new 6V6 amp

[sorenj07]

Well, it looks like some kind of project might be coming closer to fruition. I've got my hands on a pair of loose Z565 transformers as well as a PA774. I'm thinking of ditching the PA774 and getting something that runs a bit less hot, preferably in the 250mA range. A custom job from Edcor might do the trick. Choke-input will use the fabulously oversized yet rock solid Hammond 193M. If the chassis can take the weight, it's worth it. 6V6EH's or JJ 6V6S' in UL will most likely be the output tubes of choice.

I definitely want to run fixed bias. It seems to sidestep the problems associated with cathode resistor and (bypass) capacitor choice. The 6V6's probably need a bit more bias voltage than EL84's so I might want a separate supply, or a dedicated winding if I get a PT custom wound.

The phase inverter is still in the air. I like the idea of a 12AT7 in LTP, fed by a reasonably sized negative voltage and tail resistor. I'm basically ripping off the amp I just built with a couple key exceptions. I definitely want to try implementing some NFB, and I want this thing to look PRISTINE. I knocked together my last amp in a half-budget way, saving money on OPT's and the chassis, but I don't plan on sparing as much expense this time around. A nice solid custom .1" chassis with ventilation holes, possibly with a faceplate from Front Panel Express might be in order. Might get a grill or some sort of cage for the top fabbed up as well.

I've also always really liked motor run capacitors... And I have 6 more of those .22uF 1KV Russian PIO caps to use as couplers. A final bit of overkill would be to use a pair of TV damper tubes for rectification. If I'm designing my own PT, a 6.3V 2.5A winding for two 6**3's would kick butt.

Anyone who's ever seen me gripe about never having spare cash can guess that I need some funding for this to take place. I am the prototypical college student, after all. I've got a couple things I can try dumping before I need to start thinking about tarting up my (clunky Yellow_Light_Colorz_PDT_13 ) 6L6 monoblocks.

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Anyone who's ever seen me gripe about never having spare cash can guess that I need some funding for this to take place. I am the prototypical college student, after all.

Two words for you undergrads: Pell grants. Unless you got fat tax returns on your way. You might also consider looking for professors at electrical engineering dept, who might see fit to spend tax payer's money to fund your project. I heard Bush speak in his address to the congress the other day on increase in funding for basic research in physical sciences.

[EWBrown]

If you have concerns about the strength of your chassis, use a length of angle bracket, I generally use 1/2 or 3/4 inch "angle iron" (which can be aluminum)and align the front edge of the trannies so that the mounting screws are all in a straight line, and then drill the chassis and angle bracket to accomodate them, and even with some of teh rather flimsy aluminum "dumpster chassis" that I use, this makes for a very stable and rigid / solid construction. It's best to run the bracket along the long axis of the chassis.

For the "heftier" power trannie for your 6V6 project, perhaps one of the An-Tek toroids may be the way to go, mount it below the chassis, this eliminates passing the leads through the surface. The angle bracket support may have to be modified to accomodate this, however. Perhaps a length of angle along each side of the toriod.

HTH

/ed B in NH (still in the grip of a Martian Death Cold / Flu bug)

[TerrySmith]

IF your using a choke input, I would SWAG a stock PA-060 power trans might be what you need. There are usually one or two listed on ebay at any given time.

[sorenj07]

Two words for you undergrads: Pell grants.

That would be hilarious, I'll have to ask around..

If you have concerns about the strength of your chassis, use a length of angle bracket

I've been doing this with Hammond's line of aluminum chassis, but I'm tired of being able to scratch up the surface super-easily, and with the overall crappy finish. I'm going to go for a custom fabricated stainless steel chassis. Getting it with pre-cut IEC/tube socket holes would save me lots of time on this tougher material.

perhaps one of the An-Tek toroids may be the way to go

I recognize that they're good transformers, with more than enough current, but I absolutely hate the way they look, especially next to all the other E-I iron. Aesthetics in this case unfortunately steer me towards the less efficient option. But then again, who said tubes were efficient in the first place? Tellow_Light_Colorz_PDT_07

IF your using a choke input, I would SWAG a stock PA-060 power trans might be what you need. There are usually one or two listed on ebay at any given time.

That might do the trick. Does anyone happen to have the specs? I figure it's 720VCT, but what about the current? What kind of filament/bias windings might it have?

[TerrySmith]

The stock power trans would be at least 720vct @ 200ma, 2x 6.3v @ 3.5a, 5v @ 2a. The Triode and Dynakit trans is more robust than the stock if you want more.

[sorenj07]

About the PT: I've probably spent about 4 hours on the '774, making it look nice, so I think I'll just stick with that. I managed to get the bells off with some creative use of a razor and pliers. I shaved and sanded down the lumpy goop on the core to an even level, in anticipation of putting a couple coats of this really good black metal enamel. I used a razor and some chisels, then some fine grit sandpaper and a wire wheel to get the bells down to bare metal, which I will polish and clear-coat. It'll be purty to look at, and a welcome change. I'll probably leave the Z565's alone for mojo factor.

The things that remain for me to get, in other words, are the input choke, tubes, sockets, hardware, and a nice chassis. With the exclusion of the chassis, the rest of these items shouldn't be too bad. The PT, I figure, should work well in choke input. It's 330-0-330 but will probably be a bit higher due to wall voltage being higher, so even at .9*RMS, I can probably count on a steady 300-310V or so.

Digikey stocks a nice small 115+115:12V:12V 10VA toroid that I'll use for a fixed bias source. The nice thing is, I can probably grab one of the primary windings and rectify it to feed a negative rail to an LTP if I want. Since the rectified 24V windings will basically draw zero current to bias, that leaves me with plenty of room for 10mA to feed each LTP at -170V or so. Either that, or I leave the primaries paralleled, and stack the 12V's rectified, using the "bottom" 12V rectified to feed a nice CCS instead. Options, options..

The transformer:

http://rocky.digikey.com/scripts/Produc ... 96&M=62042

This could be very fun.

Note: a final bit of good news is that I finally rigged up some nice dummy loads onto some repurposed heatsinks, so I'll be able to try and fine-tune some NFB into my amps, and maybe post some square waves. Waiting on some replacement lytics for a certain Eico 379 signal generator that someone gave me a while ago... Yellow_Light_Colorz_PDT_11

[sorenj07]

Grabbed a quad of Russian 6F6S tubes to use with the '774 and Z565's. I like the ST look, they're affordable, and I can always sub in 6V6's if things don't work out.

[sorenj07]

tentative schematic, just came up with it. how does it look? if the 6F6G's are close enough to 6V6's, around what values do you think would work for the pot? I could probably use a smaller bias transformer since i'm basically not loading it at all, but that's a minor detail.




EDIT: Sorry, the bottom plate resistor on the 6SL7 is meant to be 280K...

[EWBrown]

There is a 30VA 12+ 12 toroid at Parts-Express for a similar price ($22) which is pretty small, around 2.6 inch diameter and 1.1 thickness. A little extra VA capacirty, maybe the VA / PI filaments could also be fed with DC for even lower hum / noise.

http://www.partsexpress.com/pe/showdetl ... er=122-600


I've had pretty gooresults with "prettying up" Z565s, both old and new, but the PA774s seem to always have extra junk / varnish and other nasty stuff to contend with. Even worse are the old early vintage tar-dipped trannies, they are a mess to start with and even worse to try to clean...

Just recall the old Uncle Remus "tar baby" stories, these olden trannies will leave one all covered in goop and in a somewhat less than happy frame of mind Yellow_Light_Colorz_PDT_21 Yellow_Light_Colorz_PDT_09 Yellow_Light_Colorz_PDT_02 Yellow_Light_Colorz_PDT_03

/ed B in NH