This circuit can be viewed as a combination of SRPP and CASCODE, in other words, a SRPP with top triode replaced by a "pentode like" CASCODE.
One explanation of SRPP operation is a triode loaded by a constant current source, but critics alway argue that top triode doesn't have high enough gain to be a good CCS, so naturally, a pentode is suggested (google "MU STAGE PHILOSOPHY"). Someone must happen to go through tube circuit textbook and see the notion that a cascode has pentode like high gain with triode like low noise, and decide to sub the pentode with a cascode. That is my WAG.
See the cascode excerpt in "Ultra-Linear Cascode" section half-way down at:
It's About Time & Ultra-Linear Line Stages
One explanation of SRPP operation is a triode loaded by a constant current source, but critics alway argue that top triode doesn't have high enough gain to be a good CCS, so naturally, a pentode is suggested (google "MU STAGE PHILOSOPHY"). Someone must happen to go through tube circuit textbook and see the notion that a cascode has pentode like high gain with triode like low noise, and decide to sub the pentode with a cascode. That is my WAG.
See the cascode excerpt in "Ultra-Linear Cascode" section half-way down at:
It's About Time & Ultra-Linear Line Stages
If it was an attempt at combining an SRPP and a cascode then it shows that the designer understands neither of them. A correctly adjusted SRPP (many are not correctly adjusted) is a push-pull stage - the optimum load roughly corresponds to both valves contributing equally to the output current. A cascode produces its output at the upper anode, which here is simply connected to the supply.
It could be (as kevinkr(?) suggested) a failed attempt at bootstrapping the upper valve of an SRPP, due to thinking that this is simply a CF. Or it could be simply a cynical marketing exercise: make a circuit which sort of works (in the sense that it outputs some audio) and then either claim, or let others assume, that it is so clever that normal designers can't understand it.
It could be (as kevinkr(?) suggested) a failed attempt at bootstrapping the upper valve of an SRPP, due to thinking that this is simply a CF. Or it could be simply a cynical marketing exercise: make a circuit which sort of works (in the sense that it outputs some audio) and then either claim, or let others assume, that it is so clever that normal designers can't understand it.
I think you jumped too far ahead. I merely presented a possible thought process that could have gone into such a design, which is somebody might have taken the "pentode like" aspect of the cascode configuration in an attempt to make a better current source load for SRPP. Depending on load impedance, the current source approach could be valid.
However, the connection of the capacitor in question does have a significant impact on the operation of this circuit. For a true pentode like operation, the cap needs to be connected to the cathode of middle triode. In this regard, it is better than a real pentode, since the grid of the top triode is not supposed to draw any current, you can have much higher value bias resistors, so they don't load down the output. The side effect is that the gain would probably be too high (close to mu of the bottom triode) for a typical line preamp. Connecting the cap to ground will turn the top triode into a power supply ripple filter (like a MOSFET cap multiplier) as some have pointed out, and the circuit is basically a standard SRPP with gain close to 1/2 mu. Connecting the cap to the cathode of lower triode might have some sort of bootstraping effect, but my guess is that there is little difference than connecting it to ground. A simple sim will tell.
However, the connection of the capacitor in question does have a significant impact on the operation of this circuit. For a true pentode like operation, the cap needs to be connected to the cathode of middle triode. In this regard, it is better than a real pentode, since the grid of the top triode is not supposed to draw any current, you can have much higher value bias resistors, so they don't load down the output. The side effect is that the gain would probably be too high (close to mu of the bottom triode) for a typical line preamp. Connecting the cap to ground will turn the top triode into a power supply ripple filter (like a MOSFET cap multiplier) as some have pointed out, and the circuit is basically a standard SRPP with gain close to 1/2 mu. Connecting the cap to the cathode of lower triode might have some sort of bootstraping effect, but my guess is that there is little difference than connecting it to ground. A simple sim will tell.
Someone who thinks that an SRPP is essentially a common cathode with a current source load does not understand SRPP, although this could be (as you suggest) a common view. It would be slightly more true of a mu follower, although even there it is not the whole story.
When used with a very high impedance load, the SRPP becomes a common cathode with an active load - not quite the same as a constant current as the load balances the lower triode. Increasing the value of the upper cathode resistor turns it into a mu follower, which with a very high load does approximate to a constant current source.
For some reason the Chinese do seem to like SRPP-like line stages. I can't imagine why as the SRPP makes a poor line stage - too much gain, too fussy about load impedance. Maybe the problem is that some people can't tell the difference between a line stage and a headphone amp (for which the SRPP might be quite good!).
When used with a very high impedance load, the SRPP becomes a common cathode with an active load - not quite the same as a constant current as the load balances the lower triode. Increasing the value of the upper cathode resistor turns it into a mu follower, which with a very high load does approximate to a constant current source.
For some reason the Chinese do seem to like SRPP-like line stages. I can't imagine why as the SRPP makes a poor line stage - too much gain, too fussy about load impedance. Maybe the problem is that some people can't tell the difference between a line stage and a headphone amp (for which the SRPP might be quite good!).
SRPP when directly coupled to output triode grid works like a charming driver. Though, it requires two-store B+ building...
I'll admit I use the SRPP to.. err... drive the 300Bs in the amp I am listening to right this moment, and I've been known to direct couple them too. And they work really well. In fact I have an unpublished 2A3 design that does just this. I'm quite happy with this amp despite the fact that I am building a pair of GM70 monoblocks currently. Worst case this particular amp will remain the "summer amp" 😀 (I'll note that these designs are more than 10yrs old...)
Last edited:
I guess I gotta confess too. A long time ago Kevin published a really cool push pull 300B amp design in VTV magazine. It used a really complicated tube regulated supply, two LTP's in a row (6SL7 driving a 6SN7 driving 300B's) and ultra rare OPT's.
I decided to make one using stuff that I had on hand. The tube regulated supply became a mosfet regulated supply. The 6SL7 LTP became a 5751 LTP, but the 6SN7 LTP became this funky offspring that resulted from mating an LTP with an SRPP. It was basically two SRPP's with the bottom cathodes tied together through one resistor to the negative supply using 6FQ7's. The 300B's were cheap Sovtek's and the OPT's were cheap guitar amp transformers from Ebay. I built it using cheap Mouser resistors and caps. It instantly became my favorite amp and saw daily use for several years. It resisted any attempts at upgrades. I tried several different OPT's including some UTC LS-57's, but it always sounded worse, so I learned to leave it alone. The SRPP circuit can be made to sound pretty darned good under the right conditions.
After several years of use the amp finally died (dead electrolytics in the power supply) and sits in the closet awaiting a proper redesign. That amp was the most dynamic sounding amp that I have ever built. Oddly enough some of this can be attributed to the ultra cheap OPT's which are really great sounding when driven by a low impedance tube. Petes red board with big fat sweep tubes comes close in dynamics and I am using the same OPT's.
The redesign is underway, and gone are the funky SRPP LTP's. After testing and tossing several driver designs, there are only a few contenders left. One is a dual LTP using a 6SL7 driving a 6SN7 (see the 6L6GC in AB2 thread). Kevin had this one right ater all. A test amp is up and running with that driver, and another design is in the breadboard stages.
“Others laught at me because they think i am crazy, but i laugh at the others because they don't see the true meaning of life.”--Chinese Proverb
Alternatively you could help them discover the true meaning of life, rather than laughing. I'm learning rather a lot from reading people's honest attempts to figure something out. The best of the Internet - people from all countries pooling their intellects.
From grasshopper "The student"- "Kung FU" How long will it take me to understand and learn master, "The master", probably a lifetime maybe a little longer.😛
Regards
M. Gregg
Nope, that's not how cynics behave -it seems our Chinese friend prefers to enjoy himself. I don't understand the circuit but I disagree about being a fashion statement (Chinese fashionable? LOL), there must be a reason behind it but I can't figure it out. It's very un-Chinese to use an extra tube if you don't have to, so I'm thinking it may have something to do with the power supply not being good enough (noisy); but you guys have said this isn't a regulator. Dunno.
Given the propensity to copy now mediocre ancient Western designs with a plethora of unnecessary circuitry (viz. the many bad copies of the original Marantz 7 design on ePay) I am not sure you can assume it is there for any other reason than to raise the tube count. (A marketing driven decision.) There are very talented designers in China, I'm just not convinced this is one of them, but no shortage of bad design(ers) here either for that matter.
My quick and not so thorough simulations of this circuit indicated that the extra stage rather significantly degrades the overall performance - it appears to work a LOT better without..
Adding an extra valve, which degrades circuit performance, for marketing purposes is better or worse in (a) a moral sense, (b) an engineering sense, (c) an artistic sense, than simply adding/displaying a valve which is not connected to the circuit (apart from heater supply).
Discuss.
Discuss.
- Status
- Not open for further replies.