My low watts projects

[Ty_Bower]

The latest completed project is a small 6BM8...

OK... I've got just about all the chassis work on mine done. All I have to do is figure out how to finish the wiring underneath. Any hints from the expert point-to-point builders?

[EWBrown]

Put a 7-place terminal strip at each screw on each 6BM8 socket, this will give you 12 tie points (the grounded centers aren't used) to support all teh required tube support components. A few more terminal strips for the power supply components, between 4 to 6 of the five-plate terminal strips would be good. Just do a "dry run" layout with all the components, then once you have the positioning figured out, then drill and mount as needed. Best to do a few "paper trial runs" before committing it to metal.

You're right, those Ten-Pounder $20 Schumacher power trannie are huge monsters Those trannies could easily power 8 6BM8s and maybe even 12 Your 6BM8 amp is the "Goliath" when compared to my tiny "David" version, which I can easily hold in one hand. (see it further down the page, along with a compact 12B4A "Darlikg Killer"

I really like your nice wood bases and the nicely finished chassis plates .

[Ty_Bower]

I really like your nice wood bases and the nicely finished chassis plates .

Thanks. I thought I'd try something different this time, rather than my usual framing lumber boxes. The first draft was a bit rougher...

[EWBrown]

After about 20 hour's operation, the PSE-UL 6F3P/6Ф3П amp has started sounding a LOT better - except for the right channel, which sounded a bit weaker and somewhat dull and flat. It also took a LOT longer to warm up, and that should have been a big clue

I made some more careful AC voltage measurements, and found that what I believed to be two 6.3VAC filament windings, was actually one 6.3VAC @ 3A and one 5VAC @ 2A winding (the MPS PT275 and PT285 DO have two 6.3VAC windings), so one pair of tubes was getting only 80% of the proper filament voltage. Yeah, I know, I should have caught this in the initial voltage testing, but it was late a t night and I wasn't "all there" .

I just ASSumed that both windings were 6.3VAC ; FWIW, in the newer version of the PT190-2 power trannies, MPS made the second filament winding with both 5VAC and 6.3VAC taps, so it has dual function.

I disconnected the 5VAC winding, from the R chan filaments, and then connected then to the 6.3VAC windings and all was sounding MUCH better The 6F3P / 6BM8s consume 780 mA, so I am exceeding teh current rating by 120 mA or 4%.. Big honkin' deal...

It was still running a bit too "hot" for the cathode current (about 40 mA per tube, with 210V P-K, which exceeds the 7 W PD rating by 20%. So I added a 50 ohm, 10W (actually two 100 ohm, 5W WW) resistor between the cathodes and the first 180 uF cap. This dropped the B+ by approximately 8VDC, and brought each pentode's cathode current down to 36 mA, and 195V P-K which is just about spot on at 7Watts PD.

This also noticeably improved the sound quality, which I wasn't really expecting...

Now, what to do with that "unemployed" 5VAC winding...

Well, one could connect it in series with the 6.3VAC winding, and then feed the filaments with 11VAC, and use 11BM8s. It would be relatively simple to wire in a DPDT switch under the chassis in order to make it "convertible" for either tube type. 11BM8s consume 450 mA filament current, so the 5V, 2A winding will have more than sufficient current capability. Since the filaments have a "virtual center tap", with two 150 ohm resistors at one of the tube sockets' pins 4 & 5, this will still be good for either filament voltage with no re-wiring necessary.

/ed B

[MashBill]

The latest completed project is a small 6BM8...

OK... I've got just about all the chassis work on mine done. All I have to do is figure out how to finish the wiring underneath. Any hints from the expert point-to-point builders?

Looking good!

I often look at the bottom side of the Marantz 8 as inspiration for wire routing and layout. They are a work of art.

[EWBrown]

I finally got around to starting the "DK101"" 12B4A SE amp, and decided to see if I could cram it all into and onto a surplus 7.5 X 4.5 X 3 inch cast aluminum Bud box. With a just bit of "creative license", I have made this become a reality

Why have I called it "DK101"

That stands for "Darling Killer" 101. It should have about twice the output power, and approx half of the Darling's 2HD figures.

It uses nearly the same circuitry as the "Darling" 1626 based amp, but with 12B4As instead of 1626s , and a slightly different driver, using a 12AT7WA in place of 12SL7 (or the original 8532s). I had to "shrink" things down in order to get it all to fit in its rather limited space. FWIW, 12DT8 will also work in place of the 12AT7, as it is essentially the same tube, just without the center tapped filament. It is drop-in compatible with this design with 12V filament power.


I had to change from Plan A, using the Hammond 369 EX power trannie, to Plan B, using one of the AnTek 05T200 toriods, in order to get it all to fit neatly.
.
The toriod, THE 22699 CHOKE, and the rest of the PSU components are all mounted in the bottom, and on the rear of the "tub" part of the box, and the two TF103-48 OPTss and the three tubes (2 X 12B4A, 12AT7WA) on the top cover, as well as the speaker binding posts, input RCAs and volume control. A short six-wire "internal umbilical" cable will joint the two sections. I plan to make the final assembly to "hinge" open from the rear, for easy internal access for tests and measurements, and future mods and repairs.

I have already checked out the PSU and, under resistive test load, it delivers the correct B+, and I connected the two 6.3VAC filament windings in series for 12.6VAC CT, as all the tubes have 12.6V filaments, this makes it just a tad simpler, with one less connection to each tube socket I'll keep it simple, with AC filament voltage, as I've had no problems with making this operation totally hum-free.

I have all the top cover'smajor parts, and tube sockets, and terminal strips mounted (all this done on the first day's work). Next step is to gather up all the small components (Rs and Cs) and wire it all up, point to point style.

I'm expecting about 1.65WPC, and max 2HD around 6% at full power output.

Photos of this (and my two recent 6BM8 / 6F3P builds) coming soon...

/ed B

[Ty_Bower]

...this schematic.

I've got a big sack full of Radio Shack 1/2 watt CF resistors I'd like to try to use up. Is there any reason I couldn't use them for most of this circuit? The cathode resistor R8 needs to be at least 2 or 3 watt rated, and in the power supply R12 (if used) should probably be 2 watts as well. Ed said he used a 1 watt part for R13, but I can't see how that is really necessary. There should be less than 2 mA going through there, which is only 0.08 watt.

Photos of this (and my two recent 6BM8 / 6F3P builds) coming soon...

I shiver with antici...pation.

[EWBrown]

Nothing wrong with using paralleled or series'ed resistors where needed, I've done it more than once. I just use what I have in the junque boxes

/ed B

[azazello]

My DIY in Chicago SE 6V6GT Silvania $ 12AT7 Silvania & OTs Hammond 125CSE & PT Hammond 369AX & Chocke Hammond 154M & chasse Hammond 1444
I built for my son:
http://picasaweb.google.com/azazello52/ ... 2692374338
http://picasaweb.google.com/azazello52/ ... 4232646610
Tested with test CD.... We heard 40-18000 khz, sorry we have not some special test stuff. OT tranies are very, veri good....

[EWBrown]

Two more small amps, that I completed last month.





On the left is a 12B4A / 12DT8 SET, the "Darling Killer" built on and into a 4.5X7X3 inch cast aluminum Bud Box. The power supply (using an Antek 05T200) is built into the "tub", along with the electrolytic caps, choke and power switch and fuse, and the little green LED on the front. The amplifier circuit is all on the lid, and is connected to the PSU with a six wire "umbilical" This was designed and built so that I could try other amplifier designs on the existing PSU, with a minimum of wasted efforts. OPTs are the Triode TF103-48s, 5K to 8 ohms. Good for about 1.6WPC and sounds quite nice.
The circuit design is essentially identical to the "Darling" except for the tubes used.

To the right, is my compact 6Ф3П / ECL82 (Russian 6BM8) SEP/UL. It measures about 6.5 X 5.5 inches and the wooden frame is 1.5 inches high. This also uses the Triode TF103-48-UL OPTs, The circuit is that which I described in earlier postings, with two NFB paths, the usual "global" NFB from the OPT secondary to the VA cathode, and an additional plate feedback path by means of a 1 Megohm resistor connected betwen the triode and pentode plates. This addition made a dramatic difference in the sound quality, from very good to "unbelieveably excellent" for such a simple design. It is good for about 2WPC, and I did not have any room inside fir a volume control. Compare the size of the tube box and CD to the two amps. The circuit schematic for the 6F3P amp is about 2 or 3 postings above this one.



This "thumb" doesn't link, just use the one 3 postings higher up the page.

Both amps' PSUs have the Fender 22699 choke (4H, 90 mA, 105 ohms DCR) and the 6F3P Power trannie is the diminutive 269EX (190-0-190 VAC @ 65 mA and 6.3VAC @ 2.5A) much smaller than those "ten pounder" 7019s from E-bay

Both amps have absolutely no hum or background noise, "is this thing really turned on" quiet


I'll add some "guts shots" later on., as well as some of the larger, more powerful (5WPC) 6Ф3П / ECL82SE-UL design.

/ed B in NC

[soundbrigade]

Nothing wrong with using paralleled or series'ed resistors where needed, I've done it more than once. I just use what I have in the junque boxes

/ed B

Agrre to that! Just keep an eye on voltages and wattages. Even IF a resistor can withstand the power, the voltage may be too hard for the resistors. A bunch of 1/4" resistors may cope with he heat/power but they are so small that the electric field generated across the resistor successively wears it down.

[20to20]

Two more small amps, that I completed last month.

To the right, is my compact 6F3P (Russian 6BM8) SEP/UL. It measures about 6.5 X 5.5 inches and the wooden frame is 1.5 inches high. THis also uses the Triode TF103-48-UL OPTs, The circuit is that which I described in earlier postings, with two NFB paths, the usual "global" NFB from the OPT secondary to the VA cathode, and an additional plate feedback path by means of a 1 Megohm resistor connected betwen the triode and pentode plates. This addition made a dramatic difference in the sound quality, from very good to "unbelieveably excellent" for such a simple design. It is good for about 2WPC, and I did not have any room inside fir a volume control. Compare the size of the tube box and CD to the two amps. The circuit schematic for the 6F3P amp is about 2 or 3 postings above this one.


/ed B in NC

Have you compared it with and w/o the UL connection? Wondering if small pentodes used SE benefit from UL. I read an article about a design using EL84 and the author/tester felt that particular tube didn't seen to show any improvement. Don't know if EL84 is unique in that regard or what reason it may be that way.

I finally found my choice of tools for hole punching. Here is THE BADDEST bit on the planet.

http://www.idealindustries.ca/products/tools_totes/saws_drill_bits/tko.php

I used a 1-1/8" on some 16ga steel and it cut a burrless hole in about 10 seconds. It kicks the slug out automatically with a spring.

[EWBrown]

I have found from previous experiences that 6BM8s, 11BM8s, 16A8s and 6F3P / 6Ф3П clearly sound better with 40-43% UL. (OK, it's a matter of opinion - MY opinion Both in PP and SEP.

The minor downside of UL operation is slightly lower output power, but the better sound quality more than makes up for this. It would be a relatively simple matter to add an SPDT or SP3T mode (Pentode, UL and Triode) selector switch for each channel, just don't flip it around when the unit is powered up. OK, I've done it, but it can and will adversely affect the tube's operating lifetime, plus it makes some nasty sounding POPs when you do it

Ultralinear operation is not a benefit for 11MS8s and 6GV8s, UL operation sounds dull and lifeless , but the original pentode or triode modes sound good with these tubes . Been there, done that...

/ed B

[katabatic]

Please, check this schematic Ed. I think this is what you wrote up.



I still want to see photos of your wiring.

Ty/Ed-

Excuse my well-established ignorance, but what is the component labeled I1, a bit above the power transformer in the schematic? The circle with the kind-of-down-arrow in it - I've looked through all the schematic symbols I can find and can't find a match. (Also, what does it do?) Thanks.

George

[Ty_Bower]

what is the component labeled I1, a bit above the power transformer in the schematic? The circle with the kind-of-down-arrow in it

Well, that drawing is really more of a computer model of the amp, rather than a schematic. The symbol you describe is a current sink. I put that in the model to approximate the current draw of the other channel, since I only show one channel in the model. It helps me figure out the power supply voltages. If you are planning to build this amp and use that drawing as your schematic, just interpret that symbol as the other channel.