Cathode biased 6550... anyone tried it?

[bluesbear]

I built a single ended 6L6 amp for practise that's just not quite loud enough unless I go full tilt. Bye,bye any kind of clean. I looked it up and the 6550 should be just about double the wattage of a 6L6 in class A operation. That should be just about right. The question is, will 6550's take kindly to cathode bias? I've never seen an example so, once again, I'm asking for expertise at the best place I know.
Thanks,
Dave

[phsyconoodler]

Any tube will take cathode biasing,it depends on how high the voltage is.Too high and it gets hard to control with a cathode resistor.

  To get the headroom from a single 6550 you need to anticipate the output it can produce and design the rest of the amp around it.Too small an OT and it will saturate early.An inefficient speaker will break up early too.
  I surprised myself by putting a very efficient EVM speaker in a Princeton Reverb once;it raised the headroom by about 3-4db easily.
It used to breakup at about 4-5 and with the EV it was hard-pressed to break up at 8.And as loud as any Deluxe Reverb too.

  Big OT and efficient speaker = higher clean headroom.

[bluesbear]

This thing already has a 15 watt single-ended OT and a 20 watt Hound Dog speaker.If this isn't going to work, I suppose I could add a second 6L6 in parallel. I have the room. I'd have to replace the speaker, though. As I look at the tube specs, maybe I'd be better with 2 - 6V6's. It looks like in class A operation, a 6V6 is only a little less wattage than a 6L6. 2 - 6V6's would mean no speaker change. What do you think?
Thanks,
Dave

[nateflanigan]

I came across these the other night looking around for something similar...

http://db.audioasylum.com/cgi/m.mpl?forum=tubediy&n=130609&highlight=proud+papa

http://diyaudioprojects.com/Tubes/KT88/

[DummyLoad]

buttery and i fiddled with A1 single tube KT88 a while back - not much came of it.


this looks interesting.

http://diyaudioprojects.com/Schematics/SE-6550-Tube-Amp-Schematic/

[PRR]

First make SURE your 6L6 is powered and volted to best advantage. What plate-cathode voltage? What current? What load?

6550 was intended to cover the far-fringe of audio amps: deep fix-bias push-pull HIGH power.

However it works fine cathode bias.

I spent a LOT of time fooling with a Single-Ended 6550. The manual says 20 Watts but this is at high THD, large bias-shift, and inconvenient G2 voltage. My scratchings said 17 Watts would be nice. My parts-hunt did not turn up just-right parts.

I ended up with 13 watts of fairly clean sine-wave and 25 watts of total distortion (which would not threaten a "20W" guitar-market speaker). I could get here with just 34 watts Pdiss, and the PT and OT that I had would not match-up to be able to take advantage of the 42W Pdiss rating.

With a third re-build I probably could have found my 17 Watts, but the difference on the ear 17 vs 13 is not worth the effort.

6L6GC should do 3/4 the output of 6550. That's not on the datasheet which didn't update the SE suggestion for the GC redesign.

> It looks like in class A operation, a 6V6 is only a little less wattage than a 6L6.

No. 6V6 can't sanely make 6 watts. Original 6L6 had an SE suggestion for 11W; this turned out to be too bold and was revised to 10W. However this is the 19WPdiss can; the 30WPdiss bottle can do more.

Run two 6L6GC at 25W Pdiss each (50W total, >60W for total plate power). If correctly loaded, that puts you just about 20W clean sine, and your "20W" guitar speaker will easily swallow such an amp even full-blast clipping.

Two 6V6 in push-pull will also get near 20W, using standard Fender-clone iron, with much less heat and weight than a good 20W SE amp. Lower THD too, up to clipping. So be _sure_ you need the SE overdrive sound.

[bluesbear]

Well, my problem is I have this amp I went to the trouble of making. I have 15 and 20 watt amps; they're too loud for what I need, for band practice and for a 2 piece blues/jazz at a small wine bar. This 6L6 Champ would be fine if I could turn it all the way up and still have anything but OD. It sounds great but I don't play leads all the time. I just need to eke out a little more and I'd be fine. I guess my actual question is, will I get more from a 2 - 6V6 SE than I get from 1 - 6L6, all other things being equal? If not, I suppose I could go with 2 - 6L6's and replace the speaker (to match the impedence). I was hoping for a fix that didn't involve spending any money but I guess that may be impossible.
What do you think?
Thanks,
Dave

[jjasilli]

I think you can fix the issue with a resistor from your parts draw!  In my VibroChamp, schematic posted in the Schematics Section, I "boosted" preamp voltage to about 270VDC on the plates. (I had also increased plate resistors to 220K, along with larger cathode resistors).  The result was lots of clean headroom, almost no overdrive even dimed.  (Then I added a RAW control which allows me to dial overdrive in or out at any volume setting.)

[alerich]

jjasilli - do you have a link to that? I searched but couldn't find it. Very interested in what you did. Would like to try it on my VC. More clean headroom would be fun with that little amp since I really don't like the way it overdrives.

[jjasilli]

It's here, scroll down:  http://www.el34world.com/Forum/index.php?topic=6088.0

[HotBluePlates]

Or find a method that suits you to hamstring the output power of your bigger amps, down to the volume you need.

[alerich]

Thanks, HPB!

[nateflanigan]

I also found this.  Looks like it's supposed to be a SE plexi, which sounds like fun, I guess. 

http://i9.tinypic.com/8brpco2.jpg

Any idea what voltages one would want from the PT?