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Morgan Jones and cascodes

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Re: Morgan jones and cascodes

Ryssen said:
I was reading Morgan Jones book "Valve amplifiers" 1st edition,when i saw this:
"As mentioned before the only really satisfactory valve for use as the lower valve is the Ecc88;any other type will waste HT"

Is this true?:xeye:


Not really. A triode with high gm is required at the bottom of a cascode. The ECC88 is an excellent, but not the only, choice. Rozenblit uses 2x 6s45p to form the cascode 1st gain block in his phono stage. My McIntosh MR71 FM tuner uses a 6DS4 nuvistor at the bottom and a 12AT7 at the top of its cascode TRF front end.

Jones is accurate in pointing out the ECC88's convenience, as 90 V. per section is quite satisfactory.
 
Surely the type of tube will depend on the output amplitude required? E.g. as input stage in a power amplifier usually an output signal of a few volt only is required, so wasted voltage is of little consequence. I am not suggesting all tubes will do; I do not see the ECC83 in that role. But there are also disadvantages in using too high Gm triodes at low currents required for voltage amplifiers; they could operate on a steep slope in their transfer characteristics rendering work points critical. There are other factors beside a low operating Va for the input unit.

Regards.
 
With only a few minor simplifications in the algebra, the gain of a cascode is:

gm x RL

Where gm is the gm of the lower tube and RL is the load resistor on the top tube. So to maximise gain you need a high gm tube for the lower device.

So Eli had it RIGHT (See his post above).

Cheers,
Ian
 
lndm said:
Where does the upper device fit in? Is it more or less important than the lower device vis-a-vis noise and tonal contributions?

The upper device amplifies what is fed to it. If the lower tube gain and transconductance is significant enough (as it should be) the top tube's noise will be moot.

Tonal characteristics is another story. I've played with various top tubes and some really suck and some sound great.
 
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How does the performance of a couple of triodes in cascode compare with that of a small signal pentode, e.g. EF86 or EF94 a.k.a. 6AU6? A pentode has some practical advantages, including no second heater at elevated voltage and no sharing of the B+ across two tubes. I have no experience of usng cascode, so the pentode would be my natural choice if I needed that much gain in a single stage.
 
Hi ray_moth,

the cascode was invented to give similar performance like a pentode (high gain, low CMiller), but w/o the partition noise of pentodes.

The application in mind was very high gain RF / UHF input stages with the excellent S/N ratio a triode can do at microVolt or single digit mV levels.

For audio, with usual input signal levels of some 2 digit or 3-digit mVs, the noise argument is moot, of course, except maybe for phono cartridge pre-amps.

In most small signal audio apps a cascode has no real advantage to a correctly dimensioned pentode circuit, I agree.

Tom
 
In most small signal audio apps a cascode has no real advantage to a correctly dimensioned pentode circuit, I agree.

True, but production of small signal pentodes had ceased. I suspect that at least some of the interest in cascodes is related to that fact. Thankfully, New Sensor has resumed production of what is arguably the best, for audio, small signal pentode, the EF86.
 
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Yes, Russian tubemakers offer new EF86. 6AU6 is very abundant and cheap, although not still in production. I can understand a manufacturer of amps not wanting to be reliant on the supply of NOS tubes but for DIY use it's not a problem.

6AU6 examples seem to vary considerably, in terms of microphonics and noise, but I found RCA examples were good. If one can just accept the idea of using pentodes, they do the job pretty well I think. They also make good CCSs, for the unadventurous (like me!)
 
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Joined 2004
I'd say it probably does. In its heyday it was used in preamp stages or the first stage of a main amp, which is how Mullard promoted it e.g. in the 5-10 and 5-20. I can't imagine using it to amplify a large signal, such as in a driver stage.

I plan to build a PP EL34 pentode amp with 6AU6 as the first stage. Of course, it will need a lot of NFB to get a decent amount of damping with pentode mode, so the high gain of the 6AU6 will be handy. I'm really looking forward to doing this, because I want to put every good feature I can think of into it, just to prove that pentodes aren't as black as they're painted!
 
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