If i make a near-copy of doug hammonds' firefly which uses a single 12au7 tube in self-split push-pull configuration, how do i know what output transformer is appropriate?
The schematic calls for hammond 125a, 22,500 to 8 ohms.
That doesn't seem to jive with the data sheet on the 12au7 which states 6.5k in "vertical" and 7.7k in "horizontal" (and vertical and horizontal mean zilch to me)
I don't know if i'm misinterpreting the schematic or the specifications or the operation of a push-pull amplifier.
I ask because i'm considering using a 12bh7a instead, and perhaps parallel 12bh7a if i feel greedy. The 12bh7a has a different output impedance.
I have a push/pull OT that was originally in service to a pair of 6l6 tubes, and i'd like to find a combination of tubes that will work with the OT that I have.
The schematic calls for hammond 125a, 22,500 to 8 ohms.
That doesn't seem to jive with the data sheet on the 12au7 which states 6.5k in "vertical" and 7.7k in "horizontal" (and vertical and horizontal mean zilch to me)
I don't know if i'm misinterpreting the schematic or the specifications or the operation of a push-pull amplifier.
I ask because i'm considering using a 12bh7a instead, and perhaps parallel 12bh7a if i feel greedy. The 12bh7a has a different output impedance.
I have a push/pull OT that was originally in service to a pair of 6l6 tubes, and i'd like to find a combination of tubes that will work with the OT that I have.
None of those tubes will work well with an output transformer that ran a couple of 6L6's. Is there a reason you want a 1-2W amp or would you like something with more power, I am assuming the OT you have will at least make 10-20W. How much do you want to spend?
The vertical and horizontal refers to TV service where they are used in an oscillator circuit. You need to look at Class A specs. Class A is when the tube is reproducing audio and current is running through the tube all the time.
The vertical and horizontal refers to TV service where they are used in an oscillator circuit. You need to look at Class A specs. Class A is when the tube is reproducing audio and current is running through the tube all the time.
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I want to understand.
I don't want to follow a schematic or build a kit.
I don't want to repair the PA amp that I found and stripped for parts.
I don't want to use 6l6 because it'll be too loud for my home (and it would also be tough to fit in the enclosure I'm hoping to use)
I don't want to spend any money because this is supposed to be a scavenged parts project.
I've collected a stash of NOS tubes, but there's only about a 100 of them and only a few of them are 'common' types. 12ax7, 12au7, 12bh7, 6l6 plus a few others that seem to have some potential.
I have 1 useful OT - so there's no other option there.
I have a poor selection of filter caps scavenged from A/C units. Best - 450V 38uF, second best - 370V 50uF
Smaller resistors I can scavenge. Smaller caps too. I've got a few different sizes of coupling caps that will work.
So I'm building from that.
I don't want to follow a schematic or build a kit.
I don't want to repair the PA amp that I found and stripped for parts.
I don't want to use 6l6 because it'll be too loud for my home (and it would also be tough to fit in the enclosure I'm hoping to use)
I don't want to spend any money because this is supposed to be a scavenged parts project.
I've collected a stash of NOS tubes, but there's only about a 100 of them and only a few of them are 'common' types. 12ax7, 12au7, 12bh7, 6l6 plus a few others that seem to have some potential.
I have 1 useful OT - so there's no other option there.
I have a poor selection of filter caps scavenged from A/C units. Best - 450V 38uF, second best - 370V 50uF
Smaller resistors I can scavenge. Smaller caps too. I've got a few different sizes of coupling caps that will work.
So I'm building from that.
Well the transformer will not work well with the triode tubes listed. How about listing the other tubes.
This may help you understand.
http://www.ax84.com/p1/P1_Theory_Document.zip
This may help you understand.
http://www.ax84.com/p1/P1_Theory_Document.zip
I don't understand how the theory doc applies to push-pull.
Separate from that:
I think I realized the source of this mistake.
I had originally been planning for a type A single-ended amp and for whatever reason I had zeroed in on 4.5k as the relevant value for the 6l6 tube. (backed away from that idea for multiple reasons) Now I see that 4.5k is the load resistance rather than plate resistance and it's only when the tube is biased a certain way. I was comparing that load resistance value to the plate resistance value of the 12bh7a.
I have no clue what the difference between the two is, but I'm starting to get frustrated.
Tubes and quantities - some of them are used:
4x: 12ax7
10+: 12au7
6x: 6bg6
2x: 6L6
5x of 7867 tubes
10+: 5u4gb
3x: 3B28
2x: 12bh7a
1x of OD 3
1x: 5751
1x: oa4g
3x: 12sn7
3x: 12sl7
3x: 12h6
1x: 6av6
1x: 6x4
2x: 12sn7gta
1x: 6sn7gtb
4x: 115n045
3x: ob2
11x: od3
2x: 6cd6ga
2x: 6aq5a
1x: 6x5gt
My enthusiasm for the topic is at low ebb after an afternoon spent trying to locate multiple shorts in the overhead lights. We must've had a power surge - we had at least 5 ballasts short out at the same time. I'm exhausted.
Separate from that:
I think I realized the source of this mistake.
I had originally been planning for a type A single-ended amp and for whatever reason I had zeroed in on 4.5k as the relevant value for the 6l6 tube. (backed away from that idea for multiple reasons) Now I see that 4.5k is the load resistance rather than plate resistance and it's only when the tube is biased a certain way. I was comparing that load resistance value to the plate resistance value of the 12bh7a.
I have no clue what the difference between the two is, but I'm starting to get frustrated.
Tubes and quantities - some of them are used:
4x: 12ax7
10+: 12au7
6x: 6bg6
2x: 6L6
5x of 7867 tubes
10+: 5u4gb
3x: 3B28
2x: 12bh7a
1x of OD 3
1x: 5751
1x: oa4g
3x: 12sn7
3x: 12sl7
3x: 12h6
1x: 6av6
1x: 6x4
2x: 12sn7gta
1x: 6sn7gtb
4x: 115n045
3x: ob2
11x: od3
2x: 6cd6ga
2x: 6aq5a
1x: 6x5gt
My enthusiasm for the topic is at low ebb after an afternoon spent trying to locate multiple shorts in the overhead lights. We must've had a power surge - we had at least 5 ballasts short out at the same time. I'm exhausted.
Don't know what level of knowledge you have with tube circuits. That document is a good one to get people started and covers most of an amplifier. P-P is just an inverter away, easy to pick up after you have the basics. Couple of good sites to learn from.
The Long-Tail Pair
The Valve Wizard -Long Tail Pair
Nice collection of tubes. I would go with the 6AQ5 for an output tube if you want to use the transformer you have. Theoretically they would be happier with 10k but I tried them with 5k and they seemed to manage as long as the supply voltage is not too high.
The Long-Tail Pair
The Valve Wizard -Long Tail Pair
Nice collection of tubes. I would go with the 6AQ5 for an output tube if you want to use the transformer you have. Theoretically they would be happier with 10k but I tried them with 5k and they seemed to manage as long as the supply voltage is not too high.
I hope i didn't seem ungrateful. I had read that once before at the beginning and it didn't click then. I've read it through twice today and did some of the calculations and it seems that i'm understanding things a little bit better.
The 6aq5 i'll get back to....
I'm still absolutely lost on the OT question.
12au7a has 6500 ohm plate resistance. Single -ended you'd want to reflect 10% back on the tubes, right? So about 650.
But in firefly it's arranging a dual triode as a self split. Does that mean each triode has it's own resistance in parallel with the other and it is therefore 13000 per triode? Then if you add them together in series you get 26000?
That's the best i can come up with.
Is the rule of thumb different for single-ended vs push-pull OT? Not 10%?
Anyways - back to 6aq5. I will start exploring that tube immediately. I have a decent collection of tubes to choose from but the numbers meant almost nothing to me just a few weeks ago. I've been randomly selecting a few and reading about them but i had not gotten around to that particular tube. I had assumed that it was a single triode version of one of the 12v tubes like a few of the others that i had looked at that started with the number 6. It seems to have some promise. Hopefully that's one of the ones that is still new in box.
And the OT has some flexibility as well. If i'm not misunderstanding things then I ought to be able to hook up an 8 ohm speaker to the 4 ohm tap and reflect 10k impedance to the tubes in that way? The OT has 2,4,8,16, 250 ohm and 70v taps on the secondary.
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Not sure where you get the 10%, unless you are thinking about not loading down a source and then you try to have the load 10X the source impedance. Not good for maximum power transfer though.
You are in luck with the transformer, that gives you more options to play with. With thew amount of tubes you have I would go for a simple guitar amp with a cathodyne phase inverter. Fender's Harvard comes to mind. Cathode biased though. Something like this, missing ground on the bridge rectifier though.
You are in luck with the transformer, that gives you more options to play with. With thew amount of tubes you have I would go for a simple guitar amp with a cathodyne phase inverter. Fender's Harvard comes to mind. Cathode biased though. Something like this, missing ground on the bridge rectifier though.
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The harvard looks like a very close match for the parts i've got. I've heard of princeton but not harvard. Thanks.
I also like the looks of the b+. I've got a PT and bridge rectifier combo that gives about 320v when combined with a filter cap. The Harvard has 305v listed to the center-tap of the OT.
Still a few problems. Would need to figure out fixed bias (or install another small PT to give me negative volts. Also need to figure out how to mount those tubes. Can't locate any 7 pin bases.
I also like the looks of the b+. I've got a PT and bridge rectifier combo that gives about 320v when combined with a filter cap. The Harvard has 305v listed to the center-tap of the OT.
Still a few problems. Would need to figure out fixed bias (or install another small PT to give me negative volts. Also need to figure out how to mount those tubes. Can't locate any 7 pin bases.
Just go cathode bias. Gibson used the 6AQ5 in some of their amps. This one is simple enough if you ignore the tremolo.
An externally hosted image should be here but it was not working when we last tested it.
The tubes look really good btw. One is rca and the other sylvania - not sure if that's bad - but they are both clean and don't appear to have been used. The getter isn't white like on some of the other tubes i've scavenged.
Thanks for helping me find drawings too. I'll have to look that over and read about phase inverters. Cathodyne doesn't mean a whole lot to me just yet. I'll get reading.
I am glad you said that about the 12bh7a as well. I thought that i might be able to but i'm kinda overwhelmed at the moment.
Thanks for helping me find drawings too. I'll have to look that over and read about phase inverters. Cathodyne doesn't mean a whole lot to me just yet. I'll get reading.
I am glad you said that about the 12bh7a as well. I thought that i might be able to but i'm kinda overwhelmed at the moment.
I found two new filter caps. 450v 110uF and not extraordinarily large. Now i'm feelin' it.
After giving it some more thought i've been thinking that i still would prefer to use the 12bh7 (for a few reasons) but i think that i can run two of them in parallel and get the plate resistance into a better range for the OT. With 2 in parallel i should be fine to connect an 8 ohm speaker to the 4 ohm tap. It would also allow me to still have a 4 ohm connection (the old 2 ohm tap) and of course 16 ohms (to the old 8 ohm tap).
And then my co-worker just brought me another dozen or so tubes. Nothing majorly interesting. A 6sn7, a 6au5 (i think?) 6bq6? Maybe i should go look again. In any event there is a tiny one that has no marking that i can make out so i'll hopefully be back with a picture of that sometime soon.
After giving it some more thought i've been thinking that i still would prefer to use the 12bh7 (for a few reasons) but i think that i can run two of them in parallel and get the plate resistance into a better range for the OT. With 2 in parallel i should be fine to connect an 8 ohm speaker to the 4 ohm tap. It would also allow me to still have a 4 ohm connection (the old 2 ohm tap) and of course 16 ohms (to the old 8 ohm tap).
And then my co-worker just brought me another dozen or so tubes. Nothing majorly interesting. A 6sn7, a 6au5 (i think?) 6bq6? Maybe i should go look again. In any event there is a tiny one that has no marking that i can make out so i'll hopefully be back with a picture of that sometime soon.
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