Thunderbass output tube bias?

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Hey, I'm curious on how the T-BASS is set up for biasing the 8417's. Anyone done this yet? My amp has, what looks to be, a trimpot by these two tubes. I'm also thinking of swapping the 12ax7's for 12ay7's to create a bit more headroom and clean up the sound a bit and am wondering if there are others here who have tried it. Please share your thoughts.
 

capnjuan

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oddball said:
Hey, I'm curious on how the T-BASS is set up for biasing the 8417's. Anyone done this yet? My amp has, what looks to be, a trimpot by these two tubes. I'm also thinking of swapping the 12ax7's for 12ay7's to create a bit more headroom and clean up the sound a bit and am wondering if there are others here who have tried it. Please share your thoughts.
Hi oddball;

Bias: The 8417 TBass is a 'fixed-', not cathode-, biased amp and has three trim pots; one marked 'hum balance' that balances the AC on the tube heaters. The other two trim pots are on the chassis deck adjacent to the output tube sockets. If you have a meter, set it to DC voltage, connect the black lead to the chassis, turn the amp on with a speaker connected to the output transformer, and, for the left output tube (as viewed from the rear), put the red lead into the left hole in the test point fitting.

You are looking for .52vdc on the meter; if you don't get it, bump the nearest-to-the-tube left pot back and forth until the meter reads .52vdc. If you don't get it spot on, leave it and try the other tube; same sequence, right hole on the test fitting, move the pot until you get the corrrect reading. If you get it on one and not the other, either the tube is too far gone or either the 1% precision 6.8ohm cathode bias, 2.7K, or 91K grid bias supply have drifted out of tolerance. If you know the tube is good, clean both pots with spray cleaner and do over. If still no good then one or more of either the bias network or bias supply resistors are out of tolerance and a technical intervention is necesary. Same process for the 6L6 except .41vdc bias voltage although I'm not sure the 8417 bias supply can reach that point; mine didn't.

This thread for 8417/6L6 power and bias discussion
Discussion of TBass 8417/6L6 Amps

This thread for locating bias pots and test point on the 8417 TBass
Includes pics of pots and test point

Unlike cathode-biased amps, the bias pots allow the use of mis-matched tubes; they don't age evenly but the pots/test point allow you to keep the bias on them the same.

Pre-amp tubes: The 12AX7 has a higher gain factor than the 12AY7; you may get cleaner sound with the 12AY7 but it will be at the expense of some gain. They are plug-and-play substitutes otherwise. It's just a suggestion but you ought to try both and leave the one that gives the most pleasing result.

cj
 

matsickma

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Hey oddball,

Your comment about going to a "12AY7 to create more headroom and clean up the sound a bit" sounds to me like you have a different issue with your Thunder Bass than a 12AX7 gain problem. These amps are typically pretty clean sounding until you max them out. I think you have something else in need of repair or maintenance.

M
 

matsickma

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Hey capn,

Your procedure for checking the bias of the power tubes...Are the DC values the same for all tubes with this feature? That is, do you use 0.52 VDC for 6L6, 5791A, EL34 and 6550's?

M
 

capnjuan

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Hey Mike: At the risk of whining about schematics ...... :evil: The bias values; 8417 .52vdc and 6L6 .41 are from the Tbass schematics. The Maverick Lead / 7591 is cathode biased, no pots and just guessing, the Maverick Bass is the same.

The Quantum Bass w/ 8417s and 6GF7 regulator (Thank you again Richard) uses the identical value resistors in the bias network as the TBass 8417 but schemo says .3vdc (-20vdc from the bias supply to the standby switch ahead of the bias resistor network). After buying nearly every Guild schematic that MusicFarts.com has (including the same mislabelled one twice but excluding the ThunderStars), no schemos for Guild EL34 or 6550 amps. I'm also not strong enough to say flat out that all EL34s bias at the same point...maybe so, maybe not ... I just don't know for sure.

The only way that I know to determine the correct bias would be to calculate it given the testable -vdc out of the bias supply and considering the drop through the resistors - but this is past me technically. Alternatively, finding an EL34 design schematic with a similar bias supply which, as I get a headache, might only be determinable by manually drawing the supply in your amp. If the bias supplies are similar, whatever that design IDs as the bias setting arguably ought to apply to any EL34 with a similar bias config. And the same for the Guild 6550 ball-knocker music monsters.

Can't think of another way; would mean drawing a schematic for an EL34 amp bias supply, loading a flock of say Marshall schematics, comparing bias configs, and noting bias settings. ugh ugh ugh Wish I had something more positive ... wish I had a few more schematics! :evil: cj
 
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Hey, thanks for the information capn!! When it comes time to replace the power tubes, it'll sure help me out. As for the gain factor on the amp Matsickma, I don't think there is a problem with the amps output, or at I least I hope there's nothing wrong...I'm playing with some friends and pretty much turn everything all the way up to get volume. What I'd like to achieve with the 12ay7's is a higher volume before the tubes start to break up...ideally they would just begin to break up with the amp at max volume. That way the tubes could be pushed further by an overdrive stompbox...I'm thinking an LPB1 or or another booster. I'll probably try just replacing one at first to see what happens with the channels daisy chained together and go on from there.
 

capnjuan

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Thanks Steve; good stuff! cj
 
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Hi oddball;

Bias: The 8417 TBass is a 'fixed-', not cathode-, biased amp and has three trim pots; one marked 'hum balance' that balances the AC on the tube heaters. The other two trim pots are on the chassis deck adjacent to the output tube sockets. If you have a meter, set it to DC voltage, connect the black lead to the chassis, turn the amp on with a speaker connected to the output transformer, and, for the left output tube (as viewed from the rear), put the red lead into the left hole in the test point fitting.

You are looking for .52vdc on the meter; if you don't get it, bump the nearest-to-the-tube left pot back and forth until the meter reads .52vdc. If you don't get it spot on, leave it and try the other tube; same sequence, right hole on the test fitting, move the pot until you get the corrrect reading. If you get it on one and not the other, either the tube is too far gone or either the 1% precision 6.8ohm cathode bias, 2.7K, or 91K grid bias supply have drifted out of tolerance. If you know the tube is good, clean both pots with spray cleaner and do over. If still no good then one or more of either the bias network or bias supply resistors are out of tolerance and a technical intervention is necesary. Same process for the 6L6 except .41vdc bias voltage although I'm not sure the 8417 bias supply can reach that point; mine didn't.

This thread for 8417/6L6 power and bias discussion
Discussion of TBass 8417/6L6 Amps

This thread for locating bias pots and test point on the 8417 TBass
Includes pics of pots and test point

Unlike cathode-biased amps, the bias pots allow the use of mis-matched tubes; they don't age evenly but the pots/test point allow you to keep the bias on them the same.

Pre-amp tubes: The 12AX7 has a higher gain factor than the 12AY7; you may get cleaner sound with the 12AY7 but it will be at the expense of some gain. They are plug-and-play substitutes otherwise. It's just a suggestion but you ought to try both and leave the one that gives the most pleasing result.

cj
The links for the Tbass discussion have gone bad.
 

fronobulax

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The links for the Tbass discussion have gone bad.

Very old thread and the board software changed. @GAD has tools that should fix some bad links so stand by.

Just FYI capnjuan died several years ago. @Default has carried on the tradition of sharing knowledge about tube amps.
 

GAD

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Very old thread and the board software changed. @GAD has tools that should fix some bad links so stand by.

Just FYI capnjuan died several years ago. @Default has carried on the tradition of sharing knowledge about tube amps.
Those links are from three softwares ago. Maybe four. That's gonna be tough.
 
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Very old thread and the board software changed. @GAD has tools that should fix some bad links so stand by.

Just FYI capnjuan died several years ago. @Default has carried on the tradition of sharing knowledge about tube amps.
Thanks, I’ll start a new thread for further discussion.
 
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