ARC ST 70 C3 MODIFICATION MANUAL Service Manual free download, schematics, eeprom, repair info for electronics
Further detail on partial feedback to Cathode.
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As for RH84, can only tell you what problems I had to fix.
Others want to pretend 4-10W from bone standard RH84???
Not unless you running with much higher B+ and/or screen.
I was stuck with existing voltages, so sandblasted A2 had
to fake it for me. Without A2 help, 1-2W was the ceiling...
I abused a 10M45 in a weird way that added 5V threshold
to the grid, I think it recharged the coupling cap whenever
pentode G1 would try to drain it. Wasn't clone of superdrive...
Not cause funky way works better, just cause I had certain
parts on hand and make minimal physical change to existing
circuit. I don't have that drawing on me at the moment.
Further detail on partial feedback to Cathode.
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As for RH84, can only tell you what problems I had to fix.
Others want to pretend 4-10W from bone standard RH84???
Not unless you running with much higher B+ and/or screen.
I was stuck with existing voltages, so sandblasted A2 had
to fake it for me. Without A2 help, 1-2W was the ceiling...
I abused a 10M45 in a weird way that added 5V threshold
to the grid, I think it recharged the coupling cap whenever
pentode G1 would try to drain it. Wasn't clone of superdrive...
Not cause funky way works better, just cause I had certain
parts on hand and make minimal physical change to existing
circuit. I don't have that drawing on me at the moment.
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Nah, ..we threw sand at another (much nicer sounding) amp (12AT7 - 12B4 ). Regardless, I concur, despite tracing out and simm'ing my version of RH84 (12AT7 - EL84), it was abysmal sounding and has since been dismantled. I couldn't have made it wose sounding if I tried (and I have a knack for that)
Maybe it's time to revist it , or try an all pentode amp this year.
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I think we forget how grids get leaky even before they reach positive voltages.
Totally mess up a high impedance passive Schade network when it happens...
One or two chunks of well placed sand can compensate.
An emitter follower from plate to top of the Schade, and another from the mid
point of Schade to G1... That path is no longer a parasite to your output, and
G1 can fwd conduct all it wants without butchering Mu.
Then again Schade from secondary to cathode requires no sand, adds to output
power, and won't severely miscalculate a Schaded Mu if G1 sometimes leaks.
Totally mess up a high impedance passive Schade network when it happens...
One or two chunks of well placed sand can compensate.
An emitter follower from plate to top of the Schade, and another from the mid
point of Schade to G1... That path is no longer a parasite to your output, and
G1 can fwd conduct all it wants without butchering Mu.
Then again Schade from secondary to cathode requires no sand, adds to output
power, and won't severely miscalculate a Schaded Mu if G1 sometimes leaks.
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Here is my attempt to trace curves in LTSpice with and without cathode bypass for 12AX7 and 12AT7.
Tube models are Koren_tubes.inc
SveinB
Attachments
Hey Svein,
This verifies what I said in my post #25, that a triodes characteristics goes towards pentode when current feedbacked. Must do the same sim with some different triodes tonight. Still there is no chance to get the characteristics of a good pentode like D3a.
The best I have found so far, when simming a complete 6L6 Schade-amp, is the triode-section of 6F12P. But this is with a large cathode resistor, 1k, and an added CCS in the cathode circuit to make it draw 15-20mA. Gain suffers though.
This verifies what I said in my post #25, that a triodes characteristics goes towards pentode when current feedbacked. Must do the same sim with some different triodes tonight. Still there is no chance to get the characteristics of a good pentode like D3a.
The best I have found so far, when simming a complete 6L6 Schade-amp, is the triode-section of 6F12P. But this is with a large cathode resistor, 1k, and an added CCS in the cathode circuit to make it draw 15-20mA. Gain suffers though.
Here is a D3a with and without cathode bypass.
Depending on the load, fully bypassed Rk might actually be too much degeneration. Bypassing only half of Rk might be sufficient for good linearity, and a bit more gain.
When we remove all unlinearity of the driver, we get no distrortion-cancellation between driver and output tube, and the total THD of the amp will probably increase.
Svein.
Depending on the load, fully bypassed Rk might actually be too much degeneration. Bypassing only half of Rk might be sufficient for good linearity, and a bit more gain.
When we remove all unlinearity of the driver, we get no distrortion-cancellation between driver and output tube, and the total THD of the amp will probably increase.
Svein.
Attachments
Svein,
When simming D3a/Schade best results are achieved with a fully unbypassed cathode resistor. 47ohm for 20mA. Testing with bigger resistors and a cathode-circuit CCS results in, as you say, worse results. The opposite is for the same circuit with a E88CC cascode at 15mA 2*90V.
Off course you must use a gyrator instead of anoderesistor for optimum results.
About triodes, sims are dissappointing due to high levels of even-order harmonics.
When simming D3a/Schade best results are achieved with a fully unbypassed cathode resistor. 47ohm for 20mA. Testing with bigger resistors and a cathode-circuit CCS results in, as you say, worse results. The opposite is for the same circuit with a E88CC cascode at 15mA 2*90V.
Off course you must use a gyrator instead of anoderesistor for optimum results.
About triodes, sims are dissappointing due to high levels of even-order harmonics.
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Sorry, I intended to say that:
Depending on the load, fully un-bypassed Rk might actually be too much degeneration. Leaving only half of Rk un-bypassed might be sufficient for good linearity, and a bit more gain.
As I do not have much experience with pentodes, I do however fully accept the recommendation from our our more learned friend Revintage that the Rk should be fully un-bypassed.
Svein.
Svein, that's a great illustration of how the curves simply change slope with a triode, which shows an increase in plate resistance. With a pentode it mostly just changes the spacing (gm).
If you work out some designs using practical circuit values you will find for the most part that any input pentode over about 5000 gm will result in too much input sensitivity without some degeneration (unless you're building a guitar amp...) The design trick is to get the right sensitivity and input tube idle point where you want it by tweaking the feedback resistor, the cathode resistor, input tupe op point, and parallel plate load (resistor or CCS) on the input tube.
I would probably go the route of a combination of resistors and diodes vs. the capacitor, but there is usually some way to arrange the circuit to require neither diodes or bypass caps.
In these amp designs using current mode feedback, front end gain and output tube grid swing are simply not a challenge and the circuit optimization can be focused in other areas...
Cheers,
Michael
If you work out some designs using practical circuit values you will find for the most part that any input pentode over about 5000 gm will result in too much input sensitivity without some degeneration (unless you're building a guitar amp...) The design trick is to get the right sensitivity and input tube idle point where you want it by tweaking the feedback resistor, the cathode resistor, input tupe op point, and parallel plate load (resistor or CCS) on the input tube.
I would probably go the route of a combination of resistors and diodes vs. the capacitor, but there is usually some way to arrange the circuit to require neither diodes or bypass caps.
In these amp designs using current mode feedback, front end gain and output tube grid swing are simply not a challenge and the circuit optimization can be focused in other areas...
Cheers,
Michael
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Svein, that's a great illustration of how with a triode the curves simply change slope, showing increased plate resistance. With a pentode it mostly just changes the spacing (gm).
If you work out some designs using practical circuit values you will find for the most part that any input pentode over about 5000 gm will result in too much input sensitivity without some degeneration (unless you're building a guitar amp...) The design trick is to get the desired sensitivity and input tube idle point where you want it by tweaking the feedback resistor, the cathode resistor, input tupe op point, and parallel plate load (resistor or CCS) on the input tube.
I would probably go the route of a combination of resistors and diodes vs. the capacitor, but there is usually some way to arrange the circuit to require neither diodes or bypass caps.
In these amp designs using current mode feedback, front end gain and output tube grid swing are simply not a challenge and the circuit optimization can be focused in other areas...
Cheers,
Michael
If you work out some designs using practical circuit values you will find for the most part that any input pentode over about 5000 gm will result in too much input sensitivity without some degeneration (unless you're building a guitar amp...) The design trick is to get the desired sensitivity and input tube idle point where you want it by tweaking the feedback resistor, the cathode resistor, input tupe op point, and parallel plate load (resistor or CCS) on the input tube.
I would probably go the route of a combination of resistors and diodes vs. the capacitor, but there is usually some way to arrange the circuit to require neither diodes or bypass caps.
In these amp designs using current mode feedback, front end gain and output tube grid swing are simply not a challenge and the circuit optimization can be focused in other areas...
Cheers,
Michael
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He is saying that pentode loaded on low resistance is more linear than triode.
It is what I did an year ago in Pyramid VII. But plate resistors define feedback ratio, so they must be as strict as feedback resistors.
In my Pyramid- VII amp 17K load resistors are fed by regulated +400V, while 240K feedback resistors are connected to GU-50 anodes with 800V. That means, both +400 and +800V supply currents to LTP driver.
Why it is Tubecad's idea, I can't understand.
Actually, many Tubecad's ideas were expressed and even published many times before, by other people.
There were few pages in WW magazine where they published "Circuit Ideas"; they honestly printed where that ideas come from. But I don't remember somebody calling them just "Wireless World Ideas".
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Keeping with something Tubelab mentioned, I to have built SE amps using shade
style feedback with a triode driver and UL connected pentode, they can not only
perform quite well in the spec department but sound very nice as well.
I am not going to begin to say a pentode driver is not a better device to get the
the most effective feedback out of this design, but having built these amps with
a triode driver an pentode ran in UL I have found most amps do not "need" much
additional feedback beyond the UL connection that a triode driver can work well.
12AT7 or an 6AQ8/ECC85 have done extremely well in this config, we have tried
a few other "cleaner" drivers and gotten more distortion out of the final amp, so
it goes to show that some specs don't tell the whole story, and there is some of
complimentary distortion cancellation that can happen there as well.
When yu can get 10watts out of a single ended KT88 amp with a triode driver at
less then 3% distortion I would consider that an pretty darn good performing amp.
To me most of the ones we've tried on other pentode output tubes rigged in single
ended ul sound quite nice as well, so while a pentode driver may be more effective
in this circuit I just don't feel it is needed or even beneficial. IMHO
Just saying that not all circuits running shade feedback necessarily need a pentode
driver to perform rather well when the whole/final design is taken into account here.
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Not suggesting one should only build UL style amps, just saying the circuit does work.
(I use UL, Pentode, Triode, whatever I feel the piece/circuit works with)
But this Triode driven UL connected pentode output with shade style feedback I like.
style feedback with a triode driver and UL connected pentode, they can not only
perform quite well in the spec department but sound very nice as well.
I am not going to begin to say a pentode driver is not a better device to get the
the most effective feedback out of this design, but having built these amps with
a triode driver an pentode ran in UL I have found most amps do not "need" much
additional feedback beyond the UL connection that a triode driver can work well.
12AT7 or an 6AQ8/ECC85 have done extremely well in this config, we have tried
a few other "cleaner" drivers and gotten more distortion out of the final amp, so
it goes to show that some specs don't tell the whole story, and there is some of
complimentary distortion cancellation that can happen there as well.
When yu can get 10watts out of a single ended KT88 amp with a triode driver at
less then 3% distortion I would consider that an pretty darn good performing amp.
To me most of the ones we've tried on other pentode output tubes rigged in single
ended ul sound quite nice as well, so while a pentode driver may be more effective
in this circuit I just don't feel it is needed or even beneficial. IMHO
Just saying that not all circuits running shade feedback necessarily need a pentode
driver to perform rather well when the whole/final design is taken into account here.
--------
Not suggesting one should only build UL style amps, just saying the circuit does work.
(I use UL, Pentode, Triode, whatever I feel the piece/circuit works with)
But this Triode driven UL connected pentode output with shade style feedback I like.
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It's nothing special, an the latest amp is still on the bench, when finalized can post.
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But basically it's the attached schematic, values "tweaked" here and there, roughly
440v plate to cathode and 82ma on the output with 5.5ma and 180v on the drivers
with a relatively small amount of "schade" feedback using that 270k resistor.
Makes a fairly simple circuit that I think works pretty well.. (about 1v sensitivity)
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But basically it's the attached schematic, values "tweaked" here and there, roughly
440v plate to cathode and 82ma on the output with 5.5ma and 180v on the drivers
with a relatively small amount of "schade" feedback using that 270k resistor.
Makes a fairly simple circuit that I think works pretty well.. (about 1v sensitivity)
Attachments
Since it is still on the bench, have you been able to form an opinion of the sonic results of the different feedback arrangements, esp. the effect of the rather modest anode-grid feedback.
Svein.
I've been running this same basic circuit arrangement on various pentode output
tubes, an only 1 so far did not sound as good or better with the mini anode-grid
feedback scheme in place and that was an EL500 for whatever reason.
I misquoted I believe as well earlier on the output and distortion on the KT88, it
was I believe 10w at 3% not at less then 3%, with clipping and 5% at 12.5 watt.
(it's not actively on the bench as we speak, others have taken precedence)
tubes, an only 1 so far did not sound as good or better with the mini anode-grid
feedback scheme in place and that was an EL500 for whatever reason.
I misquoted I believe as well earlier on the output and distortion on the KT88, it
was I believe 10w at 3% not at less then 3%, with clipping and 5% at 12.5 watt.
(it's not actively on the bench as we speak, others have taken precedence)
I'm a little surprised that 12AT7 can drive KT88 to full output
swing with two kinds of strong local NFB wrapped round that
output? Would have one grid or the other too easily tip-over
into forward conduction before full output could happen?
No proof to back up that statement, just a gut feeling....
swing with two kinds of strong local NFB wrapped round that
output? Would have one grid or the other too easily tip-over
into forward conduction before full output could happen?
No proof to back up that statement, just a gut feeling....
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