Gremlin 2 !! Please Help

[ian_r]

Davy,

I had some instability problems at first with my KEL84. Hard to track down as problem would disappear when I made measurements with scope around the input stage. The "cure" was to attach a short length (about 5cm) of unterminated screened cable to the input pin on the PCB (with screen grounded). The few pF of additional capacitance sorted it out. I had some advice from others on the old WAD BB - don't know if there's any way to access this? I could have a look about tonight and see if I still have a copy of info received if you want.

Regards,

Ian

PS I may come back to tap into your knowlege of yacht maintenance!

[David Mccallum]

Hi Ian

Thank's for the tip i may well find that it work's on mine too,at least i've got a few things to try now.Any other info from the old site will be welcomed by me and maybe others as i see there's quite a few people posting that their building this amp.
My new amp turned up today,an Icon Audio stereo 40i, i traded an old digital amp in for this exellant sounding device(no whine) more powerful sound but maybe not quite as detailed.The extra bass could be covering it to some extent though.Anyway it means i can fiddle with the KEL84 at leasure and hopefully sort it,upgrade/tweek it and then compare the two.I'ts a bit uneven though as the icon offering is rated at 40wpc linear and 18 in triode mode.
Im a self employed marine engineer to trade so i'll help advise on any related problems that you or any forum member's or admin may have.It may be an idea to pm me though as i doubt it would be of much interest to many others.

[ian_r]

Davy,

Please see attached file that may be of interest. Someone kindly sent me this via the old WAD BB, unfortunately I no longer have the original post. Hope it helps.

Regards,

Ian

[David Mccallum]

Thank's Ian

Something will work im sure,i get ressonance with just the speakers and powerlead connected.That's why i previously thought it was caused by the mains transformer,to make matters worse it seemed to be a cure when i changed it.After a few day's however it was back to it's usual self,Doh.. Maybe i need some of Russ Abbots shorties! to stop the gremmelin's running in the unused phono socket's eh

[David Mccallum]

Just finished modifing my KEL84,i increased R4/5 to 10K & the same for the EL84's.I've carried out the feedback mod & i fitted a 220Mfd cap to C3 (bass mod)& ran the test's.Still getting low voltage at pin 2 of the ECF80's,should be 0v,other voltages seem reasonable.I waited 10 min's and guess what ,it still screeches at me .I've resisted the temptation to total it with a sledge hammer, so far,not for much longer though.It's just as well i bought annother amp,or i'd be really scunnered.I think i might have to take Richard up on his kind offer to fix it as im totally stuck.I tried Ian R's suggestion about soldering short length's or coax to the input pins to no avail too,i hate to admit defeat on what should have been a simple build,but

[NickG]

How annoying. If not Richard, send it here, I think it needs a prod with a scope to see just what is going on.

[David Mccallum]

Thank's your a pal,I've already PM'd Richard so i'll wait for his reply and see what he suggest's.

[KennyK]

David, have you tried removing the feedback to see what effect that has?

[David Mccallum]

Hi Kenny,

I've reduced but not removed feedback,to no avail.I increased the grid stoppers to 10k, no use,so i've sent it to Richard to test it for me.I'll await his finding's and post the answer,hopefully quite soon.

Cheers

[David Mccallum]

Hi folk's,
good news at last,the gremlin is dead!.
I thought it strange that it did'nt misbehave for Richard,but did again on return to me.My last mod/try for a cure,was to order and fit the parts for Richards limiter/suppressor.On the morning the part's turned up however we had a powercut!,being self-employed,i decided to give myself the day off, and set to work with daylight and my battery powered soldering iron.The part's fitted and everything put back as was(with a few mod's) ,i ran the amp for three day's with no whistling noises!.Aparently as the power company switched over the grid to a new local system,a circuit breaker along the road from us blew and had to be replaced.Trumpton's best had their hands full as the traffic lights failed in Oban,and some twenty thousand people lost power.
The good news is the gremlin is dead though and the light's are back on,the amp however need's a good burnin period again as i've replaced several wires and re-soldered a lot of connection's.At present my Iconstereo forty trounces it on sound stage and definition,itwas'nt runin when i bought it(second hand),brand new Electro-harmonix 6CA7 output tubes and JJ ECC83's + Mullard ECC81 drivers.These are clearly upgrades on the chinese valves originally suplied with this amp.I cant believe someone would sell an amp as good as this one obviously is without waiting to hear it (runin),it sounds far better now than when it first arrived.However back to the KEL84,i'll be keeping it as it's dam fine backup should i need it.I would like to modify it further though,maybe i'll replace the internal signal wires with silver coax?.
Im open to sugestions,???.Thank's again to Richard, and all else who helped me with this wierd problem.

Cheers