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SRPP - issues understanding how to increase current

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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.
 
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?

😕
 
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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.
 
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+.
 
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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?
 
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?
 
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.
 
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.
 
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

I agree. sounds good to me.

I was playing around with the caps this morning actually trying to economise with 100uF caps but the 480uF caps just sounded better.

Also got FREDs.

Using this as a headphone amp and sounded very sweet so far.

My play around with a CF after the SRPP.
 
😀😀😀

Just did some tweaking. Listened to 'The Chain' by Fleetwood Mac and heard something right at the beginning of the track I've never neard before.

Sounds like one of the band members adjusting something or something like that before the guitar and drums come in.

I've got 220uF coupling caps with 0.1 MKP10.

On the cathode resistors I've got 1000uf bypass cap with a 0.01 MKP4 cap.

15mA on the 6N6P's.

Very impressed so far.

But maybe a bit bright for some. I might try bypassing the coupling caps with some small PIO.
 
😀😀😀

Just did some tweaking. Listened to 'The Chain' by Fleetwood Mac and heard something right at the beginning of the track I've never neard before.

Sounds like one of the band members adjusting something or something like that before the guitar and drums come in.

I've got 220uF coupling caps with 0.1 MKP10.

On the cathode resistors I've got 1000uf bypass cap with a 0.01 MKP4 cap.

15mA on the 6N6P's.

Very impressed so far.

But maybe a bit bright for some. I might try bypassing the coupling caps with some small PIO.

I read the links about the operating point, with mine currently at 8mA they sound dynamic and detailed, certainly not dull. I run without cathode capacitors - never seen the need for them, try without those and you may find the same. I'm using PIO caps bypassed with silver mica into them and PIO caps bypassed with polypropylene on the way out.

I fitted my extra transformer (A Clairtronic 0-6 0-6V 6VA L/P encapsulated transformer) that just neatly tucks in the side of my chassis and hooked it up to the top 6N6p at 7Vac measured. No more time tonight but tomorrow I'll string it to the cathode with some Rs (i.e. heater/cathode voltage = 0) and hook up the top tube to see how SRPP sounds with these, should be nice if the current sound I'm getting from common cathode is anything to go by.

ETA:
Why are you using 220u for coupling - is that direct drive to headphones?
 
Hi Brit1

As you mention the combination of headphone and SRPP I immediately thought about a recent blog of Broskie on the SRPP+ optimized for low impedances (such as the 300R of his sennheiser headphones). I did some calculations on it once and, IIRC, it would not work well for lower impedances, but maybe you have got some 300R sennheisers as well!

Well, here is the link to the blog, with a practical example using the 6H30P, which is in fact not very different than the 6N6P. If you are interested we can work it out together (let me know the impedance of your headphones!)
 
Wish I had some sennheiser's🙁

Just got a cheap pair, not even sure what impedance they are.

How can I measure this?

I'll have a look at the blog later. But this is an amp I'd really like to perfect and tweak.
Had a quick look and a very interesting blog I need to read.

Family issue just came up.
 
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The majority appear to like this tube at high currents ~20mA
I guess just play aorund with it and see what you like.
Just another view:

K&K Audio / Lundahl Transformers - My 6n6p experiments - KurtP - December 29, 2007 at 10:41:29

This morning I put in 150R resistors and get 18.6 mA. Sounding very good. Very tight bass and sweet highs.
Wish I had equipment to measure distortion though!
I'll have to give it a try, the guy in the article said he saw a big change going from 85V to 108V on the plate, so at 200V I'd expect to be out of the effects of needing that much current, but we'll see 😉

A bigger idle current will mask the current taken by the next stage and/or phones so as long as the PSU is up to it, it will give a lower impedance. Not sure my PSU is up to it, I need a tube that's perfect at 5mA 200V really 🙂
 
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