30VA bias transformers

the thermionic watercooler

30VA bias transformers

Postby kheper » Mon Aug 13, 2007 3:14 pm

I purchased 2 new, little transformers.

http://nekhbet.com/new_amp/bias_trans.jpg

They are secondaries are supposed to be:

40-0-40

and

20-0-20

(The guy who sold it to me is not sure about
this, but he is checking.)

Ok. So, take the 40-0-40 secondaries

(40 * 2) * x = 30VA

80 * x = 30

x = 30 / 80 = .375

The max amperage should be .375 A,
more than adequate for any bias supply.

How should I proceed? A capacitor input
full-wave rectifier, on the 40-0-40 should
yield about 57Vdc. Or, a single diode on
the center tap should yield 80Vdc.

Would it be best to hook this up as in the
stock Dynaco 70 schematic with 1 bias
transformer and 2 pots for the 4 output
tubes? Or, since I have 2, use 1 bias
transformer and one pot per side with 2
output tubes per side?

The output tubes are going to be 807 tubes.
I have been reading that the 807 tubes do
not like more than 25ma idle current.

The amp will contain ALL seperate
transformers. I have a UTC 10A 6.3V
filament, the 80VCT bias, and a soon to
be had 700VCT high voltage transformer.

Since I'll be going with a SS rectifier, I'll be
able to put a standby switch on the high
voltage transformer's primary. A better
solution than trying to switch high voltage
secondaries.

However, I am going to have to build or
buy a very large chassis to accomodate
all of the trannies. Something on the
order of 17" W x 15"L x 3"H. Maybe even
larger.
User avatar
kheper
KT88
 
Posts: 1252
Joined: Wed Dec 21, 2005 10:14 pm
Location: Philly, PA

Return to diy hifi

Who is online

Users browsing this forum: No registered users and 18 guests