Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fazeka on January 05, 2025, 06:05:35 pm
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Hi there,
Apologies for the seemingly simple question in advance.
I have a newly acquired 1964 Vox AC10. Original electrolytics so changed them out and installed a fuse.
I was comparing the amp with the schematic and noticed something that I could use some help in trying to understand.
I see that pin 5 (heater) on the ECF82 appears to be grounded.
(https://i.imgur.com/QfrK93N.jpeg)
(https://i.imgur.com/sRu32JC.jpeg)
The schematic shows pin 4 grounded:
(https://i.imgur.com/lQ4T4iF.png)
Is there an issue with this amp having pin 5 grounded instead of pin 4? I'm not sure if the schem has an error or the amp was wired incorrectly but I'm fairly confident it doesn't matter for the other tubes but pin 9 on the 12AX7 is confusing me here and overall I don't think it matters whichever pin is grounded but want to confirm.
More importantly, *why* do the filaments need to be grounded?
Thank you for your patience, wisdom and understanding.
Chris
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The heater elements don't care which pin is grounded. And for the heater to work, neither one needs to be grounded, that's just there as a ground reference for the rest of the amp to reduce noise.
The 12ax7 could have either 4+5 or 9 on either side, as long as both sides of the heater elements see a 6.3V differential.
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There's no polarity on AC filaments / heaters, so any leg can be plugged in anywhere.
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... *why* do the filaments need to be grounded?
They don't.
But the insulation inside the valve between the cathode and the heater inside it is only rated up to 100V, sometimes 200V.
If it fails, hum and buzz from the heater piwer will contaminate the signal path, and if it's a hard short, valve operation may be affected.
So, the heater and cathode circuits both need a path reference to the same point, so that the voltage between them can't drift too far apart.
Circuit common 0V / ground is a convenient point that meets that requirement, so is often used.
Another requirement is that the heater circuit has a low impedance path to circuit common. If it doesn't, it will pick up spurious ghost voltage from magnetic fields in the local area (eg mains transformer, output transformer, choke).
That spurious voltage can't deliver current, doesn't affect the voltage applied to the valve heaters, but it will tend to cause interference / noise / oscillation.
So either a direct connection to common, or a reasonably high value capacitor between the heater circuit and common, is needed.
It would be really beneficial for you to read and get your head around http://www.valvewizard.co.uk/heater.html
If you have any queries, ask! :icon_biggrin:
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Thank you very much Pete for the explanation!