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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

topology and PSRR

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firstly, i'm still basically learning electronics. in any case i'm working on a project that interest me that i want to use as a learning experience. its an overdriven tube pre-amp and a gainclone. why? most people are saying that the SPIKE protection circutry in the national power ICs is sonically bad and should be avoided.

so i was thinking about what would happen if you basically pre-distorted the input so that it is compressed/limited a bit. from more research i see that tubes have some of these abilities, and more so at low plate voltages.

so i was looking at requirements for this combo:
1.) the spike protection must never be activate during even clipped input signal. so output power will be limited, and i accept this for a prototype/fun test.
2.) the PSRR of the tube pre-amp limiter would have to be high because i'd like to keep this thing small. having multiple chokes that can't be within a half foot of anything is bad for size!
3.) there are 3 tubes that come to mind for this application: ecc86. ecc88. 12u7. i've heard the ecc88 is decent for low voltage apps. nothing about the 12u7.
4.) a minimum number of tubes should be used per side i wanted 1 tube per input channel in order to reduce any error between tubes.. i may make amps for bi amping that use a common tube preamp but i'm worried that the effects of the audio crossover will affect the signal and accidently trigger SPIKE in rare cases.

but i was looking into the topolgy for the input, like i've seen some long tailed pair designs, and at low voltage i feel i could use a current source in the tail instead of a resistor, but i forgot if that affected PSRR any. cathode follower is ok, but i'm worried it may be harder to overdrive to get the pre-distortion i am trying to test. and i'm not sure if it has better PSRR then anything else.

so the main question is which topolgy has the best PSRR. the purpose is so i can try pre-distorting a signal in the hopes that the added distorting sounds better than an alternatively added distortion. the PSRR is becasue i hate hum.
 
pre-distorting a signal in the hopes that the added distorting sounds better than an alternatively added distortion. the PSRR is becasue i hate hum

outch. It would be easier to use a software limiter than to actually build one.

PSRR is a bit different than hum. You need good filtering of the AC signal to minimize hum. A good wiring is also critical. Feedback will be your best friend in reducing overall hum.
 
6JH8
Just one tube contains the differential stage, CCS tail, and adjustable limiting (by current level). Requires magnetic shielding if near xfmrs, and it is a good idea to demagnetise the tube itself initially. Use gyrator loads for PSRR. (Mu around 6, but could cascode for high gain)

It only requires a RC filter to fix the B+ PSRR for any front end tubes.

https://frank.pocnet.net/sheets/093/6/6JH8.pdf

Compact, then there are the subminiature tubes, like 6021 or 6111 etc.

https://frank.pocnet.net/sheets/141/6/6021.pdf
 
1.) the spike protection must never be activate during even clipped input signal. so output power will be limited, and i accept this for a prototype/fun test.

Look for diode soft clipping circuit. It will limit the input signal before the SPIKE activates (which i assume is some kind of hard limiter on the chip amp?) and give you the rounded clipping behavior found on valve amps. This can be applied independently to your tube pre-amp.
Soft Clipping

Since the diode clip provides you the predistorted signal, then a tube input stage is redundant. If you still insist on having one with good PSRR and minimum tube count (though you will most likely end up with too much gain), look for hybrid mu-follower. It uses one triode per channel, excellent PSRR and has low enough output impedance to drive the diode soft clipping. Works nicely with ECC88. Aim for around 100V for the plate-cathode potential.
01a Preamp (Gen2) – Bartola Valves
 
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