Pros and cons of Cascoded front ends

[Wavebourn]

you must be joking ) you drive your output-tubes with a GU-50???

I answered your question, about +-200V swing from pentodes.

I use 6J51P to drive Gu-50 tubes. Before that used 6P15P. Before 6P15P used 6N6P. However, they don't need 400V swing, but I suppose you need less of driving power so there are many options.

 

[gorgon53]

wavebourn, sorry if my question confused you the way it did. I will have a look into 6J51P (wich I am unfamiliar with). If any of the tubes you mentioned is able to produce results that come even close to results obtainable by a e288cc + 6bx7 cascode I will be very deligthed and thanck you for your tip and certainly use a more cost-effective option (e288cc is costly). The only "power" I need is the usual reactive power
related to miller capacitances, but voltage swing, high amplificatian and good linearity are essential.

 

[atmasphere]

I've gotten some good results using a cascode LTP with active tail loading. Not so common for hollow state, but they use them a lot in SS designs. That's where I got the idea in the first place.

We've been using that idea for the last 25 years. In that time, we've found that cascodes don't have to have a high impedance output, and you could easily get them to drive output tubes.

The trick is to run some current, but to do that you need the ability. Usually this means doubling up some tube sections on the bottom portion, this allows you to decrease the plate resistors, which will result in wider bandwidth and *increased* gain, possibly also lower distortion, which should approach that of a single triode if its set up correctly.

I've experimented with various biasing means for the top grid of the circuit. So far the most effective have been with a zener diode controlling the voltage. This eliminates the need for a series resistance between the plate of the bottom tube and the cathode of the top tube.

In differential cascodes, the CCS is paramount to performance. I prefer to run a 2-stage vacuum tube circuit and a B- supply. The power supply immunity that results is prodigious!

 

[Wavebourn]

As far as I know differential cascodes come to SS world from analog computers made on tubes, they were used to solve differential equations.

 

[Miles Prower]

We've been using that idea for the last 25 years. In that time, we've found that cascodes don't have to have a high impedance output, and you could easily get them to drive output tubes.

Back in 2007, I got nothing about this at all, and every Google reference to hollow state cascodes was in reference to its use as a VHF amp. Nothing on the sonic performance, so it was a case of "try and hear" -- and hope it wasn't used because it was "wierd", not because it had been tried and found to sound like

An externally hosted image should be here but it was not working when we last tested it.

So that's what I did: 6SN7s didn't have enough gain, and 12AT7s had a gm that rolled off too fast, so it was back to the RCA manual to look for different types, and I came up with the 6BQ7 -- designed for cascode use (good to 300MHz) and decent audio loadlines (though the type isn't mentioned as an audio type, not even as an after thought). The gain was right, and the final design gave excellent sonic performance.

Much later, I found that the Atmasphere line of OTL amps used them, and another design was out there "in the wild", and that it was mentioned in an old article by someone named "Hedge" back in the 1950s, hence the name "Hedge phase splitter".

In differential cascodes, the CCS is paramount to performance. I prefer to run a 2-stage vacuum tube circuit and a B- supply.

I prefer the Vkk supply, but with cascoded BJTs to make the active tail load.

"The power supply immunity that results is prodigious!"

Are you quoting Izzy from Digimon Adventure 02? In any case, yes it is.