6P14P-V = 1000hr
6P14P = 3000hr
6P14P-K = 3000hr and vibration resistant
6P14P-EV and 6P14P-EB = 5000hr
6P14P-ER = >10000hr
I have 6P14P sitting here. At some point, I'll put them in, turn out the lights, and see if there is any red plate glow.
6P14P = 3000hr
6P14P-K = 3000hr and vibration resistant
6P14P-EV and 6P14P-EB = 5000hr
6P14P-ER = >10000hr
- maximum rating for filament voltage is 5.7 - 7 for 6P14P, 6P14P-V (6n14p-B) and 6P14P-EV (6n14n-EB)
- maximum rating for filament voltage is 6.0 - 6.6 for 6P14P-ER (6n14n-EP), typical for long life tubes
- 6P14P-ER (6n14n-EP) also has higher filament current - 800 ±60mA vs. 760±60mA for the others
I have 6P14P sitting here. At some point, I'll put them in, turn out the lights, and see if there is any red plate glow.
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First of all, the concertina plate load is 27k.
For a concertina, the plate load should be the same as the cathode load.
Part of the concertina cathode load is 33k R7, but that is in parallel with 150k R4, that goes to 1.3k R3 and 20k R10.
33k in parallel with 150k = 27k.
At DC, C3 prevents any signal feeding from either of the two cathodes of the 7247; and there is no feedback there at subsonic frequencies, or at DC.
The easy way to remove the positive feedback, is to disconnect the 150k R4 at both ends, and then wire the 150k R4 in parallel with the 33k, R7.
But then you may have to decrease the 300k R5, because the concertina is turned on harder with the cathode reistor 27k instead of 33k.
I would expect that you might try and replace the 7247 with a 12AU7 or 12AT7; but that will require changing all resistors in the gain stage and concertina.
The open loop gain will go down, but you may have to change the feedback resistor R10 and C7
Not an easy task.
For a concertina, the plate load should be the same as the cathode load.
Part of the concertina cathode load is 33k R7, but that is in parallel with 150k R4, that goes to 1.3k R3 and 20k R10.
33k in parallel with 150k = 27k.
At DC, C3 prevents any signal feeding from either of the two cathodes of the 7247; and there is no feedback there at subsonic frequencies, or at DC.
The easy way to remove the positive feedback, is to disconnect the 150k R4 at both ends, and then wire the 150k R4 in parallel with the 33k, R7.
But then you may have to decrease the 300k R5, because the concertina is turned on harder with the cathode reistor 27k instead of 33k.
I would expect that you might try and replace the 7247 with a 12AU7 or 12AT7; but that will require changing all resistors in the gain stage and concertina.
The open loop gain will go down, but you may have to change the feedback resistor R10 and C7
Not an easy task.
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Not worth all that simply to experiment, and I don't have the proper test equipment to evaluate results.
The positive feedback is quite a curious thing to me.
Never Get Old,
Re-read my Post # 142, I added an edit that gives a simple way to eliminate the positive feedback.
Re-read my Post # 142, I added an edit that gives a simple way to eliminate the positive feedback.
If I ever get proper test equipment, I'll try things like this. I haven't touched an oscilloscope since 1988, and even then I didn't fully know what I was doing, just basics.
Positive feedback in the (very linear) driver stage is used to increase open loop gain in order to have more feedback available, for a given choice of closed loop gain. It's a very elegant piece of work, but it's pooh-pooh'd because Dynaco isn't hip with the in crowd. You'll notice that the circuit has no low voltage electrolytic capacitors in circuit, and Dynaco's preamps don't either. That's how you build if you want people to still be discovering your secrets two or three generations later.
All good fortune,
Chris
All good fortune,
Chris
Your description of the ER's is pretty much how the Sovtek EL-84M's sound in my my ST-35. The P's I'm using in a moddified console amplifier have a bit fatter low end bass compared to the old stick Mulla4ds that were being used. I prefer the 6P14P-P's over the Mullards. I get the same overall sound from a pair I'm using in a scratch build SE amp.
Is it possible to add a 100k volume control at the input, so you can attenuate the input a bit?
If you lower the open loop gain of the amplifier, but at the same time have the same global negative feedback resistors Dictate the amplifier's gain . . .
you will still have the same gain.
In other words, the global negative feedback sets the gain, unless there is not enough open loop gain.
In that case, global negative feedback does not do anything, so just take it out.
Suppose you were to remove the negative feedback resistor and parallel capacitor from the output transformer secondary.
Now the open loop gain is higher (so far not what you wanted).
But then, disconnect the screens from the Ultra Linear Taps; and re-connect those screens through 100 Ohm resistors to their respective plates.
That lowers the open loop gain by perhaps 6dB or more.
I think the UL taps are 25%, that is 25% local negative feedback. Triode Wired mode is 100% local negative feedback.
Triode Wired outputs, not Ultra Linear outputs.
It certainly would be stable, no matter which loudspeaker you use (well, it would not work well with a 2 Ohm load no matter what circuit you use with the parts you have).
** Do not be afraid to do this. It will be easy to go back to the original configuration, if you do not like it.
The amplifier will not oscillate, etc. when you try this.
Remove the global negative feedback resistor and capacitor (just disconnect one end of each)
AND re-wire from Ultra Linear to Triode Wired mode (a 100 Ohm resistor from screen to plate of the same tube [two of those, one for each output tube]),
You will probably get what you want . . . lower gain. 6dB, 8dB, 10dB.
And, you will not need an oscilloscope, it will be stable.
The tradeoff will be Lower Output Power.
There is no free lunch! But boy, those Triode Wired Sandwiches taste really good!
The only easy choices are:
Live with the excess gain;
Put a volume control at the input (just like Old Hector said);
Purchase another amplifier.
Or try the method I mentioned above. **
you will still have the same gain.
In other words, the global negative feedback sets the gain, unless there is not enough open loop gain.
In that case, global negative feedback does not do anything, so just take it out.
Suppose you were to remove the negative feedback resistor and parallel capacitor from the output transformer secondary.
Now the open loop gain is higher (so far not what you wanted).
But then, disconnect the screens from the Ultra Linear Taps; and re-connect those screens through 100 Ohm resistors to their respective plates.
That lowers the open loop gain by perhaps 6dB or more.
I think the UL taps are 25%, that is 25% local negative feedback. Triode Wired mode is 100% local negative feedback.
Triode Wired outputs, not Ultra Linear outputs.
It certainly would be stable, no matter which loudspeaker you use (well, it would not work well with a 2 Ohm load no matter what circuit you use with the parts you have).
** Do not be afraid to do this. It will be easy to go back to the original configuration, if you do not like it.
The amplifier will not oscillate, etc. when you try this.
Remove the global negative feedback resistor and capacitor (just disconnect one end of each)
AND re-wire from Ultra Linear to Triode Wired mode (a 100 Ohm resistor from screen to plate of the same tube [two of those, one for each output tube]),
You will probably get what you want . . . lower gain. 6dB, 8dB, 10dB.
And, you will not need an oscilloscope, it will be stable.
The tradeoff will be Lower Output Power.
There is no free lunch! But boy, those Triode Wired Sandwiches taste really good!
The only easy choices are:
Live with the excess gain;
Put a volume control at the input (just like Old Hector said);
Purchase another amplifier.
Or try the method I mentioned above. **
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I must not have been clear. What I meant to say was that I wouldn't mind losing some gain if I removed the positive feedback, not that the gain is a problem. Sorry for the confusion. There is no problem with the gain of the ST-35.
The folks here were just responding to this statement. Options would appear to be attenuate or tweak the design, which might not be trivial.
Removing the 150k resistor, and then forgetting to wire it across the 33k cathode resistor . . .
Will cause a little bit of 2nd harmonic distortion . . .
Which for the most part, will be corrected by the global negative feedback.
In any case, because of the global negative feedback, the gain will be almost identical.
1. 150k connected per the schematic.
2. 150k completely removed.
3. 150k re-wired across the 33k.
Set up one channel with 1. the other with 2. Do a double blindfold test (keep both speakers connected, and send the Left channel signal from a CD player, and switch between Left channel 1 and Right channel 2).
Rewire the channel from 2. to connect as. per 3. Repeat the double blindfold test of 1. and 3.
Then wire the L and R channels to 2. and 3. Repeat the double blindfold test with 2. and 3.
Try to hear the difference.
Good luck!
No scope is needed, if you believe math and physics.
Will cause a little bit of 2nd harmonic distortion . . .
Which for the most part, will be corrected by the global negative feedback.
In any case, because of the global negative feedback, the gain will be almost identical.
1. 150k connected per the schematic.
2. 150k completely removed.
3. 150k re-wired across the 33k.
Set up one channel with 1. the other with 2. Do a double blindfold test (keep both speakers connected, and send the Left channel signal from a CD player, and switch between Left channel 1 and Right channel 2).
Rewire the channel from 2. to connect as. per 3. Repeat the double blindfold test of 1. and 3.
Then wire the L and R channels to 2. and 3. Repeat the double blindfold test with 2. and 3.
Try to hear the difference.
Good luck!
No scope is needed, if you believe math and physics.
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A tale of two Dynaco amplifiers:
Left: Dynaco Stereo 70 series ii (circa 1991 by Panor Corp.) gutted and rebuilt by me in 1993, wired point-to-point, 0.1% resistors, and doubled power supply capacitance. Tubes are NOS General Electric 6GH8A drivers and 2009 Electro-Harmonix EL-34 outputs.
Right: Dynaco Stereo 35 series ii by Dynakit Inc, recently finished by me with all of the upgrades listed in this thread. Tubes are JJ 12DW7 drivers and either 6P14P or 6P14P-ER outputs.
The amplifier on the left always has been amazing. I just dusted it off and hooked it up again tonight to compare to the amplifier on the right. Sigh. 🙁
Something just doesn't sound correct with the ST-35. The 6P14P-ER tubes are too light on the bass, so I put in 6P14P instead. I'm not sure how long those will last, but in the dark I don't see any traces of red glow on the plates. The bass improved, but with either type of output tube, the amplifier has a harsh, annoying edge in the midrange. It's very annoying. The sub-bass also is light. The little Chinese amp kit that I built a few months ago sounds great with the 6P14P.
Out of desperation to get rid of the harsh edge, I tried the 4 ohm outputs of the ST-35 instead of the 8 ohm outputs since my speakers do dip to 4 ohms across their range (8 ohm driver pairs wired in parallel). The hard edge disappeared, but so did all of the sub-bass. All traces of sub-bass simply vanished. I did get a wonderful, open midrange sound with the 4 ohm output though.
So, I lowered the annoyingly heavy and extremely hot-running ST-70 series ii down from it's perch on a high closet shelf. It yielded the usual perfect sound, despite having 30 year old power supply capacitors in it and running as hot as a fireplace. It has gutteral sub-bass that drives the music and open and spacious midrange with no hard edge at all. I always use the 8 ohm output.
I wonder if the ST-35 has a problem driving my speakers? They aren't anything difficult to drive, just a pair of Vifa 8 ohm bass drivers and a pair of Vifa 8 ohm midrange drivers with first order crossovers, plus an 8 ohm tweeter with a second order crossover. So the load presented is nothing crazy, and no other amplifier has ever had a problem with them.
My other thought is that maybe the JJ driver tubes just sound bad. I have had bad experiences with JJ tubes in the past, but they were affordable so I decided to give them a try.
Any thoughts on this harsh edge to the sound anyone? It goes away with the 4 ohm outputs, but so does all of the sub-bass, which is not acceptable at all. Something just doesn't seem right with this situation. On the 8 ohm output, it's the type of harsh sound that I would associate with early solid-state gear and early CD players. Is this type of sound actually normal with the ST-35? Maybe that's just the way it is?
Has anyone tried NOS General Electric 12DW7 tubes in the ST-35? GE seems to have kept making those for some time compared to other tubes. Electro-Harmonix 12DW7's are available, but they are relatively expensive.
For anyone interested, here are the innards of the ST-70 series ii after I removed the stock circuit board and replaced and rewired everything. 30 years later it's still going strong, with 20 years of those running 8-12 hours per day as I was using it as front left-right in a 5-channel surround system that was always on.
Left: Dynaco Stereo 70 series ii (circa 1991 by Panor Corp.) gutted and rebuilt by me in 1993, wired point-to-point, 0.1% resistors, and doubled power supply capacitance. Tubes are NOS General Electric 6GH8A drivers and 2009 Electro-Harmonix EL-34 outputs.
Right: Dynaco Stereo 35 series ii by Dynakit Inc, recently finished by me with all of the upgrades listed in this thread. Tubes are JJ 12DW7 drivers and either 6P14P or 6P14P-ER outputs.
The amplifier on the left always has been amazing. I just dusted it off and hooked it up again tonight to compare to the amplifier on the right. Sigh. 🙁
Something just doesn't sound correct with the ST-35. The 6P14P-ER tubes are too light on the bass, so I put in 6P14P instead. I'm not sure how long those will last, but in the dark I don't see any traces of red glow on the plates. The bass improved, but with either type of output tube, the amplifier has a harsh, annoying edge in the midrange. It's very annoying. The sub-bass also is light. The little Chinese amp kit that I built a few months ago sounds great with the 6P14P.
Out of desperation to get rid of the harsh edge, I tried the 4 ohm outputs of the ST-35 instead of the 8 ohm outputs since my speakers do dip to 4 ohms across their range (8 ohm driver pairs wired in parallel). The hard edge disappeared, but so did all of the sub-bass. All traces of sub-bass simply vanished. I did get a wonderful, open midrange sound with the 4 ohm output though.
So, I lowered the annoyingly heavy and extremely hot-running ST-70 series ii down from it's perch on a high closet shelf. It yielded the usual perfect sound, despite having 30 year old power supply capacitors in it and running as hot as a fireplace. It has gutteral sub-bass that drives the music and open and spacious midrange with no hard edge at all. I always use the 8 ohm output.
I wonder if the ST-35 has a problem driving my speakers? They aren't anything difficult to drive, just a pair of Vifa 8 ohm bass drivers and a pair of Vifa 8 ohm midrange drivers with first order crossovers, plus an 8 ohm tweeter with a second order crossover. So the load presented is nothing crazy, and no other amplifier has ever had a problem with them.
My other thought is that maybe the JJ driver tubes just sound bad. I have had bad experiences with JJ tubes in the past, but they were affordable so I decided to give them a try.
Any thoughts on this harsh edge to the sound anyone? It goes away with the 4 ohm outputs, but so does all of the sub-bass, which is not acceptable at all. Something just doesn't seem right with this situation. On the 8 ohm output, it's the type of harsh sound that I would associate with early solid-state gear and early CD players. Is this type of sound actually normal with the ST-35? Maybe that's just the way it is?
Has anyone tried NOS General Electric 12DW7 tubes in the ST-35? GE seems to have kept making those for some time compared to other tubes. Electro-Harmonix 12DW7's are available, but they are relatively expensive.
For anyone interested, here are the innards of the ST-70 series ii after I removed the stock circuit board and replaced and rewired everything. 30 years later it's still going strong, with 20 years of those running 8-12 hours per day as I was using it as front left-right in a 5-channel surround system that was always on.
Also, does anyone know what the maximum voltage applied to pin 9 of the output tubes might be? Schematic is in post #1. I have some Chinese output tubes that will blow instantly if it's over 300V.
Interesting comparison. I suppose the only real proof of the pudding is some form of investigation with an oscilloscope and signal generator, with a dummy load, and at least eliminate any artifacts that might be contributed from the ST35.
I have realised recently that I will need to bolster my skills to do qualitative and quantitive testing as part of verification. Without it there is no real end game, just woolliness and subjectivity.
Could it be possible to try the input stage from one of your other amps with the output stage of the ST35 and see where the colouring of the sound originates?
I have realised recently that I will need to bolster my skills to do qualitative and quantitive testing as part of verification. Without it there is no real end game, just woolliness and subjectivity.
Could it be possible to try the input stage from one of your other amps with the output stage of the ST35 and see where the colouring of the sound originates?
I'll add a third item to the comparison: the $400 ultra budget EL-84/6P14P Chinese tube amp kit that I built a few months ago. It sounds great even though the output transformers distort badly above 6 watts/ch from 20Hz to 30Hz. The ST-70 also sounds great, as it always has. The oddball here is the ST-35 with the harsh midrange. I'm surprised.
Since it's smooth sounding on the 4 ohm output (with zero sub bass though) and it has plenty of power output, and all components were the highest quality and were tested individually to spec before installation, and I didn't change any values from the circuit as designed, I am going to assume that there are no failed components involved or anything major like that. Everything is well grounded, and I get zero hum or buzz.
Correct. All I can do is compare the sound of the amplifier to the other two that I have. I don't have access to any test equipment. It's a game of which one of these is not like the others among the three, and the ST-35 is the oddball.
Too much NFB? Not enough NFB? It's a weird harshness that I didn't expect.
I suppose step 1 is to try some different driver tubes.
Would this be .75 * 380V = 285V?
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Perhaps this picture from the Dynaco ST-35 build manual can help (see voltage chart on left lower hand corner of pic):
Best,
Anand.
Measure voltages at pins 1,2,3 of the 7247s, compare to the chart.. Grid (pin 2) should be several volts negative with respect to the cathode (pin 3). Some Russian tubes are different enough from vintage ones that they don't bias properly in a direct coupled circuit; I don't know abut JJ. Measure with a good DMM - the cheapo Harbor Freight type have a 1 meg input, not suitable for measuring high impedance circuits.
Thanks. My copy of that has become corrupted, and row 9 isn't labelled. 375V is a no-go for cheap Chinese 6P14 tubes, so they will stay in the parts box.
I'll measure all of the voltages now that I have removed it from the system and replaced it with the ST-70 for a while.
Welllllllll ... the one I have will have to do for now. It's a budget model but maybe one step better than HF.
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