GK-71 as a pentode.

Cressy Snr · Post by **Cressy Snr** » Fri Apr 17, 2015 2:04 pm

Have you got any feedback in there Phil?

If not, it could sound bright at the top and loose at the bottom.
Not saying it will, but it is a distinct possibility.

pre65 · Post by **pre65** » Fri Apr 17, 2015 2:07 pm

No feedback Steve. Now you tell me.

Loose bottom ? Yuk.

pre65 · Post by **pre65** » Fri Apr 17, 2015 5:42 pm

On the RH813 Alex has two 220K 2W resistors in series (440K) between driver anode and power valve anode.

Would that be worth a try ?

And does having a CCS on the driver cock things up feedback wise ?

http://rh-amps.blogspot.co.uk/2014/06/r ... oject.html

EDIT

I see Steve uses 440K on his statement amp.

pre65 · Post by **pre65** » Fri Apr 17, 2015 5:47 pm

Another question.

On the G2 feed could I drop a few volts with a resistor instead of adding another zener ?

EDIT

This may be irrelevant now as the voltage has dropped down to around 380v, don't know why.

Cressy Snr · Post by **Cressy Snr** » Fri Apr 17, 2015 7:34 pm

pre65 wrote:On the RH813 Alex has two 220K 2W resistors in series (440K) between driver anode and power valve anode.

Would that be worth a try ?

Yes, but you need to use, as a minimum, 220K x 2 in series because of the voltages that could be generated across the feedback resistors by the feedback current itself, so don't skimp in this area. With a GK71, I would use a longer chain of lower value resistors if at all possible eg 4 x 110K, or something.

And does having a CCS on the driver cock things up feedback wise ?

Depends where it is. If it is being used in place of the driver anode resistor, I can't see there being a problem.

I see Steve uses 440K on his statement amp.

Yep my 440K is, as I explained above, made up of a series combo of 4 x 110K.
I use this combo in my "Barbarik" amplifier.

It does take quite a bit of faffing to get a power pentode SE to sound right, but when set up properly, they offer truly great sound. Nothing like a SET but equally valid.

pre65 · Post by **pre65** » Fri Apr 17, 2015 8:14 pm

Thanks Steve.

3W 110k are unobtainum so I will parallel loads of 220K.

pre65 · Post by **pre65** » Sat Apr 18, 2015 10:58 am

I've been thinking.

Re the feedback resistors.

At 220K X 2 the voltage drop for 1.5ma would be 660v

So, as my HT is around the 600v mark how can any current flow for feedback purposes ?

My bwain hurts.

First day of the outdoor bowls season today, I'll be ready and waiting at 2pm.

I won a final in the indoor club championships, drawn pairs. Got my name on a trophy and £10 cash.

Cressy Snr · Post by **Cressy Snr** » Sat Apr 18, 2015 12:08 pm

pre65 wrote:I've been thinking.

Re the feedback resistors.

At 220K X 2 the voltage drop for 1.5ma would be 660v

So, as my HT is around the 600v mark how can any current flow for feedback purposes ?

My bwain hurts.

That's an easy one and the answer is surprisingly easy to miss.
There is 600V at one end of the feedback resistor and at the other, is whatever voltage is at the anode of the driver stage. Subtract the 2 and that is the voltage drop across the feedback resistor chain.
Let's say there was 300V at the anode of the driver stage, so you have a 300V potential difference between the ends of the feedback chain. This PD and the feedback chain value will govern the amount of feedback current that will flow through those resistors.

pre65 · Post by **pre65** » Sat Apr 18, 2015 12:24 pm

Silly me.

I've used two 3W 220K resistors in the end, and both amps have been modified now.

Not got time to try them before I go out, so maybe tomorrow.

Cressy Snr · Post by **Cressy Snr** » Sat Apr 18, 2015 12:32 pm

Enjoy the bowls Phil

pre65 · Post by **pre65** » Sun Apr 19, 2015 9:47 am

SteveTheShadow wrote:Enjoy the bowls Phil

I did, thanks Steve.

The feedback pentode configuration is up and running. I'm using it on the sky box while Jean catches up with "the voice", just to run it in.

Will try some proper (for me) music later.

Alex Kitic · Post by **Alex Kitic** » Mon Apr 20, 2015 7:47 am

pre65 wrote: And does having a CCS on the driver cock things up feedback wise ?

http://rh-amps.blogspot.co.uk/2014/06/r ... oject.html

Yes it does change things a lot. I do not know whether you have a CCS above the anode of the driver tube, or below the cathode?

If above the anode the ratio between load and feedback is skewed (with ill effect). It's easy to check, just follow my schematics for the driver accurately and check what is better for you.

If below the cathode, the feedback ratio is skewed (much lower feedback). Again, easy to check in practice.

If the amp was great or fantastic or utterly satisfactory, you would have left it as it was. Now that you have started your journey, I guess you should go all the way...

pre65 · Post by **pre65** » Mon Apr 20, 2015 9:14 am

I will have another listen this afternoon when I get back from my physio class.

Unless it sounds better than it did yesterday it's back to triode mode.

I was no way unhappy with triode mode, but one has to try things just in case.

PS the driver valve CCS is above the anode.

Cressy Snr · Post by **Cressy Snr** » Mon Apr 20, 2015 4:41 pm

What is the problem with the sound?

If it is bright, fizzy and boomy, then your feedback is too low.
Try strapping one of the 220K resistors out and see what happens.
It takes a good deal of experimentation to get the sound right, which you might not have the inclination to do.

However if you try just the one 220K and still don't like it, then just go back to triode.

Alex Kitic · Post by **Alex Kitic** » Mon Apr 20, 2015 5:06 pm

pre65 wrote:I will have another listen this afternoon when I get back from my physio class.

Unless it sounds better than it did yesterday it's back to triode mode.

PS the driver valve CCS is above the anode.

I am not aware of the schematics of your amp.

If you are trying to reproduce the RH813 with GK71 (somehow) than you should be using (at least) an ECC81 as driver for the output GK71. In that case, you can adopt the Ra and Rfb of the RH813 schematics.

Otherwise, it's pointless. Pentode without feedback is not going to sound any good, and just putting the Rfb from the RH813 to a totally different driver configuration (like, different driver tube, different anode arrangement, etc.) is not going to lead to good results.

Based on your dissatisfaction with the sound of the GK71 in pentode mode, I am inclined to think that you amp is nowhere near the RH813 schematics...

audio-talk

GK-71 as a pentode.

#31

#32

#33

#34

#35

#36

#37

#38

#39

#40

#41

#42

#43

#44

#45