6550's in a EL34 Solarus (or other similar) type amp

[MusicFox]

I was informed by a senior member that the 6550's will work in a EL34 40 watt amp (quirk of fate?) like my 69 solarus with a different (hotter?) bias setting and a slightly reduced overall output. I was also told it was safe for the output transformer. I'm guessing that the tube and curcuit specs have it running "Cooler" than a EL34 in same amp. This Idea does not work other way around (EL34's in a 6550 type amp) without rewiring the socket I am told as well. Go figure....

My question is.for all concerned ..what current level should the 6550's be biased at if in a 40 watt (EL34) amp?

Thanks.

[Soundmasterg]

The 6550's should be biased somewhere between 60% and 70% of the maximum dissipation level. I can't tell you what current that would be because it depends on what the plate voltage is in the amp in question.

Look up the specs: http://www.nj7p.org/Tube4.php?tube=6550

and you'll see the max dissipation of the 6550 is 35 watts. So 60% of that is 21 watts, and 70% of the max is 24.5 watts. So measure the plate voltage, multiply by the current (in milliamps) and you end up with the dissipation.

So an example:

550v times .035ma = 19.25. This would be a cold bias...below 60%. So we turn up the bias and remeasure, BUT, the plate voltage has now changed. So 545 times .040ma = 21.8. This would be in the range that is good sounding and safe. Adjust towards the 70% for a fatter and more harmonically rich sound, and shorter tube life....adjust towards the 60% for a colder sound with longer tube life. My example was made up....you should really check the amp for yourself and see what the voltages and currents are to get an accurate reading.

If your amp has the smaller power transformer, then you may have to be careful running the bias really hot and running the amp wide open....as the power transformer may not be able to supply the increased plate current. If this is the case, then you may get lucky and just blow a fuse after months of use, but you can adjust the bias a little on the cooler side (around 65% and less) to be safe.

Greg

[MusicFox]

Awesome Bias info. My restoration tech will be thankful for all my research with your help.....Thank You.

I'm also wondering what tonal or other characteristics anyone has experienced using both types of tubes 6550's or EL34's in the 40 watt amp (Smaller transformers). I intend to use my Solarus and 2X12 Marslands for Guitar as well as a Bass amp backup for my Laney Bass stack as I play both instruments regularily. If the Sunn sounds good with bass I'll use it a lot more there. 40 watts is plenty most small rooms I play. I used to have a faulty Ampeg V4 which used to cut out after warming up a frustrating 10 minutes under hard playing with Bass. Huge tube tone with any cabinet. My hope is the Solarus will sound good with bass too. Tube tone!!
Thanks