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In Reply to: RE: phasing issue posted by Tre' on June 28, 2017 at 14:16:54
It's the schematic in the link in the OP of this thread, and all the changes I made are listed in the OP. Just read the OP and look at the schem and you are up to speed.
Follow Ups:
OK, this schematic with a 6dj8 instead of a 6n1p?
To start with if you have 150vdc and 222vdc on the plates of the 6dj8 you will kill the tube.
A 6dj8 is only rated for 130vdc plate.
What about the 130 ohm cathode resistor instead of 400 ohms for the 6bq5s?
What voltage do you have, cathode to ground?
What is your plate voltage on the 6bq5s?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Got a little time to mess with the amp, so I can answer your points in order.1. Got rid of the high plate voltages by changing the 4.3k ohm resistor at the top of the schematic to 20k. This dropped the 150vdc to about 100 vdc and the 222vdc to about 120vdc. That is measuring from plate to cathode to get plate volts. Should be good there now.
2. The cathode resistor is now 130 ohms, as per recommended in the tube manual. This gives a plate current of about 67mA (8.8vdc/130 ohms).
3.Cathode to ground is 8.8 volts.
4.Plate volts (plate to cath) is 320vdc on the 6bq5, vs max of 300vdc in the tube manual, but I can and will reduce that when we get back from our trip.
As a side note, lowering the plate voltages seems to have smoothed out the highs a tad. Could just be me, but it sounds a bit smoother.Next step is to do a a/b comparison with my se 26/26/45 when we get back.
Edits: 06/29/17 06/29/17
Are both 6bq5 cathodes connected to ground through one 130 ohm resistor?At 320 vdc plate to cathode and 8.8 volts bias you should have about 67ma per tube.
I don't get it.
If you're looking for 33.5ma per tube the common cathode resistor should be about 223 ohms. 15 volts / 223 ohms = 67ma.
If you look at the triode plate curves in the link below you will see that about 15 volts bias with 320 volts plate to cathode gives about 33.5ma of current per tube. 2 tubes, 2 times 33.5ma is 67ma.
2 X 33.5ma is 67ma flowing through the one cathode resistor.
Could your tubes be worn out/weak?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 06/29/17 06/29/17 06/29/17
Isn't that what the schematic shows, both to ground through one resistor? That's what I did.
The tube manual says 67mA for the 6bq5. Should I be trying to have that through each tube for a total of 134 mA? I am confused.
No Vinnie, I'm not trying to tell you what to do.I'm saying that if both cathodes are connected to the one common cathode resistor (BTW that is the right way to do it) and there is ONLY 8.8 volts at the cathodes then (according to the plate curves) there should be a lot more current flowing through the tubes.
Something is wrong or your tubes are worn out.
Please look at the triode plate curves that I posted a link to.
Look at 320 volts and 8.8 volts grid.....do you see where the current is?
"The tube manual says 67mA for the 6bq5"
That's Max current and not where you want to operate each tube.
Also, where the data sheet says 130 ohms cathode resistor for 2 tubes that's pentode connected. This is not what you are doing.
The data sheet shows a cathode resistor of 270 ohms for one tube at 250vdc plate.
That would be 135 ohms for 2 tubes but you plate voltage is higher so that's not what you want.
Again look at the triode curves. Everything you need to know is there.
The red dot is at about 8.8 volts bias and 320 volts plate and it's 80ma.
That would be for one tube but you have two tubes.
The way you have it now there should be 160ma of current total.
That's not what you want to do.
Again, the triode plate curves show that at 320 volts plate the bias should be at -15 volts to get about 33.5ma from each tube.
That should take a 223 ohms resistor to give the 15 volts bias and if it doesn't then something is wrong.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 06/30/17
That's what my question was from the start, do I use 67mA or 33.5 mA per tube. Now that I know the answer I will adjust the value of the cathode resistor to give me about 30 mA per tube so I am not on the ragged edge. Thanks.
Edits: 07/03/17
That sounds about right.320vdc times 30ma is 9.6 watts
The 6bq5 will dissipate 14 watts (12 for the plate and 2 for the screen grid)
You could go a little higher safely.
320vdc times 35ma is 11.2 watts and that would be 80% of max dissipation (still in the "safe zone").
BTW with 8.8 volts at the top of the 130 ohm cathode resistor each tube IS running at 33.5 ma but that just shows that something is wrong.With only a 130 ohm cathode resistor those tubes should be running 80ma each.
I think your tubes must be worn out or the plate voltage is not what you said it is or something else is not what you say it is.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 07/03/17
I have to go back and start at the B+ and work forward from there. I want to try the unit staying within the recommended limits in the tube amp, so I want to try 250 vdc on the plates. I will report back with some new numbers when I think I have everything right and see what you think then.
The GE data sheet shows 250vdc plate, 34ma., 270 ohm cathode resistor with a 3.5k plate load for single ended triode connected Class A.So for PP triode connected Class A that would be 250vdc plate, 34ma per tube, 135 ohm common cathode resistor with a 7k plate to plate OPT.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 07/03/17
Thanks. I will start there.
I will try to get back to you tomorrow, but we are leaving to visit my wife's family for several days so it may be next week before I can check them all out out. They all sound like valid points. I did use 130 ohms for the cathode resistors on the 6bq5's, that's in the list of changes I made. Thanks!
Edits: 06/28/17
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