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Old 7th August 2009, 10:19 AM   #21
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Thanks Rod, hard to keep up with all the insignia..
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Old 7th August 2009, 10:39 AM   #22
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I've run the cheap 'unbreakable' Chinese 6L6GCR tubes with 640V B+!
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Old 7th August 2009, 11:15 AM   #23
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Digital,

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I've run the cheap 'unbreakable' Chinese 6L6GCR tubes with 640V B+!
How much of that 640V was on the screens? And how long do they last like that??
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Old 7th August 2009, 02:21 PM   #24
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I've run the cheap 'unbreakable' Chinese 6L6GCR tubes with 640V B+!
I have extracted 90 watts from a pair of these tubes without red glow, but I didn't do it with voltage. I used AB2. I ran 500 volts on the plate and 400 volts on the screen. Both within the ratings.

The real limitation of the 6L6GC and other tubes of the same pinout is not the internal structure. The exact same guts that was used in the 6L6GB is put in a bottle with a plate cap and called the 807 or the 6BG6GA depending on the base. The plate voltage rating is 750 volts. Clearly the "guts" can handle the PLATE voltage, not screen voltage. Why the low rating on the 6L6GC?

Pin 3 is the plate. It carries a voltage of up to twice the B+ when a normal inductive load is applied and the tube is operated with a reasonable distortion level. I have seen much higher voltages in guitar amps run into clipping with less than optimum load impedances.

Pin 2 is the filament it should be grounded or bypassed to ground and operated at a low voltage.

When the tube voltage is high and the amp clips with a high impedance load an arc can occur between pin 3 and pin 2. It doesn't have to be a guitar amp. Many HiFi speakers have an impedance peak near resonance in the bass region. Play some techno music loud and you may see fireworks. What happens?

The peak plate voltage can go into the 1 to 2 KV range. Something will fail and an arc will occur. Sometimes the arc will occur inside the OPT from the plate winding to ground usually toasting the OPT. This is not uncommon in guitar amps. Usually the arc will occur inside the base of the tube, inside the socket, or across the pins at the bottom of the socket from pin 3 to pin 2. If the filament winding is grounded the amp may recover undamaged. If the filament winding in not grounded the arc may continue somewhere else. Common places are H to K of another tube or inside the power transformer. Both cause bad things to happen.

Yes, I have seen all of the above happen. Whenever I build a guitar amp I ground the filament windings and use ceramic sockets for the output tubes. Avoid the black phenolic sockets for the output tubes if you are pushing the voltage limits and ground the filament winding. The phenolic sockets will carbonize in an arc and often burn.

Quote:
As for comparisons of NOS, I don't think there's enough 5881s left to make the discussion worth having.
I have a bunch of Tung Sol 5881's. I find many of them gassy and they will often go into a runaway condition at plate voltages approaching 400 volts. Many may be used but all came from military spares so they should have been used well within their ratings and tested when installed (40 or 50 years ago).

I have seen several older Russian tubes relabled and sold as 6L6GC. Many of these will not live under conditions that wouldn't break a real GC.
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Old 7th August 2009, 02:55 PM   #25
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Please consider 6384. Although expensive these are very, very tough. Same socket, different pinout !

Durability and good sonics, exactly what you want
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Old 7th August 2009, 02:58 PM   #26
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The peak plate voltage can go into the 1 to 2 KV range. Something will fail and an arc will occur.
Peavey and other guitar amp makers (including me!) fit HV diodes from 6L6 anode to ground (diode's anode connects to ground) - to limit this inductive flyback voltage.

This limits the negative peak on one side of the trafo to <-1V, which controls the positive peak on the opposite half of the trafo-primary. with a B+ of 470V the peak positive should be less than 1000V.

I use one or more series-connected UF4007s to do this - a few pennies to protect your amp from serious damage!

If anyone tries this, make double sure you get the diode(s) the correct way round!

This is a good trick to apply to Marshalls - their output trafos are not very robust against voltage peaks.
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Old 7th August 2009, 05:20 PM   #27
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TubeLab is quite correct: I forgot the HV limitations of the IO base. The UX base of the 807 is thus king: TC anode.

richy
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Old 7th August 2009, 05:34 PM   #28
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The UX base of the 807 is thus king: TC anode.
The king of all "6L6" types is this:

http://www.vacuumtubes.com/6BG6.html

Do not confuse this with a standard 6BG6. It is one of the "stuff anything in the glass to fulfill contracts" tubes from the end of the vacuum tube era. It is a 7027A in an octal base with a plate cap. I put a pair, with adapters, in a guitar amp for a friend about 2 years ago. They are still going strong. They work like sweep tubes with a 450 volt screen grid!
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Old 7th August 2009, 10:36 PM   #29
chrish is offline chrish  Australia
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Quote:
Originally posted by tubelab.com


The king of all "6L6" types is this:

http://www.vacuumtubes.com/6BG6.html

Do not confuse this with a standard 6BG6. It is one of the "stuff anything in the glass to fulfill contracts" tubes from the end of the vacuum tube era. It is a 7027A in an octal base with a plate cap. I put a pair, with adapters, in a guitar amp for a friend about 2 years ago. They are still going strong. They work like sweep tubes with a 450 volt screen grid!

Has anybody built these converters? I just got a set of these tubes and the adaptor kits in the mail yesterday. The kit is an octal base, an octal socket, some wire and a top cap. The octal socket is much smaller diameter than the base, so does not sit snug inside. How have people put these together?

Regards,

Chris
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Old 8th August 2009, 12:01 AM   #30
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Originally posted by DigitalJunkie
I've run the cheap 'unbreakable' Chinese 6L6GCR tubes with 640V B+!
Dude! Party on!

Win W5JAG
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