Hi All,

As an added bonus of asking RAT's about adding a choke to my
Heathkit aa-151 power supply, Patrick Turner had a once-over
of the schematic and made a few excellent suggestions. The
next one I'm going to tackle is changing the single cathode-
resitor-bypass-cap to the 6BQ5's.

The aa-151 is a PP 6BQ5 "ultra linear" integrated with a 5AR4
rectifier and cathode biasing, and aside from 6EU7's instead
of 12AX7's is like the Heathkit SA-2;
http://www.mods.com/heath-hifi/sa2ma...2schematic.gif

I've done the obligatory newsgroup search for this topic.
Maybe surprising, but some people have experienced negative
effects from going from one cathode resistor-cap to four
separate ones. The general argument being about improved DC
balancing with shared resistor-caps per channel (in PP).

So... I'm going to go half way, changing the one resistor-cap
into two, one for left, one for right.

Currently, shared between all four 6BQ5's cathodes is a 100ohm-
7watt resistor and a 50uf-25V bypass-cap. Patrick suggested
four separate pairs with 200ohm and 1000uf, a BIG change!

The 6BQ5 spec-sheet states for PP class AB two tubes, a typical
cathode-bias resistor of 130ohm.

What I'm wondering is the RAT's-eye-view of the values for these
resistor-cap pairs. I'm thinking 165ohm-5W and 80uf would be OK,
but maybe a bit conservative.

Thanks!

Hi:

You note that the 6BQ5 spec sheet shows 130 ohm cathode resistor for 2
tubes, class AB PP. Be aware that this is for designs at approx max ratings
of 300V A-K: the AA151 runs the 6BQ5s WAY over max plate voltage at
something like 360V A-K (similar to Dyna ST/SCA35). It seems reasonable to
assume that if the original design used 100 ohms for four cathodes,
equivalent for two pairs of two cathodes ea would be 200 ohms and this is
still running the BQ5s pretty hot. I'd be inclined to run more like 220-250
ohm @10W ea.

A real benefit of separating the common K resistor is that a matched quad of
BQ5s is no longer required, just two matched pairs. Heath (and Dyna) don't
really do a good job of alerting the user to this issue. Individual K
resistors reduce the need for matched pairs but if it were my amp, I'd still
go for them.

--
Steve

"Steve O'Neill" wrote in
:

Hi:

You note that the 6BQ5 spec sheet shows 130 ohm cathode resistor for 2
tubes, class AB PP. Be aware that this is for designs at approx max
ratings of 300V A-K: the AA151 runs the 6BQ5s WAY over max plate
voltage at something like 360V A-K (similar to Dyna ST/SCA35). It
seems reasonable to assume that if the original design used 100 ohms
for four cathodes, equivalent for two pairs of two cathodes ea would
be 200 ohms and this is still running the BQ5s pretty hot. I'd be
inclined to run more like 220-250 ohm @10W ea.

A real benefit of separating the common K resistor is that a matched
quad of BQ5s is no longer required, just two matched pairs. Heath
(and Dyna) don't really do a good job of alerting the user to this
issue. Individual K resistors reduce the need for matched pairs but
if it were my amp, I'd still go for them.

--
Steve




Good point! I was wondering about the reduced current sharing the
resistor 4 ways...

Hi RATs!

MTBF (Mean Time Between Failures) vs parts count is not the linear relationship
direct logic might presume.

As systems endure the rise and fall of the random ravages of time, larger parts
populations exhibit more stable performance than smaller parts populations.

Parallel resistors and other stupid wastes of time and money are not so stupid,
statistically.

The interesting thing to me about circuits is how they sound at this moment.
All the rest is engineering mumbo-jumbo and only of interest to off world
intellectuals and money grubbing employees.

Each of us hears what we listen to and only learn anything epiphanally useful
by happenstance and ironic cognitive dissonance.

Pretending MTBF has any useful application relative to listening to Bach is
simply posturing one's ego as greater than one's soul.

A common pose, but, hardly noble. Just smug

If you like what you hear, you are in the right place.

If you think you know what others hear, you have your head in a dark, smelly
place.

If you know what others should hear, your head IS a dark, smelly place

Listen and let listen!

Happy Ears!
Al


Alan J. Marcy
Phoenix, AZ

PWC/mystic/Earhead

Wow. Another great post, hopefully to appear in Al's "Zen in the Tube
Garden." I especially liked the line, "Each of us hears what we listen
to and only learn anything epiphanally useful by happenstance and ironic
cognitive dissonance."

Thanks, Al. Live long and prosper!

Cheers,
Fred

TubeGarden wrote:

Hi RATs!

MTBF (Mean Time Between Failures) vs parts count is not the linear relationship
direct logic might presume.

As systems endure the rise and fall of the random ravages of time, larger parts
populations exhibit more stable performance than smaller parts populations.

Parallel resistors and other stupid wastes of time and money are not so stupid,
statistically.

The interesting thing to me about circuits is how they sound at this moment.
All the rest is engineering mumbo-jumbo and only of interest to off world
intellectuals and money grubbing employees.

Each of us hears what we listen to and only learn anything epiphanally useful
by happenstance and ironic cognitive dissonance.

Pretending MTBF has any useful application relative to listening to Bach is
simply posturing one's ego as greater than one's soul.

A common pose, but, hardly noble. Just smug

If you like what you hear, you are in the right place.

If you think you know what others hear, you have your head in a dark, smelly
place.

If you know what others should hear, your head IS a dark, smelly place

Listen and let listen!

Happy Ears!
Al


Alan J. Marcy
Phoenix, AZ

PWC/mystic/Earhead

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

oooooooooooowow!
longggggg posssssssssst!

they sing -or they not sing.
pure facts
we agree.

--
Choky
Prodanovic Aleksandar
YU


"TubeGarden" wrote in message
...
Hi RATs!

MTBF (Mean Time Between Failures) vs parts count is not the linear
relationship
direct logic might presume.

As systems endure the rise and fall of the random ravages of time, larger
parts
populations exhibit more stable performance than smaller parts
populations.

Parallel resistors and other stupid wastes of time and money are not so
stupid,
statistically.

The interesting thing to me about circuits is how they sound at this
moment.
All the rest is engineering mumbo-jumbo and only of interest to off world
intellectuals and money grubbing employees.

Each of us hears what we listen to and only learn anything epiphanally
useful
by happenstance and ironic cognitive dissonance.

Pretending MTBF has any useful application relative to listening to Bach
is
simply posturing one's ego as greater than one's soul.

A common pose, but, hardly noble. Just smug

If you like what you hear, you are in the right place.

If you think you know what others hear, you have your head in a dark,
smelly
place.

If you know what others should hear, your head IS a dark, smelly place

Listen and let listen!

Happy Ears!
Al


Alan J. Marcy
Phoenix, AZ

PWC/mystic/Earhead

Hi RATs!

Perhaps one of us is wise?

Happy Ears!
Al

we agree.



Alan J. Marcy
Phoenix, AZ

PWC/mystic/Earhead

Hi RATs!

Well, it is true that extremely short MTBF would make listening to an entire CD
impossible My position is simply that as audio experimenters, we can focus
simply on sonics, and leave other engineering miracles for the production
types.

None of my circuits have died, they have all been murdered by me in the name of
sonics. Oops, the 315A transmitter tubes for IFFR didn't last long as audio
tubes, on the order of weeks, 24/7, but that was one out of hundreds. And, they
did look cool as anything I have tried

My point was using MTBF as some sense of quality in a private system is simply
not very useful. Nor am I interested in watts consumed to produce the
milliwatts that drive my speakers. Those are indeed real things, but, I only
have ears for the music. The techno parallel universes are not of concern for
me.

I do not hope to leave this life with a place in history, just a smile

Happy Ears!
Al


Alan J. Marcy
Phoenix, AZ

PWC/mystic/Earhead