Hello,

I'm a new subscriber here and this is my first post so be kind...

I'm a lifelong electronics hobbyist and computer programmer by trade
and jazz guitarist by night. I've been soldering since I was 6 but I
could not explain Analog theory if my life depended on it, so I guess
I'm dangerous but I love the hobby.

I have a 1965 VOX AC10 that was not powered up for 32 years, I'm 50 and
I last used this amp when I was 18...

I just posted the schematic to alt.binaries.schematics.electronic to go
with this post.

Last week I replaced all the tubes with new ones and after 32 years
flipped on the power to this puppy (while jumping back for caution). I
know now that this was probably not the best thing to do, but luckily
it went ok. The amp warmed up fine, I plugged in my guitar and it
sounds great. I especially like the clean channel, the vibrato is
channel does not sound as good. I've got about 12 hours of playing
time on it with no problems except...

Well the sound is great but after warm up there is a fair amount of
hiss, enough to annoy me when playing jazz. I would like to at least
get the hiss minimized on the clean channel which consists of only a
triode ECF82 preamp with 5 other parts, feeding the 12AX7 PI, feeding
the dual EL84's. This path looks very simple and elegant without a lot
of parts.

Looking at the schematic and ignoring the vibrato channel for now...
Which resistors and capacitors should I replace to lessen the hiss?

The hiss actually sounds a little like undulating water running, not
real constant like tape hiss. It occurs on both channels so I am
assuming that it is occurring in 12AX7 or EL84 stage. If I turn the
vibrato channel volume to ground the hiss is there when clean volume is
turned up. Conversely if I turn clean volume to ground the identical
hiss is there on vibrato channel. The tone just cuts the hiss but its
still there. The hiss only starts about 30 seconds after warm-up, ie.
I can play hiss-free for 30 seconds, if this means anything.

I'm going to replace parts but wanted to start with the ones that are
most likely to be the cause. If anyone has the time to take a cursory
look at this beautifully symetrical and simple circuit I would really
appreciate it. I will probably use low-noise metal-film resistors and
good caps to get the noise down as much as possible even if it means
changing the "vintage" tone slightly.

Any help in prioritizing my part replacements would be greatly
appreciated. And I hope I can contribute back to the group in the
future.

Thanks,
Rick

First of all, You probably didn't need to re-tube, so if You still have the
old ones around, try to put 'em again in place: some people would pay many $
for original Mullard EL84s.

Noise is due to many causes:

- 50Hz hum from filaments, which is reduced by proper grounding; in Your
Vox amp this is not possible 'cause the same supply feeds the rectifier too
- microphony, which can be reduced selecting tubes and/or mounting the tube
sockets on rubber supports
- electro-magnetic interference, which can be reduced by proper lay-out and
wiring techniques (spiraled AC filament connections, short paths etc.)
- rectifier noise from the HV rail, which can be reduced by a good DC
supply filtering

Components, as long as they're on-spec, usually do not cause noise.
Unfortunately You swapped the only components that generally do not
deteriorate with age, 'cause they're sealed under vacuum. Now I'd say it's
time to replace all the electrolytic capacitors, increasing a bit the DC
filtering. In detail (possible producers suggested)

- First capacitor connected to the rectifier: NEVER increase this one, it
will over-stress the rectifier. I'd use a 16uF/475V (Sprague Atom)
- Second capacitor (305V source): increase a bit. I'd use a 47uF/500V
(Sprague or Illinois), but larger ones (80uF/450V) are not a problem if they
fit inside
- Third one (255V source) and C10B: increase a bit. I'd use a clamp-mount
32+32uF/500V (JJ or F&T)
- C21 (EL84 cathode bypass): 47uF/25V (not to be increased)
- C3 (ECF82 cathode bypass): 22uf/25V (not to be increased)

Other parts:

- Signal capacitors: use polypropylene film caps, rated for 450V minimum,
respecting same values (Sprague Orange Drops, Wima)
- Anode load resistances: replace old carbon composition ones with metal
oxide resistances having the same ohmic value and a power rating of 2W
(Neohm-see RS Components)

Partial filament hum fix:
- ground the 6.3V supply via a small 450V cap, use "LPS" (spiral-wound
heater) type 12AX7s

Final filament hum fix:
- buy a small, dedicated 120/6.3 transformer, disconnect the EZ81 filament
from the "general" supply and feed it separately, then ground the existing
filament supply thru a 10R resistor.

Watch Your fingertips, that's not SS stuff. 400V hurt.

Ciao

Fabio

PS: very nice amp indeed!


"RickH" ha scritto nel messaggio
oups.com...
Hello,

I'm a new subscriber here and this is my first post so be kind...

I'm a lifelong electronics hobbyist and computer programmer by trade
and jazz guitarist by night. I've been soldering since I was 6 but I
could not explain Analog theory if my life depended on it, so I guess
I'm dangerous but I love the hobby.

I have a 1965 VOX AC10 that was not powered up for 32 years, I'm 50 and
I last used this amp when I was 18...

I just posted the schematic to alt.binaries.schematics.electronic to go
with this post.

Last week I replaced all the tubes with new ones and after 32 years
flipped on the power to this puppy (while jumping back for caution). I
know now that this was probably not the best thing to do, but luckily
it went ok. The amp warmed up fine, I plugged in my guitar and it
sounds great. I especially like the clean channel, the vibrato is
channel does not sound as good. I've got about 12 hours of playing
time on it with no problems except...

Well the sound is great but after warm up there is a fair amount of
hiss, enough to annoy me when playing jazz. I would like to at least
get the hiss minimized on the clean channel which consists of only a
triode ECF82 preamp with 5 other parts, feeding the 12AX7 PI, feeding
the dual EL84's. This path looks very simple and elegant without a lot
of parts.

Looking at the schematic and ignoring the vibrato channel for now...
Which resistors and capacitors should I replace to lessen the hiss?

The hiss actually sounds a little like undulating water running, not
real constant like tape hiss. It occurs on both channels so I am
assuming that it is occurring in 12AX7 or EL84 stage. If I turn the
vibrato channel volume to ground the hiss is there when clean volume is
turned up. Conversely if I turn clean volume to ground the identical
hiss is there on vibrato channel. The tone just cuts the hiss but its
still there. The hiss only starts about 30 seconds after warm-up, ie.
I can play hiss-free for 30 seconds, if this means anything.

I'm going to replace parts but wanted to start with the ones that are
most likely to be the cause. If anyone has the time to take a cursory
look at this beautifully symetrical and simple circuit I would really
appreciate it. I will probably use low-noise metal-film resistors and
good caps to get the noise down as much as possible even if it means
changing the "vintage" tone slightly.

Any help in prioritizing my part replacements would be greatly
appreciated. And I hope I can contribute back to the group in the
future.

Thanks,
Rick

Wow, thanks for all this info. I replaced the tubes because I did
remember that the EL84's were blown when I was 18. They were cheap
Radio Shack Korean made tubes. I do have all the old tubes which are
all original and made by some English company that begins with a "B".
Except the ECF82 was a Mullard. So I will try swapping the 2 preamp
tubes. The 12AX7 is a new LPS with spiral filiment, but I'll give that
a try too.

The 60Hz line hum is practically non-issue on this amp, it is quieter
hum-wise than every other tube amp I have. Also it seems to not pick
up EMI hum even through a single-coil hollow body guitar.

It's just the hiss. Does rectifier noise ever show up as a midrange
undulating hiss?

Thanks for the info.

RickH wrote:
Wow, thanks for all this info. I replaced the tubes because I did
remember that the EL84's were blown when I was 18. They were cheap
Radio Shack Korean made tubes. I do have all the old tubes which are
all original and made by some English company that begins with a "B".
Except the ECF82 was a Mullard. So I will try swapping the 2 preamp
tubes. The 12AX7 is a new LPS with spiral filiment, but I'll give that
a try too.

The 60Hz line hum is practically non-issue on this amp, it is quieter
hum-wise than every other tube amp I have. Also it seems to not pick
up EMI hum even through a single-coil hollow body guitar.

It's just the hiss. Does rectifier noise ever show up as a midrange
undulating hiss?

Thanks for the info.



My guess for the cause of the hiss is the old carbon resistors.
They tend to drift,and get noisy over the years.
Changing the coupling caps,and possibly filter caps should help aswell.
(though is seems the filter caps may be in good shape,they didn't
explode when you turned it on again,and there's not much hum.)
I suppose another (maybe remote) possibility could be HF oscilation.

Oops I neglected to mention that the amp did on 2 occasions break into
oscillation, the first day, I attributed this to the caps re-forming
after 32 years. It was not feedback because I know that all too well
by playing hollow bodies for 35 years. No oscillation break occurred
since then but the hiss is almost like a "verge of oscillation" hiss as
opposed to the usual hiss you hear. What would cause oscillation?
Part value drift I assume?

Also the hiss does not start until 30 seconds or so after the amp is
producing sound.
Could that indicate the large 8 watt 130 ohm resistor on the EL84's
heating up and changing value? Dont know if that one is wirewound or
carbon.

The ECF82 (split triode/penthode), penthode section is used as the
vibrato oscillator and the triode is used as the clean preamp. Could
there be crosstalk from the penthode into the triode if the vibrato
oscillator is running wild, even with the vibrato switch off and
vibrato channel grounded? Seems unlikely but I will try out the
Mullard ECF82 tonight. The vibrato, though hissy, seems to be
vibratoing fine though, depth and speed pots work.

I'm thanking everyone for the help, I think ultimately the cause is
several old parts drifting and clanking as they get warmer. I may wind
up just replacing most of the R and C's since the trannies are fine. I
think it's worth it, it does have a tone better than all my other amps
that have no noise. Of course none of my other amps are Class A with a
Rec tube either.

I'd start by replacing R27, and check the value of R25.

"Ptaylor" wrote in message
...
RickH wrote:
Wow, thanks for all this info. I replaced the tubes because I did
remember that the EL84's were blown when I was 18. They were cheap
Radio Shack Korean made tubes. I do have all the old tubes which are
all original and made by some English company that begins with a "B".
Except the ECF82 was a Mullard. So I will try swapping the 2 preamp
tubes. The 12AX7 is a new LPS with spiral filiment, but I'll give that
a try too.

The 60Hz line hum is practically non-issue on this amp, it is quieter
hum-wise than every other tube amp I have. Also it seems to not pick
up EMI hum even through a single-coil hollow body guitar.

It's just the hiss. Does rectifier noise ever show up as a midrange
undulating hiss?

Thanks for the info.



My guess for the cause of the hiss is the old carbon resistors.
They tend to drift,and get noisy over the years.
Changing the coupling caps,and possibly filter caps should help aswell.
(though is seems the filter caps may be in good shape,they didn't explode
when you turned it on again,and there's not much hum.)
I suppose another (maybe remote) possibility could be HF oscilation.

"RickH" ha scritto nel messaggio
oups.com...
Wow, thanks for all this info. I replaced the tubes because I did
remember that the EL84's were blown when I was 18. They were cheap
Radio Shack Korean made tubes. I do have all the old tubes which are
all original and made by some English company that begins with a "B".

I suppose it is Brimar.

Except the ECF82 was a Mullard. So I will try swapping the 2 preamp
tubes. The 12AX7 is a new LPS with spiral filiment, but I'll give that
a try too.

The 60Hz line hum is practically non-issue on this amp, it is quieter
hum-wise than every other tube amp I have. Also it seems to not pick
up EMI hum even through a single-coil hollow body guitar.

It's just the hiss. Does rectifier noise ever show up as a midrange
undulating hiss?

Thanks for the info.

Yes, thats it. I was not at home to see the tubes earlier.

RickH wrote:

Oops I neglected to mention that the amp did on 2 occasions break into
oscillation, the first day, I attributed this to the caps re-forming
after 32 years.

The most sensible thing to do with a tube amp after a 32 year sleep
is to take it to a tech who is a pro who works everyday with these things
to get
all the elctrode voltage checked and all DC voltages, and power supply
ripple voltages,
and all signal voltages, and to see if it can be stimulated to oscilate
some way.
Sometimes increasing the presence setting and HF tone stack boost will make
it oscillate at some HF,
perhaps above what you can hear.

It was not feedback because I know that all too well
by playing hollow bodies for 35 years. No oscillation break occurred
since then but the hiss is almost like a "verge of oscillation" hiss as
opposed to the usual hiss you hear. What would cause oscillation?
Part value drift I assume?

Oscillation is a function of feedback which isn't wanted.

Hiss could be coming from noisy resistors, but its more likely coming from
a noisy input tube
somewhwere. Again a tech with a CRO would find out what's happening fairly
quick.




Also the hiss does not start until 30 seconds or so after the amp is
producing sound.
Could that indicate the large 8 watt 130 ohm resistor on the EL84's
heating up and changing value?

Very unlikely.

Dont know if that one is wirewound or
carbon.

With the amp turned off and cold, the 130ohms can be measured,
like all other resistors used from a circuit point and terminating at a
tube electrode.
When not turned on, the tube is totally not conductive, and connected
resistors
can be measured accurately. Carbon types usually rise in value.

I suggest you replace all R with the same wattage types now used,
or multiples of 1/2 watters to make up what are 2 watt types.

So for 2 watt 50k, use 4 x 1/2 watt 220k in parallel.

New metal film R would be fine.

Some of the caps may have changed value, or become leaky,
especially if they are paper&film caps.
Use 630v rated polyester througout.
Small value mica caps may be OK, they don't change value much or degrade.

Replace all electros as a matter of course.

That should use up two sundays' worth of your life, hey,
but you have about 40 years to go.....

Do one component at a time, so you KNOW what is connected what.

If you were really keen, you'd get a schematic, and figure out how each
section works.
Its only a conglomeration of systems based on simple subsystems built
inside
glass envelopes, and if you start at the input, and work out how the furk
the input stage works
along with a little study about how a basic triode works, then working out
the rest
shouldn't take a computer programmer very long ( he sure has to be good
with logic, eh ).



The ECF82 (split triode/penthode), penthode section is used as the
vibrato oscillator and the triode is used as the clean preamp. Could
there be crosstalk from the penthode into the triode if the vibrato
oscillator is running wild, even with the vibrato switch off and
vibrato channel grounded?

Unlikely. Vibrato tubes are usually phase shift oscillators.
The R values are high, and can drift, and it either oscillates and gives
you vibrato, or it won't
oscillate at all but usually it is a circuit incapable of any HF signal,
since all it can do
is oscillate at LF.

Seems unlikely but I will try out the
Mullard ECF82 tonight. The vibrato, though hissy, seems to be
vibratoing fine though, depth and speed pots work.

Maybe the vibrato tube is noisy, because the tube is gassy.
Maybe the tube current is feeble; too little.



I'm thanking everyone for the help, I think ultimately the cause is
several old parts drifting and clanking as they get warmer. I may wind
up just replacing most of the R and C's since the trannies are fine. I
think it's worth it, it does have a tone better than all my other amps
that have no noise. Of course none of my other amps are Class A with a
Rec tube either.

BTW, the rectifier tube cannot cause hiss in the amp.

But with all the gains turned up high, nearly all muso amps are noisy,
some hum, and some hiss, and some do a lotta both, but very little signal
is needed to cause full power
at the output, so the larger the input signal from your guitar pick up,
the less you have to turn up the master and input stage gain,
ie, if there are master and gain controls, and the less noise you will get.

I have never met anyone who likes completely clean tone from a
muso amp. Hence they like tubes, because tubes distort musically, well,
some of the time maybe, George Benson and Mark Knofler know how to do it,
but the heavy metal freaks don't.
However, rather than deafen everyone with driving the output
stage into over drive to create the hard rock power tone, some musos
prefer to have enough gain in a preamp tube stage so they can over load
that
easily by controlling the guitar level control, then they have a pre-amp
gain control on the amp, and a master control so the power stage gain is
turned down
and the output stage only amplifiers the predistorted sound of the
overloaded
preamp. Nobody is deafened, and the tubes don't mind, and the sound can
have
some real nice gooey warmth, and there won't be much hiss or hum because
the
signal to noise ratio is fine, and there isn't much gravelly tone added by
the
rail hum modulating the signal.

So obviously, using a tiny signal input with gains all set to maximum will
give you a hissy hummy sound.

If you have not got a spring box type of reverb unit, beg, borrow or steal
one,
they will make a signal sound like hot chocolate, or like
what one dude said, like hot cow****.
Maybe you know what I mean, but a dash of reverb goes a long way.

Patrick Turner.

Thanks Patrick,

I think I am leaning towards sending this chassis out to be re-C'd and
re-R'd with all inherently low-noise and over-rated parts by a pro.
(And build a cheap kit amp to satisfy my tube-electro-hobby desires.)
I know this may change the "vintage" tone but IMO there are as many
undesireable aspects of vintage as there are desireable. All I really
want to preserve is the soft clipping (at mezzo forte) and even-ordered
harmonics, and getting more clean headroom instead of less is the side
I would want the rebuild to err on. On stage, an overly clean amp is
much more useable than one that is always in overdrive. I am after an
old Jimmy Raney tone, this vox can do it with the hiss gone and a
little more clean headroom before soft clipping.

Since my regular solo gig is in a small martini bistro/bar and I play
90% jazz on a George Benson GB12, I prefer as clean of tube sound as I
can get, I generally only use the neck pickup. The only effects I
sparingly use are a little reverb and/or chorus. I always use 2 amps
in stereo through a passive A/B/both switch, one gets the clean dry
guitar signal all night, and the other amp plays the reverb/chorus
"wet" signal (this amp is currently SS a Roland JC120 jazz chorus),
which also serves as the acoustic guitar amp, (am using a high-end
tubed mic preamp as a pre-amp for my acoustic piezo). If one amp goes
dead on the gig I still have a backup. I'd like to use this vox for
the base dry signal, because it has the soft clipping that the JC120
cant do. And it sounds a lot better than my Peavy Classic 30 tube amp
I use now which is too scooped in the midrange.

I have always run the volume pots on my guitars at 8 and set the amp
volume to be at forte (for the room) with normal picking. That way I
have 2 more clicks at the guitar to go to mezzo forte and 8 more clicks
towards pianissimo, and I dont have to touch the amp volume again all
night. If for some reason I need to get real loud I turn up the PA
master.

I mic both amps with SM57's and my vocal into a Mackie PA that the bar
owns and the female singer and occasional sax player use. I use CD's,
MIDI (band in a box, Roland sound canvas), and a sampler/sequencer for
bass/drum/piano backing tracks into 3 of the stereo channels on the PA
mixer, so I can easily select where my backing track comes from.

I keep my fake book low at my side in case I forget a chord or melody
(I say if you can read use it). I'm working on an invention I call the
"electronic music stand" for solo acts (basically a touch LCD on a
music stand with a small near-field monitor) which I hope to use with
band in a box, sheet music, and all these computer controlled gizmos
coming out now. Trying to find an affordable touch sensitive LCD panel
and small factor computer is next to impossible however. Many
manufacturers are eliminating controls on devices and making them
computer-controlled, but trying to use that stuff on a stage is
literally impossible. I'm hoping my electronic music stand invention
will cure that and with some large-button custom software will let me
control everything by touch panel.

With this setup I can play every request that comes, as I have about
5000 MIDI sequences in all genres, most my own arrangements, for
backing tracks. If I dont know the tune I can still improvise over the
backing track for a great fake. If a bass player wants to join me one
night then I can just unckeck the bass player on the MIDI sequence.

BTW for some of your questions... I did post the schematic at
alt.binaries.schematics.electr=ADonic

thanks again,
Rick