KT88 conversion to integrated

[Greg.]

Ha, you are particularly right about the choice of resistor and this argument could be extended to the shunted pot application, because of the difference different resistors bring to the sound and how it is presented. I sometimes wonder if those who have rejected the shunt pot mod have listened to the best options, because in my experience, using basic carbon or metal film resistors doesn’t work (for me) sound wise. Experiment with some alternative or possibly better quality resistors and you might be convinced this is a mod worth going with and, you can retain your motorised pot. The resistors I originally favoured are no longer available, but there are plenty of options out there. you could discover something that lights up your ears.

Having said all that, yes, baseline is a quality stepper and my preference, my Pre 2 is fitted with a stepper made up with an Elma switch using (affordable from RS, Farnell or elsewhere) Welwyn RY55 precision resistors.

[bikerhifinut]

Thanks to Bob O, I received a nice working Motorised RK271 Alps "Blue" pot a couple of days ago
So seeing as I bottled Tonbridge this morning, and if you can see what I can out of the window you'd understand why, its Biblical weather up here and its travelling Southeast. I aint suicidal!
Anyway I used my time to shoehorn the Blue alps in, I was loth to buy a new one as I had an idea it would be a tricky fit due to its extra length over the newer RK168 models.
It did fit, with just enough space above (below?) the Octal Valvebase behind the pot mounting hole. To do this you must rotate the whole Potentiometer assembly by one quarter of a turn (90 degrees) clockwise as you look from the front of the panel. This moves the motor up and out of the way of any dangerous high voltage wiring and exposed solder joints.
So then it was a reatively simple if fiddly job of soldering the signal wiring in.
So its done and so far no dramas and works a treat. As to whether its better than the previous RK168 is open to debate but my measuring tends to show that the Blue alps pot has much better channel matching right round the rotation.
The WD88VA is back in and driving my ESL989s without drama and sounds sweet and clear.
I don't think I was really getting anything extra using a preamplifier. Although the caveat here is that you need sources with a decent output level as the amp is about 450mV sensitivity for full output, and you'll have the volume cranked well round anyway. Which was partly what I wanted to get the usable volume levels in the middle area where the balance is optimum.
On that score alone I wouldn't recommend a shunt mod as it will halve the sensitivity if the usual 47k resistor is used with a 50k pot.
Anyway, the experiment is concluded and I suppose I should get a photo uploaded soon.
I think its a good modification to the integrated amps, I no longer have a KEL84 and I suspect that's a bit tighter for a motorised alps blue, but you might get the shorter RK168 in. Those come in at just over a tenner or so online so its not an expensive mod apart from the £55 ona Hi Fi Collective remote control board wot works with any Philips RC5 remote. I tried a Chinese ebay special but it was a bit flaky for my liking and the HFC one is very reliable and has useful features like wind down on switch on and adjustable mute settings.
I think Matthew could offer this as an option on the Kits but obviously John C or someone would need to give it the once over from an electrical safety viewpoint due to the proximity to an output valve socket. I'm happy with it but I kind of know where the wirings routed.
A.

[Bonky]

Not wishing to infiltrate into this good thread but I'm going to convert my KEL84 PowerAmp to the Integrated version. At present I have a Schiit Saga V1 pre-amp serving it so I wish to try to 'park' that for the time being. I have a Trichord Dino for the phono pre-amp.

Matthew, as usual, has been excellent at getting back to me. As I have a slight issue with hum I'm going to try the simple 'earth-lifting' technique at the same time.

I was tempted by the Alps Pot mod mentioned in this thread but will wait for Matthew and/or Colin to OK it.

BW

Richard.

[bikerhifinut]

Originally Posted by Bonky

Not wishing to infiltrate into this good thread but I'm going to convert my KEL84 PowerAmp to the Integrated version. At present I have a Schiit Saga V1 pre-amp serving it so I wish to try to 'park' that for the time being. I have a Trichord Dino for the phono pre-amp.

Matthew, as usual, has been excellent at getting back to me. As I have a slight issue with hum I'm going to try the simple 'earth-lifting' technique at the same time.

I was tempted by the Alps Pot mod mentioned in this thread but will wait for Matthew and/or Colin to OK it.

BW

Richard.

Are you asking on the basis of wanting to try fitting a remote volume control board and motorised pot Richard?

If that's the case drop me a PM and I'll assist in any way that I can.

Otherwise the process is very simple, you are buying the conversion kit off Matthew, and it will have ample of everything you need. There's no faffing about needed in any way. It should come with a manual Alps "Blue" RK27 pot.

I don't understand the "earth lifting" bit. The KEL84 in common with all WD amplifiers already has its signal earth lifted from mains/chassis earth as part of the design, it's a 10R resistor in parallel with 0.1uF from signal earth to the ground screw. This should be plenty and under NO CIRCUMSTANCES should you disconnect this and have the circuit "floating". It's there for your safety.
I went one further with my homebrew power amp,and am considering the same with the WD one. That's to wire in a pair of back to back rectifier diodes across the earth in the unlikely event of the resistor going open circuit. The diodes provide a nice DC path to earth in a fault situation but because a silicon diode usually doesn't start conducting until (usually) theres 0.6 Volts across it, then any small hum currents from earth loops are effectively blocked. It works on my gear. There's another version of this using a bridge rectifier module but its effectively the same thing.

A.

[bob orbell]

Two diodes back to back, either anode to anode or cathode to cathode makes a clipper diode. Bob

[Bonky]

Thanks Bikerhifinut for the full response...and apologies to you for hi-jacking this thread.

I'm now confused (easily done). You imply that the 'earth-lifting' idea is unwarranted and possibly dangerous, and yet Matthew has written to me thus:

"The earth lift is completely harmless, (10R 3W + 100nF 400V cap in parallel between PCB Earth and Chassis Earth) but if you don’t have any hum issues, I’d recommend you leave things as they are."

He has even posted the cap and resistor to do this. I'll contact him again re the Alps pot.

Thanks again,

R

[bikerhifinut]

Originally Posted by bob orbell

Two diodes back to back, either anode to anode or cathode to cathode makes a clipper diode. Bob

Sorry Bob I wasn't clear about it, it's 2 diodes in parallel back to back i.e. anode to cathode in parallel with each other, it's a known way of "lifting" signal earth from mains/chassis earth whilst maintaining the path to earth.
My bad bob, hope that clarifies it.

Andy.

[bikerhifinut]

Originally Posted by Bonky

Thanks Bikerhifinut for the full response...and apologies to you for hi-jacking this thread.

I'm now confused (easily done). You imply that the 'earth-lifting' idea is unwarranted and possibly dangerous, and yet Matthew has written to me thus:

"The earth lift is completely harmless, (10R 3W + 100nF 400V cap in parallel between PCB Earth and Chassis Earth) but if you don’t have any hum issues, I’d recommend you leave things as they are."

He has even posted the cap and resistor to do this. I'll contact him again re the Alps pot.

Thanks again,

R

No I didn't say its unwarranted, as far as I am aware the Power amplifiers incorporate the 10 ohm plus 0.1uF earth lift as part of the design and therefore you should not need to add an additional one. My KT88 WD amp certainly has it. As Matthew says this provides the safety path to mains earth should any fault condition occur. If for any reason the earth lifting resistor and capacitor have been omitted then its a good idea to reinstall them.
What is dangerous is if you remove these components therefore leaving the circuit "floating" with no path to safety earth.
I hope that clears things up Richard.

You haven't said if its just the straightforward manual Volume control you were after doing or whether you were interested in fitting an aftermarket remote control board to operate a motorised potentiometer.

And you haven't hijacked anything Richard your postings have all been relevant

A.

[Greg.]

Can I add that for full safety, mains earth must go direct to chassis earth and a link from there goes to the earth lift network.

[bikerhifinut]

Absolutely Greg