Re: Problem with a Fisher 400

>I can guarantee you that I don't have a matched set of tubes...so I am
>interested in adding the trim resistors. However, I not sure what,
>exactly I am balancing...do I need to make the current the same at each
>bias resistor?

Mpfffff.......

Matched tubes is about 30% real fact, 70% myth and 80%
not-quite-necessary in a well-designed system. If you question as to
that adding up to 180%, you are right. It is about that way.

Some systems greatly benefit from matched pairs/quads due to sometimes
marginal design or lack of adjustable bias, or a marginal bias
adjustment process, lack of general maintenance, etc. Carefully matched
tubes greatly reduce the stress on the entire system as long as the
match holds. The problem is that these days, the match typically is
badly done initially and rarely holds all that long either. (Often
tubes are sold as 'matched' with identical lot and date codes, no
more). Assume that no tubes are other than reasonably matched (20%),
and work from there with the rest of the circuit.

Clarification:

Match = w/in 5% on the following values, at least:

Plate current
Grid Current
Bias Voltage (usually externally fixed)
GMhos
Filament Current (at a fixed voltage)

At a range of operating voltages. It can be done, but it is a snap-shot
condition, not permanent after significant use.

Added Trim Resistors are a last resort. Your amp has survived lo, these
last 40+ years as-designed. With proper maintenance, it can last
another 40.

It is 'written up' in several venues. rec.antiques.radio+phono will
direct you to many good sites on recapping and drifting resistors, tube
bias, and so-forth.

At some point the Chorus will start ranting here. Not such that you
should ignore them, not hardly. There is considerable wisdom herein.
But beware the overly complicated. You could shotgun the small-value
caps, check all the critical resistors, and test all your tubes on a
good GM-type tester, replacing individual and *pairs* as needed and be
90% there. Were your unit mine, I would go further and carefully test
all the electrolytics, clean the controls, replace any selenium diodes,
and (again) install a dropping resistor to get the voltage that the
unit 'sees' to about 110V. And, where it mine, when I found that magic
resistor value, I would obtain a 30-watt Dale finned aluminum resistor
for permanence. I would also use my current meter to install the
correct DUAL-ELEMENT fuse such that I could actually protect the
equipment, not just the real-estate. Dual-element fuses are NOT
"slow-blow" fuses, BTW. They are fuses that at turn-on can absorb a
massive overcurrent (cold filaments conduct well) but once the surge
has passed function as quick-blow fuses thereafter.

On a detail note: Jon suggests pots in series with fixed resistors so
bias becomes adjustable. This can be (is) an excellent expedient,
however and CAUTION: Use the best-damned-quality pots you can find at
any cost. Cheap carbon-element pots can drift with heat, deteriorate,
go intermittent, all sorts of problems such that you can introduce a
condition that could be fatal... such as 0-bias voltage... can you say
Runaway? . Make sure that the pots are rated at the current they will
be carrying, with a margin of at least 40%.

Music/sound/noise at the headphone jack could be so many things *right
now* that I would not be concerned until I had dealt with all the
small-value caps. Just one leaky cap could explain both the heat,
noise... and any other wildness so that ab-initio, I would ignore that
issue until far down the restoration process. Cap leakage _can_ be
temperature-dependent as well.

Do you have a means to measure operating current draw in real-time?

I have rebuilt any number of tube amps, tuners and pre-amps. I have
rebuilt dozens of vintage radios and more than a few solid-state amps
and tuners. It ain't nohow rocket science if common sense and common
cautions are followed. But, and for the record, I would not take on
your unit without some pretty basic equipment including an Isolation
Transformer, a metered Variac capable of very fine and accurate
measurement of voltage and current, and a good VOM. A scope is rarely
necessary unless/until you need an alignment, and then much more than a
scope is needed. I have all of the above and much more, but really,
most of it is unnecessary at this point.

Keep in mind that you are NOT the manufacturer, you do not need to
purchse your restoration parts from the lowest bidder and at minimum
specifications. The incremental cost between say... a 0.022uF cap at
100V, and that same cap at 600V is negligible. So, when replacing
non-electrolytic caps, you want to match the CAPACITANCE value, but you
are permitted (and should by one increment at least) to increase the
operating VOLTAGE. Modern caps are so much smaller (and generally
better) than the caps on your unit that going up in voltage even
several increments will not cause size issues. Electrolytics are
another dissertation. Replacement options there depend on what they are
doing and where they are in the circuit.... and their value.

Peter Wieck
Wyncote, PA

.