Ceriatone Forum

Hello,

I don't want to harras Nik with a thousand mails, so I think I should give it a try here:


In my 5X3, I swapped the GZ34 for a NOS 5Y3 and the sag is plain gorgeous.

From what I understand, with the 5y3, plate voltage is reduced by roughly 40Vs, meaning the 6L6GCs are now running a bit "cold".
The cathode bias should take care for the most of it and I shouldn't see any major problems.

But what if I dropped in 5881s (400V/25W) instead of the 6L6GCs (500Vs/30W) - would these be the adequate power tubes to use in conjunction with the 5Y3?


I'm bad at math and have little tube knowledge (as can be seen in my previous thread, haha) and i have no real knowledgeable amp guy in my area - please be patient with me.  :-[

That would get you closer, yes.  How close?  I dunno without the math.  But, it's not going to hurt the amp to run it that way long term.

You could also try a NOS 5V4 rectifier, which falls somewhere inbetween 5Y3 and 5AR4 in sag and voltage, and is still a 2A tube.  I bet a 5V4 and 5881 would dial in pretty close to "just right."

Personally, if it were my amp, I would take the measurements with both the 5AR4 and 5Y3, do the math and figure out the correct cathode-bias resistor value.  I expect it would be either a 250-ohm or a 220-ohm resistor.  BUT if you don't feel confident checking voltages, I don't want to encourage you to do so, not without someone with more experience there to observe. 

Nik lists his voltage reading to ground in a chart at the bottom of his links page, but for biasing you need to "actual plate voltage" (voltage measured between pins 3 and 8 on a 6L6GC or 5881) and the voltage across the cathode-bias resistor, then it's a matter of plugin them in here in the "Calculate Plate Dissipation Based On Plate Voltage And Cathode Current Readings" section.

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

First:
# of tubes sharing the resistor: 2
voltage across resistor with 5AR4: ??
actual resistor value: 270
actually plate voltage with 5AR4: ??

See what the calculated current and dissipation is.

The try it with the 5Y3:
# of tubes sharing the resistor: 2
voltage across resistor with 5Y3: ??
actual resistor value (start with stock): 270
actually plate voltage with 5Y3: ??

The start trying lower resistor values until you get close, without exceeding the plate dissipation you found above.

thanks for the link to the voltage chart, i didn't even know it existed!  :D

i think i now halfway understand what is going on in there... but i don't dare take the measurements myself, so i'll take it to a tech and get on his nerves with my half-knowledge.

thanks a lot, i sure appreciate!!!