Thunderbass tech questions

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Hello everyone. My first post. You have a nice little forum going here. Glad I found you guys.

I have Guild Thunderbass head that I really like. It looks to be a late model version (black tolex) without the tube rectifier. It currently has a pair of 6L6's. I have biased it to .414 VDC as the schematic suggests. I assume that would be correct bias amount for both the tube and SS rectifier versions?

I noticed this in another post from CJ:
While the 6L6 and 8417 have identical pinouts, the plate/screen voltages in the Guild 6L6 design are 460v/420v but 400v/300v in the 8417 design and 6L6s are biased hotter than 8417s. The Thunderbass has a bias test port and separate bias pots that allows accurate bias and permits the use of unmatched tubes. To run 6L6s in the amp, at least one resistor in the power supply - like the chunky one that's coming apart - would have to be reduced in value to push the screen voltage up and some consideration given to changing the bias resistor. CJ

Exactly which PS resistor should be changed, and what would the suggested value be? Would I then still bias it to .414 VDC or should it be altered for the 6L6's?

Thanks.
John
 

capnjuan

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EMT140 said:
... I have biased it to .414 VDC as the schematic suggests. I assume that would be correct bias amount for both the tube and SS rectifier versions? Hi John and welcome to LTG; yes, type of rectifier doesn't matter. Exactly which PS resistor should be changed, and what would the suggested value be? See below Would I then still bias it to .414 VDC or should it be altered for the 6L6's? Yes; see below.
The bias tap on the transformer puts out a nominal 50VAC; with the 1/2 wave rectifier, inverted cap, and with the specified value resistor - 2.7K shown in the green circle -, the output of the bias supply should be -44vdc. That voltage is fed to the bias pots and all things being equal, at idle there should be .414v appearing on the cathode; the specified bias shown in the blue boxes.

thunderbassbias.jpg


If you are trying to bias to the schematic, you have to confirm that you are getting -44vdc. You can measure it at X as shown on the left. If your'e getting less than -44vdc then the resistor value would be reduced. If more than, then the resistor value would be increased; start with the source voltage first.

If you have about -44vdc but are not reading .41v on the cathode, then either spin the bias pot until you do. If you can't get .41 by spinning the pots then you'll have to hot check the resistance of all those itty bitty resistors in that nest. My experience and that of others is that the 6.8ohm / 1% precision resistors have drifted out / up / down.

I changed the bias dropping resistor because the original value resistor was set for 8417s, not for 6L6s; unless you are not getting -44 out, then there's no reason to change the bias resistor. Good luck with your amp; pics when you get a chance. CJ
 
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CJ, thanks for the detailed response. I'll investigate further and report back.
John
 
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I'm getting a reading of -42.4 VDC. Enough difference to change the 2.7K resistor?

I am able to bias each tube to .414
 

capnjuan

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EMT140 said:
I'm getting a reading of -42.4 VDC. Enough difference to change the 2.7K resistor? I am able to bias each tube to .414
If it were me, I wouldn't mess with it anymore. Every now and then, you want to check them; as the power tube ages, it's ability to conduct is slightly reduced. Good luck! CJ
 
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