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Homebrew 6BQ5/EL84 SEP amplifier based on "RH84" circuit


mike stehr

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This was a ongoing project I started about 5-6 years ago. I acquired a piece of 1/16th inch thickness steel plate at a place I worked at, and had a guy mill out the hole for the transformer and IEC plug socket. I had some Brazilian cherry 3-3/4" hardwood flooring sitting about, so I made a wood base for the steel plate.

I wasn't looking forward to drilling socket holes through 1/16" steel plate with my cheap drill press, so I shelved the project for about 2 years...

Last spring I decided to get brave and carefully drill the socket holes. I managed to pull that off without a hitch, so I continued.

The output transformers are Grundig or some German make. I spent about 50 bucks in resistors, and had all the other parts on hand. The power xformer is from a Heathkit single-ended 6BQ5 integrated amplifier.

I maybe should have added a choke to the power supply, but I had no room left underneath, and wanted to keep things minimal on top. (plus I had pretty much committed myself by drilling holes/placement)

I had started working on the power supply last spring, working on dropping resistors trying to establish a working supply without dropping too much voltage and too much dissipation through the resistors. It was summer by then, and I tend to shelve audio projects during the summer months.

Winter is here, so I returned to the project. It took some noodling and pondering with the Duncan PS program, determining resistor voltage drop and dissipation, along with figuring out the proper load to input to the program. Finally, I got everything fingered out and come up with a RCRCRC...The first "R" is actually right off the rectifier. I had better luck coming up with a cleaner waveform using the program with the 100 ohm resistor right off the EZ81/6CA4 rectifier.

It musta paid off, dissipation through the "R"s is nice and low and safe.

The amp sounds rather good, and gets better sounding over time. It's a new amp with maybe 3 hours of use, and it takes time for things to settle in. With my 12" coaxials, the amp has a little lack of bass. But, my little Magnavox EL84/6BQ5 console amp does the same. The CDK coaxials need a little more oomph for bass than the little SE EL84 amps can give. They have good bass with a 2A3 SET stereo amp, and better bass with a Scott LK-48B integrated PP 7189 amplifier.

I need to drag out the Cornwalls and see how the RH84 amplifier sounds with them...the little Magnavox SEP console amp does very well with the Cornwalls...with respect to balance of tonality.

I have a Raytheon 12AT7, a Mullard EZ81, and Sylvania 6BQ5 black plates. I may try a pair of Mullard EL84s next...

I still need to finish and oil the wood base.

Mike

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Edited by mike stehr
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That's really beautiful work Mike,. The build quality is supper. It's obvious you spent more than a little time thinking through the layout.

For me this post couldn't come at a better time. Right now, under the supervision of our own Tubefanatic, I'm beginning a SEP project which somewhat similar to yours. Looking at your work has caused me to re-think things.

So some questions.

Why are the output trannies so close to the power tranny? Why are the speaker outputs so near the power supply? Is the copper bar rail used for a "star grounding" scheme, and if so where did you get the copper? What are those cool things you are using for solder terminals and where did you get them?

Inquiring minds want to know.

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Thanks for compliment, Marty. I am a bit curious about your project.

Why are the output trannies so close to the power tranny?

I just placed the OPTs to keep the primary leads and B+ as short as possible. You've seen plenty of tube amplifiers with the output transformers sitting right next to each other, it just depends on how one places the iron in relation to each other for minimal magnetic interference. After mentioning that, I didn't pay attention to the orientation of the OPTs in relation to the power xformer. I just took a chance and things seem fine.

Why are the speaker outputs so near the power supply?

I wanted a amplifier that had all the connections out of the back of the amplifier, and look as simple as possible on top...dunno why, just wanted the commercial look I suppose...

The optimum approach would be to park the input jacks right next to the input tube grids for minimal length. And depending where one would mount the OPTs, have the speaker outputs as near the OPTs as possible with minimum length.

I just didn't want a interconnect hanging off the front, with speaker wires off the sides...

Is the copper bar rail used for a "star grounding" scheme, and if so where did you get the copper?

I think star grounding involves running all the grounds from required components to one place, which involves a bit of ground routing/wiring layout. Star grounding works fine, and can be done very well...I was just shown this way to go about it. Both grounding schemes work just fine, (bus bar/wire, star grounding) it's just a different approach.

All the high voltage/current components are grounded at the back of the bus wire, such as the PS center tap, the PS first cap ground, etc...and you work you way towards the front, to where you have the low voltage/current ground connections. The grounds, (shield) of the input jack wires are grounded at the very front of the bus wire.

I went to the hardware store and bought about 2-3 feet of 10 (or 8?) awg solid core copper wire, and stripped it. The solid core ground wire off of Romax will work as well.

What are those cool things you are using for solder terminals and where did you get them?

They are they ceramic terminal saddles pinched from a Tek oscilloscope...I love the things!

Looks like my Decwares, nice job!

Match them with the cornwalls and see what happens... :)

I did that today. It matches up real nice with the Cornwalls....real nice...

Edited by mike stehr
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Thanks for the detailed reply Mike. I like the way you think, but I'll probably go with a more conventional layout with the speaker terminals and output transformers. I really do like your craftsmanship and I may group some of the inputs, power cord etc towards the back. I was going to use a project box, but now I think I'll come up with a metal plate and build a box. Needless to say after Maynard gives me a hand, he favors star grounding by the way, there will be a writeup on the new toy, complete with pics.

The project is using the major parts and tubes from an old Pilot console, and Maynard has actually redrawn the circuit and power supply which is pretty cool since I have no idea how one does that. Obviously way beyond my humble skill set.

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Thanks for the detailed reply Mike. I like the way you think, but I'll probably go with a more conventional layout with the speaker terminals and output transformers. I really do like your craftsmanship and I may group some of the inputs, power cord etc towards the back. I was going to use a project box, but now I think I'll come up with a metal plate and build a box. Needless to say after Maynard gives me a hand, he favors star grounding by the way, there will be a writeup on the new toy, complete with pics.

The project is using the major parts and tubes from an old Pilot console, and Maynard has actually redrawn the circuit and power supply which is pretty cool since I have no idea how one does that. Obviously way beyond my humble skill set.

You can always use a project box and build a wood frame around it. I tend to prefer that idea, because you can get better shielding from EMI and noise using a bud box than a plate with a wood base.

Seems like Maynard did most the brain work for you...it's just a matter of layout, wiring, getting things like voltages checked over, and get his blessing when it's ready to go.

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  • 2 weeks later...
  • 2 weeks later...

Nice work - the humble 6BQ5, either single-ended or push-pull, has become a real favorite of mine, up there with the 45, 2a3, and 300b. The approx. 4 watt SE circuit I made in.....what was it.....1996, I think, was capable of really nice (and surprising loud) sound with Klipschorns. Earlier this year I also acquired and then completely disassembled and rebuilt a very presentable LEAK ST20. A very precise and clean sounding amp; not the least bit overly warm, lush, or euphonic.

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  • 2 weeks later...

Got even more sidetracked and decided to breadboard up a 6V6 PP circuit, based on the Dynaco (floating paraphrase?) 6V6/EL84 PP circuit.

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I needed some sort of 6V6 PP circuit for a pair of pimped out Magnavox 6V6 mono console amplifier chassis' I have.

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The output transformer is pulled from a '61 Admiral 6BQ5 PP console amplifier. It has 16 and 8 ohm output taps, the console stereo used the 16 ohm taps for the tweeters, and the 8 ohm taps for the woofers. The Magnavox OPT's are rather small in comparision.

It took some noodling with feedback resistor values, and the frequency/fdbk compensation cap, but I did finally manage to make a decent square wave. I tried running the feedback resistor and comp cap with the 16 ohm tap, and use the 8 ohm for speaker out, dialing in for best square wave response. But things sounded harsh and sharp when listening to music.

I then went with the feedback network from the 8ohm tap, and used that tap for speaker out, dialing in for best squarewave response.

It's only mono on one Cornwall, but the second take with music sounds rather good. Strong bass, with no harsh sharpness to my ears. Good enough to continue on with the dual mono 6V6 PP project.

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Mike

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  • 2 weeks later...

It works!

Sounds really nice on the Cornwalls, even with Yugo EL-84s and a RCA 6CA4. I'll toss in a coveted Mullard EZ81 with some nice EL-84s on the next pass.

I made a mistake, and found out quick when there was no sound. The matter was rectified...

I still need to attach/repair a plate cover thingie, and it'll be done.

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Edited by mike stehr
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