No glow for me with the OD3

B

bainjs

New member
In discussing the great build with Laudanum of his FP3 in the Gallery, I discovered that the OD3 should be lighting up.  I built my FP3 a few months ago and it sounds great, but alas, no glow from the OD3. It's the RCA supplied with the kit.

We can all use a little more glow in our Bottlehead lives, so anyone have thoughts on this?  Paul Joppa gave me some things to check, but I suspect they will be ok.  Grainger and I went through the voltages extensively when I built the kit, but that's a whole 'nother story.

Thanks for any suggestions, I need the glow....

Joel
 
Hi Joel.  You can see the glow with the Hytron, but the top spacer/plate doesnt have the two slits in it like the RCA.  So, the RCA internal structure allows more of the glow to be visible.  Anyway, atleast you will confirm that it is the tube.  The Westinghouse could be a re-label so it may be good for showing the glow, or not so good. 
I picked up an RCA from ebay a couple days ago for 3 bucks BIN since I know they glow pretty nicely.  I think there was another RCA there for 3 or 4 bucks.
Grab it if it's still available, then you'll have one that you know is a pretty good glower and there isnt much risk for a few bucks.
 
The confusing thing is my original is a RCA.  I'll check the eBay listing.  One more can't hurt! 
 
I had suggested the voltages because if the tube is regulating then it is glowing on the inside and the amp will be performing properly. If it's not regulating, then the voltages will help determine why.
 
The one in my profile picture is a Hytron and is labeled JAN CHY 150-30.

The plates have a cover on top but it is translucent and when the tube is on you can't hardly tell it is there at all.
 
I have two of the same Hytrons.  I didnt notice the translucent top before but you're right, you can see the glow through it.  My RCA has a space around the bottom of the shield as well as two slits on the top and it does emit a bit more glow overall because of it.  But the Hytrons are better than I first thought they were.  I must not have looked down at the top.  Cool.
 
Per Paul's suggestion, here are the voltages I measured on the OD3 tube.  R21 - 206vdc, R25 - 407vdc,  R3 - 407VDC, R7 - 406vdc, I could not get a stable voltage on R5.  It jumped around and would finally show .005vdc.  I have the RCA tube with slits in the top, however, there is no glow and the tube does not get the least bit warm. 

If there is a problem somewhere, why does it sound ok?  Very confusing.  I hope to have some additional OD3s to try soon. 

Thanks for any help/suggestions.

Joel
 
OK, good info. You tube is not glowing, nor is it operating - if it were, then the voltage at T21 would be 150v, +/- 5v at most.

The high voltage supply comes in at R7. Inside the 0D3, that is connected to R3, which is then wired to terminal 25. Then it goes through the big resistors to terminal 21. Since you have voltage at terminals 21 and 25, this part must be working, and terminal 21 is supplying power to the preamp circuitry. (If the regulator tube were working, it would shunt some of the current to ground, reducing the terminal 21 voltage to 150 volts.)

Terminal 21 is connected to R5, the positive input of the regulator tube. R2 goes to ground at 24U and 28L. Check these connections and solder joints, especially terminal 28 which has a lot of wires. That terminal requires more heat to get all the wires connected properly, so it's easy to get a poor joint there. You should be able to measure the tube voltages of 0 and 150 at the ends of the 0.1uF bypass capacitor, R2 and R5
 
Could it be the tube itself?  I've checked all the connections and reheated the tabs.  I checked the continuity on the wires to make sure the grounds and the connections are ok  The voltages are still not correct.

Why would the preamp work (quite well to my ears), if the OD3 is not operating?

Thanks

 
Yes, it could be the tube. Based on your numbers, and the fact that you have worked on the solder joints and inspected the construction to confirm all is as it should be, that is the most likely issue. Time to give Eileen a call or an email and ask for a replacement 0D3!

Some details: You measured 407 and 206v at the two ends of the big resistors, which in parallel add up to 10K ohms. That calculates to 20.1mA being drawn by the preamp circuitry, which is about right. The circuit is powered, so it operates. It does not have the stability and drive that it would if the supply were regulated, but it still works.

When it is running right, the voltage at T21 would be 150v, as I said. If T25 is unchanged, that would indicate 25.7mA current through the 10K resistor. The amplifier circuit will draw a bit less current when its supply is reduced - maybe 16mA? - so the regulator tube will conduct maybe 9 or 10mA and should glow quite nicely.

Since this was a problem from the start,
 
I emailed Eileen about a replacement.  I still don't understand how the FP3 could be working with the OD3 not operating.  Very strange stuff!

Thanks

Joel
 
From reading this exchange, it seems as if the suspect is the regulator tube.  If it is indeed bad, the rest of the circuit is still getting the unregulated power supply, so it works but not at its best.  The regulator tube is supposed to be shunting some excess Voltage (and, current, though that is not really the main issue) to ground, and thereby establishing one fixed operating point for the amplifier circuits that stays fixed regardless of the signal.

To put it way too simply, tube amplifier circuits tend to operate with three operating parameters that are constantly changing with the applied signal; if you pin one of those three down and make it constant, as with using a shunt-regulated supply, you generally make the tube amplifier more linear, or accurate, if you will.



 
Thanks for putting it "way too simply".  That makes sense even to me. 

So, even though it sounds good, it may sound better when I get a replacement tube.

I hope that's the issue!
 
After checking and re-checking the connections going to the OD3, out of frustration and thinking I had nothing to lose, I thumped the tube and it lit up!  That's probably not the best thing to do to a tube but so far, it still working. 

Thanks for everyone's help and suggestions.

Joel
 
OK, so while it is on, do you think you can get those Voltage measurements again?

Meanwhile, do you hear a difference?
 
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