Greetings, Friends. I've been ruminating on a new power amp. I'd like to use a pair of 18w 5k SE OPTs on hand, and I'd like to use some of these Soviet tubes I've collected, namely Gu-50 power tubes and 6N6P drivers. Stereo amp, tube rectified PS, cathode biased, not too much silicone.
I like this design I found over on ValvesNMore.com
It has the 6N6P as voltage amplifier and cathode follower-driver and the Gu-50 in triode mode with g2 and g3 wired to the plate. But it calls for a 3.5k OPT. How would this circuit behave with a 5k OPT?
This chart from Vtaidy seems to look promising, with 380v on the plate, 80 mA, 52v on the cathode and a 650 ohm 20W cathode resistor
So if I can get a b+ of 420v I should be in good shape. Thoughts?
I've also considered Skunkie's 300b, which does call for a 5k OPT. Could I drop a Gu-50 into that circuit, adding a 12.6v heater supply?
I'd like to use a Noval driver tube, as I have 6N1P, 6N2P, and 6N23P, all dual triodes, as well as the 6E5P Tetrode, and I'd like to experiment with different drive styles with this amp. But the main thing is a good output stage, so my question is, Will the Gu-50 work with the 5k OPT?
Is it possible to use a voltage doubler on the 6.3v winding of the PT to power the Gu-50s? 1.8A@12.6V is required, plus another 1.8A@6.3V for the driver tubes. All tubes will have unipotential cathodes, and it would be nice not to have to buy the additional transformer for the 12.6v supply.
thanks for taking a look!
will
I like this design I found over on ValvesNMore.com
It has the 6N6P as voltage amplifier and cathode follower-driver and the Gu-50 in triode mode with g2 and g3 wired to the plate. But it calls for a 3.5k OPT. How would this circuit behave with a 5k OPT?
This chart from Vtaidy seems to look promising, with 380v on the plate, 80 mA, 52v on the cathode and a 650 ohm 20W cathode resistor
So if I can get a b+ of 420v I should be in good shape. Thoughts?
I've also considered Skunkie's 300b, which does call for a 5k OPT. Could I drop a Gu-50 into that circuit, adding a 12.6v heater supply?
I'd like to use a Noval driver tube, as I have 6N1P, 6N2P, and 6N23P, all dual triodes, as well as the 6E5P Tetrode, and I'd like to experiment with different drive styles with this amp. But the main thing is a good output stage, so my question is, Will the Gu-50 work with the 5k OPT?
Is it possible to use a voltage doubler on the 6.3v winding of the PT to power the Gu-50s? 1.8A@12.6V is required, plus another 1.8A@6.3V for the driver tubes. All tubes will have unipotential cathodes, and it would be nice not to have to buy the additional transformer for the 12.6v supply.
thanks for taking a look!
will
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GU-50's don't look all that great as SE triodes. PP comes out much better but I might suggest SE pentode with plate to plate feedback using a pentode driver. The SE pentode loadline can be found that has a lower THD than the triode, higher power and with the local feedback will give excellent distortion. I'm working up a breadboard of something like this right now. Probably be a couple of weeks yet before I have it running for tests.
Good luck with your project,
John
Good luck with your project,
John
You can use the "headroom" field at the bottom of the parameters to see a prediction of 2nd, 3rd and 4th harmonic distortion numbers and THD. Sure, it is only a prediction like any sim but by trying different op points you can see relative trends.
5K vs 3.5K transformer will result in slightly less power out and lower distortion. Some people have been satisfied with GU50 SE output.
How efficient are your speakers and what listening level is your expectation? What genre music?
GU50/EF80
Gu50 vs 300b
GU50 SE questions
The search box is your friend.
How efficient are your speakers and what listening level is your expectation? What genre music?
GU50/EF80
Gu50 vs 300b
GU50 SE questions
The search box is your friend.
Yes I imagine I'd need to elevate the heaters if I use Cathode drive...but 6N6P is a robust tube, gotta see what the datasheet will allow.
Right now I listen to a pair of Polk Monitor 60s, over a decade old. They do quite well with a DIY 6B4G SET. I also have a pair of Overnight Sensations, and a pair of Fostex 8" fill range still in the boxes. I plan to try open baffle and back-loaded horn enclosures, when I get time to build em.
Gu-50 can make way more power than I need, it's more about building an amp with parts I already have. Interested in A2 drive, but no idea how to build such a circuit.
w
Right now I listen to a pair of Polk Monitor 60s, over a decade old. They do quite well with a DIY 6B4G SET. I also have a pair of Overnight Sensations, and a pair of Fostex 8" fill range still in the boxes. I plan to try open baffle and back-loaded horn enclosures, when I get time to build em.
Gu-50 can make way more power than I need, it's more about building an amp with parts I already have. Interested in A2 drive, but no idea how to build such a circuit.
w
6N6P has a screen between the sections, according to the datasheet. Perhaps you could use both sections from one tube for the input stages and both from the other for the cathode followers, each tube with their own heater winding.
OTOH, I would question the need for the cathode follower at all. 6N6P a single sections from a 6N6P gain stage should have the "grunt" needed to drive a GU50 on its own, and since the cathode followers are cap coupled to the output tubes they won't be useful for class A2 drive anyway.
OTOH, I would question the need for the cathode follower at all. 6N6P a single sections from a 6N6P gain stage should have the "grunt" needed to drive a GU50 on its own, and since the cathode followers are cap coupled to the output tubes they won't be useful for class A2 drive anyway.
I agree on the no-need for the cathode follower, also because it runs at the same idle current of the driver, so no gain for the slew-rate as well.
You can suppress it and use ose one triode less. Pentode with plate-to-plate feedback gives more power and sounds good as well.
You can suppress it and use ose one triode less. Pentode with plate-to-plate feedback gives more power and sounds good as well.
Your schematic shows 12.5VAC for the GU50 heaters. But then you talk about using a voltage doubler to get 12.6VDC... 1.8A for the heater is going to be a lot at 12.6V - and if you use a doubler to get 12.6VDC, your transformer needs to be pretty beefy. Consider 5A rated minimum and that is for each channel if you go for the doubler. Somehow I doubt your mains transformer will have that covered...
Anyway, it would be interesting. I would go for separate heater transformer(s). Sometimes you can pick up these 12V ballast transformers cheap. You can use them to breadboard your build at least and see if 12VAC will be good enough.
I agree that the cathode follower is probably not needed. I would use grid stoppers. Your power supply design will be really important. That will decide if it sounds good or not.
Anyway, it would be interesting. I would go for separate heater transformer(s). Sometimes you can pick up these 12V ballast transformers cheap. You can use them to breadboard your build at least and see if 12VAC will be good enough.
I agree that the cathode follower is probably not needed. I would use grid stoppers. Your power supply design will be really important. That will decide if it sounds good or not.
This is a variation of what I'm testing, adapted for 5k load.
Biasing at around 80% you get around 15 Wrms at 5% THD, 1 Wrms at 0,8% THD, always 2nd harmonic dominant, DF around 3.
The pmosfet dissipates around 6W.
Just a sketch, needs further optimisation.
Biasing at around 80% you get around 15 Wrms at 5% THD, 1 Wrms at 0,8% THD, always 2nd harmonic dominant, DF around 3.
The pmosfet dissipates around 6W.
Just a sketch, needs further optimisation.
Are you kidding?
Medium mu VAS stage + CF (overall gain about 10) capacitor coupled to the power tube.
The GU50 bias is 50V, so input must be at least 5V peek up to A1 .... at about 5W output.
The amplifier's data on ValvesNMore.com shows 5.45V RMS input - 7.83V peek for maximum power: more over A1 limit.
IMO it's not optimal construction.
If you want, you could do SRPP with the 6n6p instead of the above schematic. I did it before and it made for a nice little project. One less coupling capacitor, and more gain for your GU50.
Does anyone know Mu for triode strapped GU-50? I have not found it while searching the internet, including Wiki.
I got 3.6 from analyzing a graph. Does that seem reasonable?
I got 3.6 from analyzing a graph. Does that seem reasonable?
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It's got to be something low like that Gimp.
Here, let's do a little calculation. AT 100mA, grid volts change from around -18v to -60v between 200v and 400v anode tension.
So (400v-200v)/(60v-18v) = 4.76 - BUT: I am not sure how g2 and g3 are connected in the above chart.
If I look at some other Tridode curves (from Wavebourn) where both g2 and g3 are connected to the anode, I calculate different values:
If we look at 75ma, going from 120v to around 245v on the anode, the grid voltage goes from around -10v to -40v.
This means mu is around (245v-120v)/(40v-10v) = 4.17
I would tend to trust Wavebourne's chart...
Anyway, hope this helps. As with most soviet era tubes, you can be sure there will be some variation. 😉
Here, let's do a little calculation. AT 100mA, grid volts change from around -18v to -60v between 200v and 400v anode tension.
So (400v-200v)/(60v-18v) = 4.76 - BUT: I am not sure how g2 and g3 are connected in the above chart.
If I look at some other Tridode curves (from Wavebourn) where both g2 and g3 are connected to the anode, I calculate different values:
If we look at 75ma, going from 120v to around 245v on the anode, the grid voltage goes from around -10v to -40v.
This means mu is around (245v-120v)/(40v-10v) = 4.17
I would tend to trust Wavebourne's chart...
Anyway, hope this helps. As with most soviet era tubes, you can be sure there will be some variation. 😉
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