KT120 Rg = input impedance?

[cfortner]

Hi,

in the point of view of a KT120 with fixed BIAS is Rg = BIAS-resistance + grid stopper value.

Is this resistance Rg equal to the input impedance of this KT120 looking from the LTPI or CF-driver?

I assume the input impedance of the KT120 is quite high and Rg has to be below 51k.

I‘m asking this for ac load line calculation.

Thanks

Christian

Edit: I‘m talking about ac-coupled LTPI or CF

[pdf64]

The grid circuit resistance includes the grid stopper + grid leak + bias supply output resistance.

In class AB1, the AC load would just be the grid leak resistor value  (assuming that the bias supply output was decoupled).

[cfortner]

Ok, thanks!

Why decoupled? I‘m looking for the impedance in a running amp.

[pdf64]

Power supplies generally have decoupled outputs.
Rather than trying to compose a response that covers every scenario, it would be helpful for you to provide the schematic you thinking of.

[cfortner]

Thanks for your reply!

I'm planning an amp with six KT120, referring to the Hiwatt 400. The KT120 are very difficult to match, thats the reason I want separate BIAS-pots. Due to Rg of 51k for a KT120 I'll build BIAS-circuit, which detemines a Rg of 50k max.

My question is: Whats the impedance of one KT120 in this circuit as seen from the perspective of the CF with ECC99 or 12BH7?

 

[cfortner]

I'm looking for the load impedance of three KT120 in parallel, the cf has to drive a huge signal into this (very low) impedance.

[pdf64]

I'm looking for the load impedance of three KT120 in parallel, the cf has to drive a huge signal into this (very low) impedance.

Yes indeed!
Some of the text on the schematic is a bit fuzzy, does R20 = 9k1?

[cfortner]

Yes, 9,1k

[pdf64]

I get the AC load  = 27k + (9k1 // 22k) = 27k + 6k4 = 33k4.
So 11k1 for 3 in parallel.

[cfortner]

Why do you include the 22k from BIAS power supply? Is the connection of the BIAS supply to the 22k a virtual ground?

[pdf64]

All power supply outputs are low impedance, that’s what the decoupling caps do.

[cfortner]

Thanks a lot!

[tubeswell]

You may have already pondered this, but if you’re wanting to drive a large high current signal (particularly bass guitar frequencies) with AC coupling, the tendency for blocking distortion will be greater. The smallish-sized grid stopper on your schematic might or might not alleviate this.


Another method could be to use hybrid fixed and cathode bias, with each output stage side driven by a shared DC coupled interstage (post PI) 12AU7 CF that has a low (10k) load resistor connected to a high negative voltage sink, and tweak your individual K120 bias with cathode bias. YMMV

[cfortner]

Thinking..

[cfortner]

Negative voltage at the cf I saw some times.

I have an amp with a PT 700VA and an OT 400W 1,6k Raa and I got little more than 500W with six KT120 at V+ 805V.

But - instable BIAS due to a high z BIAS circuit, Rg is 130k.

Thats the reason I‘m trying to drive three bottles per side with a LTPI + CF, I believe, it will be possible today.

[cfortner]

A mix of fixed and cathode BIAS in an 300-400W amp with 6-8 bottles
I never saw.

[tubeswell]

A mix of fixed and cathode BIAS in an 300-400W amp with 6-8 bottles
I never saw.

Something like a goosed-up SVT with (slightly) bigger adjustable cathode resistors and lower load resistance through CF driver


https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_svt_poweramp_revf.pdf