RH 84 -- SE EL 84 amp

jrebman · « **on:** 4 Jan 2008, 10:45 pm »

Have any of you folks ever built or heard one of these? It is a single-ended EL 84 amp that has an output of about 4 watts, runs in pentode mode and from what I know of it, looks pretty easy and inexpensive to build.

Just at the curiosity stage right now as we've got 3 SimpleSE projects well under way here in Colorado.

-- Jim

Dmason · « **Reply #1 on:** 4 Jan 2008, 11:16 pm »

Jim,

From what I have heard several times, it is THE SE EL84 circuit to pursue. There are som amazing anecdotes on the matter of the results. It would be the apple of my eye if the Uber Beam Machine werent evolving.

SET Man · « **Reply #2 on:** 4 Jan 2008, 11:26 pm »

Hey!

Hmm.. I was looking into to this a few weeks ago.

Is this the one you are talking about?

I have some tranformers from an old RCA SE EL84 console. So, I'm thinking of using them to make a new SE EL84. Haven't measure them yet. Hope that they will work with this design

I wonder if there are other simpler one out there?

Anyway, still this look pretty good. Oh! BTW... Angela currently having a sale on Hammond 125SE series trans right now.

These should make a nice inexpensive SE EL84 amp.

Take care,
Buddy

jrebman · « **Reply #3 on:** 5 Jan 2008, 12:11 am »

Dan, well, it's never too early to start thinking about the next amp

. I just love the EL84, and have some nice specimens lying around, so I know I'll eventually be doing something with them. Good to hear that the reports are encouraging.

Buddy, one thing I do know about this amp is that it uses an OPT with a 5k primary, where most EL84 amps typically use something more like 8k or more, but the MQ Robinhood series look like they will do the job for $100 each. Not saying that is the best choice, but at least it won't break the bank. And with a 300v 75 Ma PS, that part won't be too terribly expensive either.

-- Jim

Dmason · « **Reply #4 on:** 5 Jan 2008, 01:47 am »

Yes, there were comments from --I believe-- Bas Horneman, that as the circuit allows for a 5K output, the lower Zout was the clincher, in his opinion. I think there are notes about it at his site basaudio or something like that.

I have never heard an EL84 amp that didnt sound really damn impressive. A good one tells you that SET is, in my opinion, really just another option, and not the grail. The old Magnavox SE circuits are good too. You do not need much in the way of output irons, but the Robin Hood stuff would be choice.

What I like about the idea of using the TungSol 6550 is that it became clear that with the new circuits, and new production tubes including the 6SN7 at least the equal of the old stuff, you get a single ended "muscle amp" for not alot more effort.

floobydust · « **Reply #5 on:** 5 Jan 2008, 03:33 am »

Yes, the MQ RH iron would work well here. They also offer a 43% UL tap which might be a nice option switch on the amp. The circuit uses a bit of feedback from the EL84 plate back to the grid to clean things up. Overall... nice, simple, low parts count.

So Jim, how are the SimpleSE projects coming along?

Regards, KM

floobydust · « **Reply #6 on:** 5 Jan 2008, 03:38 am »

Almost forgot... some additional SE amps, including an oldie but goodie... ECL82, some other EL84 designs and a 6550 design also.

http://www.tubebuilder.com/schematic2.html

Regards, KM

planet10 · « **Reply #7 on:** 6 Jan 2008, 09:31 am »

I'm listening to an RH84 right now. A very good amplifier... and even better are a set of monoblk parafeed derivatives with EF86 (triode) as driver. Both built from mostly junk box parts.

dave

jrebman · « **Reply #8 on:** 6 Jan 2008, 04:35 pm »

Dave,

Care to share the schematic, BOM, and TX specs? I like the EF86. Wonder if the new production one is any good.

-- Jim

jrebman · « **Reply #9 on:** 6 Jan 2008, 04:48 pm »

KM,

SimpleSEs are coming along nicely -- just about all parts are here (except for my PT and OPTs, which are just waiting for me to be able to buy them), but the other guys have a pair of Allied PTs, and the James OPTs. Shortly we should be having a board stuffing and soldering party, and then my guess will be on getting chassis made to be able to assemble them on.

I found (actually Dave did) a nice 12" x 24" x .125" piece of copper plate, and some very nice pieces of 4/4 Koa for my chassis, but am still considering how I want to make the chassis -- simple or exotic.

I have to say that it's a really great project, and having 3 of us with a lot of intersecting, but not equivalent skills and resources is turning out to be a very symbiotic thing. Each amp is slightly different, so they won't be exact clones -- but more in line with the resources and desires of each builder -- and I'm sure they are all going to sound great.

I just bookmarked that tubebuilder site and will give it a closer look -- thanks for the link, I didn't know about that one.

Best,

Jim

andrewbee · « **Reply #10 on:** 6 Jan 2008, 05:10 pm »

I built an RH84 a while back, maybe 2 years ago by now with junk box parts, recycled chassis and 125ESE outputs. I thought it was so good I ordered a pair of Lundahl 1663 50mA S.E. outputs and Jensen copper in oil caps and planned to rebuild it. Fast forward to sometime last week and I have actually started the rebuild. Machined the top plate and painted it with Hammerite so now have to wait at least a month for the paint to get somewhat hard. I need to check and make sure I have everything I need to build it but I should. This time I am going to regulate the screen grid with an EL84 or 12B4 as a Shunt reg.

Last time I used the series resistor for the screen as per the schematic but remembered having to up the value as it was too low. I always wondered if it would sound better with the regulated screen and hopefully will know in a couple months. I certainly sounded VERY good as per the schematic.

Changing the 12AT7 for a 5965 sounds nice as well.

I am now looking for a nice piece of Mahogany to build the base with.

Andrew

JoshK · « **Reply #11 on:** 6 Jan 2008, 07:26 pm »

I've never heard an EL84 SET amp, but I've never heard a PP EL84 amp sound bad, just like I've never heard a 45 SET sound bad. They are very linear tubes with very nice harmonic profiles. The 6V6 is another.

Thanks for the link KM, that is one I didn't have, and I thought I'd seen them all. The EF86 > EL84 is intriguing too.

Andrew, take a look at Broskie's recent blog on partial regulation. IIRC, he warned against regulating screen grids without regulating B+. Don't know if you already were regulating B+, but thought I'd mention that.

floobydust · « **Reply #12 on:** 6 Jan 2008, 08:01 pm »

Regulating the screen is always a good idea, dates back to articles in ARRL to the 50's, maybe further. For simplicity you may want to consider some of the older gas regulator tubes as they have sufficient current range for use as a shunt regulator for screens. A bit of de-coupling and they should be very quiet. I don't feel you need to regulate B+ as a pre-req for screen regulation... just my $0.02. In general, voltage regulation is used when variations result in unacceptable performance or when you absolutely must have a clean supply with low-impedance and/or when the current demands are dynamic and cover a wide range. Push-pull amps (class AB) have large current swing demands as the power increases. A true class A amplifier has very little current swing demands from idle to full power. So you'll realize more benefit from regulated supplies on a P-P amp... but the cost is high.

I'd be interested to know if anyone has tried a UL connection on these smaller amps as most SE transformers generally have screen taps on the plate winding and the EL84 has been used very successfully in this mode before (Dyna ST35 is one example). Regarding the 6V6... agreed, many swear by these as those do by the EL84 (which can really get hot due to the small envelope). I've also seen (and heard) the 6AU6 used as a driver to the EL84 (my first amp at age 7 designed and built by my Dad). There's also some schematics in the old Radiotron handbook using a 6AU6 in triode mode and using the plate as a shield.... kinda neat.

Josh, one more link... some nice pieces of custom gear:

http://www2u.biglobe.ne.jp/~tossie/contents-E.010106.html

Regards, KM

planet10 · « **Reply #13 on:** 6 Jan 2008, 08:21 pm »

Quote from: jrebman on 6 Jan 2008, 04:35 pm

Care to share the schematic, BOM, and TX specs? I like the EF86. Wonder if the new production one is any good

The monobloks exist primamrily because of the "kindness" of 2 grundig console hifis. Output from the power is rectifierd with a graetz bridge using an EZ81, then CLCRC down to ~295V. Uses all Solen poly filter caps. EF86 anode is direct connected to the power supply and strapped in triode. Cathode bias. Input thru a 100k pot. the EF86 is RC coupled to the EL84 which is also cathode biaed. 100k (IIRC) plate to plate feedback. The primary of one of the grundig OPTs is used as a parafeed choke, the other as the load with a 3,3 Clarity cap (IIRC) as the parafeed cap.

All-in-all less than a $150 expended for the pair.

dave

planet10 · « **Reply #14 on:** 6 Jan 2008, 08:26 pm »

Quote from: andrewbee on 6 Jan 2008, 05:10 pm

Last time I used the series resistor for the screen as per the schematic but remembered having to up the value as it was too low. I always wondered if it would sound better with the regulated screen and hopefully will know in a couple months. I certainly sounded VERY good as per the schematic.

That too is on our try it someday list. We also have a 6V6 version on the bench (not parafeed, but much bigger East German output iron. And i should have enuff coke bottle 6V6s to match the big 5U4 coke bottle rectifier.

dave

andrewbee · « **Reply #15 on:** 6 Jan 2008, 09:13 pm »

Quote

IIRC, he warned against regulating screen grids without regulating B+

B+ or bias? As was posted above by Kevin, regulation is really when you want everything quiet and is easiest and cheapest with low current demands. I cloned the FVP5 linestage and also the superreg it uses and also used a regulator in my Aikido linestage but I don't think it was absolutely necessary in either case... should'nt hurt though.

Quote

For simplicity you may want to consider some of the older gas regulator tubes as they have sufficient current range for use as a shunt regulator for screens.

I thought about that and am still thinking about it. I have a VR150 so need VR105 (?) to get my 250V. Its not too late yet. I may try it both ways and see. I can always enlarge the 9 pin socket hole and add another to series the VR tubes. I was planning to breadboard the reg circuits anyway to see which is quieter / more stable. I have never used a VR tube but was reading up on them and asked for some advice as well. They seem easy to implement. The only con I have read thus far is that they need a little time to settle down and that they do not take kindly to being shut down and then restarted within a few minutes.
I also have a spare TIP50 left from the Superreg project which could also be used for regulation along with a few other bits.

I usually use Zeners to regulate.

Quote

We also have a 6V6 version on the bench (not parafeed, but much bigger East German output iron. And i should have enuff coke bottle 6V6s to match the big 5U4 coke bottle rectifier.

Dave, that should look real nice! a nice old school look with the coke bottle tubes.

I still prefer my Tripath amps nowadays but have so much tube stuff that I have to use some of it, plus I get the chance to play with my tools some more

floobydust · « **Reply #16 on:** 6 Jan 2008, 10:09 pm »

Andrew,

Good point.... regulating the screen with a fixed bias supply could be a problem, yet another good reason to have the bias supply provided by a tap on the plate winding. Using self-bias should not be a problem with a regulated screen supply.

Re VR tubes... I've had lots of experience with them over the years.... did a major preamp design in the 80's that was housed in a PAS preamp chassis and used a pair of VR tubes driven with a solid-state CCS. They work very well, but I also used the smaller 9-pin 0A2 (150v) and 0B2 (105v) tubes for ~250 volts. Just be sure to bypass them with a good quality film cap (at least 0.01uF but no more than 0.1uF) and ensure your current range is in the recommended range allowing for the change in screen current. Further decoupling using a basic RC setup is a good idea to eliminate the possibility of motor-boating or higher range oscillations. I've not had any problems with short duration off/on cycles but that could be due to the CCS employed.

And yes, prefer older ST glass to the later cylindrical types (ala 5V4G to a 5V4GA) and never really warmed up to the original balloon glass used for the 45. I also find the ST glass to be more immune to mechanical vibration and I've always gotten better hum balance on the ST glass.

Regards, KM

andrewbee · « **Reply #17 on:** 6 Jan 2008, 10:30 pm »

Quote

but I also used the smaller 9-pin 0A2 (150v) and 0B2 (105v) tubes for ~250 volts

Thanks for that Kevin, I did not know there was a 9 pin VR tube. I had read of the OB designation but just thought it was another naming convention of the same tube... like EL84 / 6BQ5. Maybe I will give A.E.S. a call and try some of those smaller tubes.

floobydust · « **Reply #18 on:** 7 Jan 2008, 02:16 am »

One last link for the EL84 fans... the original Mullard 3-3 amplifier design and a few extras too. This circuit actually goes back 51 years now.... talk about "what's old is new". Link #6 shows both versions of the famous 3-3, with and without control (ie, bass, treble, volume) and would also make a nice construction project.

http://www.technicalscientific.com/techinfo.htm

Regards, KM

JoshK · « **Reply #19 on:** 7 Jan 2008, 02:42 pm »

Well the reason I read why screen regulation is problematic when B+ is not regulated is this... Say your wall is running at 117V and your B+ is 400v and your screen is 255v regulated. Now suppose the wall voltage climbs to 125v (mine was 126 most of the weekend), your B+ is now 427v and your screen is still 255v. What just happened to your screen vs. plate?