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In Reply to: RE: anyone have a schematic for a Wurlitzer 6420 Tube amp? posted by mr9iron on January 21, 2017 at 23:48:46
If I read the uploaded graphic correctly, The single 6SN7 is wired as a paraphase, AKA seesaw, phase splitter. The PP 6L6 "finals" are self (cathode) biased.
There is NO way that 6SN7 can drive the O/P tubes to full power, when fed the 2 VRMS signal of a "standard" CDP. What sort of job do you plan on having that amp do?
Eli D.
Follow Ups:
Eli,
Thanks for the reply, right I was wondering if there was a way to drive the 6l6 to full power.. I have two of these I was going to build these into guitar amps for a friend.. For use as a home practice amp, probably not an amp you would play out with, unless it is a small area..
Probably drive an 8 ohm 12 inch guitar speaker in a cabinet. We will chose one of the more efficient Emmince drivers that are popular with bass and guitar players..
These were designed to drive one speaker and a field coil as I have found out.. I am using a dummy 6k load for the field coil and I can drive an 8 or 16 ohm speaker fairly well but I can tell there is a power leak..of course it's all original right now, nothing replaced..
You'd be surprised at the amount of power PP 6L6s can deliver, which is easily 30 W. Newer variants of the 6L6, like the 'GC, deliver yet more.Other than the RIAA equalization considerations, guitar pickups produce signals at the "same" levels as MM phono carts. and require lots of gain in the preamp.
Check the Fender "Bassman" out for ideas in realizing your objectives. The Sovtek 12AX7LPS is a genuine 7025 equivalent containing a spiral wound, hum bucking, heater. Mounted in an enclosure separate from the speaker (suppression of microphonics) the 'LPS will work well in that sort of circuitry.
Eli D.
Edits: 01/22/17 01/22/17
I would replace the 'SN7 with a 'SL7 and change component values to suit. A 6SL7 will drive PP 6L6s just fine.
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Buy Chinese. Bury freedom.
If I swap out the 6sn7 with a 6SL7 what would I have to upgrade as you implied?.. It's though to read the schemo, but in general what would I need to upgrade.. I thought the 6sl was a drop in replacement but folks say this will drive the 6l6's better..
again any help appreciated..
J
Easiest fix, if low gain in stage is an issue.
The input from the Wurlitzer organ into the amp prolly has more voltage than a guitar, for instance. That's why a 6SN7GT was used. Just a guess.
The original post didn't say this would be a guitar amp. Even changing to a 6SL7 won't produce enough gain for that. He'll need at least one more stage.
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Buy Chinese. Bury freedom.
Usually in the 100s of mV AC range (prolly average, ~500mV). I kinda remember organs having outputs regularly around & above 1 VAC.
stupid me I didn't have the continued threads notify me, and I just saw these new posts.. Yes, originally these were organ amps that must have driven a field coil and speaker. They have two sockets labeled for speaker 1 and speaker 2.
Yes, my idea was to see if I could turn these into guitar amps. I have a friend who is a guitar player. However up to this point my experience has been rebuilding audio amps. I am not a musician in any way and have only basic knowledge of guitar amplification.
In light of your suggestions would I be better off converting these to an audio application using a 6sl? VS. trying to convert to guitar amp? Sounds like I would have to come up with a different input stage all together.. I would however have an available socket if I had to use two tubes, as the sockets used for the speaker outputs and even the organ input, could be removed..
thanks again everyone..
He can put in a 6SL7 for an input voltage amp and he can wire the 6SN7 as an LTP and he's be in business.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Apparently, only 1 socket is available. :> (
Eli D.
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