Zeners added, voltages before and after

[bvayling]

Ok, thanks for your perseverance Josh. In reality, I was measuring C at the wrong place, where the screen grid resistor R25 and the power tube screens intersect. So yes, my C voltage is 294.

I guess right now I will put it back together and play it for a while before changing anything else. I have a new lower powered speaker to try out too.

I still want to try running the zeners through A but I need to order some 10V zeners first, as I am running out of real estate to add more 5’s.

Cheers,

Bern

[JMPGuitars]

Ok, thanks for your perseverance Josh. In reality, I was measuring C at the wrong place, where the screen grid resistor R25 and the power tube screens intersect. So yes, my C voltage is 294.

I guess right now I will put it back together and play it for a while before changing anything else. I have a new lower powered speaker to try out too.

I still want to try running the zeners through A but I need to order some 10V zeners first, as I am running out of real estate to add more 5’s.

Cheers,

Bern

Sounds like a good plan. You can add the Zeners before the standby switch if you need to. That's how I did it in the build photo I linked previously.

[bvayling]

Yes, I saw that. I actually have some space above the capcan between the PT and the board. I think I'll be able to fit everything in.

[bvayling]

Ok, so this time I used the same 17V zener chain, but I put it between the standby switch and A.

As I expected, it dropped the plate voltage to the range, actually a little below.

A: 342V
B: 326V
C: 293

I attached the voltage chart and the bias chart.
The new bias calc with the lower plate voltage shows the plate dissipation at 10.4 W, so I am tempted to put the 160 ohm cathode resistor back in and see what that does.

17V zeners before A BVA 18W classic bias.jpeg

17V zeners before A BVA 18W classic bias.jpeg

[bvayling]

mm. here's voltage chart

17V zeners before A BVA 18W classic voltage.jpeg

[bvayling]

I was surprised to see B+ of 349V at pin 3 of V6, since the zeners are connected between the standby switch and the capcan terminal A and I thought the voltage only got reduced after the zeners??

(Before I moved the zeners, I had 359V B+ at pin 3 of V6.)

[JMPGuitars]

Ok, so this time I used the same 17V zener chain, but I put it between the standby switch and A.

As I expected, it dropped the plate voltage to the range, actually a little below.

A: 342V
B: 326V
C: 293

I attached the voltage chart and the bias chart.
The new bias calc with the lower plate voltage shows the plate dissipation at 10.4 W, so I am tempted to put the 160 ohm cathode resistor back in and see what that does.

Those voltages look good.

You can leave the cathode resistor as it is. If you lower the resistor value, you'll be biasing the tube hotter. My guess is 160 will put you around 11W or so per tube.

How does the amp sound now?

[bvayling]

Oh, yes, that's why we do this!

The amp sounds great, I think it has noticeably less headroom than before. I’ll be able to make a better assessment when I put it back together.

[zaphod_phil]

Reduced B+ will always give less headroom.

[JMPGuitars]

Reduced B+ will always give less headroom.

That was his goal. He had too much headroom before.

[bvayling]

Ok, I could help myself. I left the zener array the same and put the 160 ohm cathode resistor back in.

A,B and C down 2-3 V, almost spot on the "target." B+ and plates also down a couple of volts. Cathode voltage also down to 11.9

As expected, plate dissipation back up to 11.2 watts, but within reason I think.

Can't tell if I can tell any difference.

Voltage and bias charts attached. I think I am done and will transition to playing the amp!

voltage chart 18W 160 ohm 17V zeners into A.jpeg

bias GDS 18W 160 ohm, 17V zeners into A.png