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24.22.235.48
In Reply to: RE: BB Proof 2A3 DC posted by Garg0yle on December 05, 2014 at 20:22:08
Notice I have already slightly altered the circuit; I'll post a revision after all the words are up and we see some feedback.
Power Supply
Direct coupled amplifiers need very high voltages because there is no coupling capacitor between stages. The highest voltage rating for widely available electrolytic capacitors is 450v; components that can produce and tolerate higher voltages are harder to find, so that is the voltage chosen. The fairly typical Hammond 273X power transformer is rated 350-0-350 volts at 110mADC, 5v at 2A, and 6.3v at 4A. It is also available with universal primary and 50Hz rating as the 373X. With a 5V4 rectifier, about 440 volts should be available. To keep the voltage loss low, a CLC filter is used; Hammond 156L is 5H at 135 ohms. Ripple should be small enough to produce less than 1mVrms hum, giving capacitors of about 50uF.
The first capacitor seems large, but at 60mA total current, the peak rectifier plate current is well within the maximum rating. A 47uF 450v rating would work in place of the two 100uF in series, but I've shown the more robust method. A film or motor run at 600v would be nice. The second capacitor is split, as a cathode bypass plus ultrapath capacitor; this is to keep the signal current loop short. The ultrapath capacitor is the first candidate for upgrading. A 5AR4 can substitute for the 5V4 easily; other rectifiers may need different filament current and supply voltages to get the right output voltage.
A separate 2.5v transformer such as the Hammond 166M2 is used for the 2A3 filament; Hammond does not offer a 2.5v transformer with 50Hz rating. There are a wide variety of other ways to power a 2A3 filament, however!
Follow Ups:
Is the indirectly heated rectifier really needed for the BB-proof aspect (slow start to B+) or would a directly heated rectifier be acceptable, assuming a higher transformer voltage to compensate?
you need to find a way to make sure the HV ramp up very slow and not overshoot in first couple seconds.
I use 5r4 but with a delay timer on HV, separate filament transformers for driver and output tubes plus tube voltage regulation like those vintage Tektronix power supply.
The "monkey" configuration protects the 2A3 from the main violence of a rapid start. There is still some current drawn as the filament heats, of course.
I found an ASC 50uf 515vdc PIO cap and Angela instruments.
Is 50uf close enough for C1?
Does ASC mean it's a Start cap?
I also found a Sangamo PIO cap that is rated 47uf and 600v, but it is listed as a start cap.
Any advice?
Jamie
Big speakers and little amps blew my mind!
ASC is the manufacturer and they do appear to be motor "run" caps at Angelas, at least if they are the same as these: http://www.ascapacitor.com/traditional_film.php50uF should be fine for C1.
I would pass on the Sangamo unless you can be certain you have a model that is suitable.
Edits: 12/07/14
Sangamo 772501
50uf, 720vac
Big speakers and little amps blew my mind!
I should clarify by being suitable, it is known as a "motor run" capacitor as opposed to a "start" Capacitor.
I have a bunch of 45 / 7 400 vac round motor run caps from this store. My understanding is that the DC rating is at least 1.4 times the 400 VAC rating. The store doesn't have them now but there are other caps with similar ratings.
I have had great success using Goop to attach these to steel / aluminium or polycarbonate. The bond is strong enough to hold them but not quite so strong that you can't remove them.
Maybe I will build design this since I have never done a 2A3 amp before.
ray
Hey Ray,
I knew there was a conversion. I learned a lot of this years ago, and then backed off DIY.
Thanks for the link.
Here is a film in oil cap, Obbligato, from DIY hi fi supply. 47uf and 630v. That may give me wiggle room for a 5u4g?
Jamie
Big speakers and little amps blew my mind!
The more the merrier.
Do you think those are fairly recent stock?
I will bookmark this site, although I didn't notice any run caps with a high enough voltage rating.
The link to a PDF on the Digikey site states:
RELIABILITY AND LIFETIME
AC capacitors are rated for a full service life of 60,000 h with an
estimated 94% survival rate when operated at full rated voltage,
60 Hz and rated ambient temperature.
SHELF LIFE
AC capacitors are expected to perform for their full service life
objective after being exposed to a maximum shelf life of 10+
years when stored in a controlled environment.
ray
Thanks Garg0yle,
I had just googled ASC. I'll do better next time.
Do you think 515v would allow going up to a 5U4G?
Another poster mentioned C2 as important. Is that the 100uf 250v cap?
Do you think the 100uf 350v Ultrapath cap is worth a nice cap?
I like the layout of Paul's schematic. It's the first one I ever seen done like that.
Jamie
Big speakers and little amps blew my mind!
In the revised circuit (I'll post it after a few more comments, with a new version number) I've suggested using four 100uF/350v caps, with C1 being two of them in series. Electrolytics really are happier with no more than 80% of their rated voltage on them.
Something similar is the case for film caps. I recommend digging up the manufacturer's literature and reading it; CDE is a really good starting point.
Thanks Paul.
Big speakers and little amps blew my mind!
I think, someone correct me on this if I am mistaken, that those two 100uF capacitors are actually seen as one 50uF capacitor with double the voltage rating of the lowest rated capacitor, in this case the lower rated cap is 250v, effectively making it a 500V rated cap to be a little higher then the 450V B+
Someone else will have to comment on whether these should be oil capacitors or not.
I myself am running out of room!
GargOyle,
No need for correction
http://farside.ph.utexas.edu/teaching/302l/lectures/node46.html
As I've mentioned before, the big oil caps are very good for the money. If prototyping, use em...unless you just don't have the space..
I personally have learned to totally dislike the sound of electrolytics in the PS (like a wet blanket draped over the speakers) ...with that said, much of this opinion has resulted from dealing with old commercial stuff that used cheap caps to start. I'm sure that there are some nice sounding electrolytics out there for the C1 position but they are almost certainly pricey compared to the big oil caps...
Stuben
OK Stuben, that sounds like a reasonable enough experience for me to maybe pursue the film cap route.
(Just to clarify, those little chassis mount Mr. Steady linked earlier were Electrolytic.)
My chassis is 12"x9"x2" so it will be a little tight.
Cheers.
"I myself am running out of room!"
Don't laugh, and I'll probably get flamed.
I've been collecting cigar boxes for a while as possible enclosures. I think I will do two for each monoblock. A power supply box, and the main box, connected by an umbilical cord. I think I will hard wire the umbilical.
Just playing,
Jamie
Big speakers and little amps blew my mind!
Parts Connection does have a nice selection.
I think cigar boxes would look very nice, although I'm not crazy about the all wooden box thing, it is something I have contemplated many times.
Ideally they would have a metal lining with the wood box around it, which is a little tougher in your case as you already have the finished boxes.
It's doable, just more a little more work, but hey that's part of the fun.
You might be able to get some of those Hammond chassis to fit from Parts Connection, as long as it works for the layout, IMO it wouldn't matter if the wooden box was a little bigger.
Yes, I thought about lining the boxes. I think diyhfsply has copper shielding. BH doesn't line their wood boxes, but I think they shield the signal wire instead.
After reading Paul's explanations, I decided to build it stock with the 5v4s, no modifications.
I think the Hammond OPTs might work well for me, because I would only use this amp in a very limited bandwidth, like 300hz to 1,800hz.
I want to give this a crack, but I'll go slow.
Jamie
Big speakers and little amps blew my mind!
"BH doesn't line their wood boxes, but I think they shield the signal wire instead."
Those amps do have a metal top plate to hold the parts together, maybe you could fit something like that to the underside of your cigar boxes, then use the shielding tape if necessary.
That way you won't have a problem if the cigar box splits it's lid etc.
Anyhow I'm just making suggestions, I'm sure you know what you are doing.
"Anyhow I'm just making suggestions, I'm sure you know what you are doing."
Not so much.
One step up from a bread board is what I'm looking for. I figure using two boxes (PS-main amp) will split the weight load. Maybe even mount the OPT inside the box.
Big speakers and little amps blew my mind!
47uf 600v Sonicaps were at parts connexion for $15.
Big speakers and little amps blew my mind!
How about Hammond 266M2? It's 50/60 Hz.
Any of the 025 models from MagnaQuest? The DS-025 is the basic model.
Observe, before you think. Think before you open your yap. Act on the basis of experience.
Mike is a friend, and also has had a long relationship with Bottlehead, so I can't comment beyond saying I like them.
Thanks - I missed it because it's not in my old paper catalog. Thanks!
Awesome, I am going to play around with PSUD and see what I can do with my iron.
I also should pick out a spot for the plate choke(s) to go underneath my chassis.
The dimensions of the Hammond 156C appear to be "1.5 in W x 2.81 in L x 1.69 in H" which should fit OK.
Cheers.
OK so with a little tinkering using my 370V taps, a 5AR4 and a 7.5H 220Ohm choke I can get a solid 449.7 volts.
Ripple is 29.5 mV.
It's got a little jiggle while ramping up, I've never had that before in PSUD.
I am going to post a screen shot here, see if I am missing anything.
Thanks.
I noticed last night that I forgot to parallel my chokes in PSUD, my choke is 110 ohms, not 220 ohms as shown above.
This raised my voltage around 5V, ripple also went up a couple mV.
Hey Paul is it possible for me to replace both C1 AND C2 series capacitors with 50uF 800V WIMA's?
Also the "5th" 100uF capacitor added to the choke of the 6J5, should this stay around 100uF or would 50uF work here. I know you specified that to keep them all the same, I was just wondering.
If this is possible I could order:
6 50uF 800V WIMAs
OR
4 50uF 800V and 2 100uF 800V
Either way it would be nice if I could get away with 6 caps for the two amps instead of 10.
I could fit 3 of these square WIMAs much easier then 5 round film caps.
PS I am learning lots!
Cheers.
The second power supply capacitor is split for a reason, as explained in the text.
You are always free to make a different design, of course. :^)
I will go through it again, I thought it was split for voltage rating.
No need to take it the wrong way.
I didn't take it the wrong way, but I did post in a hurry - sorry it didn't come across as I wished!
The second PSU cap is split into a cathode-coupling capacitor (high voltage to 2A3 cathode) and a cathode bypass capacitor (2A3 cathode to ground). This is done to cause the output signal current loop to include only the cathode coupling capacitor, and exclude the cathode bypass capacitor.
Gotcha, no worries.
I made some Styrofoam stunt doubles of the Panasonic DC Link caps today.
Tentative locations have been determined, they will fit within the chassis, just need to make up some sort of bracket to support them.
A bracket of sorts will also support the potentiometers under the chassis as well.
Cheers
I though maybe I would toss up a few pictures.
The 2A3 is there, but the other tubes are stand-ins.I appreciate your patience.
I have tentative locations for the DC Link caps as per the blue foam replicas. I have some aluminum stock that I will run across the tops of those capacitors to secure them.
The choke can go in that general area as shown (156), I will see if I can find a null before I attach them.
The two pots will be located at the front near their respective sockets. These will also be secured underneath with some aluminum angle stock.
The grounds and 25W resistor will go in the center area.
So the layout is more or less finished, I can now run my B+, although some joints may remained un-soldered until I get the film caps etc.
I didn't wire my AC as I think I will need most of my black solid core for ground returns. I have some flex wire for this I could use if needed, I could use clear, as to distinguish the live AC from the grounds.
It doesn't really matter but I thought it would be nice to have the wires coded.
Here is the 8 colour code I came up with, omitting the 120VAC to PT.
Red - B+
Orange - HV secondaries
Brown - 5v heater
Green - 6v heater, speaker +
Blue - Plates
Black - Grounds, grounded elements, returns
Yellow - Cathode
White - 2.5v heaters
Edits: 12/20/14
nt
A really clever hardware hack with chassis construction !
Thanks!
The rest of the tubes ordered last night. I decided against used ST glass 6J5s and ordered a pair of military spec Sylvania's.
The price of NOS 5AR4's are a hundred dollars and up so got some new production Sovtek instead.
It's apparent that I won't be putting the panel meters on these, just a little to tight for space, one model from Partsconnection may fit, but resolution wasn't there, I went ahead with the assembly without putting a big hole in for it.
I might yet build a larger cage around my chassis and mount a suitable panel meter there.
I guess I just leave the 267K and 178K resistors in place, but it is unclear if I need to put a jumper resistor where the panel meter was or not.
Hey Garg0yle,
Nice pics. It's looking good.
It seems that good analog panel meters aren't cheap. I wonder if putting in test jacks wouldn't work better. Just plug in your DMM, and adjust the bias from the deck.
I'm interested to hear how the adjustible cathode resistor works out for you, and if you use it to tweak the voltage.
Jamie
Big speakers and little amps blew my mind!
Thanks you.
"It seems that good analog panel meters aren't cheap. I wonder if putting in test jacks wouldn't work better. Just plug in your DMM, and adjust the bias from the deck."
It's looking that way, other then I will probably keep everything below deck.
Have a good new year.
OK so some parts have started to roll in.Tubes, electrolytics, the 390Ohm Rs and the bleeder resistors that have the ratio to read the 180V point for the meter/DVM terminal.
I got 270K and 180K instead of 267K/178K, hopefully that gets me close enough, maybe I will have to read 179V instead of 180VThe 6J5s look nice.
The 5AR4's are a little chincy, when I hold them to my ear and tap, they make a clanging sound similar to bumping the reverb tank on a guitar amp.
Drlomu and the Tube Wrangler would not like these! :P lol.They sound like this @ 0:20
https://www.youtube.com/watch?v=N5maWd6gm2E
Edits: 01/03/15
Thought I would post a bit of an update.
I'm just waiting on the chokes and terminal posts to arrive.
I have had a bit of design paralysis with regards to locating particular parts like the pots and large resistors. It feels like a chess match at times without having all of the parts in hand.
My overkill hum-pot is a little large, the Bournes pot for the 6J5 is shockingly smaller then I expected. To somewhat complicate matters, I want to have them underneath the chassis. so I need to accommodate room for a bit of L stock.
I more or less have my grounding scheme planed out. I debated having the chassis shield connected only at the input, as I had never tried that.
Ultimately I decided that I will connect it at the power supply caps.
The electrostatic shields are connected to the mains Earth, I wasn't sure if it mattered, just thought it might be a little quieter ground is all.
Overall I am reasonably happy with what is installed. I might move the B+ wire a little farther away from the PT though.
The last of the parts have arrived.
My power transformer does not have a center tap on the heater windings. (it has 0-5V-6.3V)
So I was wondering if I should make an artificial center-tap with some 100 ohm resistors that grounds at the 6J5 cathode pin? or on my ground buss close to the HT center-tap?
OK I guess I am going to elevate the 6J5 heaters at the cathode pin.
I was doing some grounds last night and wanted to accommodate a heater return in a certain order.
Thanks anyways.
OK chokes turret posts and RCA jacks are ordered.
I wussed out and I am going to use Electrolytic caps.
As it is I dropped almost $500 on odds and ends, also I found it next to impossible to buy film caps with paypal that would fit.
So now I just hurry up and wait.
Everything except for one package is coming from Canada, so everything should arrive in a reasonable amount of time without customs roulette.
Happy new year.
Happy Holidays.
I finally got a few parts ordered.
-390 ohm 10 watt wire wound ceramic type. (Only need 5 watt, but it was close etc)
-3K ohm 25 watt resistors. I got the adjustable kind and will just leave the wiper off. Fairly compact.
-25 ohm 25 watt ceramic potentiometers for the hum pots. Probably overkill, but it looks sturdy enough.
-1K ohm 2 watt Cermet potentiometers. Now these I was trying to get in a little higher wattage, but they were huge.
Hopefully the 2 watt pots would be OK on the 6J5 cathodes?
Going back to eBay now to find some tubes.
I have a couple 6J5 book marked.
It's a little unfortunate I need the 5AR4 in my specific application, as some of the other rectifiers are a little cheaper and available NOS in a ST jug.
The Classic Hammond 200 Series power transformers now are made with 115 VAC AND 125 VAC taps on the primary. If your home's line voltage is a typical 122 VAC, and you use the 115 VAC primary winding, you get a 1.0608 step up ratio. This makes the 350-0-360 VAC winding a 373.1 VAC winding, nominal.Also, you used 165 Ohms as the PSUD PT DCR. That, without looking, is likely the end to end ( 350 - 350 VAC ) total winding. On such a center tapped transformer, you only use EITHER end to center tap as the DCR.
But wait, it gets better, you can be more precise, and we should be. You can measure the DCR of the primary, and the step up ratio of the primary winding, to the High Voltage secondary winding, to determine the "best " DCR numbers to input into PSUD 2.
I will look for that Formula and post it herein.
----------------------------------
Hi Jeff!
When you work with Duncan amps PSU Designer II, please use the formula for Rtransformer:
Rtr. = Rsec. + (n x n x Rprim.)
Rsec. = DC Resistance secondary [Ohm], measured between (0V - high volt) of one 'leg'.
n = step-up ratio (= voltage secundary / voltage primary)
Rprim. = DC Resistance primary [Ohm]
This is the transformer resistance as it's 'seen' by the rectifier
tube.
Jeff Medwin
Edits: 12/07/14 12/07/14 12/07/14
If you download the specs for the one particular transformer, you get the open circuit voltage and the winding resistances, as well as showing the two primary taps. 273X is 381v, 85 ohms; 125v primary is 2.889 ohms. I'm seeing 461v with 48mV pk-pk ripple on PSUD-II, assuming 125v in on the 125v winding.The current specs are now in rms current, so you can use PSUD and see the increased current available with a choke-input filter compared to cap-in. Very nice!
Hammond is really being responsive to DIY needs. My power line is 123.7vAC right now (10PM local time).
Edits: 12/07/14
"The Classic Hammond 200 Series power transformers now are made with 115 VAC AND 125 VAC taps on the primary"
I didn't know that. As with the filament transformer, I am out of date with my Hammond catalog - this is really good to know. Should solve a lot of "my transformer buzzes" posts!
Hiah Paul,Have I inputted this PSUD2 simulation correctly? If so, will the Driver stage be happy with 66.84 mVAC of B+ ripple? Am I looking at it wrong, simulating it wrong, or what... both ?? Oh yes, the driver stage B+ ripple will perhaps be divided in mVAC by those two high value dividing resistors, which I was unable to display in PSUD.
I usually try for 2 mVAC of ripple, or less, to the driver. Anything under 850 mVAC is fine to the non-fussy 2A3 Finals.
I keep looking at this, and decided to ask for your help.
Can you, or anyone simulate what the ripple will be at "idle" at the top of the choke load, the B+ that feeds the input / driver stage please??
Is higher driver stage ripple a "topology trade-off" on any "Monkey On A Stick" design, and / or the 1947 Jack Robin-Chester Lipman L-W amp, where the B+ to the 6SF5 is also derived from the Final's tube cathode, with minimal power supply decoupling ??
Jeff Medwin
Edits: 12/07/14 12/07/14
In my quick try I used a higher winding resistance and got more like 50mV peak to peak; otherwise my simulation was pretty much the same.
The driver is fed from its plate choke, whose impedance at 120Hz is 113K ohms - that gives a lot of reduction. The ripple at the choke takeoff is half that of the full PSU, from capacitive splitting - more help. Not having built this, or simulated it, I can't be sure. But another cap at the high voltage end of the plate choke will kill the driver hum, if it's a problem.
I didn't show the calculation, but here it is: 1mVrms at the speaker is 2.8mV peak to peak, which at the primary is 50mV peak to peak. The filament will make about 2mVRMS at the 8-ohm tap, so this seems like a reasonable spec. Got to leave some room for the reality check once it's running! :^)
"Also, you used 165 Ohms as the PSUD PT DCR. That, without looking, is likely the end to end ( 350 - 350 VAC ) total winding. On such a center tapped transformer, you only use EITHER end to center tap as the DCR."
I thought you would be shocked by that.
They are in fact 330Ohms from end to end.
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.
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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