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.
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.
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.
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!
tubelab.com said:
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
I made my own using chassis mount sockets and the bases from dead Chinese KT88's. They are UGLY but not very tall. There is a thread about octal adapters where someone used PVC pipe.
The 6384 is the Bendix Mil spec version of the 6AR6. Tung Sol 6AR6's can still be found cheap sometimes. You can use them for prototyping before droping in the expensive tubes. I have experimented with both of these tubes and you can indeed crank them pretty hard. They are NOT a drop in replacement for a any 6L6 type no matter what that Ebay seller says. The pinout is different and the bias may wind up just outside the adjustment range of many amps. They don't like 450 volts on the screen either.
If considering something other than a true 6L6 type, I keep coming back to the Electro Harmonix KT88. Same pinout as the 6L6GC so the socket limitations still apply but I get an easy 75 watts from a pair in AB2 triode mode at 450 volts with a 2500 ohm load. These still whup the 6BG6GA's (triode mode) both in power output and sound quality, no adapter needed.
I have been working on a P-P amp and if I had to make the output tube decision today I would go the EH KT88 route. The only decision to make is 2 tubes per channel for 75 watts, or 4 tubes per channel for 150 watts. My OPT's are good for 400 watts but there are no UL taps. I don't need to make the decision today, so more experiments will occur before committing to a design. The somewhat infrequent experiments are here:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=133034&highlight=
http://www.diyaudio.com/forums/showthread.php?s=&threadid=137063&highlight=
http://www.diyaudio.com/forums/showthread.php?s=&threadid=143135&highlight=
Looking again at my adaptor kits, it looks like I only put one chassis mount socket in to one tube base, and those two happened to have the loosest fit. Also, looks like the octal base is slightly tapered, and the pins of the socket hit the bottom of the base before the fit got snug enough. I will just trim the pins, this should allow it to slide in enough for a snug fit and also help guard against shorts in the bottom.
What do you guys recommend using to glue the two together, an epoxy?
Also, why is it when I think I have done the research and found the right component then purchased something, someone comes along and says xxx is better
Regards,
Chris
What do you guys recommend using to glue the two together, an epoxy?
Also, why is it when I think I have done the research and found the right component then purchased something, someone comes along and says xxx is better
Regards,
Chris
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