LJT,
Earlier you mentioned trying a long tailed phase splitter. I prefer that over the "serial" signal phase splitter you have. But it is a tradeoff.
The splitter you have has more second Harmonic Distortion out of the first gain stage, the second gain stage of the splitter has less second Harmonic Distortion. So you are sending more 2nd HD to the 'push' EL34 / 6L6 stage; and less 2nd HD to the 'pull' EL34 / 6L6 stage.
But there is more gain in your splitter than in a long tailed spitter, so the open loop to closed loop gain ratio is larger than when using a (lower gain) long tailed splitter.
A long tailed splitter cancels the 2nd HD of the in phase and out of phase signals.
But the gain is about 1/2, so more input signal is required to drive the output stages.
The open loop to closed loop feedback ratio is less (about 6 dB less).
I could give you a schematic example of how I implement a long tailed phase splitter if you ask me (and when I have the time).
Happy listening (that is major part of what it is about).
Earlier you mentioned trying a long tailed phase splitter. I prefer that over the "serial" signal phase splitter you have. But it is a tradeoff.
The splitter you have has more second Harmonic Distortion out of the first gain stage, the second gain stage of the splitter has less second Harmonic Distortion. So you are sending more 2nd HD to the 'push' EL34 / 6L6 stage; and less 2nd HD to the 'pull' EL34 / 6L6 stage.
But there is more gain in your splitter than in a long tailed spitter, so the open loop to closed loop gain ratio is larger than when using a (lower gain) long tailed splitter.
A long tailed splitter cancels the 2nd HD of the in phase and out of phase signals.
But the gain is about 1/2, so more input signal is required to drive the output stages.
The open loop to closed loop feedback ratio is less (about 6 dB less).
I could give you a schematic example of how I implement a long tailed phase splitter if you ask me (and when I have the time).
Happy listening (that is major part of what it is about).
LJT, How could I have missed the mistake in your schematic. I have looked at it many times. But there is a missing part! The top half of V2 does Not have a grid (return) resistor. There should be a resistor from that grid to ground. The maximum grid return resistance for an ECC82 is 1 Meg Ohm.
I bet your actual circuit has a resistor there. Is that correct?
I bet your actual circuit has a resistor there. Is that correct?
One obvious question....are the output tubes vertically or horizontally mounted in that amplifier? A AM radio transmitter manufactured during the 1960's used EL34's as an audio driver for the 4-400A modulator tubes, and the EL34's were horizontally mounted. The EL34's were constantly being replaced until a plug-in adapter was built to orient the EL34's vertical. (Electrode sag was always a problem with EL34's.....)
Lots of new information here. I will address it in order.
Cathode bypass capacitors. The OEM value seem to be 64V From what I read 50-, or even 35 volt could suffice? Anyway I do have 4x polyester caps 100uF/100v that I can make fit, but it will not look nice and it will limit access for measurements, so I will not put them in until I am done playing with other modifications.
g1 coupling capacitors: I measure 152, 154, 160, and 160 volts over the 47K anode resistors of the phase splitters. The common 1K resistors measure about 6.6 volts. The note on the schematics expect this to be 7volt. Anyway, this calculates to an anode voltage of the splitters of approximately 170 volts, so I would expect the 400volt rating to be plenty. Still the PCB is prepared for physically larger caps and I do have some 330nF/630 volt in the drawer. Is it recommended to put these in or should I get hold of some 470nF?
BTW: These measurements indicate about anode-cathode voltage of about 160 and about 3.4mA in each triode. Is this OK for an ECC82/12AU7?
Phase splitter: for each grid measures either 1M or 150K to GND. There left and right circuits use different parts of the valve for inputs. Right channel splitter has input on pin7, left channel on pin2. 1M resistor is located close to the coupling cap with about 2cm (3/4") of PCB track between resistor and socket. 150K resistor has half this track length.
Tube position: Vertical mounting, socket down.
Thanks
Lars
Cathode bypass capacitors. The OEM value seem to be 64V From what I read 50-, or even 35 volt could suffice? Anyway I do have 4x polyester caps 100uF/100v that I can make fit, but it will not look nice and it will limit access for measurements, so I will not put them in until I am done playing with other modifications.
g1 coupling capacitors: I measure 152, 154, 160, and 160 volts over the 47K anode resistors of the phase splitters. The common 1K resistors measure about 6.6 volts. The note on the schematics expect this to be 7volt. Anyway, this calculates to an anode voltage of the splitters of approximately 170 volts, so I would expect the 400volt rating to be plenty. Still the PCB is prepared for physically larger caps and I do have some 330nF/630 volt in the drawer. Is it recommended to put these in or should I get hold of some 470nF?
BTW: These measurements indicate about anode-cathode voltage of about 160 and about 3.4mA in each triode. Is this OK for an ECC82/12AU7?
Phase splitter: for each grid measures either 1M or 150K to GND. There left and right circuits use different parts of the valve for inputs. Right channel splitter has input on pin7, left channel on pin2. 1M resistor is located close to the coupling cap with about 2cm (3/4") of PCB track between resistor and socket. 150K resistor has half this track length.
Tube position: Vertical mounting, socket down.
Thanks
Lars
I'll address the splitter/modification stuff separately from the questions relating to the short life expectancy of EL34 in this amp.
First I plan to do replace the g2 resistors to a higher value, replace coupling capacitors and replace sockets for the small valves.
Grounding wires are a bit all over the place, so I plan to make a nice star grounding post also before I start the other modifications.
Maybe put a switch on the NFB for measuring open loop behavior as well
After that I will measure the transformer ratio, and perform some other measurements so that I have a reference for future modifications (distortion spectra, IM and damping factor). When I start the changes (hopefully in a couple of weeks) I will start a new thread on this, and pick up on the kind offer of assistance.
The though of this is "bugging" me so much I want to do something about it out of principle (cause the amp is really sounding OK already!)
I do have some potential valves laying around (ECC81 / ECC82 / ECC88).
Maybe replacing the ECC83 at the input with a -81, -82 could bring the gain back up? I also have a single unused 12BZ7 that may be used for this purpose.
I have considered putting in an order for a pair of 12BH7 tubes for splitter and possibly the input amp.
Any opinions on this?
The amp is really sounding OK - After all it is a 25W class A, but reminds me more of a transistor thing than valve based.
It is just missing some of the fullness of my friends 300B or my RaySonic CD128. Would be nice with some of the "fun factor" of the Miniwatt as well.
So far I have just been listening thru very detailed, low sensitivity, Seas Magnesium speakers, so I may be giving the amp unfair comparison.
First I plan to do replace the g2 resistors to a higher value, replace coupling capacitors and replace sockets for the small valves.
Grounding wires are a bit all over the place, so I plan to make a nice star grounding post also before I start the other modifications.
Maybe put a switch on the NFB for measuring open loop behavior as well
After that I will measure the transformer ratio, and perform some other measurements so that I have a reference for future modifications (distortion spectra, IM and damping factor). When I start the changes (hopefully in a couple of weeks) I will start a new thread on this, and pick up on the kind offer of assistance.
I always wondered what the implication of "imbalance" between positive and negative going half-waves would be!
The though of this is "bugging" me so much I want to do something about it out of principle (cause the amp is really sounding OK already!)
As long as the stage can still deliver the voltage swing and sufficient current for the output I belive I'll be fine. I belive the PCB lend itself to the LTP changes relatively easy.
I do have some potential valves laying around (ECC81 / ECC82 / ECC88).
Maybe replacing the ECC83 at the input with a -81, -82 could bring the gain back up? I also have a single unused 12BZ7 that may be used for this purpose.
I have considered putting in an order for a pair of 12BH7 tubes for splitter and possibly the input amp.
Any opinions on this?
Listening to Bonnie "Prince" Billy at the moment!
The amp is really sounding OK - After all it is a 25W class A, but reminds me more of a transistor thing than valve based.
It is just missing some of the fullness of my friends 300B or my RaySonic CD128. Would be nice with some of the "fun factor" of the Miniwatt as well.
So far I have just been listening thru very detailed, low sensitivity, Seas Magnesium speakers, so I may be giving the amp unfair comparison.
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Cathode bypass capacitors. The OEM value seem to be 64V From what I read 50-, or even 35 volt could suffice? Anyway I do have 4x polyester caps 100uF/100v that I can make fit, but it will not look nice and it will limit access for measurements, so I will not put them in until I am done playing with other modifications.
Yes, 35V caps will work as long as you are careful when you adjust the bias. 35V / 390 Ohms = 89.7 mA (bias would have to be severely mis-adjusted to have this much current).
g1 coupling capacitors: I measure 152, 154, 160, and 160 volts over the 47K anode resistors of the phase splitters. The common 1K resistors measure about 6.6 volts. The note on the schematics expect this to be 7volt. Anyway, this calculates to an anode voltage of the splitters of approximately 170 volts, so I would expect the 400volt rating to be plenty. Still the PCB is prepared for physically larger caps and I do have some 330nF/630 volt in the drawer. Is it recommended to put these in or should I get hold of some 470nF? BTW: These measurements indicate about anode-cathode voltage of about 160 and about 3.4mA in each triode. Is this OK for an ECC82/12AU7?
The question is, will the power supply rise to more than 400V before the tubes warm up. That could potentially put more than 400V on the caps. That is why I recommended higher than a 400V rating. I would not change the coupling caps capacitance, the feedback loop is counting on a specific phase shift from the RC networks. Changing the uF value might change stability. 160V x 3.4 mA = 0.55 Watts is OK for a 12AU7
Phase splitter: for each grid measures either 1M or 150K to GND. There left and right circuits use different parts of the valve for inputs. Right channel splitter has input on pin7, left channel on pin2. 1M resistor is located close to the coupling cap with about 2cm (3/4") of PCB track between resistor and socket. 150K resistor has half this track length.
I do not see any 1Meg resistor in the schematic. Where is it? Asking for advice on a circuit that is not the same as the schematic does not allow for proper analysis. Changing grid resistors can change the RC network, and the stability. Also it can reflect back on the gain of the two splitter halves (you want the same gain for the 2 halves. The splitter cathode resistor is not a current source, and the 2nd half of the splitter gets its signal from the resistor summing network (the uneven EL34 grid resistors makes up for this).
Originally Posted by 6A3sUMMER The splitter you have has more second Harmonic Distortion out of the first gain stage, the second gain stage of the splitter has less second Harmonic Distortion. So you are sending more 2nd HD to the 'push' EL34 / 6L6 stage; and less 2nd HD to the 'pull' EL34 / 6L6 stage.
I always wondered what the implication of "imbalance" between positive and negative going half-waves would be! The though of this is "bugging" me so much I want to do something about it out of principle (cause the amp is really sounding OK already!)
Originally Posted by 6A3sUMMER .....there is more gain in your splitter than in a long tailed splitter, so the open loop to closed loop gain ratio is larger than when using a (lower gain) long tailed splitter... ....the gain is about 1/2, so more input signal is required to drive the output stages. The open loop to closed loop feedback ratio is less (about 6 dB less)..... As long as the stage can still deliver the voltage swing and sufficient current for the output I belive I'll be fine. I belive the PCB lend itself to the LTP changes relatively easy. I do have some potential valves laying around (ECC81 / ECC82 / ECC88). Maybe replacing the ECC83 at the input with a -81, -82 could bring the gain back up? I also have a single unused 12BZ7 that may be used for this purpose. I have considered putting in an order for a pair of 12BH7 tubes for splitter and possibly the input amp. Any opinions on this?
Changing from one triode to another could require a lot of changes to the rest of the circuitry. You are working with a circuit board, and certain other fixed components, such as output transformers, power supply, etc. Changing to a cathode coupled splitter, and the consequential loss of gain, will change the feedback open loop to closed loop ratio. Damping factor, frequency response, distortion will all change. After you get this amplifier working with new components, solve the EL34 internal shorts, etc. you should enjoy the amplifier for what it is. After that you can start on a new amplifier design from scratch, with different topology, or different tubes, etc. That is a major project.
Yes, 35V caps will work as long as you are careful when you adjust the bias. 35V / 390 Ohms = 89.7 mA (bias would have to be severely mis-adjusted to have this much current).
g1 coupling capacitors: I measure 152, 154, 160, and 160 volts over the 47K anode resistors of the phase splitters. The common 1K resistors measure about 6.6 volts. The note on the schematics expect this to be 7volt. Anyway, this calculates to an anode voltage of the splitters of approximately 170 volts, so I would expect the 400volt rating to be plenty. Still the PCB is prepared for physically larger caps and I do have some 330nF/630 volt in the drawer. Is it recommended to put these in or should I get hold of some 470nF? BTW: These measurements indicate about anode-cathode voltage of about 160 and about 3.4mA in each triode. Is this OK for an ECC82/12AU7?
The question is, will the power supply rise to more than 400V before the tubes warm up. That could potentially put more than 400V on the caps. That is why I recommended higher than a 400V rating. I would not change the coupling caps capacitance, the feedback loop is counting on a specific phase shift from the RC networks. Changing the uF value might change stability. 160V x 3.4 mA = 0.55 Watts is OK for a 12AU7
Phase splitter: for each grid measures either 1M or 150K to GND. There left and right circuits use different parts of the valve for inputs. Right channel splitter has input on pin7, left channel on pin2. 1M resistor is located close to the coupling cap with about 2cm (3/4") of PCB track between resistor and socket. 150K resistor has half this track length.
I do not see any 1Meg resistor in the schematic. Where is it? Asking for advice on a circuit that is not the same as the schematic does not allow for proper analysis. Changing grid resistors can change the RC network, and the stability. Also it can reflect back on the gain of the two splitter halves (you want the same gain for the 2 halves. The splitter cathode resistor is not a current source, and the 2nd half of the splitter gets its signal from the resistor summing network (the uneven EL34 grid resistors makes up for this).
Originally Posted by 6A3sUMMER The splitter you have has more second Harmonic Distortion out of the first gain stage, the second gain stage of the splitter has less second Harmonic Distortion. So you are sending more 2nd HD to the 'push' EL34 / 6L6 stage; and less 2nd HD to the 'pull' EL34 / 6L6 stage.
I always wondered what the implication of "imbalance" between positive and negative going half-waves would be! The though of this is "bugging" me so much I want to do something about it out of principle (cause the amp is really sounding OK already!)
Originally Posted by 6A3sUMMER .....there is more gain in your splitter than in a long tailed splitter, so the open loop to closed loop gain ratio is larger than when using a (lower gain) long tailed splitter... ....the gain is about 1/2, so more input signal is required to drive the output stages. The open loop to closed loop feedback ratio is less (about 6 dB less)..... As long as the stage can still deliver the voltage swing and sufficient current for the output I belive I'll be fine. I belive the PCB lend itself to the LTP changes relatively easy. I do have some potential valves laying around (ECC81 / ECC82 / ECC88). Maybe replacing the ECC83 at the input with a -81, -82 could bring the gain back up? I also have a single unused 12BZ7 that may be used for this purpose. I have considered putting in an order for a pair of 12BH7 tubes for splitter and possibly the input amp. Any opinions on this?
Changing from one triode to another could require a lot of changes to the rest of the circuitry. You are working with a circuit board, and certain other fixed components, such as output transformers, power supply, etc. Changing to a cathode coupled splitter, and the consequential loss of gain, will change the feedback open loop to closed loop ratio. Damping factor, frequency response, distortion will all change. After you get this amplifier working with new components, solve the EL34 internal shorts, etc. you should enjoy the amplifier for what it is. After that you can start on a new amplifier design from scratch, with different topology, or different tubes, etc. That is a major project.
Does anyone have a "modified" schematic for the AI s500, that includes the conclusions of this post? I have had witnessed the fluorescent carrot syndrome of this amp and regret losing one of my Sventlana =C= in the process so I do not want to see it happen again.
I can see that the minimal changes should be:
1. changing R10/11, EL34 screen grid resistors, from 100R to 1K?
2. change R9, EL34 cathode resistor, from 220R to something higher and updating it's cap from 50V to 100V?
There is more but scraping this Post is hard work.
I can see that the minimal changes should be:
1. changing R10/11, EL34 screen grid resistors, from 100R to 1K?
2. change R9, EL34 cathode resistor, from 220R to something higher and updating it's cap from 50V to 100V?
There is more but scraping this Post is hard work.
Hello, made just made this amp from Audio Innovations Series 800 power amp schematic. According to findings in this thread I installed of course 1M grid return resistor to ECC82 and increased EL34 screen grid resistors from 100 ohm to 820 ohm and all coupling caps are rated at 630V (they are little bit high, 0.68uF as they were that what I had in hand, correct 0.47uF ones are on the way here). I have main PSU cap rated at 500V and bypassed with bleeding resistor. No stability issues as far as I know from scope readings, I had only minor ground loop issue, which I manages to fix. I have made only quick listening test this evening, but very pleased so far. It needs only little finishing touches underneath.
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Nice Build Airhead. Unfortunately, mine is currently "out of commotion", but I need to get it back up and running since I prefer it greatly compared to my Shannling SPS-80 (Mullard 4-20 style amplifier).
Are those the Toroidy transformers from Poland?
Sitt back and enjoy your work. Must be very gratifying during these "lock-down" corona times
In case you are into tube-rolling you may be interested in knowing that I briefly tested using ECC99 and 12BH7 for the paraphase splitter instead of the ECC82/12AU7 as those types are supposed to be more linear.
My immediate though was that it offered an improvement.
Unfortunately the power transformer did not like the extra current draw and started humming so they did not stay in long.
Are those the Toroidy transformers from Poland?
Sitt back and enjoy your work. Must be very gratifying during these "lock-down" corona times
In case you are into tube-rolling you may be interested in knowing that I briefly tested using ECC99 and 12BH7 for the paraphase splitter instead of the ECC82/12AU7 as those types are supposed to be more linear.
My immediate though was that it offered an improvement.
Unfortunately the power transformer did not like the extra current draw and started humming so they did not stay in long.
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