Cheapfield project

[whitelabrat]

I've been working, very slowly on a project to build a DIY version of the Mr. Millet's Wheatfield HA-2. I'm trying to be cheap as far as the chassis and power transformers are concerned. Cross fingers.

So far I've been cleaning out an old Bogen PA chassis that I'd like to use for this. It's already got octal sockets and a choke so that's less drilling. I've also got a Triad C-14X (6H 120ohm 200ma) choke to use as an alternate, but the ancient choke on the chassis looks pretty beefy and reports as about 80ohms instead, so I'll give it a whirl.

I'm concerned that my power supply may be too small. It's an Allied Electronics 6K56VG (540vct, 120ma) whereas I just discovered to day that the Wheatfield can use a Hammond 270HX which is rated for 200ma. I'm curious how much the 6SN7/6AS7 setup can draw. I figure as long as the current rating is OK I can use a 5AR4 instead of the 5U4GB to get the voltage up, or worst case I can use hexafreds to minimize the voltage drop.

Anyway, HERE a thread discussing the same topic. I just didn't want to dig up an old thread:

Another thing that has caught me off guard are the Solen 100uF 400v caps I bought for this. They're enormous and won't fit in the chassis, So I figure I'll have to zip-tie the big boys to the top of the chassis and of course insulate the leads appropriately.

I've salvaged some nice terminal strips from a couple of Tektronics oscilloscope modules that will be handy if they fit in the chassis.

So now I just need to figure out a good layout and then do a bit of drilling.

Stay tuned! Pics to follow.

[whitelabrat]

Some candy for everyone. Get a load of those Solens! Turns out they'll fit under the chassis after all. Excellent.

[Eli Duttman]

I'm concerned that my power supply may be too small. It's an Allied Electronics 6K56VG (540vct, 120ma) whereas I just discovered to day that the Wheatfield can use a Hammond 270HX which is rated for 200ma. I'm curious how much the 6SN7/6AS7 setup can draw. I figure as long as the current rating is OK I can use a 5AR4 instead of the 5U4GB to get the voltage up, or worst case I can use hexafreds to minimize the voltage drop.

Use SS rectification and a CLC filter. Use as small a cap. as you can in the 1st position, consistent with an appropriate B+ rail voltage. More current can be drawn from the power trafo, when the rectifier conduction angle is large. The smaller the 1st cap., the larger the conduction angle.

I suggest you use a "cockeyed bridge" for rectification. It should cost less than FREDs, while blocking SS diode switching noise from entering the PSU filter. Assemble 2 sets of series connected UF4007 pairs. Snub each diode pair with a HIGH WVDC 10 nF. capacitor. Connect the cathode end of the assemblies to the rectifier winding's ends. Ground the anode end of the assemblies. Connect the anode of a 600 PIV Schottky diode to the rectifier winding's CT. Take the "raw" B+ from the Schottky's cathode. By itself, the Schottky diode's speed keeps PN junction diode switching noise out of the PSU filter. The UF4007 assemblies are snubbed to keep switching noise out of the power trafo, where it could get into the tubed circuitry via the heater supply.

[whitelabrat]

Excellent advice! A light bulb went off in my head along with your suggestion, where I've got another amp that makes some switching noise from it's pair of UF4007's.

How much variation can I use with the 10nF snubber? I've got a 47nF 2000vdc handy, and a couple 47nF 600vdc orange drops available.

I've already got a spare pair of IXYS DSEI12-12A FRED's. Should they be quiet enough? I was planning to use the center tap of the power transformer along with the FREDs. Can that be snubberized? I do have a mess of UF4007's too so the bridge is no problem.

[n_maher]

I have a serious soft spot for Mr. Millett's work. In fact, I'm such a fan that Pete and I cooked up a design called "The Menace" which is essentially the opposite of what you're working on. It's a modified/redesigned HA-2 with the bulk of the mods comprising a completely new power supply design and adding a second, parallel output tube.

Here's some eye candy to keep you inspired on your current build.

Power Supply


Amp Section


Amp Upskirt


And if you're still looking for a power transformer I have a 270HX that I pulled off of my old HA-2 (I modified it to use a 370HX before selling it) that I could pass along very cheaply. As far as just how much current this amp draws it's almost entirely dominated about the 6AS7 output tube and controlled by the cathode resistor (R15 on the original schematic). Make sure you have that resistor adequately sized and ventilated. A cool running amp this is not.

[edit]I just caught the part in your original post about your power trafo being rated for 120mA, that's nowhere near enough if you build the amp according to the original schematic. Current draw for the original amp is up around 180mA and if you adjust the cathode resistor to lower the current on the output tube to make your power supply work you'll affect the performance of the amp significantly.

[Eli Duttman]

I've already got a spare pair of IXYS DSEI12-12A FRED's. Should they be quiet enough?

Use the FREDs (unsnubbed) as the connections to ground of a "cockeyed" bridge. The Schottky on the CT keeps crud out of the B+ rail. By themselves, FREDs are pretty quiet (better than unsnubbed UFnnnn) and I don't know what (if anything) works to snub them. Take no prisoners and put ferrite beads on the heater wiring as close to the power trafo as can be.

[whitelabrat]

I'm glad I piped up. 180ma wow.

n_maher That's one heck of a power supply! Thats a lot of transformers! And thanks for the generous offer! I've read that paralleling the 6as7's allows helps with low impedance phones.

Eli thanks for the tips!

It appears the power transformer I've got won't cut it. I'll run some calculations to determine if a 270HX and the Triad choke will work out with solid state rectification. I've got a ton of 5U4's but they're hit-or-miss so I may go with Eli's suggestions there.

For the ps, I was going to follow the schematic fairly closely:
47uF --> 6H 120ohm --> 47k ohm bleeder to ground --> 3 x 220uF

For the cathode resistor I've got the heavy duty aluminum resistors that n_maher appears to be using too. I've salvaged a couple heat sinks with nice copper heat spreaders on them . I've also got a huge aluminum heat sink that I'm going to pull from a toasted PA amp that I could attach the resistors to.

180ma wow.

[Eli Duttman]

WLR,

At 2.5 A./6AS7 heater, your going to need a hefty filament trafo, especially if you use 2X PSE sets for the "finals".

There is another way to get the B+ current you need, in an economical manner. Follow in the footsteps of "El Cheapo". Buy a Triad N-77U and a Triad VPL16-1600. Phase the 3 secondaries up and wire them in series. Use the composite to energize a Greinacher ("full wave") voltage doubler consisting of your FREDs and 2X 250 WVDC/820 μF. 'lytics. Of course, inrush current limiting via NTC thermistor(s) is a must.

BTW, if you don't already have a suitable 'SN7, consider the Loctal 'F7.

[n_maher]

Originally posted by whitelabrat
[B]I'm glad I piped up. 180ma wow.

Try coping with the 360mA+ that the Menace draws.

n_maher That's one heck of a power supply! Thats a lot of transformers! And thanks for the generous offer! I've read that paralleling the 6as7's allows helps with low impedance phones.

It's not exactly portable, that's for sure. And actually, what you're seeing is a single transformer and 3 chokes with caps scattered about for good measure. I think it weighs about 40lbs or so.

And no worries if you don't end up wanting the 270HX, I've been mulling over building another HA-2. I have almost enough spare parts to do it and tubes coming out of my ears for this amp. It's a sickness...

And Eli has hit the mother load with his loctal advice, the 7N7's are cheap and better then an awful lot of the 6SN7s you'll find. I built an adapter for mine so that I can run some of the boutique 6NS7 variants that I have but also have the option to run 7N7s as well.

[whitelabrat]

I do have two Oneac isolation transformers, one is rated for 1A and another for 2A. A voltage doubler should give me just about 300vdc using the 1A transformer. It shouldn't buzz like the Hammond either.

I may be able to use the 6K56VG to make an EL Cheapo variant for the pile of 6aq5's so not all is lost. Radshac sells a 3A 12.6vct transformer real cheap too.

I've got five or six 6SN7's so I'm good to go there.