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Old 25th June 2003, 07:42 AM   #11
PRR is offline PRR  United States
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> Interesting post but... Frankly, I don't much care about that since I feel it to be a better approach to use independent supplies, some even regulated, for input and outputstages.

I am just spouting "Conventional Wisdom". You are of course free to be as wild and crazy and extravagant as you want.

But what do you gain by ignoring my advice? If you go to a very low-Mu tube you get a little more B+ current and output power for a given supply power. OTOH if you pick a high-Mu tube, it drives a little easier but needs higher B+ voltage and/or more heater power for a given output power. But your philosophy does not care about power supply costs. I'm sure you will happily build whatever size PS a stage needs.

And nobody is going to build a special-Mu tube just for you. The mainstream of hollow-state electronics is and always has been "one B+ supply", direct to the power-stage and decoupled to the driver and earlier stages. In that crassly commercial world, Mu= "about 4 or 5" is the only way to go for profitable power. With rare exceptions (pass-tubes the main class) you won't find many high-power tubes with Mu much different from 4-5 for triodes, 5-20 for tetrodes. Even the advent of "unprofitable power" ($5,000 for 2 watts???) has not stirred tube designers to cook-up any new tubes. (Indeed they have a hard time just imitating the old tubes.)

Anyway, if you go too far from the time-proven parameters, you do get insane fairly quick. If you are willing to build multiple supplies, you are not the kind of designer to accept rules of thumb blindly. When I say a driver voltage amp can make peak voltage about 20% of its supply voltage, that's at 5% THD or so, which is rather high. An extravagant design would aim for lower THD, which means the peak audio voltage will be a smaller percent of supply voltage. Peak of 2% of B+ gives about 0.2%-0.5%THD.

So say you pick a power tube with Mu=2 (for low plate impedance) and B+ of 250V (6080/6AS7). Grid bias and peak grid drive will be about 80V to 100V (more like 100V for 6AS7 to keep it from melting at 250V). If the driver can only swing 2% of its supply for low THD, the driver needs to run on 5,000V!!! Even if you don't care about the cost of a 5,00V supply, that will limit your choice of tubes and you may not find one you like with that kind of voltage rating.
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Old 25th June 2003, 07:57 AM   #12
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Ah yes, forgot about dB.

Hmm not sure why but I feel like putting on my psychologist hat:

I'm sure we could go on for weeks about this. That it could seems to indicate either a. interesting and constructive discussion taking place, or b. someone's hit a nerve with someone else's beliefs. Since diyAudio is often more belief (12AU7, need I say more) than engineering (when's the last time someone here gave a darn about .0001% THD), b. is easily possible.

So put it to rest. (As much as I might like to jump in the discussion.)

Tim

P.S. Psst... pentodes have lower distortion than triodes...
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Old 25th June 2003, 08:51 AM   #13
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Default ...Ouch! The truth hurts...

Quote:
P.S. Psst... pentodes have lower distortion than triodes...
True - sometimes...

But we don't listen to THD very well but we do listen to the distortion spectrum exceptionally well - that's why a lot of people like listening to DHT SE.

but again it is perfectly possible to engineer a pentode amplifer to have low distortion and a nice distortion spectrum... but it's easier to get a nice sounding DHT SE and it's fashionable.

PBR.

Interesting posts - Thankyou. And you reinforced why I don't like resistor loaded drivers

A question - In terms of higher mu triodes being easier to drive ...how do you balance miller capacitance against gain ?

ciao

James
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