I could use some advice on tube preamp voltage. I just
built 4 tube preamps (Pultec MB-1) and the schematic calls
for a particular 600 Volt Center Tap transformer etc. After some
tinkering
with half wave, full wave rectification, filtering and more filtering,
pondering using a choke etc., I haven't come up with anything I feel
comfortable enough to connect to the preamps that were built with high
quality parts etc., so I tried connecting a Power One IHB-200 linear
200 volt power supply to the plates and used a 6.3V transformer for
the heaters. It works great, but I'm wondering if anyone sees any
problems with using only 200 volts when it's supposed to be 345 volts
or so. Any help would be appreciated.

Rifa Roederstein wrote:
I could use some advice on tube preamp voltage. I just
built 4 tube preamps (Pultec MB-1) and the schematic calls
for a particular 600 Volt Center Tap transformer etc.

So order one from Hammond.

After some
tinkering
with half wave, full wave rectification, filtering and more filtering,
pondering using a choke etc., I haven't come up with anything I feel
comfortable enough to connect to the preamps that were built with high
quality parts etc., so I tried connecting a Power One IHB-200 linear
200 volt power supply to the plates and used a 6.3V transformer for
the heaters. It works great, but I'm wondering if anyone sees any
problems with using only 200 volts when it's supposed to be 345 volts
or so. Any help would be appreciated.

Yes, you'll get reduced headroom, and you'll need to change the cathode
resistor values so that the grid bias voltages are correct. Measure
the grid/cathode voltage and compare it with the ones in the handbook
for the tubes you are using. Adjust the cathode resistor until it's happy.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Thanks once again Scott for your free advise and tips! I ended
up getting a power supply kit with more voltage so I won't
botch up the Pultecs. It's not a bad deal for the money, hard to
see how the guy's making a profit. http://www.stf-electronics.com/Page0015.Html

Take Care,
Paul


(Scott Dorsey) wrote in message ...
Rifa Roederstein wrote:
I could use some advice on tube preamp voltage. I just
built 4 tube preamps (Pultec MB-1) and the schematic calls
for a particular 600 Volt Center Tap transformer etc.

So order one from Hammond.

After some
tinkering
with half wave, full wave rectification, filtering and more filtering,
pondering using a choke etc., I haven't come up with anything I feel
comfortable enough to connect to the preamps that were built with high
quality parts etc., so I tried connecting a Power One IHB-200 linear
200 volt power supply to the plates and used a 6.3V transformer for
the heaters. It works great, but I'm wondering if anyone sees any
problems with using only 200 volts when it's supposed to be 345 volts
or so. Any help would be appreciated.

Yes, you'll get reduced headroom, and you'll need to change the cathode
resistor values so that the grid bias voltages are correct. Measure
the grid/cathode voltage and compare it with the ones in the handbook
for the tubes you are using. Adjust the cathode resistor until it's happy.
--scott

I could use some advice on tube preamp voltage. I just
built 4 tube preamps (Pultec MB-1) and the schematic calls
for a particular 600 Volt Center Tap transformer etc. After some
tinkering
with half wave, full wave rectification, filtering and more filtering,
pondering using a choke etc., I haven't come up with anything I feel
comfortable enough to connect to the preamps that were built with high
quality parts etc., so I tried connecting a Power One IHB-200 linear
200 volt power supply to the plates and used a 6.3V transformer for
the heaters. It works great, but I'm wondering if anyone sees any
problems with using only 200 volts when it's supposed to be 345 volts
or so. Any help would be appreciated.

Assuming we're talking about a 12AX7 here, 200V is a healthy plate voltage,
though 250V is typical. Some crappy pre's even run them at 50V. 345V seems
almost excessive.

As long as parts aren't being damaged, there's always tweaking to be done to
tube amps for different sounds. A lower plate voltage than spec is one of
many. Still a perfectly valid application of the preamp. If you like the
tone, don't feel compelled to match specs.