Kat88 is it any good?? with Klipschorns

[pre65]

Perhaps it was at the time,and for the money ?

[Richard]

Originally Posted by Ianm2

I find it strange that noel said it was a gem of an ouput trnasformer, yet rich's findings indicated otherwise.

Hi Ian,

Noel didn't use the word "gem" afaik but do let me know if it's written elsewhere. In the supplement measured performance he said, "Just as important though is the transformer with which the KT88 is paired. Ours offers suberb results."

I said, "The sound was excellent and the standard TXs were good even though they rang a bit on SW test." So I don't find any disagreement

Sure, the £400 Sowters are better but you won't find that quality as standard on any kit amp I know of. The whole WAD Kit88 kit only cost that amount!

Rich

[Grog]

well it looks like i've just bought a Kat88,
it's built.... i guess i'll find out how well when i pick it up.
it's got kimber RCA sockets, circuit breaker instead of mains fuse.
audio note tants, black gates, kimber coupling caps, main rectifier replaced with high speed diodes as per kel34, and DH labs internal wiring.
valves are (new apparently) EH KT88 JAN philips 6au6 and Telefunken ecc82. pearl coolers

is there anything reccommended to do / upgrade, i was planning to check it over for poor construction, and check all the working voltages, then hook it up to the klipschorns.
i hope i cen get the hum low enough not to be a problem with 105dB/w/m

[Global]

I had not jumped into this thread previously but maybe I should have said earlied that unless you are listening in a barn, the combination might well blow the walls out! The low hum level might well be audible at that sensitivity.

I think there is something in the FAQ about regulating the 6AU6 heater supply which needs only a few components - 2 Caps, 2 diodes and a 5v regulator. This should substantially reduce any residual hum. I have a circuit somewhere if you need it.

Mark

[Grog]

I'll see what it's like, but i might well need to go regulated DC on the heaters.
Thanks for the offer of the circuit, I presume it's the standard rectifier then 5v reg with 2 diodes in the gnd leg.
the klipsch also double up as home cinema, so i need a bit of headroom, rooms 12-13' x 25' so a fair size.
i should really measure how much power i'm using at the moment.
i was a bit wary of going for sub 20w as i'd hate to buy the amp and run into headroom problems.
next on the agenda will be a DIY center speaker to kinda match, was thinking a jbl 2441 on a 2350 horn, 200Hz - 20k !

[Richard]

I don't think you'll have (excess) power problems. As you say, it's best to have headroom and you can always turn the thing down!

You won't be able to go reg heaters from any of the included power supply as the heater ciruit is all 6.3v ac so there isn't enough over-voltage even for an ldo reg. Paulo did it many moons ago using a separate 9v tx for the 6au6 heaters. He didn't get lower overall hum though than my efforts using schottkies and extra rc filtering section which are in the faq.

So, certainly there is a good improvement from improving the dc heating, to a point, and the amp can be very quiet indeed. Something I didn't try was dropping a small 10H choke in the ht line after R1 to smooth the small valve supply more but mine was inaudible at the listening seat anyway.

Do you have the circ and build sheets? If not I can email them.

Rich

[Grog]

Hi Rich, i think i have all the documentation from the original build, i think there were some mods and improvements published of which i have no copies.

i was assuming i could use the 6.3v rectify to 8-8.5v ish (LDO diodes) and then use a LDO reg?
if not i guess i could use another 9v tx.

I think the amp already has the upgraded HT bridge (i'll check when i've got the amp home) some extra RC or LC smoothing should cure it i guess.

has anyone managed to convert it to use a valve recitfier? i like the idea of a couple of 5u4 type diodes mounted on top, i guess i'd need a new mains tx or could i wind on a few more turns to the torroid to get the higher voltage needed.
this would give a nice slow start up, and save me having to run a inrush current limiter (those things run realy hot!)

[Richard]

OK I'll post the stuff I have George.

Somehow 8+v dc from 6.3v ac never seems to happen (losses are fixed and the voltage is low so they are higher proportionally than with ht for example). By all means try it, schottkies will be your best bet, 1N5822 or similar. For a reg LM1084 is good. You could use the adjustable version and set it to 6v instead of 6.3v which might help - you'll be looking for every fractional volt. Do scope it as the filaments will smooth the output and you may not realize if it's actually dropping out.

Thinking further, there may be another way that we didn't try back then. Use just 2 diodes in a voltage doubler rect with 2 caps. The downside is drawing twice the current but the supply is from the main 9A heater winding and the 6au6 current totals only 0.6A of this so it should cope ok. Again the 1084 adj with resistors or 5v with 2 diodes would be capable.

Valve rects will mean a complete redesign as the supply is presently ss onto bulk caps which valves wouldn't like. Actually, the HT is too high if anything, so a valve rect dropping it a bit would be fine but will be tricky and will change the character of the amp.

Rich

[pre65]

If the rectification is a diode bridge you could use a valve rectifier (GZ34/37) in place of two diodes,or a couple of 6D22s TV damper diodes and 2 SS diodes,to create a hybrid bridge with slow warm up.

I use two 6D22s in a hybrid bridge on each of my 845 power stage monoblocks.

One advantage of the 6D22s are that they are 6.3V rather than 5V.

http://i111.photobucket.com/albums/n...rsupply001.jpg

[Grog]

hmmm, that sounds tempting, so do you get about 1/2 HT through the SS diodes until the others warm up?
whats the wiring for this? and voltage drop?