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I would like to build an amplifier

jvdk01

jvdk01

I'm with Flat Stanley
Hi

I recently got the transformers shown in the following photographs. I would like to build an amplifier around them. I believe they were either for or from a PP 6L6 amp project. I'm not sure if the photos alone are enough for someone to point me to a schematic that is both simple and sounds good. I'm not sure what sort of rectification is appropriate for the power transformer, but not opposed to either SS or tube. It doesn't have to be a 6L6 design. Any approprite output tube would be fine, but availability and cost is certainly a concern. I don't have much experience, but I did recap my SC ASR 433 with a lot of help from AK members. I've also built a headphone amp.

Thanks in advance
John

Power transformer

3372854695_59f6ee2e0f_b.jpg


A couple shots of the outputs

3372854265_6f10b5d2c4_b.jpg


3372854475_912eee1d21_b.jpg
 
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Looks like that power TX would like solid state rectification (no 5v tap, and no center tap (CT) on the high voltage (HV) winding). The first step, unless you 100% sure what the outputs came from, is to find out their primary impedence (Pri. Z).
 
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Black and green are your speaker connections. Red is the center tap on the output transformer (OPT), while blue and brown are the tail ends of the Pri..

Green is typically the color code for an 8ohm secondary. Knowing that, will make it fairly easy to measure the Pri. Z.. Knowing the Pri. Z will help us select your output tube.:yes:
 
Actually Jay it looks like (If you look real close) there is a black wire in the center of the
HV red wires, so I would assume that be a center tap, then there is plenty 6.3v heaters
to do say an EZ81 tube rectifier.

Since he believes the outputs are from a 6L6 amp and I see no UL taps let's assume the
primary impedance is 6.6K pentode outputs with a single 8ohm tap.

Now 110ma high voltage means say, 25ma per output when using 4 tubes for this stereo
amp leaving some for the front end.

Let's give this some thought and see what output tubes to suggest then..:scratch2:
 
300 volts would seem a little better option for PP 6V6, or 6BQ5 possibly....to me....

I like the four 6.3 volt heater windings.
 
Kegger said:
Actually Jay it looks like (If you look real close) there is a black wire in the center of the
HV red wires, so I would assume that be a center tap, then there is plenty 6.3v heaters
to do say an EZ81 tube rectifier.

Since he believes the outputs are from a 6L6 amp and I see no UL taps let's assume the
primary impedance is 6.6K pentode outputs with a single 8ohm tap.

Now 110ma high voltage means say, 25ma per output when using 4 tubes for this stereo
amp leaving some for the front end.

Let's give this some thought and see what output tubes to suggest then..:scratch2:
Click to expand...

Ah, I do see the CT now.:yes: I still like SS rect. but that's just me.:yes: Now, 25mA per tube, could be a lots of things.:scratch2:
 
Mike Stehr said:
300 volts would seem a little better option for PP 6V6, or 6BQ5 possibly....to me....

I like the four 6.3 volt heater windings.
Click to expand...

I thought about that 300v rating too, but you could always run the 120v wall power into the 100v primary to get some more voltage out of the sec.. I like my 6BQ5s on ~ 340v, even though some books say that's high.:music:
 
The two wire side has green and black, while the other side has brown , red and lt blue. I do have have a meter. I don't have a variac at home, but I can borrow one next week. I do have one of those poor man light bulb things if that would work.
 
After some more thought this is what I would suggest. :)
(Given the power tranny and what it takes to power the amp)

6V6 Triode connected Push pull, with say a 5965/12AT7 voltage amp/direct coupled
concertina phase splitter an a simple plus fairly inexpensive but affective Pwr supply.

(Using one of those $10.00 Radiodaze chokes you could do something like this)
 

Attachments

  • 6v6_pp_ps.GIF
    6v6_pp_ps.GIF
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Kegger said:
After some more thought this is what I would suggest. :)
(Given the power tranny and what it takes to power the amp)

6V6 Triode connected Push pull, with say a 5965/12AT7 voltage amp/direct coupled
concertina phase splitter an a simple plus fairly inexpensive but affective Pwr supply.

(Using one of those $10.00 Radiodaze chokes you could do something like this)
Click to expand...

I'm over my head, so I have to ask a couple stupid questions. Is the EZ81 tube(s) shown two tubes, or is it two halves of one tube. What is the symbol on the far right of the schematic with two intersecting circles. Why do the components in the schematic (T1, C1 and C2) have a resistance value. Is that just a default value that the model assigns. Why is a choke better than a resistor. Sorry for the questions, and thanks for the help.

John
 
jvdk01 said:
I'm over my head, so I have to ask a couple stupid questions. Is the EZ81 tube(s) shown two tubes, or is it two halves of one tube. What is the symbol on the far right of the schematic with two intersecting circles. Why do the components in the schematic (T1, C1 and C2) have a resistance value. Is that just a default value that the model assigns. Why is a choke better than a resistor. Sorry for the questions, and thanks for the help.

John
Click to expand...

The EZ81 is just the one tube with two sets of plates inside (aka full wave). The interconnecting circles are the load. The R values are required to get a result ( I think most people use 1 for caps as a default). Chokes offer better regulation and superior filtering than a resistor.
 
I figured that was the reason for the resistance values. Is there any advantage to one form of rectification over another. If I were building thousands, I can see how a couple SS diodes would be a huge savings over a tube and socket and whatever else is associated with a tube rectifier, but for a one off project, it's only a few dollars. Also, how do I go about measuring the impedence of the OT. I know I've seen it before, but I can't find it now.

Thanks
John
 
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I think SS or tube rectification is purely a matter of taste. I personally like to run as high a voltage as a transformer will allow, so I use SS quite a bit. Diodes have come a long way too, being faster and quieter than those of years ago. That being said, I have several pieces that are tube rectified, and like them very much as well.

For measuring your OPT Pri. Z, you will need to apply a small amount of AC voltage to the tails (brown and blue) of your OPT Pri.. This will help us find the turns ratio (N). Measure your voltage at the primary (lets say you used 100v), then divide it, by the voltage you read at the secondary (lets say it is 2.5v). The equation would be 100/2.50=40, with 40 being the turns ratio (N). Impedences are changed by N squared, so, 40x40=1600. Since you measured the secondary voltage at an 8ohm tap, the primary Z would be 1600x8=12.8k.
 
110 mA is too low for a stereo amp - you could get more with a solid state rectifier and a smaller input cap (approaching choke input). But even 150 mA is on the light side...

Let's see what the output transforners are - 1OK would be good.
 
What Tom say's is true, if your trying to build a "correct" amp.

But if you just want to build something that will sound halfway decent you can build
what I'm uploading as a schem and get that amp working, if later you want to use a
bigger power transformer, maybe even go pentode and use loop feedback I provided
a place for that as well.

I changed the driver tube also.

This would be a very easy amp and inexpensive 1 as well to play with while you learn.
 

Attachments

  • 6V6_333V_CB_m1.GIF
    6V6_333V_CB_m1.GIF
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That schematic along with the power supply I listed before would let you build an amp.

Just building a simple amp like this with inexpensive and some parts you have would be
a good first step an learning experience, you don't even need to know what the output
transformers impedance is, it will work regardless of what it is, so if you just wanted to
do an amp build and make it work this will do.

You have just enough current in the high voltage and heaters to build this, doing a tube
rectifier will let you learn that portion as well.

Later you can experiment and upgrade and measure all you want. ;)
 
jvdk01 said:
Hi

I recently got the transformers shown in the following photographs. I would like to build an amplifier around them. I believe they were either for or from a PP 6L6 amp project. I'm not sure if the photos alone are enough for someone to point me to a schematic that is both simple and sounds good. I'm not sure what sort of rectification is appropriate for the power transformer, but not opposed to either SS or tube. It doesn't have to be a 6L6 design. Any approprite output tube would be fine, but availability and cost is certainly a concern. I don't have much experience, but I did recap my SC ASR 433 with a lot of help from AK members. I've also built a headphone amp.

Thanks in advance
John
Click to expand...

This seems like designing an amplifier this way is a lot like reverse engineering, IMO. Usually, I would think the circuit is designed, current is calculated and/or measured and the transformer would be chosen to withstand the demands of the circuit around real world and worst case scenarios.

Usually, the amplifier is designed to use the best characteristics of the circuit or tube type, biasing method, tone stack (if it has one) and gain stages. If it's a PP design, the phase inverter type will need to be considered, too.

Antique Electronic Supply has software for amp design and I think that kind of thing is available from other sources, too. I think that would be a good place to start and when I begin a design for a guitar amp that's similar to one I bought, I'll do it that way. The one I bought is from the late '30s, as far as I can tell and uses tubes that aren't always available but their equivalents are. The problem is that whatever I substitute with undoubtedly change the sound and I really like the way mine sounds..
 
There a various ways to design/build amplifiers, many build based on what they have,
be it tubes, chokes, speakers, output transformers or even power tranny's.

Most of my builds are based on what I have laying around that I want to build something
with rather then buy all new things based on what is wanted. :)
 
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