Q re: mixing circuit for Marshall’s

[RobBozic]

Hi all,
Hope everyone is doing well.


Couple of questions regarding my attachment.
Will the 220K mixers in series with the 470k / 470p and 470k / 330p provide enough isolation between channels so the treble boost on each channel won’t be affected by the other channels treble boost? Goal is to voice each channel like a bright channel with then further tweaks with cathode circuits and bright caps etc… Hope that makes sense.


I’m not sure on the math for this, or does it just come down to trial and using your ears.


Also is there a limit to the value of the overall mixer resistors? Ie: can I increase the 470k’s to say 820k to provide more isolation? (With new capacitor value providing the same frequency boost).


Thanks in advance.
Rob

[pdf64]

I don’t think the arrangement you’ve come up with may provide much of a treble boost for both channels.
Each side’s mixing circuit is the 0V return for the other.
And then there’s the Miller effect of the following stage rolling off more high frequencies as the total effective grid circuit impedance increases.
To achieve your goal, I use a bright cap on each channel vol control, 270k mixer resistors unbypassed.
It works well enough for my purposes, given that bright caps become less and less effective as their volume control setting increases.

Oops, typo, edit in bold 

[RobBozic]

Hi,
Thanks for your reply.
I understand what you’re saying, sort of.
However if I have the other control up half way this provides some resistance to ground like a tone control. Then again I guess there’s no need for the 220k mixers in series in my sketch.
As per your advice I’ll just put a bright cap on a switch for the Normal channel, say 250-470pf and this should sort of balance out the treble roll off from the bright channel mixer bypass cap.



Another question is the attached Ampeg J20 mixer arrangement. I assume the Normal channel will not have highs bled to ground somehow via the Bright channel 470p mixer cap?


Thanks
Rob

[sluckey]

Also is there a limit to the value of the overall mixer resistors?

Some Magnatone amps used 1M for mixing resistors. The M12 is one example...

     https://el34world.com/charts/Schematics/files/Magnatone/Magnatone_m12.pdf

[pdf64]

…
Another question is the attached Ampeg J20 mixer arrangement. I assume the Normal channel will not have highs bled to ground somehow via the Bright channel 470p mixer cap?
…

You assume incorrectly.
What’s boosted on one channel gets cut on the other. So truthfully, the channels are bright and muffled, rather than bright and normal.

Anodes are connected to 0V common via the anode load (DC and AC) and the internal anode resistance.
It really helps to get your head around small signal AC circuit analysis, reducing things down to their Thevenin and Norton equivalent circuits.

Apologies, see edit to reply #2 in bold.

[shooter]

It really helps to get your head around small signal AC circuit analysis, reducing things down to their Thevenin and Norton equivalent circuits.

WARNING!!
there is a whole generation of ppl like me irreputable damaged by classes like that 

[PRR]

> a whole generation of ppl like me irreputable damaged by classes

Come now. This is no different from:  if you run a ditch from your upper field to your lower field, to drain the upper field, your lower field is gonna get wetter. Network theory lets you say how much wetter. That may not be precise help in wet and dry years, or loud and soft music. But trends can be useful.

[RobBozic]

Thanks everyone.
the Magnatone amp looks interesting, mixers off the plates.
Yeah I need to read about Thevenin circuit analysis. I’ve always skipped over those sections of Merlin’s
Thanks again
Rob

[jordan86]

I found a sample page of merlins book on google books. I got one sentence in to thevenin theory and my brain hurt. Didn’t even bother scrolling down to Norton. 

But what my small mind does know is that the earliest JTM45’s used 270K mixing resistors. So it must have worked well enough to release that way. I also use 220K mixing resistors with 500k volume pots in my single ended plexi and it seems to work.

https://www.drtube.com/schematics/marshall/jtm45tr.gif

[shooter]

I got one sentence in

now imagine your whole career rests on "getting it right!!"
I did it, but after cranking all the math, the components were nothing but equations, which just sucked the life outta  "Electronics"!!!


I've found you can swap 1/2 doz parts and test in less time than it takes to math out an answer in most cases

if you run a ditch from your upper field to your lower field,

LOL, this was the year I finally got the inflow - outflow dialed in.  No math, just lots and lots of digging soggy peat, some stone work.  I've even determined 1ft drop in pond level requires a 1.5" rainfall to break over the outflow.  I did use a ruler n rain gauge though

[jordan86]

Can someone dumb this down for me...are we saying that even with mixing resistors, the channels still interact and cause some sort of losses on their opposing channel? Like phase cancelling of frequencies? I would like to practically learn more about that.

[pdf64]

The other channel’s mixing resistor connects, via the stage’s output impedance, to 0V common.
So with 2 identical channels there’s a 6dB loss in the mixer.

[Ed_Chambley]

Can someone dumb this down for me...are we saying that even with mixing resistors, the channels still interact and cause some sort of losses on their opposing channel? Like phase cancelling of frequencies? I would like to practically learn more about that.

Let us dumb it down.  If you remove any ANY part of the circuit, the tone and frequencies will change.  If you find an amp that you really love and make sure you each stage works identically, you will have the amp!  No Shift!


If you can hear what I cannot, sorry but I remove myself from your debate!  Do Output Transformers sound different?  I suggest they all sound better than ClassicTone does now!  Look for Nice Components Baldwin OT's for 6v6 at 10 K reflected and run your El84's and 6v6 circuits at 320 Plate Voltages in keeping with RCA 6v6 (nice) or malladr duck EL84's for more saturation.  Then after you have built this 15 watter, fins the best cabinet, speaker you can find.  Heaven awaits!