Very basic PF question

[Jim R.]

I know I should know this and it''s somewhat embarrassing to ask this at this point, but better to ask than assume:

In a parafeed configuration, where does the end of the plate choke not connected to the plate of the output tube, connected?

Thanks,

Jim

[vetmed]

The choke provides the load to the tube, so unless I'm not understanding your question the other end connects to the B+ supply Magnequest chokes used to be marked, and may still be, with one lead or solder terminal with a dot and in practice the "dot is hot" so you would know what end connects to the B+. Hope this helps.

[Jim R.]

Hi Robert,

Thanks for the confirmation -- I figured it out reasoning that the primary of the OT in a series feed amp goes to the B+, and then when I thought about it a bit further, it was really the only way that made any sense. I was looking at it from a different perspective -- that which said that the choke "absorbs" the dc load on the output tranformer and the cap blocks the DC.  Now that may be true or it may be what to me is an awkward to say it. Never mind that without it there would be no B+ connection to the output tube ;-).

Thanks again,

Jim

[VoltSecond]

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.siteswithstyle.com%2Fvoltsecond%2FParamour_cathode_mod%2FPARAMOUR_CATH_MOD_DETAIL.gif&hash=c7603b79040679a162234221d3ce8099f04ffaa4)

I prefer to put the Parafeed Capacitor to the plate and transformer to the cathode.

The DrP version:

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.siteswithstyle.com%2Fvoltsecond%2FParafeed_fun%2FXLS_SCH2.gif&hash=2eb537abd93b077ee75301a76a3d3ef1b1a13d7d)

V2 isn't a voltage source, it should be a resistor.

[Grainger49]

Jim,

A little more exposition of the pictures above. 

V-S uses "the alternate Parafeed return" where the output transformer "ground" is at the cathode of the output tube but at the output of the hum pot.  That is at the junction of the cathode resistor and cathode bypass cap.

The plate choke is attached to the B+ and the plate.  The Parafeed cap is at the plate and the dot of the primary of the output transformer.

[Jim R.]

It's interesting that this is how I figured the rest of the output section should be configured, but thanks for the confirmation. What is V2?

-- Jim

[Grainger49]

Sorry, V2 is the voltage at the cathode. He shows it as a voltage source but it is the result of whatever combination you have there.

[Paul Joppa]

... Magnequest chokes used to be marked, and may still be, with one lead or solder terminal with a dot and in practice the "dot is hot" so you would know what end connects to the B+. Hope this helps.

Unfortunately that can be confusing. The dot actually indicates the start of the winding, and is preferred for the highest AC voltage - "hot" in the signal sense. So the dot would go to the plate, and the other end to the B+, which is at AC (signal) ground even though it's at the highest DC voltage.

I believe the reason for this convention is that the capacitance to ground at the start of the winding is determined by the design of the bobbin and this will remain as intended in any amplifier, and also that the end of the winding on the outside should have a low voltage and low sensitivity to other voltages inside the amplifier, so it can neither cause nore respond to other electrical fields (e.g. hum).

[VoltSecond]

I prefer to test the hookup.

Temporarily float the core and secondary, play ~2 kHz test tone and measure the voltage from signal ground to core and to the secondary.  Power down, check that that B+ is at zero, flip the primary leads and retest.  Leave the leads in the position that measured the lowest voltage.  If the core and secondary measure differently, I normally use the one that causes the core to measure the lowest.

Be careful when doing this. Its easy for fingers and leads to get across high voltages. Leads are replaceable, fingers are not.

If you are >>not<< using feedback, leave the core floating and hook the secondary to which ever ground it is going to hook to and measure the core voltage with one secondary lead grounded at a time. Hook the lead that causes the lowest core voltage to ground (even if it makes the output polarity inverting, the speaker leads can always be swapped for polarity concerns.)

Then. . .You can also play with the orientation of the core (turn it in ~30 degree increments) to find lowest power supply pick up (with all of the leads floating again.)

[RPMac]

VoltSecond, could you explain the purpose of the wire from 22 to 12.

If the parafeed cap is below the primary of the OPT, won't the OPT see the DC plate voltage and is this why you put the parafeed cap between the plate and OPT? I assume that because the parafeed cap blocks DC current, it works in either position.

[VoltSecond]

The wire from 22 to 12 connects the output winding to the single point ground. If the transformer has an insulation failure or a high voltage wire breaks loose and hits the output winding, it keeps the voltage at the speakers safer to touch.

C2 is the parafeed cap.  It couples the AC on plate to the output transformer while blocking DC.
C1 is the cathode bypass cap. It keeps the cathode at an AC short. With the parafeed XFMR attached to the cathode, 11,  instead of ground, 14, there is 100X less voltage change on the cathode when the music plays which makes just about everything better.

[Jim R.]

Thanks VS -- I got a whole lot more from this than I was originally asking for. Much appreciated.

-- Jim

[RPMac]

One last question...what is the difference between the parafeed cap being before as oppose to being after the OPT?

[Grainger49]

The Parafeed cap blocks the DC from the OPT.  This allows the OPT windings to be optimized for audio signal path and not beefed up to carry DC.

If you put it after the OPT the voltage would be there requiring a different insulation factor on the winding but it would block the DC from moving through the windings.

[RPMac]

Thanks, Grainger. I had wondered if the DC voltage on the winding would have an effect on the OPT even though the parafeed cap after the OPT blocked the DC current.