k5881

[Ianm2]

I was lucky enought to get ahold of 2 of these, so my valve enthusiasm is rekindled.

I plan to do a little fiddling, not too much, but here are my thoughts.

They are the original mk1 with the ef86 triode wired.

I plan to pentode wire one.

Then do the mark 2, perhaps a srpp ecc83, and then mooo! it

maybe even an anode into cathode follower first stage direct coupled.

or parallel the anodes up

I may fiddle with the output putting el34s in and kt88s to try.

and then some capacitor experiments with couplers, I plan to get some OLD sprague caps. a la 1950s amps to try.

and maybe some modern couplers, too.

And maybe obtain the k5881 pse output transformers and pse one or just bridge them both..

or perhaps do a williamson implementation with ecc82s its very straightforward, so I can compare a williamson with a mullard amp.

should be great fun...watch this space if interested.

Its a really simple easy to understand circuit, mullard 5-20 basically, and easy layout so fiddling is invited, I may even try some really low value psu caps vs some big ones to see what sounds better, the joy of these is its all pretty simple to play

[NickG]

With all due respect, why not buy a bit of MDF, and build from scratch on that, far more fun, no hassle getting it to all fit in a box, no holes to drill/punch, and the worlds supply of WAD K5881 remain undimished.

[NealG]

I have to agree with Nick. I don't want to dampen your enthusiasm Ian but just look what happened to Dave D and his 5881 PSE experiments! He ended up scratch building an amp. Youre wasting your time with the PSE version IMHO, been there, done it got the T shirt.

[Richard]

Hi Ian

Saw them myself and they look nice .

My advice would be not to change the circuit unless you have sig gen and scope and can adjust the feedback loop and compensation each time.

They look ideal for EL34s with 470R cathode Rs, and link g3 with k of course. I'd also remove C1 blocking cap if you're using a good source without dc offset. Fit shottkies and try favoured coupling caps.

Worth trying 5751 for splitter and adjusting the anode loads for best balance. I matched the OP 470k gridleaks then used 180k and 187k (180k+6k8 seried).

I'm going to try my EF86s in triode like this next, I don't think the extra gain of srpp is a good thing.

regards
Rich

[NealG]

If you're determined to play the best thing to do is slap a CCs in the tail of the splitter and use identical RL for each half, works wonders.

[Ianm2]

hi its ok, I am not going to do anything radical, just a few minor variations on the input stage really, to see which is best whilst keeping the feedback the same.

With this sort of thing, I have a sneaking hunch the mullard standard circuit is going to be the best.

but it will be interesting to either compare triode ef86 with pentode, and then the wad mk2 which is ecc83srpp (which souded better in mu form to me)

And comparing it with the other.

Just out of interest, Richard, srpp doesn't really alter the gain of the standard triode much, typcially around half the mu of the valve

[andrew ivimey]

I was surprised when I saw you had bid and won them, Ian.

You have seemingly had thousands of amps and so much that is far better than these amps.

There will be fun to be had twiddling but where on earth do you find the time?

[Ianm2]

yes its been a journey Andrew, learning experience, I enjoy learning how they work and all that. I suppose more so than actually listening to them. My heart really is with valves, and some transistor amps.

Actually, from all the amps I have had, and I had one of these in the past, it was one of the most enjoyable sounding amps. it was the srpp version and very neutral. I have a feeling the earlier wad amps were better designed, less hair shirt.

[NickG]

Yes, that why I suggest a breadboard build, its more fun, and none of the hassle with casework. And you can pick what parts you want to use from the start rather than having to work with anothers choices.

[andrew ivimey]

It took a while for me to realise how easy and valid it is to use breadboarding rather than all that horrible, cutting, drilling, filing and blood all over the place.

Some of us have achieved wonderful looking projects when finished, just not me. That requires patience the right tools and a fair degree of skill.

I have a quad of 5881s and I like them but someone in an earlier thread posted on the musical virtues of el34. I only once tried SE with either type of valve and was not that impressed so passed on.

I am finding I don't even want to tweak either of my two main amps and although the WE91 clone isn't finished, I know what it sounds like. It is good.

I have found that my interest in building more amps, because they may be different or better has seriously waned.

So I went on the playing with record decks and yes the Quad 303.