wd88va low output

[Gerome Macri]

hi guys, so i collected my 88 from the repairers and they informed me that because of the close proximity of c7 to r16 the heat from the resister caused the c7 to fail. a design flaw apparently. this caused one of my output valves to blow. they fixed the problem and also swapped out the other capaciters and shifted them slightly to keep them away from the heat. they gave the amp a check over and noticed everything seemed to be as it should exept for the fact that its only out putting 10 watts .Even with new valves. they said that it was always only ever out putting around 10 watts even before the fault. In the time they had the amp nothing was really obvious as to why the output was so low. He did say they could, given the time find the cause but could take quite a while and get costly. So i took the amp as is. Any one had the same experience with this amp or have any idea as to why the loss of power?
Gerome

[david]

Hi Gerome,
First of all i would say how does it sound? Loud? And what speakers are you driving?

Has the sound changed over the tine you had the amp, or does it seem as loud now as ever?

Who built the amp?

I don't know the circuit of this amp but to be so down in power-I assume on both channels-some fairly major wiring or component fault would have had to be made on both channels.

I don't see why working that out should be costly. Correct voltages are a good place to start and if all these are correct, I would start with the feedback wiring and then just check all components, links, connections are correct.

Hope that helps and there are otehrs here who know that circuit well.

[John Caswell]

Hi Gerome
I think it is much more likely that the valve failed destroying the capacitor. I have built a good many of these and the bias capacitors are not really near enough to the bias resistors to cause problems.
Far be it from me to cast aspersions on their capabilities but finding out why the output is so low should be a relatively simple job. For it to be that low, would require the HT amongst other things to be very wrong.
So....... are you competent enough to measure voltages inside the amplifier and report them here.
I realise that you are 12000 mls away in sunny Perth but we should be able to do something about this.

John

[Gerome Macri]

Hi Dave, John. The Gents I sent my amp to once had a very small company making valve gear. They are still making one off or small run units and selling them on ebay and gumtree. They are a father and son team, making single ended stuff. The father who started the business is getting on a bit (76) so he's not as active with coming up with new designs so they scaled things right back. His son, the gent I dealt with but who did not fix my amp is an electrical engineer. He did say all the voltages checked out fine and all the obvious things that would result in output drop were checked and were ok. Now he didnt double check after his father but has alot of confidence he hadn't made any mistakes.
To answer your question David, i built the amp about 5 or 6 years ago. Well half built it. Got to the stage were i had to check voltages and wasnt getting correct readings so was completely dis heartened at that stage and not very confident with my VERY limited abilities with a volt meter, so i sent it to WD to have it finished off professionally. Turned out i missed a link somewhere on the tag boards, but was told i did a great job with the soldering and everything else was fine.
A house move then immigrating back to australia meant I would have been lucky to put 200 hrs on the amp. It has spent most of its life in a box. After hooking it up today it sounds good if not better than before. c7 and c8 on both channels were replaced with ELNA cerafine caps, havent noticed a drop in power. seems normal. Do you think the ol boy could be mistaken?

[John Caswell]

Hi Gerome,
Yes of course he could have made a mistake, we all do many times in our lives.
I suggest that you run the amp and see how it goes.

John

[Richard]

Hi Gerome, agreed with John about the bypass caps, there seems plenty of room on the tag boards to get them spaced away from the cathode resistors.

The low output is a bit tricky as you're at the mercy of your engineer there unless you can check the stages through yourself. As the others have said this is not difficult given the right gear and shouldn't cost you more than an hour to diagnose.

You could try a new Ecc83 as that provides most of the gain, and check pins 1 and 6 for 185VDC and pins 8 and 3 for 1.4VDC. Go through components double checking values for the likes of R5 which should be 1M and not, say, 100K by mistake etc. Also wiring connections to TXs and valve bases. More than that really needs a signal putting through and signal voltages checked at each stage.

Did he make a mistake, possibly, as it would be unusual for both channels to be equally low if the common supply HT voltages are correct. I recently had 2 incidents of low power and both were just on one channel.

[Gerome Macri]

hi guys, so with some help from my good mate and a little courage i went through all the voltage checks, ground checks, basically all the specified checks that were in the build instructions and everything is completely within range. double checked ecc83 anode and cathode voltages aswell Richard. the only thing that didnt match up was the reading from the chassis ground to to the IEC earth pin, which was showing .1 ohm instead of 10 ohms have been listening to the amp most of the day and sounds good. My only gripe is I have to turn volume up to about 10 o'clock to get high volumes. That's through a set of two way floor standers and a set of Yamaha bookshelves. Still seem to be a little under powered. Could just be my imagination though. thats me for a day i think. Have a heap of kids toys to build before i head to bed...joy... thanks for your prompt replies gents, if theres any other advice i'd love to hear it. for now i'll just enjoy my amp and i must say i really enjoyed the craic of poking around inside the 88. Was a great learning curve. All the best for Christmas to you and your families. Wer're expecting 40 degrees so it'll be down the beach for me and mine.
Slan!

[bob orbell]

Hi Gerome, I had a problem with mine some time ago, a simple test you can do is switch on and leave for about ten minits playing or not, them put your hand close to the KT88 valves, they should be VERY HOT, which means they are all working, if two are much cooler this would explane the low power, these amps will work with just one valve in each chanel, give it a try, but remember if all are working they will burn your skin on contact. Good luck. BOB

[Gerome Macri]

i'll check that out Bob. when i get a moment i'll give you guys the exact voltages. Happy Christmas!

[Richard]

Ugh far too hot for me much nicer here at 6C well ok I'm a bit jealous


"the only thing that didnt match up was the reading from the chassis ground to to the IEC earth pin"


Not quite sure what you mean there as the chassis earth post should be solidly bonded to the mains earth IEC pin but if you mean Signal Ground to the IEC earth pin you should find 10R.

Anyway you might find it fun to check the earthing (which may hold a clue to the problem) as you don't need the amp powered up, just an ohm meter, so it's a safe investigation.

Basically, start to think in terms of Mains Earth (rather than earth) and Signal Ground or 0V (rather than earth).

Firstly the amp circuitry should all be isolated from the chassis by insulating all parts using insulated RCA connectors and wiring the black speaker earth posts to Signal Ground not chassis etc etc.

To check, disconnect the 10R earth lift resistor. Then put probes on amp Signal Ground and Chassis Mains Earth post. They should not be connected at all.

If they are still connected hunt down the connection and break it. Then reconnect the 10R and check you now get 10R reading between amp ground and chassis mains earth.