------------------------------ From: dunker@Lise.Unit.NO (Thomas Dunker) Subject: Re: New DAC tube stage Date: 27 Feb 1994 15:11:37 GMT In article <2kl213$mep@introl.introl.com>, thk@tyres.asd.sgi.com (Tom Kong) writes: > In article <2kidgn$10ro@introl.introl.com>, > wrote: > : > : 330V > : | > : plate > : --------grid > : | cathode 10 mFd > : | |------||--------- out > : | \ > : | / > : | \ 1.25 k > : | / > : ----------| > : plate > : In -----------grid > : cathode > : |----------------- > : \ | | > : / | | > : 1.25 K \ = 1mfd = 2200 mfd > : / | film | 'lytic > : |----------------- > : GND > : > > I am surprised that the above circuit works well at all. > Firstly, the grid needs to have a resistor to ground, otherwise > the grid will be floating, and the quiescent current will be completely > unpredictable when the input is not connected. Secondly, a 12AT7 should > normally run a quiescent current of a few milliamps. With the values > of the plate resistor given, the plate resistor will only drop a few volts, > making the clipping characteristers extremely asymmetric. Finally, with a > fully bypassed cathode, the voltage gain of the circuit is 100% depending > on the particular tube you have, is temperature sensitive, and will change > as the tube ages. From this circuit it should get a voltage gain > of Gm * 1.25k. I don't have the 12AT7 specs with me, but the gain > will probably be between 10 to 100. I think the above circuit > sounds nice because you are listening to the added even harmonics > from the asymmetric transfer function. My limited experience with > tube is that anything will sound decent as long as it is not oscillating > or badly clipping. > > From: the! > Hey, this is the classic SRPP (Shunt Regulated Push Pull) voltage amp. Pardon me, but even if you criticize tubes or biasing, you can't seriosly suggest that the circuit is a bad idea. You are right about the grid to ground resistor, which should always be remembered, in order to avoid negative charge to build up on the grid and thus interfere with the biasing. The SRPP circuit is, provided that the biases aren't totally misplaced, one of the finest tube voltage amps there are. The principle can also be applied to FETs, but not to BJTs. It was developed for use in linear UHF amps, and has become popular in tube audio. There are several advantages to this hookup: 1) High input impedance, low output impedance, making it suitable in buffer applications. 2) Extreme linearity (true) > very low distortion, down to 0.01% for small signals (phono applications), up towards 0.1% for typical line level signals, possible without any feedback needed. 3) Not critical with regard to tube type used. (True: I actually built a pair to replace the output vert amp in a 40 MHz 'scope. The supply voltage was well below 200 volts, should have been over 300. I removed the cathode bypass cap, so it would amplify DC. I tried two different tube types, first 12AU7s - worked fine, I could have used some more gain. Still, loads of overload margin. Then I tried a pair of rare twin triodes called 13D7 (ECC807) which has an amplification factor of 140, more than twice that of the 12AU7. Without changing the bias resistors, I plugged them right in, and they still worked fine, now with much more gain. Worked great, no problem.) This circuit sort of cancels out the unlinearity of the two tubes, and the resulting transfer characteristic is very linear. 4) Voltage gain AND current gain. Perfect for a line stage, where both are often needed. Looking for a cheap alternative to $2000 line preamps (how do they do it)? Here it is. Throw in a good potmeter or attenuator and a good selector switch and you have a great preamp. Conclusion: Tom Kong's propaganda should be disregarded. This is indeed a very fine line amp. One thing to remember: The cathode of the upper triode will be at approximately half of the B+ potential. Care must be taken so that you don't exceed the maximum heater to cathode voltage. This means that you can't use one twin triode tube for one channel unless the heater connections come out separately, which they never do. So you must use one twin triode for the two lower triodes, and another for the upper ones if you're doing it in stereo. Then you must get a separate heater voltage for the upper ones and somehow elevate it to the required potential. The easiest way would probably be to make a voltage divider with two resistors between the B+ and ground, and connect the middle to the center tap of the upper tube heater connection (usually pin 9), and then connect a 'floating' 12.6 volts to the end heater terminals. I'm outta here! Tom Dunker ------------------------------ From: peterca@ento.csiro.au Date: Tue, 1 Mar 94 13:51:06 EST Subject: SRPP circuit comments Tom Dunker writes in defense of the SRPP circuit and I agree almost entirely. Two points however. 1) My limited experience is that the circuit worked wonderfully with and 12AX7 (passive RIAA) then 12AT7 amp in my phono stage; also with a 12AT7 in my CD player. I also had a 12AU7 version of the circuit as a line amp. I had an intermittent whistle from the line stage. It may have been a subtle stability probem due to some stray capacitance somewhere. I never tracked it down because I found that I had enough signal level from all sources to drive my power amps direct without a line amp and it sounded better like that too. 2) The following advice is important but it is not true that you can't use the two halves of the same dual triode for the upper and lower triode. : One thing to remember: The cathode of the upper triode will be at : approximately half of the B+ potential. Care must be taken so that you :don't exceed the maximum heater to cathode voltage. This means that you :can't use one twin triode tube for one channel unless the heater :connections come out separately, which they never do. So you must use :one twin triode for the two lower triodes, and another for the upper :ones if you're doing it in stereo. Then you must get a separate heater :voltage for the upper ones and somehow elevate it to the required :potential. The easiest way would probably be to make a voltage divider :with two resistors between the B+ and ground, and connect the middle to :the center tap of the upper tube heater connection (usually pin 9), and :then connect a 'floating' 12.6 volts to the end heater terminals. The maximum cathode to heater voltage for an ECC81 (12AT7) is 150V, for a ECC82/83 (12AU/X7) it is 180V (Mullard data book). In my phono stage the regulated DC heater supply is floated on a voltage divider of 180K and 62K across the 310 volt supply. This puts the heater voltage at about 1/4 of the supply voltage or half way between the two cathodes. This works out at about 40 V from each cathode and well within the maximum ratings. The voltage divider is decoupled from the B+ supply with a 100uF cap. The voltage divider also acts as a bleeder resistor for the B+ supply. This arrangement has proved reliable for the last five years. An advantage of using the two halves of the one dual triode is that they are likely to be a better match than you might get otherwise unless you have selected and they are necessarily operating under the same conditions of temperature, gassyness of the capsule or anything else that might have a subtle effect (How big a difference this is in practice I don't know but it seems like a good idea). A further thing which seems a good idea is that the circuit can be made compact. IE the lower anode the upper grid connection can be made very short and with minimal wire, the resistor from lower anode to upper cathode can be connected between the socket connections. Also the left and right channels' signal paths don't have to be interleaved. Peter Campbell (peterca@ento.csiro.au) C/O Div. Entomology, C.S.I.R.O. Box 1700 Canberra A.C.T. 2601 Ph.61-6-2464158 (w), 61-6-2516213 (h) Australia 61-6-2464173 (fax)