RH UNIVERSAL AMPLIFIER

[roren]

Wonderful ! I got it.
Thanks for explaining it so well Alex.
I have absolutely nothing against having a silicon based CCS. I was more interested to
know if it's purpose was only for flexibility. But now I know that it comes handy to set
the optimal current for the O.P.transformer.
I don't see how I could resist to build me one of these RH amps. I have a pair of
'Hammond 125E SE' and some old chassis and a lot of tubes, all I need is time.

No, that was the story about the driver tube... you need a driver tube with a relatively high transconductance and relatively high mu... while the ECC81 fits the bill ideally, you can use 5965 and E180CC just as well. ECC83 will not drive to full power, 6SL7 even less suitable... etc.

Oops. I must have got that wrong. Thanks for correcting me.

[Alex Kitic]

Here is a picture of my personal RH Universal, here in action with 8417 output tubes and 5AS4 rectifier, 6201 Philips driver.

[roren]

Nice amp Alex.
I am a bit curious to see how big your PSU inductors are
Do you have a picture with the hood off?

For now my plan is to build with 12AT7 as driver. And for output, I have some
NOS Russian 6P3S beam tetrodes. They seem to be quite similar to 6L6GT. Why
not use them?

[Alex Kitic]

Nice amp Alex.
I am a bit curious to see how big your PSU inductors are
Do you have a picture with the hood off?

Under the hood you can see no chokes, they are housed on the upper level with the output tubes...



The output transformers are in the back of the box, and the two consume the width of the box... the chokes are in the front part of the box... unfortunately, I do not have a picture of that...

But I have made a couple of pictures of the smaller (second) choke compared to a well known rectifier tube:





All my chokes are usually custom wound on recycled EI laminations. The fist choke is larger and has slightly higher DCR (which comes as no surprise, since it has more windings).

[Alex Kitic]

For now my plan is to build with 12AT7 as driver. And for output, I have some
NOS Russian 6P3S beam tetrodes. They seem to be quite similar to 6L6GT. Why
not use them?

If you are going to use 6П3С (haha, cyrillic characters at hand, we do a lot of writing in cyrillic characters), I would suggest you to stick to the schematics and power supply, but introduce take care of two important issues:

1) Until you get some higher anode dissipation tubes, change the 12.4 ohm resistors to some value that would give you 80mA total cathode current draw, 15.6 ohms would be ideal, but since that value does not exist, adopt the fist standard value above that, or use two 33 ohm resistors in parallel. The same setup will be just fine for other 20W anode dissipation tubes you might find...

2) Use only 5R4 rectifiers as these will yield the lowest B+ (you would get probably some 250V across the tube). If you get some EL34s, you could use them with 5U4 in the same modified setup.

Once you find more powerful tubes (25W+ anode dissipation) you can change the current setting resistor back to 12.4 ohms. In theory, you might install a switch allowing to choose between the two setups... I have postponed that solution, since I have little confidence in switches when it comes to setting cathode current :)

[Alex Kitic]

Note the 6550 ST shape marked RCA on the metal base (the marking is on the other side) and the anodes without holes.

Also note the 5U4 rectifier with black anodes marked RCA (one of the best sounding 5U4 in ST shape).

Last but not least, the tubes were a gift from a friend who has recovered them from the bombed TV building... the NATO bombing back in 1999 when several people died in that building, in case you wonder "which bombing". A little tribute to friendship and memories.

[roren]

Thank you for the pic's, Alex.
Yes you're pretty handy with the cyrillic characters

2) Use only 5R4 rectifiers as these will yield the lowest B+ (you would get probably some 250V across the tube). If you get some EL34s, you could use them with 5U4 in the same modified setup.

First I must pick a power transformer. And then from there I must decide which rectifier
to use, from what voltage it gives. If I remember correct the transformer I have only
have about 330 AC Volt. That's maybe a bit to low for these kind of PSU?

Thanks again!

Rolf

[Alex Kitic]

First I must pick a power transformer. And then from there I must decide which rectifier to use, from what voltage it gives. If I remember correct the transformer I only have about 330 AC Volt. That's maybe a bit to low for these kind of PSU?

A 330V AC secondary can be used in a CLC configuration similar to that of the RH807 or RH84 - do some similations in PSUD to define the correct values.

Choke first PSs offer better sound in my view, but the cap first approach should be just fine for an SE amp with constant current draw, provided the secondary of the transformer is powerful enough (I presume that you are talking about a CT transformer, therefore what you would need is i.e. 660VCT "chosen current draw" mA x 1; if the transformer is 330V AC no CT, than it should be about "chosen current draw" mA x 1.5 rated).

[roren]

It,s a CT and it is 330V between center and end. I think it's for some 'Fender'
amp. The HT winding is 250mA. That should be enoughfor 2 x 80ma plus the pair of 12AT7.
I have done some simulations in PSUD and CLC seem to be the way for me.
I hope to get the time to build this, and when I do I will let you know about it.

Thanks.

Rolf

[Alex Kitic]

It,s a CT and it is 330V between center and end. I think it's for some 'Fender'
amp. The HT winding is 250mA. That should be enoughfor 2 x 80ma plus the pair of 12AT7.
I have done some simulations in PSUD and CLC seem to be the way for me.

I would say that it would be enough for the "full" 100mA as well. You could include a switch allowing you to choose between 12.4 ohm (100mA) and 15 ohm (85mA)... or similar. Possibly the best way to do ti would be connecting 12.4 and 3.3 in series and allowing the 3.3 to be shorted when the switch is on: that way the worst possible condition is that the tubes will not draw 100mA, but only 80mA (did not hurt any tube... but might hurt the sound).

[roren]

Possibly the best way to do ti would be connecting 12.4 and 3.3 in series and allowing the 3.3 to be shorted when the switch is on: that way the worst possible condition is that the tubes will not draw 100mA, but only 80mA (did not hurt any tube... but might hurt the sound).

Sounds like a good idea! That's the way I will do it, when I do it
I hope that I in the near future could report about my experience with this amp.

Rolf

[Alex Kitic]

You are welcome! I am looking forward to hearing your's and other diyers' impressions on the RH Universal... I like mine very much and feel that it's universality and versatility are yet to be discovered - while the sound is definitely the best of the series.

[Vermeero]

Build the RH Universal, what a great performer... the sound is a tad more precise as with the RH84. More power too!
See attached files for the result.

[roren]

Nice build Vermeero!
I have been away from this forum (new PC) so I haven't seen your pic's until today.
Would you mind tell us about the chokes, where you got them?

Again, nice and tidy build.

Rolf

[mr2racer]

Hey All,

I've built two RH84 and they both sounded great. I just found this thread and am really interested in building this amplifier. Does it sound as good as its smaller brother?

Kevin