Little amp

[davidwpack]

Just took a quick look...pin 5 isn't connected to ground. I did miss a resistor on the 50L6 socket that connects pin 1 to pin 5. So the only 2 things connected to pin 5 are the .02? cap and the 470k resistor from pin1 to pin 5.
Edit: pin 8 of the 12SL7 is directly connected to pin 1 of the 50L6 via a black wire.

[davidwpack]

Added some humps and added the resistor to the socket.

[sluckey]

This is what I see...

[Tony Bones]

I think Sluckey's right. I was trying to figure out to express it when he posted a great diagram.

Note that pin 1 on the 50L6 is not connected to anything in the tube, but it is used as a tie point for the signal common a.k.a. floating ground. Pin 8 on the 12SL7 is the last heater pin in the chain and is connected to the floating ground.

I can see in the first photo you posted that there is a 4.7Meg grid leak resistor on the input stage. Is there really a 4.7K cathode resistor too? Seems like both would provide too much bias...

[davidwpack]

Yeah. Those numbers are right. Looks like yellow, violet, red unless the colors have faded. Sluckey, the pin numbers need rearranged on the 12sl7 on yours. Also there's a cap to ground on the power switch. I found a mistake in mine. I put a jump where there should have been a connection but I think it still works if I fix that. Or am I way off? This amp/tubes are weird.

[davidwpack]

Like this. I know it's a weird approach but is it remotely right or just a mess?  Just trying to figure out if I grasped anything with this amp.

[PRR]

You have two B+ and two "grounds", mixed-up. (I know how it is.)

[sluckey]

Yeah. Those numbers are right. Looks like yellow, violet, red unless the colors have faded. Sluckey, the pin numbers need rearranged on the 12sl7 on yours. Also there's a cap to ground on the power switch.

Ha. I thought I fixed those pin numbers before doing anything. They're fixed now. The cap has always been there.

[davidwpack]

Thanks guys!!! I guess drawing up a schematic for tubes that I'm not familiar with must require drawing a schematic I'm not familiar with. Shoulda went with what I already knew. Makes much more sense now. Thank all you guys for your help!!!

[DummyLoad]

i see same as slucky except i see 2x 47K grid stoppers for V1a.

see attached.



--pete

[kagliostro]

Ciao Pete

I see someting strange

R3 - R11 connection 
or other way, R3 is also connected to ground 



Franco

[DummyLoad]

Ciao Pete

I see someting strange

R3 - R11 connection 
or other way, R3 is also connected to ground 

Franco

Franco, i believe that it is drawn correctly. 


--pete

[sluckey]

I like your drawing better Pete.

[DummyLoad]

I like your drawing better Pete.

thank you. 


--pete

[davidwpack]

I got 47k too. Looks like yellow violet orange.
Edit...Sluckey you have 50sl7 for the power tube.

[davidwpack]

If the current draw is .15A per tube (=.5A), I should add probly a 1A fuse...and a 110VA  or better isolation transformer? (I'm going to build a bucking transformer). Is that about right? Thanks!

[sluckey]

If the current draw is .15A per tube (=.5A), I should add probly a 1A fuse...and a 110VA  or better isolation transformer? (I'm going to build a bucking transformer). Is that about right? Thanks!

The filament current is .15A for each tube, but since the tubes are wired in series the total filament is still only .15A. Isolation transformer is a good idea. If you want a fuse put it on the primary of the Iso tranny. You don't need a bucking transformer. If you are concerned about the very slight increase in filament current due to todays higher wall voltage, just increase the size of that big series resistor. If that resistor is really 150Ω it's already the correct size for 120VAC. I wouldn't bother with it.

[davidwpack]

Oh ok. So the 35VA transformer from tubes and more should be fine then it looks like. Thanks again.

[kagliostro]

... I belive that It Is drawn correctly

Oh, I too, that kind of PS Is really confusing

Franco

[PRR]

> someting strange ... R3 - R11 connection  ... that kind of PS Is really confusing

Draw it out "conventional". DC power flows top down.

As an amplifier: instead of ~~110V of DC supply, this stage feels more like 60V total supply. But it is the 1st stage, signal level is low, this is OK (for a *cheap* amp). The 0.1uFd cap keeps the V1A cathode "grounded" for audio frequencies. The stage will work as an audio amplifier.

Why would we do such a terrible thing to an audio amplifier stage?? Well, this transformerless power supply can be plugged-in either way. If the 120V wall outlet is 120VAC and "zero", with pre-1960s plugs you had a 50:50 chance that B- would be 120VAC. This might hum, and you might flip the plug for less hum. However the "Neutral" wire is never dead-zero. And there are some conditions where it has a substantial voltage to dirt-ground.

Look at this user. The bunny is barefoot, on concrete, leaning on a steam radiator. These are all good grounds, or at least better conductors than a living body. And this body is holding bare-metal connected to the amplifier and its uncertain connection to lethal voltages.

This particular amplifier seems to use a trick that was legal for a few years. If you work it through, the "only" path from user to wall-outlet is through R11 C3. As drawn, this is not a lethal shock. 4.3mA is less than 5mA and 5mA is still the standard for anti-shock GFI cut-outs.

But parts fail, and the cheap capacitor often fails SHORT. (Likewise, heater-cathode insulation can short, and a later standard apparently required this first heater be isolated.)

And the amplifier is less good than it could be. And the 0.1uFd is not a great ground for hum.

1920s-1940s automobiles had no seatbelts or dual tail-lights. Most of the old cars I see fixed-up and run on the road have seatbelts and full tail-lights now. This is basic save-your-ass thinking. Likewise these old shock-hazard amplifiers *MUST* be re-rigged with isolation transformers... this isn't 1959 anymore, and you are not broke young parents trying to encourage your child with music while also feeding the kid.

[kagliostro]

Many Thanks PRR

Now all is way clearer

So this circuit tried to supply to the intrinsec defect of such kind of Power Supply

The improvement due to the use of a standard Power Transormer will diminuish the electrocution hazards, and, I suppose, also the noise added to the signal

In this case, will an isolation transformer solve both problems or it will be only an insurance against electrocution without effects on the noise ?

Thanks

Franco


BTW: Steam radiator is common in your country ? We have and had hot water heated radiators, I spent 24 years working at wholesale of hydraulic, sanitary and heating equipment and never hear about steam radiators in civil houses

[davidwpack]

I was reading this site and this guy mentions adding a diode for full-wave. I wasn't sure if it was advisable.
https://www.instructables.com/id/Isolation-transformer-upgrade-for-old-guitar-amps/
Edit: not diode. SS bridge?

[sluckey]

Hogwash! His whole premise for adding the FWB is based on the fact that the rectifier tube is only 1/2 wave and he says" a transformer is designed for full wave operation, blah, blah, blah. He totally missed the fact that the series connected filament string is still operating at full wave and  draws .15A @ 120V and that is an 18 watt load on the transformer. This is a much greater load than the puny halfwave rectifier tube has to supply.

IMO, it's a total waste.

[davidwpack]

Ok. I figured you guys would have mentioned that if it were true. Thanks!

[kagliostro]

I agree with Steve talking about the consumption related problems

but ....... looking the whole thing from one other side

Will the use of a FWB (also to feed heaters) give as result less noise and hum ??

Franco

[shooter]

less noise and hum ??

the series connected filament string is still operating at full wave

Without a "re-design" it is what it is

[davidwpack]

Oi! I've got my parts and got it together finally. Sounds really good but not nearly loud as it should be. By comparison, cranked, it's as loud as my vibro champ on 4. The on/volume is scratchy and spotty as hell though. I'll get it cleaned up and see what happens. Here are some pictures for you. I used a pedal enclosure as a junction/switch/fuse box. Mounted them with Velcro and a couple screws to the back of the rear panel. Thought it'd hum but not very much at all. Thanks for everyone's help!

[sluckey]

 

[shooter]

Nice, way to stick with it!

not nearly loud as it should be

I'm thinking based on the schematics above, 4 on a vibro-champ is about all you're gonna get without some Mods

[davidwpack]

Oh ok. I was thinking it would be at least as loud. I mixed up some tube data. I see now that it's about 4 watts for the 50L6.

[shooter]

so you have a 5F1-2 fenderish, put volume at 10, Marshall 4X12 cab and jam 

[PRR]

> about 4 watts for the 50L6.

Not with 110V power. More like 2W fully optimized. Less with lowest-price parts like a cheap amplifier would use.

Fender never made anything this small.

It can still annoy the neighbors.

[DummyLoad]

> about 4 watts for the 50L6.

Not with 110V power. More like 2W fully optimized. Less with lowest-price parts like a cheap amplifier would use.

Fender never made anything this small.

It can still annoy the neighbors.

aha! amp build competition: who can build an amp with the most power using cheap tubes for lowest cost.    tubes must be available on fleabay or from the NOS houses; e.g., tubes and more, etc.. you must use an output transformer; not a power transformer for the output transformer. surplus Ok. $200 limit. 


--pete

[PRR]

DIYaudio.com $100 amp competition
https://www.diyaudio.com/forums/instruments-and-amps/190738-hundred-buck-amp-challenge.html

[shooter]

 \
now you start, I already built at least 5, they all got re-purposed to something useful 

[DummyLoad]

DIYaudio.com $100 amp competition
https://www.diyaudio.com/forums/instruments-and-amps/190738-hundred-buck-amp-challenge.html

IMO, 200bux is more realistic and would likely increase probability of results that would be worth keeping and actually sound worth a crap.

you'd be ahead if you fab your own chassis or use the cheap hammond project boxes. scrap toaster ovens work.      old radio chassis' too.

--pete

[DummyLoad]

\
now you start, I already built at least 5, they all got re-purposed to something useful 

wuss! you've got one more shit-build in ya! probably already got the parts pulled.    


--pete

[shooter]

wuss!

I'm playing in the sand(amp)box quit picking on me 
I do have a nice preamp TUBE board all wired n waiting, but I spent 1/2 my allotted funds, or all if I limit to $100, does it count if they've already been expensed off the books?

[DummyLoad]

wuss!

I'm playing in the sand(amp)box quit picking on me 
I do have a nice preamp TUBE board all wired n waiting, but I spent 1/2 my allotted funds, or all if I limit to $100, does it count if they've already been expensed off the books?

cook the books however you like... uncle sammy may no likey tho.   

--pete