New Tung-Sol 6L6 GC STR, Red Plating Mystery???

T

tube-a-lou

Addicted Member
Hi all,

I just received today a quad of Tung-Sol GC (Russian) tubes brand new and
tested by a well known West Coast Dealer and put them in my Bogen MO-30 amps
and three of the four tubes went red plate. Now the Chinese Shuguang tubes I took
out were perfect no red plating and the voltages were great. Can anyone explain to
me what happen here??

Thanks
 
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some of the Russian tubes bias differently. If this is cathode bias, it may need the resistor changed in order to make things happy.

also why I like fixed bias amps that have adjustments :)
 
Maybe bump the existing resistor by 10-20% and see what it does? The exact value would probably be determined by what I found in my junkbox to try. Even if it ends up being some mish-mash of series/parallel nonsense to come up with a total value for experimenting.

Knowing the voltage across the resistor, resistor value, and plate voltage might be some help here.
 
The Bogen schematic shows them biased right at 30W dissipation with a common 200 ohm 7w cathode resistor. Something at or over 250 ohms should get you to a better dissipation on the tubes assuming they are well matched for balance.
 
Bad thing is those red-plated tubes can not be trusted now. Most of new production tubes do run hotter for given bias. Good reputable seller warns about it...
 
Here's a update with the tubes, I had in both amps 2 pairs of CDE PIO Capacitors (vintage), and purchased another
quad of caps from a different manufacturer, I put a pair in one amp and decided to try them out again and
tonight the Tung-Sols didn't redplate, I thinking the CDE caps are leaking.
 
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Did you measure the caps for leaking DC voltage? PIO are kinda notorious, for early failure --- in blocking DCV function.

Thanks!
 
tube-a-lou said:
Here's a update with the tubes, I had in both amps 2 pairs of CDE PIO Capacitors (vintage), and purchased another
quad of caps from a different manufacturer, I put a pair in one amp and decided to try them out again and
tonight the Tung-Sols didn't redplate, I thinking the CDE caps are leaking.
Click to expand...
Are you measuring / calculating anything with these new tubes installed? If you don't know how we can describe the process. All you need is a multimeter and a calculator.

I have no experience with new production output tubes but from what I've read they just don't seem to meet the same specs as old production. If I were to use them I would measure and make sure I was not running them at more than, say, 75% or so of the dissipation listed as "typical" on the data sheets.

As for leaky coupling caps, that is also easy to check. If you have a proper cap tester thay can be tested before installation. If not, they can be checked in circuit. Again, all you need is a multimeter. Just measure the DC voltage on the grid. It should be zero or in the very low mV range.
 
I did a test with the Robinette Bias cal.

Chinese/ Russian
Voltage across cathode resistor 30.15 / 33.80
Total Cathode Current 150.8mill/ 169 milliamps
Total Plate Current 142.5 / 159.7milliamps
Plate Current Per Tube 71.3 / 79.9
Plate Dissipation Per Tube 26.9 / 29.6
Plate Dissipation Per Tube % 89.7 / 98.7

If you Tubes is a Triode or
Dual Triode
Plate Current 75.4 watts / 84.5watts
Plate Dissipation 28.4 watts / 31.3watts
percentage 94% / 104%


That's the readings I get, you could see the Russians soak up a lot
more that the Chinese Tubes.
 
both of those numbers are hotter than I'd prefer honestly. Even if you had to compromise and set the Chinese ones at 80%, that should get the Russian ones down to maybe 90%.
 
gadget73 said:
big reason I don't use old caps without doing a full voltage leak test. If I don't have access to a tester, they hit the trash and I use a modern film cap.
Click to expand...

I agree. The only NOS PIO caps I've used are the Russian military surplus. They are completely sealed. Never had a bad one. I have read that some have non-insulated exteriors but I've never found that to be the case.
 
tube-a-lou said:
I ordered a few resistors 300ohm and 330ohm to lower things down.
Click to expand...

What are the values to the grid-to-ground (grid leak) resistors? Like 220K-ohm? I noticed that some of the newer made 6L6GC are more stable with more grid-to-ground voltage. Maybe try 120-150K-ohms?
 
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