Modding the KEL84

[jonjin]

Hi,

Had some problems with my KEL84. One of the EL84s went cherry when I swapped them and melted one of the resistors before I could turn it off. Anyways, small fix.

But I was wondering if there was any mods I could do to it at the same time. I've almost decided to replace the coupling caps to Jensen PIO and do the approved DC heater supply mod.

Was wondering if there's any merit in changing the resistors? Kiwanes? Tantalums?

JJ

[murray johnson]

I haven't got/seen the KEL84 schematic but typically when an o/p valve goes 'carrot' it tends to take out the cathode resistor and (because it has gone over voltage) any associated bypass capacitor. It might also be worth doing the other channel's cathode components too as they may also have been stressed. I'd try to find some nice Non Inductive power resistors (double the power rating needed) and some 50v bypass caps (if it has them) maybe Elna Cerafine or Silmics.

It would be worth putting in tantalum film resistors and do consider the Jensen or AN copper foil PIO's (if they'll fit!).



EDIT. I've found the circuit & it does use bypass caps on the o/p valve cathodes.

Plenty of other scope for tweaking.

[jonjin]

Bugger, I've got 50V Black Gates as the bypass resistor. Anyway of me checking before binning them away?

Edit - yup, one of the cathode resistors is fried. The rest seem ok. Anyone here with a capacitance meter I could post the black gate to?

JJ

[murray johnson]

The end of the cap may be showing signs of stress ie trying to expand outwards. On the other hand it may be ok. 50v may have beenenough coverage. You can at least check it hasn't shorted internally with a meter.

[jonjin]

Schematic attached.

JJ

[murray johnson]

these sound good & are reasonably priced. good seller too.

http://cgi.ebay.com/ws/eBayISAPI.dll...B:FSEL:US:1123

[jonjin]

murray johnson wrote:

these sound good & are reasonably priced. good seller too.

http://cgi.ebay.com/ws/eBayISAPI.dll...B:FSEL:US:1123

Thanks... I'm thinking that I'll only need to replace one of the bypass caps so I could stretch to a Black Gate. Parts Connexion still do them. Trying to figure out which resistors to replace with Halcos now...

Thinking at least:

R4,5
R10,11
R12,13

Am I right in those?

And for some reason they previous builder had Tantalums for the bypass resistor that got fried which I'll need to replace as well. Either that or actually it'll be cheaper to replace the 4 bypass resistors back to Halcos.



JJ


edit - and some Jensens of course

[murray johnson]

If you are replacing R12/R13 you should also really replace R14/R15.

If the whole pcb needs to be removed to do this, there might be a case for doing R16,17,18,19,20,21,22,23 too!

I do wonder about the LF bandwidth of the coupling between the phase splitter driver and the o/p stage. It has an Fc of about 1.5Hz, ie pretty low. It'd be interesting to see whether others thought that was a bit wide.



EDIT : Actually, looking at a few other EL84 circuits (Mullard 5-10, Leak Stereo 20,) they are run pretty 'wide open'

[oldphrt]

I'm wondering why they bothered with a separate cathode resistor and capacitor for each EL84. Use of a common resistor means no bypass cap is needed at all.

[jonjin]

jonjin wrote:

edit - and some Jensens of course

Actually, probly going for some Ruskis now... PIO for a fraction of cost

Ruski PIO eBay

JJ

[jonjin]

Okay here's the part orders...
Flipping 'eck, resistors and capacitors aren't cheap and I had to go to Part Connexions, hificollective and ePay for them!

1. C15 cathode bypass cap (the old one probably dead) - replaced with the same Black Gate (std)
2. R27 cathode bypass res (the old one fried) - replaced with the same Tantalum (don't know why the previous owner went with Tants here )
3. C6,7,8,9 coupling caps - el cheapo Ruski PIOs
4. R4,5,20,21,22,23 grid stoppers - Allen Brady 2W carbon comp (sourced from eBay )
5. R12,13,14,15, 28, 29, 30, 31 - Halco (UK not the crappy US ones)
6. R10,11 - PRP metal film (there were no Halcos at this value)
7. And the parts for the DC heater mod from Maplins

Hope it'll work not to mention sounding okay...

JJ

[i_should_coco]

jonjin wrote:

jonjin wrote:

edit - and some Jensens of course

Actually, probly going for some Ruskis now... PIO for a fraction of cost

Ruski PIO eBay

JJ

Get the Russian Teflons, rather than PIO. They are excellent.

[jonjin]

Considered this over and over... but heard the Teflons sound harsher (?more truthful). PIOs were warmer but can sometimes be muddier/smeary but I suspect this could be due to long burn in time? Anyways, who knows without trying...

I'm hoping the combination of lean Halcos and warm PIOs will work out well.

JJ

[i_should_coco]

jonjin wrote:

Considered this over and over... but heard the Teflons sound harsher (?more truthful). PIOs were warmer but can sometimes be muddier/smeary but I suspect this could be due to long burn in time? Anyways, who knows without trying...

I'm hoping the combination of lean Halcos and warm PIOs will work out well.

JJ

I wouldn't say the Teflons are harsh. I guess you could accuse them of being a bit lean in the wrong circuit, but they have a clarity and sweetness that is hard to match. At $2 each, you can afford to get them as well and try them!

Removing the metal case improves them no-end.

[jonjin]

i_should_coco wrote:

jonjin wrote:

Considered this over and over... but heard the Teflons sound harsher (?more truthful). PIOs were warmer but can sometimes be muddier/smeary but I suspect this could be due to long burn in time? Anyways, who knows without trying...

I'm hoping the combination of lean Halcos and warm PIOs will work out well.

JJ

I wouldn't say the Teflons are harsh. I guess you could accuse them of being a bit lean in the wrong circuit, but they have a clarity and sweetness that is hard to match. At $2 each, you can afford to get them as well and try them!

Removing the metal case improves them no-end.

I think I opened a can of worms on this (PIO v Teflon bla bla bla). I thought cable wards were never-ending... it's a good thing most people don't know about the never ending combinations of capacitor materials

FWIW, I'm going to have a punt on some Teflons as well. All I need to do now is learn how to dissect them.

JJ

[i_should_coco]

jonjin wrote:

i_should_coco wrote:

jonjin wrote:

Considered this over and over... but heard the Teflons sound harsher (?more truthful). PIOs were warmer but can sometimes be muddier/smeary but I suspect this could be due to long burn in time? Anyways, who knows without trying...

I'm hoping the combination of lean Halcos and warm PIOs will work out well.

JJ

I wouldn't say the Teflons are harsh. I guess you could accuse them of being a bit lean in the wrong circuit, but they have a clarity and sweetness that is hard to match. At $2 each, you can afford to get them as well and try them!

Removing the metal case improves them no-end.

I think I opened a can of worms on this (PIO v Teflon bla bla bla). I thought cable wards were never-ending... it's a good thing most people don't know about the never ending combinations of capacitor materials

FWIW, I'm going to have a punt on some Teflons as well. All I need to do now is learn how to dissect them.

JJ

A pipe cutter (as in one of those rotary ones which you twist, for plumbing) works really well.

[jonjin]

Thanks... but did you reseal them and change the pins/leads?

JJ

[i_should_coco]

jonjin wrote:

Thanks... but did you reseal them and change the pins/leads?

JJ

It was a friend actually - he wraps them in roofing felt type stuff and seals the ends with epoxy or such. They sounded better than theun-messed-about-withones.