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In Reply to: RE: II. Power Supply posted by Garg0yle on December 06, 2014 at 17:05:10
Make sure that choke is well spaced from the power transformer and filter choke, and as much as possible with its axis perpendicular to the power transformer axis. It's more sensitive than the output transformer, which should also be well spaced and oriented.
What I have often done is to mount the power transformer on some plywood and connect its primary to a line cord. Then put a voltmeter on the choke or OPT primary and move them around, reading the voltage picked up and looking for nulls.
Follow Ups:
OK so I applied power to the primaries to find a good spot for the plate choke as recommended.
I was able to obtain a steady 0.004 mV while switching the amp on and off at one particular location.
It's pretty well isolated from the PT, but I do have some latitude between OPT and the 6J5 socket, so I think I will split the difference between the two.
...one to go.
I decided to not elevate the 6.3V heater instead just connect a couple 100ohm resistors for an artificial center tap grounded back at the bus, instead of passing the heater current through my signal ground.
That's my reasoning anyways.
I am going to putter away at the second one tonight then double check my work.
I forgot to get some 1.5A fuses, I will have to scrounge some up.
I took the completed amplifiers out to the garage for a smoke test.
The two pots were set roughly half way. One amp had a little more output then the other, but I am not going to get excited just yet. My 2A3's are used and are marginally matched, see what happens after I adjust the pots are properly adjusted with a DVM.
Tomorrow I will figure out how to prop the amps upside down so I can do some tweaking with the tubes in place. The 2A3 is a touch taller then the transformers.
In the garage I wired a couple of 92db speakers in series to get a mono speaker roughly 16 ohms for the trial. The source was a cell phone.
-17 is not an ideal temperature for the speakers with rubber surrounds, but what I did hear was very good!
Hopefully tomorrow I will get them into the system. I will update after I get a chance to give them a real listen.
Eventually I will make some plinth bases for them to cover up the bottom and maybe add some ventilation.
Big high five to Paul Joppa, Thanks.
Couldn't wait until tomorrow lol.
Dialing in the cathode pot on the 6J5 was easy as pie. This fixed up the differences in output I heard earlier.
I did not play with the hum pots yet, I can hear a bit on the left but good enough for tonight.
Not to sound cliche, but I did hear some things I have never heard before. lol Slight nuances in a song called Beyond by Daftside/Nicolas Jarr.
There is definitely an increase of presence in the mids and high, (or a loss of lows relative to the RH84 I was using.)
In fact I padded down my xovers a bit to balance it out with the lows.
I will have to optimize the tuning of my speaker cabinets for the dampening factor of these amps compared to the RH84 I was using.
Anybody sitting on the fence I recommend making some. If I had to grade it against the RH84, the RH84 would be a 7/10 and the BBProofs would be a solid 8 maybe 8.5/10
While I should give it some time before making any grandiose claims, it's probably reasonably safe for me to say that they are the best sounding amplifiers I have heard to date. FWIW.
This morning with some fresh ears I can definitely hear piano chords with more clarity, the individual notes are a little more tangible.
I want to say that the snare drums don't quite have the snap, or audible skin contact of the RH84, I'm not sure.
Don't read too much into this, I feel the sound of snares may be a little fuller, perhaps they decay is a little more noticeable then before with the cap-coupled amp.
I do like having the isolation from the monobloc chassis again, pretty sweet.
Got a chance to put the pedal down this afternoon.
The track was Metatron by Darkside/Psychic.
These babies got really big sound!
The RH84 has a sound-stage that starts low in front of me, going higher and farther away from there.
These amps by comparison represent more of a wider, taller wall of sound that I can only describe as a cross between a warm humid breeze and standing 5 feet in front of a speeding transport.
I feel dwarfed and enveloped. Sublime even.
I dialed the hum out today.
It's pretty easy to get ~6mV of hum by ear, I couldn't get much lower then 3.9mV - 4mV with the voltmeter using the low pass filter setting.(I get an even lower result without the low pass filter, but it was more erratic, so I used the LPF to effectively find a null as it was more stable.)
They are effectively silent.
Obviously they sound a little better with the hum eliminated.
For shits and giggles I decided to separate the chassis, earth ground and electrostatic screens from audio ground. This introduced a bit of higher frequency hash, so I put it back to how it was.
Also worthy of note, regular 1A fuses have been working in my application, they are well behaved little amps.
Edits: 01/15/15
It appears Paul may have abandoned this thread.
Though I don't know specifically why, I suspect it is because of my sabre rattling with Drlomu and Tube mangler.
So I will take the opportunity to say sorry to Paul, it appears you may not want to be associated with somebody the likes of me, whatever reason that may be.
Nonetheless, thank you for your efforts.
On the topside of my chassis I have the PT-choke-OPT all in a row along the back. The chassis is kinda small in an attempt to keep it rigid.
The choke and OPT are potted, so hopefully they will be OK.
The 156C can go underneath toward the front in a corner close to the 6J5 and/or 2A3.
I will try your suggestion to use the DMM to see if I can find a sweet spot for the 156C. I do have to order these first though.
Hey Garg0yle, years ago a company called Welborne Labs came out with a DRD style amp (similar to the Monkey). You can find chassis layout photos on line if you poke around. They had both a solid state and tube rectified version of it. IIRC it could be ordered for a 45, a 2A3 or a 300B. It was reported to be very quiet.
I actually picked up a couple of those chassis from Welborne on a clearance one time...always wondered how to figure out the placement of the items 'under the hood'...think I might add a "BB proof half monkey ala DRD chassoise" to my hot stove on deck circle...
If you go to their website (still in business I guess) you can buy a PDF of the construction manual. I did it weekend but it didn't open, but a day later they sent me an email copy. Anyway, clearly Paul's circuit is different / better, but this is a great guild to the layout.
re: "...clearly Paul's circuit is different / better", mine is nearly identical in conception - I bypass the 2A3 cathode resistor instead of the driver cathode resistor; that's about it. The topology is so simple, there are few variants possible.
Mostly what I have tried to offer here is some specific parts values as a help in getting started, and a description of how they were chosen for those who want to make modifications. Plus of course I could not have put someone else's commercial design in the public domain!
Thanks.
OK so I had a look at a few of the Welborne DRD via google image search.
My layout is similar, the differences being:
-I have my choke jammed in between the PT and OPT on the topside.
-The 2A3 is in between the input tube and rectifier, placing the 6J5 on the opposite end of the power supply area. (Although I see some variations, both 2 and 3 tube versions of the Welborne DRD)
-The plugs and terminals are going to be along the "long" side of the chassis. The long sides on mine will be the front and back. I pondered turning them 90 degrees like the Welborne, but I will leave that for now as I don't see an apparent advantage to do so at this point.
Although I might put the power switch near the IEC socket like the Welborne has. I haven't had a problem with front mounted power switches in the past, however I think it might be prudent on this one.
I don't know what the orientation is of my OPT and choke though, since they are enclosed. I will orient these by their lead-outs at the bottom to minimize crossed wires etc.
I noted the plate choke on the DRD appears to be on the "front" wall near the tube sockets, similar to where I will place mine.
Given the size of my chassis the layout is fairly decent, time will tell.
If it doesn't work out I would have to go to a bigger chassis as opposed to rearranging the 12"x8"x2" chassis I have now.
Paul, thanks for doing this! You mentioned some issues regarding choke and transformer placement. Even the best circuit in the world can be ruined by poor chassis layout. It will be interesting to see what people come up with to achieve lowest hum.
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