transplanted output transformers

[Wharfcreek]

Hello......I'm new to all this......and attempting to build my first ST-70, or perhaps I shoulds "rebuild" my first ST-70. I have been accumulating a number of parts....and just acquired a new set of output transformers. These transformers only have 4 and 8 ohm outputs, thus the 16 ohm feedback tap has no where to go.....other than the 8 ohm lead. This begs the question: What changes have to be made in order to use these transformers most effectively.....on a "stock" board, or on your DIYTube board? I will likely install these transformers in an amp with a "stock" driver board first.....but eventually may pull them out and install them in a "Final Product" version.......new chassis, new P/T, the works!! I'd appreciate any advice you can offer. Thanks, TSD

[kheper]

For the diytube.com driverboard, I used 2k
resistors as subs for R9 & R10 and 330pF
mica caps as subs for C3 & C4. The amp
is very, very stable.

The mathematical breakdown is: Going
from 16 ohm to an 8 ohm feedback
taps, the resistor value should be:

2.74K / 1.414 = 1,909 = 2k

Note: 2.74k is default for 16 ohm taps.

The cap value should be:

220pf * 1.414 = 311 = 330pf

Note: 220pf is default for 16 ohm taps.

Just divide the resistor value by the
square root of 2 and multiply the cap
value the square root of 2.

See the parts list, the BOM, on this
site for the original values and the
resistors and cap board numbers.

Sonically, the old, stock driverboard
does not contend with the diytube.com
board. I did not like the muddiness of
the st70 before I upgraded.

[mesherm]

On the stock ST70 driver, tack solder a 2400 ohm resistor across each of the two 1000 ohm feedback resistors and use the 8 ohm speaker tap and see how it sounds. 2400 in parallel with 1000 gives very close to 707 ohms which is 1000/1.414.