SRPP - issues understanding how to increase current

[Brit01]

Trying to increase the Cathode current through a 6N6P of a simple SRPP design.

I have 250 B+ on the top plate and 134 V on the cathode.

Bottom tube I have 300 ohms cathode resistor with about 4.1 V giving me around 13mA.
I'm trying to increase this.

Also have 180 ohms on the cathode resistor of the top tube.

Having issues understanding how to increase the current to about 17/18 mA.

[DF96]

Smaller resistors.

[Brit01]

If I use smaller resistors on the bottom tube the voltage on the cathode of the top tube is pulled down and appears to balance out the current on the bottom tube hence no increase in current.

Maybe I need to go down to much smaller resistors on top and bottom tube and keep them equal for a SRPP?

[DF96]

Yes, smaller resistors at top and bottom and usually keep them the same value. As far as DC is concerned the SRPP is basically two constant current sources connected in series. To get the output to settle at about half HT (which is usually what is wanted) the two CCS need to be set to the same current, which means the same resistor.

[Brit01]

Thxs DF96. Got it now.

Same resistors. had to boost the B+ quite a bit so I get 280V on the top plate.

Now I'm getting around 15mA. Bit better for linearity of this tube.

Thinking this is getting too high B+.

Maybe lower cathode resistors with a lower B+.

[Globulator]

Originally Posted by Brit01

Now I'm getting around 15mA. Bit better for linearity of this tube.

Funnily enough I'm looking at 6N6p in an SRPP configuration.
One tube for the lower triodes, a 2nd tube on an independent heater transformer (arrived today!) for the upper triodes.

I've got around 400V to play with, so with a centre point of 200V at 10mA gives me a Vk of 9.5V, allowing a 9.5/10mA = 950R or actually some 1K resistors that I have 'in stock'!

What makes you think that the 6N6p is more linear at 17mA than at 5mA or 10mA? Is it because you are running at a low voltage?

[Brit01]

Correct me if I'm wrong (because I'm a bit of a noob when it comes to the lines and data graphs) but looking at the data sheets they look more linear above 15mA even at higher voltages. Heard many opinions also that they like a higher current.

I've used these tubes before and love the sound especially when they are driven up to 20mA.

So you have 400V at the plate of the top tube and 200V at the cathode?

[Globulator]

I'm not sure if they will be more linear - has anyone done tests?
Also the SRPP circuit will use them differently to a regular common cathode one.

Mine are currently half built SRPPs, with a 22K resistor serving as the top tube, as my heater transformer arrived today hopefully soon I can hook up the top half.. also known as tube #2!

I have a 400V supply so yes the top plate will be at 400V, the lower plate at 200V (all things being ideal). These will then be driving a GU50 pentode into an SE OPT for my highly modded Sweet Peach amp.

Looking at the curves I'd put your 280V/140V SRPP with a 6N6P slap bang in the middle with a 12.5mA current and 5.5V cathode - giving Rk = 440R, so I'd go with 470R if it was I

Also you may need a bigger PSU capacitor with bigger loadings - I found I had to do this.

[Globulator]

Originally Posted by Brit01

Correct me if I'm wrong (because I'm a bit of a noob when it comes to the lines and data graphs) but looking at the data sheets they look more linear above 15mA even at higher voltages.

P.S.

Careful of interpreting the transfer function graphs for linearity - I made that mistake, those graphs are at constant anode voltage, it is not the transfer function of (any kind of) amplifier stage they are measuring there.

[Brit01]

There are quite a few threads on this.

Try this (someone said it sounded muddy below 10mA which I found true also;

6N6P vs. 6N6P-I