I'm working on an amp which has 2 X cathode biased EL34's and a pair of 6N2P's in the input stage and phase splitter stages.
Originally it had an ECC81 and ECC82, one of these was dead so I replaced both with the 6N2P's.
Doing some testing I've noticed around 16v VDC on pin 2 of the phase splitter. I replaced the 0.22 blocking capacitor suspecting this may be the issue but the problem persists.
There's a grid resistor 1.16M which goes to a tag which is shared with the cathode resistor from pin 3, then a 22.6K to ground.
I've tested continuity from pin 2 to ground to make sure the grid resistors have a path to ground which they do.
As a newcomer to this I am unsure how to proceed.
Hope someone might be able to shed some light.
Regards 👍
Originally it had an ECC81 and ECC82, one of these was dead so I replaced both with the 6N2P's.
Doing some testing I've noticed around 16v VDC on pin 2 of the phase splitter. I replaced the 0.22 blocking capacitor suspecting this may be the issue but the problem persists.
There's a grid resistor 1.16M which goes to a tag which is shared with the cathode resistor from pin 3, then a 22.6K to ground.
I've tested continuity from pin 2 to ground to make sure the grid resistors have a path to ground which they do.
As a newcomer to this I am unsure how to proceed.
Hope someone might be able to shed some light.
Regards 👍
Remove the capacitor, or disconnect one end. Is the 16VDC still on the grid?
What is the DC voltage at the other end of the 1.16M resistor?
If it is 16VDC, then the 16V grid DCV is expected and normal for your circuit,
since there is no other DC path from the grid to another circuit node.
Please post the complete schematic.
What is the DC voltage at the other end of the 1.16M resistor?
If it is 16VDC, then the 16V grid DCV is expected and normal for your circuit,
since there is no other DC path from the grid to another circuit node.
Please post the complete schematic.
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The cap you replaced was a polycarbonate metallized in a thermoset case, normally stable and good quality. Unlikely to cause the problem.
As Ray says, post the schematic, the voltage there might be perfectly legit
As Ray says, post the schematic, the voltage there might be perfectly legit
A positive voltage on the grid is perfectly normal for a cathodyne ... of course the cathode is more positive 🙂
This is the reason why we can use direct coupled cathodyne and first stage .
In this case 16V seems too low , a complete schematic with voltages is needed
https://www.valvewizard.co.uk/cathodyne.html
This is the reason why we can use direct coupled cathodyne and first stage .
In this case 16V seems too low , a complete schematic with voltages is needed
https://www.valvewizard.co.uk/cathodyne.html
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ECC82 is very, very different from the 6N2P. Depending on your topology some voltages may go astray. Schematic please.
A complete and accurate schematic is worth 1,000 words.
Schematics are necessary in order to troubleshoot most problems.
Consider one factor of amplifier design:
At power-on, some of my amplifier's B+ is nearer to 500V than it is to 400V. Then the tubes warm up, draw current, and the B+ settles to the normal operating voltage.
Driver tube plates and phase inverter tubes plates connect through a plate load resistor to B+.
When the tubes are cold, the higher B+ voltage goes through that plate load resistor to the coupling cap, and the other end of the coupling cap goes to the grid return resistor, or grid resistors network which returns to ground.
So, until the tubes warm up, the Total Higher B+ voltage is applied across the coupling capacitor.
Therefore, I never use any coupling capacitor that is not rated for 600V or more.
Design for reliability.
Who said the circuit in question is a cathodyne?
As far as I know, no self-respecting cathodyne phase splitter has Only 16V on the grid. (90V or 100V or more is what is needed).
Perhaps the original tubes died, because there is a problem with the amplifier.
Back to . . . Schematic please.
Schematics are necessary in order to troubleshoot most problems.
Consider one factor of amplifier design:
At power-on, some of my amplifier's B+ is nearer to 500V than it is to 400V. Then the tubes warm up, draw current, and the B+ settles to the normal operating voltage.
Driver tube plates and phase inverter tubes plates connect through a plate load resistor to B+.
When the tubes are cold, the higher B+ voltage goes through that plate load resistor to the coupling cap, and the other end of the coupling cap goes to the grid return resistor, or grid resistors network which returns to ground.
So, until the tubes warm up, the Total Higher B+ voltage is applied across the coupling capacitor.
Therefore, I never use any coupling capacitor that is not rated for 600V or more.
Design for reliability.
Who said the circuit in question is a cathodyne?
As far as I know, no self-respecting cathodyne phase splitter has Only 16V on the grid. (90V or 100V or more is what is needed).
Perhaps the original tubes died, because there is a problem with the amplifier.
Back to . . . Schematic please.
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I have only the schematic/sketch I did myself. Go easy, it's my first attempt at drawing one, most of the resistor values are as measured, I can't guarantee everything is 100% accurate but hopefully it gives an idea on the circuit for those who asked. I appreciate your help, this is a steep learning curve but with the help of this forum and uncle Doug on YT, I am getting my head around it one step at a time!
I guessed that would be the case and expected to have to make some revisions to the circuit to accommodate. I obviously need to do some further reading on this, if you point me in the right direction I'd be grateful.
This is something I was going to mention. The readings I get here seem to fluctuate, a lot. I've taken a few readings and recorded the results and I've seen this as high as 33v .
^^^^^^^^^That!
Also the grids of a LTPI (differential amp PI) typically sit at a positive voltage.
The cathode voltage would give some clue.
Another thing is the reliabilty of the voltage reading.
To roughly measure the grid voltage of a PI, a cathode follower or a fixed biased power tube, the meter input resistance should be at least 10M. Otherwise the reading is likely to be low.
If no full schematic is available, a drawing of the PI would help.
Edit: Missed 2 posts above. So it looks like a cathodyne but the pin 1 wiring is wrong. Coupling cap should go directly to plate.
Also the grids of a LTPI (differential amp PI) typically sit at a positive voltage.
The cathode voltage would give some clue.
Another thing is the reliabilty of the voltage reading.
To roughly measure the grid voltage of a PI, a cathode follower or a fixed biased power tube, the meter input resistance should be at least 10M. Otherwise the reading is likely to be low.
If no full schematic is available, a drawing of the PI would help.
Edit: Missed 2 posts above. So it looks like a cathodyne but the pin 1 wiring is wrong. Coupling cap should go directly to plate.
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Measure the voltage at the lower end of the 1.13M grid leak. That's your real grid voltage.
What is plate supply ("HT")?
What is plate supply ("HT")?
Rikaro,
That circuit with the coupling cap to the cathodyne is a valid topology (there are 2 classical cathodyne circuits: DC coupled from the earlier stage; and cap coupled, as this amplifier uses).
The 1.13 / 1.16 Meg and the (0.76k?) self bias cathode resistor are in a loop from the grid to the cathode. That is OK.
Because there does not appear to be any global negative feedback that is applied before that coupling cap to the cathodyne,
there is no danger of too many coupling cap poles to worry about (like there would be in a Williamson circuit).
Yes, most meters have trouble measuring the grid voltage when the grid resistor is 1.13 / 1.16 Meg. So, instead . . .
Measure the B+ at the top of the cathodyne's plate resistor.
Measure the Plate voltage of the cathodyne.
Measure the Cathodyne voltage.
If that stage is OK, then the voltage across the resistor to the plate; the voltage of the plate to the cathode; and the voltage of the cathode to ground . . .
Will each be approximately 25% to 35% of the total voltage of B+ to ground, with the total adding to 100% of the voltage of that B+ to ground.
That circuit with the coupling cap to the cathodyne is a valid topology (there are 2 classical cathodyne circuits: DC coupled from the earlier stage; and cap coupled, as this amplifier uses).
The 1.13 / 1.16 Meg and the (0.76k?) self bias cathode resistor are in a loop from the grid to the cathode. That is OK.
Because there does not appear to be any global negative feedback that is applied before that coupling cap to the cathodyne,
there is no danger of too many coupling cap poles to worry about (like there would be in a Williamson circuit).
Yes, most meters have trouble measuring the grid voltage when the grid resistor is 1.13 / 1.16 Meg. So, instead . . .
Measure the B+ at the top of the cathodyne's plate resistor.
Measure the Plate voltage of the cathodyne.
Measure the Cathodyne voltage.
If that stage is OK, then the voltage across the resistor to the plate; the voltage of the plate to the cathode; and the voltage of the cathode to ground . . .
Will each be approximately 25% to 35% of the total voltage of B+ to ground, with the total adding to 100% of the voltage of that B+ to ground.
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Speaking of the output coupling cap (0.15µ). In the drawing it's connected the the HT node instead of the plate.
Rikaro,
Good catch!
That would make the push pull amplifier into a . . .
__ pull, __ pull, ____ pull amplifier.
Good catch!
That would make the push pull amplifier into a . . .
__ pull, __ pull, ____ pull amplifier.
Ok, I've made a couple of changes, firstly I've taken the 6BR7's out, these won't be needed. They where for 2 mic inputs and are redundant.
These where the original HV resdings
I've removed the resistor you can see crossed out and changed the resistors marked blue. The reason for this, I only had 216v supplying the valve marked as ECC81 and the specs say 250v.
I assumed (wrongly?) that the resistor (marked 47k) had drifted causing the voltage drop....
I've changed the individual anode resistors to try and get closer to 250v.
These where the original HV resdings
I've removed the resistor you can see crossed out and changed the resistors marked blue. The reason for this, I only had 216v supplying the valve marked as ECC81 and the specs say 250v.
I assumed (wrongly?) that the resistor (marked 47k) had drifted causing the voltage drop....
I've changed the individual anode resistors to try and get closer to 250v.
A good rule of modifying a tube amplifier is to get it working properly first.
Then modify it.
Questions that should have been asked earlier:
Which tube stage has the dead tube?
Is one channel, L or R working, and are you happy with the sound of that working channel?
If you are unhappy, what needs to be improved?
What are the model number of your speakers?
All tube substitutes are equal, but some tube substitutes are more equal than others.
Then modify it.
Questions that should have been asked earlier:
Which tube stage has the dead tube?
Is one channel, L or R working, and are you happy with the sound of that working channel?
If you are unhappy, what needs to be improved?
What are the model number of your speakers?
All tube substitutes are equal, but some tube substitutes are more equal than others.
The amp should be left with original tubes , for beginers is difficult to modified it properly .
It is completly wrong to change resistors like that ... all the blue plate resistors
It is completly wrong to change resistors like that ... all the blue plate resistors
Not the supply voltage is important but biasing the tubes right . The 47K resistor must be modified if the voltage is unacceptable low . 216V vs 250V is not such a big issue , not a reason for the amp not to work .
But if is an old amplifier the filter cap after that 47K resistor could be dried out , that's why the voltage is lower .
To avoid useless questions you should check/change all the electrolytic capacitors before working in the amp .
But if is an old amplifier the filter cap after that 47K resistor could be dried out , that's why the voltage is lower .
To avoid useless questions you should check/change all the electrolytic capacitors before working in the amp .
All electrolytic's have already been replaced, except for those circled. I've ticked the blocking cap on the cathodyne which wasn't an electrolytic but it's been replaced.
The 2 remaining electrolytic's have been confirmed to be not passing DC, they're going to be tricky to replace so I've left these alone for now.
The 2 remaining electrolytic's have been confirmed to be not passing DC, they're going to be tricky to replace so I've left these alone for now.