Hello everybody,

I have fully resored my quad II.
And i have 0.25 - 0.26 VDC on the KT66 grids (leaking capacitors).
I bought russian pio, the sound is good but they are leaking a bit(and
they are quite inexpensive).
Is this a problem in your experience ?

thank you,
Best regards,
Luc D.

wrote:

Hello everybody,

I have fully resored my quad II.
And i have 0.25 - 0.26 VDC on the KT66 grids (leaking capacitors).
I bought russian pio, the sound is good but they are leaking a bit(and
they are quite inexpensive).
Is this a problem in your experience ?

thank you,
Best regards,
Luc D.

I don't like pio caps.
I have seen too many old japanese amps using them where
oil has leaked all over the chassis and the caps became electrically
leaky.
I use polypropylene caps.

0.25v at the grids isn't always caused by leaky caps.
In new tubes the Eg1 is usually a bit negative.
Then as the tube ages, Eg1 goes a bit positive with power tubes.
With signal tubes, like 12AX7, biasing was sometimes achieved with
5M or 10M used for a bias resistor and the grids would be 1 volt negative.

Power tube gride tend to drift positive, even where the coupling cap is
perfect.
The 560 k grid bias R used in Quad II is a very high value, and
can be reduced to 470k without bothers.
The 560k often drift up to 750k with age, making static bias grid current
cause a worse rise in Eg1, since it causes more Ia to flow, and that
causes more
Ig1 to flow.......

Patrick Turner.

I have fully resored my quad II.
And i have 0.25 - 0.26 VDC on the KT66 grids (leaking capacitors).


** No way - that amount of DC on the KT 66s grids is NORMAL and as
designed.

Check the schematic and see how the ( 680K ohms ) grid leak resistors
return to a point that is sitting at that DC voltage.




.............. Phil

May i contradict you ?

I have also tested polypropylen caps and the tension was 0.02 - 0.03
VDC on the grid.
So i imagine that this is due to the pio caps that are leaking.

Anyway from what value of leaking voltage should i replace the caps ?
What is the dangerous eaking voltage for my quad ii ?

Thank you,
BR,
Luc D.

wrote:

May i contradict you ?

I have also tested polypropylen caps and the tension was 0.02 - 0.03
VDC on the grid.
So i imagine that this is due to the pio caps that are leaking.

Anyway from what value of leaking voltage should i replace the caps ?
What is the dangerous eaking voltage for my quad ii ?

If you want peace of mind, replace the oil caps with some
Auricaps, or Wima, both polypropylene, and rated for
630v DC, 0.1 uF, which should be OK for 100 years.
I have never heard a bad sounding polyprop cap.
BTW, the grid bias R stabdard are 680k, not 560k.
Replace them with at least 1/2 watt metal film along with all other old
carbon composition
resistors.

I also remove the 180 ohm Rk and bypass electro.
Then I ground the CT of the tertiary winding of the OPT.
Then I connect 390 ohm or 470 ohm x 5 watt
R between each end of the tertiary winding and cathode, and bypass these
with at least 470 uF
at 50v rating.
This will make sure the bias current in each KT66 stays nearly equal much
better than the original design,
but still allow the amp to work exactly the same way in class A.

If more than 1v dc appears across the 680k bias R, and the caps are not
leaky,
the tube is ageing. At 3 volts, the tube is stuffed.

One way to test for grid current at idle, ie, no signal, is to disconnect

coupling caps from grids, just leave bias R, then measure the DCV across
the 680k
after 10 minutes, then 1/2 an hour, then 1 hour.
If it rises slowly to over a volt in an hour, replace the tubes
because they are ageing, if you want peace of mind.
Old tubes with a lot of +ve DC on their grids will
not often be well matched, and thd may be 4 times higher than a new
matched pair.
It still will sound OK, since the extra thd is usually only 0.1% of 2H.

if a tube conducts too much current with rising DCV on the grids,
the tube may go into thermal runaway
and sit in the amp glowing red hot with 250 mA
of current for sometime before it internally melts down and short
circuits,
or goes open or causes the OPT a lot of damage, and that is really
expensive to fix.
The OPT might be damaged before the mains fuse blows.

Active protection against such traumas is an excellent idea, but it
wasn't possible in 1950.


Patrick Turner.




Thank you,
BR,
Luc D.