Even order harmonics versus tube matching

[5150loveeddie]

Here is an other view of the subject:

Sorry to come off a bit rough.... But a lot of what has been said does not apply to this PI circuit....
The analysis does not take into acount the FEEDBACK LOOP !!!!
This changes everything...
The Tail in the P.I. which also includes the 5K presence pot recieves a CORRECTION signal from the output that forces the P.I. to go back into balance..... The correction signal is out of phase, therefore it pushes back in the opposite direction at the tail, thus puting back the system into symmetry... This is a dynamic AC operation that takes place when signal is present... monkeying with the plate resistors is a waste of time...
Yes it is true tha the 82K should be a bit bigger for starting out in balance but that would only be valid if you had perfect 12AX7 tubes that were matched so both halves had dead on perfect gm and your output stage was gm matches as well..... Thats why this P.I. circuit was designed, so that the feedback gain makes up and corrects for the "normal" imbalance found in P.I. and output tubes... the correction signal takes into account the output valves in addition to the P.I. tubes, so it looks at the entire output stage..
There are a number of things that can go wrong and cause the closed loop system to get a false sense of thinking it's in balance, when it's not actually... addressing these issue is critical to dialing in a Marshall instead of all the "band-aid" tricks that try to mask the issue....
Try disconnecting the feedback loop to see where the symmetry is at..also the more off the symmetry is with feedback loop dissconnected then the more feedback needed to push it back in..so the amount of feedback can be critical as well.....but too much correction feedback can sound cold....so try starting off close to being balanced...also the less gain in V3, then the less it is able to make correction....
For increasing Even order harmonics look to the triode gain stages....the biasing and loading play a major role...

What do you think?

[novosibir]

You could just put another resistor across the tail resistor. That'd give you a bit mo' gain too. The longer the tail the better the balance and vise versa. If you got enough gain going on in you preamp you can change the cathode resistor on stage right before the cathode follower. That will make the preamp clip assymetrically. Just messing with the bias on you power tubes will get you different hamonics as well, and you don't need to be clipping for it to happen.

What is THE tail resistor? And what value should we put accross it to add K2s??

The resistor right before the the cathode follower, the 820ohms right? So we change it with what? 1.1k, 1.5k????

The "tail resistor" is the 10K in the cathode row of the driver tube. Still lower it to maybe 6.8K like in some Tweed Fenders leads to a more rugged sounding amp pretty like an increased FB resistor and makes the driver stage still unstable working.

Increasing to maybe 22K like in the Blackface Fenders stabilisizes the driver more than it is now, but you won't like the sound. You'd tame the beast and you'd missing some punch! But great for ONLY clean playing.

I wouldn't recommend any change in the cathode row of V3!

The 820 ohms cathode resistor of V2 anyway doesn't set the tube to the middle of it's working line (there's another word, but I don't know it right now out of my head), so by driving hard the V2 it's anyway clipping one side more than the other and is producing K2. Increasing to 1.1K or higher would make the amp sound cleaner with less K2, with less even order harmonics.

The only you can do is what I've already mentioned above in this thread, to set both the plate resistors more out of symmetry - increasing the 100K, decreasing the 82K - but more or less of the obtained asymmetry would be stabilized out again from the feedback loop...

... the feedback loop always compares: what's going out, what came in, and tries to regulate - so in this case less feedback would be better that too much. But when the power stage is driven into saturation, the feedback loop anyway mostly is out of function, because it can't handle so much "regulation" as needed in this situation

Larry

[5150loveeddie]

Thanks Larry................

[novosibir]

Thanks Larry................

You're welcome

Larry

[5150loveeddie]

Larry,

I've found a 112K and a 103k in my 100ks cc Allen Bradley resistors, before I start twinking for ever, which one should I go first OR would the 112k be to much and lows might get too muddy??

As for the 82ks I found one at 80.0K.......

Thanx again!!!!!!!

[rockstah]

very interesting

[Bainzy]

I think finding this is one of the great problems people have with wanting better tone - all the 'magic' amps that people talk about seem to have this more than others.

You could try driving the power tubes even more with the resistor method ****** described, but I wouldn't risk blowing parts in my amp just to get that tone.

[novosibir]

I've found a 112K and a 103k in my 100ks cc Allen Bradley resistors, before I start twinking for ever, which one should I go first OR would the 112k be to much and lows might get too muddy??

As for the 82ks I found one at 80.0K.......

Every amp is reacting different to a change in the drivers symmetry, what depends on the symmetry of your driver tube, the OT and the feedback loop.

But the 80K/112K sound good. Solder it in and try alternately to parallel the 112K with a 1.0 meg with alligator leads, to "make" it back to a 100K again and hear to the difference!

You also can go the other way and don't go with the 82K under, but over the balance point, make both plate resistors 100K and try alternately to parallel the first one with a 470K, to make it back again to a 82K

Happy experimenting!

Larry

[5150loveeddie]

Larry!

What would be a good driver tube in your opinion, I just found myseft two 12ax7A from Westinghouse (one black plate and one long plate)??

[novosibir]

I don't have any experience with Westinghouse tubes, but anyway any NOS would be a good choice.

Good experiences I also had with EI's in the driver position and microphony "there" (nearly) isn't an issue.

BTW: I'm missing your last post on this place, where you spoke about your experience with the inbalance of the driver tube Who stole it off of this thread?

Larry

[5150loveeddie]

Yeah I was repeating myseft a little to much so I will repost in a clearer manner once I finish my testing, I'm still trying to figure out if the 80k/112k is the best, i don't notice any changes when piggy backing the 112k with a 3M or neither a 2M but around 1.5M it does kind of change thing getting closer to the stock 100k, should I try higher then 112k or stop AND go lower then 80k instead, is there a SIDE to consider more then the other or it is irrelevant??, anyway the sound is ultra rich, so far so good Larry

BTW I gave your email to a guy in the UK he needs a replacement output transformer, he might contact you....

[novosibir]

... should I try higher then 112k or stop AND go lower then 80k instead, is there a SIDE to consider more then the other or it is irrelevant??, anyway the sound is ultra rich, so far so good Larry

The side doesn't matter, to get the inbalance - only when the plate resistors are too much different, also the current becomes very different.

So try the following, to maybe get out still more "richness":

- Reduce the 112K at the second system again to about 100K.

- Replace the first plate resistor with a 68K

- and instead of a simple cable from the board to the tube socket's plate pin swap in a 33K resistor.

Then both plates will "see" a 100K plate load and will suck about the same current, but the AC signal from the first system to the output tubes is diminished by the voltage divider to about 2/3

Larry

[5150loveeddie]

... should I try higher then 112k or stop AND go lower then 80k instead, is there a SIDE to consider more then the other or it is irrelevant??, anyway the sound is ultra rich, so far so good Larry

The side doesn't matter, to get the inbalance - only when the plate resistors are too much different, also the current becomes very different.

So try the following, to maybe get out still more "richness":

- Reduce the 112K at the second system again to about 100K.

- Replace the first plate resistor with a 68K

- and instead of a simple cable from the board to the tube socket's plate's pin swap in a 33K resistor.

Then both plates will "see" a 100K plate load and will suck about the same current, but the AC signal from the first system to the output tubes is diminished by the voltage divider to about 2/3

Larry

Do you mean to install a second cable from the 68k (board) with a 33k in serie to the plate (pin), so there will be two wires hook up to the plate pin??

Remember Larry I'm a frog and your German, btw my very first girlfriend came from Wisbaden...25 years ago!!!!

[novosibir]

Do you mean to install a second cable from the 68k (board) with a 33k in serie to the plate (pin), so there will be two wires hook up to the plate pin??
Remember Larry I'm a frog and your German...

Ok, once again, for all the dogs, frogs and Spoks out there :

NOT a second cable, but INSTEAD of the cable should be the 33K. In other words, on the board you swap in a 68K instead of the 82K plate resistor. From the 68K usually a blue cable is running to the plate's pin - because cables don't have so much resistance, put in the 33K on one end of the cable - or if the leads of the 33K are long enough, remove the cable entirely and put in the 33K instead of the cable.

... btw my very first girlfriend came from Wisbaden...25 years ago!!!!

Ah ha, you've joined the Army here in Germany What about your kids with your first girlfriend

Larry

[5150loveeddie]

Got it..........

No kids Larry I was only 15, well I think there is no kids, I hope....she left Canada back to Germany no news since, she was my first girlfriend we only kiss.......