RH84 Ver.2 PSU - Where and what to buy?

[Richard]

Hi Lee, Al,

Not wishing to disagree with Al's reading of the specs but I think you'll be fine with 5R4 running that lowish voltage. I have a pair I used in 5-20 monoblocs and swapped with GZ34 and 5U4 without issue. Further, these monos had first caps of 150uF(!), but they did have 100R on each anode, the mains TX impedance sometimes needs consideration.

That was a good few years ago so I had a quick google and this thread throws more info on 5R4,

https://www.thegearpage.net/board/in...r-tube.694885/

[bikerhifinut]

I'm with Al on this one.
I would be more than a bit nervous pushing a valve rectifier out of its recommended maximum capacitor value, notwithstanding series resistance to limit inrush, you're putting a big strain on the valves, and you can always increase the value of the capacitor downstream of the choke or smoothing resistor.
Before you go out and buy a new transformer for this project I'd think very hard about using something like an EZ81 if you are determined to use valve rectification, well up to the job of supplying a couple of EL84 at full chat and readily available as NOS if you hunt around. And best of all, relatively cheap!
These days I would only use a valve rectifier if the circumstances dictated it, i.e. the transformer was already in my posession and in order to achieve the correct HT I would need to use a valve.
My SE project coming up is utilising an old transformer plucked from a Mullard 5-10 design and thus will use an EZ81 as its gota 1A x 6.3V heater winding.
Otherwise I'd use a 250V secondary and a silicon rectifier. I really don't have the religious convictions about Valve rectification that others do.
Theres a really good PSU simulator in the Duncans Amps website as I remember and its an interesting exercise to go through.
Anyway if you have decided to buy those majestic transformers then its all academic and I'd counsel using a readily available GZ34/5AR4 rectifier if its a 5V heater rect. You're going to be running it well under its ratings and thats no bad thing for a rectifier.

A word from one who got bitten expensively, make sure these transformers from majestic are beefy enough in the laminations, I had a pair made for my original EL34 monoblocs and they were noisy as hell mechanically and as far as I am concerned a mains transformer with any kind of quality audio pretensions has to be totally silent in operation.

If you think about it, all you need is a transformer specified for a pair of Mullard 3-3 (sometimes described as a 5-6 when built as stereo.)
Or a Transformer built for the classic Mullard 5-10 circuit, both will be spot on.


Andy

[snowman_al]

Richard thanks for the info on the 5R4, yes I read 'Typical' as 'Maximum'...

There is however another robust, cheap 5 volt Octal rectifier that would be ideal in this application the Russian 5ц4м / 5C4M (http://r-type.org/exhib/aaa1820.htm).
Not as ugly or large as the 5R4 and at $6 or $7 each way cheaper than the GZ34.

[Richard]

Hi Al, Andy,

Yes I’ve seen short form data for 5R4 which shows “Max 4uF” but think it probably means if the supply impedance is not known as these are high voltage rects.

The full data sheets show “Typical 20uF” with the supply impedance shown for different voltages, and say **, in effect, that more uF can be used if the supply impedance is increased and/or the voltage is lower.

Mullard’s 5-20 with GZ34 has 125R for 8uF and their 10W amp with EZ81 has 200R for 50uF, Mullard and 5R4 info attached.

Reading Lee’s posts he wants to build the amp shown in post 1 but we don’t know the mains TX impedance so probably 100R on each plate would be sensible.

[eldarvanyar]

Originally Posted by snowman_al

I would choose the 300v-0-300v.
The final HT+ (B+) will depend on the current draw, transformer regulation and rectifier 'droop'. You will not know that until you build it.
At the end of the day a few less volts means a cooler running amplifier and you will not notice any loss of performance. Alan

Thanks Alan,
I will go with 300v . Should I ask for 2.5 or 3 amp on the 6.3v secondary?

Also how do Majestic a transformers compare to Hammond? One of the posts above did not sound as though the transformers were that great.

Sorry to make a mountain out of a molehill

Thanks

[BOONDI]

[/left] quote. Richard.... Quote.

Yes I’ve seen short form data for 5R4 which shows “Max 4uF” but think it probably

The full data sheets show “Typical 20uF” with the supply impedance shown for different voltages, and say **, in effect, that more uF can be used if the supply impedance is increased and/or the voltage is lower.

20uf is for B series 5R4 wgb. But the op has already bought 5r4wga, and as he is only at approx 1/10th V, then 20 would prolly work ok. I guess it would be suck it and see.....

5r4 wba sheet here..... https://www.google.com/url?q=https:/...r6_5iYoE76QAx9

Would defo want some big heafing resistors in there though.....

[BOONDI]

Originally Posted by eldarvanyar

Thanks Alan,
I will go with 300v . Should I ask for 2.5 or 3 amp on the 6.3v secondary

Sorry to make a mountain out of a molehill

Thanks

el34 heater 760 ma X 2 plus 300 ma for 12ax7 run in pllel gives 1820 ma so 2 amp would work ok but I like to spec a little over as 10% would put you right on the limit. 3 amp would be better as this is 30% over rated.

Just remember you will need a 2amp 5 volt heater winding for the 5r4 as well. I would give it 2.5 amp here.