Heater Current Draw?

[MX117.MC7150]

Have recently picked up a pair of McIntosh MC30s and would like to try the Gold Lion reisue KT-66 power tubes. However, the original 1614/6L6GC output tubes the amp was designed around draw a nominal 900mA of heater current and the GL KT-66 reissue draws 1300mA of heater current. Anyone have any idea if the power transformer of the MC30 is good for this extra 400mA of current draw without suffering any long-term problems? Thanks.

 

[jaymanaa]

Have recently picked up a pair of McIntosh MC30s and would like to try the Gold Lion reisue KT-66 power tubes. However, the original 1614/6L6GC output tubes the amp was designed around draw a nominal 900mA of heater current and the GL KT-66 reissue draws 1300mA of heater current. Anyone have any idea if the power transformer of the MC30 is good for this extra 400mA of current draw without suffering any long-term problems? Thanks.

You can either find the original specs, which could be conservative anyway, or, you can compare the voltage at the filaments with the 66s verses the 6L6s. If you don't get any "sag" indicated by low voltage, you're probably fine.

 

[Tom Bavis]

Mac transformers run pretty cool, so I wouldn't be worried about the extra heater draw. I MIGHT worry about the bias voltage... You can measure the idle current indirectly, by measuring the drop across the 150 Ohm resistor in the power supply.

 

[dcgillespie]

With two output tubes, that would be 800 ma extra draw. If there is a preamp power take off socket on the MC-30s, they usually allow 1 amp for the filament power requirements of a mono preamp that draws its power from the power amp. If the socket exists, and obviously with no preamp connected to it if it does, then by design the power transformer should be able to handle the extra current draw just fine.

Dave

 

[GordonW]

Dave expressed my take on this very well. I just verified, by looking at the MC30 schematic... there is definitely a power-take-off socket for this use, in these amps, made to run the McIntosh C4 or C8 preamp... so you should be fine.

The same applies to stuff like Heathkit W4 and W5 amps... both of those were made to take the WA-P2, so they can handle some extra heater current within the amp, if that external preamp is not connected. The WA-P2 takes 9/10 of an amp on heaters (2- 12AX7s and a 12AU7)... so, you can use KT66s in place of the 6L6s in those, too. Or, in the W5, you can change out the 12AU7s for 12BH7s, as long as you use 6L6s (changing to 12BH7s AND using KT66s would be too much current total- one or the other is OK)...

As Tom mentioned, you may have to change the cathode bias resistor to keep current in check. I had to do this, when putting GL KT66s into a pair of Heathkit W5s- they biased up a bit "hotter" than the 7581/6L6GC type tubes, in the case I had on hand...

Regards,
Gordon.

 

[MX117.MC7150]

On which tube(s) are your thinking the cathode bias resistor may need changing? To what value? Am assuming I will need to make voltage measurement across the said resistor before knowing. Much appreciated.

Thanks for everyone's input. Much appreciated.

 

[dshoaf]

There really isn't a cathode bias resistor on the MC-30s on the output tubes. There's one of the tri-filar windings in that position.

The bias, however, is developed with the tube prior so there may need to be some minor adjustment to use the KT-66s. I'd suggest you use the rebiasing technique found in Hints and Kinks section of the Mac forum to get that set correctly.

Cheers,

David

 

[GordonW]

There really isn't a cathode bias resistor on the MC-30s on the output tubes. There's one of the tri-filar windings in that position.

The bias, however, is developed with the tube prior so there may need to be some minor adjustment to use the KT-66s. I'd suggest you use the rebiasing technique found in Hints and Kinks section of the Mac forum to get that set correctly.

Cheers,

David

This is what you need to do for the MC30, yes.

Sorry to add confusion- I was referring to what I did to the Heath W5. Same principle, different amp, different procedure...

Regards,
Gordon.

 

[MX117.MC7150]

After the above comments about the need for rebias I ask the restorer, Yves Beauvais (Mekons) about using the GL KT-66. Luckily, he has a pair and a pair of restored MC30s on his bench. He will be making the measurements on his pair and will let me know the results. He will also measure using original RCA & GEC UK KT-66s for a comparison. In the meantime I'll continue to enjoy the Telefunken/GE-RCA 'clear-tops' he provided. They are certainly singing sweetly.