cathode bias my twin? Will it work?

[dragonbat13]

I have dan torres book inside tube amps. It has a mod to convert an amp to cathode bias. The mod looses the vibrato which I don't use anyway. I have already converted the amp from hum balance to true bias adjustment. I want to try and get some of the sound of my tweed princeton. Is it possible to cathode bias a 100 watt 74 twin? I run two kt66 tubes and also plan on using one of the empty sockets to install a tube rectifier. Thanks for any help and comments.

[bueller]

Sure you can cathode bias anything. But it may not change the sound much, and it may not last long, depending on how well built the rest of the amp is. I've made two amps that are switchable and frankly there isn't much difference to me between the bias method. On the other hand there is lot more difference in the bias point. Meaning where you set the bias. Typically for a cathode bias designers seem to stray more toward maximun plate dissapation numbers, like 25-watts for a EL34. Where-as fixed biasing seems to be more conservative like 12.5 to 17 watts dissapation (as measure through the cathode including both plate and screen currents). I'm not sure why this is. I use a more conservative bias point even when using cathode biasing.

Bogner has a cathode bias option on the XTC and it's 120 watts running at 540VDC! But he uses 50-watt resistors with heat sink grease directly mounted to the chassis. You have to oversize the resistors, use one for each tube, and keep any bypass caps away from the heat producing resistors. Because they do get hot. Adding a cooling fan does wonders for tube life, but then you need a DC source for a fan. E.g. another transformer or pull it off the heaters. Speaking of which KT66s draw a lot more heater current than 6L6s is your power transformer up to it?

[dragonbat13]

I never run four tubes. I figure the current of two kt66 tubes is less than the current of four 6l6s. Is this correct?

[dragonbat13]

I just checked the specs. 2 Kt66 tubes draw 2.6 amps. 4 6l6s draw 3.6 amps. I never ever run four tubes due to the fact that it is just too loud. I even run two 16ohm celestions in parallel to account for the impedeance change with only two tubes.

I have not run the amp this way for long but I will say this:

Two KT66 tubes and a pair of vintage30's is quite impresssive. I have also done a cap job, blackfaced the amp with a torres kit and installed all new sprague filter caps. Next step is the Rectifier.

Anyone got any opinions on a good rectifier tube for my setup?

[dartanion]

Why not just add a sag resistor and save yourself the hassle of adding another tube. Same effect, only cheaper to implement.

[jaysg]

The changes you're making are pretty cool, but I wouldn't bother with a rectifier or going cathode biased. If you're looking for sag, dartanion is right. The 25W gold backed bolt-on type Dale power resistors are a good thing to try.

Word of warning...I just had a 25uF, 100V Sprague Atom in a bias supply, take out an amp. The part was about 2 years old. I've heard similar experiences from others with the smaller black atoms. I suppose cathode bias might negate that possibility. No more Atoms for me.

[David Root]

That's interesting.

I have been using Black Gate electrolytics in my bias circuits lately, but I have always used three caps, each 10uF/100V, one on the diode and one each on the wiper of the two pots, regardless of brand, without problems, in the past.

Do you believe this is a systemic problem with the black Spragues, and if so what was the mfg date on the defective one?

[chief mushroom cloud]

you can mod to cathode bias AND keep the vibrato if this is a silverface 74 twin......
I've done it.
You just need to keep the neg bias for the vibrato circuit.
Torres is an effing idiot.