I've started this build and have some questions that perhaps some of the members here can assist me with.
This started out as a clone of the PA from an Univox 1221 but after reviewing Sunns schematics and tones of the older Sunn Sceptre amps I decided to go with that instead. At some point I'll post the entire schematic, however I'll likely use a different preamp circuit.
For now I want to get the PA up and running.
I'm converting the output to regular PP rather than UL. In this case I'll simply add screen resistors, use the OT I've used with 6L6 tubes in the past and call it good?
On the pentode section of the 6AN8 the output is coupled to the triode via a 12pf cap. That seems very small and I don't see how that value can possibly work for lower frequencies. Is that the correct value or should I use a .1 value?
Still working out the details of the Sceptre schematic and there must be a typo; is the Sceptre-2 the correct one regarding the connection of the anode (pentode side) to 'B' ?
edit: I've looked closely at several variations of the circuit produced by Sunn and it is becoming more confusing due to the fact, every schematic is close but slightly different in the anode connection and the 12 pf cap. Hmm..
I'd like to have some opinions regarding the as built progress. I've got most of the phase inverter complete now and used point to point. Not too concerned regarding noise or FB as this is no worse than a Fender or some of the older P to P radio gear from the past. ??
Thanks for any assistance-criticisms, offered,
Ccat.
This started out as a clone of the PA from an Univox 1221 but after reviewing Sunns schematics and tones of the older Sunn Sceptre amps I decided to go with that instead. At some point I'll post the entire schematic, however I'll likely use a different preamp circuit.
For now I want to get the PA up and running.
I'm converting the output to regular PP rather than UL. In this case I'll simply add screen resistors, use the OT I've used with 6L6 tubes in the past and call it good?
On the pentode section of the 6AN8 the output is coupled to the triode via a 12pf cap. That seems very small and I don't see how that value can possibly work for lower frequencies. Is that the correct value or should I use a .1 value?
Still working out the details of the Sceptre schematic and there must be a typo; is the Sceptre-2 the correct one regarding the connection of the anode (pentode side) to 'B' ?
edit: I've looked closely at several variations of the circuit produced by Sunn and it is becoming more confusing due to the fact, every schematic is close but slightly different in the anode connection and the 12 pf cap. Hmm..
I'd like to have some opinions regarding the as built progress. I've got most of the phase inverter complete now and used point to point. Not too concerned regarding noise or FB as this is no worse than a Fender or some of the older P to P radio gear from the past. ??
Thanks for any assistance-criticisms, offered,
Ccat.
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Comparing your two PA schematics I suspect the First is incomplete, and won't work as drawn. The 2nd is more hopeful and shows the 12 pF is not a coupling cap but an anode bypass cap. It will roll off some of the top end well above guitar frequencies.
Cheers
JimG
Cheers
JimG
Found a missing link
Figured out what you noticed too, regarding the incomplete schematic. The power connection between pins 2-6 of the AN8 needed to be added in the first schematic.
I guess what confused me about the 12pf relates to understanding how an AC signal would be rolled off to the HV. Not disagreeing, just don't understand. I'm going to read up on hf roll off.
Thanks for the info.
Ccat.
Figured out what you noticed too, regarding the incomplete schematic. The power connection between pins 2-6 of the AN8 needed to be added in the first schematic.
I guess what confused me about the 12pf relates to understanding how an AC signal would be rolled off to the HV. Not disagreeing, just don't understand. I'm going to read up on hf roll off.
Thanks for the info.
Ccat.
As far as the AC goes there is no difference between "Earth" or HV+. You could just as effectively place the 12pF from anode to ground, but might then need a cap with a higher voltage spec.
As always, read everything the valvewizard has to say on his website. I have bought both his books (Thank you Merlin ��😉
As always, read everything the valvewizard has to say on his website. I have bought both his books (Thank you Merlin ��😉
Actually, the pentode section is directly coupled to the triode section, there is no coupling cap. Notice that the triode section has a cathode resistor of 47K. This high value elevates the voltage at the cathode of the triode to 80 volts while the plate of the pentode is at 70 volts. To me, it seems that a plate resistor for the pentode is missing, I don't see how this cap behaves like a plate load resistor. Perhaps some of the gurus out there can explain.
I have found quite a few of the old schematics bit dodgy. Don't know if it is poor draughtsmanship or deliberate misdirection.
I'm not biased... Really.
I'm using this basic circuit as my bias feed and I've built the output as two channels. Duplicated everything including the 1K resistor onward.
I can't get it to adjust below something like -53 on one channel and -34 on the other. (one chan has an incorrect resistance value. Instead of 47K on the output I mistakenly used a 4.7K)
The adjustment pots are 22K.
The real problem is I don't know enough about divider circuits to calculate the correct values for the resistors... How does the math work here??
The output tubes are 6L6 at about 450 plate volts, grid resistors are 100K so I need something like -26 on each tube. Neighbourhood guess.
Thanks for any assistance; in the mean time I'm going to read up on dividers and bias circuit design,
Ccat.
I'm using this basic circuit as my bias feed and I've built the output as two channels. Duplicated everything including the 1K resistor onward.
I can't get it to adjust below something like -53 on one channel and -34 on the other. (one chan has an incorrect resistance value. Instead of 47K on the output I mistakenly used a 4.7K)
The adjustment pots are 22K.
The real problem is I don't know enough about divider circuits to calculate the correct values for the resistors... How does the math work here??
The output tubes are 6L6 at about 450 plate volts, grid resistors are 100K so I need something like -26 on each tube. Neighbourhood guess.
Thanks for any assistance; in the mean time I'm going to read up on dividers and bias circuit design,
Ccat.
Attachments
Colorcat,
This should probably be a new thread, but I'll try to explain this voltage divider concept.
For example: you have a 10 volt power supply and you have 2 resistors of equal value, say 5k each and you connect the resistors in series. You then connect them to the power supply and measure the voltage across each resistor, you would read 5 volts across each resistor. Essentially, the voltage was halved between the two resistors because they were equal in value. So if you had another pair of resistor, say a 5k and 15k (total resistance is 20k), and you connected them to the same voltage source, you would read 2.5 volts across the 5k (1/4 of 10 volts) and 7.5volts across the 15k (3/4 of 10 volts).
On the circuit you posted, notice that R103 is 47k and the pot is 50k. So if the pot was rotated for full resistance, then the voltage for your bias will be roughly half of input voltage. If you measure across C55, that would be the voltage.
With your plate voltages, I think your bias should be in the -30 to -40 volt range depending on how you like you amp to sound. With -26, your tube may not have along and happy life,I suspect they would be pretty hot at that bias voltage.
This should probably be a new thread, but I'll try to explain this voltage divider concept.
For example: you have a 10 volt power supply and you have 2 resistors of equal value, say 5k each and you connect the resistors in series. You then connect them to the power supply and measure the voltage across each resistor, you would read 5 volts across each resistor. Essentially, the voltage was halved between the two resistors because they were equal in value. So if you had another pair of resistor, say a 5k and 15k (total resistance is 20k), and you connected them to the same voltage source, you would read 2.5 volts across the 5k (1/4 of 10 volts) and 7.5volts across the 15k (3/4 of 10 volts).
On the circuit you posted, notice that R103 is 47k and the pot is 50k. So if the pot was rotated for full resistance, then the voltage for your bias will be roughly half of input voltage. If you measure across C55, that would be the voltage.
With your plate voltages, I think your bias should be in the -30 to -40 volt range depending on how you like you amp to sound. With -26, your tube may not have along and happy life,I suspect they would be pretty hot at that bias voltage.
Well that explains it
Thanks for the explanation. I should be able to work something out tomorrow based on that.
In the explanation I mentioned the actual resistor values used to build the circuit. I didn't follow the schematic due to the fact I had only a 22K pot on hand. I'll change the other values in the morning to try to get the voltage I need. So then perhaps the other two resistors need to be 15K?
I'll have to think on this tonight, but it seems to me the pot has to then range in the upper 60% of the total resistance...
Not sure but I'll have to think on it...
Thanks and if I get through this I may be able to create a thread on determining bias resistor values. If not I'll start a thread for that purpose.
Regards,
Ccat.
Thanks for the explanation. I should be able to work something out tomorrow based on that.
In the explanation I mentioned the actual resistor values used to build the circuit. I didn't follow the schematic due to the fact I had only a 22K pot on hand. I'll change the other values in the morning to try to get the voltage I need. So then perhaps the other two resistors need to be 15K?
I'll have to think on this tonight, but it seems to me the pot has to then range in the upper 60% of the total resistance...
Not sure but I'll have to think on it...
Thanks and if I get through this I may be able to create a thread on determining bias resistor values. If not I'll start a thread for that purpose.
Regards,
Ccat.
It's Done- Nothing Fried!
I finished the power amp build tonight and although I haven't set the bias exactly yet it sounds okay.
There are some problems the first of which may be a non issue.
One power tube runs hotter than the other. Significantly. In searching this symptom on the various Forums I've come across several different answers:
a. Not really a problem;
b. A tube, a screen resistor, that's the way it's supposed to be, a bird, a plane, it's the Miller effect- Stop drinking Miller!
I double checked everything and the component values are good and the screen resistors read 1K, the bias is the same on both tubes and I've swapped the tubes. The problem stays with the socket. I also checked and tightened the tube socket connections.
Please note: This is not built as an Ultra Linear output. It is p p with 6L6 tubes with about 430 VDC on the plates. Currently the bias is set at -26 V. (I'm going to calculate that precisely in the morning.)
Another problem- The FB circuit is a little different; more traditional, (4.7K in par with a 750 pf cap, the resistor-cap pair is connected between the 16 ohm tap and FB on the schematic. A 1K resistor connects between 16 ohm tap-(resistor-cap) and the remaining end goes to ground. A switch that serves to ground point FB cuts Boost. Maybe I don't need the 1K resistor to ground and that is causing oscillations.
When I engage this circuit by disconnecting the ground from FB, I get an oscillation that varies as I turn the volume control, (remember, I only built the PA portion to this so far and I put in a 1 M audio volume control. Just looking for a speculative answer. I don't expect anyone to try to diagnose this problem until I post the finished schematic so save the scream... If you were going to.
Regards, thank you everyone assisting me so far, and good night to all,
Ccat.
I finished the power amp build tonight and although I haven't set the bias exactly yet it sounds okay.
There are some problems the first of which may be a non issue.
One power tube runs hotter than the other. Significantly. In searching this symptom on the various Forums I've come across several different answers:
a. Not really a problem;
b. A tube, a screen resistor, that's the way it's supposed to be, a bird, a plane, it's the Miller effect- Stop drinking Miller!
I double checked everything and the component values are good and the screen resistors read 1K, the bias is the same on both tubes and I've swapped the tubes. The problem stays with the socket. I also checked and tightened the tube socket connections.
Please note: This is not built as an Ultra Linear output. It is p p with 6L6 tubes with about 430 VDC on the plates. Currently the bias is set at -26 V. (I'm going to calculate that precisely in the morning.)
Another problem- The FB circuit is a little different; more traditional, (4.7K in par with a 750 pf cap, the resistor-cap pair is connected between the 16 ohm tap and FB on the schematic. A 1K resistor connects between 16 ohm tap-(resistor-cap) and the remaining end goes to ground. A switch that serves to ground point FB cuts Boost. Maybe I don't need the 1K resistor to ground and that is causing oscillations.
When I engage this circuit by disconnecting the ground from FB, I get an oscillation that varies as I turn the volume control, (remember, I only built the PA portion to this so far and I put in a 1 M audio volume control. Just looking for a speculative answer. I don't expect anyone to try to diagnose this problem until I post the finished schematic so save the scream... If you were going to.
Regards, thank you everyone assisting me so far, and good night to all,
Ccat.
Attachments
Last edited:
What Tests??
What tests should I run to determine whether one output tube is operating in a faulty manner.
I've checked the bias and both tubes are set equally to -26.
What else could cause one tube to run hotter than the other besides a bad coupling cap and I would like to get some other checks to perform.
I have checked the screen and grid resistor values. The grids are at -34.
Already swapped tubes and it stays with the socket.
I have not checked the cap yet but about to start that now.
I should have a schematic done by days-nights end however, the final is not that far removed from the original power amp section listed above.
Ccat.
What tests should I run to determine whether one output tube is operating in a faulty manner.
I've checked the bias and both tubes are set equally to -26.
What else could cause one tube to run hotter than the other besides a bad coupling cap and I would like to get some other checks to perform.
I have checked the screen and grid resistor values. The grids are at -34.
Already swapped tubes and it stays with the socket.
I have not checked the cap yet but about to start that now.
I should have a schematic done by days-nights end however, the final is not that far removed from the original power amp section listed above.
Ccat.
Great
That's a fantastic suggestion. The power supply voltages are all lower by about 10 - 15% since I used a different PT.
I've got to map all that hopefully tonight but I'm still stuck on other stuff.
Thanks,
Ccat.
That's a fantastic suggestion. The power supply voltages are all lower by about 10 - 15% since I used a different PT.
I've got to map all that hopefully tonight but I'm still stuck on other stuff.
Thanks,
Ccat.
I'll check again
Now that you mention it, I probably kept both values in memory. When I first powered up the amp it was set at -34. Then I reset it to -26 for a bias value. Being somewhat new to tube amp electronics I forgot and confused the grid signal voltage, which I never measured, with a bias signal assuming the original bias value was a signal value.
It should currently be set to -26 as that was the last setting. Please excuse the confusing info,
Ccat.
Now that you mention it, I probably kept both values in memory. When I first powered up the amp it was set at -34. Then I reset it to -26 for a bias value. Being somewhat new to tube amp electronics I forgot and confused the grid signal voltage, which I never measured, with a bias signal assuming the original bias value was a signal value.
It should currently be set to -26 as that was the last setting. Please excuse the confusing info,
Ccat.
Okay then. Whatever the bias is, both should be the same. If you find that one tube is still running hotter than the other after replacing the coupling cap, then maybe that tube is oscillating. You might not hear it, and you can only see it with a scope. Did you put the 1K grid stopper resistors? If you did, try increasing the value of the resistor to maybe 2.2K, you can even go higher. Some amps I've seen has 6.8K on the grids. Just make sure the grid stopper is connected as close to the grid pin of the tube.
Good luck, you're almost there.
Good luck, you're almost there.
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