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#1
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Grounding Choke?
This is from the Amp Slab website.
http://www.ampslab.com/SCHEMATICS/Altec260A.gif What is the purpose of the transformer/choke(s) just before the output tubes? The only thing I can think is that they are there for stability. I've seen schematics with a small value cap in this place, we're talkin' a few nanoferads, to roll off unwanted highs. Is this for rolling off lows? And since they are wound on the same core there will be reactance between the push and pull sides. I've never seen this before. |
#2
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In article .com, "Karl"
wrote: This is from the Amp Slab website. http://www.ampslab.com/SCHEMATICS/Altec260A.gif What is the purpose of the transformer/choke(s) just before the output tubes? The only thing I can think is that they are there for stability. I've seen schematics with a small value cap in this place, we're talkin' a few nanoferads, to roll off unwanted highs. Is this for rolling off lows? And since they are wound on the same core there will be reactance between the push and pull sides. I've never seen this before. I would guess that there are two purposes, the main purpose of the chokes is to provide a low impedance bias source for the output tubes to prevent thermal runaway under conditions of grid emission. A second purpose is to couple the two driver tubes together in a push-pull configuration for increased drive to the output tubes along with lower distortion. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
#3
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"Karl" wrote in message oups.com... This is from the Amp Slab website. http://www.ampslab.com/SCHEMATICS/Altec260A.gif What is the purpose of the transformer/choke(s) just before the output tubes? The only thing I can think is that they are there for stability. I've seen schematics with a small value cap in this place, we're talkin' a few nanoferads, to roll off unwanted highs. Is this for rolling off lows? And since they are wound on the same core there will be reactance between the push and pull sides. I've never seen this before. I got this far on the web. I'll look again later. This appears to be a theater amp and L1 is some sort of elaborate feedback. The core of the inductor passes through both windings so this probably balances the push-pull with emphasis on a certain bandwidth. Possibly an inductor is used because of the phase lead. This type of circuitry is very interesting. Probably most of those who really understand it are passing away. Theater amps worked very hard and they surely made them ultra-stable so they wouldn't give out due to overheating. Mark http://alteclansingunofficial.nlenet.net/Library.html |
#4
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"Karl" wrote in message oups.com... This is from the Amp Slab website. http://www.ampslab.com/SCHEMATICS/Altec260A.gif What is the purpose of the transformer/choke(s) just before the output tubes? The only thing I can think is that they are there for stability. I've seen schematics with a small value cap in this place, we're talkin' a few nanoferads, to roll off unwanted highs. Is this for rolling off lows? And since they are wound on the same core there will be reactance between the push and pull sides. I've never seen this before. I found something else, but not a whole lot. http://www.voiceofthetheatre.com/260A.th.htm so the 260 is a theater amp. I don't know if the spec sheet will enlarge or not. Mark |
#5
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Karl wrote:
This is from the Amp Slab website. http://www.ampslab.com/SCHEMATICS/Altec260A.gif What is the purpose of the transformer/choke(s) just before the output tubes? The only thing I can think is that they are there for stability. I've seen schematics with a small value cap in this place, we're talkin' a few nanoferads, to roll off unwanted highs. Is this for rolling off lows? And since they are wound on the same core there will be reactance between the push and pull sides. I've never seen this before. I tried that link yesterday but no luck. This AM I Googled & got a good schema at http://www.triodeel.com/al260a.gif but it is a very large file. As John B has already pointed out the split choke is in there to provide a low DCR to the control grids of the 813's. The spec is 30K max in the 813 G1 circuit, given in RCA Transmitting Tubes TT-4. It also assures good drive balance to the 813's should the output from the 6AU6's differ for whatever reason. However, it will not result in more drive. The 6AU6's, being pentodes are quite capable of that on their own. To get more drive the split choke would need to be in the plate circuit of the driver tubes. In that case the drivers would more than likely be medium mu triodes such as a pair of 6J5's, 6C5's or a 6SN7. Even so, the split choke will need to have a very large inductance in order not to degrade the LF response & gain of the rather high impedance of the driver (6AU6) plate circuit. See also http://www.voiceofthetheatre.com/images/260A.1.jpg and http://www.voiceofthetheatre.com/images/260A.2.jpg Cheers, John Stewart |
#6
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"John Stewart" wrote
As John B has already pointed out the split choke is in there to provide a low DCR to the control grids of the 813's. The spec is 30K max in the 813 G1 circuit, given in RCA Transmitting Tubes TT-4. It also assures good drive balance to the 813's should the output from the 6AU6's differ for whatever reason. However, it will not result in more drive. The 6AU6's, being pentodes are quite capable of that on their own. To get more drive the split choke would need to be in the plate circuit of the driver tubes. In that case the drivers would more than likely be medium mu triodes such as a pair of 6J5's, 6C5's or a 6SN7. Even so, the split choke will need to have a very large inductance in order not to degrade the LF response & gain of the rather high impedance of the driver (6AU6) plate circuit. Good explanation, thanks. Not difficult to get the high inductance, considering there is very little current and the windings are in series. Are those trimmers that the choke windings pick up the bias from? That seems to me the strange part. That would put a variable resistance between the two choke windings. Diagram isn't very clear. Otherwise the inductor can be seen as a differential mode choke, and hence a common mode shunt. A perfectly balanced signal is pure differential mode, so the choke should improve balance and offer a high impedance differential mode load to the drivers. Incidentally, it provides a low impedance path for both DC and AC components of the bias supply, which therefore needs to be reasonably smooth I suppose, even though consequent hum would be mostly cancelled in the output transformer. cheers, Ian |
#7
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Ian Iveson wrote:
"John Stewart" wrote As John B has already pointed out the split choke is in there to provide a low DCR to the control grids of the 813's. The spec is 30K max in the 813 G1 circuit, given in RCA Transmitting Tubes TT-4. It also assures good drive balance to the 813's should the output from the 6AU6's differ for whatever reason. However, it will not result in more drive. The 6AU6's, being pentodes are quite capable of that on their own. To get more drive the split choke would need to be in the plate circuit of the driver tubes. In that case the drivers would more than likely be medium mu triodes such as a pair of 6J5's, 6C5's or a 6SN7. Even so, the split choke will need to have a very large inductance in order not to degrade the LF response & gain of the rather high impedance of the driver (6AU6) plate circuit. Good explanation, thanks. Not difficult to get the high inductance, considering there is very little current and the windings are in series. Are those trimmers that the choke windings pick up the bias from? That seems to me the strange part. That would put a variable resistance between the two choke windings. Diagram isn't very clear. That is a good point Ian. I had to magnify the schema to see what the circuit is. One end of the Pots P1 & P2 goes to -125 volts while the other goes to common thru 10K, so that sure seems to be the bias for 813's setup. Otherwise the inductor can be seen as a differential mode choke, and hence a common mode shunt. A perfectly balanced signal is pure differential mode, so the choke should improve balance and offer a high impedance differential mode load to the drivers. Yes, since both windings are on a common core the split choke performs very well as a common mode sink. AC Balance to the 813 G1's should be very good. But it would be nice to be able to control that should the 813's differ, for whatever reason. Incidentally, it provides a low impedance path for both DC and AC components of the bias supply, which therefore needs to be reasonably smooth I suppose, even though consequent hum would be mostly cancelled in the output transformer. cheers, Ian Another interesting circuit feature IMO is what appears to be shields on the coupling caps C1 & C2. Each shield? is returned to common thru what looks to me like a 150K resistor. The G1 to plate cap for the 813 is large at 0.25 picofarad, so that may be one of the HF corners. But 0.25 pf in a tube that size ain't bad, at all!! Cheers, John Stewart |
#8
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"Karl" wrote in message oups.com... This is from the Amp Slab website. http://www.ampslab.com/SCHEMATICS/Altec260A.gif What is the purpose of the transformer/choke(s) just before the output tubes? The only thing I can think is that they are there for stability. I've seen schematics with a small value cap in this place, we're talkin' a few nanoferads, to roll off unwanted highs. Is this for rolling off lows? And since they are wound on the same core there will be reactance between the push and pull sides. I've never seen this before. The 1937 Thordarson sound amp manual shows this type of inductor. At least three of the smaller amps use this. Part number T-7431 is called "tapped input impedance." in T-7530-2A3 amp. http://www.audiophool.cjb.net/Misc/SoundAmp.pdf |
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