My low watts projects

[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.

[katabatic]

Thanks. As I've said before, the more I learn, the less I realize I know....

George

[katabatic]

OK, a couple more questions, as I try to work through and understand this amp. I assume that the upper grid shown in the V2 image of the tube is pin 7, so that pin goes to R9 and on to the screen grid tap on the OPT. Right?
Second question is whether other OPT's, such as the Edcor GXSE10-8-5k would work (I used them in building a Darling, but there the SG tap isn't used). Is there something special about the Triode TF 103-48 that makes is a better choice here? Thanks for the help.

George

[EWBrown]

The latest "low watts" project, all of the gory details are posted in the get*set*go topic. Basically I took an early GSG project from 2008, which had been partially stripped to donate organs to a build on a 12X10X2 chassis and MQ OPTs, so I had a plate with iron and the power and I/O conenctors, and octal sockets remaining.

I put in a new board, and rewired the sockets for 6AV5GAs, and gave it a test ride. Safe to say, it sounds just as good as 6B4Gs, with triode-strapped GE 6AV5GAs. REctifier is Sylvania 6AX5GT, with the two separate sections (like a 5Y3GT) and the drive is a vintage Tung-Sol 6SL7.

I also cobbled together a crude wood base to set the chassis plate in - the original config just had four three-inch long metal legs in the corners.



What better way to spend a "snowed-in"day than to bring another amp to life

/ed B

[TomMcNally]

"crude wood base" ! That is fine craftsmanship. I wish I had that
talent with wood.

What is a 6AV5GA ? TV tube ?

[EWBrown]

What is a 6AV5GA ? TV tube ?

I figure this is a much higher purpose than just serving as a horizontal TV sweeper

Just think of them as two-buck 6B4G substitutes.

BTW, the published 11 W PD rating is very conservative, the "Tube Lab Tube Torture Tests" prove this out http://www.tubelab.com/6AV5.htm he took some of them up to nearly 40W PD and 25Watts, they were still playing well with no red plates or glowing grids. 40W got some red plate action , but it was evenly distributed and the G2s were not glowing.

Note that not all 6AV5GAs were created equal, some have huge slots betwen the plate structure and the heat-dissipating fins. From my own experience, the Sylvania and DuMont brands seem to handle high power operation a lot better, and some of the RCAs were in the "wimpy" class.

/ed B

[TomMcNally]

The eBay sellers must have read your post, since many
of the 6AV5GA's are $ 49.99 a pair. There are still a
few cheap ones, but some of these guys think they are
selling gold. Jeez

[EWBrown]

Strange, I got most of mine for a couple bucks, the most was $4 each and the cheapest, for free.

Friend of mine down in central FLA just came across a large "stash" of NOS tubes, asked if they were worth anything. I told him to send me a listing, to see if they are all plinkers or if there is any "gold" in there.

The use of 6AV5GAs is no secret, they are all over the other audio foums, and some vintage Bogen amps use these in PP operation, that may be their "desirability".

12AV5GAs are even cheaper, and just as good. Same for 25AV5GAs, etc.

6FW5s are a heftier version with a 18W PD rating. The compactron 6GE5 and 6GT5 are also near equivalents to 6FW5.

I suspect that the 6GE5s, 6GT5s, 6GV5s, etc, are also very good in triode mode, just have to try them out sometime, in triode strapped mode. These all have 12V and 17V filament versions.

I started the "next generation" 6AV5GA amp today, chassis drilling and punching is complete, and mounted all the iron, sockets, etc, just have to wire it up under the chassis. It all goes onto and into a Hammond 12X10X2 black powder-coated steel chassis. Transcendar3K, 10W OPTs, Hammond horizontal mount PT and a hefty PSU choke. I got a good hand & wrist workout with the nibbling tool

I bought myself a cheapo drill press at "Tractor Supply" a couple weeks ago, and it definitely makes the metal mangling a LOT easier and quicker.

/ed B

[EWBrown]

I wired up the power suply and have it working. PT is a Hammond 290CX, 325-50-0-325 VAC @ 180 mA, 5V @ 3A and 6.3VCT @ 4A.
This was obtained through an earlier "pre-bay" transaction

With UF4007 rectifiers, and with the 5V winding "bucking" the primary winding, I still get 400VDC after the choke, with a 100 mA (4K, 50W) DC load. And I don't really want to add an extra R and C in the PSU chain. I also used a CL-90 Inrush Current Limiter in teh primary side, which does give a gentle start-up.

400VDC under load seems a bit high for even "extreme" 6AV5GA usage, so I may go ahead with EL34s, KT88s or 6550s instead, as the voltage would be a better match for these tubes.

/ed B

[EWBrown]

The eBay sellers must have read your post, since many
of the 6AV5GA's are $ 49.99 a pair. There are still a
few cheap ones, but some of these guys think they are
selling gold. Jeez

They have $5 6AV5GAs here:

http://www.esrcvacuumtubes.com/vacuumtu ... -6CZ5.html

Plus about 3,499,999 other tubes

/ed B

[Geek]

They have $5 6AV5GAs here:

http://www.esrcvacuumtubes.com/vacuumtu ... -6CZ5.html

Plus about 3,499,999 other tubes

Stan's a good guy. I've bought from him off fleabay

Cheers!