Gremlin 2 !! Please Help

[Greg.]

Hi Dave,

That's good news and I hope it stays good for you. if you do have further problems, it might be worth looking closely at the 'Leads' thread in the General section. I did hint when expounding my preferences for Cat5 cables that they are high capacitance and some amps can be thrown into oscillation although I didn't expect that to happen to yours, but if the problem persists, it might be worth experimenting with an alternative speaker cable (cheap mains will do for the test) just to be sure. I doubt this is the problem because of your positive voltages on 0v valve pins, but good to be doubly sure.

Best wishes,

Greg

[David Mccallum]

Hi Greg.

I've tried other types & length's of cable,it made no difference at all. I've been away most of the day so my test had stopped,but i'll continue with it tomorrow.So far so good,though i cant explain the voltage where their should be none(i'm no expert as you know),but the amp sound's dam good when it's not screeching at me.i'll post my results tomorrow night,my fingers are crossed for now.

[David Mccallum]

Hi Folk's

Bad new's im afraid,the amp still whines even with a change of loudspeaker and cables.I tried it with the supplied ecf80s and telefunken versions and it built up to quite a loud screech from the valve and through the speakers with both makes of valve.
I have reconnected my quad 12ls to the amp and fitted two mullard ecf80s for now as they seem to suffer the least from this problem.I'll use the amp untill my new amp turns up(hopefully wednesday),then ill be trying some of the suggestions KennyK and Richard have made,thank's for the help folk, i was beginning to despare at first but im more hopeful about finding the cure to this problem now(i might need some more help though)


(Confuseus say)--man with deathwatch beetle in wooden leg is better off than man with tin leg in lightening storm.

[hooger]

Just a thought, have you checked the values of the resistors R8/R9? In the kit I was sent they were 220R but they were supposed to be 22R. I found that I got a screech but only on one channel and only when the volume was up beyond a quarter. I think this was a problem with the original WD parts list and has now been put right.

[KennyK]

I knew somebody else had mentioned getting the wrong resistor value before, but couldn't remember who, or which ones were wrong...

(I was supplied a 22R instead of a 2k2 for the PSU section, which should have been a 2k2. it lasted a couple of seconds before going O/C)

Anyway, the small dc voltage at the EL84 grids, is this a red herring, or could this provide some clues as to what's causing the whine.

(thinking back to my PhonoII when I built it, I got a 'seagull' whine from one channel for short while - swapping the '83s over cured it.)

[David Mccallum]

Hi Folk's

I was sent the 22R resistors separately,but in time to fit them when i first built the amp.
The reading i've got written down from my last check of R1 was 2.19ohms so no problem there.Thank's for your thought's though it's apreciated,im going to order some part's from farnel on one ,to attempt Richard's suggestion about raising the values of R4/5 to 6K8 or 10K. I might also try lowering the feedback although i cant understand why this might help .
I flunked out of an electronics servicing course in the early nineties,i couldnt afford to complete it due to student housing benefit being scraped.
If im truthful the math's and binary nonsence fried my brain to a certain degree.Still i dont think that there's a living to be made repairing television's and videos these days so it's probably just as well .
Keep the suggestion's comming though,cheers folk.

[NickG]

AFAIK, the voltage was on the ECF grid not the EF84, or am I getting confused. There are a number of reasons why the amp may become unstable. Feedback is one of them, so removing feedback as Richard suggested will eliminate that. The other obvious one, is the pentode oscilating, the easy way to address this, is the grid stopper, and increasing thats a good way forward. Remember to get a carbon not a metal film resistor for that. And make sure the resistor is as close to the valve pin as possible, make the lead from the resistor to the socket as short as you can.

[David Mccallum]

Hi Nick

Sorry if i seem a bit dumb but im going to ask anyway,I've gone and ordered some metal film resistors. They are 6K8 & 10K to fit in R4/5 ,this was before i read your last reply.Can i not use them instead of carbon type's,or do i have to try to navigate through Farnel on one's web site again .

[NickG]

Oh, well, I did mention it in post #13 :-)

Try metal film, at least we can see if it has any effect. after all we are still guessing at the moment. Let me see if I can find a circuit, R4/R5 doesn't mean much to me.

EDIT: OK, I just checked, yes. R4/R5 are the grid stoppers, I wouldn't bother with the 6k8. you won't hear any difference with the 10k. ist just the VHF response of the valve you are trying to control.

[David Mccallum]

I'll try the 10K's when they arrive then,something's got to work.My other half has asked me twice today to switch that dam thing off it's driving me mad!.
Sorry i did'nt understand your mention of using carbon resistors as grid stoppers,i was more than a bit puzzled ,at the time as to what they(gridstoppers) were.
If it was a yacht or power boat i could fix it no prob,but im a bit out of my depth with even fairly simple electronics.I got as far as using the basic functions of an oscilloscope at college.
I do hope to learn more through this process,because i just built the amp exactly as the instructions direct, without really knowing how the thing worked.