Hi. Decided to see what all the flea-power noise was all about, and had a
pair of '45s, so I thought I'd build a really simple old -school SE stereo
amp (which turned into a more complex, and not 100% old school amp.
The question I have is what to do about the fil /cath heat. I've read
Steve Bench's stuff on his solutions to the AC fil hum (I can't hear it
through the speakers, but it does show up on the headphones), but it seemed
a bit of an overkill for a simple amp. He's mentioned overfiltration of the
bias supply, and, since my amp's cath. biased, this would mean
overfiltration of the plate supply (which is *really* overfiltered - I had
the parts... But not regulated, just chokes & caps & R's). So, perhaps,
introducing some ripple (BTW, it's 120Hz hum) from the B+? Through a cap?
I've seen some schemes in old GE books where one side of the fils is
bypassed to ground, while the other is bypassed to B+ with low-value caps.
Something like that? Or simply take the B+ from a less-filtered source, and
select the filter cap values? The thing's running on a single 80 rect.,
with an R - CLC(c,c,c,c) [plate B+ & driver B+] RC [first stage B+]. I
want the 80 there, 'cos I have a neat one (the radio gets a boring plain-old
80), and the transformer came from the same place the '45's came from, and
has 2 CT 2.5V windings, along with the 5V for the 80. It's an old trany,
tons of iron & set off ~3/4" off the chassis by the weird bells, no
shielding to speak of . Could some sort of C-couplin' from the B+ & mains
windings be going on?
Anyhow, I don't really understand DC fils on DHC tubes. reduced to
absurdity (narrowly avoiding Latin), I imagine a tube with 6V fil. voltage,
and required bias of, let's say, 1.5V. And plate voltage of (get ready...)
4V. For simplicity's sake, I imagine a center tap on the heater. now, if
we bias that to 1.5V, we get goofiness, 'coz 1 side of the fil will be at
4.5V, while the other's at -2.5V, which means that ... Oh , you get the
drift. Granted, 2.5V is a small part of 50+V of the '45 bias, but ...
wouldn't the curves be different? Would the lifespan of the cathode be
lower? Would non-regulated DC (I kind'o want to keep the amp SS-free) cause
more problems than it solves? (always has been for me on high-gain stages,
though cheap SS regulation rox, if the point is *not* making an old-school
amp. OTOH, it gets to be a slippery slope - how I'd *love* to use ss
regulation in the signal path, use op amps in 1st gain stages of a RIAA
preamp (I'm going to break down and build one next, I'm yet to hear one
which could compete with not-so-high-end SS RIAA preamps... My previous
efforts were ... Junk.)), and soon the toobs become an unnecessary dangle on
an otherwise SS design. /ramble
I know DC vs. AC on DHC toobs is a topic which has been talked to death,
the only thing bein' ... I wasn't interested before, and thought flea-power,
elephant-weight amps were an absurd waste of time & money (I still think
that if you're buying all the components, and only have *1* toob amp, a
Dynaco ST 70 is the most bang for the buck, especially if one is willin' to
do some truly cheap mods), but I'm really impressed with this thing as a
listenable "background" amp, and if I can iron out a few glitches, and build
some more appropriate speakers (it's not the efficiency, but a few hotspots,
which were not bothersome with other amps. The speaks which sound good with
this amp are huge single-driver things, which would leave no room in this
shop / bedroom... Another post on that one...)
Of course, I'd appreciate any input (and will post pics / schematics
soon), but I'm mainly interested in comments from people who had practical
experience in building /designing around DHC toobs, and old timers who've
been around this stuff. I think I've learned more *handy* tricks from old
HAMs & TV repair guys (one of the reasons swap meets are so much better than
buying stuff online - old guys who are willing to chat for hours & pass on
info they thought no one was interested in. And then introduce you to
*other* guys who built gear before you were born, and who'll say stuff like
"Well, I guess that would work [looking at a quickly penciled diagram of
some "clever" solution I came up with], but why not try this? [drawing up a
much simpler & more elegant diagram, which, apparently, is real old-hat]".
Sorry for the ramblin' post, thanks for readin' it. One thing I should'a
mentioned earlier - the hum I'm talking about is coming from the output
tubes themselves, with the grids grounded, and ground loops are not a
possibility. The choke's well within it's rating, as are the caps, though I
do have a nice old mil. 600V 8+8+8 oil cap I can replace the first 'lytic
with...The power transformer is an unknown, though it runsreally cool, seems
to have ~180mA HT, and doesn't "buzz", though it's ~60 yrs old. So
interwinding C couplin' is a possibility.
-dim

Anyhow, I don't really understand DC fils on DHC tubes. reduced to
absurdity (narrowly avoiding Latin), I imagine a tube with 6V fil. voltage,
and required bias of, let's say, 1.5V. And plate voltage of (get ready...)
4V. For simplicity's sake, I imagine a center tap on the heater. now, if
we bias that to 1.5V, we get goofiness, 'coz 1 side of the fil will be at
4.5V, while the other's at -2.5V, which means that ... Oh , you get the
drift.

Some filament tubes were designed to expect one end of the
filament to be fed by the negative filament supply, and the
other end the positive supply. Portable battery radio tubes
like 1T4, 1U5 and such specify which pin is the positive
filament. Also some submini filament tubes also specify
which is the positive end of the filament. Data curves
are usually referenced to the negative filament pin. See my
web page
http://pw2.netcom.com/~wa2ise/radios/tubedac.htm#dhtdac
where I used a filament tube as a DAC "preamp" circuit.
BTW, the filament must be well filtered of hum. You probably
could do better to get the harmonics down than I did.

"shiva" a écrit dans le message de news:
E28Nd.17389$W16.6573@trndny07...
Hi. Decided to see what all the flea-power noise was all about, and had a
pair of '45s, so I thought I'd build a really simple old -school SE stereo
amp (which turned into a more complex, and not 100% old school amp.
The question I have is what to do about the fil /cath heat. I've read
Steve Bench's stuff on his solutions to the AC fil hum (I can't hear it
through the speakers, but it does show up on the headphones), but it
seemed
a bit of an overkill for a simple amp. He's mentioned overfiltration of
the
bias supply, and, since my amp's cath. biased, this would mean
overfiltration of the plate supply (which is *really* overfiltered - I had
the parts... But not regulated, just chokes & caps & R's). So, perhaps,
introducing some ripple (BTW, it's 120Hz hum) from the B+? Through a cap?
I've seen some schemes in old GE books where one side of the fils is
bypassed to ground, while the other is bypassed to B+ with low-value caps.
Something like that? Or simply take the B+ from a less-filtered source,
and
select the filter cap values? The thing's running on a single 80 rect.,
with an R - CLC(c,c,c,c) [plate B+ & driver B+] RC [first stage B+]. I
want the 80 there, 'cos I have a neat one (the radio gets a boring
plain-old
80), and the transformer came from the same place the '45's came from, and
has 2 CT 2.5V windings, along with the 5V for the 80. It's an old trany,
tons of iron & set off ~3/4" off the chassis by the weird bells, no
shielding to speak of . Could some sort of C-couplin' from the B+ & mains
windings be going on?
Anyhow, I don't really understand DC fils on DHC tubes. reduced to
absurdity (narrowly avoiding Latin), I imagine a tube with 6V fil.
voltage,
and required bias of, let's say, 1.5V. And plate voltage of (get ready...)
4V. For simplicity's sake, I imagine a center tap on the heater. now, if
we bias that to 1.5V, we get goofiness, 'coz 1 side of the fil will be at
4.5V, while the other's at -2.5V, which means that ... Oh , you get the
drift. Granted, 2.5V is a small part of 50+V of the '45 bias, but ...
wouldn't the curves be different? Would the lifespan of the cathode be
lower? Would non-regulated DC (I kind'o want to keep the amp SS-free)
cause
more problems than it solves? (always has been for me on high-gain stages,
though cheap SS regulation rox, if the point is *not* making an old-school
amp. OTOH, it gets to be a slippery slope - how I'd *love* to use ss
regulation in the signal path, use op amps in 1st gain stages of a RIAA
preamp (I'm going to break down and build one next, I'm yet to hear one
which could compete with not-so-high-end SS RIAA preamps... My previous
efforts were ... Junk.)), and soon the toobs become an unnecessary dangle
on
an otherwise SS design. /ramble
I know DC vs. AC on DHC toobs is a topic which has been talked to death,
the only thing bein' ... I wasn't interested before, and thought
flea-power,
elephant-weight amps were an absurd waste of time & money (I still think
that if you're buying all the components, and only have *1* toob amp, a
Dynaco ST 70 is the most bang for the buck, especially if one is willin'
to
do some truly cheap mods), but I'm really impressed with this thing as a
listenable "background" amp, and if I can iron out a few glitches, and
build
some more appropriate speakers (it's not the efficiency, but a few
hotspots,
which were not bothersome with other amps. The speaks which sound good
with
this amp are huge single-driver things, which would leave no room in this
shop / bedroom... Another post on that one...)
Of course, I'd appreciate any input (and will post pics / schematics
soon), but I'm mainly interested in comments from people who had practical
experience in building /designing around DHC toobs, and old timers who've
been around this stuff. I think I've learned more *handy* tricks from old
HAMs & TV repair guys (one of the reasons swap meets are so much better
than
buying stuff online - old guys who are willing to chat for hours & pass on
info they thought no one was interested in. And then introduce you to
*other* guys who built gear before you were born, and who'll say stuff
like
"Well, I guess that would work [looking at a quickly penciled diagram of
some "clever" solution I came up with], but why not try this? [drawing up
a
much simpler & more elegant diagram, which, apparently, is real old-hat]".
Sorry for the ramblin' post, thanks for readin' it. One thing I
should'a
mentioned earlier - the hum I'm talking about is coming from the output
tubes themselves, with the grids grounded, and ground loops are not a
possibility. The choke's well within it's rating, as are the caps, though
I
do have a nice old mil. 600V 8+8+8 oil cap I can replace the first 'lytic
with...The power transformer is an unknown, though it runsreally cool,
seems
to have ~180mA HT, and doesn't "buzz", though it's ~60 yrs old. So
interwinding C couplin' is a possibility.
-dim

Hi,
[snip]

I don't know if I was born before you , but anyway my experience is recent.
I've build an SE amp using 6A5 (swapable with 6B4) and I don't succeed to
remove residual noise without rectifying and filtering ( but not reulating)
the filament voltage.

My opinion is that hum was the result of too low inertia of the filament.
The tubes I used need 6V at 1 Amp and have a less "massive" heather than
they 2.5V counterparts like 2A3 or the near equivalent 45.

If possible, try to shut down the HV supply with heather still powered so
you will be able to dertermine the source of hum during the few seconds the
caps discharge.

Or even simpler using 2 x A cells and a serie resistor, that should be
sufficient to feed the heathers for a little while ;)

Then, you may decide !

Yves.

"Yves" wrote in message
...
:
as long as w'ere snippin':
: I don't know if I was born before you , but anyway my experience is recent.
: I've build an SE amp using 6A5 (swapable with 6B4) and I don't succeed to
: remove residual noise without rectifying and filtering ( but not reulating)
: the filament voltage.
:
: My opinion is that hum was the result of too low inertia of the filament.
: The tubes I used need 6V at 1 Amp and have a less "massive" heather than
: they 2.5V counterparts like 2A3 or the near equivalent 45.
:
: If possible, try to shut down the HV supply with heather still powered so
: you will be able to dertermine the source of hum during the few seconds the
: caps discharge.
:
: Or even simpler using 2 x A cells and a serie resistor, that should be
: sufficient to feed the heathers for a little while ;)

Small sealed 6 V lead-acid cell : when fresh, expect about 20 uVtt or less hum &
noise
Rudy;-)

: Then, you may decide !
:
: Yves.
:
:

In article E28Nd.17389$W16.6573@trndny07, "shiva" wrote:
Hi. Decided to see what all the flea-power noise was all about, and had a
pair of '45s, so I thought I'd build a really simple old -school SE stereo
amp (which turned into a more complex, and not 100% old school amp.
The question I have is what to do about the fil /cath heat. I've read
Steve Bench's stuff on his solutions to the AC fil hum (I can't hear it
through the speakers, but it does show up on the headphones), but it seemed
a bit of an overkill for a simple amp. He's mentioned overfiltration of the
bias supply, and, since my amp's cath. biased, this would mean
overfiltration of the plate supply (which is *really* overfiltered - I had
the parts... But not regulated, just chokes & caps & R's). So, perhaps,
introducing some ripple (BTW, it's 120Hz hum) from the B+? Through a cap?
I've seen some schemes in old GE books where one side of the fils is
bypassed to ground, while the other is bypassed to B+ with low-value caps.

If you use DC on the filament, one side will emit more that the other,
possibly affecting the sound quality. I would try getting 40 khz off of a
switching supply and using that. The harmonics of the supply, at least,
will be out of the audible range. Something to think about.

"Jim Coulter" wrote in message
...
: In article E28Nd.17389$W16.6573@trndny07, "shiva" wrote:
: Hi. Decided to see what all the flea-power noise was all about, and had a
: pair of '45s, so I thought I'd build a really simple old -school SE stereo
: amp (which turned into a more complex, and not 100% old school amp.
: The question I have is what to do about the fil /cath heat. I've read
: Steve Bench's stuff on his solutions to the AC fil hum (I can't hear it
: through the speakers, but it does show up on the headphones), but it seemed
: a bit of an overkill for a simple amp. He's mentioned overfiltration of the
: bias supply, and, since my amp's cath. biased, this would mean
: overfiltration of the plate supply (which is *really* overfiltered - I had
: the parts... But not regulated, just chokes & caps & R's). So, perhaps,
: introducing some ripple (BTW, it's 120Hz hum) from the B+? Through a cap?
: I've seen some schemes in old GE books where one side of the fils is
: bypassed to ground, while the other is bypassed to B+ with low-value caps.
:
: If you use DC on the filament, one side will emit more that the other,
: possibly affecting the sound quality. I would try getting 40 khz off of a
: switching supply and using that. The harmonics of the supply, at least,
: will be out of the audible range. Something to think about.
:
I thing Berning uses HF filament heating, does seem like a fine idea
Rudy

My beloved 2A3 SE uses AC and a simple 100R pot to balance filaments. It
hums just a very little bit when connected to 96 dB/W TQWT loaded
loudspeakers, I don't find it questionable in practical terms.
I like to keep it simple. I just provided two small transformers (230V to
2.7V) to feed each filament separately, 'cos it's very easy to have a false
V out at such transforming ratio (a half turn "more" or "less" on the
secondary is enough to get a wide difference) and I wanted to be able to
replace or to modify the minimum amount of stuff. It turned out that the
nominal 2.7V trannies gave precisely 2.5V when loaded by my Fivre 2A3's
filaments.
AC provides better uniformity of emission from both halves of the cathode,
resulting in a more "vivid" sound, IMHO.

The real problem is to have highly efficient and good sounding
loudspeakers...

Ciao

Fabio


"shiva" ha scritto nel messaggio
news:E28Nd.17389$W16.6573@trndny07...
Hi. Decided to see what all the flea-power noise was all about, and had a
pair of '45s, so I thought I'd build a really simple old -school SE stereo
amp (which turned into a more complex, and not 100% old school amp.
The question I have is what to do about the fil /cath heat. I've read
Steve Bench's stuff on his solutions to the AC fil hum (I can't hear it
through the speakers, but it does show up on the headphones), but it
seemed
a bit of an overkill for a simple amp. He's mentioned overfiltration of
the
bias supply, and, since my amp's cath. biased, this would mean
overfiltration of the plate supply (which is *really* overfiltered - I had
the parts... But not regulated, just chokes & caps & R's). So, perhaps,
introducing some ripple (BTW, it's 120Hz hum) from the B+? Through a cap?
I've seen some schemes in old GE books where one side of the fils is
bypassed to ground, while the other is bypassed to B+ with low-value caps.
Something like that? Or simply take the B+ from a less-filtered source,
and
select the filter cap values? The thing's running on a single 80 rect.,
with an R - CLC(c,c,c,c) [plate B+ & driver B+] RC [first stage B+]. I
want the 80 there, 'cos I have a neat one (the radio gets a boring
plain-old
80), and the transformer came from the same place the '45's came from, and
has 2 CT 2.5V windings, along with the 5V for the 80. It's an old trany,
tons of iron & set off ~3/4" off the chassis by the weird bells, no
shielding to speak of . Could some sort of C-couplin' from the B+ & mains
windings be going on?
Anyhow, I don't really understand DC fils on DHC tubes. reduced to
absurdity (narrowly avoiding Latin), I imagine a tube with 6V fil.
voltage,
and required bias of, let's say, 1.5V. And plate voltage of (get ready...)
4V. For simplicity's sake, I imagine a center tap on the heater. now, if
we bias that to 1.5V, we get goofiness, 'coz 1 side of the fil will be at
4.5V, while the other's at -2.5V, which means that ... Oh , you get the
drift. Granted, 2.5V is a small part of 50+V of the '45 bias, but ...
wouldn't the curves be different? Would the lifespan of the cathode be
lower? Would non-regulated DC (I kind'o want to keep the amp SS-free)
cause
more problems than it solves? (always has been for me on high-gain stages,
though cheap SS regulation rox, if the point is *not* making an old-school
amp. OTOH, it gets to be a slippery slope - how I'd *love* to use ss
regulation in the signal path, use op amps in 1st gain stages of a RIAA
preamp (I'm going to break down and build one next, I'm yet to hear one
which could compete with not-so-high-end SS RIAA preamps... My previous
efforts were ... Junk.)), and soon the toobs become an unnecessary dangle
on
an otherwise SS design. /ramble
I know DC vs. AC on DHC toobs is a topic which has been talked to death,
the only thing bein' ... I wasn't interested before, and thought
flea-power,
elephant-weight amps were an absurd waste of time & money (I still think
that if you're buying all the components, and only have *1* toob amp, a
Dynaco ST 70 is the most bang for the buck, especially if one is willin'
to
do some truly cheap mods), but I'm really impressed with this thing as a
listenable "background" amp, and if I can iron out a few glitches, and
build
some more appropriate speakers (it's not the efficiency, but a few
hotspots,
which were not bothersome with other amps. The speaks which sound good
with
this amp are huge single-driver things, which would leave no room in this
shop / bedroom... Another post on that one...)
Of course, I'd appreciate any input (and will post pics / schematics
soon), but I'm mainly interested in comments from people who had practical
experience in building /designing around DHC toobs, and old timers who've
been around this stuff. I think I've learned more *handy* tricks from old
HAMs & TV repair guys (one of the reasons swap meets are so much better
than
buying stuff online - old guys who are willing to chat for hours & pass on
info they thought no one was interested in. And then introduce you to
*other* guys who built gear before you were born, and who'll say stuff
like
"Well, I guess that would work [looking at a quickly penciled diagram of
some "clever" solution I came up with], but why not try this? [drawing up
a
much simpler & more elegant diagram, which, apparently, is real old-hat]".
Sorry for the ramblin' post, thanks for readin' it. One thing I should'a
mentioned earlier - the hum I'm talking about is coming from the output
tubes themselves, with the grids grounded, and ground loops are not a
possibility. The choke's well within it's rating, as are the caps, though
I
do have a nice old mil. 600V 8+8+8 oil cap I can replace the first 'lytic
with...The power transformer is an unknown, though it runsreally cool,
seems
to have ~180mA HT, and doesn't "buzz", though it's ~60 yrs old. So
interwinding C couplin' is a possibility.
-dim