hi watt cathiode biased amp build

[menger66]

what are some higher watt cathiode biased amps i could check out for insperation? mark

[roberto]

Order of magnitude of the power? 10, 100, 1000 W?
Any preference on the power tubes, or sound?

[Cliff Schecht]

Some really early amps run about 40W through a cathode biased pair of 6L6's. I think Fender even did this in one very early amp, but I don't have my cheat manual in front of me to confirm this. It's definitely been done by a few manufacturers but it isn't popular in vintage amps because of the amount of power you lose and how easy the output stages compress and distort compared to fixed bias.

I built a 50W Rocket with two 6550's or Telefunken EL34's in cathode bias operation. I really don't remember the plate voltage on that amp but if I was using that big of an output tube, I'm sure it's at around 420-450V on the plates (and the dropping string adjusted accordingly to get the preamp voltages right). That amp was strange, but not at all in a bad way. It's very touch sensitive but not like the Express because it never really truly cleans up. The character of the distortion changes as you play it harder though and I really enjoy the complexity it gives to chords that usually don't take so well to distortion. A lot of the touch sensitivity does come from the output stage, you see a rather large jump in cathode voltage on the output tubes when you start playing hard (meaning super sqquuuiisshhhyyy!!).

[Stevem]

why would one want a 1000 watt amp?
For one thing it would only be 2 times a loud as a 100 watt amp, yet some 4 times greater in weight, not to mention far far greater heat output!

A somewhat low wattage amp ( some 40 watts) with 4 output tubes could use 2 tubes in fixed bias and 2 tubes in Cathode bias for a great all around amp other than loud metal rock!

[Leo_Gnardo]

what are some higher watt cathiode biased amps i could check out for insperation? mark

Vox AC100, bass amp from mid/late 60's. I modded mine to have individual Rk's. Sounds terrific, uses EL34. Only problem - Vox put it in a tiny box so it bakes itself to death if left on for more than an hour. Give yours lots of room to breathe & I'm sure you'll be just fine.

[roberto]

You just remembered me that I have to try the mod suggested a while ago on this forum, to split the Rk/Ck combo on the power tubes with the following:
- a resistor (with 84% of the value of Rk) in parallel with Ck;
- a resistor (with 16% of the value of Rk) to ground.

Otherwise, with fixed bias:
- a resistor (with 16% of the value of Rk) to ground.

[menger66]

I have some 50watt output transformers and 50 watt fixed bias amp are just to loud to turn up to the sweet spot, so i was thinking id try a marshall 2203 with cathiod instead of fixed bias, as far a tubes i have el34 kt66 6550 and 6l6 and some nos tung-sol 5881, thing is these tubes in cathiode bias would be real luckey to pull 40watt from in cathiode bias and sometimes 50watt output transformer won't sound that great if not pushed enough so i'll have to check it out,

I was just wondering if any past or present builders have larger 40-50 watt amps out there using cathiode bias? i''ve already built many of the fender amps im more of a marshall guy

LEO the ac100 is it a 4 el34 cathiode biased amp?

[Leo_Gnardo]

LEO the ac100 is it a 4 el34 cathiode biased amp?

Yes, that's what it is. I changed it from all 4 tubes on one resistor to one Rk per tube.

Something about cathode bias vs fixed, I've installed switches in amps to select one or the other on the basis of "half the power more or less in self bias." I don't bother with that anymore because there wasn't enough difference in perceived loudness between the two methods. Despite the obviously lower power measured by meter in self bias, they sounded much the same.

[sluckey]

LEO the ac100 is it a 4 el34 cathiode biased amp?

All the schematics I've seen show a fixed bias amp.

[Leo_Gnardo]

LEO the ac100 is it a 4 el34 cathiode biased amp?

All the schematics I've seen show a fixed bias amp.

Vox weren't paying attention to their schematic I s'pose.

When I opened it up after first getting it, resistor biased it was.

FWIW I also have an AC50 head, self-biased. Both appear to be @ 67-68. And they sound terrific!

[KellyBass]

Leo, do you roughly recall the plate voltages of your Vox's?

[menger66]

The vox ac100 sounds pretty cool, but the amp im putting together is set up for 2 powertubes ill dig around for the ac100 schematics