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Old 29th April 2006, 03:28 PM   #11
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Default anything like this?

is there any output tubes that have high mu, run at low plate voltage and higher current...

and run it at positive grid bias?

thanks!
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Old 29th April 2006, 03:57 PM   #12
Ryssen is offline Ryssen  Sweden
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Quote:
is there any output tubes that have high mu, run at low plate voltage and higher current...
Maybee 6C33?or?
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Old 30th April 2006, 08:50 PM   #13
mwiebe is offline mwiebe  United States
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The 6HV5A this thread is about has a mu of 300, which makes a 12AX7 look like it's struggling. In a different context a 211 is considered a hi-mu tube with its mu of 12. So if you can make any further suggestion about the mu, plate voltage and current constraints you have in mind maybe someone will have a suggestion.
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Old 30th April 2006, 10:39 PM   #14
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Default how about?

looking to just use a cathode follower direct coupled to output tube...similar to schematic posted.

problem is i do not want to build a 600v power supply... maybe a 300-400vdc power supply.

output tube could be: triode, pentode, tetrode, S.E., U.L. etc... i don't really care except it is Class A SE.

paralleling output tubes is ok too... as i will dc the input buffer to them.

ending up with a low impedance output transformer would be ideal to allow for wide BW.

I guess it could be positive grid bias, 0v grid bias, or negative grid bias....

Thanks!
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Old 1st May 2006, 04:22 AM   #15
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You want high gain, high transconductance, and high current capability at low voltage in one package. The 6HV5A and related tubes look to be special cases that need high plate voltage to realize their true potential. If you want to operate at lower voltage, you may be stuck with a pentode of some sort, but I kinda doubt you'll get enough single stage gain so that you can operate with just a cathode follower input, unless you want to settle for low output power. The 6C33 is not a contender because of its low gain.

What I would do first is to calculate the peak voltage and current desired into your chosen load. Work back through the turns ratio of your chosen transformer, and that will tell you voltage and current swing at the plate. This will the capabilities you need for your output tube. If you choose too low of a plate voltage, you'll hamstring yourself in terms of output voltage swing, as even the best of tubes won't pull down near zero like a semiconductor.
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Old 1st May 2006, 08:27 AM   #16
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Problem with high-mu tubes in SE is they are difficult to make sound good.

Just my experience.
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Old 1st May 2006, 04:48 PM   #17
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Well, this 6HV5 effort is a bit of perversity on my part. This familiy of beam triodes is an odd bunch, and it would be interesting to see what can be wrung out of them. I plan to use local feedbck to tame some of the gain. It's the prudent thing to do, elsewise I may have the FCC on my tail for operating an unlicensed transmitter...
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Old 3rd May 2006, 08:16 AM   #18
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Beware Mr. Miller: 1.8pF grid-plate x mu of 300 gives you 540pF... needs a low-Z driver. The CF should do it.

Pete
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Old 3rd May 2006, 08:33 AM   #19
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Thomas Mayer had some spud amps (driving mids/highs I think) using 6HS5 beam triodes and 1200V B+ at EFT.05. There's a photo here: http://www.vinylsavor.de/etfamp.jpg

I think he was driving them from a very low-Z source (600 ohm environment).

It sounded very good...

Pete
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Old 3rd May 2006, 01:53 PM   #20
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The king of Mu has to be the 6BK4. Mu of 2000, to bad it is totally useless for audio (I tried).

I tried an experiment once, that may be useful. Take a sweep tube and tie G1 and G2 together. This becomes the control grid. The result is a high Mu beam triode. Some tubes are more linear than others with this connection. They must operate in the positive grid region at normal plate voltages. Svetlana used to have curves for this mode on their web site. Curves for this mode (called High Mu mode) were published for some WWII era pentode transmitting tubes. Common audio pentodes (6L6, EL-34, KT-88 and others) are not linear enough.
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