Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: johno9 on November 30, 2020, 04:17:21 pm
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I have completed a 5E3 build and was going to follow up with a CJ11. The schematic for the CJ11shows a center tapped 5V winding for the GZ34 rectifier, with the HV coming from the center tap. On the 5E3 there is no center tap on the 5V winding, and HV is taken from Pin 8 of the 5Y3GT rectifier. The CJ11 uses a 290-0-290 HV transformer while the 5E3 uses a 355-0-355 HV transformer.
I believe I understand the differences between the 5Y3 vs 5AR4 (construction,voltage drop, and current).
Is the center tapped 5V used to have less AC filtering (2.5V vs 5V) on the HV? Or lower HV?
My PS transformer (290-0-290) does not have the center tapped 5V. Would it be safer to substituted the 5Y3 and take HV from Pin 8?
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The center tap on the 5VAC winding helps to balance the ripple pulses from the rectifier. If you take the B+ from one side of the rectifier (either pin 2 or 8) and look at the unfiltered pulses you will see that every other pulse will be slightly bigger. This will have the effect of putting a small 60Hz fluctuation on top of the unfiltered pulses, so you will have a big 120Hz ripple component and a tiny 60Hz component as well. Don't mean too much in a lo-fi guitar amp but might be worth a dime in a hi-fi amp.
If I had a center tap I would use it, if only for bragging rights, "Mine's smoother than yours". The center taps works the same for all those 5V rectifier tubes.
Maybe PRR will come along and untangle what I just said. :l2:
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There is a descriptive write up of that topic in section 7 of linked doc - it elaborates on what sluckey says.
https://www.dalmura.com.au/static/Power%20supply%20issues%20for%20tube%20amps.pdf (https://www.dalmura.com.au/static/Power%20supply%20issues%20for%20tube%20amps.pdf)
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If you’re going to use a GZ34 then I suspect that a CT wouldn’t be of any benefit.
I think that the schematic linked in post 1 misrepresents the GZ34’s indirectly heated cathode.
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...Maybe PRR will come along and untangle what I just said. :l2:
Ah, I thought I nagged you to try it, near a decade ago, and neither of us could see the difference, +/-2.5V 60Hz bumps in 400VDC output.
I think it is a cost thing. And 2 wires is usually cheaper than 3.
It may also have been more urgent in days when 4uFd was a "big" cap.
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...Maybe PRR will come along and untangle what I just said. :l2:
Ah, I thought I nagged you to try it, near a decade ago, and neither of us could see the difference, +/-2.5V 60Hz bumps in 400VDC output.
I did try it. Even posted scope pics that showed the difference in amplitude of the pulses. For some reason I took the position there would be no difference. You poked me, I dug in with the scope and ended up proving your point. Checkmate! :laugh:
Wish I could recall enough of the discussion to search for the thread. Pics may even still be around. Not sure what the cutoff date is for Doug's purge.
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The center tap on the 5VAC winding helps to balance the ripple pulses from the rectifier. If you take the B+ from one side of the rectifier (either pin 2 or 8) and look at the unfiltered pulses you will see that every other pulse will be slightly bigger. This will have the effect of putting a small 60Hz fluctuation on top of the unfiltered pulses, so you will have a big 120Hz ripple component and a tiny 60Hz component as well. Don't mean too much in a lo-fi guitar amp but might be worth a dime in a hi-fi amp.
If I had a center tap I would use it, if only for bragging rights, "Mine's smoother than yours". The center taps works the same for all those 5V rectifier tubes...
I think it will work like that for direct / heater cathode types, eg 5Y3 / 5U4, but not for indirectly heated cathodes such as the GZ34 in the post #1 schematic.
Marshall made the mistake of taking the HT output from pin 2 when they designed/built the 90s AC30TBX for Korg Vox. The combination of their stupid hot switching standby and passing the HT current via the heater filament tended to result in a blown heater for the GZ34.
With the HT output (and hence reservoir cap) correctly wired to pin 8, the 5V heater signal at pin 2 is forced to swing above and below the Vac reference at pin 8. ie it doesn’t add any ripple to the HT.
Taking to HT from a 5V winding CT wouldn't be a good move, as it would force some HT current to flow via the filament (which as Marshall proved can shorten heater life) and it would add a portion of the 5V on to the HT output, which wouldn't be there if taken from pin 8. I can't see whether it would be 2.5V or 1.25V, so a negligible amount but whatever, it's a retrograde change.
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Thanks all for the information.