Hey group,
I am still having some trouble understanding how to select an output transformer for my build project. I have two 6L6GC output tubes, I will probably grid-bas them and I'd like to get around 30W output.
I've looked through the forums, books, internet, etc. and haven't come across a "this is how to select an output transformer" instruction.
Anyone got any tips or advice? I'd like to get the TX ordered soon so trying to nail down a unit once I know what I should buy.
Output Transformer Selection
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Output Transformer Selection
Last edited by JJH0906 on Sun Dec 26, 2021 7:32 pm, edited 1 time in total.
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Stevem
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Re: Output Transformer Selection
Output transformers are not just selected by stating that the output tubes will be grid biased and how many watts your looking to achieve!
Wattage is produced by a combination of voltage applied to the plate of the tubes and the current they are allowed to draw ( just like you see in the ohms law wheel you can find on line) in terms of driving a given speaker load.
If you go to the Duncan amp site and do a search in there tube data section you will find the info you need .
In my opinion you need to learn how to do this and not rely on others so much.
This part of amp building is not hard to understand with charts like are set up on the Duncan site, plus you will reap the benefit of feeling much better about yourself.
I would also suggest that you buy a reprint of the RCA rc30 tube book, you will love it and gain a lot of knowledge from it little by little!
Also do you want this output stage to be class A , or class ab1.
A class A type output transformer capable of providing 30 watts of rms power will cost a arm and a leg, so I will assume you are looking at using a class ab1 output stage and this is how you will find the beginning of listings on the Duncan site once you get to the 6L6GC listing .
Wattage is produced by a combination of voltage applied to the plate of the tubes and the current they are allowed to draw ( just like you see in the ohms law wheel you can find on line) in terms of driving a given speaker load.
If you go to the Duncan amp site and do a search in there tube data section you will find the info you need .
In my opinion you need to learn how to do this and not rely on others so much.
This part of amp building is not hard to understand with charts like are set up on the Duncan site, plus you will reap the benefit of feeling much better about yourself.
I would also suggest that you buy a reprint of the RCA rc30 tube book, you will love it and gain a lot of knowledge from it little by little!
Also do you want this output stage to be class A , or class ab1.
A class A type output transformer capable of providing 30 watts of rms power will cost a arm and a leg, so I will assume you are looking at using a class ab1 output stage and this is how you will find the beginning of listings on the Duncan site once you get to the 6L6GC listing .
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Output Transformer Selection
As Steve said, being able to understand and calculate a load line is an important skill to learn when you transition from solder-by-numbers to designing an amp.Stevem wrote: ↑Sun Dec 26, 2021 1:42 pm Output transformers are not just selected by stating that the output tubes will be grid biased and how many watts your looking to achieve!
Wattage is produced by a combination of voltage applied to the plate of the tubes and the current they are allowed to draw ( just like you see in the ohms law wheel you can find on line) in terms of driving a given speaker load.
If you go to the Duncan amp site and do a search in there tube data section you will find the info you need .
In my opinion you need to learn how to do this and not rely on others so much.
Read this:
http://www.valvewizard.co.uk/se.html
...then this:
http://www.valvewizard.co.uk/pp.html
Single ended output transformers have to deal with a DC bias and are gapped to prevent core saturation. A larger core mass is needed while a push-pull transformer does not have to cope with an offset since its crossover point is 0V. So a SE OT ends up costing as much or more than a comparable push-pull transformer. You can develop far more power for a lot less money with push-pull. However, there are those who simply prefer the sound of single-ended amps, so the added cost is just the price of the tone.Also do you want this output stage to be class A , or class ab1.
A class A type output transformer capable of providing 30 watts of rms power will cost a arm and a leg, so I will assume you are looking at using a class ab1 output stage and this is how you will find the beginning of listings on the Duncan site once you get to the 6L6GC listing .
Re: Output Transformer Selection
I have the SE and DE information from Merlin's website and have been reviewing that. I agree, I do need to get better at load lines!
Just to clarify a couple of points, I intend on doing a class AB1, for what I understand is greater efficiency and less breakup at higher volumes.
Thanks for passing along this information, I'm going to start chewing through that.
Just to clarify a couple of points, I intend on doing a class AB1, for what I understand is greater efficiency and less breakup at higher volumes.
Thanks for passing along this information, I'm going to start chewing through that.
Re: Output Transformer Selection
You at least would have to specify the HT for your project. What's the power transformer's spec? Solid state or tube rectifier? If tube rectifier - which tube?
Re: Output Transformer Selection
Well... f*ck if I know, ha ha! Been working on amp crap all day and about burned out.
I assembled my initial power circuit the other day (see attached) and I had a whopping +496VDC for HT+ but that's unloaded. I am also trying to figure out the damn nodal resistors to supply whatever in the holy heck preferred voltages are needed for the 12AX7s, reverb transformer and.... probably some other stuff. Love some of the references I've been trying to learn from: In one case, the author uses 290V for preamps but oh wait, he changes it to 270 volts for effects loops... and I'm sure he changes it elsewhere that I've since forgotten due to burn out.
ANYWAY.... I am going to guess I can supply a HT+ of 360V to the 6L6 tubes, which is again confusing because in order to draw the load lines, I need to know the Ra-a, right? So, as I skip on down through the 6L6 specs, I am presented with three columns for Class AB1: two for what appear to be a plate voltage of 360V and a third for 450V. I don't think I can supply 450V with what I've got so I'd use the 360V columns to determine the Ra-a to create load lines BUT one column shows 6600 ohms, while the other column shows 3800 ohms! Um, say what? Also see attached.
I am going to look for another datasheet to see if there is a misprint or something. To throw something else into the mix, an existing datasheet I had for JJ shows a Push-Pull Ra-a of 5500 ohms... great... just great....
I assembled my initial power circuit the other day (see attached) and I had a whopping +496VDC for HT+ but that's unloaded. I am also trying to figure out the damn nodal resistors to supply whatever in the holy heck preferred voltages are needed for the 12AX7s, reverb transformer and.... probably some other stuff. Love some of the references I've been trying to learn from: In one case, the author uses 290V for preamps but oh wait, he changes it to 270 volts for effects loops... and I'm sure he changes it elsewhere that I've since forgotten due to burn out.
ANYWAY.... I am going to guess I can supply a HT+ of 360V to the 6L6 tubes, which is again confusing because in order to draw the load lines, I need to know the Ra-a, right? So, as I skip on down through the 6L6 specs, I am presented with three columns for Class AB1: two for what appear to be a plate voltage of 360V and a third for 450V. I don't think I can supply 450V with what I've got so I'd use the 360V columns to determine the Ra-a to create load lines BUT one column shows 6600 ohms, while the other column shows 3800 ohms! Um, say what? Also see attached.
I am going to look for another datasheet to see if there is a misprint or something. To throw something else into the mix, an existing datasheet I had for JJ shows a Push-Pull Ra-a of 5500 ohms... great... just great....
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Stevem
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Re: Output Transformer Selection
Class ab1 does not per say have less break up, but it can have a different harmonic content as compared to class A when it does start to clip from being overdriven.
This is another slice of info you should learn about that can have a big impact on what you hear!
When any tube ( or SS device) clips it produces small amounts of harmonic over tones above the root note, or notes you are playing.
This is where the somewhat crazy term of good sounding or bad sounding distortion comes into play.
It not uncommon for combinations of these odd and even harmonic overtones to produce beating sounds against each other which makes for out of tune sounding notes and harshness to your ears .
This is another slice of info you should learn about that can have a big impact on what you hear!
When any tube ( or SS device) clips it produces small amounts of harmonic over tones above the root note, or notes you are playing.
This is where the somewhat crazy term of good sounding or bad sounding distortion comes into play.
It not uncommon for combinations of these odd and even harmonic overtones to produce beating sounds against each other which makes for out of tune sounding notes and harshness to your ears .
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Output Transformer Selection
I believe I want to operate the amp in AB1, not Class A, as I'd like to have as little breakup as possible.
I read on one of the many resources I've been using that the Class AB tends to sum the odd order harmonics, which essentially adds up as an even order harmonic. Maybe I read that wrong, so please correct me if that's not the case.
Another piece of info I should mention is that the intention of this amp is to be a single-channel, low clipping circuit to be used as a pedal platform amp. It'll be nice to have a good clean sounding tone by itself but should also handle distortion pedals that will be coming in on the front end of the amp.
I read on one of the many resources I've been using that the Class AB tends to sum the odd order harmonics, which essentially adds up as an even order harmonic. Maybe I read that wrong, so please correct me if that's not the case.
Another piece of info I should mention is that the intention of this amp is to be a single-channel, low clipping circuit to be used as a pedal platform amp. It'll be nice to have a good clean sounding tone by itself but should also handle distortion pedals that will be coming in on the front end of the amp.
Re: Output Transformer Selection
As a starting point I entered the little information available (tube type + HT) into G.Amato's Loadline Calculator, derived screen voltage from HT and grid bias and aimed for the knee zone of the 0V line with these parameters: 360V HT, 320V Screen. OT impedances around 5k look about right. This is for a cathode biased setup. With fixed bias you'd probably have a slightly higher screen voltage, resuting in a somewhat lower impedance ~ 4k..5k. That's where I'd start from.
Hammond makes a number of suitable OTs...
Hammond makes a number of suitable OTs...
Re: Output Transformer Selection
Wow, that was very nice of you to do that! I didn't know that website existed but I've saved it now! I will look through that a bit more but it sounds like you have suggested a good direction to follow.tschwarz wrote: ↑Mon Dec 27, 2021 8:30 am As a starting point I entered the little information available (tube type + HT) into G.Amato's Loadline Calculator, derived screen voltage from HT and grid bias and aimed for the knee zone of the 0V line with these parameters: 360V HT, 320V Screen. OT impedances around 5k look about right. This is for a cathode biased setup. With fixed bias you'd probably have a slightly higher screen voltage, resuting in a somewhat lower impedance ~ 4k..5k. That's where I'd start from.
Hammond makes a number of suitable OTs...
I didn't see an expected power output in that chart. Is more info needed to get that or do the results provide values that can be used to estimate power?
Re: Output Transformer Selection
Look again!I didn't see an expected power output in that chart. Is more info needed to get that or do the results provide values that can be used to estimate power?
Right hand column, Output Power at g1=0. Theoretical value is 30W for the parameters I set...Re: Output Transformer Selection
See note d on nickb’s IVDS page (ie expect around 30% less than the theoretical output power) http://bmamps.com/ivds.html
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Re: Output Transformer Selection
Couldn't see the forest through the trees on that one!
The reading I've done suggests getting an output transformer about1.5 times larger than the expected output. So, should I select a TX that can handle around 50W?
The reading I've done suggests getting an output transformer about1.5 times larger than the expected output. So, should I select a TX that can handle around 50W?
Re: Output Transformer Selection
Can't go wrong with that strategy. More iron, more boompf - i.e. low end. Although I've heard that some builders like the sound of OTs somehow "magnetically breaking up". I have no first hand experience on this, maybe someone else here does...
Re: Output Transformer Selection
Interesting! I have heard something about that too, I think it was some sort of relationship to the speakers distorting due to the high output from the transformer though?
I wanted to be as clean as possible so certainly upping heat transformer capability to prevent that from happening is more in order for my plan.
I wanted to be as clean as possible so certainly upping heat transformer capability to prevent that from happening is more in order for my plan.