Series heater wiring oddity

[Bieworm]

hey guys

I have a question about series heater wiring. I built the lamington reverb unit from valveheaven.  It called for 12VAC wiring the heaters for the 12ax7 and 6bx6 in series. Now, when I put in a 12ax7 along the 6bx6 the voltage on the heaters is 7v ont the 6bx6 and 5v on the 12ax7.  I tried everything.. zenerdiodes, resistors ... but the voltages dropped equally in size...and was up to a point to wire the lot in parallel. But I discovered that when I put in a 12at7 the problem is solved. 6v each. And I tried all 12ax7 tubes I own.. same discrepancy.
Of course I have less reverb with the 12at7, so is there a fix of some kind?

[sluckey]

That is odd. 12AX7, 12AT7, and 6BX6 all have the same filament requirements. Voltage should split equally between any combination of the three types. If the unit sounds good with the 12AX7 I would not worry about the unequal split.

 

[Bieworm]

Don't I need to worry about the over and under value of the voltages? They're both out of spec.. tube life?
I thought tubes had to maintain within 10% margin of the 6.3V spec or otherwise they wear too quickly significantly?

[sluckey]

Try another 6BX6.

[Bieworm]

Try another 6BX6.

Did that. Same result. Even wired it for a 6AU6  pentode, same result. I'm puzzled...

[sluckey]

If the unit sounds good with the 12AX7 I would not worry about the unequal split.

[Bieworm]

Ok I will see this out.  Have a spare or 2 just in case . Thx Steve

[sluckey]

schematic attached...

[Bieworm]

schematic attached...

Thx, but I've built the unit based on that schematic. There is a buzz I can't get rid of though. Tried all kinds of grounding schematics. Added filter caps,... did the wiring a few times all over . Moved the parts all over ..  nothing helps.
Using it through the fx loop keeps the buzz less audible.. but I would like to solve it... sigh...

[sluckey]

Thx, but I've built the unit based on that schematic.

I know that. I posted the schematic so others could easily reference it and maybe offer some ideas.

I would disconnect the power cord earth ground wire to see if that helps. Worked fine for my Revibe unit.

PS... This thread is in the proper forum but I can move it to the main forum where it will get much more exposure (and comments). Let me know.

[Bieworm]

Thx, but I've built the unit based on that schematic.

I know that. I posted the schematic so others could easily reference it and maybe offer some ideas.

I would disconnect the power cord earth ground wire to see if that helps. Worked fine for my Revibe unit.

PS... This thread is in the proper forum but I can move it to the main forum where it will get much more exposure (and comments). Let me know.

There is no power cord ground wire. It's powered by a 12VAC wall wart. Then it's back to back connected to a 12VAC-240VAC toroidal tx. Tried 3 different tx.. same buzz
If you could move it here,  that would be awesome

[sluckey]

There's a lot of AC ripple voltage on your B+ rail. Measure the AC voltage at "HT1" to see it. Now install this additional filter node. Ripple at "HT1" should be much less. I bet the buzz will be less also.

[Bieworm]

There's a lot of AC ripple voltage on your B+ rail. Measure the AC voltage at "HT1" to see it. Now install this additional filter node. Ripple at "HT1" should be much less. I bet the buzz will be less also.

Thanks .. i'll try that. I already put an extra filter cap in front , but I don't remember how I connected it...
I don't have a 100uf cap spare.. would a 150uf 450V be all right too?

[sluckey]

yes

[Bieworm]

yes

Darn.. still buzzing . I am gonna pull out my hair on this one...

[j_bruce]

. . . Measure the AC voltage at "HT1" to see it. . .

Can I hijack this thread breifly to ask a question?

If I use a meter set on VAC at that node which has some VDC on it will it only read the AC part that is "riding along the top" of the DC? In other words will an AC meter "remove" the DC offset from the reading?

If so that is a very useful, if fairly fundamental, thing for me to know.

If I had a working amp opened up right now I would just go play around and see what I can see, or wreck a meter. But, alas all my amps are working or beyond salvage at this point.

Jerry

[Bieworm]

. . . Measure the AC voltage at "HT1" to see it. . .

Can I hijack this thread breifly to ask a question?

If I use a meter set on VAC at that node which has some VDC on it will it only read the AC part that is "riding along the top" of the DC? In other words will an AC meter "remove" the DC offset from the reading?

If so that is a very useful, if fairly fundamental, thing for me to know.

If I had a working amp opened up right now I would just go play around and see what I can see, or wreck a meter. But, alas all my amps are working or beyond salvage at this point.

Jerry

Jerry, your DMM will only read the AC fraction. There is no extra math involved. It will also be nowhere near the exact ripple size due to the limitations of the device. You'll only know that there is AC ripple present.
Well.. that's about what I understand from this material 😉

[sluckey]

Short answer is yes, but...

Use a high quality DMM. A $20 DMM may not do this. My Fluke 87V produces a useful "relative" reading. A good Klein may produce a different but still useful relative reading. Practice doing this with your meter on good working amps until you have a feel for what's acceptable or unacceptable.

I have worked on several complex electronics systems that specify to measure power supply ripple with a specific DMM. The TO also specified the max allowed ripple.

But if you need to know the absolute voltage level of the ripple you will need a scope and measure the peak to peak AC ripple (actually more of a sawtooth waveform).

edit... more info...

https://www.google.com/search?ei=5-1EX8uALu-QwbkP4seAqAw&q=measuring+ac+ripple+with+multimeter&oq=measuring+ac+ripple+with&gs_lcp=CgZwc3ktYWIQAxgAMgYIABAWEB5Q_UhYlVBgi29oAHAAeACAAWqIAf4DkgEDMS40mAEAoAEBqgEHZ3dzLXdpesABAQ&sclient=psy-ab

[Bieworm]

Steve.. here is the layout. It's not complete, groundings of parts on the tube sockets are connected to the center metal tube on the socket and then directly to the star ground... except the RC cathode of the 6au6... that one goes to the ground wire coming from the filter caps, the heater wires to ground too. Should these 3 wires go to star ground too maybe?
Fwiw.. I twisted the heater wires properly 🙂

[PRR]

> will an AC meter "remove" the DC offset from the reading?

The DC may overwhelm the meter. Most will read wrong and it is hard to predict.

Old VTVMs often had a serious series capacitor on AC and might read AC consistently in the face of any amount of DC. Most VOM needle-meters on AC rectified everything whether it was AC or not. DMMs may use algorithm to ignore small DC with AC but may not do well with very large DC/AC ratio such as amp power.

Yes, something like 0.22uFd 600V with 100k may extract the AC from the DC so a less-clever meter gives a consistent reading. I hope we don't have this problem so often that it is worth making-up the adapter.

Most meters are marked in "sine RMS". They may actually read average, peak to peak, or RMS (always with a crest factor). Rectifier ripple is not sine-like, so it won't read the same on different meters.

[j_bruce]

Thanks guys for the detailed responses. I do have a scope so that is an option if I need to look at a power supply closely.

I always appreciate the opportunity to learn something!
Jerry

[acheld]

Could you explain the heater wiring per your diagram?  Is it receiving 12V ( or just the positive side of the 12VAC supply, eg 6VAC)? 

[sluckey]

Could you explain the heater wiring per your diagram?  Is it receiving 12V ( or just the positive side of the 12VAC supply, eg 6VAC)?

The filaments of the two 6V tubes are connected in series. Then the series string is connected across the 12VAC transformer. Theoretically the 12VAC will split evenly across the two tubes, allowing the tubes to correctly operate with 6VAC on each tube.

Ignore the ground connection. It's only there to provide a ground reference for hum reduction and has nothing to do with lighting the filaments.

[Bieworm]

when you look at the layout in post #18..
Are there wires that should definitely be shielded? I have a shielded wire from the input to 12ax7 pin 2.
Is there a problem with the grounding connections/ positioning..sequence ?

[sluckey]

If the input wire is over 3 inches I'd use shielded cable and connect the shield at the input jack. Same rules for the output jack.

If the pots are viewed from the rear, you need to swap the green and blue wires on the DWL pot.

Need three more connections to V1.