Quick Question RE: Data Sheets

[adamshame]

I am looking at tube information on Duncan's site, and does Va mean Anode/Plate voltage?

And if so, why does it specify a max of 300v when I see voltages on the voltage chart being between 325-340v?

Sorry if this is a dumb question, but I am just trying to learn a bit more about HOW these amps work rather than just how to build them.

Thanks!

[zaphod_phil]

First of all you need to bear in mind that all voltages in a tube are relative to cathode. So if in a cathode-biased amp like an 18W , the power tube cathodes are at 12V say, then this 12V needs to be subtracted from the voltages measured at the anodes or screen grids, to get their true values.

It also seems that in many cases the manufacturers were either over conservative, or else they assumed that certain tubes like EL84s, 6V6s and 12AX7s would only be used in lower power audio applications, where higher voltages would be unlikely to be used (in their opinion). Guitar amps tend to break all the norms. In some amps (eg Marshall 20W or Messa F30) EL84s are being run at 400V. And many 6V6s will work fine at 450V, and even as high as 500V. I also know of a Seymour Duncan amp that ran a quad of EL84s at 560V to get 50W out - the secret is the screen grids were kept at 280V.

[adamshame]

Phil.. thank you for explaining this so simply!

I just converted a Hammond AO-39 with 6CM6 power tubes (I love these things!!!) and a 330 ohm "bias resistor" (I think that is what it is called), and due to the higher voltage of the PT (315v), I was getting about 355v on the plates. I looked at the data sheet, and it was showing the maximum plate voltage being 315v. So, with 24v on my cathodes, I am running them around 330v. These should be ok, right?

I found other posts on how to calculate "wattage" and I have them running about 10.75W. I know it is a little hot, but I am tired of tweaking this thing because I wired it the preamp and PI p2p with a terminal strip and the filter caps are on a small piece of tagboard. It sounds pretty good now, so I think I am done. I now just need to build a head cabinet!

[zaphod_phil]

I just converted a Hammond AO-39 with 6CM6 power tubes (I love these things!!!) ... So, with 24v on my cathodes, I am running them around 330v. These should be ok, right?...I found other posts on how to calculate "wattage" and I have them running about 10.75W. I know it is a little hot....

330V and 10.75W sounds all OK to me.