Heater voltage issue

[NotBen]

I have been trying to diagnose my low heater voltage on my new Seduction build.

With the stock 1.2r resistor I am getting 4.78vdc at T6 and 4.28vdc across the heaters. I am getting a good 6.7vac across the PT terminals.
First I burned in the EHX tubes overnight to see if they settled in.
Today I checked things with the EHX tubes and then some well tested Matsushitas and Teles and got the same thing.

I checked all my solder joints and then began adding parallel resistors and then substituting resistors.

First I paralleled a 3.3r and got 4.98vdc/4.3vdc; no real change. PT still showed 6.7vac.

Next I replaced the 1.2r with a .47r and got 5.57vdc/5.28vdc. PT=6.7vac.

Next I replaced the .47 with a .22 and got 5.86vdc/5.71vdc, PT still 6.7vac.

What worries me is that if I keep dropping, the plate voltage will get way too low. It went down from 75, to 70, to 65.

I tested the diodes in circuit with a MM and they all showed  1.71v.
Do yall think I should start pulling the diode legs and checking them individually, or just keep going with the resistors.

BTW, I built it with the C4s and did not build it with the Rp first, check things, and then add the C4s. Possible mistake with this? (just curious, when adding the C4s, do you really leave T44u empty - where one of the plate resistors would be. mine has the B+ feeding T44b with nothing in the upper terminal. Could this be an issue that is causing an odd plate current issue and rippling back to the power supply?)

Thanks,
Benton

 

[Paul Joppa]

This does not sound right. Check your power line voltage, which should be around 120 volts AC - that will give us some information.

The 1.71 volts measure across the diode is suspicious - makes me wonder if you have the wrong diodes. Can you read any marking on them? They should be 1N5818 Schottky diodes (might be 5819 or 5820). If you got UF4007s which are the same size and shape, that would explain the voltages!

Also, since you said 1.71 volts, I assume this is a diode test function on the meter. If you are reading resistance and getting 1.71 kOhms in one direction, near infinite resistance in the other, then this test only tells you the diodes are good, not whether they are Schottkys. Do make sure the diodes are oriented correctly and are all soldered well - the leads are copper and conduct heat very well, making it somewhat difficult to get a good solder joint.

Those are the most likely sources of this problem I can think of - 1) low power line voltage, 2) wrong diodes, and 3) bad solder connections. My money is on 2) ...

The plate voltage is not a problem; many Seductions run at 60v or even lower. The C4S sets  the correct current, and the LEDs set the correct bias, so correct operation is assured. The voltage is dropping because the cathode is getting closer to an adequate temperature.

 

[Grainger49]

The good news is that the transformer looks like it is doing well, and you seem to have gotten in the C4S in right from the Get Go! 

The 1.2 ohm resistor does help smooth the DC a little, not very much.  It mostly sheds a little voltage and brings it to where you need it.

If you have the wrong diodes you can just jumper around the resistor all together till you get the right ones in. 

I'm confused that the plate voltage is dropping as you raise the heater voltage.  However PJ is fine with it, and he is THE MAN when it comes to the Seduction.  You have to be OK.

Getting different voltages at the two tube heaters (if I am reading this right) is an oddity too.  They are connected with wire, which doesn't drop voltage unless it is 100 feet long or more.  So that says either you didn't get good contact when reading the voltages or a solder joint needs to be touched up.

Sadly those heater pins are the most crowded points in a Seduction so be careful when you go in there, take your time.  Say, imagine performing surgery on an eyeball. 

 

[NotBen]

Bad diode. They are the correct ones, but I had some spare as I use those same ones on heater rectifiers for some other preamps.

The 1.71v across the diodes in circuit using the diode function didn't find the culprit. The impedances also showed fine, except the bad one was showing a much different impedance than the other three, but still had a low impedance one way and a high to OL the other.

Line voltage this morning is 119. I usually get 121.
Now I need to stop at the supply store to get some other resistors to try out just to take a bit off, as I was getting 6.8vdc before the resistor and 6.4vdc on the heaters when I paralleled a 3.3r. I'll pick up a few different sizes to try, because I must be calculating the current wrong or it's changing on me.

Thanks for the help.

 

[Grainger49]

There is no target heater voltage, from 4 to 5 on the tubes.  So this is from my foggy memory.  IIRC, Paul wanted the tubes to be run conservatively, he targeted 6V between those pins.  If I'm wrong PJ can post and I'll delete this as misinformation.  I won't leave misinformation on the board.

 

[NotBen]

Previous posts from PJ have said the range should be 5.7 to 6.3 volts.

 

[Grainger49]

Yes, I didn't remember the range but knew the mid point was ~6V.  It, of course, for those of us who understand transformers (power, not audio output, they are black magic), the incoming voltage affects the output proportionally to the windings ratio.

 

[NotBen]

It's working great. I left the 1.2r in. I checked the line voltage multiple times during the day and as it fluctuated from 119-125 the heater voltage was 6.2-6.5. I may use a 1.3 or 1.4 later, but I will let it run for a few days first.