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Little Sweetie Forum amplifier project


henry4841
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24 minutes ago, Curious_George said:

Both can work well. If you series wire, ensure you connect the output tube first (because this will usually be the tube that draws the most current, then subsequently wire other tubes in decending current draw.

 

How can you have two different currents in the same current loop?

 

To wire filaments in series they need to have the same current requirement. i.e. you cannot place a 6Y6 and 6SJ7 in series since they have totally different filament current requirements.

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Take note of the series filaments in the All American Five radio's run directly off the mains power.

 

We need power in watts for heaters, power tubes require more power than preamp tubes. To maintain the same current draw since all the tubes filaments are in series you will notice the power tubes will have much higher voltage across the filaments.

 

To increase power you either increase voltage or current. In this case the current remains the same in order to run them in series but the power tubes have more voltage across them.

 

Typically 150mA filament ratings.

 

Rectifier tube 35v at 150mA

Power tube 50v at 150mA

Preamp tubes 12v at 150mA

 

35+50+12+12 = 109v

 

There was often a pot to drop a little voltage if mains was higher than 110v.

 

For simplicity of only having one 6.3v winding in our amps we build the voltage obviously stays the same but the current changes and we run them in parallel.

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11 minutes ago, captainbeefheart said:

 

How can you have two different currents in the same current loop?

 

To wire filaments in series they need to have the same current requirement. i.e. you cannot place a 6Y6 and 6SJ7 in series since they have totally different filament current requirements.

Let me clarify, Cappy is right.

 

However, when I responded to Capo, I had meant to imply you can wire the tubes in series, daisy chaining from one tube to the next. This IS NOT series, but parallel. 

 

Instead of the way Henry showed in his build, wiring to each tube then going back to the transformer secondary with 4 separate connections.

 

Sorry for the confusion.

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10 minutes ago, Curious_George said:

Let me clarify, Cappy is right.

 

However, when I responded to Capo, I had meant to imply you can wire the tubes in series, daisy chaining from one tube to the next. Instead of the way Henry showed in his build, wiring to each tube then going back to the transformer secondary with 4 separate connections.

 

Sorry for the confusion.

 

I got ya. When he was saying series and parallel I took the terms literally.

 

Yes I never run "separate" wires for tubes heaters, nobody really does.

 

Instead of saying they are in series I'd say they share the same feed wire as it is a better way to state it to keep people learning properly. Because technically they are all still in parallel with one anothe just sharing the same feed from the source. With the same feed wire you want the largest load closest to the source or else if it were after the more sensitive lower load preamp tubes the larger current can possibly pollute them.

 

Thank you Nick for clarifying to me, I hear series and parallel and think too literal. Technical terminology and all.

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Good discussion guys. This is what we want here. I honestly did not see the B connection on the amplifier schematic after a very quick look for it. I have made a virtual ground before on a schematic of power supply without a virtual ground doing it the way I did eliminating a hum I had on that particular amplifier. I believe it was with one of those Chinese amplifier kits I bought. Like George said it would work both ways but putting it where Maynard designed this amplifier is going to be where it will go for this build. 

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2 minutes ago, Khornukopia said:

The words series or parallel are very specific. If one reads them in the description or instructions, the definitions need to be very clear to everyone involved.

I Apologize for asking questions with incorrect terminology. I'll refrain from doing so in the future.

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1 hour ago, Curious_George said:

My Musical Power Supply output transformers have shipped out. I should have them Monday. That was a quick lead-time. I can't wait to test them. 

 

I ordered the OT10SE-HF (HiFi version).

I am very interested in how they look and compare with say the Hammond 125C's and the Edcor 10 watt ones. From the pictures they look very good. I mistakenly ordered the wrong OPT from Tubedepot the first time and immediately turned around and bought the Hammond 125C's from Mouser before getting the OPT's from Tubedepot. When the OPT's came from Tubedepot to my house I noticed that Tubedepot now carries the Hammond 125DSE version so I sent the wrong ones back and swapped them for the 125DSE. It is quite a bit bigger than the Hammond 125C's and heavier. Nice looking OPT's for about the same price as the OPT's from Edcor and the price of 125CSE's at Mouser. I like ordering from Tubedepot, good people. This means I have an extra pair of good looking OPT's to do something with in the future. This is from someone who was not really planning on building another tube amplifier after doing this project. I am just having as much fun with this project as the ones following along. Good clean fun among friends. I might have started something. Let's face it our Klipsch speakers were made for a good little SET amplifier. 

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Back to the chokes vs resistors in power supply.

 

Since this is a SE amp which has very poor power supply ripple rejection we want as clean a DC supply as possible. The 6Y6 gets it's power from after an RC filter, the R is 180 ohms and the C is 220uF.

 

220uF @ 120Hz = 6 ohms

 

We know the load of the amp is 50mA

 

This get's tricky because this is from the stock mono amp schematic but Henry's amp is stereo and has a higher resistor value of 560

 

Two 6Y6's will draw 100mA and across the 560 ohm resistor we will have a drop of 56v, that's 5.6 watts of heat so make sure you use at least 10 watt type.

 

Looking at this as a voltage divider ratio we get:

 

6 / 566 = .01

 

A 10H choke is 7.5k ohms at 120Hz. With a DC resistance of 150 ohms.

 

Now we only drop 15v which is 1.5 watts of heat, much better.

 

Viewing as a voltage divider we also now get:

 

6 / 7500 = .0008

 

With say an input ripple of 10v the RC filter will have an output of

 

.01*10 = 100mV

 

With the choke filter we now have an output ripple of

 

.0008*10 = 8mV

 

That's more than 10 times lower ripple

 

For preamps that draw very little current there reaches a point where the voltage drop across the resistor is low enough where resistors are preferred and a choke without being huge isn't as good an option. With larger power amp type loads a choke is by far the better filtering option which is why I said for anyone building this amp I'd cough up the extra $20 for a choke.

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2 hours ago, capo72 said:

Regarding filament wiring, Is there an argument for parallel wiring ... vs. series (daisy chaining) the filaments together? I have seen both ways and pondered if series was better to minimize AC wiring lengths. Probably a matter of preference, but I'm enjoying following along and trying to learn.

I am learning also, and think you asked a good question.

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6 minutes ago, captainbeefheart said:

Back to the chokes vs resistors in power supply.

 

Since this is a SE amp which has very poor power supply ripple rejection we want as clean a DC supply as possible. The 6Y6 gets it's power from after an RC filter, the R is 180 ohms and the C is 220uF.

Interesting Captain but you left out the choke filtering of the OPT's before reaching the 6Y6's. For those that do not know DC and AC can exist together in that OPT. It is being used as a choke (inductor which does do an excellent job cleaning up DC) for cleaning that power supply section as well as handling the audio signal which is AC. So in effect we have a CRCL circuit for cleaning the DC for the 6Y6. Seems to work as Maynard designed it in the one I have used for a number of years. I am sure Captain's way is better but debatable if it makes a lot of difference. Interesting reading his thoughts though. This is what discussions are about in this thread of mine without ego's getting in the way. Good work Captain. I do not do all that figuring like Captain. That is electronic geeks thing. I am more a technician repairman, hobbyist builder. My formula calculations are limited to the ones in the Ohm's law wheel. Simple ones. 

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15 minutes ago, henry4841 said:

It is being used as a choke (inductor which does do an excellent job cleaning up DC)

 

This is not really true, maybe a half truth.

 

There is no capacitor on the other side of the output transformer primary, only the resistance of the tube so it's not a great filter. I can give you an exact ratio and the ripple that would be present on the plate.

 

Going back the the example, if you have 100mV ripple at the filter node feeding the output transformer we still view the impedance of the OPT and tube plate as a voltage divider.

 

OPT = 5k

6Y6 = 1.3k

 

That's a voltage divider ratio of

 

1300 / 6300 = .2

 

You'll still have 20mV of ripple on the plate of the 6Y6 which will be present at the output albeit at a reduced level.

 

If you have 8mV at the node feeding the OPT then you will end up with only 1.6mV of ripple at the plate of the 6Y6, much less noise at the output.

 

A 5k transformer has a voltage ratio of 25:1

 

With the 20mV of ripple on the 6Y6 plate you'll end up with a little less than 1mV of ripple at the speaker terminal, remember that's as much as what was on the primary side of the 6Y6 plate with the better filtering of the choke.

 

With the 1.6mV of ripple present on the 6Y6 plate you end up with only 60nV (.06mV) of ripple at the speaker terminal.

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Like I said this morning when you designer geeks start talking complicated stuff you are leaving me out. My brain has a hard time keeping up with simple electronic stuff. Ya'll are welcome to discuss the designing part of amplifiers among yourself here but you are getting above my wanting to understand this kind of talk at this stage in my life. Ya'll are welcome to on this this thread though but remember this thread was started for a beginner. I gets too much like the diyaudio pass thread for me to want to get involved when ya'll get into discussing designing circuits.

 

Maynard designed it and I just build it using his schematic and the PS circuit he designed for stereo use. Seems to work good to me in actual use.  🙂

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Like I have also said I am sure you geeks can improve this design putting your personal touch to it like increasing to power of the 6Y6 tube. That is what they do with a design at diyaudio among those geeks. Get's over my old head. I would like you geeks to use this design to work with improving it to suit you idea of circuits. You will have a hard time finding cheaper NOS tubes to work with for a inexpensive amp design. Maynard has given his blessing for his design to be used by anyone. You guys can work with it and make a good little design a very good design. I am almost certain you can get more power out of the 6Y6 tube. I really like the sound of the tubes being used for this build. Good place to start a design using these tubes to work with. The idea of using a choke in the PS is an example that may improve it some. Some may prefer to work with a rectifier tube instead of a bridge rectifier. Lot of ways for a country boy to get to town.  

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