Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: punkykatt on March 15, 2014, 01:59:41 pm
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Hey Guys, For a friend I`m going to be building an AA1164 PR circuit (2- 6V6`s 3- 12AX71s and 1- 12AT7) in a solid state amp chassis that is very small and will be omitting the rectifier tube due to lack of space. On the turret board I have made room to install the rectifier diodes. Can I get away with using two 1N4007`s or should i use four? I also have some 1N5408`s. My brain is in slow motion today. Thanks for your help. Punky
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I would use four if they are rated for 1000V.
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Thanks Sluckey
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Old Traynor amplifiers had 6. Three on each side. There were built like a tank. I've never seen a busted diode bridge in a Traynor amp, never. To save more space, use a one piece diode bridge, the one with a hole in the center so you can screw it directly onto the chassis, close to the PT. Choose a 1,000V 5A for lifetime operation. Maybe cheaper than 6 diodes ( or 4 ? ). BE CAREFUL: there are two AC inputs and 2 DC outputs ( - and + ). If you tie the center tap of the PT to ground you OUGHTA cut the minus leg of the one-piece diode bridge and avoid using it, this way it is the same arrangement as 4 or 6 diodes. You just can't use simultaneously the minus leg of the diode bridge all along with the center tap of the PT.
Colas
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Colas, thanks for the reply and info on the bridge diode, thats good to know. I have plenty of 1N4007`s on hand and already made room on the board for the 4 diodes. Punky
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4-1n4007 will work great Marshall uses this on almost every amp they have built .
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Mr. Merlin say that is better to use a single cap after the bridge (or full wave rectifier) or a fast diode in series (like UF1007)
one of the two solution, not both at the same time
he say that a cap in parallel for each diode is prone to have problems (I didn't remember which kind of problem)
K
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Mr. Merlin say... that a cap in parallel for each diode is prone to have problems (I didn't remember which kind of problem)
There's more than 1 reason to use the caps in parallel with each diode. The most important reason, though, would be to enforce voltage-sharing among the series diodes.
That's what the 220kΩ bleeder resistors across the series filter caps in Fender amps also do: they divide the total applied voltage evenly so that each cap sees half the total voltage. In the same way, the caps paralleled across the diodes form an a.c. voltage divider, and cause each diode to see an equal portion of the total applied voltage.
It is possible for a diode failure mode to cause one diode out of the string to see most/all of the applied voltage, and create a cascading failure taking out the whole rectifier circuit.
Anyway, you usually see those only in circuits with very high voltage (like rectified to 1800vdc, where no single diode has enough voltage rating to operate) or where they are required by regulation (like in some EU products). You only need series 1N4007's in circuits with an input a.c. voltage greater than 1000v / 2 * 0.7071 = 353v RMS, or as extra safety margin for lower PT secondary voltages. So yeah, if you are worried about switching noise, you're better off using fast rectifier types, rather than the paralleled caps.
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Mr. Merlin say that is better to use a single cap after the bridge
I APOLOGIZE
Mr. Merlin say that is better to use a single cap before the bridge
the reason (he say) is because the value of the capacitor to be put in parallel with the diodes is critical
and with a wrong value the problem tend to be worse or else the capacitor has no effect at all
The use of a small capacitor (usually from 10nF till 100nF) before the bridge didn't solve the problem but reduces the
frequency of the ringing and fix the amplitude at a value determined by the transformer
He say that a modern better solution is to use fast diodes (this reduces the amplitude of ringing, but not frequency)
and you are not obliged to use all fast diodes for the bridge
you can achieve the same result putting a fast diode in series, after the bridge
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And more, I'm not sure if this applies also to cap in parallel with in series diodes
(http://www.valvewizard.co.uk/bridge2.jpg)
I apologize again for the mistake
K