Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Stuff4bikes on January 23, 2022, 12:36:20 am
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Can someone confirm/explain this circuit....
Why no coupling cap between the first two stages?
Is that negative bias on the third tubes grid and cathode?
What is going on below the 809 (output tube)?
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The 2nd stage cathode resistor is a very high value. So although the VDC on the grid will be high, the cathode voltage can also be high. And what matters for the operating point is the differential between them. Akin to a DCCF, although this is a DCCC :icon_biggrin:
Yes, negative bias for the 3rd stage grid, actually dual rail operation. Maybe to allow bigger signal swing on its cathode. It’s a CF power amp stage, operating with mixed bias, both cathode bias and adjustable fixed bias.
The output stage cathode perhaps has a dedicated NFB winding on the OT.
The output valve looks to be intended for A2 operation, ie the 3rd stage cathode signal can maintain signal linearity when the output stage grid signal rises above its cathode voltage and it draws significant current.
This looks to be a SE triode hifi amp. Those are pretty specialised things, a design goal here looks to be to minimise phase shift in the audio range. The OT is likely to be super high spec and expensive.
Dunno how happy it would be with being overdriven :icon_biggrin:
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Thank you for the response....I am almost positive it is hi-fi...I am somewhat new to all this and just
Wanted to know how "different" (to me) circuits can still achieve the same goal...
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I hope my post made sense, please ask if you’d like any clarification.
You won’t see any guitar amps with the design features you asked about.
That subset of valve hifi is pretty esoteric :icon_biggrin:
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I guess I should say that I have this tube in an old piece of working non-audio equipment......
And it's an impressive looking tube.....just wondered if the tube could be ran in a more conventional way.
I have read somewhere with around 400v you can get around 10w out of it.....
Yes, I know there may be easier roads to 10w but I like to tinker.
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It's a hum dinger pot. The tube has a directly heated cathode. The cathode serves as the heater element. Per the datasheet heater voltage is 10 volts AC or DC.
https://frank.pocnet.net/sheets/049/8/803.pdf (https://frank.pocnet.net/sheets/049/8/803.pdf)
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...The output valve looks to be intended for A2 operation, ... grid signal rises above its cathode voltage and it draws significant current.....
The 809 can't hardly pass any plate current unless the grid is well positive. It was designed to cut-off near zero grid. It really only makes sense in class B or C, making big distortion. In radio transmitters that distortion can be filtered-off.
Yes the datasheet shows audio conditions, 120 Watts in push-pull. But the grids are swung over 80V positive and suck over 100mA of current. The only way this can make sense is in a communications (military, police) transmitter system where it is expedient to use the same power tubes in the AF and RF stages, and where some unpleasant distortion won't matter.
@66Strat -- I think you have linked 803 data instead of 809 data. They are similar periods and indeed an 809 might be the RF driver for an 803 final, but the 803 is a real beast.
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Thank you guys for destroying my "pipe dream".....
I tried posting a picture of the piece of equipment this tube is used in but my pics are to large...
It is used in something called a "chronosonic ultrasound" it is some kind of portable massager from the 50's(?)
It is in a finger jointed wooden enclosure with anodized gold faceplate and faux gator tolex. Shaped very much like the early combo amps
Of the same era.
I bought the item off offer up just for the faceplate, knobs and pilot light, didn't realize it was tube driven....
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...The output valve looks to be intended for A2 operation, ... grid signal rises above its cathode voltage and it draws significant current.....
The 809 can't hardly pass any plate current unless the grid is well positive. It was designed to cut-off near zero grid. It really only makes sense in class B or C, making big distortion. In radio transmitters that distortion can be filtered-off.
Yes the datasheet shows audio conditions, 120 Watts in push-pull. But the grids are swung over 80V positive and suck over 100mA of current. The only way this can make sense is in a communications (military, police) transmitter system where it is expedient to use the same power tubes in the AF and RF stages, and where some unpleasant distortion won't matter.
@66Strat -- I think you have linked 803 data instead of 809 data. They are similar periods and indeed an 809 might be the RF driver for an 803 final, but the 803 is a real beast.
I did. When I first looked at the schematic, I read 803 instead of 809. Both tubes have directly heated cathodes. I have seen 803 tubes triode connected and used in single end triode amps.
https://frank.pocnet.net/sheets/049/8/809.pdf (https://frank.pocnet.net/sheets/049/8/809.pdf)
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The 2nd stage cathode resistor ...
... the 3rd stage grid ... The output valve looks to be intended for A2 operation, ie the 3rd stage cathode signal can maintain signal linearity when the output stage grid signal rises above its cathode voltage and it draws significant current. ...
The 809 can't hardly pass any plate current unless the grid is well positive. It was designed to cut-off near zero grid. It really only makes sense in class B or C, making big distortion. ...
Meanwhile, 2nd gain stage is split-load (though only using one output) and 3rd gain stage is a cathode follower. So we have a 1st gain stage that actually has gain, followed by two stages that have no-gain.
One would need to investigate whether the output of the 1st stage can even be large enough to drive the 809 (since it will get no help from the later stages).
... The output stage cathode perhaps has a dedicated NFB winding on the OT. ...
I am not yet convinced this is anything other than a drawing of the heater winding, with a 1Ω resistor indicated as a means of measuring idle current in the 809.
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There are plenty of "hi-fi audio" plans that paint themselves into a corner to address some supposed disadvantage in "normal circuits" then need some serious heroics to dig themselves back out. I suspect this circuit is something along those lines.
Thank you guys for destroying my "pipe dream".....
The 809 has a plate dissipation of 25-30w. Use a 6L6 variant and you save yourself a lot of trouble from a lot of directions.
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The 2nd stage cathode resistor ...
... the 3rd stage grid ... The output valve looks to be intended for A2 operation, ie the 3rd stage cathode signal can maintain signal linearity when the output stage grid signal rises above its cathode voltage and it draws significant current. ...
The 809 can't hardly pass any plate current unless the grid is well positive. It was designed to cut-off near zero grid. It really only makes sense in class B or C, making big distortion. ...
Meanwhile, 2nd gain stage is split-load (though only using one output) and 3rd gain stage is a cathode follower. So we have a 1st gain stage that actually has gain, followed by two stages that have no-gain.
Thank you guys for destroying my "pipe dream".....
The 809 has a plate dissipation of 25-30w. Use a 6L6 variant and you save yourself a lot of trouble from a lot of directions.
I have built a couple single and PP 6l6gb and gc amps, just wanted to see if I could try something different..
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I have built a couple single and PP 6l6gb and gc amps, just wanted to see if I could try something different..
Understood, but I also floated a similar idea here almost 20 years ago. In my case, I was looking at using an 813 (with a 125w plate dissipation).
So I'm on your side when I tell you using an 809 will be more hassle than it's worth, and you will end up with output you could have gotten with a 6L6 (with immeasurably less difficulty).
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You are definitely correct, probably the reason there's only a few 809 builds on the net...but hey, dare to dream right?
Well back to "obscure guitar tube" drawing board.....maybe 6AS7g or Eimac 4-65a's surely they aren't complicated...lol
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The 4-65A makes a good night light/hand warmer. :l2:
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:laugh:
mine are only good for day-light
they'd probably make a hellava SE amp though :icon_biggrin:
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… 2nd gain stage is split-load (though only using one output) and 3rd gain stage is a cathode follower. So we have a 1st gain stage that actually has gain, followed by two stages that have no-gain.
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I acknowledge that the 2nd stage, with equal value anode and cathode resistors, looks kinda cathodyne-y, but the cathode is fully bypassed.
So no cathode degenerative NFB, and the stage will have gain.
Due to the low value of the anode resistor it’ll probably be lower than that of the input stage. And the max available voltage swing will be restricted by the elevated cathode.
I am not yet convinced this is anything other than a drawing of the heater winding
But if it were, no heater current could flow?
The heater supply needs to be connected across the heater-cathode pins, rather than to 0V common.
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....it's an impressive looking tube.....
Another.
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....it's an impressive looking tube.....
Another.
Guessing a 30 watt cathode resistor wouldn't cut the mustard on that... :laugh: