[Patrick Turner]

On Aug 18, 10:00*am, NX211 wrote:
On Aug 17, 7:38*pm, Patrick Turner wrote:





On Aug 18, 8:17*am, NX211 wrote:

On Aug 16, 10:37*am, charles rollo charles.rollo.

wrote:
Recently my amps trannies were melted when the 211 failed. With less
than 150 hours on the tube but past the 30 day warrantee.
I bought the tubes in Oct. 2009 from Grant Fidelity. however never
used them until well after the 30 day time frame. Which Grant was aware
of.
One tube shorted and took out the output and high voltage trannies.
Costing me $1500 to repair.
Neither Grant Fidelity nor Full Music will care after the 30 days
expires. For $500 one would think that the Manf. or north American
distributor would make ammends because of the damage done. Something
fishy going on as the distributor.
No no and no is the answer from them. Buyer beware. Stay away.

charles

--
charles rollo

I had a new KT90 arc in the base from the plate to the filament and
literally melt down with only 50 hrs on the tube. How do you explain
that to the guy you bought the tubes from. *Not too cool.

I bought about 20 KT90s from New Sensor some years ago and fitted 8 to
a 8585 stereo amp which has B+ at 480Vdc.
Not one has failed early. *But KT90 made in the old EI yugoslavia
factory were very prone to malfunction and failure if they become red
hot due to a speake cable short.

Also had a new KT120 with about 75 hrs on it fail from a bad
connection on one of the filament pins. Then again, bought some JJ
KT77's with something floating around in one of them and it arced
internally with a big flash when installed. *I knew I should have
immediately sent it back before installing it but I didn't - and I got
burned. * These aren't operational problems with the amp but poor
quality control from tube manufactures.

New Sensor test most tubes during the grading and matching process and
they don't rely on the makers in Russia to do it. Maybe New Sensor
discard 5% of the tubes sent to them from the factory. The number of
failures is not publicised for obvious reasons lest it create the
opinion that tubes are all dodgy and you are lucky to get any which
work properly.

But Quality Control during tube production ALWAYS was a problem in
itself because HUMANS are much involved doing repetitious work. And
its mainly womens work. Anyway, if you worked in a tube factory you'd
know just how easy it is for **** to happen.

I have had bad soldering of wires to tube bas pins and its easily
fixed by re-soldering. In all my amps there is active protection and
the amp just cannot remain turned on if any single output tube
malfunctions in a manner which leads to excessive cathode currents.

Also bought some cheap Chinese 6L6GC's with hundreds of hrs on them
now and the perform flawlessly. *You don't necessarily get what you
pay for when it comes to vacuum tubes.- Hide quoted text -

If you get 95% of what you pay for then this is good because if all
tube production was being done in the USA or Oz or UK like it once
used to be done then you'd be paying +20dB more for the tubes because
of the much higher cost of labour and compliance with workplace safety
regulations.

I cannot understand someone buying an expensive KT120 when 2 6L6GC can
give similar power. But in Guitar amp land there are those who will
buy anything offered.

Patrick Turner

- Show quoted text -

Ah - KT120's are neat tubes. *They'll work in most amps that use
KT88's, etc. and you'll run out of power supply way before you run out
of tube. *$100.00 and they'll last for years if you get a good pair.
Put 600 or 700 volts on them and *they'll rock your neighborhood.- Hide quoted text -

- Show quoted text -

Indeed you can get a huge amount of AB1 power from KT120 and no doubt
they might be popular with guitar amp and PA people.

They may be cherished for where a high amount of class A power is
wanted rather than a huge unecessary amount of class AB power.
Audiophiles have somewhat more preferance for a very clean first 10
watts from a class A amp and for this you rarely need Ea more than
400V. I have used KT90 and KT88 in Quad-II amps and with much revision
of the PSU and amp circuits so that the sound is better than the
original amps with KT66, and even with Ia less than used in the KT66.

The higher the Ia can be, the lower the Ea must be to keep the idle
Pda within safe working limits for long tube life.
With KT66 or 6L6 in amps like Quad-II, the Pda is about 22W per tube
at idle. For KT88, idle Pda could be safely 35W and for KT120 about
40W I guess. But one would not want to set up KT120 in a Quad-II amp
so idle Pda was 40W or else the PT would overheat from internal
winding losses and the radiated heat from the tubes. So in fact with
KT120, one would run them with less idle Pda than KT66, about 20W
would be OK and probably you'd get better sound than with KT66- if you
modify the rest of the amp circuit and PSU circuit to give less THD
and lower PSU noise.

My latest modded Quad-II amps have plain old Sovtek KT88 and a pair of
Polam EF80 input tubes and with the amps set for 8 ohms, there is 32W
available at 0.5dB above clipping with a sine wave input. There is
cathode biasing with Dynamic BIas Stabilisation. Instant output power
at clipping without the B+ rail sagging due to class AB higher Ia draw
is about 34W. Using KT90 or KT120 would not give any PO increase of
THD reduction. KT88 or 6550 are sufficiently capable of giving as
great an anode voltage swing as is possible and KT120 will not
increase it. Using fixed bias would increase possible class AB output
power but I have enough and I like having no adjustments to make.

KT120 are supposed to have heater currents betaeen 1.7 and 1.95A which
is close to KT88/6550. Most amps able to take KT66 or EL34 can take
the higher heater current draw because usually there is some over
design of heater windings.

But this needs to be checked out. I have had no trouble using KT88 in
Quad-II amps but then in most I have deleted the damn hot running GZ32
and used slightly less Ia than originally specified so the amps I
modify run cooler so the slight extra heating from warmer heater
windings is accomodated.

There would be many who would say Sovtek KT88 or KT120EH might sound
terrible compared to a pair of original NOS KT66, but I have not heard
any evidence to support the nostalgic opinion.

Presumably, KT120 could be used in Dynaco ST70 and such like, but of
course the bias setting has to be re-set so as to not overload the B+
supply.

When EL34 or 6CA7 are used in cathode biased amps meant for KT66, 6L6,
KT88 or 6550, the bias current is lower and perhaps acceptable but in
amps designed for EL34/6CA7 the use of KT66 etc will cause higher Ia
and bias MUST be adjusted somehow.

I've not yet seen a live KT120, and so I don't know exactly how they
bias up in a circuit designed for EL34 or 6L6/KT66 etc.

Using KT90, KT88, 6550, or KT120 in amps meant for tubes which have
lower saturation currents such as KT66 and 6L6 could damage a fragile
OPT if there is bias failure. So one shoud always fit active
protection measures to stop precious old OPTs from being damaged when
hotting up the amp with tubes more powerful than original.

Patrick Turner.