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anyone have a schematic for a Wurlitzer 6420 Tube amp?

76.121.59.220
Posted on January 21, 2017 at 23:48:46
mr9iron
Audiophile

Posts: 454
Joined: March 13, 2002
 ...best one I can find is this guy but it's too small for my not so great eyeballs...

if anyone has ever modded one of these any way to make a better input stage than the single 6sn7?

as always thanks!



 

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RE: anyone have a schematic for a Wurlitzer 6420 Tube amp?, posted on January 22, 2017 at 04:38:02
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 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.

 

Sent the schemo through Photshop, posted on January 22, 2017 at 04:38:34
FenderLover

HTH!


photo wurlitzer 6420 parts-1_zpsyrs2yczs.jpg

 

RE: anyone have a schematic for a Wurlitzer 6420 Tube amp?, posted on January 22, 2017 at 06:00:12
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 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

 

1/3rd and 1/5th watt resistors, posted on January 22, 2017 at 08:51:52
FenderLover
 Never heard of such power values. Wonder if these resistors were specially made for Wurlitzer?

 

RE: anyone have a schematic for a Wurlitzer 6420 Tube amp?, posted on January 22, 2017 at 10:45:43
Triode_Kingdom
Audiophile

Posts: 10041
Location: Central Texas
Joined: September 24, 2006
 I would replace the 'SN7 with a 'SL7 and change component values to suit. A 6SL7 will drive PP 6L6s just fine.

 

RE: anyone have a schematic for a Wurlitzer 6420 Tube amp?, posted on January 22, 2017 at 11:33:15
mr9iron
Audiophile

Posts: 454
Joined: March 13, 2002
 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..

 

RE: Sent the schemo through Photshop, posted on January 22, 2017 at 11:42:46
mr9iron
Audiophile

Posts: 454
Joined: March 13, 2002
 Hey I can at least see it now...thanks! Yeah I have never seen those values before, will be interesting when I check them...

 

RE: anyone have a schematic for a Wurlitzer 6420 Tube amp?, posted on January 22, 2017 at 13:53:29
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Apparently, only 1 socket is available. :>(


Eli D.

 

RE: anyone have a schematic for a Wurlitzer 6420 Tube amp?, posted on January 22, 2017 at 17:33:08
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000



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.

 

RE: 1/3rd and 1/5th watt resistors, posted on January 22, 2017 at 17:36:00
Jim McShane
Dealer

Posts: 5910
Location: Chicago
Joined: January 13, 2003
 They aren't that uncommon. IIRC, Fisher and/or Scott used them as did others.

 

That's what I'd try., posted on January 24, 2017 at 07:12:49
FenderLover
 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.

 

RE: That's what I'd try., posted on January 24, 2017 at 08:11:07
Triode_Kingdom
Audiophile

Posts: 10041
Location: Central Texas
Joined: September 24, 2006
 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.

 

Yeah, just using guitar AC output voltage as an example., posted on January 24, 2017 at 08:16:22
FenderLover
 Usually in the 100s of mV AC range (prolly average, ~500mV). I kinda remember organs having outputs regularly around & above 1 VAC.

 

RE: anyone have a schematic for a Wurlitzer 6420 Tube amp?, posted on January 25, 2017 at 15:15:31
mr9iron
Audiophile

Posts: 454
Joined: March 13, 2002
 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

 

RE: Yeah, just using guitar AC output voltage as an example., posted on January 25, 2017 at 15:34:35
mr9iron
Audiophile

Posts: 454
Joined: March 13, 2002
 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..

 

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