How much is too much MC30's Bias?

[jmprest]

Hello,

I got one of those quad digital bias meters which inserts between the socket and tube.

When I have RCA 6L6's in it reads around 33-35ma ( maybe wrong terminology ).

But when I put in the gray GEC KT66's I see 49-51ma in one amp and 54-56 in the other.

My question is this to high, I use Telefunken 803 in first 12ax7, Mullard 10M 12ua7, RCA 12bh7 foil, Telefunken Selected in 2nd 12ax7, and the rectifier is National Union VT244.

These are just back from complete rebuild. I can post pictures in the morning if thats helpful.

Thank You in advance for any advice,

John Prest

 

[dcgillespie]

From a pure performance standpoint this current draw level is fine for KT66 tubes, as the Mac amplifier is rather insensitive to bias setting in this regard. That's why no bias setting is provided for in the first place. From a tube safety standpoint, operation at this current level is also likely fine as well, although the B+ voltage level would also need to be known to provide a more specific answer. This, together with the current draw information, would then allow the plate dissipation level to be determined. From the standpoint of tube life over the long term, assuming that plate dissipation is acceptable, operation at 55 ma average is well within the capabilities of the KT66 tube, such that no long term life issues should be of concern.

Dave

 

[GordonW]

Output tube plate voltage at idle on the MC30 is shown to be 440v, by the factory service manual. Of course, that's probably at slightly lower plate currents. As the plate current goes up, the voltage will probably drop a little...

But, assuming "worst case" condition: 56ma, assuming 440v (as mentioned, it may be a little lower, with that much bias current), that'd be 24.6 watts. Given that a KT66 is rated at 25 watts, that's still below the plate wattage dissipation limit of the tube. KT66s tend to be "rugged" tubes (like pretty much all of the "6L6 family")... they can take their share of hard use- i.e, running them at their stated limit dissipation will usually not have detrimental effect, other than they might only last 25 years, instead of 50. IME, they should be just fine like this.

Best thing is to measure the voltage at pin 3 of each KT66, while the amp is idling. If it's 440v or less, and drawing no more than 55-56 ma per tube, I'd let it go.

Regards,
Gordon.

 

[dshoaf]

There is a procedure mentioned over in the Mac forum's Hints and Kinks stickey. This involves using a variac to set the output tube plate voltage to spec as Gordon mentions.

Then you measure and adjust the bias voltage to what's called for in the schematic - I don't recall what it is right off without looking it up. The key, here, is to keep readjusting the plate voltage when you make a bias adjustment. This, then, sets the relationship of plate voltage to bias voltage on the output tubes.

The adjustment technique ideally uses a resistor decade box hooked inline with the diode after the resistor that's inline is removed. This box allows you to change resistance as low as one ohm at a time. Once the settings are correct, you then read the value of the decade box and replace the resistor with that value.

Terry DeWick mentions in the Hints/Kinks thread that there's pretty much a standard resistance he typically sees when swapping out the selenium rectifier, which prompts the whole procedure.

Hope that helps.

Cheers,

David

 

[jmprest]

Thank you everyone who responded! I feel more confident now and will post the pin 3 voltages.
John