I have two Fisher 400’s, both of which I picked up for very little money. One is immaculate (and where it’s not, parts from the other will make it so. My “donor” will put Shell Silverstein’s tree to shame. I also have a super cool chassis from another amp that I want to build a stereo amplifier into. The Fisher 400 will provide the power and output transformers.
My question is, am I limited to building my amp with 7868’s (which is what the Fisher uses)? Or can the output transformers potentially drive other tubes? What parameters can I measure and how can they be correlated to an appropriate output tube?
I am less concerned a put the power transformer because as long as the max voltage is high enough, i can knock it down as needed.
My question is, am I limited to building my amp with 7868’s (which is what the Fisher uses)? Or can the output transformers potentially drive other tubes? What parameters can I measure and how can they be correlated to an appropriate output tube?
I am less concerned a put the power transformer because as long as the max voltage is high enough, i can knock it down as needed.
The 7868 is a member of the 7591 family. O/P "iron" for 7591s works with 6L6 family tubes, provided you don't exceed power handling ability. The Russian 6Π3C-E (6p3s-e) is a nice 5881 equivalent that will easily produce all the power Fisher 400 O/P trafos can handle.
A Mullard style circuit and O/P tube g2 B+ regulated at a fraction of anode B+ rates to yield a satisfactory result.
A Mullard style circuit and O/P tube g2 B+ regulated at a fraction of anode B+ rates to yield a satisfactory result.
Fisher 400's used 6k primary transformers in their early serial number units and 10k primary transformers in their later serial number units. Best thing to do for starters is to measure the primary impedance of the transformers you will be using before you get too far into the design.
7868 and 7591 are the same tube in different bases. Also 6L6 types could be used with either the 6k or 10k primaries, but it will require attention to drive, bias, and screen voltage selection for the primary impedance of the output transformers used.
7868 and 7591 are the same tube in different bases. Also 6L6 types could be used with either the 6k or 10k primaries, but it will require attention to drive, bias, and screen voltage selection for the primary impedance of the output transformers used.
The 7591 and 7591A 'Beam Power' tubes maximum screen ratings are 440V, 3.3 Watts.
The 7868 'Pentode' tube maximum screen ratings are 440V, 3.3 Watts.
The 5881 'Beam Power' tube maximum screen ratings are 270V, 3.0 Watts.
The 6L6, 6L6G, 6L6GA, 6L6GB 'Beam Power' tubes maximum screen ratings are 270V, 2.5 Watts.
The 6L6GC 'Beam Power' tube maximum screen ratings are 450V, 5.0 Watts.
Pick your tubes, 7868 (Pentode), or all the others above (Beam Power). Then be sure to keep them within their maximum ratings.
The 7868 'Pentode' tube maximum screen ratings are 440V, 3.3 Watts.
The 5881 'Beam Power' tube maximum screen ratings are 270V, 3.0 Watts.
The 6L6, 6L6G, 6L6GA, 6L6GB 'Beam Power' tubes maximum screen ratings are 270V, 2.5 Watts.
The 6L6GC 'Beam Power' tube maximum screen ratings are 450V, 5.0 Watts.
Pick your tubes, 7868 (Pentode), or all the others above (Beam Power). Then be sure to keep them within their maximum ratings.
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The 6L6GC is capable of producing more O/P power than the "iron" can handle.
As the "O/P "iron" lacks UL taps, full pentode mode "finals" are pretty well forced. Max. open loop linearity in full pentode mode is obtained, when g2 B+ is regulated at a fraction of anode B+. Therefore, the 270 V. g2 limit of the 5881 is of no practical significance. Regulate g2 B+ at 270 V.
This listing looks interesting. Approx. $60 for the 2 matched pairs of Op tubes strikes me as being reasonable.
As the "O/P "iron" lacks UL taps, full pentode mode "finals" are pretty well forced. Max. open loop linearity in full pentode mode is obtained, when g2 B+ is regulated at a fraction of anode B+. Therefore, the 270 V. g2 limit of the 5881 is of no practical significance. Regulate g2 B+ at 270 V.
This listing looks interesting. Approx. $60 for the 2 matched pairs of Op tubes strikes me as being reasonable.
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new 7868 EH work well but have a flat sound NOS 7868 are expensive 7591 s JJ work very fine with big bass and smooth medium but beware to heating,no more 6,5vac. 7591 tung sol are the best compromise in all case of this family (7591/7868) 6p3S-E are my best choice budget/pleasure,big bass,pleasant voices ,good high
It depends on frequency too. A 50W OT @50Hz will work pretty satisfactory at 100W @ 100Hz as the induction in the core is the same.
Some great info here!! I just happen to have a NOS quartet of the 6n3C which I know work well because I have run them on my Leben recently. A great tube. I am unsure if they are the -e variant, though. I have to look.
I will need to look up what this "Mullard" style circuit is. I have a Knight (Allied) 93-670 (the version without the phonograph on top) in immaculate condition that I really like the late 40's industrial look of. So, my plan is to build this stereo amplifier into this chassis. This has, I believe, 7 octal tube sockets in it (which can be converted to whatever I need, if need be). I don't want or need tone controls, and due to this being my first "build", I am going to do away with the complexity of a phono stage. I have four dials I want to retain on the fascia. They don't all have to be functional - but I am thinking input, balance, power (rotary power switch) and volume.
What drivers and pre-amp tube (if any) does the Mullard type circuit envision? Would love to do it with something that uses octals. . .
I'm going to check on my versions when I get home. Assuming I cannot get it from a serial number, is it simply a matter of putting an ohmmeter across the primaries?mily tubes, provided you don't exceed power handling ability. The Russian 6Π3C-E (6p3s-e) is a nice 5881 equivalent that will easily produce all the power Fisher 400 O/P trafos can handle.
A Mullard style circuit and O/P tube g2 B+ regulated at a fraction of anode B+ rates to yield a satisfactory result.
Assumptions:
Fisher 400 Output transformers, Right?
6,000 or 10k Ohms plate to plate.
How about low power, simple, and no negative feedback? Triode wired Pentodes. Triode wired Beam Power tubes. Direct Heated Triodes. A pair of 6CK4 or other indirectly heated triodes. RF Tetrodes anyone?
When someone mentions a 6L6, I think of the Original Metal one, painted in black. 3 newer types (6L6x and 6L6xx) plates are still only good for 19 Watts. But the 6L6GC plate is good for 30W. Saying 6L6 is neither specific nor accurate (Well, if you mean that Black painted metal one).
I have seen circuits on this forum that use a resistive divider and MOSFET with Pentodes or Beam Power Tubes in Ultra Linear mode.
The creativity and experience of the many on this forum is practically unlimited. We are not bound by assumptions. We are not Forced into anything.
My brother had a 1955 or ’56 Chrysler New Yorker with a Small Hemi Head engine . . . Zoom! In ’62, a person down the street mentioned that Hemi in a Dragster did 175 mph in the quarter (Much more than Zoom).
A big uncut diamond had a bad chip on it. The master cutter looked at the stone, split it, and finished it into 2 very beautiful diamonds.
We assume that an object is good for . . . This, But do not look any further to . . . That.
Now there was a violin player that built an amplifier to put out as many watts as a Small output transformer could muster (there are no low notes in a violin). Now he can play his violin at Yankee Stadium, and every one can hear him play (even in the parking lot). . . . I bet you did not see that coming.
By the way . . . 5881 plate is good for 23 Watts 7027 plate is good for 25 Watts 7027-A plate is good for 35 Watts
And one I had never noticed, until mentioned in this thread: 7581 plate is good for 30 Watts 7581-A plate is good for 35 Watts
Fisher 400 Output transformers, Right?
6,000 or 10k Ohms plate to plate.
How about low power, simple, and no negative feedback? Triode wired Pentodes. Triode wired Beam Power tubes. Direct Heated Triodes. A pair of 6CK4 or other indirectly heated triodes. RF Tetrodes anyone?
When someone mentions a 6L6, I think of the Original Metal one, painted in black. 3 newer types (6L6x and 6L6xx) plates are still only good for 19 Watts. But the 6L6GC plate is good for 30W. Saying 6L6 is neither specific nor accurate (Well, if you mean that Black painted metal one).
I have seen circuits on this forum that use a resistive divider and MOSFET with Pentodes or Beam Power Tubes in Ultra Linear mode.
The creativity and experience of the many on this forum is practically unlimited. We are not bound by assumptions. We are not Forced into anything.
My brother had a 1955 or ’56 Chrysler New Yorker with a Small Hemi Head engine . . . Zoom! In ’62, a person down the street mentioned that Hemi in a Dragster did 175 mph in the quarter (Much more than Zoom).
A big uncut diamond had a bad chip on it. The master cutter looked at the stone, split it, and finished it into 2 very beautiful diamonds.
We assume that an object is good for . . . This, But do not look any further to . . . That.
Now there was a violin player that built an amplifier to put out as many watts as a Small output transformer could muster (there are no low notes in a violin). Now he can play his violin at Yankee Stadium, and every one can hear him play (even in the parking lot). . . . I bet you did not see that coming.
By the way . . . 5881 plate is good for 23 Watts 7027 plate is good for 25 Watts 7027-A plate is good for 35 Watts
And one I had never noticed, until mentioned in this thread: 7581 plate is good for 30 Watts 7581-A plate is good for 35 Watts
Easiest way, is to drive the primary with a small AC voltage (a few dozen volts perhaps, I use an old 25.6VAC Radio Shack power transformer) with a small load on the secondary, like a 10 ohm, several watt resistor. Then you simply do the math. The impedance ratio will be the voltage ratio squared.
For example-
25 volts in, 1 volt out is a 25X ratio. 25x25=625. That's a 625:1 impedance ratio. 625 times the speaker load in ohms. This makes for a 5K load with an 8 ohm speaker, or a 2.5K load with a 4 ohm speaker.
For example-
25 volts in, 1 volt out is a 25X ratio. 25x25=625. That's a 625:1 impedance ratio. 625 times the speaker load in ohms. This makes for a 5K load with an 8 ohm speaker, or a 2.5K load with a 4 ohm speaker.
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Measure impedance across the plate to plate connections. Be sure to load one (and only one) of the secondaries (i.e. an 8 Ohm resistor across the 8 Ohm tap). Plate to Plate impedance is how push pull transformers are rated.
The center tap to plate impedance is 1/4th of the plate to plate impedance. Half the turns = 1/4th impedance (and 1/4th of the plate to plate inductance).
When you are in class A push pull, the plates see an impedance load that is effectively 1/4th of the plate to plate primary impedance. But the essentially 'parallel' action makes each plate seem to be driving 1/2 of the plate to plate impedance (and 1/2 of the plate to plate inductance).
However, in the AB region, where one plate is cut off, the other plate has to drive 1/4th of the plate to plate impedance all by its self. And one tube has to drive 1/4th of the inductance all by itself.
As the push pull transitions from class A to class AB, the load on a tube shifts 2:1.
Surprised?
Oh,
I suggested using a proper load on the secondary. That gives the 'effective' impedance which includes the DCR of the primary, and the DCR of the secondary.
I like Lingwendil suggesting using a 25VAC transformer to drive the OPT (low impedance drive versus the device' higher impedance primary). With the secondary loaded, you get the effective impedance and actual In / Out ratio that includes DCR losses, which is what you get in a real amp. (don't ask about what happens with a real loudspeaker, it is not the same as a load resistor). But with the secondary un-loaded, you get the actual turns ratio. Those are two different quantities.
The center tap to plate impedance is 1/4th of the plate to plate impedance. Half the turns = 1/4th impedance (and 1/4th of the plate to plate inductance).
When you are in class A push pull, the plates see an impedance load that is effectively 1/4th of the plate to plate primary impedance. But the essentially 'parallel' action makes each plate seem to be driving 1/2 of the plate to plate impedance (and 1/2 of the plate to plate inductance).
However, in the AB region, where one plate is cut off, the other plate has to drive 1/4th of the plate to plate impedance all by its self. And one tube has to drive 1/4th of the inductance all by itself.
As the push pull transitions from class A to class AB, the load on a tube shifts 2:1.
Surprised?
Oh,
I suggested using a proper load on the secondary. That gives the 'effective' impedance which includes the DCR of the primary, and the DCR of the secondary.
I like Lingwendil suggesting using a 25VAC transformer to drive the OPT (low impedance drive versus the device' higher impedance primary). With the secondary loaded, you get the effective impedance and actual In / Out ratio that includes DCR losses, which is what you get in a real amp. (don't ask about what happens with a real loudspeaker, it is not the same as a load resistor). But with the secondary un-loaded, you get the actual turns ratio. Those are two different quantities.
I think I already know the answer to this, but if I don’t have a small voltage transformer laying around, I could use my signal generator, yes?
Actually, maybe I can’t. On a slightly unrelated note, why is it that when i set my signal generator to 20 volt sine wave at 60 cycles, my multi tester is only reading about 7 volts?
Could my signal generator be bad? My Multitester? Or am I missing something?
Actually, maybe I can’t. On a slightly unrelated note, why is it that when i set my signal generator to 20 volt sine wave at 60 cycles, my multi tester is only reading about 7 volts?
Could my signal generator be bad? My Multitester? Or am I missing something?
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