Thunderbass amps: Convert 8417s to 6L6s

capnjuan

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Tim said:
... only getting -21 v (right for 8417), not -44 as shown on the 6l6 print. This make sense to me as I'm only getting -34v after the diode, only 25vac out of the transformer. How did you deal with this?
Hi Tim: try subbing in bias resistors (trial and error) in sequence: 2.2K / 1.7K / 1.5K / 1.2K to bump the bias voltage towards -44.
Tim said:
When I check the bias points I'm getting about .85vdc turned all the way down, nearly a full volt the other way.
??? Confirm that you have the (+) terminal on the bias supply filter caps connected to ground and (-) end connected to the line.

If manipulating the value of the bias resistor doesn't give you a voltage at least in the ballpark, then you might need to change that 5 ohm/5 watt resistor to either 1 or 3 ohms. I'm not engineer enough to calculate the resulting voltage out but less resistance in that 'snubbing' resistor will raise the voltage out. John
 

somecallmetim

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Thanks Again, I'll try as suggested.


edit-

I went all the way down to a 1k resistor, got -28v, bias all the way down it's .56 & .52v, close enough that I felt comfortable to play it for a minuite or two, sounded good & made me feel much better.

I'll drop by radio shack tonight to see if they have anything to replace the 5ohm power resistor to gain a few more volts.

I really doubt that I'll ever get -44 though. It seems to me that the tranny is built to deliver the -20 requried by the 8417s.
-t
 

somecallmetim

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I ended going all the way down to a 500ohm resistor, I had about -31v iirc, & got the bias at about .44v iirc.


I've been playing it that way & I'm pretty happy with the tone as a bass amp, especially with my DA Starfire of course :). Compared to my Fender BXR 115 (apples & oranges, but it is all I've got) it seems very clear with a nice growl when you really dig in, though the fender is louder...

-t
 

capnjuan

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somecallmetim said:
Power supply... 1) Changed the 10ohm 10w to 5ohm
Hi Tim; the only other suggestion I have would be to reduce the 5ohm/10W to 2 or 3 ohm / 10W. These guys http://tubesandmore.com/ click Resistors on left then wirewound ... drop down menu ... 2/3/4/ and 5 ohm ... besides dropping the voltage, I think the function of the resistor is just snubbing inrush current ... otherwise ... not sure what the next move might be. J
 

capnjuan

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Another 8417 to 6L6 conversion Thunderbass head in this Ebay auction. Two sweet channels of Guild 50 watt power. Master volume, aux out, but no effects loop.


!CEOuDuwEGk~$(KGrHqYOKjoE0,VHIs6FBNRR2VYHb!~~_3.JPG



Also fresh caps using JJs ... a good way to go:

!CEOuK)!EWk~$(KGrHqYOKjgE0dTfB+V5BNRR3OT1O!~~_3.JPG



Good physical condition but pretty much up there as far as the asking price goes.
 

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Is that the one pictured with the Guild cab? If the cab comes with it, that might make the price a little less extravagant.
 

capnjuan

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Eh? The recent eBay Tbass is being sold head-only ... cab? Can I call you a cab?
 

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Haven't been spending too much time on ebay. I must have had another hallucination. In that case, that's probably in line for the amount of work, but not in line with the market. Besides, they screwed with the bias test resistors. Is that Dymo labels I see?
 

capnjuan

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Default said:
.. Besides, they screwed with the bias test resistors. Is that Dymo labels I see?
Looks like a Dymo in action ... I took the '.39 - .47' to mean 'not less or more than' but it also could have meant 54-40 Or Fight. :D
 
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