How to bias a 6l6 in a 5e3?

[pete_b]

Hi,

I have a DIY 5e3 with a choke (approx. 340V on the plates of the currently installed 6v6 tubes and 335V on the screens). At which dissipation would you bias 6L6 tubes in that amp.

I know the 70% IPD rule, but that would result in too high current, as the voltages are low. Fender used 250 Ohm cathode resistors in their Tweed 6l6 amps. Should I aim for that value, too?

PT and OT are 6l6 ready.

Cheers & a happy new year to all

-Pete-

[stokes]

The 70% is based on the max plate disipation at idle,you determine this number by multiplying your plate volts times your current in ma's.So to say you will draw to much current with the 70% number doesnt make sense.70% is 70% regardless of the plate current.

[Steve Conner]

Well, a current that would make 6L6s run at 70% of their rated dissipation could easily overheat a set of transformers that were designed for 6V6s. I guess that's what he meant by too much current.

[stokes]

Steve,read his original post,he says "PT and OT are 6L6 ready".

[pete_b]

Steve,read his original post,he says "PT and OT are 6L6 ready".

Yes, but that still doesn't mean you can apply the 70% rule to each and every P-P AB amp blindly. This rule is a true most of the time, but not always - it depends on the design of the amp. You can read the articles on aikenamps for a detailed explanation.

The 5e3 has low voltages, this means in order to bias at 70% IDP, you would need high current, according to this formula: P=U*I. For that amp (read Aiken) the 70% rule would most likely not result in the proper bias point.

In order to bias correctly, I would need a scope, which I don't have. That's why I posted the question. I was hoping that people who already biased their 5e3 with 6l6 correctly would chime in with their concrete experiences. Anyone?

Very best wishes & a happy new year

-Pete-

[Ray Ivers]

Pete,

I've only worked on one original 5E3, a long time ago - I can't recall what the idle current was.

I've biased many, many 6L6 P-P amps, though - and my advice would be to bias the amp the way it sounds best to you, using the 70% "rule" as a maximum for Class AB operation, and exceeding it only when other operating conditions are suitable (my $.02, YMMV, etc.).

Happy New Year to all,
Ray

[Steve Conner]

If they really were 6L6 ready, they would be twice the size, give 60 watts with 6L6GCs, and put out enough voltage to destroy 6V6s on sight. So I assumed "6L6 ready" was a marketing term that meant pretty much nothing.

[stokes]

Yes, but that still doesn't mean you can apply the 70% rule to each and every P-P AB amp blindly. This rule is a true most of the time, but not always - it depends on the design of the amp. You can read the articles on aikenamps for a detailed explanation.

The 5e3 has low voltages, this means in order to bias at 70% IDP, you would need high current, according to this formula: P=U*I. For that amp (read Aiken) the 70% rule would most likely not result in the proper bias point.

In order to bias correctly, I would need a scope, which I don't have. That's why I posted the question. I was hoping that people who already biased their 5e3 with 6l6 correctly would chime in with their concrete experiences. Anyone?

Very best wishes & a happy new year

-Pete-

The 70% is in reference to what the tubes plate is idling at,if the OT can handle 6L6's there is no reason you cant set the idle at 70% as long as your plate volts dont exceed what the tube can handls.To say the volts are too low to idle at 70% makes no sense.You can idle a 6l6 in class A at 250 plate volts,class A is closer to 90% plate idle,so how is idling a 340 v plate at 70% going to cause the tube to draw too much current?If you are saying that your OT cant handle the current of 2 6L6's biased at 70%,then your OT is not "6L6 ready".A scope is only needed if you want to bias the amp by the "crossover notch",which is way too cold.As Ray said,idle it at 70% and use your ears from there.The fact that your volts are lower does not mean you cant approach the 70%,you only run into trouble if your plate volts exceed the design ratings,and even that is not a hard and fast rule.Many amps run plate volts that exceed the tube manuals design max,and depending on the tube,handle it just fine.