HiFish II PSE 6V6

[shooter]

Just one more

Parts are coming, the paperwork's done.  I'll call it my take on a guitar karaoke amp.  Or just a basic mono block for music, or practice amp for the bedroom.  The owner of my last version is hunting down a small - 8" cab, then wants me to tune it.   I started by trying to get a 25W edcor to fit on a stout chassis, it did, but there wasn't room for tubes!  So I hunted up K's SE Ot wiring diagrams and we'll see how a couple 22905's cross-wired will sound.  Shooting for 300 plate, 12 - 13 bias and around 30mA on the 6V6s.

One *thing*, on the layout, I have the grid leak and grid stop R's for the 6V6s on the preamp ground, should I by-pass that turret and just run it to the PA ground section?

The TS will be built off the pots, probably with a terminal strip.

[PRR]

Why so many gain stages?

A Champ plays with two gain stages.

As drawn, input sensitivity is near 0.1mV. Classic Fender is 20mV. 0.1mV is extreme thrash turf, and probably can't be made stable.

[Fresh_Start]

Yes, grid leak (grid return) resistors should be grounded where the tube they precede is grounded. But grid stoppers shouldn't be grounded.

I can't do the math but it sure looks like you need several voltage dropping dividers between preamp stages.

Cheers,
Chip

[kagliostro]

I can't do the math but it sure looks like you need several voltage dropping dividers between preamp stages

I do not get along with math, but those preamp tubes are 12AU7, not the usual 12AX7, didn't this changes things ?

Franco

[DummyLoad]

Why so many gain stages?

A Champ plays with two gain stages.

As drawn, input sensitivity is near 0.1mV. Classic Fender is 20mV. 0.1mV is extreme thrash turf, and probably can't be made stable.

yes 12AU7. but still going to need to pull a bypass or two in that string... if he wants somewhat clean, even with 2nd stage as input. 


--pete

[shooter]

input sensitivity is near 0.1mV

That's one of the things still to learn, what to look at to guestimate sensitivity

The original pre, I stole from RCA, see attached.   A few ideas and re-works with wise inputs from here and me became what's drawn.  I fully expect to be modifying stage gains, might even  use V2b as a CF.

[Willabe]

Yes, grid leak (grid return) resistors should be grounded where the tube they precede is grounded.

They should be grounded with the same tubes cathode R.

From Merlin below;

[kagliostro]

The big difference I see in your schematic, respect the "source" schematic is that in the source V1a and V2b are is CF whose gain is max 1

in your version V1a and V2b are Anode Followers and there the gain is much more





BTW I've never seen a CF at the input of an amp, that is curious, may be that solution is due to an attention to have an impedance adaptation

What about to go V1a input > V1b CF > James Tone Control > V2a gain stage > V2b switchable OD ?

Franco

[shooter]

Thanks K, the original *design* was to drive a KT88, here's the link and *progression* to this *design*
http://el34world.com/Forum/index.php?topic=19097.0

I didn't bother re-calc'in gains from 88 to pse 6v6, figured I could scrap one stage, or convert to CF if needed.  If it works fine but I can only turn up the volume to 5, that's not a big deal to me.  Being able to have a clean reasonable range is.  It's main *job* will be playing digital music, but, when I hear *that* song, I know I'm gonna want to plug in the 6 strings, loop the song, and see just how bad I can butcher it.

[Paul1453]

OK, so now I get your concept.
The Karaoke term had me thinking you would Mic the AUX jack.

So you want a Hi-Fi tube amp that can mix in your guitar when you hear "that" song. 

Cool!  That seems a better solution then plugging in another amp to play along.

[PRR]

> I stole from RCA

I like plagiarism.

Some of RCA's sketches were throw-aways. Or worse.

And here you've totally repurposed half the stages.

> those preamp tubes are 12AU7

So I am blind.

Say gain of 15.

15 * 0.5 * 15 * 0.1 * 15 * 15 = 2,500

Say 25V needed at 6V6. 25/2500 is 0.010V, 10mV, hot but not excessive.

Three stages of Gv=36 would be the same gain, less work.

> I've never seen a CF at the input

RCA sold tubes. Plans with extra tubes may have seemed like good business.

[kagliostro]

Plans with extra tubes may have seemed like good business.

   

---

Some time ago I've read something that sounded like more tube more harmonics

There is some truth on that ?

Franco

[shooter]

K, you are reading my mind!

more tube more harmonics

I've been wondering IF tubes add harmonics, or simply amplifies them

Say 25V needed at 6V6. 25/2500 is 0.010V, 10mV, hot but not excessive

So this is input sensitivity for the amp, 10mV?
I understand sensitivity as smallest input signal  that can be, measured, amplified, cleanly,
like if I get much below 10mV on my scope I'm viewing more grass than signal.

[PRR]

> I understand sensitivity as smallest input signal  that can be, measured, amplified, cleanly,

No, this is smallest signal to get FULL OUTPUT. 10mV in will give 25V at the power tube grid, which is +/- what most power tubes need to reach Full Power Out. (EL84 typically less, hi-Volt 6L6 a bit more, but not a big difference.)

The "smallest signal which can be recovered cleanly" is the noise level. Tubes have about 2 micro-Volts self-hiss, and 1uV-20uV of hum induced from heaters or poor B+ filtering.

Your 'scope is wide-band. A tube will be about 2uV hiss in a 20KHz band, but about 20uV hiss in a 2MHz band. Still you should be able to resolve <1mV at the 'scope terminals. (10X probe pads or test stages in front will degrade this.)

[shooter]

Thank you PRR
got parts today, just as an artic blast shows up!, drillin and cutting in the house!

[shooter]

Got the board put together and as I was ringing out everything I noticed a + sign on the .022uf cap.
Is this something to indicate foil side, or is this a .022uf E-cap, stray + on the silkscreen

[sluckey]

Definitely not a polarity indicator.

[shooter]

figured it wasn't, that's why I left it *backward*

[HotBluePlates]

... I noticed a + sign on the .022uf cap. Is this something to indicate foil side, or is this a .022uf E-cap, stray + on the silkscreen

The Dijon is a film & foil cap, so possibly a mark for the outer foil. A 0.022µF electrolytic would be exceedingly tiny.

[shooter]

A 0.022µF electrolytic would be exceedingly tiny.

so it'll just make a tiny puff since it's backward
I was guessing Ecap  a stretch, stray silkscreen was my 1st *guess* since it's the only Dijon I have
with a +

[shooter]

got some in-door work done while we freeze
formed up the copper overlay, but need some 70f before I do any glue-sniffing.
need to hunt-down some graphic wiz to help-print my logo after the glue dries!

[Willabe]

That looks real nice. 

Just 1 possible suggestion, using different colored fly lead wires would make it easier to wire up.

I go with a separate color for the preamp tubes plate/grid/cathode/B+/ground/etc.   
 

[shooter]

different colored fly lead wires

Those, like the copper *overlay* is sorta "my signature" design.  This is my 3rd coppertop, red-lead build.
And, ya, you DO have to pay attention, and Ya, I do feel sorta bad for the poor tech in the future

[shooter]

I gave up on waiting for the weather so I spent the evening sniffing glue, solder, and resin.
Still gotta clean the initial solder and re-flow.  Time for a green beer to clear my head

[shooter]

More paint sniffing

[Paul1453]

Your copper coating is interesting.   

I'm wondering if I could just screw down a thin sheet of copper to cover the holes on my swiss cheese chassis'? 

[shooter]

You probably could, sorta part of why I do it.  the copper sheet is about 4X stiffer than a sheet of paper with a thick paper backing.  glue it though!   I paint instead of play, so my songs or Riffs are done with oils, acrylics, rattle-cans instead of F bodies or acoustics, otherwise the *process* is the same.

[shooter]

some progress, I now have a somewhat better view of mechanical engineers.  I set the board back from the tubes so there would be less PA tube *interaction*, thinking the OT's were not an issue, till I got the wires basically laid out.  So i'll be re-drilling board holes and moving it about 1/2 - 5/8 inch fwd.  Should be testing power by the weekend.

[shooter]

PS up and running, still need to scope ripple, but meter is showing <20mV on the grid-preamp tap.
2  errors, the 1st was easy, just a miss-wire, the 2nd, just dumb!, I used a 5k R as load to verify PS, good.  Put in the 6V6's, bad, plate current 85mA, B+ like 220!, then I realized G1 was just floating

put in a temp 820k, now * normal* is 268plate, 15.5 bias, Ip 33ish mA. using node B.  moved to node A, 324plate, 19 bias, Ip 40mA.   Probably stay with node B, but I was shooting on paper for 315 plate, 13 bias, 35mA Ip

hopefully have the TS built later tonight then solder in the board and see.

[shooter]

 

She's up and playing, dead quite!, The TS needs a *range* tweak, still gotta do the paperwork.
Thanks to PRR for the 1st *tweak*, the 2 inputs work well, the *guitar* side sounds 5F1-ish, the *aux* side plays albums as well as you can play albums.

Need to scope it, the output sounds thin, but overall it's close enough to *tweak* to taste.

[shooter]

Need some wiser than me help;
ran initial voltage checks, all the DC appears happy, The AC, mostly;
outta the pre I have clean 55Vac-rms, way more than I need. (used 50mV AC-rms in)

going into the PA grids, I have to *dial* the MV to 13Vac-rms in order to *keep it clean*.  Is that an effect of tube loading, impedance mismatch?

Out of the 6V6's I get 142Vac-rms, but by the time it gets to the speaker it's 6.7Vac-rms into a 10ohm DL, which yields only ~ 4.6W.  Is the *un-common* configuration of the OT to blame? (was expecting 7ish W)
Is there a *normal* OT coupling value?  Like with 100vac in you *should* get 10 out?

(each tube is running about 7.8W plate dissipation)

[kagliostro]

I've just discovered this schem ... an HiFish (Hifi & Guitar Amp)

may be your next effort ? 

http://www.r-type.org/exhib/aaa0711.htm

The VA-12 was designed for both HI-FI and Guitar use.


Ciao

Franco