
When checking P.S. voltages, the tubes should be IN the amp, unless you're going to use a Variac and not power up all the way. I know it's sort of a 'Catch 22' situation when the tubes are unknown, and it may not apply to all amps, but far better safe than sorry (or injured). This is the only thing about your post I wanted to clarify, Scott - maybe someone else could be spared the scary experience I had yesterday, or worse.
|6/4/2000 9:12 PM|
I should add that if the amp has a tube rectifier, it's perfectly OK to check transformer voltages as Scott mentioned as long as the rectifier tube is out. In fact, to really check the transformer it's best to have no tubes in the amp, but THEN you have to make sure that the standby switch is off, if there is a standby switch, with a solid-state rectifier. It's even better to completely isolate the transformer by disconnecting all its leads except the primary winding.
Are we still having fun?
|6/3/2000 4:54 AM|
||Re: Estey/Magnatone T32|
I think it's the same or very nearly the
same as a Magnatone 431, which is in
Pittman's Tube Amp book.
There should be two wires coming off the
output transformer, a blue one and a brown
one, if they go to different pins on
the *same* tube socket,your amp uses the
These are kinda pricey if you can find any,
it's possible to rewire for a pair of
6BM8/ECL82, but it's kind of a pain.
The other tube sockets should be for
tubes (12AU7, 12DW7 or 12AX7) which all
pin out the same. If there's a *7 pin*
socket, that should be for a 6AV6.
If I'm right about this, if you get
a diagram for a 431, you should be able
to trace what is hooked to what tube
and deduce from that which tubes go where.
|6/3/2000 12:18 PM|
Sure 'nuf. OT wires to pins 3 & 8 of the same tube. Sounds like a 6HU8. I'll track down a schematic for the 431 and take it from there. Thanks a bunch!
How many watts does a 6HU8 put out? It doesn't show in my RC-22 and the brief info at duncan amps indicates on 3 watts. If it is only 3 watts, I'm not sure I'm gonna restore it.
|6/7/2000 3:56 AM|
>How many watts does a 6HU8 put out? It doesn't
>show in my RC-22 and the brief info at duncan amps
>indicates on 3 watts. If it is only 3 watts, I'm
>not sure I'm gonna restore it.
I think the 3 watts is only for 1 of the
2 pentodes inside. 7 watts should be more
The T32 I worked on, as I recall, was
pretty loud, all things considered.
Considering how many people are looking
for amps that'll break up & distort
at low wattage, I think someone could be
very happy with it...
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