Reply
 
Thread Tools Display Modes
  #1   Report Post  
Me
 
Posts: n/a
Default Caution about E-H 7591 replacements

I recently had an experience using the Electro-Harmonix 7591 tubes in a
McIntosh 1500 that I feel warrants a comment. I noticed on a couple of
occasions that one of the 4 outputs tubes would suddenly overheat to the
point of glowing bright red. I rushed to the receiver and turned it off,
and of course immediately suspected bias. A complete examination of the
bias pots and power supply shows no problem.

Later, the same thing happend, except on a different tube. Now, I think "RF
oscillation", and so I hook up instrumentation to the amp and await the next
occurence.

Result - no RF or oscillation, just tube current running away (normal 0.7
volts across the 15 ohm bias measurment resistor became 4.5 volts!,
indicating a current of 300ma, where the normal bias level is 45ma). This
time, I was fortunate enough to observe that the tube began getting red hot
inside the plate first, then the plate followed. After closely examining
the tube, I indentified the element overheating as the screen grid. So, I
had a classic case of screen grid runaway, which means all of the tubes were
sitting there like little volcanoes ready to erupt, and the choice of which
one goes first was random.

Examination of the tube specs and the schematic of the Mac 1500 shows that
the screen is being operated off of a 450V supply, with just a 900 ohm
dropping resistor and additional filter cap between it and the plate supply
(at 460 volts). This is a very high screen voltage, and the 7591 tube data
I have shows that the screen voltage should never exceed 400V.

The receiver never exhibits this behavior on the old NOS 7591's, which just
goes to show that a new tube can be designed as a match, but unless the
exact same materials are used in the exact same geometry, Murphy can come
and bite you.

The solution? Screen stopper resistors don't "stop" it (no pun intended)
until you reach a value of over 2000 ohms, which reduces the output power of
the amp to 25W and increases distortion, measured on my own unit, from 0.1%
to over 3.0%. So that's a no go.

Another solution would be to downpower the screen voltage to below 400
volts, but I suspect that would have the same result of increased distortion
and less power (the McIntosh unity-coupled circuit requires a lot of voltage
swing to drive the output tubes, and I suspect that the high screen voltage
was used to make the tubes easier to drive, and the hand selected tubes of
the era could take it. Other McIntoshs that use 7591's, like the MC-225,
have swapped screen drive (driven from the opposite plate), bootstrapping to
increase the drive and cathode bias instead of the fixed bias in the 1500.
Cathode bias would stop this dead, since it automatically limits the maximum
current thru the tube).

So, I've had no problem with these in MC 225's, but in a 1500, watch out!!



  #2   Report Post  
Phil Allison
 
Posts: n/a
Default


"Me" wrote in message
...

I recently had an experience using the Electro-Harmonix 7591 tubes in a
McIntosh 1500 that I feel warrants a comment.



** I suspect the cause is just internal temperature ( due to high plate
dissipation ) - at a high enough temp the ( imperfect) mica insulation from
G2 to G1 becomes leaky and thermal runaway sets in.

If the 7591 grids were supplied with negative bias through a lowish
resistance (or cathode drive were used) this sort of problem is avoided.




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





  #3   Report Post  
Jim Candela
 
Posts: n/a
Default


"Phil Allison" wrote in message
u...

"Me" wrote in message
...

I recently had an experience using the Electro-Harmonix 7591 tubes in a
McIntosh 1500 that I feel warrants a comment.



** I suspect the cause is just internal temperature ( due to high

plate
dissipation ) - at a high enough temp the ( imperfect) mica insulation

from
G2 to G1 becomes leaky and thermal runaway sets in.

If the 7591 grids were supplied with negative bias through a lowish
resistance (or cathode drive were used) this sort of problem is avoided.




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


I think Phil is correct. I would suggest lowering the idle current to
30ma. The heat will go way down, and the Plate B+ will go up a little. I
don't think the maximum power will drop very much, and your ears won't be
able to hear the slight rise in distortion (open to debate). The tubes will
then probably last for many years. If you are worried about the high screen
voltage, an I am too, maybe try a suitably sized zener diode(s) in series
with the screens from the high B+ to drop the idling screen voltage to
something under 400 volts. If you do this, you will have to consider the
zener power dissipation issues, and assume someone will run up both channels
to full output with square wave drive from a audio generator. A once made a
string of 5 watt zeners attached to a heatsink with thermally conductive
epoxy. It worked well, but was UGLY!

I run old style 7591's in a linear class AB1 RF circuit with 600 volts
plate, and +375 screen grid, and 30 ma idle current. I had to regulate the
screen grids. Before I did this the idle current would vary disproportionate
with just a few volts change in line voltage. The screen voltage would rise
to about 450 volts at 125 vac line voltage. I had hoped that the G1 fixed
bias change would offset the G2 change with changing line voltage. It did
not, and since I was running the tubes at max idle plate dissipation, I had
no headroom. I too had a thermal run away with a old style 7591. They are
never the same afterward. The lesson I learned was to not run G2 over 400
volts, and to regulate G1 and G2 such that they are immune to power line
variation, and G2 was regulated up to 80 ma (for two 7591's). A little DC
fan moving some air past the tubes is also useful. I used a Ti TL-431
adjustable shunt regulator for the g1 bias control. This is nice, and will
also hold the bias should you have enough grid drive to extend into class
Ab2 operation.

I am curious about one other thing. Many pieces of tube gear designed in
the 50's, 60's were expecting the AC line voltage to be 110 to 115 vac. It
seems that the AC mains today in many locations is often between 120 to 130
vac. That means lots of vintage tube equipment is running everything high,
and this needs to be taken into consideration. Besides the element voltages
being high, and the electrolytic capacitors sometimes seeing DC beyond their
ratings, we also sometimes end up with our precious filaments running at 7
volts or more instead of the nominal 6.3 vac. Thermal run away in tube
equipment that was designed to run the output tubes at maximum dissipation
is not uncommon, and after raising the line voltage up like it is these
days, thermal runaway is very probable.

Of course if running 100% stock is the goal, NOS 7591's should be used,
and have a little air moving past the tubes. a variac with a AC volt meter
attached would also be very desirable if not mandatory.

Regards,
Jim







  #4   Report Post  
Me
 
Posts: n/a
Default


"Jim Candela" wrote in message
. ..

"Phil Allison" wrote in message
u...

"Me" wrote in message
...

I recently had an experience using the Electro-Harmonix 7591 tubes in

a
McIntosh 1500 that I feel warrants a comment.



** I suspect the cause is just internal temperature ( due to high

plate
dissipation ) - at a high enough temp the ( imperfect) mica insulation

from
G2 to G1 becomes leaky and thermal runaway sets in.

If the 7591 grids were supplied with negative bias through a

lowish
resistance (or cathode drive were used) this sort of problem is

avoided.




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


I think Phil is correct. I would suggest lowering the idle current to
30ma. The heat will go way down, and the Plate B+ will go up a little. I
don't think the maximum power will drop very much....


I thought I would do a little more investigation. The thermal runaway due
to Mica insulation intrigued me, so I took one of my NOS 7591's and compared
it to one of the new E-H's on my Vacu-trace, while measuring actual screen
current. Results as follows:

In both case, plate was 380 volts, screen at plate potential and the tubes
were biases to 70 ma (I wanted to put some thermal load on the tubes and try
to spot a trend).

E-H 7591 Screen current: NOS 7591screen
current:
0 secs: 17.5 ma 6.5 ma
30 secs: 20 ma 6.8 ma
120 secs: 12.8 ma 6.9 ma
300 secs: 10.0 ma 6.8 ma

As you can see, the NOS tube stayed stable, with fairly constant screen
current flow. The E-H, on the other hand, immediately showed higher screen
current (?exposed more to the electron flow?), and then screen current began
to decrease, which I theorize is the screen beginning to emit electrons as
it heated (positive current initially measured was current flow into the
screen due to electron impingement....the only reason can think of for flow
to decrease is that the screen began to emit as it heated and this
screen -to-cathode flow (vs. the normal cathode -to-screen flow direction)
reduced the apparent current inflow). This could also be a conduction
effect across the mica to the screen spacers, of course, so maybe I haven't
proven screen emission, but I'll try to think of a better test.

No runaway, but the highest plate voltage I can get with the Vacu-trace is
380 volts, and I have no doubt that if I could push it to 450, I would get
screen current going thru 0, then negative followed by a runaway. I may try
that on my HV power supply soon.

The E-H tube is usable within standard 7591 specs ( 400 V on the screen),
but the Mac 1500 pushes it's tubes very hard at spec setup (460V plate, 450V
screen) that they apparently can get away with on the NOS tubes.

Interesting.











able to hear the slight rise in distortion (open to debate).


  #5   Report Post  
Jim Candela
 
Posts: n/a
Default


"Me" wrote in message
...

"Jim Candela" wrote in message
. ..

"Phil Allison" wrote in message
u...

"Me" wrote in message
...

I recently had an experience using the Electro-Harmonix 7591 tubes

in
a
McIntosh 1500 that I feel warrants a comment.


** I suspect the cause is just internal temperature ( due to high

plate
dissipation ) - at a high enough temp the ( imperfect) mica

insulation
from
G2 to G1 becomes leaky and thermal runaway sets in.

If the 7591 grids were supplied with negative bias through a

lowish
resistance (or cathode drive were used) this sort of problem is

avoided.




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


I think Phil is correct. I would suggest lowering the idle current

to
30ma. The heat will go way down, and the Plate B+ will go up a little. I
don't think the maximum power will drop very much....


I thought I would do a little more investigation. The thermal runaway due
to Mica insulation intrigued me, so I took one of my NOS 7591's and

compared
it to one of the new E-H's on my Vacu-trace, while measuring actual screen
current. Results as follows:

In both case, plate was 380 volts, screen at plate potential and the tubes
were biases to 70 ma (I wanted to put some thermal load on the tubes and

try
to spot a trend).

E-H 7591 Screen current: NOS 7591screen
current:
0 secs: 17.5 ma 6.5 ma
30 secs: 20 ma 6.8 ma
120 secs: 12.8 ma 6.9 ma
300 secs: 10.0 ma 6.8 ma

As you can see, the NOS tube stayed stable, with fairly constant screen
current flow. The E-H, on the other hand, immediately showed higher

screen
current (?exposed more to the electron flow?), and then screen current

began
to decrease, which I theorize is the screen beginning to emit electrons as
it heated (positive current initially measured was current flow into the
screen due to electron impingement....the only reason can think of for

flow
to decrease is that the screen began to emit as it heated and this
screen -to-cathode flow (vs. the normal cathode -to-screen flow direction)
reduced the apparent current inflow). This could also be a conduction
effect across the mica to the screen spacers, of course, so maybe I

haven't
proven screen emission, but I'll try to think of a better test.

No runaway, but the highest plate voltage I can get with the Vacu-trace is
380 volts, and I have no doubt that if I could push it to 450, I would get
screen current going thru 0, then negative followed by a runaway. I may

try
that on my HV power supply soon.

The E-H tube is usable within standard 7591 specs ( 400 V on the screen),
but the Mac 1500 pushes it's tubes very hard at spec setup (460V plate,

450V
screen) that they apparently can get away with on the NOS tubes.

Interesting.











able to hear the slight rise in distortion (open to debate).





Good test! I am curious if the grid bias required to get 70 ma was very
different between the two tube types. I also wonder what the result would
have been if your test would have inserted a few hundred ohms in series with
G2 before it connects to the plate lead. As your test was described, I would
have thought the screen grid would hog more current from the plate than it
did. Very interesting stuff.

This kind of reminds me of a issue with the Raytheon 4D32 tetrode. This is a
pulse modulator tube used in WW2, and later as a class C RF amplifier in
some ham radio transmitters built in the 1940's, and 1950's. One failure
mode on these is the indirect cathode sheds little white flakes. These can
be seen inside the glass envelope on heavily used tubes. The failure mode on
this tube is primarily reverse G1 current, and eventual thermal run away. In
class C RF service a bad tube will work fine for a while, but after a period
of time (variable) the grid current will begin to decrease, and eventually
go to zero. At this point the tube is still working, and output power is
down a bit. It is like backing off the drive from class C to Ab1. Then the
grid current reverses direction, and plate current overload soon follows. My
theory here is that a small spec of oxide has impacted the first grid (G1),
and eventually heats up a local hot spot such that the G1 becomes an emitter
of electrons.

I wonder if the EH 7591 has any visible symptoms of localized G2 heating, or
is it all around? The RF parasitic idea you had is still a possibility. One
easy thing to do is attach both leads of a neon lamp (say NE2) to each 7591
plate lead at the socket, and fold the neon bulb over till the glass is
almost touching the chassis. Go ahead and power up the amp, and play some
music. If a parasitic oscillation occurs, expect the lamps to flicker from
time to time. The color of the neon is also a good indicator of frequency
(poor mans spectrum analyzer). A amber glow occurs well into the HF region,
and gradually becomes purple at VHF frequencies.

I still think that cranking your Mac's line voltage down to nominal for a
vintage Mac will help. I have some vintage tube equipment where I require
only about 112 vac AC line to achieve 6.3 vac filament voltage.






  #6   Report Post  
Me
 
Posts: n/a
Default


"Jim Candela" wrote in message
...
Good test! I am curious if the grid bias required to get 70 ma was very
different between the two tube types. I also wonder what the result would
have been if your test would have inserted a few hundred ohms in series

with
G2 before it connects to the plate lead. As your test was described, I

would
have thought the screen grid would hog more current from the plate than it
did. Very interesting stuff.

This kind of reminds me of a issue with the Raytheon 4D32 tetrode. This is

a
pulse modulator tube used in WW2, and later as a class C RF amplifier in
some ham radio transmitters built in the 1940's, and 1950's. One failure
mode on these is the indirect cathode sheds little white flakes. These can
be seen inside the glass envelope on heavily used tubes. The failure mode

on
this tube is primarily reverse G1 current, and eventual thermal run away.

In
class C RF service a bad tube will work fine for a while, but after a

period
of time (variable) the grid current will begin to decrease, and eventually
go to zero. At this point the tube is still working, and output power is
down a bit. It is like backing off the drive from class C to Ab1. Then the
grid current reverses direction, and plate current overload soon follows.

My
theory here is that a small spec of oxide has impacted the first grid

(G1),
and eventually heats up a local hot spot such that the G1 becomes an

emitter
of electrons.

I wonder if the EH 7591 has any visible symptoms of localized G2 heating,

or
is it all around? The RF parasitic idea you had is still a possibility.

One
easy thing to do is attach both leads of a neon lamp (say NE2) to each

7591
plate lead at the socket, and fold the neon bulb over till the glass is
almost touching the chassis. Go ahead and power up the amp, and play some
music. If a parasitic oscillation occurs, expect the lamps to flicker from
time to time. The color of the neon is also a good indicator of frequency
(poor mans spectrum analyzer). A amber glow occurs well into the HF

region,
and gradually becomes purple at VHF frequencies.

I still think that cranking your Mac's line voltage down to nominal for a
vintage Mac will help. I have some vintage tube equipment where I require
only about 112 vac AC line to achieve 6.3 vac filament voltage.




I may play with the neon lamp idea. I've about convinced myself that
McIntosh pushed these things this hard to allow unity coupling without
having to bootstrap the driver tube. The driver setup in the 1500 is
absolutely "standard", which is what would be used with plate loaded P-P,
but when you throw the unity-coupled winding in the cathode circuit, it
makes the cathode voltage run up and down with the audio current, which
requires G1 to swing much greater voltages to maintain drive to the tube
than a normal P-P. I think, since this was a receiver and aimed more at
affordability than Mac's usual stuff, they did it this way to save some
parts and production cost. They drove the tube beyond spec because they
could back then.

Another interesting part of this to me, is that, seemingly, the E-H 7591,
being physically larger, should dissipate heat more effectively. I'm an EE,
and it'sbeen a while since I use thermodynamics, but an analysis of heat
dissipation modes in these tubes would be interesting. It's obvious that
some heat goes via conduction thru pins and mica, but the radiant heat loss
from the plate is what I'm talking about. Does having the glass envelope
physically closer to the plate help radiation cooling? If so, the old
"little ones" would have an advantage, but the glass isn't close on a KT-88
and they can sure dissipate the heat.

I'm also going to look at the line voltage end of things.

Thanks!

Reply
Thread Tools
Display Modes

Posting Rules

Smilies are On
[IMG] code is On
HTML code is Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
FA Sherwood @-5500 7591 Tube Amp Michael L. Marketplace 0 June 19th 04 08:42 PM
FA: McIntosh Branded 7591 Tubes The WoodMitch Shop Marketplace 0 February 18th 04 08:06 PM
FA: 4 Strong Used Sylvania 7591 Power Tubes Scott McCorkhill Marketplace 0 September 15th 03 04:14 AM
FS: 30watt 7591 McIntosh power amp Licensed to Quill Vacuum Tubes 4 August 17th 03 03:19 PM
My Introduction and Using the 7591 at RF frequencies Jim Candela Vacuum Tubes 14 August 4th 03 02:18 PM


All times are GMT +1. The time now is 03:17 PM.

Powered by: vBulletin
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 AudioBanter.com.
The comments are property of their posters.
 

About Us

"It's about Audio and hi-fi"