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Gene Leis Model 908 Tube guitar amp
« on: December 06, 2007, 12:52:03 PM »
I know this should be posted at some guitar amp forum somewhere ,but it may be of interest here in the Lab forum.

Found this amp at a music store in non-working condition for $20

I have an obscure tube labeled "KWBA" made by Raytheon , made in the USA. Used as a power output tube on a single ended tube guitar amplifier. The Amplifier is a Gene Leis model 908, made in Manhattan Beach, California dated 1968. Would love to find a schematic for this amp too.  its very simple 3-5 watt amp, kinda like a Fender champ but a different circuit inside. Tube compliment is 35Z5GT, 50L6GT, 35W4, 12AX7A. Filter cap is a 50mfd-50mfd/150V




I have looked high and low and cannot find a datasheet or cross reference on this tube anywhere on the web.  It shares same pin compatability as a 6V6 or SL6. However it cannot be substituted for
a 6v6, I have tried to plug a 6V6 in place and it did'nt work on the amp.

Below are some pictures of the KWBA tube. I believe the tube is a
power tube because it is connected to a output transformer that feeds a small 8 ohm speaker. The rectifier tube feeding the KWBA is a 35Z5GT which is being powered from 120VAC line voltage thru a pilot lamp. There is no power transformer in this amp.





NewYorkDave

Gene Leis Model 908 Tube guitar amp
« Reply #1 on: December 06, 2007, 01:02:13 PM »
Look really, really close at the glass--mist it up with your breath if you have to. There's got to be a type number lurking there somewhere.

Unfortunately, I can't see your photos because whatever site you're using to host must be blocked by my ISP.

Gene Leis Model 908 Tube guitar amp
« Reply #2 on: December 06, 2007, 01:34:23 PM »
tried misting the tube but nothing. It must have been silkscreened but you know what happens to silkcreened tubes after 30+ years - it rubs off in a instant. I would try to picture the stamp on top of the tube but its only barely visible in the correct angle of light with the naked eye.

I will gladly send more detailed photo's of the tube if you think that would improve visibility. thanks for dropping by.

Gus

Gene Leis Model 908 Tube guitar amp
« Reply #3 on: December 06, 2007, 03:09:10 PM »
Maybe a 25L6 or 35L6

How is the 12ax7 fil wired?

35V +35V +12V =82V     110 -82 = about 28V

35V + 35V +6 + 76V       110 -76= about 34V

Gene Leis Model 908 Tube guitar amp
« Reply #4 on: December 06, 2007, 03:32:20 PM »
Gus,

35+35+12  I believe. My tube skills are dull,  How can I confirm it with a DMM ? , just measure the Fil or Heater Volts  ?

The RCA receiving tube manual takes a 25L6 and refers it to 50L6GT and rates heater capability at 50v ac/dc .

35L6 Heater rated at 35v ac/dc , these may be the tubes , not sure on that yet. but they are pin compatible octal types like the KWBA in the amp now.

the 35Z5GT is passing 42.9VDC from pin 2 to the "KWBA" power tube heater pin #2. If this helps any ..... basically  "KWBA" is getting 42.9VAC.

looks like I'm closer to a 25L6 (25v) / 50L6GT ( 50v )


thanks

shabtek

huh?
« Reply #5 on: December 06, 2007, 05:25:18 PM »
edit: get you a power transformer
the design of this and the questions you ask make for a scary situation (risk to your health/property)
 I would say make sure that green wire (ground) from the power cord is tied to chassis--but I see the anti-hum switch: whats that?...looks dangerous.
"really fine players do not use stomp boxes or master volume, they match the amp to the room and turn it up to 11.  Stevie Ray, BB King, Albert King, Duane Allman, Dicky Betts, Louis Armstrong"
   -CJ

50L6GT
« Reply #6 on: December 15, 2007, 02:47:25 AM »
Bingo .....

Thanks Gus you just helped me solve this. The "KWBA" tube was really a 50L6GT or a 50V version of the 25L6 . I confirmed it by putting one in the amp and running it. It runs exactly the way the "KWBA" tube runs. this little amp really sounds great for low level guitar amplification or recording. It has a nice tube breakup sound when driven up in higher gain settings. It would make an easy inexpensive project for amp cloning.

and best of all no SHOCKS

Shabtek - whats so scary about this amp ? no power transformer  

what if I power it using an isolation transformer, which is what I did when I was checking voltages and determining what the mystery output tube was ?

7string

Gene Leis Model 908 Tube guitar amp
« Reply #7 on: December 15, 2007, 04:06:07 AM »
GUT IT

Put in the PRR Third Watt.

This is perfect for it. I would put in 16/16 dual can caps in the octal holes and PTP the rest with those phenolic terminal strips. Your parts bill will probably be less than $75.

http://www.hoffmanamps.com/

Search for 6AU6 amps. Its a couple of pages back now. Great little practice amp. Mean tones with some pedal boost.

Gene Leis Model 908 Tube guitar amp
« Reply #8 on: December 15, 2007, 11:49:47 AM »
GUT IT , no dice.... it sounds good enough as is.

As for the PRR 3'rd Watt, Looks like a fun build for a new project

thanks for the suggestion 7String

shabtek

yeah
« Reply #9 on: December 15, 2007, 02:24:58 PM »
transformer isolated power supply and chassis grounded with 3-wire power cord
is safer
"really fine players do not use stomp boxes or master volume, they match the amp to the room and turn it up to 11.  Stevie Ray, BB King, Albert King, Duane Allman, Dicky Betts, Louis Armstrong"
   -CJ


Re: Gene Leis Model 908 Tube guitar amp
« Reply #10 on: May 28, 2013, 02:24:33 PM »
Nice. I've got the 708 and I have been trying to determine the ohm rating on the speaker, you just provided it for me, 8 Ohms.  I want to upgrade to a 10" speaker in hopes of getting a little more clean headroom out of it.  Any thoughts?  Gary

PRR

Re: Gene Leis Model 908 Tube guitar amp
« Reply #11 on: May 29, 2013, 12:16:17 AM »
> upgrade to a 10" speaker

Put in a Power Transformer, and an external speaker jack.

A Ten, or Two-Twelve, or Four-Ten will make a monster out of this midget......

BUT keep the original Six where it is!! You don't get tone like that (bad or good?) from modern speakers, and too many of these cheapo amps are being molested, modded, then abandoned.


 

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