Zeners added, voltages before and after

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bvayling
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Re: Zeners added, voltages before and after

Post by 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
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Re: Zeners added, voltages before and after

Post by JMPGuitars »

bvayling wrote:
Sun 04/26/20 10:26 am
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.
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Re: Zeners added, voltages before and after

Post by 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.
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Re: Zeners added, voltages before and after

Post by 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
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bvayling
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Re: Zeners added, voltages before and after

Post by bvayling »

mm. here's voltage chart
17V zeners before A BVA 18W classic voltage.jpeg
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bvayling
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Re: Zeners added, voltages before and after

Post by 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.)
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Re: Zeners added, voltages before and after

Post by JMPGuitars »

bvayling wrote:
Sat 05/02/20 9:34 pm
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?
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bvayling
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Re: Zeners added, voltages before and after

Post by 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.
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Re: Zeners added, voltages before and after

Post by zaphod_phil »

Reduced B+ will always give less headroom.
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Re: Zeners added, voltages before and after

Post by JMPGuitars »

zaphod_phil wrote:
Sun 05/03/20 1:33 pm
Reduced B+ will always give less headroom.
That was his goal. He had too much headroom before.
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Re: Zeners added, voltages before and after

Post by 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
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