Would there be any advantage in using what is normally an output tube as a driver? Just wondering if the ability to run them very conservatively would improve distortion over a more traditional driver tube.
mike
mike
The principal advantage (in selected cases) is greater max plate voltage and hence larger swing. The more common reason is an illogical gut feeling that a power tube ought to make the sound more, well, powerful.
Using a power device as a driver has both good and bad points. Driving the output stage into grid current (as happens from time to time) likes a low output impedance from the driver stage (and high current). However, most power devices are low gain, especially old ones. It is when driving the output stage deep into cut off (grid negative) that the low mu driver may itself go up into grid current, upsetting the third last stage.
I don't think there's a clear line. What one person might call a tiny output tube another might call a beefy driver. The fat half of a 6EM7 makes a very nice 1W output or a very nice driver for a 300B. I suppose it depends on what you're trying to drive.
A high mu transmitting tube like a 211 pretty much requires a small power tube to drive it. The Miller capacitance of any triode requires a little power, especially the older directly heated triodes. A 2A3 can be driven by 1/2 6SL7, but I found that a triode strapped 2A5/42 works better.
The old RCA tube manuals recommended driving PP 42's with a single triode strapped 42 and phase splitting interstage.
The so-called Sakuma method uses 'like drives like;' a 211 to drive a 211, for example. The driver runs at lower signal levels so contributes less distortion than the output tube. Some people claim it works very well, though I've never tried it.
What are you trying to drive?
-- Dave
A high mu transmitting tube like a 211 pretty much requires a small power tube to drive it. The Miller capacitance of any triode requires a little power, especially the older directly heated triodes. A 2A3 can be driven by 1/2 6SL7, but I found that a triode strapped 2A5/42 works better.
The old RCA tube manuals recommended driving PP 42's with a single triode strapped 42 and phase splitting interstage.
The so-called Sakuma method uses 'like drives like;' a 211 to drive a 211, for example. The driver runs at lower signal levels so contributes less distortion than the output tube. Some people claim it works very well, though I've never tried it.
What are you trying to drive?
-- Dave
I played around for a while with an EL84 driving another EL84. Found it way too microphonic to be of use and didn't pursue it. Something like a 4-5 second delay with the Russian clones. A JJ or EH were 'better' if 2-3 seconds counts. I think Pete Millett found the same issue with the EL34 in one of his preamp designs.
Thinking in generalizations right now but I kind of had in mind the keyboard amp idea I posted earlier (Transformer coupled, fixed bias, PP 6CA7 connected UL). My initial idea was 6SN7 as the driver but thought that a small power triode (or strapped pentode) might have some advantages. For obvious reasons we want to watch out for microphonics but I want to be sure that the output reaches overload long before the previous stages.
mike
mike
A smaller triode will still drive your o/p well, no fear. Just remember the o/p stage's input capacitance (as Dave Cigna said) for good hf xtn. The 6sn7 is famous as a driver, anyway, and you could always play with xfmr ratios to find out for yourself.
Now I remember ... 6AS7's (pentode or UL) are pretty easy to drive. A 6SN7 would do. OTOH, a good candidate might be the 6DN7. It's a dissimilar dual triode with an octal base. One side is not too much different from a 6SN7, though with lower ratings. The other side is a small power triode with max plate dissipation of 10W. I think both sides sound very good. It even has the same pinout as the 6SN7, so you might be able to experiment with both without too much rework.
-- Dave
-- Dave
Of course you can use a power valve to drive another power valve. It may allow you to drive the output valve (or valves) into Class A2 and increase the efficiency of the output stage. However... ...try doing the sums. You'll be amazed at how quickly required driver power supply current rises and squanders the efficiency gain in the output stage.
What is really being discussed here is the ratio between a design's anode dissipation of its output valves and its drivers. Traditional designs used very high ratios (ECC83 driving EL34), whereas contemporary designs use lower ratios in order to deliver clean current into Miller capacitance and output stage grid current (for whatever reason).
What is really being discussed here is the ratio between a design's anode dissipation of its output valves and its drivers. Traditional designs used very high ratios (ECC83 driving EL34), whereas contemporary designs use lower ratios in order to deliver clean current into Miller capacitance and output stage grid current (for whatever reason).
I regulary use an ECL82 pentode section config as triode for driving power tubes with a low resistance anode load. This will snub any varactor effect (alias miller effect) created by the output tube voltage swing reflected back to the signal grids.
The beauty of pentodes config as triodes is that the harmonics are far more consistent and far simpler without the pain of the screen grid setup and they can take alot of plate current.
I'm dead against 6SN7's, ECC82 families as drivers. Ex video tubes are way better.
richj
The beauty of pentodes config as triodes is that the harmonics are far more consistent and far simpler without the pain of the screen grid setup and they can take alot of plate current.
I'm dead against 6SN7's, ECC82 families as drivers. Ex video tubes are way better.
richj
I have tried some different tubes in my Brute force line stage as the lower possition (It´s a mu follower)I tried 6BX7 and 6AS7,the more powerful the tube the thicker sound,however there is some noise with this tubes,and they are a little heavy on my filiament trafo.My question is are there any other octal triods that are so to say between 6SN7 and 6BX7,I could try?🙄
Strictly speaking, a "driver" tube is one that drives an output tube with high grid current, so it should be capable of providing some power itself. In class A audio amps, the second to last tube is technically simply another voltage amplifier.
Having said that, if you are using an interstage transformer, a tube with highish plate current (>20ma) is desireable to "drive" the output tube because of the high excitation current requirements of common GOSS IT's. Obviously, you could use an interstage transformer with a Permalloy or Mu-Metal core to avoid having to use a power tube, but they are hard to find and expensive. James sells 45% nickel permalloy interstages.
The common use of power tubes in this application is borne out of experiences with positive results, so I would recommend it.
John
Having said that, if you are using an interstage transformer, a tube with highish plate current (>20ma) is desireable to "drive" the output tube because of the high excitation current requirements of common GOSS IT's. Obviously, you could use an interstage transformer with a Permalloy or Mu-Metal core to avoid having to use a power tube, but they are hard to find and expensive. James sells 45% nickel permalloy interstages.
The common use of power tubes in this application is borne out of experiences with positive results, so I would recommend it.
John
I am intrigued by both the current needs and output impedance needs of the driver stage. I am surprised to find that tiny drivers do as well as they do.
Surely the underlying job of the driver is to control the transition into output stage saturation. A given output device seems to have a limit to how far it can be pushed.
If you look at the performance of a cathode follower as a driver you might see it driving cleanly up to the limit and then clipping hard. A small driver will begin clipping early, but this is probably not such a bad thing.
Assuming an interest in performance at the power extreme, I feel there is probably an optimum middle ground, but where?
Surely the underlying job of the driver is to control the transition into output stage saturation. A given output device seems to have a limit to how far it can be pushed.
If you look at the performance of a cathode follower as a driver you might see it driving cleanly up to the limit and then clipping hard. A small driver will begin clipping early, but this is probably not such a bad thing.
Assuming an interest in performance at the power extreme, I feel there is probably an optimum middle ground, but where?
- Status
- Not open for further replies.