Another Scope Question

[surfsup]

Paul Ruby's page on his mod seems to be misleading (to me).

http://www.paulamps.com/18watterbuzz.ht ... Distortion

Why doesn't one just measure the cathode voltage of the EL84 and round up to the nearest zener value? i.e. if I measure 10.7 volts at idle, why not just use an 11V zener with a UF4007? He states the cap charges as the negative swing of the grid signal exceeds the cathode voltage. So just measure the Vk and put in a zener slightly greater?

[surfsup]

Okay so I measured my amp and at idle the EL84s share 10.6V on the cathode pin to ground. Here's my scope shots, the clipping starts at around 11V as the second scope trace shows 4V/div and the Yellow signal is clipping already.

Third scope trace shows 4.5V/div and the traces are clipping at 14.5 volts or so. Notice the zero point has shifted upwards though, around 2V.

Fourth scope trace shows max grid input from the MV. Clipping is around 21V with the zero point having shifted up about 3 or 4 volts. Since the negative trace is not getting cutoff, I assume the shift upwards for the crossing is due to CC charging and artificially raising the grid voltage and therefore increasing the bias voltage. So shouldn't I just need to put in a zener greater than the 10.6V measured at the cathode if I want to properly utilize the PRMod?

[surfsup]

Thinking about this even more, instead of using two diodes and two zeners, why couldn't I just put a Zener across the EL84 cathode resistor? Like a 12V zener. This is one piece instead of four. Okay I'm probably missing something since Paul Ruby would have tried that himself...

Hmm, that wouldn't prevent the CC from charging though...? Okay no more posts until I think about this some more...

[katopan]

You're right about the bias point shifting because of the coupling cap charging up, and your waveforms show it. It also shifts because the cathode cap also charges up, and both mechanisms work to cool off the bias. When it goes far enough you get crossover distortion (which is completely normal and common). But with some OTs and some amps it sounds buzzy. The Ruby mod prevents charging up of the coupling caps, but not charging up of the cathode cap. But that is enough to remove or reduce the crossover distortion enough to get rid of the buzz.

As you stated, the bottom part of both side waveforms isn't cutoff or clipped. Adding the Ruby mod creates the clipping on the bottom part of the wave needed to prevent charging up of the coupling caps. Sizing the zeners is as simple as measuring your cathode bias voltage and picking the next zener voltage up, and the value actually isn't critical.

[surfsup]

Ok thanks for confirming! I ordered 12v zeners, but only ordered one instead of two so I have to reorder more. I might get an extra one for the cathode to see what happens with justbthat before I add the PRmod...

[surfsup]

Well I finally got around to doing this and it does, indeed, help. The amp is much smoother now, single string, double stops, etc. This is a nice mod. I only gator clipped it in so far. Going to solder it in now.

[tubeswell]

Hi surfsup,

just saw this sorry. Did you ever put your scope on the PI plates to see whether the spike was happening there? Just curious.

[surfsup]

Tubeswell, no spike on the PI plates (after the cc's), but, perhaps something changed and i dont remember if i tested that again after messing with this mod. I'll try that tomorrow since the amp is out already.utting kid to bed soon. Playtime for dad is o-vah!