New DIYTube SET 6B4G Amplifier !

[djunk]

Hey Tom.
Derek here. What a great looking amp! :o
How does it compare to the Darling? Now that the house project is done I'll have to find time to build my MKIII's and this looks like a good one for the work bench stereo in place of the 1626 I have. Maybe the extra 2.25 wpc would be worth the effort???? Hope all is well.

[Shannon Parks]

How does it compare to the Darling?

Djunk, checkout Tom's thread here:
http://www.diytube.com/phpBB2/viewtopic.php?t=1386

[TomMcNally]

Hi Derek -

Good to hear from you, been awhile. I think the get*set*go is really one of the best sounding amps I've heard in awhile. It has really solid low bass, and enough extra power over the Darling that it doesn't break up when a loud part comes along in a movie. The Darlings tend to run out of power on loud spikes, which of course, doesn't happen often with CDs or MP3s and never on the radio or XM, since they compress the heck outta those. The higher quality transformers make a big difference, as well as a tube actually made for audio.

You'll love the MK-III's ... don't crank them up too loud or you'll crack the walls in your new house ! Yellow_Light_Colorz_PDT_06

... tom

[ashok]

Hi Tom, Shannon:

Great project as usual. I would love to build one, although it is going to take a bit of effort convincing my better half that we need another piece of audio equipment in the house.

I notice that the BOM calls for a 2-prong AC cord. Can you tell me what is the arrangement for the safety earth in the amplifier?

Thanks,

Ashok

[TomMcNally]

Hi Ashok -

If you are concerned with grounding, you could use
a three wire line cord and attach the ground to the
chassis, or use an IEC connector.

I usually use three wire cords on my new construction,
and two wire on Dynaco rebuilds. I haven't encountered
any hum problems with anything so far, except if Cable TV
or computer connection is involved. In fact, recently
when I changed my Adcom preamp to a newer model,
I had to disconnect the serial connection for remote
control of my Polk XM receiver from the PC because
of hum. The novelty wore off anyway.

... tom

[TerrySmith]

Does the power trannie need two seperate 6.3v windings or can one be used?

The only reason I'm asking is because I have a boatload of power trans with one 6.3v winding and one 5v winding kinda like a 270HX.

[EWBrown]

Two separate filament windings are needed, as each of the 6B4Gs needs to have its own filament "floating" about 51VDC above ground, and each filament should be independent of the other. With this 51VDC, the plate current for each 6B4G is approx 56 mA. The 6SL7 takes about 1.2 mA. and the 6AX5 isn't being overstressed.

Theoretically, the two 6B4s could be fed from the a common shared filament supply, with approx 550 ohms, 20 watts resistor for the shared "cathode" biasing, but this would necessitate closely matched tubes, and besides, I've never been a fan of "shared" cathode biasing.
If a power triode has an indirectly heated cathode (6A5G, 6CK4, 6AH4 etc) then the filaments can be fed off a common AC supply, because the cathodes can be individually biased. Can't do this with Directly Heated Triodes (DHTs).

Yeah, Dynaco did it, especially in the ST35 and SCA35, but they used closely matched quads of 6BQ5s to do this, in those "ancient" days, good tubes were relatively inexpensive, and four separate 400 ohm cathode resistors and four separate bypass electrolytics would have been a more costly approach. As they say, follow the money (or perhaps, follow the monkey) Yellow_Light_Colorz_PDT_02

HTH

/ed B in NH

[Dmason]

Shannon, Tom,

this thing rips, I really, reeelly like what it is about. I look forward to rolling on this project. Keith T. will be emailing all on his list when his shipment comes in from Taiwan.

For anyone looking for great hi SPL speaker projects to go with their *SET* build, www.zillaspeak.com/bib.asp these are great. I have built the Fostex 168, and will be moving on the FF165K which reportedly works even better. An honest 94db, 10db line gain, and the BIB's load the vertical mode, propagating bass along the ceiling, unobstructed, for bass indiscernable from dipole. What a bonus.

Here is my current flea amp, www.ramlabs-musicreference.com/6EM7-13EM7.html this tube is a dissimilar triode, ~6SL7 on the driver side, and 2A3 on the big side.

[TomMcNally]

Those BIB speakers are very cool. I wish I had some woodworking ability. I built a 6EM7/13EM7 amp a while ago, I listened to it the last couple of days - it really sounds good.

Keep an eye on the forum - Shannon should have the boards in a few weeks, and there will be information on the top plates and wood bases too.

... tom

[EWBrown]

An interesting circuit "hack" to try (I already bounced this one off Shannon), and I've used it in numerous circuits:

In place of the 3K resistors and bypass caps for the 6SL7 cathodes, use a "cheap red LED", cathode to ground, anode to the tube's cathode.
This offers the same 1.8VDC offest. A good location for the LEDs is the C3 and C4 positions, orient the LEDs so the cathode is to ground, and its anode is to the + side.

Since the current we are dealing with is only 0.6 mA, either a 2mA or 20 mA red LED can be used. My LED of choice is the HLMP1700 or HLMP4700.
HLMP6000 is also a good choice, I've used these as "cathode resistors" in other linestage and phono stage designs, with good results.

LED brightness isn't a factor here, the old 3mm or 5mm dim ones with the diffused "bullet" are good enough. The super-high-brightness retina-zappers aren't needed (or desired) here. Yellow_Light_Colorz_PDT_11

These LEDs can be considered as a "poor man's" constant voltage device. They do not have to be bypassed.

/ed B in NH

[Shannon Parks]

These LEDs can be considered as a "poor man's" constant voltage device. They do not have to be bypassed.

For those wondering why the red LEDs don't need bypassing, it is due to the low impedance of the diode itself (less than 10 ohms I think for the red LED). Lots of good info out there regarding this technique, used by Doc Bottlehead and Morgan Jones.

[EWBrown]

In some of the Bottlehead designs, both the red LED cathode sink and a C4S (Camille's Cascode Constant Current Source) plate load (in place of a simple resistor)are used in line and phono stages and power amp driver (12AT7) stages.

The C4S uses two red LEDs, two transistors, and a couple resistors to set the plate current.


/ed B in NH

[EWBrown]

I'm thinking of building up a third G*S*G using 6CK4s. They're "almost" poor man's 6B4Gs or 6A5Gs. NOS or used only, there are no "aftermarket" equivalents that I know of... Yellow_Light_Colorz_PDT_20

I ran the numbers and this should get pretty close to 3WPC, (is 2.95WPC close enough) with the rest of the circuit except for the PA cathode resistors' values, being left unchanged. The overall current consumption would be around 45 mA each 6CK4, with V P-K of 250VDC, just the cathode to ground voltage would be a lot less, around 27-28VDC, so 600-620 ohm cathode resistors would be about right. OPT primary would be around 4300 ohms. (actually 4250 is the "ideal" value and I have some DECware SET OPTs that match that value almost exactly. The overall B+ current consumption is under 95 mA, so the 6AX4s should have a pretty easy time of it.

(update 18 Mar 2008)

Friend of mine just gave me four closely matched and lightly used GE 6CK4s, and in return I gave him four 6P14P-EVs for a regulator project he's working on Yellow_Light_Colorz_PDT_03

Hmmm, One Electron seems to agree with my results above:

http://www.one-electron.com/Trans/UBT3_10.pdf

Although they have the same operating voltages / currents the OPT is 3K and PO is 3.5W. Comes even closer t a "poor man's 6B4G"...



6AH4s could be used, too, but the output power is lower, around 1.75WPC at best. Reportedly a fine sounding tube, inspite of its relatively whimpy power output level

http://www2u.biglobe.ne.jp/~tossie/6AH4-E.html






These might make good P-P candidates.

Why? Just "becuzzz"... Yellow_Light_Colorz_PDT_03 Yellow_Light_Colorz_PDT_06 Yellow_Light_Colorz_PDT_03

I've got 'bout a dozen of these some UOS (used old stuff), some NOS.

Now for something completely different (and totally whacky) Yellow_Light_Colorz_PDT_09 :o

I've had good results playing around with 6S4As, but their high rp (3700 ohms) can be a real PIA to accomodate. If one or two in parallel is good, perhaps four in parallel would be better. Run them around 325V B+, and 25 mA cathode current (390 ohms, 2W is a good match here). So, bring on the"fourpulator" Yellow_Light_Colorz_PDT_06 :o Yellow_Light_Colorz_PDT_11 Or perhaps "Octopussy" for a stereo unit Yellow_Light_Colorz_PDT_01 Yellow_Light_Colorz_PDT_03 Yellow_Light_Colorz_PDT_04 That'd work out to 325V @ 100 mA per quartet, use individual 390 ohm bypassed cathode resistors, and a 4 or 5K OPT, whichcan handle 100 mA of idle current. So we're talking at least 200 mA of B+ and this brings on one small issue:

Yeah, the existing G*S*G PSU isn't sufficient to feed these lil' piggies, in those numbers, and the 6AX5 would have a serious meltdown... The PA774 struggles under a 150-180 mA DC load, so something a tad heftier would be necessary.

The 6S4As' grid drive requirements under these conditions would be just a shade less that 10V PK-PK, so perhaps a 6SN7 would be a more suitable driver tube, as the 6SL7's gain would be excessive for most purposes.

/ed B in NH

[Yves]

Another 6B4G SE Amp:

http://www.dissident-audio.com/SE_6B4/Page.html

Yves.

[Ty_Bower]

http://mcnally.cc/pics/gsgham.jpg

Why do the rectifier diodes go on top?

Edit... never mind. I see it is a full wave bridge. The other half is on the other side...