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Little Sweetie Forum amplifier project


henry4841
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15 minutes ago, henry4841 said:

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.

 

You're all good man, it's just for others interested in specifics.

 

I did crunch the actual ripple of the power supply using the specs from Antek and your modified schematic. Again specifics are good, especially with the exact circuit we are talking and so I went further to see if a choke will help this specific circuit.

 

First capacitor of your supply won't have 10v ripple, I calculated around 2v

 

Further running the numbers puts the 120Hz ripple at the speaker terminal at around 50nV which is adequate. I don't like to see even 1mV of 120Hz ripple at the speaker terminal. I just guessed at the initial 10V ripple at the first capacitor after the rectifier but it's not that high. I'm use to seeing 500v with double the load current, say 200mA which gives a much larger ripple.

 

So don't mind me, Maynard did an good job with just going with resistors in this build. Mainly because his was a mono circuit and with only 210v and less than 60mA load there's really not much ripple so easy to filter. I figured since you doubled the load by going to Stereo it may require a choke but I think you can by just fine without one so I take back my suggestion after crunching all the data.

 

It's still a good resource to show why we do use chokes in amps, higher voltages typically over 400v with much higher load currents make larger ripples so we need better filtering. Hopefully by me showing the numbers helps paint a clearer picture to when you may need to go choke and why this amp is fine without one.

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2 minutes ago, henry4841 said:

Just remember you are talking to old Grandpa about stuff over his head. 🙂

 

No, don't say that!!!  The more you read the more you learn.

 

You may not be interested in the specifics now, but in the future you may want to come back and read through it. So it's good that there is specific data about your specific amp you are building. 

 

I do apologize for jumping right in saying the amp will benefit from a choke. I just assumed that he got away with it because of the mono amps having half the load current and I just wanted you to get the same good results with the signal circuitry. I figured that doubling the load current may push you into a higher region of ripple, enough to warrant a choke but it's still low enough to get by with resistors. If this was a preamp and there was going to be a power amp with gain after the output then I would certainly recommend getting the power supply noise down further but it's just a power amp with speaker at the output and should be good to go.

 

Yes I have seen preamps using DHT tubes that have higher voltage and more of a load current than the Sweetie amp, in that specific situation it would be prudent to go with a choke, possibly several chokes in the power supply to reduce power supply noise at the output.

 

Keep up the good work, it's coming along great!!

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20 minutes ago, captainbeefheart said:

 

You're all good man, it's just for others interested in specifics.

 

I did crunch the actual ripple of the power supply using the specs from Antek and your modified schematic. Again specifics are good, especially with the exact circuit we are talking and so I went further to see if a choke will help this specific circuit.

 

First capacitor of your supply won't have 10v ripple, I calculated around 2v

 

Further running the numbers puts the 120Hz ripple at the speaker terminal at around 50nV which is adequate. I don't like to see even 1mV of 120Hz ripple at the speaker terminal. I just guessed at the initial 10V ripple at the first capacitor after the rectifier but it's not that high. I'm use to seeing 500v with double the load current, say 200mA which gives a much larger ripple.

 

So don't mind me, Maynard did an good job with just going with resistors in this build. Mainly because his was a mono circuit and with only 210v and less than 60mA load there's really not much ripple so easy to filter. I figured since you doubled the load by going to Stereo it may require a choke but I think you can by just fine without one so I take back my suggestion after crunching all the data.

 

It's still a good resource to show why we do use chokes in amps, higher voltages typically over 400v with much higher load currents make larger ripples so we need better filtering. Hopefully by me showing the numbers helps paint a clearer picture to when you may need to go choke and why this amp is fine without one.

According to Duncan's Power Supply Designer software, ripple at C1 is 2.76V. Close Cappy...

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Mind you push pull amp output stages do not have to deal with this like single ended amps.

 

Often you will see the node for the output transformer taken directly off the reservoir capacitor and I have seen as high as 20v ripple!! Mind you pentodes have awful PSRR, think about the plate being 30k ohms and transformer load being only 2k ohms, the output transformer doesn't filter anything due to the ratio;

 

30000 / 32000 = .93

 

That means the 20v ripple on the filter node will make a

 

.93*20 = 18.75v ripple on the plates

 

But why don't we hear it on the secondary?

 

That's because any common mode noise on the plates of the output tubes gets cancelled when recombined in the output transformer.

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1 minute ago, Curious_George said:

According to Duncan's Power Supply Designer software, ripple at C1 is 2.76V. Close Cappy...

 

That's why I added my "fudge factor" by saying "around 2v ripple"   LOL

 

Thanks for checking and confirming what I figured it was.

 

Curious what specs did you use for the Antek?  I used 206v for no load voltage, given the datasheet says 194v for 460mA load I calculated deltaV / deltaI = transformer winding resistance of 26 ohms.

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16 minutes ago, captainbeefheart said:

 

No, don't say that!!!  The more you read the more you learn.

 

You may not be interested in the specifics now, but in the future you may want to come back and read through it. So it's good that there is specific data about your specific amp you are building. 

 

I do apologize for jumping right in saying the amp will benefit from a choke. I just assumed that he got away with it because of the mono amps having half the load current and I just wanted you to get the same good results with the signal circuitry. I figured that doubling the load current may push you into a higher region of ripple, enough to warrant a choke but it's still low enough to get by with resistors. If this was a preamp and there was going to be a power amp with gain after the output then I would certainly recommend getting the power supply noise down further but it's just a power amp with speaker at the output and should be good to go.

 

Yes I have seen preamps using DHT tubes that have higher voltage and more of a load current than the Sweetie amp, in that specific situation it would be prudent to go with a choke, possibly several chokes in the power supply to reduce power supply noise at the output.

 

Keep up the good work, it's coming along great!!

Regardless of how you get there (CRC / CLC), I have found that if you have <20mV of ripple going to your output tube in a SE circuit, your audible hum is going to be virtually inaudible. 

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6 minutes ago, captainbeefheart said:

 

That's why I added my "fudge factor" by saying "around 2v ripple"   LOL

 

Thanks for checking and confirming what I figured it was.

 

Curious what specs did you use for the Antek?  I used 206v for no load voltage, given the datasheet says 194v for 460mA load I calculated deltaV / deltaI = transformer winding resistance of 26 ohms.

In the Little Sweetie schematic, there are no voltage references, which kinda stinks for a DIY build. The more voltage references the better (I think). Anyway, the only voltage reference is adjust R10 for 200VDC on the anode of the output tube. I used that info to back-fill data on the power transformer in the Power Supply software. 

 

image.thumb.png.9af32adcc7d505417a87966f3975b44c.png

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4 minutes ago, Curious_George said:

Regardless of how you get there (CRC / CLC), I have found that if you have <20mV of ripple going to your output tube in a SE circuit, your audible hum is going to be virtually inaudible. 

 

There may be rare cases of going with a pentode output and a very low load impedance. Say a power tube with >50k plate impedance with a 1.2k load. You'll get about all the 20mV on the plate and the secondary will see around 2mV.

 

2mV of ripple at output to some is acceptable for a power amp but when considering very efficient horn speakers I consider even 1mV not acceptable. For example I went pentode with Shakey's amp so I really need to get ripple out of the power supply. Not easy with 200mA, 400v and a tube rectifier, chokes are a must.

 

 

I have found most tinkerers use 5k for many different tubes and they strap them triode anyway so much better PSRR and OPT has high turns/voltage ratio so the output is reduced much farther down. Typically easy to get 1mV or less at output.

 

Really you reach a point where going lower with the 120Hz hum at output is pointless because the 60Hz hum is much tougher to get very very low.

 

 

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14 minutes ago, Curious_George said:

In the Little Sweetie schematic, there are no voltage references, which kinda stinks for a DIY build. The more voltage references the better (I think). Anyway, the only voltage reference is adjust R10 for 200VDC on the anode of the output tube. I used that info to back-fill data on the power transformer in the Power Supply software. 

 

Interesting.

 

I was going by Henry's amp which he used a specific Antek transformer.

 

https://www.antekinc.com/content/AS-1T200.pdf

 

I used the two load conditions to attain a winding impedance.

 

0 amps = 206v

 

460mA = 194v

12 / .46 = 26 transformer winding resistance

 

Try inputting 206 for voltage and 26 ohms for the winding. If ya don't mind ;)

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Yes, don't know much about schematics, but always appreciate voltage references.  Try to remember not to touch the ones with higher values when powered up....

 

Quick question:  Power switch:  Yay, nay, or optional ?  Both of my amps are one set with, one set without.  I always pull the plugs on my power supply, so the power switch is kind of a redundant second step for me to power up and down.

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You guys that are into this tube building design stuff really need to decide on some tubes to work with putting your heads together doing it. I am just saying these two old tubes are good ones to work from. Get the tube data on these two tubes and work on your take designing circuits for the 6Y6 and 6SJ7 NOS tubes. Captain started this idea on his thread of designing a forum tube amplifier and I say these tubes are good ones to work with. Cheap for NOS 'AUDIO' tubes. Sound really, really good. I do not believe you are going to find cheaper NOS audio tubes to work with. Get the data sheets and get your heads together and let it be a new design by the members of the forum that like that sort of thing. Captain has already started the thread and as far as I know he has not decided on the tubes to use. I bet members would enjoy listening to your input. I would like to follow that thread as far as I can understand that is. 

 

This thread is really aimed at a beginner. 

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28 minutes ago, Curious_George said:

In the Little Sweetie schematic, there are no voltage references, which kinda stinks for a DIY build. The more voltage references the better (I think). Anyway, the only voltage reference is adjust R10 for 200VDC on the anode of the output tube. I used that info to back-fill data on the power transformer in the Power Supply software. 

 

image.thumb.png.9af32adcc7d505417a87966f3975b44c.png

image.thumb.png.3aa67609e4066f5073e4d3b2b054f438.png

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Henry, if you are interested in adding voltages to Maynards schematic here ya go.

 

6SJ7

plate- 80v

screen- 45v

cathode- 1v

 

6Y6

plate- 230

cathode- 30v to 35v

 

Since Maynard calls for "200v across 6Y6 plate and cathode", we know the cathode is say 32v and the 125se has a primary DCR of 200 ohms that puts our B+ target feeding the OPT at around 245v

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