New Project Required

[Richard]

Hi David,

I was standing back too but as Andy says no problem, they are 2 separate valves within one envelope. They do have connected heaters and according to the data sheet under some conditions one valve section may have a little less hum but in WD88 they are fed DC heater current so that would not apply. The small valve heaters are also raised 45V which may be the reason they are on a separate heater winding.

The use of a double triode for separate channels in a stereo chassis is quite common. Leak ST20 uses similar, as does WAD 300BPP and WD88, I've had them all with no problems at all.

Hum shouldn't be an issue at all, my WD88 was one of the quietest valve amps I've had. Again as Andy says, the hum may be on the signal line which no amount of heater supply smoothing will fix.

Like you I had FB1+ speakers (different tweeter I think) and a WD88 not so long ago but not at the same time. I have heard K5881 at home (don't recall which version) and found it sweet and easy going and wouldn't be in a rush to change it. The FB1's though just didn't work with the WD 300BPP I had at that time. Despite that amp having quite a midrange presence it was muddled and just wouldn't show itself from the speakers and they had to go. With hindsight it was perhaps a lack of bass control from the amp without feedback, and I could have changed the amp instead, as I've ended up doing now for similar reasons with the Castles.

The synergy between amp and speakers can be a difficult one. I'd expect WD88 to work well with the FB1s and as you live so close to Matthew would ask to borrow a WD88 if possible for a listen with them.

[barnesdavid20]

Hi Andy, again!
Forgot your comment ref, The source and the loudspeakers, I am currently using PMC FB1is and Cyrus CD8se with PSx-r power supply and KLPP1 Pre, also LP12 for records,sensitivity of the speakers is 90db.
The combination works really well and i am more than pleased with the sound, plenty loud enough, and if it was just the music that was important i would stop here, however, having got the bug now I just want to build something else, and don't those bigger valves look great.
I have also just purchased an oscilloscope and signal generator and am looking to use them and experiment on a new project. Want to leave the K5881 setup alone now just to enjoy music.

David

[barnesdavid20]

Hi Richard.
Thank you for the reply, very constructive comments.
Great idea about trying a WD88 with my setup, in fact Matthew did offer to loan me one last year to try, but was still in the throws of upgrading the K5881.
By the way have still got the feedback mod incorporated, think it is a great improvement.
Once again appreciate your help
David

[John Caswell]

Hi all,
Just reading this thread,
Yes I concur with what Andy and Richard say. Hum is a big bugbear but as Andy says DC heating of the preamp valves will not necessarily eliminate it.
As a point here I have just finished the final versions of the 88-50 amp and residual H&N unweighted (ie wideband) measures about -68dB wrt 775mV//8R which equates to about -79db wrt 1watt =2.83V//8R. Weighted it was about -100dB. And all this with AC heaters everywhere in the main amp.
This is all down to... twisted heater wiring, centre tapped heater wiring, careful routing of wires particularly anode and grid wires, relatively short wiring, good layout etc etc all built up from 60 odd years experience.
This was inaudible in the workshop. Surprise, surprise in the house with ear next to L/S I could hear hum.
Removed preamp feed and put 75R phonos into amp input, result silence, and I mean silence and this with 89dB speakers, so it is coming in from Preamp.
I suppose what I am saying is, if you have hum it's no good chucking DC heating and extra capacity at it, you need to think where it may be coming in and try to treat it at source.
You wouldn't credit the number of commercially made amplifiers especially SE types that I get in where the customer says it "hums on my speakers". This is usually due to poor HT smoothing, no hum-bucking on the heater supplies and so on. Usually down to cost.
This is where we can score as home builders by taking a basically a very good design, which the 88VA - KEL84 -6550 are, and "fettling" them to something superb.
Food for thought I hope

John

[barnesdavid20]

Hi John
Very interesting comments.
Like you when i put my ear up to the loudspeaker cone i can hear a slight hum from the K5881 MK2 srpp amp, but it is always slightly more from the RH, hardly anything from the left, This has always bugged me and for months I have been trying to find out the reason, swapped valves around,checked that the wiring is the same on both sides,checked voltages, shorted out the inputs with a small resistor, and the hum never changes,have put DC onto the first two valves without any change, it always remains on the RH.The only clue that i have is when i grip the 7025 pre valve on the RH channel I notice a slight change in the hum level, so I am thinking there is something going on in there.
All heater wires are twisted the same on both sides and both are in phase.
Implemented the feedback mod, raised the star earth above mains chassis.Changed all electrolytic's to M-Cap Polypropylene's, put in schottky diodes, calibrated all resistors, in fact I don't know what else i can do.
The hum still remains the same on the RH side.
The level of hum is small and certainly cannot be heard from the listening position so does not effect the sound quality in anyway, but saying all that it would be nice to find out the reason for the difference, and eliminate it.
If you can point me in a direction that I have not taken yet John I would be very grateful. David

[John Caswell]

Ah, the 7025
I have found these very susceptible to hum pickup, the ones I tried picked up hum/interference from the fluos in my workshop, just shielding them with my hand reduced the pickup considerably. Temporarily try an ECC83/12AX7 or a 5751. A screened base and can may be needed here. You can also try wrapping a couple of layers of cooking foil around the 7025 and earthing it to see what happens.
When I raise the tech earth from chassis I always bypass the R with a 0.1µF capacitor 100-160V rating is fine.
Let us know how you get on

John

[bikerhifinut]

I've always had a slight amount of hum on the LH channel of my 88VA.

BUT:

I have to have my ear 'ole right up to the speaker to hear it, i get silence on the RH side only a bit of hiss all inaudible a couple of feet away.
There's a sight more noise from the Phono3 and indeed all the phono stages i have used valve or Solid state.

So, I decided not to get obsessed with something I don't hear in normal everyday use.

I reckon the cause of the Hum on the LH channel is possibly because I wired the heaters by allowing the 2 sides of the wiring to go around each side of the socket. I had the thing completed before I realised my error. I can't be bothered tbh to fiddle about re routing it. It sounds very very good as it is.

My (rather long winded) point is this; please don't let minutiae give you a complex and just enjoy what is probably a very fine and well fettled piece of gear. All your replacing of capacitors etc with better/different ones probably won't alter the noise floor that much but may have the effect of giving a better sound quality.

Andy.

[barnesdavid20]

Hi Andy
Sounds like yours is exactly as mine only on the left, as you say a couple of feet away and all is dead silent, so I know that it dosn't really make any difference when it comes to listening to music, however it is also a hobby for me and i just like to experiment and try different things as I am sure many of you out there do. I also initially wired my heaters up with loops around the outside of the valve basis like you, which according to the experts was a no no,so i carefully rewired them accordingly and it made no difference at all.
There is obviously a logical reason for the difference between the two channels which I am determined to find out, thats part of the fun with any hobby.Will let you know if I find the answer.

David

[barnesdavid20]

Originally Posted by John Caswell

Ah, the 7025
I have found these very susceptible to hum pickup, the ones I tried picked up hum/interference from the fluos in my workshop, just shielding them with my hand reduced the pickup considerably. Temporarily try an ECC83/12AX7 or a 5751. A screened base and can may be needed here. You can also try wrapping a couple of layers of cooking foil around the 7025 and earthing it to see what happens.
When I raise the tech earth from chassis I always bypass the R with a 0.1µF capacitor 100-160V rating is fine.
Let us know how you get on

John

Hi John
As you suggested, I have fully screened and earthed the 7025 but with no effect on hum, the only thing that I did notice was that when I touched the screen without it being earthed there was a slight increase in hum, with it earthed it did not have any effect at all.
My raised tech earth is connected to mains earth via a 10 ohm resistor with a .1 uf cap across it.
I also forgot to say that I have also elevated the heater voltage by adding a DC voltage got from the cathode of one of the output valves.
There must be an answer somewhere. HELP!!!
Hope you won't give up on me. David

[John Caswell]

Andy, David,
The problem with doing mods, to say reduce hum, is that you need to be able to measure the result, I am afraid that you cannot trust your ears good or bad (in my case) as they may be.
The problem (if it is one) with the 5881 amp is that both sides have different layout and wiring, therefore stray capacities for argument sake are/maybe different, wire may cross differently be routed differently and so on.
The only thing I can suggest is to experiment on the bad?? side to see in you can improve things, ie move capacitors wiring and the like. You are really getting down to the difficult area (95% work done in 5% effort 5% rest takes the remaining 95% effort

John