I'll second that one. I'm glad that Ebay no longer posts the bidders ID since prices went up when Tubelab bid on something. I even had someone repeatedly outbid me on a certain tube, and then email me to ask me what to do with them now that he had them.
I have tried to like the 6HV5 and its low priced brethren, but I have yet to figure out how to make a working high powered audio amp out of some. Power oscillators and TV reception jammers are quite easy though.
There are a few tubes that just don't work for audio. Those were the ones that I had thousands of. What can you do with several large boxes of 2D21's and 6AL5's. The only way to even make them glow is the microwave oven! Nixies and numerical counting tubes, may be useful to retro geeks, but I sold them all too.
not funny, but still smiling 
Hey,
a set of nice (alphanumerical?) nixie tubes could easily become an input selector display for some tube amp 🙂
Btw. I just read your 833A test setup article and enjoyed it 🙂 (As a physicist, I am used to comments from family members concerning insanity *gg*)
Greetings from Germany,
Andreas
Stay away from the russian no-brand 6H8C in blue and yellow boxes.
Mine are stamped "6h8c 8704" and are so microphony i can hear the clicks of my volume stepper..
It goes like Dziiiiinnnnnng Dziiiing every time i turn it...
Mine are stamped "6h8c 8704" and are so microphony i can hear the clicks of my volume stepper..
It goes like Dziiiiinnnnnng Dziiiing every time i turn it...
I'd avoid any indirectly heated tube. Buy plenty of chokes for filament supplies and some strong shelving and stick to DHTs all through the amplification.
Life is too short to be wasting time with indirectly heated tubes! Except for the 6 months it takes to build a system full of DHTs of course......
andy
Life is too short to be wasting time with indirectly heated tubes! Except for the 6 months it takes to build a system full of DHTs of course......
andy
While I more or less agree, there are a few indirectly heated tubes like the 5842/417A, D3A, C3G, 6SL7, 6J5, 6J7, 6SN7, as well as a few power tubes like the original KT66 - 88, etc, which have some merit..
Any of the afore mentioned types plus many others may be sent to me for a proper and fitting disposal. 😀
I may just have to get a donation of a few boxes worth (if you'd be so kind 🙂 ) and prove you quite wrong. 😀
Thyratrons/istors make great sub amps. The hifi crowd should love them, they are absolutely packed with iron cored chokes and stuff. High efficiency too. A single 2D21 can switch as much as a 6BG6, with 1/4th the voltage drop or less.
Tim
...avoid indirectly heated tubes...?
90% of all the tubes we've used since WWII have been indirectly heated. I'd like to think we've learned SOMETHING since 1930.
I'm curious what I might be missing in my performance evaluations. I've used nothing but indirectly heated tubes in my designs since ... well... forever. and I've had good luck with them- excellent performance- no problems.
12AX7 - 12AU7 - 6SN7 - 6L6 - KT88 - 6550 - and a host of others all seem fine to me.
But I'm probably missing something. Maybe I'm not looking for the right thing.
90% of all the tubes we've used since WWII have been indirectly heated. I'd like to think we've learned SOMETHING since 1930.
I'm curious what I might be missing in my performance evaluations. I've used nothing but indirectly heated tubes in my designs since ... well... forever. and I've had good luck with them- excellent performance- no problems.
12AX7 - 12AU7 - 6SN7 - 6L6 - KT88 - 6550 - and a host of others all seem fine to me.
But I'm probably missing something. Maybe I'm not looking for the right thing.
Don't worry about it, indirectly heated tubes do indeed perform better than DHTs in all purposes. Anything else is just a lie 🙂
Tim
Tim
Absolutely, there isn’t a single DHT that can’t be significantly outperformed by an indirectly heated counterpart. A clear case of fashion over function dictating consumer choice.
If you are one of the poor deluded miscreants who was suckered into buying directly heated tubes I strongly advise you to list them on ebay for a token sum while they still hold some monetary value.
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Sigh...
Just because it's a vinyl recording doesn't make it better than a CD
Just because it's a tube doesn't make it better than a transistor
Just because it's a triode doesn't make it better than a beam pentode
Just because it's directly heated doesn't make it better than indirectly heated.
Old technology is not automatically better BECAUSE it's older.
Of course, some designs using triodes are indeed better than some other designs using beam power tubes
Some designs using tubes are indeed better than some other designs using transistors
Some vinyl recordings are indeed better than some other recordings on CD.
But they don't HAVE to be. There are plenty of good designs and plenty of bad designs using either old or new technology. It ain't the age of the technology- it's how well the design fits the requirements of the application.
That being said, I full-well realize how FUN it can be to build something completely inappropriate and impractical just for the sheer fun of doing it. Nothing wrong with a lighthearted romp into the absurd every now and then- but it's always a good thing to remember to keep fantasy separate from reality.
Most of the time.
Just because it's a vinyl recording doesn't make it better than a CD
Just because it's a tube doesn't make it better than a transistor
Just because it's a triode doesn't make it better than a beam pentode
Just because it's directly heated doesn't make it better than indirectly heated.
Old technology is not automatically better BECAUSE it's older.
Of course, some designs using triodes are indeed better than some other designs using beam power tubes
Some designs using tubes are indeed better than some other designs using transistors
Some vinyl recordings are indeed better than some other recordings on CD.
But they don't HAVE to be. There are plenty of good designs and plenty of bad designs using either old or new technology. It ain't the age of the technology- it's how well the design fits the requirements of the application.
That being said, I full-well realize how FUN it can be to build something completely inappropriate and impractical just for the sheer fun of doing it. Nothing wrong with a lighthearted romp into the absurd every now and then- but it's always a good thing to remember to keep fantasy separate from reality.
Most of the time.
I used to think this but I am having trouble finding something that will deliver say 200Vrms at low distortion with a CCS load at less than 15mA. 801A seems to fit the bill. Where's the indirectly heated version of that tube? I'd gladly use an indirectly heated version if I could find it. I'm sure it would be cheaper.
In fairness, I think that the astounding linearity across wide voltage ranges that DHTs offer is likely due to their low mu and how fine the grid was wound, not some magic mojo that dried up in the 30s. Its not that it couldn't be done with indirectly heated cathodes, its just why would you go to all that effort to make the plate curves look good out to 1000V when you are just going to use it to drive a 6L6? Typical engineering practices changed and so did the tube characteristics.
As far as output tubes go, the screen grid is your friend. Restore the triode characteristic with an external local feedback network, preferably using an output transformer winding so that input impedance stays high. You can't beat that with a DHT, unless of course DHT stands for directly heated tube and you use a pentode.
How do you turn them (thyristors, the regular kind, not the GTOs) off when it's time for the other halfcycle ?
"How do you turn them (thyristors, the regular kind, not the GTOs) off when it's time for the other halfcycle ? "
The old SCR inverters had a commutating inductor for each 180 deg. phase pair so that the opposite phase created a momentary reverse voltage spike to turn off the 0 deg. phase SCR, and visa-versa. If some heavy load disturbed the commutation you would get some real fireworks.
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All the DHTs I've tried ring like carillon towers if you tap them while operating. The more musical the chimes are, the better it sounds in the amp.
Reverberation pickups.
The early tubes did use lower gm grids, which were more linear than the later high gm ones (Usually the case with IDHT tubes. But not always, some frame grid ones are pretty linear if they didn't overdo the gm). In any case, low feedback (6 dB or so) around the high gm ones can transform their curves back to the DHT like curves. You can see it on a curve tracer using variable Schading. Too much Schading and they start to look like Op Amps.
The old SCR inverters had a commutating inductor for each 180 deg. phase pair so that the opposite phase created a momentary reverse voltage spike to turn off the 0 deg. phase SCR, and visa-versa. If some heavy load disturbed the commutation you would get some real fireworks.
-----------------------
All the DHTs I've tried ring like carillon towers if you tap them while operating. The more musical the chimes are, the better it sounds in the amp.
Reverberation pickups.
The early tubes did use lower gm grids, which were more linear than the later high gm ones (Usually the case with IDHT tubes. But not always, some frame grid ones are pretty linear if they didn't overdo the gm). In any case, low feedback (6 dB or so) around the high gm ones can transform their curves back to the DHT like curves. You can see it on a curve tracer using variable Schading. Too much Schading and they start to look like Op Amps.
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Do you mean "arbitrarily low distortion", or "distortion that looks the way I want it, not actually low but rather large" (say ~5% at full output)? The first one is easy, especially with multiple stages. The second one will take some looking, especially if you want a single stage.
Offhand, a number of recieving tubes will fit the bill. 200Vrms is 288Vp or 560Vpp, suggesting a supply well over 600V, resistive coupled. For realistic voltages, a choke load is preferable. The high voltage and power (>3W) rules out most signal tubes, suggesting deflection amps, including 6BM8 (strangely rated for 900V or something like that), 6V6, etc. After that, your only other options are top-cap tubes, including sweep tubes (a small B&W compactron may be best without wasting too much heater power, e.g. 17GV5) and transmitter tubes (807 being one of the smaller, more common types).
A better solution is to simply avoid such high voltages in the first place (i.e., choose a better output tube). Then you don't run into such problems as rare and expensive output tubes, or equally rare drivers.
There is some expense in linearity when going for high performance. Some microwave triodes are truely awful, the mesh grids have strange cutoff performance. This didn't matter for single-frequency, class C operation, so it wasn't "fixed". Many frame-grid video/RF amps have less desirable curves; however, their transconductance is so high that you can burn a little with cathode degeneration, and get much better overall linearity than another tube with the same transconductance and no degeneration.
Tim
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