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    Building, troubleshooting and testing of these amplifiers should only be
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help me design an input stage for 6b4G PP

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i have an output stage with PP 6B4G's bias at 50v.

driven with a 1:1/1 phase splitter tranny made by electra print. I think these are standardized for a 5K input impedance to the primary.... currently using a ECC99 which seems to work with good bandwidth.

problem is with 1 ECC99 with a gain of 22... my gain is too low.

looking for an input with gain of 100.

probably a 2 stage design with input tube direct coupled to second tube loaded with IT tranny.

question is which tubes should I go with?? looking for low thd of course.

6C45, E810F are your tubes

I'm assuming you are using the E-P autoformers (PVA series).

Do you need mu=100? No. I assume you are shooting for
2V input sensitivity so you can eliminate the need of a preamp.

The 6C45 and 7788/E810F will do it. mu > 50. Rp < 1000. Another possible candidate might be the EC8010 which can be had for generally reasonable prices still. 2V means 2Vp-p, which
means the signal runs from -1V to +1V. This is why you want
mu=50. At +1V or -1V, there will be 0V at one grid and -100V
at the other grid - which is what you want.

This is a great project. I found the amp to be clean but vivid and
rendering a deep and holographic sound stage. However, it lacked the euphonic colour of a single ended amp - but in a good way!

-- Jim
I'm using a 14F7 (loctal 12SL7) diff pair with a Morgan Jones CCS under it. Gain is like 5751. I haven't got around to my 6B4Gs yet - at present I'm using 2E22 outputs. This is an uncommon DH pentode, which looks exactly like a 300b (I got them for a good price). 2E22 are wired as triodes. There's enough volume alright - just got it working so no measurements yet. HT is 295v, self bias is 24v on cathode for 40mA per valve (150 ohm hum pot connected to 560 ohm cathode resitor bypassed with 100uF).

For another implementation of 6SL7 input stage have a look at: http://www.diyaudio.com/forums/showthread.php?s=&threadid=20889&highlight=

well there is a slight problem...

i will be driving the amp with a TVC... so input cap of amp is an issue.

with gain its around 240pF of cap.... dunno if a TVC would like that.

anyways... still looking for a cascade gain stage... perhaps input tube direct coupled to driver tube.

ECC99 to 12B4 perhaps?

Gary Primm CCS but replace the 12AX7 with Mil Spec 5751. It has a gain of 70 that should be plenty high enough for your application.

Some favorites are:

GE 5751 3-mica black plate (grey plates suck)
RCA Command series 5751 black plate
Sylvania 5751 black plate square getter
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