KEL84: Output Transformer Upgrade / Shunt Mod

[pete_b]

Hello,

I have a few questions about these KEL84 mods:

-Did you try other output transformers? From guitar amps I know that OTs can make a huge difference. Is there a replacement I should check out?

-Shunt pot mod: Is it o.k. to use a Kiwame? Resistor type seems to make a difference. Which one did you prefer?

-WAD first sold 100K pots with the KEL84, then switched to 50K pots. Which size would you use for use with the shunt mod?

-Pot comparison: Is there a clear winner? Which one would you buy for use with the shunt mod?


Best wishes & many thanks
Pete

[buddam]

Hi Pete

can only help with the shunt mod. I tried Kiwames, which were fine (and I think I still have the approprite value for a 50K pot - I used 39K). But I settled on Rikens because I did them second & couldn't hear a diff. with Kiwames so left them in. But, owing to a mismatch between sources I ended up with Vishay 22K, plus a 30K on CD i/p phono. So I think I've got 39K Rikens and Kiwames if you're interested.

I use a Panasonic Pot which I'm very happy with (c.35 Euros from Benny in Belgium - DIY Paradiso or something like that). Others rate this very highly and regard it + shunt mod as at least the equal of a stepped attenuator.

Send a private message if you want me to look for the resistors

Adrian

[pete_b]

Hi all,

has no one experimented with different OTs in their KEL84?

Another question: Does it make sense to match resistors (e.g. the PI 27K plate and cathode resistors) or to match the resistors in the signal path for the 2 stereo channels?

Or is that overkill?

I am asking because I found someone selling _stereo_ volume pots selected for minimum tolerance between the 2 channels. But what does a selected pot help, when the rest of critical signal path resistors vary quite a bit (most have a tolerance of 1-5% depending on the resistor type / brand used).

Adrian: Many thanks for the info & offer, but I have the correct resistors already.

Peter

[BOONDI]

Hi Pete.

A good quality volume pot such as the alps blue or panasonic pot described by Adrian above will make a BIG difference over the standard 100k cheapo pot that WAD supplied. The shunt mod will also make a difference to the sound. The reason it has such a large effect on sound quality is because it is directly in the signal path. 50K in place of 100K will only mean that the volume control seems to have different sound levels for the same position. The amp should still function as before. As to matched pairs of resistors, then yes it does make a difference to the circuit, but as a cheap way of doing it, get ten metal films and measure with a multimeter, then sellect the two that are closest to each other and the value you need. Also, remember that component values drift over time so your special matched pair may only be matched at the start and may drift very far apart. Always mount them horisontaly as well, because if you mount verticaly you will get a thermal gradient from bottom to top that will create noise. Select metal films on the basis of noise figure at temprature. The lower the noise at the higher the temp the better. Also if a 1/4W part is needed, use a 1/2 or 1W part to reduce thermal noise......

Don't really know about the Tx's tho, but I suspect that top quality output Tx's will cost an arm and a leg. Search the other threads for silver amorphous and you will see what I mean.

Best of luck with the mods......

[acorn]

Hi Pete


I personaly think the most effective mod you can do is to upgrade the .22uF coupling caps for good quality Poly types , you should be able to obtain them from £3 to35 each go for the highest voltage types 630 vdw or higher, and for the output transformers you can try the 8K Hammond from Blue bell audio, if you do go down this road do lets us know I am sure we would all like to know how you got on.

Regards Acorn

[NickG]

I don't know so this is just a opinion, but I would doubt if the Hammonds were any better that the stock WAD TX's

[david counter]

IMO you could spend an awful lot of money upgrading the txs and not achieve any significant improvement for the costs involved, certainly Hammond would be no improvement, you could get silver amorphous ones at about 700 quid but to my mind that's silly on a Kel84, just my opinion, the 84 is a great amp just with the standard range of mods,

[Paul Barker]

No matching resistors in a valve power amp isn't something I conscentrate on but if I have a bunch I do pick close ones, the valves will be now or certainly after use will drift into far greater variation than your resistors and capacitors.

Great amp, Mark Jeffs found some push pull output transformers from America very reasonable price for his Amity amp which he said surpassed the original transformers. I forget the supplier. I had as pair but haven't tried them, they are on loan to someone. Mine only cost about 100 quid the pair.

I would say for that kind of money something of known reputation is worth a try, but I wouldn't go to silly extremes in your case. I don't take my own advise. Have probably spent more on output transformers than anything else maybe 3 or 4 k in total, and am still floundering in the wilderness.

Best thing I can say to you is sell up now and buy a t amp or something.

[NickG]

Paul. were they not the O-Netics from Bud Purvine?

[Paul Barker]

Them's the ones.