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6c45p question

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6

6A3sUMMER

Member
Joined 2016
You might have as much as 500 or 1000 pf in that long RCA cable.
Measure it.
In case it is a large capacitance, you might need a Cathode Follower.
1000 pF has 8k Ohms of capacitive reactance at 20,000 Hz.
The 6C45pi plate resistance, rp, is about 2500 Ohms.

rp drives the plate load resistor (or plate current source), in parallel with the
cable capacitance, and in parallel with the amplifier input resistor.

If the plate load is a current source (very high impedance), and the amplifier input is a very high resistance and low capacitance, then:
you have the rp of 2500 Ohms driving 8000 Ohms at 20,000 Hz.

The voltage divider at 20,000 Hz is 8000 Ohms/(2500 + 8000 Ohms).
The voltage divider is 0.76.

The u of the 6C45pi is about 40.
At 100 Hz the gain will be about 40.
At 20,000 Hz, the gain will be about 40 x 0.76 = 30.5
That is -2.4dB below the low and mid frequencies (too much high frequency
roll off).

The 6C45pi has about 10,000 to 20,000 micro mhos of transconductance.
If it is used as a cathode follower, it could have about 100 to 50 Ohms of output impedance, that is much more suited to drive 1000 pF of cable capacitance.
 
M

mr ed

Member
Joined 2009
audiowize said:
Are you willing to use a pair of transformers after the 6C45? The common 5K:600 output transformer (fine to go parallel feed to keep costs down) will work well to reduce the gain of the 6C45 and help you drive those cables.
Click to expand...
Great idea, I have a Sowter TVC I was going to use on the input, i need to look up those specs, sse it could work on outpout or i could use trans in and out i guess.
 
B

Bandersnatch

Member
Joined 2003
I have a comparable tube in use, type 6S4 is a small 9 pin, with about half the gm of a 6S45. The beast is rigged as LTP for each channel and a 20k:600 line driver output TX is the load. SE input to one side, and balanced output from the 600R secondary. 6S45's work just as well, and though subtle, are an improvement over the 6S4's.
cheers,
Douglas
 
A

audiowize

Member
Joined 2005
So you're saying you've measured a 6C45PI to be ~10mA/V?
6c45%202.jpg
 
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