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HELP! EL84 amp problem!

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Dear All,
HELP!
I have just finished building a pair of monoblock
power amps based on the Mullard 3-3 circuit ie
Single ended EL84 strapped as a triode driven by
an EF86.
http://www.infomaniak.ch/~bonavolt/el84_3.htm
I have added a slight mod. Directly after the rectifier
is a 10 uf cap followed by a small choke and then the
rest of the circuit (this gives a considerably quieter
psu). The problem is this. One monoblock sounds superb no
problem there. The other however has an extremely high
pitched whistle (almost ultrasonic). It still plays
music but the whistle is there. I have added to both amps
variable feedback (via a 50K pot before the 390pf cap/6.8K).
I have grid stoppers in place. When I alter the feedback pot
one amp is fine and behaves itself. With the whistling amp
the tone drops from ultrasonic to a high pitched tone.
I have tried to track down the problem but failed.
Can anyone help? I hope I have given you enough info.
What is the small value cap for in the feedback loop and
if this is faulty would this cause the problems heard?
or is it something else?
I would really appreciate any advice as I want to get back
listening to my music!
Thanks,
Matt

Follow Ups:

Re: HELP! EL84 amp problem! Shorted Turn 16:17:17 04/02/01 (0)
Matt, if you haven't yet cured the problem, please e-mail me.
Re: HELP! EL84 amp problem! KlausB 09:35:06 04/02/01 (0)
Hi,
I took a short look at the circuit. My suggestions would be:
Check the tubes themselves. I had problems with EF 86 tubes. Did you ground the shielding of the EF86?
Make sure that the grid stopper resistor for the EL84 is soldered directly and as short as possible to the socket.
In my experience galvanic couplings often tend to oscillate. Check that the caps from the screen grid of the EF 86 and from the cathode of the EL84 are properly grounded.
Good luck
KlausB
Re: HELP! EL84 amp problem! Shorted Turn 08:51:22 04/02/01 (1)
Does each monobloc have its own power supply, or is the power supply common to both amplifiers?
If you are using a common power supply, each amplifier must be properly de-coupled on the HT rail. Otherwise, instability is likely to occur.
- Re: HELP! EL84 amp problem! Matt 09:31:58 04/02/01 (0)
  Each monoblock has its own psu.
  It may be a dodgy component but I wouldnt know were to start?
  If the 390pf cap in the feedback loop were defective what would
  happen?
  Thanks,
  Matt
Re: HELP! EL84 amp problem! P�r 07:52:49 04/02/01 (3)
Hi Matt,
Just a point based on own misstakes: Have you tried to "switch" places on the secondary on the earthing amd feedback loop. If they are placed wrong wou get a variable positive feedback.
mvh /P�r
- Re: HELP! EL84 amp problem! Matt 09:29:40 04/02/01 (2)
  The wires from the transformer are colour coded. I double checked
  them. Presumably if I had positive feedback it would be deafened by the result or not necessarily?
  Both amps have their own psu. the whistling amp whisltes even when
  the other is off so no ground probs I hope.
  What would cause such high pitched whistling?
  Any help would be much appreciated.
  Thanks,
  Matt
  - Re: HELP! EL84 amp problem! P�r 09:59:35 04/02/01 (1)
    Hi,
    What I would look for:
    Cables running close to each other (can inductively connect to each other); screenings on cables being not connected to ground (i.e, defective screening); Caps that looks "defective" (especially C3 C6); override the colour code on the opt just to rule that out (PFB); reconnect the EL84 as pentode as per schematic (you said it was trioded?).
    If the amp had'nt been direct coupled from EF86 to EL84 I would have told you to triode couple the EL86 (it sounds great to me trioded) but this cant be done without calcs in this amp. Anyway it is quite a "messy" scheeme (YMMV) with generally small caps: But now it's built so...
    Further things: Exchange components from amp to amp to see if the fault changes amp. Though tedious this is a last resort type of fault finding. Though for tubes it could be done.
    mvh /P�r [I'll keep an eye on the tread]
    - Re: HELP! EL84 amp problem! P�r 10:01:54 04/02/01 (0)
      Hi,
      One further thought: You don't happen to have a scope, or dare to connect a headphone in a little here and there to listen for level of sound? (Connect it via cap and reostat, and take care)
      mvh /P�r

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