Hello all - I'd like to ask for a little advice. I recently bought a s/h
Audio Innovations 500. Got it, plugged it in - marvellous, first ever valve
amp, very pleased. Some slight rustling noise through the speakers as it
warmed up, but this got better once the amp was warm.

Now, this evening, listening to a record and then the sound of cracking
glass, similar to that made as the amp warms up in the first 10 minutes or
so. The amp had been on about 3 hours listening at quite low levels. Had a
quick look and one of the valves was glowing brightly throughout the body -
the others just glowed around the top, as usual. No affect on the sound, but
I turned the amp off just in case. It brought to mind this extract from
Border Patrol's web site: 'EL34s are prone to internal short-circuiting
which can destroy the cathode resistors and power supply components (those
who have seen their output valves glowing like fluorescent carrots before
the sound expired on one or both channels will know what I mean)'
(http://www.borderpatrol.co.uk/aiind.htm). The valve is marked National
Electric/Made in Germany/6CAT-EL34.

Could someone tell me:

Is it safe/adviseable to run the amp after this episode?
Is there likely to be an imminent valve failure?
Cause of the superglow?

TIA

Rob

Hi RATs!

Safe? no

Valve failure? maybe

Superglow? valve or bias network failure.

Cracking glass sounds are not normal.

Take amp to a shop for service.

Nothing you have said indicates you will not kill yourself trying to find the
problem.

Happy (and healthy) ears!
Al


Alan J. Marcy
Phoenix, AZ

PWC/mystic/Earhead

On Fri, 03 Oct 2003 23:59:18 +0100, RJH wrote:
the others just glowed around the top, as usual. No affect on the sound, but
I turned the amp off just in case. It brought to mind this extract from
Border Patrol's web site: 'EL34s are prone to internal short-circuiting
which can destroy the cathode resistors and power supply components (those
who have seen their output valves glowing like fluorescent carrots before
the sound expired on one or both channels will know what I mean)'

EL34's in themselves aren't more or less prone to this kind of behavior
than other power tubes.This isn't guessing or speculation, there's
something like several hundred thousand tube amps, including Dynaco, Marantz.
Marshall and who knows what else out there, using EL34's every day.
This doesn't mean that they never fail, only that jumpimg ship and
shelling out 50 quid for 5881's because you had one...and bear in
mind, it's over 10 years old, to boot, the East Germans haven't made
EL34's since there ceased being an East Germany...bite the dust,
doesn't make any sense. And if you were to replace it with
a 6L6/5881 type, there's better stuff out there than Sovtek 5881's
for hifi gear.

Note that BP doesn't make amps with pentodes at all, and they're also
trying to peddle sets of Sovtek 5881's for $80 (twice the going rate)
and upgrades for $500.

Is it safe/adviseable to run the amp after this episode?

Only if you get a replacement EL34 for the bad one.
Sometimes dud power tubes will take out screen or bias
metering resistor. If so, you have to have it replaced.

Is there likely to be an imminent valve failure?

I don't think it's "imminent".
One of your EL34's crapped out.

Cause of the superglow?

Shorted tube, if there was a problem with the bias, there'd be
more than one tube glowing.

--
Ned Carlson Triode Electronics Chicago, IL USA
www.triodeelectronics.com

Hi All. I had the same problem with a Dynaco st-70. Amp was fine for
hours and then suddenly 1 tube would run -off. At first just switching
tubes worked. Then it would happen again after a few weeks. I changed
bias caps, bias resistors, cleaned pins, cleaned contacts etc. For
months all was fine and than one night the same thing happened again.
Same socket , same tube. This time I took the meter and checked
continuity from the top of thru the socket to the end of the soldered
connection. Ah HA!!!! When the socket got hot enough, after a few hours,
internally, a contact would break and I'd loose grid voltage. Replaced
the socket and all has been fine for the last 6 months.
Good Luck
Mike M

"michael mueller" wrote in message
nk.net...
Hi All. I had the same problem with a Dynaco st-70. Amp was fine for
hours and then suddenly 1 tube would run -off. At first just switching
tubes worked. Then it would happen again after a few weeks. I changed
bias caps, bias resistors, cleaned pins, cleaned contacts etc. For
months all was fine and than one night the same thing happened again.
Same socket , same tube. This time I took the meter and checked
continuity from the top of thru the socket to the end of the soldered
connection. Ah HA!!!! When the socket got hot enough, after a few hours,
internally, a contact would break and I'd loose grid voltage. Replaced
the socket and all has been fine for the last 6 months.
Good Luck
Mike M


Many thanks all - plenty to go on there then! I'll report back with the
findings ...

Rob

RJH wrote:

"michael mueller" wrote in message
nk.net...
Hi All. I had the same problem with a Dynaco st-70. Amp was fine for
hours and then suddenly 1 tube would run -off. At first just switching
tubes worked. Then it would happen again after a few weeks. I changed
bias caps, bias resistors, cleaned pins, cleaned contacts etc. For
months all was fine and than one night the same thing happened again.
Same socket , same tube. This time I took the meter and checked
continuity from the top of thru the socket to the end of the soldered
connection. Ah HA!!!! When the socket got hot enough, after a few hours,
internally, a contact would break and I'd loose grid voltage. Replaced
the socket and all has been fine for the last 6 months.
Good Luck
Mike M


Many thanks all - plenty to go on there then! I'll report back with the
findings ...

Rob


Same can be true for the tube pins. Resolder them.

Kind regards, Eike

"michael mueller" wrote in message
nk.net...
Hi All. I had the same problem with a Dynaco st-70. Amp was fine for
hours and then suddenly 1 tube would run -off. At first just switching
tubes worked. Then it would happen again after a few weeks. I changed
bias caps, bias resistors, cleaned pins, cleaned contacts etc. For
months all was fine and than one night the same thing happened again.
Same socket , same tube. This time I took the meter and checked
continuity from the top of thru the socket to the end of the soldered
connection. Ah HA!!!! When the socket got hot enough, after a few
hours,
internally, a contact would break and I'd loose grid voltage. Replaced
the socket and all has been fine for the last 6 months.
Good Luck
Mike M

Any idea if that caused any permanent damage to the tube? I've had a
Mullard EL34 go cherry a total of 4 times, once to the point where the main
fuse blew, I'd hate to learn that I no longer have a matched quad! For me
it was a similar problem, Audio Innovations S500 circuitboard solder joints
became weak from temp change stress, had them all redone. Tempting to
replace the power tube sockets too, already replaced the preamp sockets when
I had an RFT PCC88 mysteriously die.

Hey Sugarite:

Once a tube glows 'cherry-red' it most definetly loses some of it's
longevity and may also change the quiescent and dynamic parameters
as well rendering your matched quad to sometime back when you
unwrapped and installed them.

It really all depends for how long it has glowed cherry red before
you actually saw and reacted to this by throwing the power switch.

Power Tube plates suffer alot when their rated Plate Dissipation power
is exceeded.

My Home Brew Mullard Based EL-34 Monoblocks were (now modded)
designed and built by me according to the exact schematic from the Mullard
Book.
The test data in the book runs the EL-34's at their full 25 watt plate
dissipation rating!

So.............here is a list and timeline of quads that I have ruined since
1996 by
Cherry Red death!

My EL34 Cemetery currently has the following RIP quads;

(4) Sovtek EL34 Fat Bottle; lasted from 8/96 to 9/97, real troopers these
Sovteks!
(4) RAM Tested Tesla EL34; 10/97 to 2/98
(4) JJ Tesla Quad; 3/98 to 11/98
(4) Sovtek EL34WXT; 12/98 to 2/99
(4) Svetlana EL34; 3/99 to 8/00, these lasted quite long
(4) Svetlana EL34 replacements; 9/01 to date with some life left
after the re-bias and B+ drop mod.

I have a quad of Ei Nipple Top El34's that I plan to test soon!

On my home brews I measured +410 Volts DC on the plates at
about 62 Milliamps. That equalled about 25.42 watts dissipation
which is 0.42 watts over spec. Not much but enough to stress the
plates.

Have since rebiased with 680 Ohm resistors (up from 470 Ohm) per tube
and dropped the Plate Voltage to 394 VDC.

Now I get about 55 Milliamps at 394 VDC = 21.67 Watts.

My amps now sound less strained and a bit more full in the bass. Plus
the tubes don't make any noises when I power them up (they used to
ping when warming up!)

Vintage EL34's Hi-Fi amps ofter floored the EL34's to lower distortion
With a full 25 watt plate dissipation KT77 Genalex are probably the only
true substitute, but that was 25+ years ago and newer EL34's just don't
resist the push of some vintage designs.

Funny how amps from the late 50's and 60's really ran the heck out of
El34's.

Nothing beats measuring the Plate Voltage and Plate Current to see where
your Plate Dissipation is at. In my experience, running power tubes at
60-70%
max plate dissipation is usually satisfactory (and saves $$).

I would measure your amp and see if the 120-VAC or more mains of today
(240-VAC ROW) is causing your amp to run hotter than usual.

Once again, 30 years ago one would be surprised to
see more than 117 VAC from the mains... ST-70's have a manual
bias control but I am not familiar with how much lower one can crank
down the bias from it's normal 1.56 Volts across the precision power
resistor in the bias circuit.

Bye,

Rich Sherman


"Sugarite" wrote in message
...
"michael mueller" wrote in message
nk.net...
Hi All. I had the same problem with a Dynaco st-70. Amp was fine for
hours and then suddenly 1 tube would run -off. At first just
switching
tubes worked. Then it would happen again after a few weeks. I
changed
bias caps, bias resistors, cleaned pins, cleaned contacts etc. For
months all was fine and than one night the same thing happened
again.
Same socket , same tube. This time I took the meter and checked
continuity from the top of thru the socket to the end of the
soldered
connection. Ah HA!!!! When the socket got hot enough, after a few
hours,
internally, a contact would break and I'd loose grid voltage.
Replaced
the socket and all has been fine for the last 6 months.
Good Luck
Mike M

Any idea if that caused any permanent damage to the tube? I've had a
Mullard EL34 go cherry a total of 4 times, once to the point where the
main
fuse blew, I'd hate to learn that I no longer have a matched quad! For me
it was a similar problem, Audio Innovations S500 circuitboard solder
joints
became weak from temp change stress, had them all redone. Tempting to
replace the power tube sockets too, already replaced the preamp sockets
when
I had an RFT PCC88 mysteriously die.