Wbittle wrote:

Hi,
I have used Sovtek, Tesla, and Svetlana and all work well. The
problem with matched sets is how long they were 'burned in' prior to the
final testing stage for matching. What you really want in an ST70 to
preserve sonic balance channel to channel is a matched quartet of
EL34's. Personally, I'd read the seller's feedback and see what others
think of his items. If the seller has mostly all excellent feedback,
then chances are he is a pretty square shooter. But if you see a lot of
neutrals or negatives, be careful. Also, if this amp was recovered from
a dumpster chances are it is in need of an electronic restoration before
it will ever sound correct or be reliable. ST70's are great power amps.
I have one myself. But my amp is totally refurbished. You would not
believe the difference in sound when you change out certain old
components which have drifted in value.
There is no way to know for sure that tubes balanced with a tester
will in fact perform as a matched set in an actual circuit. So, although
the set you buy could have been picked, tested and packed as a matched
set. That is no guarantee that they will remain that way. I have a set
of four EL84's in a Scott 222C that I matched out of 12 tubes and they
have remained close for over three years. I have a set of Sovtek 7591's
that remained matched for about 3 months of use then drifted apart in
value. I had to remove them and recheck them then do my best to pair
them off in reasonably matched pairs. There is no way the seller could
have known this would happen.
BIll B.



All tubes drift apart after they have been supplied matched.
But the ones that were not matched stay well apart forever,
or they only drift together accidently.

In the last pair of Quad II amps which I re-furbished, the thd in signal
at listening levels could be drastically reduced by swapping the
positions of the output tubes and input EF86 around for the lowest measured
thd.
In one position, thd could be 15 dB higher than the other, mainly 2H.
The tubes involved in the samples I refurbished were probably not well
matched.

With guitar amps, I doubt matching the tubes for the same signal
transconductance
at the same idle current would make much difference, and I doubt you'd
notice it when listening
to the radio, as Mr Valve suggests.
I haven't a clue how Mr Valve matches his valves, and although he is quick
to say he does a great job,
he leaves me completely uninformed about how he does it,
and I remain very unlikely to go to him for matched tubes.
Sure there are lots of musos who like his services, but musos mostly know
sweet ****
all about technical aspects of their amplifiers, so any muso recommendations

can only mean sfa to me.

What may be more important for guitar amp tubes is that they have equal idle
currents
at a given grid voltage. Having equal transconductance does not always mean
an equal
grid bias voltage will produce the same idle current.

A small number of tubes would need the same bias voltage applied, *and* have
the same
transconductance with signal.
If this is the case, these tubes are truly matched.

In amps with more than two output tubes, the tubes can be swapped around so
the
transconductance, Gm, of each 1/2 of the tubes on each side of the PP
circuit
do total to give very nearly equal Gm for each 1/2, and the thd will then be
lowest
when the tubes re biased individually for equal idle current.

To set up for lowest thd at listening levels for hi-fi amps takes time and
patience,
and a thd meter and volt meter. Sorry, but its all rather fiddly and
difficult.

95% of musos just buy tubes, plug them in and use them. Some bother to
adjust the bias,
or *balance* the bias via the pot at the back of their amps,
and this minimises the hum caused unequal bias currents in a pair of output
tubes
fed from a PS where there is a lot of hum at the CT, which is a feature of
so many guitar amps;
they have attrociously noisy PS compared to hi-fi amps.
And it was compounded somewhat by the practice of Fender et all to supply
export
model amps without the LC filter following the resevoir capacitor.
But the extra dirty growl caused by the supply hum intermodulating with the
guitar
notes is all musical to most musos in the world of pop.

Musos are rarely if ever satisfied with their amps unless they sound
pleasantly
distorted, so 10% of mainly 2H and 3H is normal at even low levels,
with 40% on strong overdrives, where the signal is mainly a square wave,
and the type of tubes don't make much difference.
You could have an EL34 and a 6L6 in a PP circuit, and not hear
anything spectacularly better or worse during serious over drives.
The heavy metal bands, and now dark metal types can only
survive on 40% Dn for most of the time, and tube matching is a silly concept

of no value whatsoever.

But to listen seriously to an old Italian cello being played by a master,
better you have well matched tubes, lest the cello sound worse than
if you were at the venue where it was recorded.

Patrick Turner.