Hi RATs,

Well, after running for about five years, on an almost daily basis, the
first prototype of my "RA-100" hodge-podge quit working. Blew the AC
fuse, with no audible artifacts or other indication that anything was
awry. The music just stopped.

Cause - one of the main B+ diode strings (2x 1N5408 in series) blew
(shorted)! This in spite (or perhaps because of?) of 2-meg equalizing
resistors across each diode.

Replaced the diodes, did the old "in series with a 100w light bulb"
trick to reform the capacitors just in case, and powered it up. Tweaked
the bias a little (the tubes have obviously aged, based on the quiescent
readings, but still have the same gain and sound great).

For reference, the original article on this amplifier is he
http://www.dogstar.dantimax.dk/tubestuf/ampindex.htm

I'm in process of simplifying this design, and fixing up a few little
things I didn't particularly like (that were worked around in the
prototype - see the Hacks and Tweaks section). The goal is to build an
affordable kit and/or completed product that features the clean
sweetness of this direct-coupled CCS-driven LTP UL circuit, adding
amenities like triode/UL switching, retaining feedback bypass (perhaps
expanding to variable feedback), tone control and bypass. No phono amp
(to be perhaps made available as an external module). Input switching
and optional remote control to be handled either using a external units,
or by means of Mikkel Simonsen's universal IR remonte control modules.

I'm also toning down the plate dissipation power a bit, I'm finding that
I actually prefer a lean "mix" in this particular topology; to my ear
there's a "sweet spot" at about 26 mA per tube, at 620v plate/screen
voltage (UL) for about 16 watts plate dissipation (quite gentle, but
keep in mind that the screens are working pretty hard also in UL mode).
This ends up at a bias of around -80 volts with "typical" 6L6GC's.

Well, anyway. Just a few random thoughts. It's nice to be back.

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+

Fred Nachbaur wrote in news:j5%gc.53124$dg7.52799
@edtnps84:


Fred,

You made no mention of any caps across the diodes. You might look into
that. Do you have a MOV across the line? You might want to look into
that too. I had a TV that blew its HOT nearly everytime it thundered. A
MOV across the line kept the TV going without the need for any repairs for
many years afterward.

r


Hi RATs,

Well, after running for about five years, on an almost daily basis, the
first prototype of my "RA-100" hodge-podge quit working. Blew the AC
fuse, with no audible artifacts or other indication that anything was
awry. The music just stopped.

Cause - one of the main B+ diode strings (2x 1N5408 in series) blew
(shorted)! This in spite (or perhaps because of?) of 2-meg equalizing
resistors across each diode.

Replaced the diodes, did the old "in series with a 100w light bulb"
trick to reform the capacitors just in case, and powered it up. Tweaked
the bias a little (the tubes have obviously aged, based on the quiescent
readings, but still have the same gain and sound great).

For reference, the original article on this amplifier is he
http://www.dogstar.dantimax.dk/tubestuf/ampindex.htm

I'm in process of simplifying this design, and fixing up a few little
things I didn't particularly like (that were worked around in the
prototype - see the Hacks and Tweaks section). The goal is to build an
affordable kit and/or completed product that features the clean
sweetness of this direct-coupled CCS-driven LTP UL circuit, adding
amenities like triode/UL switching, retaining feedback bypass (perhaps
expanding to variable feedback), tone control and bypass. No phono amp
(to be perhaps made available as an external module). Input switching
and optional remote control to be handled either using a external units,
or by means of Mikkel Simonsen's universal IR remonte control modules.

I'm also toning down the plate dissipation power a bit, I'm finding that
I actually prefer a lean "mix" in this particular topology; to my ear
there's a "sweet spot" at about 26 mA per tube, at 620v plate/screen
voltage (UL) for about 16 watts plate dissipation (quite gentle, but
keep in mind that the screens are working pretty hard also in UL mode).
This ends up at a bias of around -80 volts with "typical" 6L6GC's.

Well, anyway. Just a few random thoughts. It's nice to be back.

Cheers,
Fred



--
Nothing beats the bandwidth of a station wagon filled with DLT tapes.

Hi Fred,
I'm sure you have MOVs or some other AC in protection. I found that in using
MOVs a delta config. is best (hot to gnd.; neutral to gnd.; & hot to
neutral).
BTW: How do you calculate the value & wattage of the equalizing resistors?
Also what do you think of (no pun intended) ...Hex Freds?
Cordially,
west

"Fred Nachbaur" wrote in message
news:j5%gc.53124$dg7.52799@edtnps84...
Hi RATs,

Well, after running for about five years, on an almost daily basis, the
first prototype of my "RA-100" hodge-podge quit working. Blew the AC
fuse, with no audible artifacts or other indication that anything was
awry. The music just stopped.

Cause - one of the main B+ diode strings (2x 1N5408 in series) blew
(shorted)! This in spite (or perhaps because of?) of 2-meg equalizing
resistors across each diode.

Replaced the diodes, did the old "in series with a 100w light bulb"
trick to reform the capacitors just in case, and powered it up. Tweaked
the bias a little (the tubes have obviously aged, based on the quiescent
readings, but still have the same gain and sound great).

For reference, the original article on this amplifier is he
http://www.dogstar.dantimax.dk/tubestuf/ampindex.htm

I'm in process of simplifying this design, and fixing up a few little
things I didn't particularly like (that were worked around in the
prototype - see the Hacks and Tweaks section). The goal is to build an
affordable kit and/or completed product that features the clean
sweetness of this direct-coupled CCS-driven LTP UL circuit, adding
amenities like triode/UL switching, retaining feedback bypass (perhaps
expanding to variable feedback), tone control and bypass. No phono amp
(to be perhaps made available as an external module). Input switching
and optional remote control to be handled either using a external units,
or by means of Mikkel Simonsen's universal IR remonte control modules.

I'm also toning down the plate dissipation power a bit, I'm finding that
I actually prefer a lean "mix" in this particular topology; to my ear
there's a "sweet spot" at about 26 mA per tube, at 620v plate/screen
voltage (UL) for about 16 watts plate dissipation (quite gentle, but
keep in mind that the screens are working pretty hard also in UL mode).
This ends up at a bias of around -80 volts with "typical" 6L6GC's.

Well, anyway. Just a few random thoughts. It's nice to be back.

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+

Hi Rich,

Rich Andrews. wrote:
Fred Nachbaur wrote in news:j5%gc.53124$dg7.52799
@edtnps84:


Fred,

You made no mention of any caps across the diodes.

No, I originally had some there (1000 pF), but they added a bit of
additional power supply buzz, so I got rid of them again.
(Capacitor-input filter...)

Maybe faster-switching diodes might improve the odds of surviving a
similar occurence...

You might look into
that. Do you have a MOV across the line?

Yes. That's what's strange, the MOV is perfectly intact. No thunder or
even rain, and no brightening of lights or anything when the rectifiers
failed....

You might want to look into
that too. I had a TV that blew its HOT nearly everytime it thundered. A
MOV across the line kept the TV going without the need for any repairs for
many years afterward.

Yes, they can indeed help get rid of a lot of those little glitches that
can wreak havoc with solid-state stuff....

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+

west wrote:

Hi Fred,
I'm sure you have MOVs or some other AC in protection.

Yes. Plenty.

I found that in using
MOVs a delta config. is best (hot to gnd.; neutral to gnd.; & hot to
neutral).

Makes sense... except maybe in the case of catastrophic failure, the
ground might momentarily be hot...

BTW: How do you calculate the value & wattage of the equalizing resistors?

I just picked a number out of a hat. Usually about 1 megohm is used, but
since I'm running a fairly high voltage I decided on 2 megohm, 1 watt
(strictly for the physical size, to prevent arcing). The voltage across
the diodes will be about 700 volts peak, over 1/2-wave cycle, so if I'm
thinking correctly the dissipation will be 1/2 of what it would be if it
were full wave (or 500 volts RMS) = 0.5*(500^2/2meg) = 62 milliwatts -
plenty safe in any event even if my math is wrong.

Also what do you think of (no pun intended) ...Hex Freds?

I intend to give them a try in the "A" version, and compare against the
more "economical" alternatives. ;-)

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+

"west"

Hi Fred,
I'm sure you have MOVs or some other AC in protection. I found that in
using
MOVs a delta config. is best (hot to gnd.; neutral to gnd.; & hot to
neutral).


** The connection of MOVs from active or neutral to chassis ground is VERY
DANGEROUS !!

MOVs should only be connected from active to neutral and then via a
suitable fuse in case of failures.





........... Phil

Hi,
Modern diodes are self-equalizing, and the resistors were being run
over their voltage rating anyhow.

73, Alan

Phil Allison wrote:
"west"

Hi Fred,
I'm sure you have MOVs or some other AC in protection. I found that in

using

MOVs a delta config. is best (hot to gnd.; neutral to gnd.; & hot to
neutral).



** The connection of MOVs from active or neutral to chassis ground is VERY
DANGEROUS !!

MOVs should only be connected from active to neutral and then via a
suitable fuse in case of failures.





.......... Phil



Many of the 6 outlet "surge strips" use that configuration,and are UL
listed.. ?

ptaylor wrote:

Phil Allison wrote:
"west"

Hi Fred,
I'm sure you have MOVs or some other AC in protection. I found that in

using

MOVs a delta config. is best (hot to gnd.; neutral to gnd.; & hot to
neutral).



** The connection of MOVs from active or neutral to chassis ground is
VERY DANGEROUS !!

MOVs should only be connected from active to neutral and then via a
suitable fuse in case of failures.





.......... Phil



Many of the 6 outlet "surge strips" use that configuration,and are UL
listed.. ?



I used to repair Fluke multimeters for a local Authority, they
often failed short or low but were self limiting as they exploded removing
themselves from the circuit.

"ptaylor"

Phil Allison wrote:

** The connection of MOVs from active or neutral to chassis ground is
VERY
DANGEROUS !!

MOVs should only be connected from active to neutral and then via a
suitable fuse in case of failures.




Many of the 6 outlet "surge strips" use that configuration,and are UL
listed.. ?



** Would not pass basic safety tests ( ie leakage to earth ) in Europe, the
UK or Australia.

Not seen in any appliances - USA made or other.





............ Phil

Hi RATs!

SS rectifiers, in series, 600V B+++ ???

Sigh.

Failed after only five years ...

even homebrews are mortal, sob.

_____

Fred, you may be shocked (sorry) to learn that you are not the first pioneer to
search for a cheap design

Every suit on Earth thinks this is a great concept ...

me, I go for

Happy Ears!
Al

SS : Still Sux


Alan J. Marcy
Phoenix, AZ

PWC/mystic/Earhead

"Fred Nachbaur"

west wrote:

BTW: How do you calculate the value & wattage of the equalizing
resistors?

I just picked a number out of a hat. Usually about 1 megohm is used, but
since I'm running a fairly high voltage I decided on 2 megohm, 1 watt
(strictly for the physical size, to prevent arcing). The voltage across
the diodes will be about 700 volts peak, over 1/2-wave cycle, so if I'm
thinking correctly the dissipation will be 1/2 of what it would be if it
were full wave (or 500 volts RMS) = 0.5*(500^2/2meg) = 62 milliwatts -
plenty safe in any event even if my math is wrong.



** Those 2M resistors are prone to fatigue failure ( ie going open) with
such a high voltage across them.

Imagine one resistor goes open - then the associated 1N5408 diode will have
a much greater share of the 1400 volts PIV, maybe 90 %. It will then be
likely fail short from overvoltage.

The second 1N5408 diode in series will then also immediately fail short as a
result.




.......... Phil

ptaylor wrote:

** The connection of MOVs from active or neutral to chassis ground is VERY
DANGEROUS !!

MOVs should only be connected from active to neutral and then via a
suitable fuse in case of failures.

Many of the 6 outlet "surge strips" use that configuration,and are UL
listed.. ?

But what happens if you connect that 6-outlet strip to a socket without
ground, and the MOVs short?

Or what happens if the earth connection of the socket is bad?

Sockets without ground are still very common here - we have one grounded
socket in the house!

Best regards,

Mikkel C. Simonsen

TubeGarden wrote:
Hi RATs!

SS rectifiers, in series, 600V B+++ ???

Sigh.

No accounting for taste, eh Al? ;-)

Failed after only five years ...

even homebrews are mortal, sob.

How true! If anything, they tend to be somewhat more "mortal" than
production units, simply because there's only one in existence, with
little or no prototyping history.

_____

Fred, you may be shocked (sorry) to learn that you are not the first pioneer to
search for a cheap design

Every suit on Earth thinks this is a great concept ...

me, I go for

Happy Ears!
Al

Well, me too actually. This particular design was certainly not cheap...
parts alone set me back some (estimated) $2000. But it was worth it. The
first Deep Purple album sounds absolutely wonderful on the repaired amp.

That being said -- some of the "cheap" designs can work very well
indeed. For example, my very favourite guitar amplifier (sound-wise) is
the little thing I built around AA5 output tubes!

SS : Still Sux

For the most part I agree with you. But I don't mind putting SS diodes
in a subservient role to Tube Masters, and to sacrifice their measly
little black-bug lives in the event of a glitch. You can think of them
as "pre-fuses". ;-)

It's telling that not a single other component suffered as a result of
this odd failure... especially not any of my precious JJ/Tesla 6L6GC's!

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+

I haven't seen this article before - the amplifier is beautiful... from
electronics to the nice wooden box, and front panel. One failure in 5 years
is really nothing, especially if you use it all the time! Does this also
count tube failures? If so, I'm really amazed!

I find the feature where you have the ablity to elminate feedback from the
output stage very interesting. Do you use this feature? What position do you
usually keep it in? What's the damping factor of the amplifier in either
position? Would you consider making the triode/UL switch into a
triode/UL/Pentode switch? I guess it requires an extra voltage source, but
man, 6L6's can sound damn nice in pure pentode, with a little bit of
negative feedback to keep the damping factor usefully high.

Sorry for all those questions... but what a neat project!

As a fellow fan of 807s, I imagine it sounds spectacular with those 6L6's at
high voltage... and made in BC no less - very cool.

Fred Nachbaur wrote:

Hi RATs,

Well, after running for about five years, on an almost daily basis, the
first prototype of my "RA-100" hodge-podge quit working. Blew the AC
fuse, with no audible artifacts or other indication that anything was
awry. The music just stopped.

Cause - one of the main B+ diode strings (2x 1N5408 in series) blew
(shorted)! This in spite (or perhaps because of?) of 2-meg equalizing
resistors across each diode.

Replaced the diodes, did the old "in series with a 100w light bulb"
trick to reform the capacitors just in case, and powered it up. Tweaked
the bias a little (the tubes have obviously aged, based on the quiescent
readings, but still have the same gain and sound great).

For reference, the original article on this amplifier is he
http://www.dogstar.dantimax.dk/tubestuf/ampindex.htm

I'm in process of simplifying this design, and fixing up a few little
things I didn't particularly like (that were worked around in the
prototype - see the Hacks and Tweaks section). The goal is to build an
affordable kit and/or completed product that features the clean
sweetness of this direct-coupled CCS-driven LTP UL circuit, adding
amenities like triode/UL switching, retaining feedback bypass (perhaps
expanding to variable feedback), tone control and bypass. No phono amp
(to be perhaps made available as an external module). Input switching
and optional remote control to be handled either using a external units,
or by means of Mikkel Simonsen's universal IR remonte control modules.

I'm also toning down the plate dissipation power a bit, I'm finding that
I actually prefer a lean "mix" in this particular topology; to my ear
there's a "sweet spot" at about 26 mA per tube, at 620v plate/screen
voltage (UL) for about 16 watts plate dissipation (quite gentle, but
keep in mind that the screens are working pretty hard also in UL mode).
This ends up at a bias of around -80 volts with "typical" 6L6GC's.

Well, anyway. Just a few random thoughts. It's nice to be back.

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+

I'm puzzled, I may have missed something!

"Self equalising"? First I've heard of this! Are we referring to some sort
of HT diodes with integral resistor? Or is the implication that diodes from
a similar batch are roughly matched in reverse voltage breakdown
characteristics?

Si.


"Alan Douglas" adouglasatgis.net wrote in message
...
Hi,
Modern diodes are self-equalizing, and the resistors were being run
over their voltage rating anyhow.

73, Alan

Hi,

"Self equalising"? First I've heard of this! Are we referring to some sort
of HT diodes with integral resistor? Or is the implication that diodes from
a similar batch are roughly matched in reverse voltage breakdown
characteristics?

As I understand it, they have controlled-avalanche characteristics,
meaning they can break down from overvoltage without damage, as long
as their power dissipation is limited. So if there are two in series,
and a transient comes along that is too high for one, it will begin to
pass current, keeping the voltage across it from rising further. Just
like a "zener" diode ( those over 6V are avalanche diodes, not using
the Zener effect). The excess voltage then appears across the other
series diode. So there is no reason to use equalizing resistors. And
as already noted, resistors have a voltage limit of a few hundred
volts, which was undoubtedly exceeded in this circuit.

73, Alan

Alan Douglas wrote:

Hi,


"Self equalising"? First I've heard of this! Are we referring to some sort
of HT diodes with integral resistor? Or is the implication that diodes from
a similar batch are roughly matched in reverse voltage breakdown
characteristics?


As I understand it, they have controlled-avalanche characteristics,
meaning they can break down from overvoltage without damage, as long
as their power dissipation is limited. So if there are two in series,
and a transient comes along that is too high for one, it will begin to
pass current, keeping the voltage across it from rising further. Just
like a "zener" diode ( those over 6V are avalanche diodes, not using
the Zener effect). The excess voltage then appears across the other
series diode. So there is no reason to use equalizing resistors. And
as already noted, resistors have a voltage limit of a few hundred
volts, which was undoubtedly exceeded in this circuit.

73, Alan

This still begs the question for me, how does the voltage rating being
exceeded on the resistors manifest itself? Arcing across the resistor?
(No evidence of arcing in five years of usage) Leakage? Other
manifestations?

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+

Fred asks...

This still begs the question for me, how does the voltage rating being
exceeded on the resistors manifest itself? Arcing across the resistor?
(No evidence of arcing in five years of usage) Leakage? Other
manifestations?

Unstable resistance. Change in resistance WAY outside of
tolerance. Possible short (unlikely but I've seen it happen).
High noise. Arcing.

Pick some of the above.

Across a diode, the only bad ones in this batch are arc,
short and change in resistance.


Best Regards,
Steve

Check my web page .. A
HREF="http://members.aol.com/sbench101/"http://members.aol.com/sbench101/A
Remove the .gov to EMail me

Steve Bench wrote:

Fred asks...

This still begs the question for me, how does the voltage rating being
exceeded on the resistors manifest itself? Arcing across the resistor?
(No evidence of arcing in five years of usage) Leakage? Other
manifestations?


Unstable resistance. Change in resistance WAY outside of
tolerance. Possible short (unlikely but I've seen it happen).
High noise. Arcing.

Pick some of the above.

Across a diode, the only bad ones in this batch are arc,
short and change in resistance.

Thanks, Steve. Good tips.

Guess they're outa there. ;-)

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+

So were Federal Pacific circuit breakers, but you could arcwekd with a 15amp
single breaker. Done it many times!

- Nothing is so smiple that you can't screw it up! keithw...


Many of the 6 outlet "surge strips" use that configuration,and are UL
listed.. ?

Hi Max,

MaxH wrote:
I haven't seen this article before - the amplifier is beautiful... from
electronics to the nice wooden box, and front panel.

Thanks!

One failure in 5 years
is really nothing, especially if you use it all the time! Does this also
count tube failures? If so, I'm really amazed!

No tube failures, except right initially. In the process of designing
the thing, I used cheap Chinese 6L6GC's -- and ended up getting them a
little warm on a few occasions. Once stable, I put the JJ/Tesla's in,
and they've been fine ever since. (The old Chinese jobbies are actually
still in use in a friend's guitar amp.) One 12AT7 went "off-side" (in
this circuit section matching is rather critical) and was replaced early
on also.

I find the feature where you have the ablity to elminate feedback from the
output stage very interesting. Do you use this feature? What position do you
usually keep it in?

I use it very rarely -- just to see that it still works. I almost
*always* use it in the feedback position. I find it a lot easier to
listen to for long periods of time, even though the "non-FB" setting has
a bit more punch (I ascribe this to speaker response more than anything
else).

What's the damping factor of the amplifier in either
position?

It's been too long since I knew how to compute damping factor, so I'll
give you the effective series output resistance: 6 ohms (in non-NFB
mode), and about 3 ohms in FB mode. Even though a lot more FB could be
used, I found that it reached a point of diminishing returns and 6 dB is
just about right. Just enough to tidy it up without introducing any
other artifacts.

Would you consider making the triode/UL switch into a
triode/UL/Pentode switch? I guess it requires an extra voltage source, but
man, 6L6's can sound damn nice in pure pentode, with a little bit of
negative feedback to keep the damping factor usefully high.

I've thought about it. But I'm not sure if it's worth the added
complexity, and possible headaches. Triode/UL is really easy, since the
bias essentially stays the same, pentode mode tends to throw a few
curves especially if you wanted to switch "on the fly." So I'm presently
not too keen on the idea.

Sorry for all those questions... but what a neat project!

As a fellow fan of 807s, I imagine it sounds spectacular with those 6L6's at
high voltage... and made in BC no less - very cool.

Yes, 6L6GC's do fantastic at high voltages. The hardest part is
adequately insulating the socket pins to prevent arc-over from pin 3 to
pin 2. But liberal amounts of Sally Hansen Nail Polish on ceramic
sockets does the trick wonderfully!

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+