As stated above.
I am playing around with the triode of 6U8A as a driver for a 6V6 SE amplifier. It is a simple design so the pentode section is not used. What should I do with the pentode section pins? Do I leave them with no connection? Or do I connected all pins to the ground?
As far as I know, the filament heater for the pentode section cannot be disconnected. So the cathode for the pentode is working whenever the heater is on. I assume if I use only the triode, the pentode section will eventually die due to poisoning from the filament, which is OK with me. However, will it poison the triode too?
I am playing around with the triode of 6U8A as a driver for a 6V6 SE amplifier. It is a simple design so the pentode section is not used. What should I do with the pentode section pins? Do I leave them with no connection? Or do I connected all pins to the ground?
As far as I know, the filament heater for the pentode section cannot be disconnected. So the cathode for the pentode is working whenever the heater is on. I assume if I use only the triode, the pentode section will eventually die due to poisoning from the filament, which is OK with me. However, will it poison the triode too?
Connect the pentode pins to ground if you're not using them. Yes it will poison over time, but it won't affect the triode section. Seems a shame not to find a use for it though.
Many of those triode/pentodes have pentodes which look very nice when configured as triodes. Albeit a different triode - but you may find it useful in another stage somewhere.
That's exactly what I was going to say.
Or you could triode-wire the pentode section and use it as a cathode follower between the triode section and the output tube grid. That would allow you to drive the output tube a little way into grid current. It might also reduce THD in the triode just a bit.
Something like this (but I'm not saying use this exactly)...
Or you could triode-wire the pentode section and use it as a cathode follower between the triode section and the output tube grid. That would allow you to drive the output tube a little way into grid current. It might also reduce THD in the triode just a bit.
Something like this (but I'm not saying use this exactly)...
If a CCS doesn't float your boat because you are focusing on simplicity, then just use the pentode as a triode strapped CF to drive the 6V6. It'll improve the overload recovery, which can only be a good thing in a low power 6V6 SE.
I would agree with you in general. However, I heard from someone stating that the triode section of 6U8A sounds really good. I happened to have a few of 6U8A, which I bought long time ago and have no use for them yet. The triode section looks like a 12AV7 but I have none.
This 6U8A is driving a 6V6 in a single-ended amplifier, correct?
You could use the triode section of the 6U8A as the voltage amplifier/input stage.
Triode wire the 6U8A pentode section and configure that as a cathode follower.
Connect the plate output of the 6U8A triode section to the 6U8A pentode section wired as a cathode follower.
The cathode follower will do a great job of driving the grid of your 6V6 output tube.
It's funny that people say the triode section of the 6U8A sounds good if it's equivalent to a 12AV7. The 12AV7 is not very linear.
Possibly even better -- Use the 6U8A the way it was designed to be used, with the pentode section as the input stage/voltage amplifier, into the triode section as a cathode follower, driving the output tube. That way you'll have lots of gain available for applying negative feedback. (The pentode will have a lot of gain, possibly up to 75X.)
You could use the triode section of the 6U8A as the voltage amplifier/input stage.
Triode wire the 6U8A pentode section and configure that as a cathode follower.
Connect the plate output of the 6U8A triode section to the 6U8A pentode section wired as a cathode follower.
The cathode follower will do a great job of driving the grid of your 6V6 output tube.
It's funny that people say the triode section of the 6U8A sounds good if it's equivalent to a 12AV7. The 12AV7 is not very linear.
Possibly even better -- Use the 6U8A the way it was designed to be used, with the pentode section as the input stage/voltage amplifier, into the triode section as a cathode follower, driving the output tube. That way you'll have lots of gain available for applying negative feedback. (The pentode will have a lot of gain, possibly up to 75X.)
Just use it with the pins left open. But if you commit to that tube, you need 2 holes, two sockets, if you go stereo. Those are cheap tubes, no loss to sell them or pitch 'em. If you want a great sounding amp use a conventional driver.
Yeah, if you look at the 6U8A data sheet, the triode section curves are really ugly. I would pick something else.
jeff
Not exactly how it was “intended“ to be used, but you are onto something. Want a *lot” of gain before feedback? Boot strap the pentode anode load. That triode, while not a great triode, has a lot of gm so it’s Zout is damn low as a follower. Can boot strap and drive a power tube hard. Gain can be above 75x. I’ve gotten about 200x out of 6JW8’s.
Mercury99,
1. Can you please attach a complete and accurate schematic of your 6U8 / 6V6 amplifier.
Before anybody suggests modifying any amplifier, they should start with all the details and information they can get.
2. Using any cathode follower to drive g1 grid of a 6V6 from an RC coupling might give a little more peak power (in one direction/polarity of the signal).
The tradeoff is you have to draw g1 grid current to get that extra power; but the RC coupling can cause Blocking and Bias voltage shift, neither one of those is Hi Fidelity.
3. Even though I have not seen the schematic of your amplifier, and do not know what improvements there might be . . . unless you are not happy with your amplifier, I would suggest you not make any changes.
Instead, a second amplifier in your system might be a good thing to try.
What vacuum tube amplifier aficionado can only put up with one model?
4. Not using the pentode of the 6U8 is not a sin, and not a problem. I would ground the plate, and I would ground the grid.
Grounding the cathode only can increase the possibility of the pentode's cathode's electron cloud to be drawn to either the triodes plate, triodes grid, or both.
. . . Think about it gentlemen.
5. Any amplifier that is built on a PCB is harder to modify than all the point to point amplifiers that I have designed, built, and listened to.
6. Is there anything you need your present amplifier to be/do that it does not do; such as need more gain. If you do not need more gain, adding a pentode stage is going to need you to turn the volume down.
Gain increase and Power increase are not the same; only if you have very small signal voltages can you get more power out of an amplifier that does not have enough gain.
More gain might allow you to hear more noise, and hear more hum.
And Lucky . . .
7. Adding more gain just so we can also add negative feedback is an unknown, until we see a complete and accurate schematic, and until we know something of the gain/phase of the amplifier, including the output transformer that we still do not know anything such as model number.
then there are details of the 6V6 output, such as Beam Power mode, Ultra Linear mode, or Triode wired mode.
8. Something else that we should always remember, a tube amplifier is Only part of a Hi Fi Stereo System.
What loudspeaker models or diy loudspeaker details such as impedance versus frequency, efficiency, etc. do we have?
And what are the signal sources (like full scale voltage out, such as CD, low gain phono preamp or low gain phono cartridge); these details often tell us if there is likely going to need more gain.
9. Adding a pentode gain stage, pentode cathode follower, negative feedback will probably change the sound charicter of the amplifier.
Just my $0.03
1. Can you please attach a complete and accurate schematic of your 6U8 / 6V6 amplifier.
Before anybody suggests modifying any amplifier, they should start with all the details and information they can get.
2. Using any cathode follower to drive g1 grid of a 6V6 from an RC coupling might give a little more peak power (in one direction/polarity of the signal).
The tradeoff is you have to draw g1 grid current to get that extra power; but the RC coupling can cause Blocking and Bias voltage shift, neither one of those is Hi Fidelity.
3. Even though I have not seen the schematic of your amplifier, and do not know what improvements there might be . . . unless you are not happy with your amplifier, I would suggest you not make any changes.
Instead, a second amplifier in your system might be a good thing to try.
What vacuum tube amplifier aficionado can only put up with one model?
4. Not using the pentode of the 6U8 is not a sin, and not a problem. I would ground the plate, and I would ground the grid.
Grounding the cathode only can increase the possibility of the pentode's cathode's electron cloud to be drawn to either the triodes plate, triodes grid, or both.
. . . Think about it gentlemen.
5. Any amplifier that is built on a PCB is harder to modify than all the point to point amplifiers that I have designed, built, and listened to.
6. Is there anything you need your present amplifier to be/do that it does not do; such as need more gain. If you do not need more gain, adding a pentode stage is going to need you to turn the volume down.
Gain increase and Power increase are not the same; only if you have very small signal voltages can you get more power out of an amplifier that does not have enough gain.
More gain might allow you to hear more noise, and hear more hum.
And Lucky . . .
7. Adding more gain just so we can also add negative feedback is an unknown, until we see a complete and accurate schematic, and until we know something of the gain/phase of the amplifier, including the output transformer that we still do not know anything such as model number.
then there are details of the 6V6 output, such as Beam Power mode, Ultra Linear mode, or Triode wired mode.
8. Something else that we should always remember, a tube amplifier is Only part of a Hi Fi Stereo System.
What loudspeaker models or diy loudspeaker details such as impedance versus frequency, efficiency, etc. do we have?
And what are the signal sources (like full scale voltage out, such as CD, low gain phono preamp or low gain phono cartridge); these details often tell us if there is likely going to need more gain.
9. Adding a pentode gain stage, pentode cathode follower, negative feedback will probably change the sound charicter of the amplifier.
Just my $0.03
Last edited:
rongon,
Thanks!
I just finished re-editing my Post # 17 . . . After you read it.
There is even more information (or maybe more dis-information).
You may, or may not, want to re-read my post.
Thanks!
I just finished re-editing my Post # 17 . . . After you read it.
There is even more information (or maybe more dis-information).
You may, or may not, want to re-read my post.
6A3sUMMER,
I always enjoy your comments and suggestions. Let me answer some of your questions.
As you may find, I am playing around different circuits and trying to listen to them for the sound described by people who designed them.
So I built the described amp with point to point on a wooden board. Except I used half of a 832A instead of 6V6. I was told to find some rich and warm sound from 6U8A triode. It sounds OK as built. I don't quite hear the warm part. Hopefully I will discover it one day. When I built it, I did not even think about what to do with the pentode pins so they were not connected to anything. The schematic is below.
The use of triode/pentode combination piqued my interest on them. So I looked into the books and found 6KT8 tube. The triode has low rp and high gain. I did a simulation and found it to be quite nice to drive a 2A3. It might work as well as 6SL7. However, I started to realize that I should not leave the pentodes unconnected. So I simulated one simple driver circuit similar to JE Labs Simple 2A3 and one with cathode follower using the pentode section triode strapped. I am aware that, for the cathode follower, I should aim for a plate voltage close to half of the HT. But I decided to try it anyway.
This is the Simple 2A3 with 6KT8 triode:
This is with pentode triode strapped cathode follower:
I built the triode with cathode follower and found two things. First of all, the two Sylvania tubes and two GE tubes are way off from the datasheet. I got 1.9V cathode voltage for the triode instead of 1.6V from the datasheet. Secondly, the two GE tubes sound good. The two Sylvania do not sound very clear. I then discovered that both JE Labs and Thorsten Loesch mentioned the sound of cathode follower is not as natural as simple triode driver for 2A3 or 300B amps. I also learned from the Valve Wizard that grid current flows to the cathode follower and created distortion that is good for guitar amp. So I want to try the simple one next as I have them all hooked up. Thus the question on what to do with the pentode pins as I removed the resistors and capacitors are there.
I welcome any suggestions on how to improve the cathode follower design. I will be happy to try them. By the way, I could not see how the cathode follower will have grid current as described by Merlin. Maybe I am missing something.
I have several 3.5K and 5K OPTs. I have 8 pairs of different speakers from DIY to vintage. I listen to them for my own fun and learning.