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Old 15th June 2008, 06:41 AM   #41
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Quote:
Originally posted by Miles Prower

You really have to use 'em in push-pull since you can't hit the most linear part of the plate characteristic without red plate destruction in SE.
Is PP triode out of the question with such a low screen rating?

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Old 15th June 2008, 01:19 PM   #42
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Quote:
Is PP triode out of the question with such a low screen rating?
Screen rating? What screen rating!

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Though rated like a 6V6 (Pd= 12W) I run 'em a good deal hotter.
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.

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The 6BQ6GTAs, however, don't seem to be quite so robust, and are built differently
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.

Quote:
The main problem is that they like to pull some big currents
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.

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You really have to use 'em in push-pull since you can't hit the most linear part of the plate characteristic without red plate destruction in SE.
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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Old 15th June 2008, 04:23 PM   #43
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Quote:
Originally posted by vinylkid58
Is PP triode out of the question with such a low screen rating?
May be yes; may be no. Just remember: transistors have ratings, tubes have guidelines.
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Old 15th June 2008, 11:05 PM   #44
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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".
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Old 16th June 2008, 12:07 AM   #45
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Originally posted by wrenchone
Still, 6BQ6s and 6DQ6s are incredibly cheap - it gives one ideas, it does.
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.
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Old 16th June 2008, 02:44 AM   #46
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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
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.
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Old 16th June 2008, 10:35 PM   #47
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The 6BQ6GTAs, however, don't seem to be quite so robust, and are built differently (cathode is a good deal thinner) and red plate obviously at the same Q-Point that the 'GTBs tolerate without problems
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?
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Old 17th June 2008, 12:15 AM   #48
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Quote:
Originally posted by tubelab.com

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?

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.

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Old 17th June 2008, 12:48 AM   #49
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Quote:
Originally posted by tubelab.com
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?
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.
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Old 17th June 2008, 01:01 AM   #50
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Default 6BQ6GTB's

My previous post was comparing 6BQ6's only, sorry for any confusion.

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