I wrote a lovely post only to be told my token had expired..
That will do - given an LTSpice sim is about as precise and accurate as a game of pin the donkey at a state level. It does give a general picture of things working or not which can curtail the obvious and the oblivious.
It's currently configured for my 104dBV headphones and CD player which I've measured 1.5V peak on some CDs. Let's be clear - I've seen this push 24V and 450mA just as happily as the values below (I have a 24V TVS across the load - an idea stolen from UltimateX86's schematic).
Input -> Output
-10dB 447mV -> 290mV, 5.2mA
-3dB 1V -> 650mV, 11.1mA
1.5V -> 960mV, 17.5mA
+10dbV 3.16Vrms 4.5Vpeak -> 2.14V +3.6dB
Mark - thank you for the link and the graphs.
I feel like I've learnt a lot about tube amps, some solid state, and cascodes attempting this project.
At 6am I found these two interesting articles which got me thinking more and why people seem to have issue with cascodes - more importantly their misunderstanding of them. So perhaps these musings and learnings will help people interested:
* Nelson Pass's article:
https://www.firstwatt.com/pdf/art_cas_amp.pdf
* Broskie's interesting adaptations:
Retrograde Cascode
A Cascode:
* consists of a V-to-I converter then a V-to-I converter.
* use of constant votlage/ground on V-to-I means virtually no miller effect as there's a constant charge on the grid. So no capacitance issues means higher frequency response.
* linearises the tube's characteristics
* linear lines = less asymmetric distortion on the load line = less even harmonics.
* linear lines = more symmetrical distortion = more odd harmonics (but not much see next points)
* triode normal cascode has low power output so should be driving a high impedance load.
* cascode (unfolded) needs voltage headroom to convert I-to-V without clipping and adding odd harmonics or intermodulation.
* is a gain machine, with 17mu 12BH7As it's entirely possible to get 100Vpp output from under 2V for example. See my point about headroom.
The interesting piece from Broskie is essentially using the V-to-I from the bottom.. a cathode follower as a suggestion. Interesting but given I've not seen many like that.. I'm less inclined to waste time on that. However Nelson highlights the use of Dayton-Wright as a further improvement over the standard cascode that drops harmonics further. Interesting.. perhaps a change to play with in future.
So my musings this morning also covered the general structure of the amp. The use of a cascode, drivers and the output section plus the specifics of the tubes in use - the 12BH7A and the ecc99. The achilles heal of the ecc99 is the input voltage range, and to a lesser extent the the same issue occurs with the 12BH7A.
The 12BH7A can take 20Vpp but will be letting you know. The ecc99 will give you the middle finger so sticking with 10Vpp is probably recommended.
The cascode of the design can be made to provide anything from 2V to 100Vpp, with a driver requirement a good 2-5Vpeak input is useful to sit at -10V bias and have current available to drive with the BH7. However chaining a ecc99 with it's 10Vpp input is a bit like drinking a straw whilst pinching the straw. It doesn't make sense (being objective).
This is where the M60 would be driving the low-mu 6AS7 which would be very happy with 20-40Vpp or more input. So you can see the voltage gain through the amp. You could do exactly the same 6AS7 for headphones too as tube only but you would have higher Zout than this hybrid.
I've also learn a lot about cascode interactions with negative feedback and phase shift.. I can see why the M60 uses the driver output to bias the output stage due to the additional phase shift adding coupling caps would add.
I also see why the M60 uses 0.1uF coupling caps between the cascode and the driver (not just to decouple) but the value also introduces a rolloff under 10Hz so by the time it's below 1Hz it's well attenuated to prevent LF instability. Additionally the 4.73Kohm resistor on the input prevents HF noise from causing instability by rolling off - starting just before 10Khz. This makes the amp sound less forced at the top end but keeps a good airy nature.
The M60-inspired frontend is awesome and very flexible. Probably too awesome for a headphone amp (which is probably why they don't use it) and my backend is probably capable of simply taking a triode output and being just as good with all the H2 goodness you want.
However this is the achilles heel of running the same amp as a headphone and as a speaker amp:
* cascode + driver + tubes - be a swinger - free the voltage swing! The obvious point here is OTL or big output tubes with an OT.. those in a cyclotron could be run as SE..
* cascode + driver + hybrid backend with larger BJTs - this suffers from the ecc99's grid limitations that limit the voltage swing available without distortion.
* cascode + driver driving larger bjts - this would be an option, with a single driver being able to drive the larger base capacitance of the large BJTs. Simply parallel for additional wattage. This keeps with the philosophy of using the tube output to driver the speakers with the BJT assisting..
* cascade + driver + mosfet output stage - let it go.. simply drive the mosfet gate with the cascode or driver depending on the capacitance. High impedance input makes that possible.. this is just like UltimateX86's circlotron.
Given the size of the chassis, the power supply requirements and the additional supporting components real estate.. I'm tempted to keep it HP for now but test it out later with speakers..