no b+ on coming off rectifier tube

[Bugman3183]

hello everyone.  I'm back once again hoping to get some advice.  I'm building an amp using mainly scavenged parts and upon initial testing I have no reading for the b+.  This the first amp I've built using a tube rectifier and in this case it is a 5u4gb.  I have the pt secondaries soldered to pins 4 and 6, the 5v filament connected to pins 2 and 8, using pin 2 as the out for the b+ as I understand either pin 2 or 8 can be used for this tube.  When I realized I wasn't getting a reading for the b+ I tried a different tube since I am not sure that this one works.  Second tube was a 5y3 (also used) and got the same results.  Both tubes light up but no b+.  Next I disconnected the hv secondaries and took readings off of it.  I got 390vdc from ct to each leg.  Afterwards I reconnected the secondaries and got those same readings and 780vdc from pin 4 to 6, but still no b+.  My next step was to replace the tube socket for the rectifier and all results remained the same.  Also I forgot to mention that I do have a string of 5 watt 10v zeners followed by a .1uf capacitor to ground coming off the center tap of the hv winding.  The bands on the zeners are facing ground.  I'm fresh out of ideas and was hoping I could get some guidance here from someone with more knowledgeable than myself.  Thanks.  Oh yeah I also tried using pin 8 for the b+ out which did not work as well.  I would greatly appreciate any advice.  Thanks again.

[John]

I could be totally off, so don't take this as gospel. But is it possible you've got your meter set on the wrong V measurement? For instance, I *think* that 390 each leg should be AC, not DC voltage. So if you were reading that and didn't switch your meter to DC voltage, that's why it wouldn't give you a reading at pin 8. For sure, I would not touch my finger to pin 8.  I hope it is something as simple as that, IF I'm right in my assumptions.

[Bugman3183]

Thanks for the reply.  The readings I took were actually ac  that was a typo.  Also I failed to mention that I only got a reading for secondaries referenced to the ct, but got nothing while referenced to ground.

[kagliostro]

Zener diodes followed by a cap ???

or

Zener diodes paralleled with a cap ?

how can the CT be connected to ground if you put a cap in series with it ?

K

[Bugman3183]

The cap is in series with zeners, perhaps that is my problem. I have never used zeners before and was just going off some information I had read online at some point.

[Willabe]

Do you have a lamp limiter? It's a must have must use tool for new builds and fixing old amps. It can save your PT if you have a short in your B+ PSU.

Here's the link to Sluckeys site with info on how to build 1 and how to use it. It works great and is easy to build for little $$.

http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

Zener diodes followed by a cap ???

or

Zener diodes paralleled with a cap ?

how can the CT be connected to ground if you put a cap in series with it ?

K

Yes, I think K's right. Take the .1uF cap out and check it again for voltage.

Leave the B+ rectifier wired up to pin 8. That way you can use rec. tubes with an indirectly heated cathode.

                  Brad     

[tubeswell]

Thanks for the reply.  The readings I took were actually ac  that was a typo.  Also I failed to mention that I only got a reading for secondaries referenced to the ct, but got nothing while referenced to ground.

You need your V-meter set to VAC to read AC voltages, and VDC to read DC voltages. Make sure your meter is set correctly and try again.  (You should be measuring VDC for the B+ voltage, and Pin 8 of the rectifier tube needs to be connected to a reservoir/filter cap)

[Bugman3183]

Thanks fire the advice.  I go have a dim lightbulb tester and it definitely comes in handy.  After reading the previous post I put the capacitor in parallel with the zeners and viola!  B+.  412V and 317v on 6v6 plates, and according to a bias calculator I found online I'm getting 11 watt dissipation on the  6v6gt's which sounds like it's within a reasonable range.  According to one data sheet I  read, the maximum is 12 watts, although I'm sure real world applications may differ.

[tubeswell]

...B+.  412V and 317v on 6v6 plates, and according to a bias calculator I found online I'm getting 11 watt dissipation on the  6v6gt's which sounds like it's within a reasonable range.  According to one data sheet I  read, the maximum is 12 watts, although I'm sure real world applications may differ.

Depends whether the output tubes are cathode-bias or fixed-bias.  For a 5E3 (cathode bias output stage), it won't hurt to have the 6V6s idling at 100%. But for a fixed-bias amp, you want them idling between 60% and 85% (70% is about optimal) for proper Class AB1 operation

[Bugman3183]

he amp is cathode biased but I'm not sure how that affects how it should be biased.  Any insight would be greatly appreciated.

[HotBluePlates]

...  Also I forgot to mention that I do have a string of 5 watt 10v zeners followed by a .1uf capacitor to ground coming off the center tap of the hv winding.  ...

Can you just connect the CT to ground with nothing in between? An ungrounded CT would create the symptoms you describe (a.c. present but no apparent rectification, and no d.c. out).

Reason is the rectifier diodes don't have a firm ground reference, and "don't know when to turn on."

Try it without the clever stuff, then add the clever stuff once good basic operation is verified.

[tubeswell]

Or at least just have the zeners without the cap