Hi,

I'm running Sovtek 5881s in in my Quad IIs (keeping the KT88s for special
occasions :-) ).

One of them (I have swapped it around on the same amp and to the other amp
to make sure it is the valve (tube) that is at fault) has a red hot patch
visible in low light. What causes this if its not the biasing of the valve?
I assume it can't be this if the fault moves with the valve and checks
indicate the biasing is OK.

The amps sound OK whatever valve is in which place.

Could I be damaging the amps with this valve?

Thanks for any info.

John Smith.

"John Smith" wrote in message
...
Hi,

I'm running Sovtek 5881s in in my Quad IIs (keeping the KT88s for special
occasions :-) ).


That's queer! How are you able to swap between 5881s and KT88s??

(What am I missing?)



One of them (I have swapped it around on the same amp and to the other amp
to make sure it is the valve (tube) that is at fault) has a red hot patch
visible in low light. What causes this if its not the biasing of the
valve?


Global warming.


I assume it can't be this if the fault moves with the valve and checks
indicate the biasing is OK.

The amps sound OK whatever valve is in which place.

Could I be damaging the amps with this valve?


Not until it goes, then it'll drag a few bits and bobs off with it.... :-)


Thanks for any info.


Sorry about that - couldn't resist it! I'm sure someone who *knows* will be
along to advise.... ;-)

(Not here/not posting - only *lurking*.....)

John Smith wrote:
Hi,

I'm running Sovtek 5881s in in my Quad IIs (keeping the KT88s for special
occasions :-) ).

One of them (I have swapped it around on the same amp and to the other amp
to make sure it is the valve (tube) that is at fault) has a red hot patch
visible in low light. What causes this if its not the biasing of the valve?
I assume it can't be this if the fault moves with the valve and checks
indicate the biasing is OK.

The amps sound OK whatever valve is in which place.

Could I be damaging the amps with this valve?

Thanks for any info.

John Smith.



Well, when you say the biasing is ok, what do you mean by that?

I am guessing by the fact you mention kt88's you mean the new Quad's,
not the old kt66 ones?

5881's are spec'd as a may 30w plate dissapation, and kt88's 40, so if
the bias is setup for 88's then yu may be driving the 5881's close to
their limits. The fact that one glows and the other doesn't could be
down to one passing more current at the given bias point (I don't know
how the Quads bias is setup, I thought they used to be cathode bias), or
it could be that one is just over the point it starts to glow and the
other just under.

In both cases you may be working the valves too hard.

There are some cheap kt88's on ebay that may be a better solution for you.

--
Nick

"Life has surface noise" - John Peel 1939-2004

"John Smith"

I'm running Sovtek 5881s in in my Quad IIs (keeping the KT88s for special
occasions :-) ).


** You must mean KT66s - which are similar to 5881s.


One of them (I have swapped it around on the same amp and to the other amp
to make sure it is the valve (tube) that is at fault) has a red hot patch
visible in low light.


** Not uncommon at all.


What causes this if its not the biasing of the valve?


** Less than perfect manufacture of the valve - basically imperfect grid
wire alignment.


Could I be damaging the amps with this valve?


** Not unless it gets much worse and the glow is visible in normal room
light.



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

"John Smith" wrote in message
...
Hi,

I'm running Sovtek 5881s in in my Quad IIs (keeping the KT88s for special
occasions :-) ).

One of them (I have swapped it around on the same amp and to the other amp
to make sure it is the valve (tube) that is at fault) has a red hot patch
visible in low light. What causes this if its not the biasing of the
valve?
I assume it can't be this if the fault moves with the valve and checks
indicate the biasing is OK.

The amps sound OK whatever valve is in which place.

Could I be damaging the amps with this valve?

Thanks for any info.

John Smith.

Likely culprit is the 0.1uf coupling cap to the grid of each the KT66's.
One or both are likely to be 'leaking' electrically. With no signal, there
should be approx 320v DC on one side of the cap and virtually nothing on the
other. Are the caps original?? If they are, then replace them regardless.
Perhaps you could measure the voltages on either side of the 2 caps in
question and report what you get.

"Mike Coatham"
"John Smith"

One of them (I have swapped it around on the same amp and to the other
amp
to make sure it is the valve (tube) that is at fault) has a red hot patch
visible in low light.

Likely culprit is the 0.1uf coupling cap to the grid of each the KT66's.


** Err - only one valve has the faint red patch, which follows it about
like a puppy dog where ever it goes ;-)


One or both are likely to be 'leaking' electrically. With no signal, there
should be approx 320v DC on one side of the cap and virtually nothing on
the
other.


** The correct figures are 115 volts and about 0.2 volts.




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

In article , Mike Coatham
writes

"John Smith" wrote in message
...
Hi,

I'm running Sovtek 5881s in in my Quad IIs (keeping the KT88s for special
occasions :-) ).

One of them (I have swapped it around on the same amp and to the other amp
to make sure it is the valve (tube) that is at fault) has a red hot patch
visible in low light. What causes this if its not the biasing of the
valve?
I assume it can't be this if the fault moves with the valve and checks
indicate the biasing is OK.

The amps sound OK whatever valve is in which place.

Could I be damaging the amps with this valve?

Thanks for any info.

John Smith.

Likely culprit is the 0.1uf coupling cap to the grid of each the KT66's.
One or both are likely to be 'leaking' electrically. With no signal, there
should be approx 320v DC on one side of the cap and virtually nothing on the
other. Are the caps original?? If they are, then replace them regardless.
Perhaps you could measure the voltages on either side of the 2 caps in
question and report what you get.



Of course you'll need to soak 'em in the correct grade of snake oil to
get the electrons moving from up to left

Don't think the ones from RS Components will do somehow?...
--
Tony Sayer

tony sayer wrote:
In article , Mike Coatham
writes

"John Smith" wrote in message
...

Hi,

I'm running Sovtek 5881s in in my Quad IIs (keeping the KT88s for special
occasions :-) ).

One of them (I have swapped it around on the same amp and to the other amp
to make sure it is the valve (tube) that is at fault) has a red hot patch
visible in low light. What causes this if its not the biasing of the

valve?

I assume it can't be this if the fault moves with the valve and checks
indicate the biasing is OK.

The amps sound OK whatever valve is in which place.

Could I be damaging the amps with this valve?

Thanks for any info.

John Smith.

Likely culprit is the 0.1uf coupling cap to the grid of each the KT66's.
One or both are likely to be 'leaking' electrically. With no signal, there
should be approx 320v DC on one side of the cap and virtually nothing on the
other. Are the caps original?? If they are, then replace them regardless.
Perhaps you could measure the voltages on either side of the 2 caps in
question and report what you get.




Of course you'll need to soak 'em in the correct grade of snake oil to
get the electrons moving from up to left

Don't think the ones from RS Components will do somehow?...

What you mean like RS 365-7463

"Ideal for use in loudspeaker crossover networks and valve amplifiers
where pure sound is at a premium"

:-)

--
Nick

John Smith wrote:

Hi,

I'm running Sovtek 5881s in in my Quad IIs (keeping the KT88s for special
occasions :-) ).

One of them (I have swapped it around on the same amp and to the other amp
to make sure it is the valve (tube) that is at fault) has a red hot patch
visible in low light. What causes this if its not the biasing of the valve?
I assume it can't be this if the fault moves with the valve and checks
indicate the biasing is OK.

The amps sound OK whatever valve is in which place.

Could I be damaging the amps with this valve?

Thanks for any info.

John Smith.

Find out what the bias currents are in the tubes, and what the idle DC
voltage between anode and cathode.
Calculate the power dissipation with each tube.
Measure it 2 minutes after turn on, and then 15 minutes later, and again in
another 15 minutes.
It shouldn't have a slow increase after 15 mins.

If one tube has more plate dissipation than 28 watts, it might glow a bit.
The dissipation of each tube should be below the rated value.

In a few Quad II amps I have serviced, one KT66 glows a bit,
and usually because the tubes are not matched, even though they may have once
been when new.
So one tube has say 90 mA of anode current, the other 40 mA.
This is real bad, and the glowing tube causes music to be awful above a watt,
and because of the DC imbalance in the OPT.

The bias imbalance can be often cured.
Use individual RC cathode bias networks between each cathode and its end of the

tertiary cathode winding and ground the CT of this winding.
Remove the existing 180 ohms and its bypass cap.
Use 470 ohms and 470 uF for each RC network.
Bias currents then should be far more near equal, and red patches will vanish
and sound will not be mud.

Check the DC voltage across the 560k grid bias resistors to each output tube.
The grids should not be more than 1 volt positive with respect to 0V.
If say 4 volts +ve, the tube could be stuffed.

Left alone, a red tube may all to easily thermally run away,
and try to pull its maximum saturated DC current of about
200 mA, for awhile, until it melts down to a real short,
and during this episode it over heats the primary winding on the OPT, causing a
shorted turn
or the winding to fuse open, and that is *real* expensive,
so swap that reddish tube if nothing else.

Many old tube amps behaved just this badly, and owners parked the
damn things, and went out and bought solid horror amps.

The bad mannered, ungrateful, self centered, ill-considerate behaviour
of some tubes today makes us wonder what the world is coming to.


Patrick Turner.