Zeners added, voltages before and after

[bvayling]

I put 3 x 5.6V 5W zeners in series in the B+ line.
B+ still 359V at pin 3 of V6 of course, but now 328V at the capcan connection on the board.

Too low?

Voltages on preamp tubes significantly lower, more "in line" with target values on the chart.
Plate and cathode voltages on the power tubes pretty much the same as before but the screen voltage is significantly lower.

How do these voltages look to you?

Thanks.
bva

18W Combo voltage chart 180 ohm.pdf

Voltage chart with zeners & 180W cathode resistor.pdf

[JMPGuitars]

Most of the voltages look pretty good. But you should also post your ABC voltages.

Your tubes are biased slightly hot, but safe how they are.

Here's where to get your ABC voltages:

18watt-Layout-Mark-Durham-ABC.jpg

I'm assuming point B is 328V?

Thanks,
Josh

[bvayling]

Cool, thanks as I was wondering what that was all about. I’ll measure and post those tonight when I can get back to it.

[bvayling]

A, B and C measured:

A: 360V
B: 345V
C: 328V

I am still confused by the actual definition of B+.

Josh says it is voltage at pin 3 of the rectifier. In his assembly manual Graydon refers to it as the voltage at any of the points on the board where the wire coming from the capcan connects.

On my build the voltage at the insertion point on the board of the wire coming from the capcan is 328V, same as C.
The 3 other points on the board where that wire connects are at 294V.

When everybody talks about the “ideal” B+ being 340V+/-,
Where are we talking about measuring it?

On this build with the zeners inserted, the difference of 17V between the 345V at B and the 328V at C makes sense as the sum of the zeners.

Still, where am I looking for that 340V?

Thanks!

[JMPGuitars]

B+ is the DC voltage coming from the rectifier.

Everything after it, on the same line, is a node of B+.

A is usually directly B+ (unless something like voltage dropping resistors, sag resistors, or diodes are placed between B+ and A).

B and C are nodes of B+, but the resistors and decoupling caps separate them.

Take a look at this schematic: files/JMPGuitars_18_Watt_Lite_2b_Schematic.pdf

B+ is pin 3 on the rectifier, and you can see how A, B, and C follow. It's much easier to understand on a schematic than a layout.

Side note: the term "B+" is from a million years ago when tube radios used batteries. A, B, and C, are just reference points, not directly related to the old terminology.

The ideal B+ is 345VDC (+/- 5V), and the other target voltages that follow are in the voltage test chart.

Thanks,
Josh

[JMPGuitars]

The 3 other points on the board where that wire connects are at 294V.

It sounds to me like that 294VDC is actually your C voltage reading. If I'm correct, then your voltages are fine. The tubes all look pretty good.

[bvayling]

Thanks Josh. This is good, as I am attempting to get more familiar with schematics.

C is definitely 328V

So, looking at the “target” numbers at the bottom of the voltage sheet for A, B and C with C being at 290, my C of 328V (after the zeners) still seems way too high.

Like almost 40V too high? I must be missing something or I need to add zeners to reduce this C number?

After adding the 17V of zeners, it seems my preamp tube voltages have come more in line, but power tube voltages still off?

Trying to get as close as I can.

Thanks.

[bvayling]

Another thing: is there any reason not to run the zeners into the first capcan leg (A) rather than after the second (B)? That way you would also be reducing the voltage on the plates.

[JMPGuitars]

Another thing: is there any reason not to run the zeners into the first capcan leg (A) rather than after the second (B)? That way you would also be reducing the voltage on the plates.

They're less efficient that way, but I do it. Usually because of convenience or the lack of space anywhere else. If you want to drop 20V, you might need to use as much as 40V Zeners total in series. I would probably try 30V total and see where that puts you.

Take a look at my most recent build: https://www.instagram.com/p/B_Q42I3n2d-/

If you look closely, you can see a small turret board to the right of the PT. That has my SS rectifier, and Zener voltage dropping diodes for both rectifiers. That's all before the standby switch.


Which layout did you build with?

Thanks,
Josh

[bvayling]

Ok thanks. I think I’m going to experiment a little. I think running the zeners through the A capcan terminal will help. I can see how it’s less efficient but looks like it would ultimately help all the voltages. I’ll do it and measure and see what is what.

I used a combination of Graydon’s and Richie’s 2008 layout.

[JMPGuitars]

What's your voltage where the PI plate resistors come together at their voltage source?

measure.jpg

[bvayling]

294V

[JMPGuitars]

294V

That's what I thought. That is your "C" voltage, so that string is getting close enough to the correct voltage.

It also makes sense. Your B+ was 19V high, you dropped 15 volts, and you're 4V high at C.

If you like the sound of your amp, there's no big reason to change anything.

[bvayling]

Well actually I liked the sound best when I first built it. It seems each step of increasing the cathode resistor has made the amp cleaner. So since I really want this amp to break up at moderate “studio” volume, I am looking to restore that feel while keeping the plate dissipation within spec.

I read your C as the point where C2 and R6 intersect. There I have 328V.

Thought I would run to the zener chain from the capcan terminal A instead of B, measure voltages and plate dissipation with the lower voltages and see where I am. I am actually hoping that the zeners will reduce the plate voltage enough so that I could put the 160 ohm cathode resistor back in and be in spec.

[JMPGuitars]

Well actually I liked the sound best when I first built it. It seems each step of increasing the cathode resistor has made the amp cleaner. So since I really want this amp to break up at moderate “studio” volume, I am looking to restore that feel while keeping the plate dissipation within spec.

I read your C as the point where C2 and R6 intersect. There I have 328V.

Thought I would run to the zener chain from the capcan terminal A instead of B, measure voltages and plate dissipation with the lower voltages and see where I am. I am actually hoping that the zeners will reduce the plate voltage enough so that I could put the 160 ohm cathode resistor back in and be in spec.

That's correct, but you're still somehow reading the voltage in the wrong place to get 328V. Look at the schematic: storage/18watt%20Schematic%2008-28-03%20-3.pdf

C2 and R6 show R18 and R19 between them. You read 294VDC at the point where R18 and R19 are, so that's fine.

The way the B+ affects the tone is this: higher B+ means less distortion, and more headroom. Lower B+ means earlier breakup and less headroom. Clearly you want to lower your B+.

The reason I believe you're not happy is because your zeners are between B and C points. This leaves the power tube voltages high, giving you more headroom, while placing the preamp closer to the ideal specs. So you should place your Zeners either between A and B, or before A. Before A (between the standby switch and filter caps) is the easiest, but will require extra Zeners for the same voltage drop.

Thanks,
Josh

[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