Heater voltage strangeness

[Ampzone]

Anyone ever encountered a variation in heaters on amps where say e.g. the output tube heaters (like a Marshall style amp) are reading a touch higher say 6.25-6.3vac but down stream at say V1 across the heater pins its around 6.15 - 6.20v ac?

[g1]

You can have some losses across the wires. So the furthest from the PT winding would have the lowest voltage. But you could have some iffy solder on the way too, so you might want to check those connections.
As far as the wiring goes, you can measure the AC voltage across the wires between the sockets, and see if there is drop.

[Ampzone]

You can have some losses across the wires. So the furthest from the PT winding would have the lowest voltage. But you could have some iffy solder on the way too, so you might want to check those connections.
As far as the wiring goes, you can measure the AC voltage across the wires between the sockets, and see if there is drop.

Good point, the amp is behaving fine I just checked and noticed this. Yes the drop starts on phase V3 after V4 output. I will try your suggestion of going from say V4's pin 7 output wire to pin 9 V3 ditto V4 pin2 feeding V3 pin 4-5 and see what gives. I may also re-flow the solder on the pins in that area.

Stay tuned.

[Chuck H]

What is the nature of the amp? I remember looking into wire gauge WRT voltage drop after considering the current demands of filament circuits and the popularity of tightly twisted wire pairs to the sockets. Using something like 18ga the voltage drop is negligible for almost any amp, but if something like 24ga wire were used, the amp has a lot of real estate inside and the pairs are tightly twisted (increases overall wire length) you could end up with a volt lost to wire resistance.

[Ampzone]

What is the nature of the amp? I remember looking into wire gauge WRT voltage drop after considering the current demands of filament circuits and the popularity of tightly twisted wire pairs to the sockets. Using something like 18ga the voltage drop is negligible for almost any amp, but if something like 24ga wire were used, the amp has a lot of real estate inside and the pairs are tightly twisted (increases overall wire length) you could end up with a volt lost to wire resistance.

Its a Marshall 100w Super lead with 22awg CSA heater wires. I just checked with wall voltage 121.2 VAC at the moment it went from 6.38 first output tube seeing heater direct from the PT to down the line 6.23 on V1 first preamp tube. For reference I opened up a known working stock 70 Marshall Super lead 100w 22awg wire and did the same wall voltage at that moment was 120.1 vac 6.25 down to 6.09 on the first premap tube. Hmm

[Chuck H]

What's most interesting to me is that you don't have MORE voltage drop. According to an on line calculator I just plugged the numbers into you should have .39VAC dropped over 3 (or 6?)* feet of 22ga. wire with 6.3vac@4A across it. For a tightly twisted pair those amps might have four or five feet total wire length on the heater circuit.

*the calculator allows wire length and THEN selection of how many "sets" of conductors for different phase opertaion but does not specify if the length is total or the length of a "set" of wires. If I felt jiggy about it I suppose I could just download a wire resistance chart and do the simple math.

[Ampzone]

What's most interesting to me is that you don't have MORE voltage drop. According to an on line calculator I just plugged the numbers into you should have .39VAC dropped over 3 (or 6?)* feet of 22ga. wire with 6.3vac@4A across it. For a tightly twisted pair those amps might have four or five feet total wire length on the heater circuit.

*the calculator allows wire length and THEN selection of how many "sets" of conductors for different phase opertaion but does not specify if the length is total or the length of a "set" of wires. If I felt jiggy about it I suppose I could just download a wire resistance chart and do the simple math.

Im just guessing (eyeballing the amps but its about 2-3' of wire twisted. So can we maybe assume its just what it is then, resistance. So 6.38 ish down to 6.20 is not a big deal then?

[g1]

No big deal.

[Ampzone]

No big deal.

Cool thanks guys