Ampeg "Guitaramp", 1956?

[jjmt]

jjmt,

for each tube, following the process explained by Uncle Doug in his youtube video:

- (power off) measure the resistance between pin 8 of the 5Y3 and pin 3 of the 6V6. This is the impedance on half of the input side of the output transformer ®
- (power on) measure the voltage drop across that same region (V).
- calculate plate current as V/R (PC)
- measure plate voltage relative to ground (PV)
- calculate Power Dissipation (PD) PD = PC*PV = 10 watts

6V6 tube has a max rating of 12 watts. 10/12 = .83

I installed independent bias resistors (550ohms and 680 ohms) and capacitors for each 6V6 to make the outputs approximately equal at 10 Watts. The amp has plenty of volume, so you may be correct that I can reduce the output of the tubes further.

I did not chart the results after I replaced the cathode bias resistors and caps, I just assumed that the results would fall out of my plot at 10 watts per tube (red dots on plot I posted earlier). To get down to 70%, I would be at 8.4 watts. To do that I think I would need 680 and 1000 ohm bias resistors by interpolation for the outer tube and extrapolation for the inner tube.

I am happy to hear your thoughts. Am I on the right track in your mind?

Larry

Hi. That all seems correct, although the splitting of the cathode resistors might have been a bit of overkill. You haven't mentioned what your plate voltages are, but some quick guestimates put the difference in plate current of your two tube at under 5ma originally. That's on the high end of what you can get away with, but dropping the joint current draw by 30% would likely have gotten you down to around 3.5ma, which is pretty standard for a "matched pair", at least in the current state of NOS.

In any event, if your tubes aren't red-plating, then I would leave it at the current levels. On a different subject, you didn't mention if you by-passed the split resistors? The original circuit had a bypass cap to eliminate interaction of the two tubes that can result in some nasty oscillation. If you bypassed the independent resistors, all is well, and if not, all is also well . But without the bypass caps your plate impedance is going to increase (I think), which is going to affect what it sounds like. More compression, I think. Maybe that is a good thing, but just curious as to your experience.

Beyond that, there is the whole Class A versus AB discussion that is a minefield that I usually try to avoid, but there are those that argue that a shared cathode resistor effectively imposes class A on the amp (much debate can be found on this), whereas splitting the resistors is more consistent with class AB, which would support a bias level down towards 70% and which would allow your tubes to run cooler. Given that you seem willing to experiment, you have somethings to consider.