Dangers Of Exceeding 6V6GT Maximum Ratings

[PRR]

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[sluckey]

 

[Ed_Chambley]

I wouldn't laugh too hard.  A blackface Deluxe playing one every day and hitting it hard, I have never had tubes last a year.  It runs them too hard IMO.  I have a 36 watt, 4 tube 6V6 amp.  I built it 6 years ago and still proudly stands with the original Tung-SOl 6V6GT and I am running about 355 VDC plate.

[PRR]

I have a feeling in the heyday there was 300V plate-stuff(*) and costlier 500V plate-stuff. Which is why original-spec 6V6 is rated around 300V. Why use the good stuff? Let them pay more for 6L6!!

The rise of TV may have forced large stocks of the better plate-stuff. So it may have got into the 6V6 machinery. Leo may have noted that some 6V6 stood 400+V OK, and by 1959 was buying enough tubes to get that written into his supply contract.

But half a world away, Australia used a LOT of 6V6 in table-radios (where we would use 50L6) and often down around 200V for couple-watt output. They were not backward but they were thrifty. Why use the fancy stuff for a mundane tube?

(*) Not really just "plate-stuff". Oxide and grid need extra care for higher voltage without back-erosion or stray emission. But same at our end. Radiotron (Australia) was rushing into more exotic radar and TV types, the dull old 6V6 was not getting the best processing. And just-maybe they had a lapse in their process, revealed in the most over-volted applications, and they blamed it on designers/users not following max specs.

[Ed_Chambley]

You thread, so I will ask a question you know the answer.  I have never checked a Bias Tap on a PT for voltage change.  Did last night.  I traditionally simply use a Marshall Type Bias using HT, not a Bias Tap.  I didn't want to tear into it, but I have seen a bias tap inside before.  Why doesn't it change in proportion due to + or - proportion.  I checked 60 Bias Tap at 120V and read 62 VAC from the tap. I say good.  Dropped the variac to 100VAC, this is on an tubed up amp.


The tap is unused and didn't change much at all.   


Would you say it is best to use a Bias Tap when available?

[sluckey]

The tap is unused and didn't change much at all.

Really? You decreased the input by 16.67%. I would expect that secondary tap to decrease by the same percentage.
 

[Ed_Chambley]

The tap is unused and didn't change much at all.

Really? You decreased the input by 16.67%. I would expect that secondary tap to decrease by the same percentage.

Me too! The HT voltage did almost exactly which changed the Bias.  I did learn something I was always a tad concerned about.  You know I have a couple of old original marshalls and have never checked them, I always run the off a variac since I cannot really use them to play anywhere.  The change in bias is not enough for concern.


I plan to check a new PT I have for another build.  This one has a 120V Bias Tap, so should be close to 1:1.  No load either.