Help required with WAD KaT88 please!

[Greg.]

I've got an original WAD KaT88 here for repair. With no disrespect to the owner, it's needed a hell of alot of work just to get it functioning. Mostly the problem has been that it has been subject to upgrade as per Richard's original upgrade spec and the PCB has been visited so many times it is now more or less shot away. Reflecting on the upgrades, it includes most of the original stuff and also reduced feedback. KT88's are still in use so R13,14,15,16 are still set at 180R. Having said that, with some hard wiring and rebuilds of upgraded BR1,2,and 3 together with some fettling elsewhere, I've got the amp working and frankly playing music very nicely.

However there is increased (although not bad) hum on the right channel and certainly voltages are not right. In particular I'm very unhappy with Rt Ch V1 g3 cathode at 27vdc when it should be near 10vdc. This is significant as C9 cathode bipass, although rated at 35v is bulging (about to blow). This suggests to me that on switch on, power surge pushes this cap to the limit. Some of the other voltages are also suspect. Please look at the chart below and any suggestions to sort this will be gratefully received. I'm expecting Richard and Dr John to come in here considering the passage of time from when this kit was current.

[Ianm2]

if the pcb is so bad, I would rip it out and use tag boards, with new R's and C's.

I find if an amp is in such a bad way, only a full rebuild done properly will satisfy.

have you tried swapping valves, too?

wait a minute...on this amp its a mix of fixed and kathode bias...that may account for something kathode to grid won't be just the kathode to ground volts.

also check values of bias resistors..and

if you are wondering about surge, put your meter on the suspect bit whilst amp is off and discharged, and then switch on to view effects.


shouldn't a valve be more open circuit b4 stabilisation of bias? I don't know


I would also wear goggles if that cap is looking dubious, if working whilst on, molten flying metal and eyes do not get along as bedmates

oh, of course, if you haven't done, chuck the duff cap in the bin, and replace, the hum may miraculously vanish

finally, perhaps returning feedback components to original may make some improvement

[Greg.]

I should have included that I have tried moving valves and the problem does not travel with them and remains fixed around the V1 area. I've also checked values of all corresponding components and they are within spec. I've also continuitied the curcuit both to earth and power supply to read satisfactorily.

Best wishes,

Greg

[NickG]

on switch on, power surge pushes this cap to the limit

There shouldn't be a surge that side of the valve. Maybe check its matching cathode resistor.

[Richard]

Hi Greg

As Nick says check the 180R cathode resistor is actually measuring that value. If it is and you have 27v then that valve is passing 150mA. It will be glowing red anode and you'd expect hum from a mismatch with it's pair. Otherwise it may be the cathode R has gone high so change R and cap. All the electrolytics may be getting tired now, that amp came out 2000 and gets warm inside doesn't it.

[Black Stuart]

Richard,
this is an amp I bought s/hand in 99 and it has not been used since.

I did a lot of mods in one hit over two years ago now, yes I know but that is why I'm dead set against PCBs, quite simply they are a pain in the rear - taking them out and putting them back in again.

My fatal error was in not realising that the Pi-filter created by John Caswell was not made using a s/hand piece of vero board - why because to break the track he had drilled away the copper conducting strip. Wrongly I had assumed the above.

When I had done all the mods and switched on to test this blew and ironically did away with the offending copper conducting strip on thePi-filter board. It also took with it a cathode bi-pass cap (a beloved Silmic).

So the caps, that were'nt damaged should be fine.

This amp was a nightmare for me - I've modded other pieces of kit with no problem both before and since.

As I said no more PCBs for me - tagboards and point-to-point are if anything far easier to work with and importantly change comps.- all that is required is good photo copied pictures to work from for those without the technical knowledge.

I have my eye on a particular piece of kit building eminating from the Netherlands which uses the methods I prefer and uses easily understood graphics - changing caps or Rs will be a doddle.

This still leaves Greg with this problem. Ian's suggestion was one I would dearly love to have the technical understanding to take up.

For solid state equipment where there are a veritable ream of small components to solder fine ibut for valve amps of any kind - please tagboards and point-to point wiring - and the amp will run cooler without a horizontal PCB which impedes air flow.

[Ianm2]

its not difficult, stuart, worst bit is actually drilling the chassis to mount the boards, you just need a nice drill and very good bits.

then mount on standoffs.

and b4 hand, obviosly draw your tagboard out, most components will go vertically, but a select few horizontal. common 'earths' together, making sure all commons are on one side of the board, takes a little spatial manipulation

do it logically in order, say parts attaching to the grid first, then cathode, then anode, and onto next stage, keep the order however you decide to arrange it, so you can see what's going on.

look at the schematic and tick parts off you have done, and work along the tagboard, and amp schematic, both from left to right

just look at an older wad kit instructions on tag and analyse, and you get to know how, and can perhaps even improve the orientation

most components can be separate tags, but a few will be a little diagonal.

and then wire upto sockets and other points.

it really isn't difficult.

now if you are feeling adventurous, you can do it chinese style and mount direct to the valve bases.

to do that, there are a few spatial orientations for neatness, left to right, back to front, all components straight, and at 90 degrees, some may even be mounted in the top to bottom plane.

its a nitemare for riaa's tho'. I have one in plan, and that's going on tagboard, if I ever get it done.

my prior efforts failed with riaa's the chinese way.

[richardcooper2k]

its always gets interesting when you've got several bits to solder to the same place, i try to solder another one on and one or more of the others spring away. at times like that i wish i was an octopus !

often you can twist the bits together before you solder to stop this happening but that can make it difficult/messy to unsolder. besides it spoils all the fun !

[pre65]

Hi Stuart-i have used the tag boards that WD sell and have bought 4 more for my PT15 amp.

The other night i sat down with the circuit diagram to try and work out which components go where,and after about 6 tries i think i have a reasonable layout.

Its not that difficult once you get started,might be even easier if you have a pcb layout to refer to.


Philip

[Greg.]

Hey, come on you guys, Stuart is in Spain and I'm in the UK with his amp and you support him to have it hardwired Give me a break I have actually got the thing working and sounding good even if the PCB is shot away. A bit of dextrose hardwiring has helped here. I'm down to sorting this one last problem and advice from John, Richard and Nick concurs that I need to explore things around the cathode resistor albeit it tests to correct value in situe and I'm happy it's properly soldered in location. Thanks for all the advise on that. Simply, work has got back in the way and I've not had time to revisit the amp and explore any more for now. Certainly I need to check that I'm geting 180R connected back to chassis earth but at present the board is out the chassis (again) and a new cap needs to be fitted (using cooking caps so as not to spoil Stuart's last Silmic before solution found).

I won't be back on this until next week with work commitments and then Owstfest at the weekend. Be sure I'll keep you posted and if the infernal thing does defeat me, I can visit John at the end of the month for an expert approach. Incidentally, I don't think Stuart properly explained on John's previous involvement on the pie filter bit. What John had done was absolutely correct as you'd expect. Subsequently Stuart misinterpreted his work which led to other problems with the B/rectifiers but this is now happily sorted.

Best wishes,

Greg