50L6 guitar amp

[hex69]

Hey guys,

I'm new to the forum, but I've learned alot from reading on here.

I've done some electronic projects including many guitar pedals, I know schematics and circuits well enough, I'm no stranger to DIY.

I've built 2 tube amps from scratch so I have moderate amount of tube experience, quite good soldering skills and I know what not to do.

I'm looking to built a new AA5 based guitar amp and want to use as much parts as I can from my junk box.

Here's what I found in there :

- An isolation transformer 120->120 VAC (60VA rated)
- Power tubes: 50L6
- Preamp tubes: 12au7, 12ax7, 6n2p-ev, 6av6, 12av6.
- A nice metal box which can be use for the chassis
- All the stuff to built an eyelet board
- Some usable sockets

Only 2 parts are missing though, the output transformer.. and I don't have any 35Z5.

I thought about skipping the 35Z5 and replace it with a SS rectifier. I found this schematic using one 50L6 (from Tino Zottola's book) :

http://www.kirtland.com/guitar1/guit...50L6_Amp-1.gif

Seems like a nice little amp to built, and by luck my isolation transformer has multiple wires on the primary (COM-120-208-230-460), so I'm pretty sure by using the COM/230 wires it will fall near the perfect 60VAC at the secondary which would be perfect for this project.

Okay now is this the correct way to 'guestimate' the things?

50L6 filaments -> 150 ma
Max plate -> 50 ma
Max Screen -> 10 ma
12ax7 filaments (12V serie) -> 150 ma
Max plate -> 2.4 ma

Roughly 362.4 ma, so my 60VA isolation PT should be okay?

Am I correct saying this design uses an half wave rectifier circuit (kinda doubler).. I wonder if this can cause some heat in the power transformer, would it be better instead to use a full wave bridge? I read somewhere that power transformers are designed to be symmetrically loaded.

On the other hand, the plate voltage suggested in the schematic seems a little low?

I would really appreciate some advices and comments from the more experienced people.

Thank you!

[nosaj]

Voltages seem fair for an AA5 design.
Check here for some parts you may need there are plenty of people there from canada to minimize shipping cost. If you don't see what you want make a WTB ad (want to buy)
The guys there are just as helpful and friendly as they are here.
http://antiqueradios.com/forums/viewforum.php?f=15
Welcome to the Forum

nosaj

[J M Fahey]

Tino Zottola´s is a *terrible* design.

1) Using series mismatched primaries means it will have 4 *four* times the proper DC resistance, so it will heat 4 times as much as expected. Just do the Math.

2) voltage doubling is actually 2 half wave rectifiers in series, current peak will be twice as much as before, heating will be 4 times as much, since dissipation is proportional to current squared

1 + 2 means that now your 60VA transformer can only handle 1/4th of that, or 15VA.

3) so just use the transformer the way God intended, with a full wave bridge rectifier for +B and series 150mA filaments as needed to reach as close as possible to 110VAC ,

"Wasting" extra voltage on some filament series resistor is less wasteful than Tino Zottola´s kludge.

[hex69]

@nosaj

Thank you for both the reply and the invitation!

@J M Fahey

Thanks you very much for the advises!

Although I not sure to understand point 1) Here's the 60VA iso transformer I'm about to use in the project, which is a single transformer. I'm working on a schematic right now, could post it later to get some advises, my technical knowledges are not near of what I hope they should.

Which part of the schematic can be qualified as terrible? I thought about using my iso transformer and a SS full wave rectifier, maybe 4Xuf4007, my transformer will be okay right? As you can see on the pic it has multiple primary wires, is it okay to use COM/230 wires hooked on the 120VAC (wall) so that I could get near 60VAC at the secondary, that would be perfect for the project.

Thanks again all!

[nosaj]

@nosaj

Thank you for both the reply and the invitation!

@J M Fahey

Thanks you very much for the advises!

Although I not sure to understand point 1) Here's the 60VA iso transformer I'm about to use in the project, which is a single transformer. I'm working on a schematic right now, could post it later to get some advises, my technical knowledges are not near of what I hope they should.

Which part of the schematic can be qualified as terrible? I thought about using my iso transformer and a SS full wave rectifier, maybe 4Xuf4007, my transformer will be okay right? As you can see on the pic it has multiple primary wires, is it okay to use COM/230 wires hooked on the 120VAC (wall) so that I could get near 60VAC at the secondary, that would be perfect for the project.

Thanks again all!

Honestly What I would do is (giving that mains is 120v) I would clip meter wires on the 2 secondaries and connect the primaries to the 120 connection.
60VAC is assumed to be VRMS.

Personally I wouldn't be worried too much about heat at this point, but then I'm a build it together and play with things more than I am on the technical aspects.

And as far as Juan a most excellent person to listen to. He can seem brash sometimes but that's only when you ask too many questions instead of doing what he asks( but if you don't understand what he's saying please state it) He is rather direct because the aim is to get to the end game with as little frustration as possible.

How do I know you ask because I have been guilty of tying up his time before

There are many excellent techs here who graciously lend their knowledge and time both of which are expensive to give back to all of us.
nosaj

[hex69]

No issue here, people have personalities. I have such respect for knowledged people, I've learned a lot from reading on here like I said, and surely don't want to tie anyone time.. trying to understand when he says "Using series mismatched primaries" since I'm gonna use a single isolation transformer, and I don't want to begin something that will not work in the end. I'm a peaceful man at learning.

Honestly What I would do is (giving that mains is 120v) I would clip meter wires on the 2 secondaries and connect the primaries to the 120 connection.
60VAC is assumed to be VRMS.

That's what I did Sir, roughly got 68 VAC at the secondary.

At this point I'm still not sure of what I'm gonna built, just looking the options I could have starting from this schematic

And Here's the schematic of the power supply I have in my head for this project.

Seems correct? I have no clue on how to calculate R2,3,4 and C1,2,3,4

Even I'm stuck using PSU designer because of the filaments sharing the circuit.

Edit: By adding the full wave bridge, the doubled voltage will be higher for sure than what's in the original schematic, maybe the key is to keep all voltages the same by playing with R2 ? I dunno

Thank guys!

[nosaj]

No issue here, people have personalities. I have such respect for knowledged people, I've learned a lot from reading on here like I said, and surely don't want to tie anyone time.. trying to understand when he says "Using series mismatched primaries" since I'm gonna use a single isolation transformer, and I don't want to begin something that will not work in the end. I'm a peaceful man at learning.



That's what I did Sir, roughly got 68 VAC at the secondary.

At this point I'm still not sure of what I'm gonna built, just looking the options I could have starting from this schematic

And Here's the schematic of the power supply I have in my head for this project.

Seems correct? I have no clue on how to calculate R2,3,4 and C1,2,3,4

Even I'm stuck using PSU designer because of the filaments sharing the circuit.

Edit: By adding the full wave bridge, the doubled voltage will be higher for sure than what's in the original schematic, maybe the key is to keep all voltages the same by playing with R2 ? I dunno

Thank guys!

No rectifier tube typicall aa5 designs use somewhere between a 80 uf an 40uf cap for filtering I have seen them as high as 220uf more filter less hum.
B+ will go to your power tube R3 will drop the voltage for the plate to what you need for the next tube and R4 does the next. The guys around here will tell you it all Ohm,s law(which I'm still learning to correctly use it . Here's a link on how to figure voltage drop https://sciencing.com/calculate-volt...s-6128036.html
You will also need to pull tube data sheets to determine what values you need.

Well you need to know where your going decide on what to build first.
i started with a junkbox champ, then a kalamazoo model one, then a tweed deluxe all from jusnk I harvested from organs going to the dump or sitting in a thrift store for 5 or 10 dollars. You get everything to build and amp and then some.

nosaj

[Helmholtz]

The voltage doubler arrangement needs to be fed with AC. It makes no sense after the bridge rectifier, it won't double the voltage. Using the bridge requires higher secondary voltage, i.e. a different PT.
I think you may try your transformer with the voltage doubler (without the bridge) shown in the schematic, as the tapped primary will not perform quite as bad as the primary arrangement JMF described. A tapped primary transformer uses a larger wire cross section than the double primary type. Apart from that, connecting to the 230V tap reduces the magnetizing current (by a factor of about 4) and thus copper and iron losses - in other words temperature load.
I would recommend to monitor transformer temperature, though. Good mechanical/thermal contact with the chassis may further help to avoid excessive transformer temperatures.

[hex69]

The voltage doubler arrangement needs to be fed with AC. It makes no sense after the bridge rectifier, it won't double the voltage. Using the bridge requires higher secondary voltage, i.e. a different PT.
I think you may try your transformer with the voltage doubler (without the bridge) shown in the schematic, as the tapped primary will not perform quite as bad as the primary arrangement JMF described. A tapped primary transformer uses a larger wire cross section than the double primary type. Apart from that, connecting to the 230V tap reduces the magnetizing current (by a factor of about 4) and thus copper and iron losses - in other words temperature load.
I would recommend to monitor transformer temperature, though. Good mechanical/thermal contact with the chassis may further help to avoid excessive transformer temperatures.

Ohhh my it's all crystal clear now! Thank you very much Sir I understand 100%

That's what I thought, I'm gonna built this amp like it is in the schematic and then after will see if I can do mods to my taste.

I could also use the 120:120 tap on the transformer and built a nice push pull.. maybe the next built, I have several iso transformers exactly the same!

You all rock! Thanks again!

[Helmholtz]

Ohhh my it's all crystal clear now! Thank you very much Sir I understand 100%

That's what I thought, I'm gonna built this amp like it is in the schematic and then after will see if I can do mods to my taste.

I could also use the 120:120 tap on the transformer and built a nice push pull.. maybe the next built, I have several iso transformers exactly the same!

You all rock! Thanks again!

You're welcome! Looking forward to hear/read about the results.

[hex69]

You're welcome! Looking forward to hear/read about the results.

Now I'm doing some tweaking on my last project then I'm gonna start this one and post pics/soundtracks

Thank you!