MC240 Using bias probe - some interesting observations...

[Paul K]

Did some measurements and that need explaining to me please.

I had a matched set of JJ 6L6GC in my MC240 and they were showing ma readings per tube of approx 20ma on each tube on one side and approx 15ma per tube on the other....this was weird - also the fact that when switching sided those numbers balanced out a little more and changed their values a slight bit...

BTW these tubes were matched by The Tube Store (lovely folks as we all know) and they were sold to me as low current drawing tubes - apparently 21ma using whatever they use to measure with...so if anybody is wondering why the values are low with them, that is why)

Checked using some old GE 6L6GC tubes and those came into balance with approx 16ma on each tube...

All transconductance measurements on my Stark 9-66 had most tubes at 5000...so really strong testing that way...

Ok - I had no idea whether those GE's were matched or not...but I pulled an older version of that tube out to further be a bit more precise in my measurement and that tube measured about 41ma through the probe...So I went back to another tube and put that in and it was back to about 16ma on each for that side...I decided to switch the tubes on this side so that they were still the same pair and for some reason that put them on the button as far as ma was concerned....

Now I have questions....

This was restored in 2012 and went over by a tech who knows and has worked on Mac gear (he owns MC30s and is an amp designer so I trust him...) and the voltages were correct then.
The resistors across the pins joining two 6L6GC tubes were never changed nor were they ever tested for drift....would this now be a problem to look at?

Also - what was up with the one GE tube drawing that much current? I have seen others using bias probes in an MC30 and seeing that kind of ma reading - why are mine so low?
Interesting stuff...I believe the vendor did match my JJ tubes so I am suspecting the amp...which btw sounds fantastic still - just wondering what is up.

It is being run at 115VAC using a bucking transformer and is set to the 117VAC tap for info...

All help I thank you for in advance people...

 

[Bill Ferris]

According to Mc. & my own experience owning and repairing their vintage tube amplifiers for yrs. because of the unique nature of their output transformer stage design (AKA Unity Coupled) matched output tubes are not required for best performance of their tube amplifiers, and that design runs the output tubes very conservatively, but using matched output tubes certainly won`t degrade the amplifiers performance, it won`t improve it either over a pair of decent quality tubes. But if purchasing and using pricier matched tubes make you feel better about the amplifier`s performance, go for it, who am I to judge.

I run my MC 30`s & single MC 60`s power transformer using the internally provided 125 volt provided power transformer tap so I don`t need to be concerned about more modern higher mains voltages so then I don`t have to get involved with external bucking transformers, or a variac.. Just plug and play..
The amps all factory + spec`d out and seem to be happy, with yrs. of trouble free operation running them connected to the 125 volt tap instead of the factory default 117v, which was the more common household voltage many decades ago when the amps were designed and built.

And if I understanding your concern about the two devices connected between the 2 output tubes, that you seem to think are resistors, from your not very good & poorly lighted thumbnail picture, they are in fact what are called chokes (a coil of wire wrapped around a insulated tubular substrate in this case) and extremely rarely ever fail !

Try not to over think your amplifier !

Enjoy your MC 240

 

[Paul K]

According to Mc. & my own experience owning and repairing their vintage tube amplifiers for yrs. because of the unique nature of their output transformer stage design (AKA Unity Coupled) matched output tubes are not required for best performance of their tube amplifiers, and that design runs the output tubes very conservatively, but using matched output tubes certainly won`t degrade the amplifiers performance, it won`t improve it either over a pair of decent quality tubes. But if purchasing and using pricier matched tubes make you feel better about the amplifier`s performance, go for it, who am I to judge.

I run my MC 30`s & single MC 60`s power transformer using the internally provided 125 volt provided power transformer tap so I don`t need to be concerned about more modern higher mains voltages so then I don`t have to get involved with external bucking transformers, or a variac.. Just plug and play..
The amps all factory + spec`d out and seem to be happy, with yrs. of trouble free operation running them connected to the 125 volt tap instead of the factory default 117v, which was the more common household voltage many decades ago when the amps were designed and built.

And if I understanding your concern about the two devices connected between the 2 output tubes, that you seem to think are resistors, from your not very good & poorly lighted thumbnail picture, they are in fact what are called chokes (a coil of wire wrapped around a insulated tubular substrate in this case) and extremely rarely ever fail !

Try not to over think your amplifier !

Enjoy your MC 240

Thank you for your very informative and quite humbling writing. I will not overthink this amp. I have a tendancy to do that.