(OvrReactor) wrote in message ...
Anyone care to comment on which tube would be "better" for a high powered bass
amp? By "better", I mean more reliable when abused. You know how we bassists
are! I've been wanting to try my hand at building a high powered all tube bass
amp, but I figure someone else's experiences will help save me a lot of time
and money. I've built a few bass amps in the 100 to 200 Watt range, but I'd
like to take things up a few notches. And if I'm happy with the results, MAYBE
I might actually make a few for sale...

But I sure as heck couldn't call them "Mitchell Amps". People might think I'm
the "other" Tom Mitchell that made amps in Riverside, California back when I
was still in High School, and then people would expect me to FIX all those
bargain-basement Boogie clones...

By all means, POST your response, but PLEASE respond by EMAIL too! I;m only
drop in to this newsgroup two or three times a month.

Best regards,
Tom Mitchell
website - http://members.aol.com/ecc81
website - http://www.over-reactors.com

Going higher in power will require custom transformers which will get
expensive. To double power, there is a 3 dB increase in volume e.g.
100 dB to 103 dB. The KT series are better than the conventional
numbers at an additional cost. For critical applications, the KT
versions are advised. For standard applications, the standard number
will work. If you insist on 200 watts, use transmitting valves. The
B+ will be 750 volts. The ceramic-metal outputs can be used but
remember the fins are connected to B+ and the MUST be fan cooled using
a socket made for fan cooling and the B+ MUST be delayed enough time
for the heater to get to full temperature (30 seconds). The size and
weight of the power transformers, heater transformer, reactors
(chokes) and output transformer will make a large amplifier very
heavy. A waist high steel rack cabinet with good quality wheels, two
chassis (power supply and amp connected by a cable) will be needed.

Power supply regulation is a must with bass! The current swing is high
so the B+ must not vary too much. A reactor input filter is highly
recommended. The ouput filter condenser should be large e.g. two 200
mfd, or higher, condensers in series (total voltage rating 1,5 times
the B+ voltage) with equalization resistors across them. The large
condenser value will keep the B+ more stable at high volume low pitch
frequencies. I know these work from extensive work in audio design.
Typical pi filters are fine for loads that do not vary greatly and
heater-cathode rectifiers give better voltage regulation than those
witha filament and also delay the B+ thus preventing cathode
stripping (greatly reduced valve life and shorts developing from the
cathode to G1).

I do not recommend using the 211 or 845 if you are going to be rough
with the amp. The tall envelope gives more surface area to get hit and
break. The filament is not designed for rough usage. These will also
require 1250 volts B+ for 100 watts output as well as a bias of about
100 volts thus a large signal is required for full output.

A stacked supply for 6550 is easier to work with than having dropping
resistors generating heat and the voltages from a stacked supply will
be enough to drive the screens and preamp stages. Two 300 volt
supplies with pi filters will work in this case. Keep the output
condenser large as above. The input condenser should be 20 mfd 450
volts (TVA1709). It is easier to find a pair of transformers with 300
volts output than 600 volts. They will also have the 5 and 6,3 volt
windings. The heater windings can be done a couple of ways, full-wave
center tap or two (ouput stage and preamp stages). Using filtered d-c
on the heaters WILL make a difference in the hum. D-c is MUCH better
than a-c for hum reduction/elimination.