Identifying Power Transformer leads for FENDER Amp

[Enzo]

Two things:
1. do the tubes light up more or less normally? If so, whatever the readings, it is right for the amp.

2. 3.2 between WHERE? One probe on 2 the other on 7, you should see 6vAC.

As to the 44v, you said earlier you had 45v on the cathodes of the power tubes. I suspect someone wired the faux center tap on the heaters to that cathode for elevation.

[czech-one-2]

Two things:
1. do the tubes light up more or less normally? If so, whatever the readings, it is right for the amp.

2. 3.2 between WHERE? One probe on 2 the other on 7, you should see 6vAC.

As to the 44v, you said earlier you had 45v on the cathodes of the power tubes. I suspect someone wired the faux center tap on the heaters to that cathode for elevation.

Ok, I was measuring one pin at a time. 2 to 7 is 6.2VAC
Yes, the artificial center taps are wired to that cathode. Should I move those from the cathode and wire them to the pilot light?

[Enzo]

It is working, why change it?

Elevating heaters is an old favorite way to reduce hum potential., at least in the first stage or two.

[czech-one-2]

Right, just wondering if one center tap location is preferable?
Heater wires are lifted.


and tilt back legs installed...

[Enzo]

By elevated, I mean they are at a DC voltage elevated above ground. Totally unrelated to elevating the wires up off the chassis.

[czech-one-2]

By elevated, I mean they are at a DC voltage elevated above ground. Totally unrelated to elevating the wires up off the chassis.

Aha, referring to the artificial center taps?

[Enzo]

If the heaters are at a raised DC voltage, it CAN help reduce certain types of hum. One may connect the heater center tap - real or artificial - to the DC voltage source, or even just connect one side of the heater string to the DC.

[czech-one-2]

If the heaters are at a raised DC voltage, it CAN help reduce certain types of hum. One may connect the heater center tap - real or artificial - to the DC voltage source, or even just connect one side of the heater string to the DC.

I might as well learn something else from you Enzo. You mean only one side of the heater string is connected to the 6.3v tap, and the other side not? Sorry if its yet another stupid question but why would you not connect both strings to the 6.3v tap?

[Tom Phillips]

I might as well learn something else from you Enzo...

I think you will also enjoy reading about the basics in an article that lays out all the common design approaches. It really helps to see all the information in one place. Check out the well written article at http://www.valvewizard.co.uk/heater.html particularly the Hum in Heater Supplies section.
Cheers,
Tom

[czech-one-2]

^ Thanks!
But this is pretty deep for my simple mind-
'Heater Elevation
Elevation means referencing the heater supply to a DC voltage other than ground or zero volts. The heaters still operate at 6.3V or whatever, but this floats on top of the elevation voltage. Some valve stages such as cathode followers require the heater supply to be elevated to avoid exceeding the valve's Vhk(max) rating. But even when not explicitly needed, elevation can reduce hum in AC-heated circuits by reducing or saturating the leakage current between heater and cathode.*

The DC voltage is applied to a transformer centre tap, artificial centre tap, humdinger, or whatever reference connection the heater supply would normally have.

The elevation voltage can be taken from a potential divider across the HT (it doesn't matter where you position the divider), and an elevation voltage around 30 to 60V is typical. The divider should have a fairly high resistance so as not to waste current, although the lower arm (R2) should not be excessively large or Rhk(max) may be grossly exceeded, so it is advisable not to make it greater than 100k. The elevation voltage should be decoupled/smoothed with an arbitrarily large capacitor (C1), say 10uF or more.

Another convenient source of elevation voltage is the cathode of a cathode-biased power valve. No current flows ‘into’ the heater supply from here, so the power valve bias is not affected.'

I'm really unclear on how best to wire up my heater string. I have 2w 100ohm center taps on tube #1, but may try 220ohm 1/4watt on the pilot light instead? The amp is not overly noisy, but I feel it could still be quieter.

[Tom Phillips]

...I'm really unclear on how best to wire up my heater string. I have 2w 100ohm center taps on tube #1, but may try 220ohm 1/4watt on the pilot light instead? The amp is not overly noisy, but I feel it could still be quieter.

This most recent part of the discussion was to explain a possible cause for the readings you were getting when you measured the heater voltage. It is seeming like your amp is already wired to produce DC elevated heaters. The Valve Wizard article explains the various ways that heater circuits can be configured. Once you understand the options then you can recognize which version is installed in your amp. DC elevating the heaters can reduce hum but that is not guaranteed. It all depends on what is causing the hum in the particular amp.

Anyway...It is unlikely that removing the 100ohm artificial center tap resistors on tube #1 and adding 220ohm 1/4watt artificial center tap resistors on the pilot light will be any better than what you have.

[Steve A.]

If the heaters are at a raised DC voltage, it CAN help reduce certain types of hum.** One may connect the heater center tap - real or artificial - to the DC voltage source, or even just connect one side of the heater string to the DC.

For a cathode-biased amp you have a good source of DC voltage at the cathode(s) of the power tube(s). It's been almost 20 years since I connected the two 100R resistors to the cathodes on my Mission Amps 5E3 kit instead of to the center tap of the PT heater windings...

Steve Ahola

P.S. If you have a 6VDC lantern battery (or a holder with four 1.5VDC batteries) you can always try connecting the heater string to that to see if the AC filament supply is causing the hum.

EDIT: I see that Enzo already mentioned the power tube cathode trick in Post #59 here. I believe that you can also elevate a DC heater supply... or does that not work?

[Enzo]

Don't obsess over "better". The value of the 100 ohm or 220 ohm resistors doesn't matter. All they need to be is more or less the same as each other. 100, 220, 330, 176.2, whatever.

Referencing them to ground works, referencing them to a positive voltage works. You ALREADY have it wired to a positive voltage. STOP RIGHT THERE. Now go work on some other issue.

There are 100 sources of hum and noise in an amp, and each source has its own cure. More filter caps will do nothing to help heater hum, and elevating heaters will do nothing to help power supply ripple. So if there still is some hum in the amp, find out where it comes from. Don;t just start changing center tap resistors in hope we hit upon a solution.

And as with heater hum, there is more than one way to go about it, and if it works, it works, no need to look for "better".

[czech-one-2]

^ Ok, got it. It is working fine and the noise is completely within normal tube amp toleration.
Hope you liked my 'elevating the heater wires' joke!

Great to learn a bit more,keeps me coming back to this place!