• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

What other tubes can I use this OPT for?

If you want to be picky all paralleled tubes should have separate bias adjustments, so no need to match 100% anyways.
Another option is to bias slightly below max to give some slack if one tube runs much hotter than the other.
I'm one of those guys who do not believe I'll hear the difference if the tubes are matched or not. (yeah thats from actual testing, but perhaps my years as an aircraft electrician has messed up my hearing without me noticing;)
To answer your question tho, I would think any serious tube dealer that has 300Bs in stock will do the matching/testing for you.
 

jlsem

Member
2003-06-17 1:12 am
Dallas,TX
Hi
from what I read the 6528 is hard to get and plate resistance is 245 Ohms.. In parallel that would mean 122.5Ohm? How would that interface with the 1.5k primary of the transformer?
Looks like a solid tube though.. real kick ar$#

If you run the tube at around 300V on the plate and biased at -35 volts, which is a good operating point for an audio amplifier, the plate resistance is around 560 ohms. Two sections in parallel will have a plate resistance of 280 ohms. A good rule of thumb for push pull triodes is to use an opt with a plate to plate impedance of 4X the plate resistance of the tube. 4 X 280 = 1120. So, 1500 ohms will give somewhat less power but with less distortion. Run somewhat more conservatively at -38V to -40V gives a plate resistance of around 750 ohms. Run like this, 6528s will last for years.

John