AC hum from heaters.
Travis Gardiner
have hum, some of them loudly. With all the controls at 0, including
the MV, I get some 60hz(I think) hum, the level is constant, at louder
volumes the hum doesn't get louder. It seems certain tubes when I pop
them in the amp just make it hum. Is it from the heaters? a
NOS-Jan-Phillips 12AT7 I have creates A LOT of hum in the phase inverter
position. Other preamp tubes that have caused it are a Sovtek 12AX7LPS,
and one 12AX7WA I have, but not the other? I have new filter caps, but
maybe I should go for MORE filtering; of course the push-pull nature of
the amp means balanced power amp tubes cancel out any hum, but the
preamp hum irritates me. The main B+ filter is two 100uf/350v Sprague
Atoms in series, plus 3 20uf/500v Atoms for the rest. Or maybe I should
go with a choke; I've heard that helps filter P/S ripple. Opinions?
Travis Gardiner
yet ;)
RonSonic
<travis.gard...@sasktel.net> wrote:
>P/S I'm not sure if its 120hz or 60hz hum, my ears haven't learned that
>yet ;)
Is it higher or lower than your low E string, it's about 83Hz
Ron
Stephen Cowell
"Travis Gardiner" <travis.gard...@sasktel.net> wrote in message
news:vqtdan...@corp.supernews.com...
> I've been trying various preamp tubes, and it seems a few of the ones I
> have hum, some of them loudly. With all the controls at 0, including
> the MV, I get some 60hz(I think) hum, the level is constant, at louder
> volumes the hum doesn't get louder. It seems certain tubes when I pop
> them in the amp just make it hum. Is it from the heaters?
yes, called h-k leakage. The tubes are bad. Use them
for trem oscillators, or chunk 'em.
__
Steve
.
Travis Gardiner
Miles O'Neal
> I've been trying various preamp tubes, and it seems a few of the ones I
> have hum, some of them loudly. With all the controls at 0, including the
> MV, I get some 60hz(I think) hum, the level is constant, at louder volumes
> the hum doesn't get louder. It seems certain tubes when I pop them in the
> amp just make it hum. Is it from the heaters?
Almost certainly.
> a NOS-Jan-Phillips 12AT7 I
> have creates A LOT of hum in the phase inverter position. Other preamp
> tubes that have caused it are a Sovtek 12AX7LPS, and one 12AX7WA I have,
> but not the other? I have new filter caps, but maybe I should go for MORE
> filtering; of course the push-pull nature of the amp means balanced power
> amp tubes cancel out any hum, but the preamp hum irritates me. The main
> B+ filter is two 100uf/350v Sprague Atoms in series, plus 3 20uf/500v
> Atoms for the rest. Or maybe I should go with a choke; I've heard that
> helps filter P/S ripple. Opinions?
Doesn't matter - those relate to B+ - plate voltage DC.
Filament hum is raw AC. If the amp has a hum adjust control,
try that. Otherwise, you can try playing with the filament
grounding or add a DC bias to it, but that's about it.
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Travis Gardiner
> On Sun, 09 Nov 2003 15:46:32 -0600, Travis Gardiner wrote:
>
>
>>I've been trying various preamp tubes, and it seems a few of the ones I
>>have hum, some of them loudly. With all the controls at 0, including the
>>MV, I get some 60hz(I think) hum, the level is constant, at louder volumes
>>the hum doesn't get louder. It seems certain tubes when I pop them in the
>>amp just make it hum. Is it from the heaters?
>
>
> Almost certainly.
>
>
>>a NOS-Jan-Phillips 12AT7 I
>>have creates A LOT of hum in the phase inverter position. Other preamp
>>tubes that have caused it are a Sovtek 12AX7LPS, and one 12AX7WA I have,
>>but not the other? I have new filter caps, but maybe I should go for MORE
>>filtering; of course the push-pull nature of the amp means balanced power
>>amp tubes cancel out any hum, but the preamp hum irritates me. The main
>>B+ filter is two 100uf/350v Sprague Atoms in series, plus 3 20uf/500v
>>Atoms for the rest. Or maybe I should go with a choke; I've heard that
>>helps filter P/S ripple. Opinions?
>
>
> Doesn't matter - those relate to B+ - plate voltage DC.
> Filament hum is raw AC. If the amp has a hum adjust control,
> try that. Otherwise, you can try playing with the filament
> grounding or add a DC bias to it, but that's about it.
>
Hmm, maybe I can replace the two 100ohm resistors to ground with a
couple of better matched ones, I bet they're not that close being old
carbon comp types. Would that help?
Mike Schway
Travis Gardiner <travis.gard...@sasktel.net> wrote:
Travis, I think you and I are the only folks who have had problems with
the Philips NOS-JAN 12AT7s as PIs. It's definitely heater-cathode (H-K)
leakage, and in my case would only start in after a couple days of
service. It's happened to me **6 different times, 6 different amps**
out of a single batch of the Philips 12AT7s. MASSIVE 60Hz hum. These
tubes also had rather high Gm for a 12AT7. Maybe I got a batch of
mismarked 12AX7s? What's your date code? :-)
The 12AX7 LPS isn't a good tube for high H-K voltages. It's not even a
good tube for cathode followers. The 12AX7WAs are fairly inconsistant
in manufacture. Besides, any AX7 is a poor choice for Fender P/Is due
to high H-K potential).
I've had really good luck with the recent ElectroHarmonix 12AT7s as
phase inverters. Try them.
Increased filtering won't help if it's a heater problem. If it's 60Hz
and not 120Hz, it's the heater (or you've lost half your
rectifier...unlikely if it's not shorted).
Re-matching the filament balance resistors won't help with a H-K leakage
problem in the P/I...(in a BF Fender,) the cathode is near 100V.
--Mike
--------------------------------------------------------------------
Mike Schway | [Picture your favorite quote here]
msc...@nas.com |
--------------------------------------------------------------------
Travis Gardiner
>
> Travis, I think you and I are the only folks who have had problems with
> the Philips NOS-JAN 12AT7s as PIs. It's definitely heater-cathode (H-K)
> leakage, and in my case would only start in after a couple days of
> service. It's happened to me **6 different times, 6 different amps**
> out of a single batch of the Philips 12AT7s. MASSIVE 60Hz hum. These
> tubes also had rather high Gm for a 12AT7. Maybe I got a batch of
> mismarked 12AX7s? What's your date code? :-)
I'll check on that
>
> The 12AX7 LPS isn't a good tube for high H-K voltages. It's not even a
> good tube for cathode followers. The 12AX7WAs are fairly inconsistant
> in manufacture. Besides, any AX7 is a poor choice for Fender P/Is due
> to high H-K potential).
>
> I've had really good luck with the recent ElectroHarmonix 12AT7s as
> phase inverters. Try them.
>
I should have qualified this better, but I haven't been using AX7's for
PI, the AX7's I referred to causing the humming were in the V1 position.
The cathodes on the PI in my SF fender are above 115V actually , I do
have an EH 12AT7, I don't believe it hums too much. The quietest AT7 I
have is some "made in Japan" job that I know nothing about, it was in
the amp when I bought it, but the tone is not that great.
Phil Farano
What kind of amp is this?
Who serviced this amp?
Phil Farano
Ampmedic
Las Vegas, NV
me...@ampmedic.com
"Travis Gardiner" <travis.gard...@sasktel.net> wrote in message
news:vqtdan...@corp.supernews.com...
Miles O'Neal
> Hmm, maybe I can replace the two 100ohm resistors to ground with a couple
> of better matched ones, I bet they're not that close being old carbon comp
> types. Would that help?
It could. Unsolder them from each other and
ground, and check their values. If they're
not really close, get either some 1% resistor,
or just get a handful of 5% and get as close
a match as you reasonably can.
Are you sure you have the 12AX7LPS and not
the 12AX7LP? The latter is made for use
with DC on the heater, and hums like crazy
with AC. Of course, it would do that almost
anywhere, but the first gain hole will be
the worst, because the hum gets amplified
with every stage after it's injected.
RoccaforteAmps
the 12AX7LP? >>
Both hum like madd in AC filament circuits.
Im still using old Chinese 12AX7s. FWIW
Travis Gardiner
Miles O'Neal
Not mine. The LPS has always worked as
advertised for me. Then again, I almost
always DC bias filament circuits. But
even stock I haven't had that problem.
You buying these off ebay or something,
Doug? 8^P
Stephen Cowell
"Miles O'Neal" <m...@rru.com> wrote in message
news:pan.2003.11.12....@rru.com...
> On Tue, 11 Nov 2003 16:44:17 +0000, RoccaforteAmps wrote:
>
> > <<Myles:
> > Are you sure you have the 12AX7LPS and not the 12AX7LP? >>
> >
> >
> > Both hum like madd in AC filament circuits. Im still using old Chinese
> > 12AX7s. FWIW
>
> Not mine. The LPS has always worked as
> advertised for me. Then again, I almost
> always DC bias filament circuits. But
> even stock I haven't had that problem.
> You buying these off ebay or something,
> Doug? 8^P
You 'member when Doug and LV kissed and
made up? Start of a beautiful relationship...
__
Steve
.