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Post subject: 1 meg grid-to-ground resistor on original 6G15 Reverb Unit
Posted: Wed Apr 11, 2012 5:24 pm
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Went back and added the 1-meg ohm, 1/2-watt grid-to-ground resistor to the 12AX7 (7025) output tube of my old Reverb Unit (6G15). You can seen the resistor on lower right side, connecting pin 2 of the 12AX7 to ground. I used the unused strap of the foot pedal jack to ground out the resistor.

No added hum or other issues. Pretty neat location. Out of the way of jacks, tubes, and signal or heater wiring. Good idea for those of you that own these older units. The resistor helps center the idle of the tube and will dump an excess current load, if the tank should short out or draw a high transient current peak (and maybe save a good 12AX7).

http://www.webphix.com/schematic%20heav ... _schem.pdf

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Post subject: Re: 1 meg grid-to-ground resistor on original 6G15 Reverb Un
Posted: Wed Apr 11, 2012 6:02 pm
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Pretty smooth installation, B-Man.

Saves you the grief of having to "puddle" a glob of solder on the chassis to secure a decent ground.

Rawk on!

8)

Arjay

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Post subject: Re: 1 meg grid-to-ground resistor on original 6G15 Reverb Un
Posted: Wed Apr 11, 2012 10:19 pm
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Very nice Beemer! That's very similar to the way I put the 1meg in the Weber 5G15. And yeah, there is no change in performance. Thanks for the tip! Art

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Post subject: Re: 1 meg grid-to-ground resistor on original 6G15 Reverb Un
Posted: Thu Apr 12, 2012 11:00 am
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Thanks for the pic BMW...we've talked about this before but I haven't done it yet to my '63 reverb,I like the way you hooked into the jack for ground...I porbably wouldn't have thought of that. :)


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Post subject: Re: 1 meg grid-to-ground resistor on original 6G15 Reverb Un
Posted: Tue Apr 24, 2012 12:56 pm
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I'm going to find the right value resistor to do this BMW,within the next few days...I should change the filter caps while i'm working on it...is there anything else you recommend while I'm performing surgery?


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Post subject: Re: 1 meg grid-to-ground resistor on original 6G15 Reverb Un
Posted: Tue Apr 24, 2012 1:05 pm
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btw,where can you find a 3/4 amp fuse???


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Post subject: Re: 1 meg grid-to-ground resistor on original 6G15 Reverb Un
Posted: Tue Apr 24, 2012 2:21 pm
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Look for 750mA (either 125 or 250VAC, either voltage is ok) glass BUSSMAN or Littlefuse:

http://www.nextag.com/750ma-fuse/products-html


Last edited by BMW2002Ti on Tue Apr 24, 2012 3:10 pm, edited 2 times in total.

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Post subject: Re: 1 meg grid-to-ground resistor on original 6G15 Reverb Un
Posted: Tue Apr 24, 2012 2:37 pm
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Rebelsoul,

I'd recommend changing the caps under the hood. This is really a single-ended amp, like a Champ. So, those main power/filter caps are very important. Now-a-days, I use the 47mfd/500VDC F&T. But, the Sprague Atom 40mfd/500VDC are fine, too. Also, going to a good WW cathode-tied resistor under the 6K6GT is a good idea. Both tone-wise and for good tube life. Change any bypass cap that looks old, leaky, or dried-out. And resistors way out of spec. Usual amp precautions.

Maybe, a good idea just to change-out all 'lytics at the same time (I usually do this). Then, the old ones aren't stressed by the new caps under the hood. And you know exactly when all the caps were done. For future reference.

I used a 250mfd/25VDC rated bypass cap under the 7025 (12AX7).

Look for a good 6K6GT (BP RCA or Sylvania). And a quiet 12AT7 (like a Sylvania 12AT7WA).

http://www.webphix.com/schematic%20heav ... _schem.pdf

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Post subject: Re: 1 meg grid-to-ground resistor on original 6G15 Reverb Un
Posted: Tue Apr 24, 2012 3:01 pm
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Thanks Beemer,I'll get my parts list together and have some fun with the soldering iron when they come in.
I've been meaning to do this but lately the '67 Vibrolux has had my attention...I'm through changing things on it unless something happens....need a new project!
BTW,I've been planning on trying the F&T caps,they're supposed to be really nice...some of the places I've bought Spragues from in the past have stopped getting new supplies in because of a price hike,but offhand I can't remember on what site I read that. :D


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Post subject: Re: 1 meg grid-to-ground resistor on original 6G15 Reverb Un
Posted: Tue Apr 24, 2012 3:06 pm
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OH, BTW... notice the 820-ohm WW resistor replacing an OEM 1K-ohm (cathode-to-ground on 6K6GT)? I like the idle bias with this ohmage better, esp using the BP RCA or Sylvania 6K6GT (my favorite for this unit). Mills are wonderful for cathode-biasing, IME.

Good luck! These units are definitely worthy of a good restoration.


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Post subject: Re: 1 meg grid-to-ground resistor on original 6G15 Reverb Un
Posted: Tue Apr 24, 2012 4:45 pm
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gotcha,on the 820-ohm,I'm going to give it a try also,I use the old RCA 6K6GTs....also some Sylvanias in reserve. :D


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