Steve,
FYI, I was quite impressed with your previous publication about improving the Eico ST-70.
Andres' approach is outside my realm of comfort (which probably speaks for itself). I'm even more skeptical about your statement:
The really big news is that this technique is applicable to almost any push-pull, fixed-bias, hifi, tube amplifier, using the original transformers!
Really? What others have you modified and what were the outcomes?
And what would be the (practical) benefit of a patent, if you were able to somehow secure one?
FYI, I was quite impressed with your previous publication about improving the Eico ST-70.
Andres' approach is outside my realm of comfort (which probably speaks for itself). I'm even more skeptical about your statement:
The really big news is that this technique is applicable to almost any push-pull, fixed-bias, hifi, tube amplifier, using the original transformers!
Really? What others have you modified and what were the outcomes?
And what would be the (practical) benefit of a patent, if you were able to somehow secure one?
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To TheGimp:
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To jono1:
Thank you for the kind words about the ST-70A.
The reason that the power transformer can support the load is is NOT because it is overspecified. It is because the peak-to-average ratio of music is typically 14dB, meaning that while the peak level might be 124W, the average is less than 5W. Of course, the solid state rectifier works the same in almost any such amp, as does the extra heater transformer.
Implementing the extraordinary negative feedback does require careful attention to frequency compensation on a case-by-case basis. For output transformers with poor high-frequency characteristics, high frequency distortion might not turn out as nicely as it did in the Eico ST-70. You get what you pay for. Nevertheless, the high negative feedback can be implemented over a reasonable audio range with almost any hifi output transformer.
So that is why the Supermods™ technique will work with almost any push-pull, fixed-bias, hifi, tube amplifier, which has room for more tubes. (You left out the last part.)
---Global negative feedback has been increased by 10-12dB, to a value of 30dB. Very few commercial amplifiers use much more than 20dB. So it is extra-ordinary, when an amp uses 30dB or more.
---Name two commercial designs which use 30dB or more of global negative feedback. Name even one that uses 40dB or more.
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To jono1:
Thank you for the kind words about the ST-70A.
--- The reason that the technique is so broadly applicable is because the principles upon which it works (covered in the article) are quite general. Contrary to the people who assume that it relies only on overspecified transformers, it in fact develops far more power at lower frequencies than most people thought was possible from ordinary transformers. It turns out that stiffer drive can extend the effective low frequency end of an output transformer. Also, operating at half-impedance reduces the effect of core saturation by a factor of two. That is applicable to any almost any push-pull, hifi, output transformer.
The reason that the power transformer can support the load is is NOT because it is overspecified. It is because the peak-to-average ratio of music is typically 14dB, meaning that while the peak level might be 124W, the average is less than 5W. Of course, the solid state rectifier works the same in almost any such amp, as does the extra heater transformer.
Implementing the extraordinary negative feedback does require careful attention to frequency compensation on a case-by-case basis. For output transformers with poor high-frequency characteristics, high frequency distortion might not turn out as nicely as it did in the Eico ST-70. You get what you pay for. Nevertheless, the high negative feedback can be implemented over a reasonable audio range with almost any hifi output transformer.
So that is why the Supermods™ technique will work with almost any push-pull, fixed-bias, hifi, tube amplifier, which has room for more tubes. (You left out the last part.)
"The combination of techniques here will only work for an OT that has extra heavy copper wire internally and excessive bandwidth for the original application. How many amplifiers are built this way?"
"--- I don't see how you can justify that statement. "
Operating at lower impedance requires higher current thru the xfmr. If the wire resistance is not appropriately lowered also, then you have higher wire resistance losses and poorer damping factor. So you need an xfmr with excess copper wire sizing (like a 2X power rated one would have). Operating at lower Z -AND- 2X power will really stress the wire, requiring 2X the current, causing 4X the copper losses. The music crest factor will allow one to get away with this overload "most" of the time, true, but not all of the time. Put some loud disco music on for a while and you may have a toasted OT. The original OT was designed to take advantage of the crest factor already. So was the power supply.
An output transformer will have some leakage inductance between the primary and secondary. This acts like a low pass L-R filter when loaded by a load R. If you halve the load, then the lowpass frequency is halved also. Placing say a 4 Ohm load on the 8 Ohm tap will lower the high freq. end similarly. This is in conflict with increasing the neg. Fdbk, since that will require more bandwidth. Only if you had excess bandwidth to begin with will this technique work out.
Commercial amplifiers generally don't use more expensive parts than they need for their advertised specs.
"--- I don't see how you can justify that statement. "
Operating at lower impedance requires higher current thru the xfmr. If the wire resistance is not appropriately lowered also, then you have higher wire resistance losses and poorer damping factor. So you need an xfmr with excess copper wire sizing (like a 2X power rated one would have). Operating at lower Z -AND- 2X power will really stress the wire, requiring 2X the current, causing 4X the copper losses. The music crest factor will allow one to get away with this overload "most" of the time, true, but not all of the time. Put some loud disco music on for a while and you may have a toasted OT. The original OT was designed to take advantage of the crest factor already. So was the power supply.
An output transformer will have some leakage inductance between the primary and secondary. This acts like a low pass L-R filter when loaded by a load R. If you halve the load, then the lowpass frequency is halved also. Placing say a 4 Ohm load on the 8 Ohm tap will lower the high freq. end similarly. This is in conflict with increasing the neg. Fdbk, since that will require more bandwidth. Only if you had excess bandwidth to begin with will this technique work out.
Commercial amplifiers generally don't use more expensive parts than they need for their advertised specs.
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I have to agree here. If I was to hotrod an existing HiFi OR guitar amp, I could and would:
(1)add additional output tubes or replace the originals with higher rated tubes.
(2)increase the B+ and boost the heater capacity needed to support the additional or larger output tubes by one or more of the following:
(A)switching to solid state rectifiers.
(B) adding an auxilliary power transformer for boosted B+ or extra heater capacity.
(C)switching from a FWCT ciircuit to a FW bridge.
(D)switching from cathode bias to fixed bias.
(3)Change the primary or secondary tapping on the OPT (this means using the UL taps for the plate or putting the speaker on a different impedance tap.
(4) change a UL amp to pure pentode configuration.
(5) Modify the driver configuration to allow grid current flow in the output tubes. This usually means adding a mosfet to drive the grid. This is explained in the PowerDrive section of my web site. Yes this sometimes requires an auxilliary transformer, which can be used to provide the boosted B+ and additional heater capacity.
What I would NOT do is blindly apply large amounts of negative feedback in order to get the amplifier to measure good on the test bench. Yes it is possible to apply enough feedback to correct a lot of different sources of distortion. This usually removes much of the "life" from the music making the amp sound just like most of the solid state designs with a bucket load of open loop gain and a ton of feedback. In ANY amplifier design or major modification I start with ZERO feedback and increase it until it sounds right with the speakers that I intend to use. Some speakers need more feedback than others to clean up the bass.
This hyena also has an MSEE and 38 years of analog and RF design experience at Motorola. I blew up a Fender Champ about 49 years ago by stuffing a 6L6 in place of the 6V6, a 5U4 in place of the 5Y3 and wiring two speakers in parallel thus reducing the effective load impedance. Granted I was about 10 years old at the time and had no clue what I was doing (or even what impedance was) but the amp just screamed until the power transformer melted down. I have been hotrodding, building and designing tube amps ever since.
I purchased over 100 of the previously mentioned budget OPT's about 12 years ago. They were intended for guitar amps and rated for "6600 ohms, 80 VA from 80 Hz to 4 KHz". I was intending to use them in guitar amps but my curiosity prompted me to explore their limits. I did build several guitar amps, but those transformers have found their way into all sorts of HiFi applications.
I discovered the phenomenon that the low frequency corner and the low frequency saturation point were grossly different depending on what was driving the transformers. A 6L6GC configured as a pentode with zero feedback was about the worse managing about 60 Hz at 30 watts, while a 300B triode could crank a clean 30 watt sine wave through the same OPT's down to 20 Hz without feedback. It doesn't take a rocket scientist to figure out that the negative feedback is whats extending the usable frequency range. It is largely doing this by lowering the effective plate resistance of the output tube. Yes, paralleling tubes, or using a bigger tube also lowers the plate resistance.
The usual application of large amounts of GNFB is one way to accomplish this...with the usual side effects associated with large doses of GNFB. I tend to prefer other ways. I use cathode feedback in the output stage only to reduce the output impedance succesfully in my SSE design. It works wonders on some OPT's and can be used in triode, UL and pentode mode. Schade feedback across the output tube is a good alternative and is used in Petes red board. I am running the same OPT's in a red board wired for 3300 ohms. It can hit 125 WPC at 1KHz and 80 watts at 30 Hz because of Schade feedback. No GNFB is used at all.
I have plenty of experience blowing stuff up. Overpowering an OPT will generally not hurt it. The easiest sure fire way to kill an OPT is to run it without a load. This will allow the plate voltage to rise to kilovolt levels if the amp ever clips. An arc over inside the OPT will start a destructive meltdown. Excessive DC current is also a possible killer. It is possible to overstress an OPT with too much plate voltage, but it takes a lot of voltage. I have succesfully extracted 250 watts from that "80VA" OPT with 750 volts of B+.
Remember as the OPT saturates, its inductance drops rapidly. As the inductance drops the current through the output tubes skyrockets. Continued operation into saturation can cause output tube failure and possibly damage the OPT.
Good point! Although I don't abuse my amps in quite the same way as you...
Interesting discussion, in a sort of "peer review" kind of way. It was all in good fun until the OP took himself a little too seriously and declared it an "invention" with no "prior art".
Seems to me there is no shortage here of competent and experienced EEs and equivalent, and the overwhelmingly one-sided feedback pretty much spells out the verdict.
Seems to me there is no shortage here of competent and experienced EEs and equivalent, and the overwhelmingly one-sided feedback pretty much spells out the verdict.
"how does one avoid saturation"
Saturation sets in at so many volt-seconds. For a given low end frequency, the seconds part is constant. So we are left with a constant voltage max at that freq. By going to a lower impedance, more power can be transmitted since the current increases and Watts are volts x current. (the primary source current is cancelled by the secondary load current, so it is not the transmitted current/power that causes the saturation, but rather the magnetizing inductance current, which is set by the volt-seconds)
By using a lower source Z as well (and increased feedback), one can drive it further into saturation too, giving more power yet.
Saturation sets in at so many volt-seconds. For a given low end frequency, the seconds part is constant. So we are left with a constant voltage max at that freq. By going to a lower impedance, more power can be transmitted since the current increases and Watts are volts x current. (the primary source current is cancelled by the secondary load current, so it is not the transmitted current/power that causes the saturation, but rather the magnetizing inductance current, which is set by the volt-seconds)
By using a lower source Z as well (and increased feedback), one can drive it further into saturation too, giving more power yet.
Never published before?
30dB NFB isn't unheard of, does anyone actually know the NFB level of a stock ST-70? I'd bet it's not far off 30dB. Your other mods are utterly trivial, I'm amazed you even deem an extra heater transformer worthy of discussion.
All this thread has shown me is that 4 output tubes can produce more power than two. Which is sort of what I knew already.
that's what you think, tube builders have been doing that way before.....
i read an article in the "Popular Electronics" Magazine in the 70's stating that using solid state rectifiers, your amp gains around 9 watts more power...
that's about the power being consumed by he tube rectifier becoming available to the power amp....
The original ST70 used a 1K FB resistor from the 16 ohm tap to a 47R in the cathode of the input stage.
Since the voltage at the 16 ohm tap is SQRT2 times the voltage at the 8 Ohm tap (which we are using as the output), the feedback factor (Beta) should be (47/1000)*SQRT2.
Feedback it 20 Log ((47/1000)*1.414) = -23.55dB
Add another 6dB and your are near the 30dB level that has historically been recommended as the maximum feedback which can be used without sucking the life out of an amp.
The proposed procedure is interesting and well incorporates several standard tecniques for improving amps.
Documentation on the web site is very good, and the procedure is well presented.
As pointed out, one or more other amps should have this procedure applied to them to support the generalization that it can be applied to any amp (or similar wording).
However this said, I do not find it to be anything out of the ordinary.
Since the voltage at the 16 ohm tap is SQRT2 times the voltage at the 8 Ohm tap (which we are using as the output), the feedback factor (Beta) should be (47/1000)*SQRT2.
Feedback it 20 Log ((47/1000)*1.414) = -23.55dB
Add another 6dB and your are near the 30dB level that has historically been recommended as the maximum feedback which can be used without sucking the life out of an amp.
The proposed procedure is interesting and well incorporates several standard tecniques for improving amps.
Documentation on the web site is very good, and the procedure is well presented.
As pointed out, one or more other amps should have this procedure applied to them to support the generalization that it can be applied to any amp (or similar wording).
However this said, I do not find it to be anything out of the ordinary.
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I think I know of one. HK Citation II... 30dB feedback. I don't know if it's global.
Steve
Yeah, there's been an amazing amount of debate on the details of marketing taxonomy. Terms like that are perfect for "No true Scotsman" arguments.
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