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waner 19th May 2006 04:29 PM

Lowering HT
I plan to build a Vox AC30 clone with a tube rectifier/choke PS.
The problem is that I have a Power tranny that delivers 340V.
I would like to lower that to about 280-290V.

Could I connect a 60V zener to the centertap of the Tranny?


kevinkr 19th May 2006 10:02 PM

I'd use a power resistor instead, high power zeners are pretty expensive. Another thought would be to use a partial choke input (very small cap after the rectifier. I can't really say what sort of effect this might have on the performance of a guitar amplifier though.. Caveat emptor! :D

ray_moth 19th May 2006 11:05 PM

I agree, a choke-input filter seems like a good solution. You should get a bit less than 0.9 x the secondary RMS volts, which would be in the ball-park

waner 21st May 2006 09:52 AM

Thanks for your replys!

I was planning to use this config:
Tranny - 5AR4 - Cap - Choke - Cap + additional rc filters

I want about +350V at the first cap (this point should go to the center tap of the Outputtransformer). The output tubes are EL84`s.
And the cap after the Choke should go to g2 of the EL84`s

I think that the HT voltage would get to high.


AndrewT 21st May 2006 10:24 AM

hang on, your description in post4 is a capacitor input supply. The peak voltage on the first cap is still root2 times Vac.

A choke input supply has the choke FIRST and this produces 0.9times Vac after the choke BUT only for a small range of output currents. The output current is very dependant on the choke size. If the current draw drops below the choke input minimum the voltage starts to rise alarmingly.

Dave Cigna 21st May 2006 12:02 PM

Mattias, you can try a choke input supply, but you might be disappointed. I have found that with guitar amps a power supply that is too stiff tends to ruin the playability and tone. The most common 'mistake' is adding too much capacitance. A choke input supply would be even more extreme. It won't play like a Vox, though you might like it.

Adding series resistance kills the dynamics and makes the amp lifeless.

I have used zeners in the center tap just as you suggest. It has worked well in vintage amps that were designed for lower voltage from the wall, like 110V when we're getting 120V - 125V these days.

The easiest ones to use are the 50W stud mounted versions. Bolt the stud through a hole in the chassis. And just solder the CT lead to the zener. It's almost too easy when the CT would normally be connected directly to the chassis through a solder tab. The chassis makes a suitable heat sink.

When used this way you need a zener oriented with the cathode connected to the stud and the anode at the solder tab. The 'standard' configuration is the other way around, so be careful.

Mouser sells a selection of suitable parts made by NTE for about $15 each. (That's cheaper than a choke suitable for choke-input use.) The NTE5240AK (3.9V) through NTE5296AK (200V) Are the ones to look at. The 'K' at the end means the cathode is connected to the stud, which is what you want.

IIRC, the AC30 used a 300V transformer, so it might seem that you would want to use a 40V zener with your 340V transformer. In reality you'll need more like 55V or 60V. This is due partly to the fact that the diode won't drop the full zener voltage at 250mA, and partly due to the fact that the sine wave won't be a sine wave anymore. Never mind, just trust me. :)

You might try the NTE5278AK (56V)

-- Dave

edit: after some thought I've convinced myself that even more zener voltage might be needed to get down to 350V B+. As much as 70 or 80 volts. If you measured your transformer at 340V unloaded then you might need the 82V NTE5283AK

knubie 29th May 2006 06:34 AM

Use a rectifier tube with a much higher voltage drop like the 5R4 instead of the 5AR4, this will help a bit and not mean big changes...

Merlinb 29th May 2006 10:31 AM

Firstly I wouldn't add zeners to the centre tap of the transformer as it is interfering with the ground connection. Instead, add them in series with the HT after the rectifier.
A power resistor along with a valve rectifier is going to cause a LOT of sag, especially when trying to drop the sort of voltage you want to. Perhaps a MOSFET regulator could be used? Something like:
Or you could try using a small transformer on the primary of your power transformer to act as an auto-tranformer, to bring down the secondary voltage. You could also use the auto-transformer to tap off a heater supply.

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