Swapping between output tubes in an AX84 amp

[simonallaway]

A couple of months ago I built an AX84 P1 eXtreme amp. All is well. I added extra 9 pin sockets for future tweakage (I am currently using a 6V6 in an 8 pin socket for the output stage). So now I am thinking I'll try some other tubes to compare tone etc.

I am thinking I need to do the following:

1) Wire up one of the empty 9 pin sockets with heater power, and the appropriate anode/grid/screen wires 'jumped' over from the existing 8 pin socket. This 9 pin would be for an EL84.

2) Do not jumper the cathode wire over, but instead use it's own resistor and parallel capacitor.

If that's true, l have a question.... Looking at the schematics I see that on the P1, the plate voltage is 255v, but on the schematic for the P1 eX it is 376v. Would 376v kill an EL84? Looking at tube charts shows me various voltages so I don't know which to obey (and guitar amps seem to ignore plenty of these implied limits anyway).

Thanks in advance,

Simon

PS When I say 'swap' I don't mean I want to use a switch of any kind. I mean 'pull one tube, put other tube in'.

[simonallaway]

If that's true, l have a question.... Looking at the schematics I see that on the P1, the plate voltage is 255v, but on the schematic for the P1 eX it is 376v. Would 376v kill an EL84? Looking at tube charts shows me various voltages so I don't know which to obey (and guitar amps seem to ignore plenty of these implied limits anyway).

I was looking at the table at the bottom of this image: http://www.triodeel.com/6bq5p1.gif  which tells me a maximum plate voltage of 300v, so I think I've answered my own question.

[Fresh_Start]

Tube data search:  http://www.mif.pg.gda.pl/homepages/frank/vs.html

While the data sheets show a 300 volt limit, IIRC there are several amp designs which run EL84s with considerably higher voltage.  Just a guess, but you might be better off biasing the tube a bit cool, IOW not 100% of plate dissipation, if you're going to run over the 300 volt mark.

Also, I believe that voltage limits for the screen grid are more important to pay attention to than the plate voltage "maximums".  However, let's get at least one other opinion on that...

Query:  With a cathode biased amp, is plate voltage the absolute value of B+ at the plate or the difference between voltage at the plate and voltage at the cathode?  (I think the 2nd hypothesis is correct but want to be sure)

Cheers,

Chip

[John]

With my EL84 cathode biased Harmony 415 (surely, you all remember that one?  ) the final measurement on plate voltage was 398. But, it sounded so sweet I went ahead and buttoned it up, figured I'd see what happened. So far it's got probably 50 hours on it with no harm or change in tone. However, I also  increased the cath resistor to 250 ohm/10W to bring the dissipation down to a safe level (12 watts IIRC). I read somewhere on here that voltage doesn't kill tubes, current does. So far it seems to be the case.

If I do start replacing tubes more than once a year, I'll start playing with some resistor/cap combos coming off the rectifier to try to reduce the B+. But for now, I was anxious to start playing, and it seems fine. When I get around to building a little 5 watter, I really think I'm gonna try to reproduce what I have, just with half the stuff. Don't worry, you'll know when I start that project too. 

[simonallaway]

Tube data search:  http://www.mif.pg.gda.pl/homepages/frank/vs.html

Cool! One place to find it all. Thanks!

While the data sheets show a 300 volt limit, IIRC there are several amp designs which run EL84s with considerably higher voltage.  Just a guess, but you might be better off biasing the tube a bit cool, IOW not 100% of plate dissipation, if you're going to run over the 300 volt mark.

So if I used the calculators here...

http://www.webervst.com/tubes1/calcbias.htm

...to show the bias for a Class A EL84 at 90% dissipation with 376V at the plate, it suggests 28.7mA. Do you think 90% is conservative enough given the plate voltage being > 300V ? I think I'll measure the cathode current in the stock P1 eX circuit across a 1Ohm resistor, just to see what it is now, and how that value compares with this calculator.

Query:  With a cathode biased amp, is plate voltage the absolute value of B+ at the plate or the difference between voltage at the plate and voltage at the cathode?  (I think the 2nd hypothesis is correct but want to be sure)

Also, the page notes something that seems to confirm your second hypothesis:

" Note: For calculating plate dissipation in cathode biased circuits, subtract the measured cathode voltage from the measured plate voltage and use that as the plate voltage in the calculator above."

And John, almost 400V on the plate sounds crazy when compared to the spec!  I do realise guitar amp designs aren't too good at observing tube specs, but that seems way over the top. But if it works, then it works I guess 

[John]

Well like I said, I'll see how long the tubes last. I don't know why it's as high as it is; the only thing that's different than the schematic is higher cap values (47 instead of 20uf) but I can't see that accounting for that much higher voltage.

[Rev D]

 I believe one of the Traynors ran over 400 volts as I recall. I'm sure it'll be fine, EL84's fortunately are fairly cheap too.

Regards,

RD