My Testing an External Plate Voltage Test Point for a Bias Reference

[Platefire]

I always thought this was an good idea to have a ready readable plate voltage to go along with external Bias test points and external bias pot adjustment. I didn't know quite how to do it until PRR showed me a Voltage Divider Circuit that would reduce the measurement down to safe "mV" to avoid a possible melt down/short on the chassis. So I have attached the circuit PRR provided below.

So going by my schematic I picked up some 3 watt, 1Meg, 1% Vishay Metal Film resistors(Red). I measured the 5 I had and found one the measured exactly 1Meg, so I used that one. I already had some metal film 1K, 2 watt resistors(gray) and use one that measured .986. So that one would fall in the 1.4% range---pretty tight!
So I've included a picture of my layout arrangement. May be hard to see but you can see it if you look close.The 1Meg goes from #3 Plate pin to a empty term on an existing terminal strip and then a red wire from terminal strip to test point. Where I put the test point just happened to have an empty terminal strip ground handy, so I was able to install the 1K fairly easily even though in tight quarters.

The results can be seen on my pictured Multi Meter readings. The direct reading of the plate voltage was 416.5DCV and the reading through the test point circuit was 410.1mV=410.1DCV. So the test point circuit reading had a 6.4DCV shortfall from actual.

So in conclusion I guess you could note the 6.4VDC offset and add that your readings taken from now on taken on this test point or either call the 410.1VDC close enough??? I just had to try this out! What do you all think???? Platefire 

[Platefire]

The Test point that was added is to the far right. Tried to get it lined up with the others but it's sagging a little:>(BTW---The amp is a Dukane PA Head converted to 5F6A Bassman back in 2006.

[2deaf]

Divide your result by .000985

[Platefire]

.41634517

[2deaf]

.4101/.000985=416.35V

[Platefire]

Where are you deriving your .000985 from?  1K resistor measurement?

[2deaf]

R1/(R1+R2)=X   R1=986   R2=1M   986/1,000,986=X   X=.000985

[Platefire]

Thanks!

[sluckey]

Now you gotta do one for the other tube. 

[Platefire]

Not even tempted

[sluckey]

How can you ever know the dissipation of the other tube if you don't know the plate voltage for the other tube? To assume the plate voltage of tube 2 is equal to the plate voltage of tube 1 is just wrong. May as well use a single cathode resistor and assume the current splits equally through the two tubes.

Come on plate. Just do it. You know you want to!   

[jjasilli]

Do 5 test points for each tube and then take the average.

[Diverted]

This is not to the main topic of this post. But man, I love that old school blue PA head!

[shooter]

Divide your result by .000985

that's not even a number!  that's like spiting hairs with a hatchet 

[Platefire]

I'm Happy with it

[sluckey]

Divide your result by .000985

that's not even a number!  that's like spiting hairs with a hatchet 

Sure it is. If that was seconds it could represent 79.7 miles on a radar screen. That's a BIG number! 

[PRR]

Platefire> 1K, 2 watt resistors... measured .986

2deaf> Divide your result by .000985

2deaf has a point. You selected your "1K" low and your result is low exactly past the second significant digit.

If you are going to (foolishly?) fret about the third significant digit, pick your "1K" a hair high.

[PRR]

> To assume the plate voltage of tube 2 is equal to the plate voltage of tube 1 is just wrong.

I think you are teasing about over-exactness. 

In a *transformer* output stage, if Vp1 is not super-close to Vp2, something is very wrong.

The normal DC drop across a plate winding is 10V-20V, with maybe 20% unbalance between sides. That's a 4V difference on about 400V, a 1% difference of Vp. Tube work can be more like 20% slop and the music still plays.

A minor ill effect of reading the plates individually: it loads-down the signal. True, 1Meg on a typical 6K winding is about nothing, so here it does not matter.

[2deaf]

. . . pick your "1K" a hair high.

R1/(R1+1M)=.001   1000/0.999=1001.001 Ohms

[shooter]

it could represent 79.7 miles

now that's a REAL number, trackin an inbound F-14 at mach 1ish all you could see on the nixies were the big numbers and they were going down real fast, and your heart rate went up proportionally 

[2deaf]

. . . call the 410.1VDC close enough?

Now that everybody has had their fun, let's take an example.  Say you have a 25W tube that you want to bias at 70%, which is 17.5W.  You think the plate voltage is 410.1V and you set the current to 42.67mA.  But really the plate voltage is 416.5V and the 42.67mA makes it 17.77W which is 71%.  Close enough.

[Platefire]

I didn't mention this but on this amp, the plate voltages are almost identical. So the OT is pretty well balanced on this one. Even if it wasn't, like its already been said---not a Nat's hair worth of difference to be concerned about.

I keep good records, so I have records of original plate voltage/bias setting readings when originally completed and record changes anytime I change tubes. This test point IMHO is just a good easy access reference to check if things are still copacetic!  So even if you wasn't satisfied with a simple sample, you could use the equation already outlined to account for the known offset to nail down to actual or just simply add 6 volts to the reading. This is certainly good enough for my purposes and if anyone else wanted to do it, they could take it to what ever extreme they felt they needed. Thanks, Platefire

[Platefire]

Diverted
Thanks ! There is a story behind this PA Head. It was actually was the PA Amplifier in the High School Auditorium where I graduated in 1965 and I think It may have been there even when my older Sister graduated in 1959. It was made in 1958. About 14 Years ago a contractor did some renovations in the old Auditorium and the old PA head was given to a guitar picking buddy(Charlie) of mind I graduated with by the contractor. When Charlie found out that I was into tube amp building and restoring, he gave it to me. So to me it's amazing that I even ended up with it and is really special to me. Platefire
BTW---Here is a picture of Charlie and I (Bob) probably playing Penetration:>)           I'm the one playing the Harmony Arch top Acoustic on the right with the Kent pickups added on.

[shooter]

to me it's amazing that I even ended up with it

, I'm thinking you're probably from the same school of thought that say's;
"There's no such thing as a coincidence" 
It's always fun when things "wind up" where they belong
When I give ppl my art, a lot of times i'll tell them, I don't think this is really yours but can you hang onto it for the real owner, sometime in the future they'll see me and say, hey, that painting...........

[Platefire]

Yelp, in total agreement! As stated in James 1:17

[pdf64]

I always thought this was an good idea to have a ready readable plate voltage to go along with external Bias test points and external bias pot adjustment. I didn't know quite how to do it until PRR showed me a Voltage Divider Circuit that would reduce the measurement down to safe "mV" to avoid a possible melt down/short on the chassis. So I have attached the circuit PRR provided below.

So going by my schematic I picked up some 3 watt, 1Meg, 1% Vishay Metal Film resistors(Red)...

Bear in mind that with signal, there can be big Vac on power tube plates; when overdriven, back emf spikes may be up to 2kV.
What's the voltage rating (limiting element voltage) of your 1M resistors; most regular resistor ranges are 350V or 500V?
I think it would be much better to take a Vdc sample of the HT / B+, ie at the OT CT - reservoir cap node. As noted, under idle conditions it should only ever be a few volts above either plate Vdc, which may be seen to be a negligible difference.

[Platefire]

My 1Meg resistors I bought for this purpose are rated at 750Volts. Here is the link if you want to checkum out!
https://www.mouser.com/ProductDetail/Vishay-BC-Components/PR03000201004FAC00?qs=sGAEpiMZZMtlubZbdhIBIEEdsE%252bzDFm9xwpKNsJJfwI%3d


You know I could just use my Military Tube Socket Adapter Kit shown in picture! It has Numbered Socket pins test points on each of them. To use them you simply pull your tube, insert the adapter and plug your tube into the adapter and you have access to working plate voltage.

Well I've been wanting to do this plate voltage test point for several years, so Ahhhh, feels so much better
BTW---Yeah, there I go editing again%>/