ok I Bought it ... what now :D

[Grog]

the Kat88 that is.
just got it home and unpacked it, Bought off Hugo Cass (anyone know him?)
I have a few comments / questions for the experts on here.
looks like it's got the later gridstopper values, the fixed bias is standard, the cathode resistors are 180R, original was 150R (40W dissipation), later value 220R (32W dissipation)
am i right in thinking this puts the dissipation around 37w??

It came with EH KT88's which are (apparently) very nearly new, are these regarded as good, or should i change them to something else, i don't want to go spending money for the sake of it, i'm tempted go live with it as is for a while and see.

the main HT caps are 'BC' i think, i also have in the box some 'jj' looks like jj tesla logo, caps 500uf @500v and 100uf @ 500v, are these regarded as much better than the standard ones? the 500uf's look a little seconhand, they have a few small dents in the can, 5mm x 8mm oval ish. are these going to be ok.

one last question, whats the best plan for checking the working voltages, run the amp on it's side (right hand side down) i'm used to transistor projects where they run fine upside down!!

[Greg.]

To test voltages, it's easier with the amp inverted using items such as a stack of books as a tempory support jig. If you are happy however and in this case, testing with the amp on it's side it's not a problem. This method is not favoured with valves that have long filaments which can sag when heated horizontally leading to destructive contact with the grid.

Best wishes,

Greg

[Grog]

ah, so it's ok to test upside down. I did wonder about the valve heat warming the socket and the valve loosening. i'll give it a go with a pile of books.

[David Mccallum]

Cant see the HT caps having that much bearing on sound quality, id use the ones fitted unless their faulty.

Hav'nt tried many different KT88's but i find EH make a dam fine specimen, clear and powerful sounding (provided their within spec).

Enjoy what should be an excellant amp and dont rush into modifing it before you've gotten well used to it.

(the book-jig method works well for me too)

[Grog]

well i tested it last night, i found 2 jewlers screwdrivers with heatshrink over the ends fittied perfectly into the top plate (the pcb mounting boss holes) and held the amp up very well.
i'll post up test voltages when i get home, but they all seemed within 10%.
i noticed when using the variac that i had a rustling through 1 channel only at about 1/2 mains voltage, it went when at full 230v. ??
the hum is quite noticeable using dummy speakers (2 bare 6" drivers 6ohm 86db sensitivity)
so i've decided to do the heater BR-CRC mod before plugging it into the 103db/w/m klipschorns!
i'll grab the scope this week and check it out for oscillations etc after the mods.

i've also got an in ruch current limiter to go in, can this go between the mains input socket (unfused) and the 2A circuit breaker. this would give me 2 solid points to solder it to, or should it go after the circuit breaker?

[Richard]

Hi George,

Hugo1 is a member here and may be able to answer your questions. I think he used regulator units in it for a while, not sure, lost track The shottky BR and CRC mod is good.

I used standard fixed bias divider and 220R cathode resistors, 180R may be fine depending on valves and local mains, check for red spots in a fully dark room.

Perhaps keep all the valves and other parts in until you get it working and can have a listen, or unless they show themselves to be faulty/worn out. EH are good and you can tune sound with a different set later if you prefer.

Rustling at that voltage may be dirty 6au6 valve pins, test by rocking to see if they are noisy, good connections won't make noise, clean with a glassfibre pencil.

Thermistor can go in either place but away from wiring as it gets hot. Short the inputs as well as loading the outputs when bench testing as mine used to take off without, presumably due to rf. I'm amazed at your self control btw, I'd have plugged the thing in by now and gone from there

Rich

[Grog]

thanks for the feedback Richard,
i'll put the br and crc in and have a listen, i think the hum will be too loud with 104dB/w/m without the mods
it was more noticeable than i'd hoped on 86dB/w/m.
hugo removed the regulators before i got hold of it.
i had a good tap about and wiggle of the valves and nothing seemed to change, might clean up the pins as you suggest, just for good measure.
i might put a pot in the fixed bias, that way i can adjust the overall bias a little, would there be any point in triming each induvidual valves bias with the compound bias arrangement used?
i guess i could go fully fixed bias, which would get rid of the cathode bypass caps, did you ever try this???

[Richard]

Check the earthing carefully as mine didn't hum, one of the quietest amps I've had with the heater mod done and matched valves.

You could use a 50k pot as you say or go four pots. No-one here tried either as I recall. If staying mixed keep enough cathode bias to keep the stage balanced.

The bias isn't really an issue so long as the plates aren't glowing. It was originally intended to be held fully in class A but bias had to be increased to lower plate dis when some of us found the mains a bit high and red plates. Sound-wise the change made little difference. If anything, I though it was a bit better, a little freer and more open sounding. Output power-wise again not much change, maybe a W or two extra from running AB at the highest output, can't remember, but the circuit was certainly good for it as I got 43W across the whole band later with the Sowters.

Rich

[Grog]

ok, i think i'll check the voltages (i've got written down at home) on the op valves, to see if they are still in balance.
maybe i should lower the fixed bias as well as keeping the 180R cathode R's.
i'm itching to get it in the system, but i want to make sure it's right first.
might also put the little maplins 2A circuit breakers on the outputs just in case anything goes wrong, i really don't want to kill the Klipschorns...... or put more than 32W through them when i'm in the room!

[Grog]

ok for the bias voltages i got
V1 g=-9.3 k=12.85 bias=22.15
V3 g=-9.5 k=12.38 bias=21.88

V2 g=-9.3 k=12.85 bias=22.15
V1 g=-8.8 k=12.85 bias=21.65

these seem ok?
everything else was well matched and within 10%
although this was running with the variac set at 230v, i think the mains here is a little higher i'll check next time i run it.


as for the heater mod, with the caps i've got either 3300uf 25V or 4700uf 16V.
these are Phillips/BC with ripple current 2600mA.
also got some panasonic 6800uf 16V 105degree caps, not sure of the ripple curent on these.
am i right in asuming the 16v ones will be fine?