VOX AC30CC2 combo amp

P

Phil Allison

Guest
** Hi,

the " VOX AC30CC2" is made in China and designed by Korg of Japan. The two
12 inch speakers say made by Wharfedale - but that too has been a Chinese
brand for a long time now.

The only European parts in the whole thing are the Russian made valves, Korg
must have developed have a low opinion of Chinese ones. No surprise.

Some of the many oddities include using multiple op-amps in parallel for
reverb drive and 500V, TO92 mosfets acting like small triodes for the
tremolo phase-shift oscillator.

Plus - the amp is exquisitely engineered to destroy EL84s and GZ34
rectifiers in short order.

There is a STANDBY switch that lifts the CT on the secondary of the power
tranny - but in my example a 39kohm 5W resistor has been added across switch
by the makers. It's not in the schem I found on-line.

The ONLY thing the resistor does is make the amp hum when in standby mode
!!!!!

Takes about 10 seconds after you move the toggle for an annoying hum to
appear - by this time the main HT has dropped to 20V.

Things that stop the hum:

1. Remove the 39 kohm.

2. Remove the GZ34.

3. Ground any of the grids of the EL84s.

Removing the phase splitter valve has no effect.

Variacing the AC down reduces the hum in proportion.

Barking mad.



.... Phil
 
"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:b5k27kForh8U1@mid.individual.net...
** Hi,

the " VOX AC30CC2" is made in China and designed by Korg of Japan. The two
12 inch speakers say made by Wharfedale - but that too has been a Chinese
brand for a long time now.

The only European parts in the whole thing are the Russian made valves,
Korg must have developed have a low opinion of Chinese ones. No surprise.
Click to expand...

I heard that Sovtek bought all the old Mullard valve manufacturing
equipment, and carried on producing to the original Mullard design specs,
but how true that is, I really wouldn't like to say


Some of the many oddities include using multiple op-amps in parallel for
reverb drive and 500V, TO92 mosfets acting like small triodes for the
tremolo phase-shift oscillator.

Plus - the amp is exquisitely engineered to destroy EL84s and GZ34
rectifiers in short order.

There is a STANDBY switch that lifts the CT on the secondary of the power
tranny - but in my example a 39kohm 5W resistor has been added across
switch by the makers. It's not in the schem I found on-line.
Click to expand...

This is an odd technique, and I'm not sure that it's one that I like. Until
recently, I had never seen it. Then I got given an old American juke box amp
to repair, that used it to 'standby' the amp when the jukebox was idling. I
also then found it a week later on an AC30 reissue like yours. Unless you
realise what's going on with this odd arrangement, you get some mighty
strange voltage readings with the switch off.


The ONLY thing the resistor does is make the amp hum when in standby mode
!!!!!

Takes about 10 seconds after you move the toggle for an annoying hum to
appear - by this time the main HT has dropped to 20V.

Things that stop the hum:

1. Remove the 39 kohm.

2. Remove the GZ34.

3. Ground any of the grids of the EL84s.

Removing the phase splitter valve has no effect.

Variacing the AC down reduces the hum in proportion.

Barking mad.
Click to expand...
Yes

Arfa

... Phil
Click to expand...
 
"Phil Allison" wrote in message news:b5k27kForh8U1@mid.individual.net...


** Hi,

the " VOX AC30CC2" is made in China and designed by Korg of Japan. The two
12 inch speakers say made by Wharfedale - but that too has been a Chinese
brand for a long time now.

The only European parts in the whole thing are the Russian made valves, Korg
must have developed have a low opinion of Chinese ones. No surprise.

Some of the many oddities include using multiple op-amps in parallel for
reverb drive and 500V, TO92 mosfets acting like small triodes for the
tremolo phase-shift oscillator.

Plus - the amp is exquisitely engineered to destroy EL84s and GZ34
rectifiers in short order.

There is a STANDBY switch that lifts the CT on the secondary of the power
tranny - but in my example a 39kohm 5W resistor has been added across switch
by the makers. It's not in the schem I found on-line.

The ONLY thing the resistor does is make the amp hum when in standby mode
!!!!!

Takes about 10 seconds after you move the toggle for an annoying hum to
appear - by this time the main HT has dropped to 20V.

Things that stop the hum:

1. Remove the 39 kohm.

2. Remove the GZ34.

3. Ground any of the grids of the EL84s.

Removing the phase splitter valve has no effect.

Variacing the AC down reduces the hum in proportion.

Barking mad.



.... Phil



Interesting you mention destroying GZ34's, I have changed a few of these
now. (Sovtek originals)
One Sovtek replacement only lasted a few months. I replaced it with a JJ
Tesla one, which seems a far more robust and heavier build, and haven't seen
it back since.


What's the design flaw that compromises output and rectifier valves?
I seem to remember the wiring/PCB around the transformer/rectifier circuit
being a bit shonky.


Cheers,

Gareth.
 
"Gareth Magennis"
"Phil Allison"
the " VOX AC30CC2" is made in China and designed by Korg of Japan. The two
12 inch speakers say made by Wharfedale - but that too has been a Chinese
brand for a long time now.

The only European parts in the whole thing are the Russian made valves,
Korg
must have developed have a low opinion of Chinese ones. No surprise.

Some of the many oddities include using multiple op-amps in parallel for
reverb drive and 500V, TO92 mosfets acting like small triodes for the
tremolo phase-shift oscillator.

Plus - the amp is exquisitely engineered to destroy EL84s and GZ34
rectifiers in short order.

There is a STANDBY switch that lifts the CT on the secondary of the power
tranny - but in my example a 39kohm 5W resistor has been added across
switch
by the makers. It's not in the schem I found on-line.

The ONLY thing the resistor does is make the amp hum when in standby mode
!!!!!

Takes about 10 seconds after you move the toggle for an annoying hum to
appear - by this time the main HT has dropped to 20V.

Things that stop the hum:

1. Remove the 39 kohm.

2. Remove the GZ34.

3. Ground any of the grids of the EL84s.

Removing the phase splitter valve has no effect.

Variacing the AC down reduces the hum in proportion.

Barking mad.


Interesting you mention destroying GZ34's, I have changed a few of these
now. (Sovtek originals)
One Sovtek replacement only lasted a few months. I replaced it with a JJ
Tesla one, which seems a far more robust and heavier build, and haven't
seen it back since.

What's the design flaw that compromises output and rectifier valves?
Click to expand...

** Max points to you for asking good questions.

Not one, but numerous design errors ( never seen in a British VOX amp) are
involved.

1. Destroying the EL84s.

The Korg AC30CC operates EL84s beyond their safe dissipation limits - with
a DC supply of 320V and a cathode current of up to 60mA each at idle, the
poor valves simply cook.

PLUS there is a BIAS switch on the back panel labelled "HOT"and "WARM".
Hot = 50 ohms common cathode resistor, warm = 82ohms - both are too low,
so the valves fry.

Then there is the tremolo circuit which drives all four EL84 grids with a
sub sonic sine wave to modulate gain - a crude idea not seen since the 60s
in a few low powered Fender models. It kills valves.

2. Destroying the GZ34.

The standby switch does that - by creating massive surges at each switch on,
well beyond the safe limit.

British AC30s, with a GZ34, simply never had such a switch.

I have read on VOX forums that the 39kohms is a kind of fuckwit solution to
the problem dreamt up by the Chinks.

Well, it aint.

It just makes the amp hum.

BTW:

There is a simple fix for the EL84s overheating.

I do it routinely, but never tell owners.



.... Phil
 
"Arfa Daily"

I heard that Sovtek bought all the old Mullard valve manufacturing
equipment,
Click to expand...

** Sovtek is the trading name of a US based valve importer - they make
nothing.

The "New Sensor Corporation" own that name and Mullard too.

They could print either on dog's turds if they wished.



..... Phil
 
"Phil Allison" wrote in message news:b5kkmnFspu6U1@mid.individual.net...


"Gareth Magennis"
"Phil Allison"
the " VOX AC30CC2" is made in China and designed by Korg of Japan. The two
12 inch speakers say made by Wharfedale - but that too has been a Chinese
brand for a long time now.

The only European parts in the whole thing are the Russian made valves,
Korg
must have developed have a low opinion of Chinese ones. No surprise.

Some of the many oddities include using multiple op-amps in parallel for
reverb drive and 500V, TO92 mosfets acting like small triodes for the
tremolo phase-shift oscillator.

Plus - the amp is exquisitely engineered to destroy EL84s and GZ34
rectifiers in short order.

There is a STANDBY switch that lifts the CT on the secondary of the power
tranny - but in my example a 39kohm 5W resistor has been added across
switch
by the makers. It's not in the schem I found on-line.

The ONLY thing the resistor does is make the amp hum when in standby mode
!!!!!

Takes about 10 seconds after you move the toggle for an annoying hum to
appear - by this time the main HT has dropped to 20V.

Things that stop the hum:

1. Remove the 39 kohm.

2. Remove the GZ34.

3. Ground any of the grids of the EL84s.

Removing the phase splitter valve has no effect.

Variacing the AC down reduces the hum in proportion.

Barking mad.


Interesting you mention destroying GZ34's, I have changed a few of these
now. (Sovtek originals)
One Sovtek replacement only lasted a few months. I replaced it with a JJ
Tesla one, which seems a far more robust and heavier build, and haven't
seen it back since.

What's the design flaw that compromises output and rectifier valves?
Click to expand...

** Max points to you for asking good questions.

Not one, but numerous design errors ( never seen in a British VOX amp) are
involved.

1. Destroying the EL84s.

The Korg AC30CC operates EL84s beyond their safe dissipation limits - with
a DC supply of 320V and a cathode current of up to 60mA each at idle, the
poor valves simply cook.

PLUS there is a BIAS switch on the back panel labelled "HOT"and "WARM".
Hot = 50 ohms common cathode resistor, warm = 82ohms - both are too low,
so the valves fry.

Then there is the tremolo circuit which drives all four EL84 grids with a
sub sonic sine wave to modulate gain - a crude idea not seen since the 60s
in a few low powered Fender models. It kills valves.

2. Destroying the GZ34.

The standby switch does that - by creating massive surges at each switch on,
well beyond the safe limit.

British AC30s, with a GZ34, simply never had such a switch.

I have read on VOX forums that the 39kohms is a kind of fuckwit solution to
the problem dreamt up by the Chinks.

Well, it aint.

It just makes the amp hum.

BTW:

There is a simple fix for the EL84s overheating.

I do it routinely, but never tell owners.



.... Phil



Thanks.


So what's your fix - adding more cathode resistor, OPT CT to the downstream
side of the choke or adding R in series, adding R to G2 grids, a fan?!



Cheers,

Gareth.
 
"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:b5kll2Ft0eaU1@mid.individual.net...
"Arfa Daily"


I heard that Sovtek bought all the old Mullard valve manufacturing
equipment,


** Sovtek is the trading name of a US based valve importer - they make
nothing.

The "New Sensor Corporation" own that name and Mullard too.

They could print either on dog's turds if they wished.



.... Phil
Click to expand...
Well, that *is* splitting hairs a bit. Even if the name is owned by an
American company, the valves were, if not now, at least previously
manufactured in Russia, and the manufacturer that was doing that, and
sticking the name "Sovtek" on them, may well have been using equipment that
was previously owned by the original Mullard company.

Arfa
 
"Gareth Magennis"
"Phil Allison"

So what's your fix - adding more cathode resistor, OPT CT to the
downstream side of the choke or adding R in series, adding R to G2 grids,
a fan?!
Click to expand...

** First I replace the cathode resistor with one of about 120 ohms, 5 or
10W.

The cathode voltage rises a bit at idle and quite a bit more when the amp is
driven to clipping - resulting in some crossover like distortion and loss
of power output.

Next I add a couple of 5W zeners in series across the 120 ohm - picked so
they barely draw current at idle but clamp the voltage at about 13 or 14V.
So we now have *fixed bias* done at the cathodes.

The result is each EL84 now dissipates a comfortable 9W at idle.

The output power *increases* to over 30W with no sign of crossover.




..... Phil
 
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