Pros and cons of Cascoded front ends

[Burnedfingers]

I can't return the board because its already stuffed. Therefore I am looking for other options.

 

[SY]

What's the tube layout (the question I asked before)? In the left channel, are the upper and lower sections in the same envelope? Or do left and right bottom share one envelope and left and right top share an envelope?

 

[Burnedfingers]

The cascode va is in one tube for each channel. The phase inverter for each channel is one section of a single 6sn7.

 

[SY]

OK, then, remove the 1u/470k from the upper tube's grid circuit. Remove the 620R resistors. Substitute a 1.2V drop IR LED for the bottom 620R. Build a CCS set for 1.5mA (adjustable will help here- the diyAudio/Morgan Jones cascode ones will be perfect for this). Connect one end to the 300V source, the other end to the pad where the 620R used to be that's connected to pin 5 (the lower tube's plate). From that junction, jumper to pin 2. This leaves the top section unused and connects the plate of the bottom section to the phase splitter.

Adjust the CCS to get something like 120V or so at the cathode of the 6SN7 phase splitter. That ought to be a bit more optimal.

 

[Wavebourn]

What about SRPP option?

 

[SY]

I'm unconvinced that, given the almost-no-loading of the phase splitter, that the SRPP will have any linearity advantages over a plain CCS-loaded voltage amplifier.

 

[ray_moth]

I have modeled with LTspice (but never built) a Morgan Jones 'Beta Follower' using a 6SL7. This is a mu follower with a PNP transistor wired as a crude but effective pseudo-CCS to serve as a plate load for the lower triode, in place of the usual resistor.

The reason I went for that design was shortage of B+, which meant it would be unrealistic to have a high value load resistor for the lower triode. Gain and linearity seemed very good, and I believe it would make an excellent direct-coupled front end to a Mullard-style splitter.

 

[Burnedfingers]

Can I use a 6SL7 as a phase splitter in place of the 6SN7?

 

[ray_moth]

Sure you can, but it's best to isolate it from the OP tubes - either with direct-coupled cathode follower drivers or with a differential stage, as in the Williamson.

 

[SY]

Can I use a 6SL7 as a phase splitter in place of the 6SN7?

Yes, but why? The 6SN7 will work better- higher current, lower source impedance, and lower mu.

 

[gorgon53]

lower tube E288CC, upper tube 6BX7 is a excellent combination for big swing lowest distortion. E188CC + 6BX7 comes close

 

[Wavebourn]

I still vote for pentode, 3 years after.

 

[gorgon53]

well, i would not say i choose nails over screws, i use both depending on the application

 

[Wavebourn]

well, i would not say i choose nails over screws, i use both depending on the application

Me too. For UHF frequency converter I would prefer cascode. It fits perfectly.

 

[gorgon53]

I use cascodes in audio because of (among other things) the possibilitie to use fb on the second grid.

 

[gorgon53]

sometimes maybe a pentode would be useful (and certainly more cost effectiv) but i did not yet find a pentode-model that would giv even close results to the real world when g2 is used in unusual ways. Triode cascode simulations come much closer.

 

[gorgon53]

wavebourn, do you know of any pentode with strain gauge g1 that can swing at least +-200V without the anode going below g2? If you do, I would be grateful to hear of it

 

[Miles Prower]

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.

 

[Wavebourn]

wavebourn, do you know of any pentode with strain gauge g1 that can swing at least +-200V without the anode going below g2? If you do, I would be grateful to hear of it

GU-50 in my amps swing +-700

 

[gorgon53]

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