NFB necessary in SE amp?

[John]

Hey fellas, I'm drawing up plans for a very small SE conversion from an old relic. Schematic is attached for your comments and critique. My question is the topic. If I don't implement it, is it much more likely to hum? Or is the feedback used more for tonal properties? My vast experience  does not include even playing a SE amp, ever. Thanks in advance!

[stingray_65]

Tonal properties.

Basically the NFB is giving more clean headroom.

It is a popular mod to add a switch to defeat the NFB.

http://www.el34world.com/Forum/index.php?topic=5621.0

Ray

[stingray_65]

I just looked over your schematic,

That looks like a very gritty amp! (good IMHO)

I'm not all that familiar with the 6SL7 tube, but I am curious about your choice for Rk.

Where did you come up with 1.5K?

Shouldn't it be around 470R if your sharing the cathodes and using a 20uf Ck?

I'm really not sure about that, maybe someone with better technical knowledge can chime in.

http://www.tubezone.net/pdf/6sl7.pdf    pg.4

Ray

[DummyLoad]

you need a cap from 6sl7 plates to top of top of volume control. using the wiper for a grid leak is not wise, it works, but not a good idea. if the pot fails wiper open (as they usually do), the 6V6 will be quite unhappy. you'd get more gain cascading the 6SL7 triodes, but that may not be what you're looking for. attached a .sch with the cap added.

respectfully,

--DL


 

[John]

Ray, thanks for the replies. I just pulled 1.5k outta my nether regions. I'll change that value on the schem. This is a 2-be-built-sometime thing- working on a slightly larger SE right now (slowly) using tubes I'm more familiar with. I still have to test the transformers that came with the derelict to make sure they're good, but drawing is free. Thanks again!!

[John]

DL, oops! Thanks, posted while you were,  I guess. I always forget that danged cap, and Sluckey has pointed that out to me in another schem I drew. And yes, I forgot the grid leak resistor as well. I think using something like a 1M to ground is good? Thanks again!

*edit* attached updated schem

[Willabe]

It also dampens the speaker. Tightens up the bass so it's not so loose or floppy sounding.


                   Brad        

[stingray_65]

Hey John,

Verify that 470R with someone who can state it with confidence.

before your build you know.

Ray

[echuta13]

I generally prefer the sound of SE amps without NFB.  They have a bit more midrange emphasis, and distortion onset is more gradual (provided you drop the gain of the preamp to compensate).

[PRR]

One stage 6SL7 (even paralleled) won't bring guitar up to 6V6 level.

[John]

One stage 6SL7 (even paralleled) won't bring guitar up to 6V6 level.

Good to know. Since the PT and OT both are quite tiny, I'll research some of those smaller power tubes. I was *ass*uming I could make a sortakinda Valve Jr with it something. Thanks for taking a gander!

[stingray_65]

One stage 6SL7 (even paralleled) won't bring guitar up to 6V6 level.

Good to know. Since the PT and OT both are quite tiny, I'll research some of those smaller power tubes. I was *ass*uming I could make a sortakinda Valve Jr with it something. Thanks for taking a gander!

OK, so where did you get these trannies and what do you know of them?

Working the way you are, those transformers will determine most of your circuit.

Once you determine their ratings, the rest will be pretty straight forward from there.

Ray

[John]

Ray, it is a CSC amp I got off eBay a while back, bought it for the chassis really (cheap). There's no markings or anything like that, I can't find a schem anywhere. I think it was a phono amp or something like that. Tubes were what came with it - 6sl7, 6v6, 6x5 rect. Original trannies too.  Like I said, I haven't even tested them yet to see if they're good or not. I won't be putting any real money into this project, fer sher! If nothing's good, I paid 20 bucks for a little chassis. Thanks!

[Colas LeGrippa]

I'd like to open ''.sch '' files but my computer doesn't recognize them. It's a drawing schem. program I guess that I 'd like to get.

[Willabe]

Here, you need to get a good grip to open those files Colas.

http://www.el34world.com/Forum/index.php?topic=11457.0


          Brad        

[TIMBO]

Hi John, I bought a CHEEPIE awhile back (valve Jr.) and was not sure what sound i could get out of it but the money i couldn't buy the parts for the same money. The sound stock was not great so i invested a few bucks and ended up with a neat little amp  The reverb could be deleted to save some money but sounds great and the feedback on a switch really adds a great effect.I think that a 6V6 for the power tube would make sound even better as the EL84 can sound a little brittle but works well in this little amp.I have not used the 6si7 tube so i can't comment on its sound.Thanks

[stingray_65]

John,
lets look at the 5C1 champ for inspiration.

it uses a 6SJ7, an octal pentode for v1 instead of a 6SL7 dual triode.

http://tdsl.duncanamps.com/show.php?des=6SJ7
http://tdsl.duncanamps.com/show.php?des=6SL7

both have the same draw on the heaters, no issues there.

If I'm reading the chart right, both draw similar current from the power supply.

[John]

I'm probably misunderstanding what I'm reading, but it looks as though the 6sl7 has a mu of 70, while the 6sJ7 has a mu of 19 (?). Transconductance is similar, p-diss for SL is 1 watt max, for SJ is 2.5 watts max.

I never should have started this thread.... now instead of finishing my other project first I'm gonna get sidetracked by my curiosity! 

[Willabe]

One stage 6SL7 (even paralleled) won't bring guitar up to 6V6 level.

I'm probably misunderstanding what I'm reading, but it looks as though the 6sl7 has a mu of 70, while the 6sJ7 has a mu of 19 (?). Transconductance is similar, p-diss for SL is 1 watt max, for SJ is 2.5 watts max.

6SL7/12AT7 = 70, 6SJ7/12AU7 = 19.


               Brad     

[John]

One stage 6SL7 (even paralleled) won't bring guitar up to 6V6 level.

I'm probably misunderstanding what I'm reading, but it looks as though the 6sl7 has a mu of 70, while the 6sJ7 has a mu of 19 (?). Transconductance is similar, p-diss for SL is 1 watt max, for SJ is 2.5 watts max.

6SL7/12AT7 = 70, 6SJ7/12AU7 = 19.


               Brad     

And there's the source of my confusion. If PRR says the 6SL7 won't drive the 6v6, I believe him! But then how does the 6SJ7 get it done?

[stingray_65]

One stage 6SL7 (even paralleled) won't bring guitar up to 6V6 level.

I'm probably misunderstanding what I'm reading, but it looks as though the 6sl7 has a mu of 70, while the 6sJ7 has a mu of 19 (?). Transconductance is similar, p-diss for SL is 1 watt max, for SJ is 2.5 watts max.

6SL7/12AT7 = 70, 6SJ7/12AU7 = 19.


               Brad     

And there's the source of my confusion. If PRR says the 6SL7 won't drive the 6v6, I believe him! But then how does the 6SJ7 get it done?

The SJ7 is a pentode. The mu of 19 if the pentode is wired as a triode

[John]

Ohhhhh. *slaps hand to forehead*

[Willabe]

One stage 6SL7 (even paralleled) won't bring guitar up to 6V6 level.

1 stage, not 1 + 1, cascaded. I think thats what he ment.


            Brad     

[Jennings]

Your little amp reminds me of one I had until recently...same valve line up of 6sl7, 6v6, 6x5 rect...Made a nice little gritty sounding garage style amp with an 8" 50s/60s Celestion alnico speaker.  Now, if only I knew where I put the circuit diagram for that to compare with yours...?!  I'll see if I can find it.

[John]

One stage 6SL7 (even paralleled) won't bring guitar up to 6V6 level.

1 stage, not 1 + 1, cascaded. I think thats what he ment.


            Brad     

Yes, I was thinking about that. Again showing my lack of knowledge, I thought that a paralleled dual triode gave more mu, but not necessarily more "gain" or distortion than a cascaded stage. I read so much stuff I can't keep everything straight, and then start these topics without checking myself. 

Jennings, if you find that diagram that would be great! Meantime, I'll re-draw mine using a cascaded 6sl7. I need the practice.

[Willabe]

I read so much stuff I can't keep everything straight, and then start these topics without checking myself. 

Me too John, me too.       


            Brad     

[PRR]

> I think it was a phono amp

Did you rip it up?

An older phono-amp, stock, might be a fine mellow practice amp.

I'm sure you'll find the 6SL7 sections are wired as separate stages, cascaded. Very likely there's knobs between to diddle the lows and highs. And of course the volume control.

[John]

It is the same as that one, except in much worse shape.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=200609455198&category=119019&_trksid=p5197.c0.m619#ht_5235wt_905

I did already take out most of the stuff last summer figuring on all new caps/Rs and stuff, but also found a bare wire coming out of the PT along with cracked insulation on most of the other wires.

I did get a variable AC supply, so when I dive into the amp first I'll check the PT to make sure it's not cooked. The bare wire I *think* is because the insulation just gave way- nothing smelled bad IIRC.

I will for sure cascade the 6sl7.

I drew up a diagram by pencil as I was taking the stuff out, and for fun I'll make a .sch of it. I didn't understand some of the stuff at the time, but looking at it now I think the filament wires were grounded on one side? I'll do that schem and then let you tell me where all I've got it wrong!

[stingray_65]

I just looked at that e-bay amp.

Where did they hide the OT? under the chassis?

Probe the PT with an ohm meter before you put power to it.

Verify that crumbling  insulation hasn't allowed a short to the transformer itself.

Some shrink tube will shore up the wires, don't be too afraid to remove the end bells off the tranny if you think you have to. Just remember those old  trannys often used aluminum screws to hold them together and they are easy to wallow out the slot. Use a wrench to loosen the nut first without putting a screwdriver in the slots.

I've reconstructed a lot of trannys  that are worth while that I've harvested from vintage equipment. Just this last couple weeks I've rebuilt 3 PT's and 2 OT's and have went as far as converting 4 of the 5 from laydowns to uprights. ( a big collection of end bells from junk trannys come in handy)

OH!, while the end bells are off, cheap satin black spray paint makes them look real nice. For the laminations, don't scrape them or try to clean them up in any way, a cheap disposable sponge brush and a LIGHT coat of regular varnish brings them back.

[stingray_65]

OH!

From the size of that PT, you can push a couple of 12AX7's no worry instead of a 6SL7.

If you skip the 5U4 and go SS you can up the B+ rail a few volts into guitar amp range. Most audio stuff like that amp were not running things as hot as you see in good sounding guitar amps, so skipping the 5U4 will avoid it's 50V loss,

That PT is likely over rated a bit for that circuit, back in those days no electronics were inexpensive, even the lesser quality items were designed with longevity in mind

The small OT you speak of may not like being pushed as hard as we might want to, But it's small size is more typical of a guitar OT than a HiFi OT and might sound pretty darn good. Most Fender OT's (IMHO) are small and under rated, especially compared to a good HiFi amp of the same rating, but I also feel that is part of the Fender sound.

The other question, not that I saw a pic of the amp, did you get 3 cool vintage chicken head knobs too?

[PRR]

> same as that one

That's a PA amplifier.

The "Microphone" input "is" a guitar input, change the connector.

The "Phono" input needs two resistors to be your iPod input.

[John]

Ray, the OT is indeed inside the chassis. Good advice about the PT, thanks! And a SS rectifier would be easier than the tube anyway. I'll have to look in the box, but I think I did get a couple knobs, not all 3.

PRR, thanks. I was working on the schem last night a little bit. Lots of stuff doesn't make sense to me with the way it was wired. I remember at the time thinking a lot of the solder joints did not look original. Could be why the seller said "needs work, plugged it in and got the crap knocked out of me".

[stingray_65]

One other thought about SE amps in general.

Tube rectos serve a purpose on P-P amps ran in class AB. When there is a surge in the signal, the power tubes put a large current surge to the B+ rail and it "sags" and adds compressive qualities to the tone.

SE amps on the other hand are ran in class A (generally). They draw near max current at all times and don't experience this sag. So (IMHO) the only advantage of running a tube recto is to add a big expensive glowing tube.

[John]

Okay, attached is the schematic, taken from my hand drawn tracings when disassembling. I believe the PT is still in there with the heater wiring intact, so I will double check that part of it, but I remember being very puzzled about how the heaters were wired. In no way will I guarantee the accuracy of the schematic! I wasn't sure how to draw the on/off/bass/treble dual pot.

Like I said, I thought quite a few of the joints were not original, I'm guessing someone "modded" this before I got it. When I had looked inside the first time, there was no way I was even plugging it in, not even in a "hey y'all, watch this!" fashion. 

It'll be a while before I dig into this, but figured I'd give you something to laugh at meantime.

[PRR]

Makes sense. RE-draw:

[John]

Ah, things become clearer. Thank you!

[PRR]

> 5B6 PT was likely a 300-0-300.  My guess is that a higher output would fry the 6SL7s and the 6SC7s.

No. The ~~360V was for the power tubes. Driver and preamp tubes are always fed through a dropping resistor. This can drop any amount needed. Indeed you often "need" 10%-30% for ripple filtering.

Also the actual tube voltage is less than 70% of the local supply voltage.

So if the tubes did die at 300.0V  (they don't), you could have 428V at local B+, 30% for dropping/filtering, is 557V at the raw B+.

Or ~~360V raw, drop the 30% for filtering leaves 280V, and Vp at 70% of that gives just 200V across the tube.

For other reasons (electrolytics) we like to stay under 450V no-load so maybe 400V operating B+. When you find "much" higher B+ on the output bottles (Ampeg VT-4 595V!) you may find a power divider going to the small-stuff so the cold-start voltage isn't much over 450V settling toward 410V in use.

BTW, 12AT7 is rated 300/330V yet many/most Fender/copy reverb drivers put 380V-445V on it.