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Jolida 502 plus Hammond OPTs.
Jolida 502 plus Hammond OPTs.
I did report recently here on the qualities I found in Jolida 502 OPTs.
There must be many ppl with these amps and a Google search will
now bring up my experience with the Jolida 502 and its repairing.
One of the pair in an amp got some shorted turns and both have
now been replaced with Hammond 1650P OPTs,
which are 6k6 : 4, 8, 16 ohms.
The original OPT was 5k : 4&8 ohms, a load that is too low IMHO,
and the Hammond give a better load match for real hi-fi.
The Hammonds fitted in without drilling any new holes or pinching any
wires.
I used 5mm machine screws and nuts instead of the 4mm used by Jolida.
The Hammond OPT has the same core pattern, 32mm tongue E&I wasteless
like the Jolidas, but the Hammond has a stack about 7mm taller, 51mm,
so there is more iron in the Hammond.
When fitted straight into the Jolida amp, I got squegging RF oscilations
above
low levels even without global NFB.
I fitted 560 ohm series R between OPT screen connections and 6550
screens, and no more
queer oscillations.
But the stability with NFB wasn't any better than the good channel of
the Jolida.
A 0.22uF or 0.47 uF cap cause violent HF oscillations even with an 8 ohm
load connected.
I placed a zobel network between the 12AX7 anode of lower triode in the
SRPP input tube set up.
Correct values for R&C were 3k9 and 270pF.
No more violent HF oscilations with a pure C load and I still get over
50kHz BW into an R load.
To slightly improve it I placed zobels across each 1/2 primary of the
OPT.
The values are 3k3, 5 watt, wire wound, plus 0.0022uF, 2kV rated.
There was no benefit to increasing the C of 20pF comp cap across the 27k
Rfb
between output and V1 cathode.
If this C is increased to say 56pF, RF oscillations begin to appear.
A zobel across the OPT secondary increased the ringing with C loads, so
I didn't use
any zobel at the output.
A also place a 0.22uF and 100k dc blocker HPF before the V1 input triode
grid
to block any DC leaking into the balance gain pot network which causes
scratchiness
and noise.
I don't think the Hammond has such high winding losses as the Chinese
made Jolidas.
The 1650P is a fine subsitute for the Jolida OPTs.
I would say the shunt C and leakage inductance of the Hammond is no less
than I found on the Jolida OPT.
Primary inductance is high, and saturation began occurring at the 50
watt
watt level at 22 Hz.
Anode winding resistance was very similar to the Chinese OPT.
I would add that the best performance is with the 4ohm outlet from the
Hammond.
The Hammond does not have a divided bobbin like the Jolida OPT,
but produces similar final outcomes, so why Jolida ever bothered to use
a vertically
divided bobbin is a mystery, because usually the bean counters all say
NO to such things.
But with labout in China at $2 per DAY, who cares?
I would suppose that the config of the Hammond is S-P-S-P-S at best.
I got 75 watts even into 2 ohms, 50W into 4 ohms, and 25W into 12 ohms,
and the amount of class A PO is high if speakers are above 6 ohms.
The only bother is the hum at the output.
The Jolida PSU has a pair of 470uF caps in series to make 235uF and
these are charged by silicon diodes
from a bridge, giving +518Vdc with Ia at 44mA in each 6550 output tube.
There is 1.8Vrms of saw toothed ripple voltage at the cap, and this is
where the
CT is connected.
If the idle Ia dc of pairs of 6550 are equalised by careful bias
adjustment, I get about 3mV of 100Hz saw toothed hum in the 8 ohm
outlet,
with slightly less at the 4 ohms outlet.
With 34mA and 42mA in tubes of each channel, the hum could be nulled
almost all away.
This isn't right though to have a dc net offset, so I will have to add
more B+ filtering to the
Jolida PSU, using 2 x 470uF in series plus a 250 ohm feed resistance to
make a CRC.
The 250 ohms will be a quad of 10 watt x 1k in parallel, because the 250
ohms must dissipate 9 watts.
But noise at the output will fall by 30 times, and enough to get the
ripple noise
lower than other noise and all < 0.5mV .
B+ will be a lower 470V, allowing slightly higher Ia, and hence greater
% of class A1 PO.
My client is quite happy with a 25W triode AB1 amp I have lent him while
I fix his Jolida.
He doesn't need the instant 80 watt power capability.
In fact I had mains voltage = 253vrms all day to day here where I should
have 240Vrms,
so lowering the B+ from 518V to 470V is only doing the right thing.
The power tranny hums somewhat with 252V supply, but little can be done.
Its probably been designed for 220/110V mains, with the idea that
"she'll be right" when its used in Oz. There are no adjustment taps and
input tube
heaters needed a 0.68 ohm R in series to reduce the heater voltage
nearer to 12.6V.
Running bean counter designed power transformers at 10% above their
design voltage
does usually make them hum as the iron gets pushed into partial
saturation.
But for 9 years PT has not overheated, so I am leaving it there.
Maybe 22 ohms in series with the rectifier diodes might reduce the
PT mechanical rectifier noise, but it doesn't always work.
Patrick Turner.
I did report recently here on the qualities I found in Jolida 502 OPTs.
There must be many ppl with these amps and a Google search will
now bring up my experience with the Jolida 502 and its repairing.
One of the pair in an amp got some shorted turns and both have
now been replaced with Hammond 1650P OPTs,
which are 6k6 : 4, 8, 16 ohms.
The original OPT was 5k : 4&8 ohms, a load that is too low IMHO,
and the Hammond give a better load match for real hi-fi.
The Hammonds fitted in without drilling any new holes or pinching any
wires.
I used 5mm machine screws and nuts instead of the 4mm used by Jolida.
The Hammond OPT has the same core pattern, 32mm tongue E&I wasteless
like the Jolidas, but the Hammond has a stack about 7mm taller, 51mm,
so there is more iron in the Hammond.
When fitted straight into the Jolida amp, I got squegging RF oscilations
above
low levels even without global NFB.
I fitted 560 ohm series R between OPT screen connections and 6550
screens, and no more
queer oscillations.
But the stability with NFB wasn't any better than the good channel of
the Jolida.
A 0.22uF or 0.47 uF cap cause violent HF oscillations even with an 8 ohm
load connected.
I placed a zobel network between the 12AX7 anode of lower triode in the
SRPP input tube set up.
Correct values for R&C were 3k9 and 270pF.
No more violent HF oscilations with a pure C load and I still get over
50kHz BW into an R load.
To slightly improve it I placed zobels across each 1/2 primary of the
OPT.
The values are 3k3, 5 watt, wire wound, plus 0.0022uF, 2kV rated.
There was no benefit to increasing the C of 20pF comp cap across the 27k
Rfb
between output and V1 cathode.
If this C is increased to say 56pF, RF oscillations begin to appear.
A zobel across the OPT secondary increased the ringing with C loads, so
I didn't use
any zobel at the output.
A also place a 0.22uF and 100k dc blocker HPF before the V1 input triode
grid
to block any DC leaking into the balance gain pot network which causes
scratchiness
and noise.
I don't think the Hammond has such high winding losses as the Chinese
made Jolidas.
The 1650P is a fine subsitute for the Jolida OPTs.
I would say the shunt C and leakage inductance of the Hammond is no less
than I found on the Jolida OPT.
Primary inductance is high, and saturation began occurring at the 50
watt
watt level at 22 Hz.
Anode winding resistance was very similar to the Chinese OPT.
I would add that the best performance is with the 4ohm outlet from the
Hammond.
The Hammond does not have a divided bobbin like the Jolida OPT,
but produces similar final outcomes, so why Jolida ever bothered to use
a vertically
divided bobbin is a mystery, because usually the bean counters all say
NO to such things.
But with labout in China at $2 per DAY, who cares?
I would suppose that the config of the Hammond is S-P-S-P-S at best.
I got 75 watts even into 2 ohms, 50W into 4 ohms, and 25W into 12 ohms,
and the amount of class A PO is high if speakers are above 6 ohms.
The only bother is the hum at the output.
The Jolida PSU has a pair of 470uF caps in series to make 235uF and
these are charged by silicon diodes
from a bridge, giving +518Vdc with Ia at 44mA in each 6550 output tube.
There is 1.8Vrms of saw toothed ripple voltage at the cap, and this is
where the
CT is connected.
If the idle Ia dc of pairs of 6550 are equalised by careful bias
adjustment, I get about 3mV of 100Hz saw toothed hum in the 8 ohm
outlet,
with slightly less at the 4 ohms outlet.
With 34mA and 42mA in tubes of each channel, the hum could be nulled
almost all away.
This isn't right though to have a dc net offset, so I will have to add
more B+ filtering to the
Jolida PSU, using 2 x 470uF in series plus a 250 ohm feed resistance to
make a CRC.
The 250 ohms will be a quad of 10 watt x 1k in parallel, because the 250
ohms must dissipate 9 watts.
But noise at the output will fall by 30 times, and enough to get the
ripple noise
lower than other noise and all < 0.5mV .
B+ will be a lower 470V, allowing slightly higher Ia, and hence greater
% of class A1 PO.
My client is quite happy with a 25W triode AB1 amp I have lent him while
I fix his Jolida.
He doesn't need the instant 80 watt power capability.
In fact I had mains voltage = 253vrms all day to day here where I should
have 240Vrms,
so lowering the B+ from 518V to 470V is only doing the right thing.
The power tranny hums somewhat with 252V supply, but little can be done.
Its probably been designed for 220/110V mains, with the idea that
"she'll be right" when its used in Oz. There are no adjustment taps and
input tube
heaters needed a 0.68 ohm R in series to reduce the heater voltage
nearer to 12.6V.
Running bean counter designed power transformers at 10% above their
design voltage
does usually make them hum as the iron gets pushed into partial
saturation.
But for 9 years PT has not overheated, so I am leaving it there.
Maybe 22 ohms in series with the rectifier diodes might reduce the
PT mechanical rectifier noise, but it doesn't always work.
Patrick Turner.
