Screen rating? What screen rating!
Yeah, I ran them a bit hotter. The tubes in the photo were cranking along at about 40 watts dissipation. The current meter was pegged (300+ mA for both channels) and the plate and screen supply voltage was 450 volts. I am guessing that I was dumping about 60 watts into each tube and extracting 20 watts of audio for 40 watts of dissipation, but I wasn't making careful measurements.
I found a really toasted looking pair of these. Both Sylvanias. One had a spot on the plate where all of the coating was gone. There was a shiny silver spot. There was almost no getter left. This one glowed right on the silver spot at 20 watts. The other could take some abuse, but nothing like the 6BQ6GA.
In all seriousness, I discovered when I "tested" some 6AV5's that all tubes with the same numbers are not created equal. If you intend to operate a tube in excess of the ratings you really need to do some testing on the tubes that you intend to use, at your intended operating point, and beyond. Some sweep tubes can operate far in excess of the screen voltage rating (some 6AV5's and 6LW6's) and some get really excited if the screen voltage is exceeded (6CD6's).
During my testing at extreme voltages I did not see any screen grid glow in the 6BQ6's that I tried. This indicates that these GE sourced tubes can handle voltages in excess of the ratings, but other brands may not. When you operate a tube at a high screen voltage it will require a lot of negative bias to control the plate current. These experiments were in the -50 to -70 volt range. This is clearly wasteful with cathode bias, but cathode bias is helpful in these situations because the screen to cathode voltage is "controlled" too. This helps to avoid runaway.
Just about any tube will have a "real maximum" voltage limit. This is the point where the plate current will run away regardless of the control grid voltage. This is often dependent on the care taken when assembling the tube, and thus varies from tube to tube. Some early vintage Sovtek 300B's would run away at less than their 450 volt maximum rating even with the grid held at -150 volts. Poor grid alignment is to blame here. I saw some tendency towards "bias creep" when operating these tubes in the severe overload shown here. I don't think that is was severe enough to be an issue at normal operating conditions.
Big currents are an issue, and an advantage. I have some big Antek toroids that are capable of nearly an amp of current. The power supply is not an issue. The large bias voltage and large current will make cathode bias resistors glow like the tubes did last night. From my experience tubes capable of "big currents" make "big sound". The bass and transients are not limited by wimpy cathodes. This is especially true as you get away from class A operation.
I may have seen some hint of this last night. The distortion keeps dropping as the plate current is increased. This is how I got to the edge of red (white?) plate destruction. More experiments are in order to see if a happy medium can be reached.
I did also "test" some 6DQ6's. There were 5 NOS Sylvanias in the box. Of course I am not going to subject some NOS tubes to the 'white glow" test. The first two tubes that I tried performed quite differently. The picture shows two tubes humming along putting out 20 WPC at 3% THD. One is glowing, one is not. Later I played musical tubes and found that 4 out of 5 can do this without glowing. There was no grid glow either. The bias voltage required was -80 volts. My 63 volt cathode bypass caps would go bang if cathode bias was tried. I have some 160 volt caps here somewhere. Again, more experiments are needed.
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The last time I saw a tube glowing that brightly was a few seconds before it popped and became one with the atmosphere (no great loss, as it was gassy to start with)... Still, 6BQ6s and 6DQ6s are incredibly cheap - it gives one ideas, it does. I still have yet to see how screen driven 6CD6s will perform. My 6CD6 sreen drive amp -in-process is sitting belly-up on my bench as I write this and calls out forlornly every time I go down into the basement, "Richard, finish meeeee".
wrenchone said:
Better get 'em now while they're still cheap because they ain't stayin' cheap. The price is already up where I got a couple dozen, and after posting about using them as audio finals. Once George and I get done talkin' 'em up they may go up a whole lot more.
6BQ6GTBs would also make good finals for a gee-tah amp.
I have tortured those same tubes on and off all day. They are in the amp as I write this and still running! For now thay are operating at a much more conservative, non glowing bias point. I have come to believe that Miles may be right. The 6BQ6's may best be used in a P-P amp. Measurements and listening tests concur, these tubes sound and measure best in SE mode when they glow like light bulbs. Right now they are running in triode mode in a Simple SE board with 560 ohm cathode resistors, CFB enabled with a 3 K ohm OPT (UBT-3). The B+ is 380 volts with 360 volts on the plate and 45 on the cathode. This is a tube dissipation of 25 watts. There is no visible plate or screen glow. Power output is 6 watts at 3% distortion. Earlier I was extracting about 10 watts in UL mode with a 2.5K load. Not too shabby for a $2 tube. The distortion at 1 watt is 1.4%. It drops to 0.7% when the glow is increased. I have seen no reason not to use these tubes in triode mode. I must state that I have hammered about 6 of these guys today, but ALL of them have been GE's. I have not tested any other brand.
I was playing CD's through them and I found that loud rock, techno and other extremely dynamic music just sounded better if I grabbed the power supply knob and rotated it clockwise. Metallica was generating some serious tube glow!
I have cranked through about 10 6DQ6's, mostly Sylvania's. Beware of Japanese 6DQ6's. The tube current is all aver the place and some glow brightly in one small spot (bad, very bad). The Sylvania's like to run hot in SE mode as well but they sound good at more conservative operating points. In the same amp with the same 380 volts of B+ the tube current is about 90 ma and the dissipation is 27 watts. Power output is 7.2 watts in triode and the distortion at 1 watt is 0.6%. Power in UL mode (2.5K OPT) was about 12 watts. Here the amp sounded good without any trace of glow.
I turned up the power supply (with Sylvanias) until the tubes just started to glow, and then backed off about 20 volts. The B+ was 430 volts, tube current was 107 mA, wit 335 volts across the tube (36 watts). Power output was 8.4 watts. The distortion at 1 watt was 0.37%. I listened for about an hour and I must say it rocked! At one point I turned the power supply up to 500 volts (Metallica again). The plates were glowing, but the grids were not.
I have seen evidence that both of these tubes may do well at lower voltages and higher currents with a lower output impedance. At this point I don't have any transformers to confirm this, so some parafeed experiments are needed. Here I don't use an OPT, just a big bag of large resistors to find the optimum load. Again, more experiments are needed, but the weekend is over. Next time.
Miles, Anyone. I have been "testing" 6BQ6GA's they are a nice fat tube with plates that are the same size as the 6AV5GA or some 6DQ6's. These are the ones seen glowing in a previous post. They are obviously robust but I don't have many. I also have some of the wimpy 6BQ6GT's and 6BQ6GTA's. They are skinny tubes about the same diameter as a 6V6. They can't take any of my S***.
I don't have any 6BQ6GTB's. AES has them on sale for the ridiculous price of $0.98! Are these similar to the 6AV5GA?
tubelab.com said:
The difference appears to be physical size only, no difference in the electrical specs, according to the data sheets. The GTB's are smaller in diameter.
BTW, AES wants $1.00 each to select specific brands. I hope the GTB's are as robust as the GA's, cause that's what I'm looking for.
Jeff
tubelab.com said:
According to the spec sheets, the design center values are very similar. It looks like the 6BQ6GTB was designed for a slightly higher g(m) and max peak pulse voltage (5.5KV for the 6AV5 6.0KV for the 6BQ6GTB) which would seem to account for the plate cap connection. Other differences are a higher nominal Vsgsg (175Vdc v. 200Vdc). The 6AV5 uses the fatter T-12 bottle, and the 6BQ6GTB the skinnier T-9 bottle. The plate characteristic looks similar as well, but runs a bit under the 'BQ6 characteristic, but that's expected since the 'BQ6 has a slightly higher g(m).
At that price, go get some. And I thought they were cheap at a $1.30 a pop.
vinylkid58 said:
Here's what I've tried so far:
6BQ6GTA (Sylvania black plates) -- Run these with a cathode current of 55mA, and they red plate obviously. You can see the glow under normal lighting.
6BQ6GTB (Sylvania black plates -- control grid radiator wings) -- No glow with the cathode current as high as 70mA when viewing in the dark.
6BQ6GTB (RCA Grey plates -- no grid wings, slightly taller plate structure) -- Run at 70mA in the dark, these show a trace of colour. No glow when operated at 55mA.
6BQ6GTB (Realistic Lifetime -- factory seconds of unknown origin) -- Same as the RCA grey plates.
Well, inspired by this thread, I took advantage of a sale at AES and grabbed a couple dozen 6BQ6s at just under a buck each- all are RCA, GTB flavor. They look kinda small next to some of the beasties I've been playing with lately. I also took the opportunity to nail down some more 6LR8s, one of my favorite tubes of the moment. The 6BQ6s will wait around for a while until I figure out an appropriately nefarious use for them. Projects in the works:
1) 6CD6GA screen drive SE with Hammond 125ESE output iron - This was my first-ever tube amp, but got stalled on the bench while I worked on other stuff. The finishing concept is quite different from the citcuit I started... The monster is sitting at my job waiting for a couple of ground wires, then I'll fire it up.
2) JFET-6LR8 hybrid push-pull ultralinear - stalled on the bench waiting for me to match the input JFETs and buy the output iron - probably Hammond 1620.
3) 6LR8 triode-connected SE using Hammond 125CSE output iron - waiting for me to do the rest of the cabinet work (holes for jacks and such, and some crackle paint). I found out the hard way that winter is not the best time for crackle paint.
1) 6CD6GA screen drive SE with Hammond 125ESE output iron - This was my first-ever tube amp, but got stalled on the bench while I worked on other stuff. The finishing concept is quite different from the citcuit I started... The monster is sitting at my job waiting for a couple of ground wires, then I'll fire it up.
2) JFET-6LR8 hybrid push-pull ultralinear - stalled on the bench waiting for me to match the input JFETs and buy the output iron - probably Hammond 1620.
3) 6LR8 triode-connected SE using Hammond 125CSE output iron - waiting for me to do the rest of the cabinet work (holes for jacks and such, and some crackle paint). I found out the hard way that winter is not the best time for crackle paint.
So did I, but there were only a few left. Too many people are reading this! I have been using 6LU8's but the 6LR8 is the same tube with a few less pins on the bottom. I also have a single tube "spud" amp, haven't finished optimizing it yet.
I got some more of those $0.98 6BQ6's. I have some P-P experiments in mind.
They said that they have "a lot" and they aren't selling. Stan told me the same thing.
I usually tell people to be prepared to experiment whenever you wander from the beaten path. You won't find too many schematics for unusual tubes, and you certainly won't find any commercial equipment to just plug and play. If and when schematics for these, or any other uncommon tubes show up, people will start buying them, and the prices will go up.
Miles' schematic looks like a winner, but you would still have to build it. I have successfully made them play in a Simple SE, but they were operating above the published maximum ratings. I got some coming, but I have no way of knowing if I will ever find a realistic operating point for SE use, or when that may happen. I got them for eventual P-P experiments.
They also have some other uncommon sweep tubes for small money, but I resisted the temptation. I have far too many tubes already. The 6GV5 for $1.80 looked good too, and don't forget my old favorite the 6AV5GA for $3.75.
I was wondering George if you've tried torturing or know anything about the reliability of the Tung-Sol reissue tubes:
www.thetubestore.com - Tung-Sol 7581 (6L6GC) Audio Tubes
I have some old production tubes that would work in my son's Fender but I was wonder what of new production might be the best.
I've read a few good things about them but otherwise don't know much about them.
www.thetubestore.com - Tung-Sol 7581 (6L6GC) Audio Tubes
I have some old production tubes that would work in my son's Fender but I was wonder what of new production might be the best.
I've read a few good things about them but otherwise don't know much about them.
George, I'd like to have a little of all your knowledge just to be able to make this kind of experiment ...... amazing !!!
I haven't tried any, since I tend toward cheap tubes. They are made in the same factory as Sovtek, EH, and Mullard reissues, so I can assume similar quality.
Maybe 6 or 8 years ago I got offered a box full of "previously auditioned" Russian tubes for $10 each. All were Sovtek, or EH branded and had the TubeStore matching stickers on them. Most were matched pairs.
I stuck 4 of the Sovtek EL34's in a Marshall 100 watt amp, and they were still alive when I sold the amp 2 years ago.
I stuck a pair of EH KT88's in my original "Industrial" SSE. That amp runs the tubes hot (and the Hammond power transformer hotter) at 100 mA and 430 volts each. After several years of near daily use, they are still in there, but have lost their cool blue glow.
None of the other tubes have died, but they haven't seen heavy use. I believe the TubeStore screens and matches their tubes to weed out the duds, but I don't know anything about their process.
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