How to bias a Fender Bassman 59 Ltd guitar amp?

[Philbo King]

Here's the schematic:
file:///C:/Users/User/Downloads/'59%20Bassman%20LTD%20Schematic.pdf

Bias should be adjusted (via R49) for 60mV at TP19. This corresponds to an idle current of 30mA per 6L6GC.
Use matched power tubes.

A link to a local file on your C: drive won't help him much.

 

[Rikaro]

Hope this works.

 

[turk 182]

It's illuminating to hear how adjusting the output tubes' bias changes the sound of an amp. Higher bias results in more gain and more "crunch." Lower bias results in less gain and a cleaner, more sparkly sound.

are you telling me your running signal through the amp when doing a bias adjustment?

 

[Larpy]

I'll play through an amp at different bias levels to see where it sounds the best to me, yes.

 

[turk 182]

not quite what i was asking but ok, as to biasing for "sonic flavour" i've seen far too many badly biased amps, in both directions, starved or red plating all because it's "possible" with little to no regard for proper operation.

 

[Larpy]

OK, I see where you're coming from. Yes, I like to tune tube bias by ear as well as measurement. I almost always like the sound of an amp biased between 70% and 100% of the output tubes' maximum safe dissipation (determined by whether the circuit is class A or AB and whether it's cathode or fixed bias), but it's interesting to listen to different levels of bias. No argument that going above max safe dissipation counts as "badly biased," but sometimes a colder bias of an amp (say 50% of max dissipation) sounds good to me, given the guitars I play, the way I play them, and the way I hear.

 

[Sorento]

... But if the amp sounds better to you with lower bias (the range between –52v and –25v or so), you can leave it there. A bias setting higher than –52v (–60 for example) will shorten the life of the output tubes...

outch ... ! ... it's the other way round ...
the -52V goes directly to the grids of the 6L6s in fixed bias;
so, -60V is running the tubes cooler, not hotter ... less current, extending tube life
while -25V would run them super hot, probably way into red plating ... shortening tube life by orders of magnitude ...

 

[Rikaro]

It is always a good idea to follow OEM instructions (i.e. the Fender LTD schematic).
Grid bias voltage is only loosely related to the actual tube current and dissipation.
Adjust to 60mV at the test point and be fine.

 

[Larpy]

Sorento, yes, of course you're right. I stupidly was thinking about bias current instead of voltage while I was typing. I'd delete that part of my earlier post but too much time has elapsed to edit it.