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Old 8th March 2008, 10:30 PM   #1
pchw is offline pchw  United States
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Default Is 450V the max of triode strapped 6l6gc

2 years ago, I purchased a DIY tube amp from a Chinese student who was in hardship, but it required a step up transformer (included) to operate. I wasn't comfortable to even open up the bottom plate at that time, and I didn't bother to try it. DIY and tube were like ET to me :-) This morning I was bored, so I took it out and played with it. It hummed badly, and the 3 out of the Chinese KT88 were no good. After some fiddling, inspection, and fix up, it sounded again with my Chinese coke bottle 6L6GC. Since the OT's are PP and have no UL tap, I want to strapped them as triodes. The current B+ is at 430 which is quite close the design max, but since this is PP instead of PP, I wonder it is safe (to the tubes) to do so. There is a filter choke in the CLC PS. If I am not mistaken, this choke is only rated at 200ma; therefore, I will be biasing the output tubes at around 45ma each. Does it sound right?

Thanks!!
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Old 8th March 2008, 11:24 PM   #2
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Fred,

Well, the ratings are dependent on which valve you are using and which manual and set of specifications (guidelines?) you choose to go by. RCA specify for a genuine 6L6GC that 450 volts is the maximum on G2 unless in ultra-linear mode, and then it is 500 volts. But they do suggest that in triode mode you keep it at or below 450. 7027As are specified at 500 volts but 7581As are once again specified at 450 volts. If you've ever carefully inspected and used these it is pretty evident that the internal construction is identical. The later heavier 6BG6s are also the same internally.

Now, if your amplifier is cathode biased there will be a drop of some voltage across the cathode resistor(s) that will be subtracted from what the anode and G2 will actually see. At the B+ voltages you are talking about this may be around 40 or 45 volts so this should be considered.

For what it's worth, I've run Sovtek 6L6WXT+ valves in UL cathode bias at 525 volts and not had any problem with them. I run them at just over 50 mA.

I can't provide any recommendations regarding China 6L6 valves. The only ones I've tried were horrible quality but the ones you have might be better.

Wade
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Old 9th March 2008, 12:58 AM   #3
pchw is offline pchw  United States
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Hi Wade,

Thanks for the reply. The specs sheets that I looked up were from GE and Tung Sol. Both are "text book" specs, same as RCA that you found. This amp uses fixed bias. So, it is very close to this "text book" specs. The Chinese 6L6GC's that I have look like those Tubelab used. If so, they maybe able to take some beating. I also have some EL34's (Svetlana and SED Wing C), maybe they are better suit to this voltage as triodes?

Thanks again!!
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Old 9th March 2008, 03:07 AM   #4
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Quote:
The Chinese 6L6GC's that I have look like those Tubelab used. If so, they maybe able to take some beating.
I have operated these tubes at 525 volts, but this was cathode bias, so counting the loss across the cathode resistor and the OPT the tubes see about 460 to 470 volts. I haven't blown one yet.

Quote:
I also have some EL34's (Svetlana and SED Wing C), maybe they are better suit to this voltage as triodes?
I have tested several flavors of EL34 at the same voltage in the same amp (SimpleSE). All had no issues except the Winged C tubes. I believe that the tubes that I received were from a defective batch since other users have good results with them.
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Old 9th March 2008, 04:28 AM   #5
pchw is offline pchw  United States
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Thank you, George!!
The B+ is a touch below 430. If they are the same Chinese 6L6GC, then they will survive.

Since they will be bias no more than 50ma, they will be nowhere near the max plate dissipation. I hope this will increase their chance of survival. Time to find out!!
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Old 10th March 2008, 02:54 AM   #6
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The internal construction of the 6L6GC, 7027A and 7581A is identical but for the cooling fins on G1, at least in the examples I have examined. Heat radiation from those make a difference. Also (I have not examined) it is said that the 7027A has a copper layer sandwidged in the anode metal, for better dissipation - unlikely to have an effect on maximum voltages.

But then one finds that the size of the G1 cooling fins also differ from 6L6GC to 6L6GC! (nothing is ever simple ). One also finds that the Russians can be quite liberal with their numbering. Some marked 6L6GCs are 6L6GBs; internal construction mounted on a glass tongue instead of posts going directly through the bases.

I have operated 6L6GCs (GEC) with 500V on screens and 600V on anodes without problems, but the dissipations were well within the limits. The ratings are conservative, e.g. allowing for some abuse; I would not go there in amplifiers bashed around on stage and in the back of a transporting truck. But as said by others, these days it is advisable to make sure of the construction; a difficult task if one cannot inspect the tube before purchasing.
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Old 10th March 2008, 03:25 AM   #7
pchw is offline pchw  United States
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Quote:
Originally posted by Johan Potgieter

But as said by others, these days it is advisable to make sure of the construction; a difficult task if one cannot inspect the tube before purchasing.
Yeah, it is almost impossible to "see" the tube in person before purchase these days. It is almost the case that the money must be paid first and determine what we get later. GEC 6L6GC? Those must be like gold if they are found in the market at all!!!
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Old 10th March 2008, 04:31 AM   #8
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Yep. I ordered them from the UK some 15 years ago.

.... so you can imagine just how I felt when I dropped one on the floor some time ago
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