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

[eldarvanyar]

Hi All,
I want to build a RH84 V2 valve amp and have been gathering the parts for the project and now have most of them. I don’t know if you are familiar with the amp the web page is here:





http://rh-amps.blogspot.com/2013/02/...-2_26.html?m=1

I have bought Lundahl ll1663 output transformers for the project
https://www.audiohobby.eu/en/lundahl...1663-50ma.html

I have also bought a Audionote Choke 160 10H 120ma

https://www.hificollective.co.uk/com...choke-160.html

I have struggled to find a suitable power transformer although I know the Hammond RX372 BX or DX would be suitable according to the designer.

From Alec: The RH84 requires 100mA DC B+ and you should look into HV secondaries of more than 100mA. For instance, the 372BX or 372DX are what you need for an RH84 - with universal primary that can cover US voltages as well as all combinations of European voltages (220-230-240V).

Hammond 372 DX
Power Rating (VA) 127
Primary VAC
Universal - 100, 110, 120, 200, 220,230, 240 VAC 50/60 Hz
A.C. High Voltage Secondary RMS 600V C.T. @ 144ma.
Bias Tap on High Voltage Secondary w/ C.T. Grounded (VAC) 50
Filament #1 (VAC)
5V C.T. @ 3A
Filament #2 (VAC)
6.3V C.T. @ 4A

I think the Encore XPWR066-240v would also be suitable.

I have bought the necessary valves 2 x EL84 and 12AT7 Driver, including the rectifier a 5R4WGA CHATHAM NOS VALVE

Can anyone point me in the right direction to source an open frame type psu as I would like to build the amp in an enclosed box rather than have everything on show. The Hammond and Edcores are Bell type transformers for sitting on top of the amp.

I have sent two emails to majestic transformers and Primary Windings who I have ordered from before but have not had any replies.

I would like to be able to complete this build this year and would appreciate any thoughts or help in the right direction.

[bikerhifinut]

I've built a couple of versions of this circuit.
I did one with a fairly standard silicon bridge rectifier and CLC filtering. Used a standard 240V AC secondary transformer and that should get you into the 300V zone ok.
It all depends on your religious and other beliefs about power supplies, these days I'm more than happy to use a silicon rectifier and avoid the expense and extra current needed for a valve rectifier. You can also have a nice big reservoir capacitor without worrying about the rectifier valves maximum value.
You pays your money and you takes your choice.
I reverted to the original circuit rather than the zener diode screen grid feed and the LM317 based CCS on the cathodes, I used the standard 270R cathode resistor and a simple resistor feed to the screen grid. I had odd issues with the LM317 not switching on at times which I assume was due to the time taken for the valve to start conducting on switch on. I cant honestly say I noticed much difference.
You also have to use sensitive high efficiency loudspeakers, this amp may claim 5W, but a SE EL84 is only ever really good for about 3W tops before distortion becomes a real issue and I reckon its closer to 2W for anything like a reasonable quality.
But I will say if you take very good care on your heater wiring and grounding you'll get an amp that is astonishingly quiet in terms of background noise and hum, especially for an SE design.
I dont want to sound like I am raining on your parade, it is a reliable circuit in as much as it works pretty well and I did torture mine by driving a pair of Quad ESL989 which was really an unfair fight. But it did make a pleasant sound, just quietly! And didnt give up the ghost. (I usually drove a pair of Kudos Cardea1 with it and it coped fine, given that small standmounts aren't terribly sensitive)

I fiddled around a bit with mine and I use a version of it that I tweaked around a 6V6 pentode, operating conditions aren't a lot different to an EL84 anyway. It gets used for playing back radio and background music from the PC or media player when I'm faffing about in the workshop/radio shack and is pretty damn nice there.

By the way Alex kitics design isnt by any stretch unique, and Alex doesn't claim it is anyway to give him credit. I recently found the MO (GEC) data sheet for their Loctal KT81 output pentode as I found a few in a job lot of ex BBC valves I bought on spec a few months back, and their SE application for that valve looks awfully similar to the RH84 design with the anode feedback. I rather like that idea as it opens out the choices on output transformers in as much as you dont have to scratch your head working out feedback component values if the transformer doesnt quite have the turns ratios that the design specced or used.
I'll have a go at that application as I have some decent transformers asking to be used. I shall try and fish out the data sheets and post here.

For my money, and again this is just my personal feelings, not the holy gospels, I'd advise anybody wanting a serious stereo amplifier to build something based on a pair of EL84 or 6V6 types in Push pull. You won't pay very much more, especially as decent SE output transformers are not cheap to make, and you'll get a genuine 10W plus at less than 1% distortion.
Look at the KEL84 if you want to see where I am coming from, you'd be hard pushed to build something anywhere near as good from parts sourced independently for anything near the price. trust me on this, I know from experience.
Good luck, and have fun whatever you decide on as a suitable PSU, just dont get hung up on the design posted with the circuit.

Best Regards,

Andy.

[bikerhifinut]

As to mains transformers, and as you are wanting to build it as an enclosed case type, then consider using a Toroid or even 2 toroids. One for the HT and there you can use a standard 240V isolation type, and for the heaters you have choices galore. 6V for parallel heaters and 12V for series heaters. An over specified 6V toroid will usually deliver a bit more than 6V under load and 6V is enough to get decent emission from the cathodes. I think its possible to find toroids wound for 6.3 and 12.6 Volts also.
Theres a Polish manufacturer, Toroidy, you can google em up on the internet and they'll even custom wind you a nice encapsulated transformer with the Voltages and VA you need for a surprisingly reasonable fee. The downside is since brexit there will be unavoidable red tape and duty to pay now. Worth a look all the same?

Andy.

[bikerhifinut]

link to thread on my original build of the RH84, might throw up some pointers.

http://www.world-designs.co.uk/forum...?t=6993&page=2

[eldarvanyar]

Hi Andy,
Thanks for taking the time to reply.

I have looked at your build and from the various users reports on the web the RH84 seems to have a good reputation with many happy users and Alex Kiltic is very helpful. I will progress with it and complete the project this year. I do have some Foster FE126EN speakers that I could use with it.

I would rather buy a good built and competed second hand KEL84 at some point to add to my VA88XL that I have. I already have a EL34PP hardwired amp that I bought from Home Kong about 2006 so I am quite well catered for.

I have sent an email to Nick at HiFi Collective to see if he can get one made for me.

[eldarvanyar]

Hi again,

As ever things are progressing slowly. However Majestic Transformers can make me a psu at a very reasonable price and I am going to go with them.

I have decided I will use the 5R4 rectifier as I have two NOS that I have bought.

Majestic have quoted on the following supply

Primary : 0 - 230V - 240V 50Hz

Secondary 1 : 315V - 0 - 315V 120mA

Secondary 2 : 0 - 6.3V 2A

Secondary 3 : 0 - 5.0v 3A

Rating : 88.2VA

Style : Fully shrouded, top chassis mounting with flying lead termination. Earthed electrostatic screen between primary and secondary windings.

Size : W 84mm, D 102mm, H 102mm, weight ~ 3.1kg

This is higher than the 300v - 0 - 300v but they can wind it with this rather than 315v -0- 315v.

The designer Alex has said that 120mA is more than enough for the RH84, and suggested the HT to be 630-640V CT.

I think where I am slightly confused is that the Hammond Transformer is 300v-0-300v.

I know that I can adjust the voltages with resistors etc but would rather keep it as simple as possible.

The other thing is since my stroke 8 years ago my mind seems to have gone to mush when it comes to numbers, physics and equations. I can no longer get my head around it. So grateful for any help. It would be great for me to complete this project.

One last question what valve base do I need to buy for the 5R4 rectifier valve?
Thanks.