Feedback fun with K5881 - Re-visited!

[Richard]

There's still a lot of interest in K5881 which was one of WAD's earlier designs from Oct 1993. The amp was based around the Mullard 5-20 circuit and later changed to the MKII version using a 7025 in SRPP instead of the EF86.

Later AG designs for WAD; Kel34, Kit88, and Kel80, were also based around the 5-20 but developed further to use a clever arrangement for applying global feedback.

This can be done with K5881 as explained below by NealG back in 2006. The original thread now has some corrupted text and no drawings but Neal's post is copied below with a new drawing added for anyone wanting to try it. The original thread is here for anyone wanting a read,

http://www.world-designs.co.uk/forum...read.php?t=410

Originally Posted by NealG

Following on from this thread, I thought I would try AG’s cct on my own 2a3 PP and also K5881MKII to see what effect it had. AG’s cct is clever and designed to work around LF instability by adding more feed back at LF. The amp actually becomes more stable due to LF phase correction, bass is improved as a result. I have to say I agree with this, on both amps I've tried Bass is noticeably improved, as is the midrange for some reason. Both amps seem more tuneful and ‘in control’. K5881 still sounds a little closed in to my ears as do most valve amps with high feedback but the mod has managed to bring out more controlled Bass and midrange detail.

This following should be applicable to any amp which implements loop feedback in the same manner as the K5881, which is based on the Mullard 5-20 cct.

Here’s what you need to do using K5881 MK2 as a reference.

Remove R3, Remove R4, Remove C2, Remove R18

Put R3 in the place of R18.

Put a 1000uf 10v or 16v quality ‘lytic cap in place of R4.

Add a 11R resistor in place of R3 (two 22R in parallel)

Move the feedback wire from the junction of R3/R4 to the junction of R3 and Cathode of V1




For K5881 MK1 the procedure is the same except that R3 becomes 33R not 11R as with K5881 MKII.

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Editted in after following posts in this thread,

<< Also it will be necessary to adjust C10 for best Sqr Wave performance. Change from 220pF to 1.2 ~ 1.5nF. This is best performed wile under test with a 'scope, however, if this cannot be done use a 1.5nF polystyrene cap. For K5881 MK1 use 1.5nF ~ 2nF, this has not been tested. >>

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If you’re thinking of using this in another similar cct work out the PD ratio formed by the existing f/b resistor and the shunt resistor (that is R4 divided by R4 plus R18). When you move the BIAS resistor (R3) to the f/b resistor position you need to recalculate the shunt to get the same ratio as before. If you don’t your f/b level will be different. I aimed for a 10Hz rolloff with the cap value. It can be tweaked a little in value to change Bass response.

[barnesdavid20]

Cheers Richard,
Can"t wait to have a go.
Any idea why it shows C3 and C7 separately,currently I have only got one cap of 22uf 450 volts, plus of course the one on the anode of the 7025, I think this is how it came with the kit. you would have thought they would have just shown one Cap of 47uf 450v.

[Richard]

Hi David,

That drawing was taken from the MKII upgrade supplement article so I imagine it was drawn like that to show how the first stage HT connection was changed. It doesn't have the cathode bypass value on it or the new component numbers either. If you check it against the original K5881 circuit R8 220K has been taken out from between C3 and C7 and, from what you say, it sounds like the MKII kits then used the C3 cap on the anode of the 7025.

[barnesdavid20]

Hi Richard
Ok understand about the two Caps now.
Do you think it is worth trying out the feedback change, it was said in the earlier thread that it increased bass and made the amp sound more musical.
I have tried various changes now as a result of browsing the internet, but apart from upgrading components have not really found anything that really improves the sound, who knows this might be the exception.
Cheers David.

[Richard]

Well there is a definite electrical difference. I've had 2 WAD amps which used that method, both designed by AG, and both sounded very good in the bass department. OTOH my 5-20 monoblocs were also good using the more usual method and other parts in the circuit will affect the result. The 'blocs had large high spec OPTXs which may have already given them good bass. Who knows though, they may have been even better with that connection but I never tried it. Yes I'd try it

[barnesdavid20]

Hi Richard
right will give it a go, have just received the parts in the post today, will let you know the results.
Cheers

[NealG]

Wow that takes me back! Sheer coincidence finding this thread, I was following a link to another and just hit Todays Posts! I still have this mod on my K5881, and although I don't use the amp much these days, it gets an airing now and then and still performs well. I must check as well as I think I junked the SRPP input stage as I found the sensitivity to great.

Anyway, Hi Richard, its been a while, hope you are yours are all well!

[NealG]

There's another tweak you can try, the kit is supplied with equal size anode resistors for the splitter R9/R10 at 150K 2 or 3W. This gives a slight imbalance to the phase outputs at high frequency. To achieve a better balance change R9 for a 147K resistor or buy up a load of 150K resistors and select the highest reading one for R10 and the lowest for R9.

[barnesdavid20]

Hello Neil, glad you found the thread.
Interesting about the unbalance caused by R9/R10, could you explain the reason for this, how did you find it, do you think it will be the same on all K5881s or could it be caused by slight unbalance of valves, I do have a sig gen and scope but not to familiar with there use, or how to check the amount of unbalance, as i would assume it would be impossible to actually hear the difference.
You said you may have removed your SRPP triode input stage and reverted back to the MK1 version, this is very interesting as NK describes this change as being a vast improvement over the original version, obviously you didn't find this so, I have just used a simple 75K/25K resistor network at the front end to solve the sensitivity issue and it seems to work well with the KLPP1.
Will the feedback mod still work with the SRPP set up?
Regards
David

[barnesdavid20]

Hi Neil,
Have just noticed on the K5881 circuit that there are two different anode voltages on the ecc83 phase splitter. 333v and 330v has this got anything to do with the unbalanced situation? why is one different to the other if they should be balanced .

David