5B6 started

[martin manning]

I like to wire heaters for the power tubes out of phase with each other.

This can cancel out some noise.

John

With two power tubes in push-pull the heaters should be wired in-phase to get common mode cancelation. If you have a quad, then you can wire each side out of phase to minimize hum from each side.

[tictac]

telentubes: I'm sourcing parts for a 5B6 build myself. Seem like a with 700V PT your B+ is going to be too high for a pair of cathode biased 6L6's with a 250 ohm cathode resistor...

Is this PT what other 5B6 cloners are using? Or maybe you're planning on using 655o power tubes?

Sorry if you already mentioned and I didnt read carefully enough

TT

[tictac]

Martin - can you explain futher about the heater wiring...

Do you mean with a quad of power tubes wiring each pair of output tubes out of phase with themselves; or do you mean one pair of output tubes is wired out of phase the other pair?

I assume the preamp tubes would still be wired in phase with each other.

TT

[ToneMerc]

telentubes: I'm sourcing parts for a 5B6 build myself. Seem like a with 700V PT your B+ is going to be too high for a pair of cathode biased 6L6's with a 250 ohm cathode resistor...

Is this PT what other 5B6 cloners are using?

TT

Generally yes, I used MCI Valco iron.

https://tubeamparchive.com/viewtopic.ph ... hlight=5b6

TM

[martin manning]

Martin - can you explain futher about the heater wiring...

Do you mean with a quad of power tubes wiring each pair of output tubes out of phase with themselves; or do you mean one pair of output tubes is wired out of phase the other pair?

I assume the preamp tubes would still be wired in phase with each other.

TT

Sure, a pair of output tubes on one side are working in parallel, so if they are wired out of phase then they will tend to cancel each other's filament-induced noise. I don't know if that would be any better than just wiring them all in-phase and doing the canceling in the OT or not. Perhaps it would depend on how well the tubes are matched.

[telentubes]

OK, I yanked the 470 Ohm screen resistors, which weren't on the schematic in the first place.

Tictac, The PT uses the specs in Tino's build guide. Others have used the same.

Do I need to ground pin 1 of the 6l6s as described in the build guide?

[martin manning]

Do I need to ground pin 1 of the 6l6s as described in the build guide?

Only if you think you might want to run EL34's at some point, but you would need screen resistors and another 1.2A of filament current.

[tictac]

Martin - seems like the heater filaments, being a seperate circuit from the plate circuit would not actually be effected by the push-pull cancelations of the OT.

I hear quite a few comments about filament noise but with good layout practise and tight twisting of the filament wires AC filaments shouldn't present any noise problems...

TT

[martin manning]

Wiring filaments in- or out-of-phase is an effort to eliminate filament circuit noise that is coupled into the audio path inside the tube, via inter-electrode capacitances.

[tictac]

Here's a comment from an expert tube guy at my LinkedIn Vacuum Tube forum....

Jürgen E. Ewert • Jack, that's right the PP cancels out the hum that comes in through the heater-cathode in the power stage because each tube in the PP stage amplifies the hum-signal with 180 degree phase shift...

So I guess PP really does cancel the heater noise too...

TT

[martin manning]

So you don't want to believe me, but you'll take some random guy on an internet forum's word for it?

[Cliff Schecht]

Lol. There is always interelectrode coupling via capacitance in a vacuum tube. Putting some DC voltage on the heater center tap is another way to reduce the capacitive coupling effect (and this helps reduce the requirements for CMR in push-pull stages).

[tictac]

Martin, I posted what Jurgen said to try and say I'm agreeing with what you said...

I've know Jugen for quite awhile, he knows his stuff and is not some random internet guy...

I'm simply thinking out loud, I'm sorry if that offends you... I'm 53 years old but I'm still trying to learn and be open minded so please don't take it personally...

TT

[martin manning]

TT, no offense taken, I was joking... after all I'm just some random guy on an internet forum!

The more I think about wiring the filaments on each side of a pp pair in- or out-of-phase the more I think it doesn't make any difference.

[telentubes]

A cry for help to the intrepid trouble shooter builder gurus.

Got my tubes today. Went over the wiring one last time and got out the "light bulb limiter".

Plugged in the AC, threw the switch, without any tubes installed, and the bulb did not loose its intensity (nor did the jewel light go on). I tried again with all the tubes installed, with the same results.

Went over the wires again and checked the fuse...
One side of the IEC goes to the fuse, then to the "on/off" switch, then from the other side of the "on/off" switch through the transformer, and into the other side of the IEC.
The Black/Red Center Tap and the White/Brown Center Tap wires are grounded to the chassis.
The two brown wires go to all the filaments and jewel lamp.
The red 350V wires go to pins 4 and 6 of the rectifier tube.
The yellow 5V wires go to pins 2 and 8 of the rectifier, with a wire from pin 8 to B+1 (I had that wire on pin 2 first, then changed it).

Checked AC to the IEC, both sides... good
Unplugged the AC and checked continuity and switch for the 120V loop into the PT... good
Checked continuity of the 5V yellow wires... good
Checked continuity of the brown 6V filament wires... good
Checked continuity of the jewel lamp... good

Now here is what I don't understand,
Checked continuity of the 350V red wires... not good. I then I checked several other Power Transformers that were either "new in box", or ones that I knew were good, and I got some that tested with a closed circuit, and others that tested open. The filament, 5V, and 6V on all transformers had closed circuits. Only the red wires were one or the other. I hope that makes sense.

I hope there is something simple that I'm missing. Maybe someone can spot an error. Maybe the transformer is a dud?
Thanks. Here are some pics.