Changing Marshall bass to one channel only? Shared/Split Cathode

[g1]

Perhaps SGM meant that the lack of full cathode bypassing on the 'high treble' input triode may result in increased susceptibility to picking up heater hum (on that triode)?

Last time he mentioned it, he was talking about the actual tube, not the circuit. As if the "A" side of a dual triode was always noisier than "B" or some such.
So it was supposed to matter whether you wire up 1,2,3 or 6,7,8 in all cases, but it was some secret that most "mere techs" were unaware of.

[g1]

So I put an resistor parallel to the last 50uF to ground? Let's say I am measuring 300V at this point I will use R = 300V/0,001A = 300k ? So I am simulating what the other triode will draw?

Exactly. If it was 300V at that node when both sides were running. So you need to know what the voltage was stock.
You are simulating what the now unused side was drawing before.

[J M Fahey]

One small detail.

47k plate resistor, 430 ohm cathode resistor

47K combined plate resistor is fine, but cathode resistor is already meant for 2 tubes, so keep the 820r .
And agree that parallelling both triodes is better than just leaving one unused.

[pdf64]

Sorry, I was thinking of the 1987 http://schems.com/manu/marshall/jmp_lead_50w_1987.pdf so some of my previous post is nonsense, ie the partially bypassed cathode hum and (as Juan points out) the value of cathode resistor.
Pete

[shocki]

And agree that parallelling both triodes is better than just leaving one unused.

This means the signal runs through both triodes but with less gain? But shouldn't it be about 27k for about half the gain?

[soundguruman]

That's probably what shocki had in mind.
Let's see.
Now we have an unterminated triode with an open grid, happy picking radar signals from Mars or whales talking, it's messing with the other triode's bias, we have unexpected +6 dB gain because we eliminated a resistive mixer and the sound of the remaining channel is like none of the original ones.

Here's one proper way to do it, at least keeping the bright channel:
[ATTACH=CONFIG]28759[/ATTACH]

EDIT: well, while I was busy editing the schematic other answers clicked in.
The mod I suggested is the one most faithful to the original circuit and sound , not getting into discussions whether it's good or bad.
Pity is, both channels do not sound the same, one is bright and punchy, the other is mellow, dark.
Now you lost the option to plug into one or the other.
Even worse, you lost the unique Marshall option of plugging into the bright one, jumpering the unused input into the dark one, and mix both sounds at will to get other flavours.
Personally I would save a little panel space by using just *one* jack, wired to both triode inputs, and keep the dual volume controls.

"Now we have an unterminated triode with an open grid, happy picking radar signals from Mars or whales talking,..."

Hahahhahahhahhahhahhhaaha! that's funny. I thought you had no sense of humor, but indeed I was very very wrong...
hahahahahhahhahhahhahhhhah. ILICISCCCAOMKB.
good one.

You might think the unterminated triode would do that, but no. The 1/2 tube cathode plate being UN-connected, it is not dropping a voltage, not an amplifier. Good point though.
Perhaps thing that you could terminate the UN-used grid to ground?

I wanted to leave the V2 series grid resister / parallel bypass cap in. Not out. I mean I am thinking distortion, and not the added input signal.
But more "gain," or input signal would be OK with me, sans the series grid resistor / cap, if not for the tendency to drive the grid temporarily V2 too far positive...producing what I am calling undesirable distortion. Or some would call it cut off distortion.
But in order to test for this, the amp is going to be driven by guitar, and is being judged as sound quality, instead of just a usual sine wave.

[J M Fahey]

You might think the unterminated triode would do that, but no.

Oh yes it would.
You should refresh a little your Electron Tube theory, which you show you don't understand.

The grid voltage *still* controls the electron flow, from cathode to plate.

Does it have a predictable voltage, referred to ground or cathode, so we are able to establish beforehand *how much* current will that triode pass?
No.

Unless some lab starts measuring and publishing such data.

Will it have *some* voltage present?
You bet.

To begin with, and to be pedantically precise, we should talk about an *electrical field* there (go Google that difficult term ) which is what will actually control the electron flow.

Of course, applying an external voltage there (yes, grounding, a.k.a. "0 volts" is *also* a voltage) *will* establish an electrical field.

In the absence of a purposelly added voltage or reference, THERE WILL *STILL* BE THERE *SOME* ELECTRICAL FIELD , which by definition will control electron flow.

That point will also be ***VERY*** HIGH IMPEDANCE and so extremely sensitive to any electric field, which happens to reach that point in space.

And that electric field may be static or any frequency (that's what's implied in the "whales talking to Radar transmissions" range).

Sorry for using examples way beyond your capacity to understand.
I should use simpler examples.

Although, in fact, why should I?
The explanation about unterminated grids was not meant for you anyway but for legitimate posters who *might* find it useful.

And you are forgetting something else, that grid, even if not physically connected to anything else, will still get some negative bias all by itself.
But don't want to confuse you even more, let's go on with what you wrote.

The 1/2 tube cathode plate being UN-connected, it is not dropping a voltage, not an amplifier.

Don't know what a "cathode plate" is, not such element in any triode description available (although you probably write your own books and invent your own words to support your own funky theories)

But if you mean that tube is passing no current, you are WRONG again

That cathode is near ground, that plate is near +B , there's presumably some 200 volts difference between them, I guess the cathode is hot and spewing electrons all around it so .... yes ..... current will pass.

Go back to study Electron Tube Theory, it was demonstrated over 100 years ago.

And "passing some current while having some voltage difference between its terminals" *does* meet the definition of "dropping a voltage" , unfortunately for you.

Perhaps thing that you could terminate the UN-used grid to ground?

Go get a new pair of glasses or at least wipe some of the puke that covers your monitor screen, in my edited schematic I DID connect the unused grid to ground.

Does that qualify as "terminated"?
I guess so.

I wanted to leave the V2 series grid resister / parallel bypass cap in. Not out. I mean I am thinking distortion, and not the added input signal.

I beg your pardon?
Not clear what are you hinting at.

But in order to test for this, the amp is going to be driven by guitar, and is being judged as sound quality, instead of just a usual sine wave.

I beg your pardon?
The amp is being judged AS sound quality?
What does that mean in English?

being judged as a sine wave?
You mean the amp is a sine wave?

Wouldn't it be :
being tested/judged USING a sine wave?

You know, English DOES have some grammatical rules to write phrases which convey meaning, that's the main purpose of language.

[pdf64]

My experience is that an (accidentally) unterminated 12A_7 grid can cause the section to redplate.

'And agree that parallelling both triodes is better than just leaving one unused.
This means the signal runs through both triodes but with less gain? But shouldn't it be about 27k for about half the gain?'

Arranging like triodes in parallel, whilst keeping the operating point the same as for a single triode, tends to increase gain and reduce noise. It's nothing massive but worth at least trying if there's a spare section.
Pete

[soundguruman]

[QUOTE=J M Fahey;345894]Oh yes it would.
You should refresh a little your Electron Tube theory, which you show you don't understand.

The grid voltage *still* controls the electron flow, from cathode to plate.

Does it have a predictable voltage, referred to ground or cathode, so we are able to establish beforehand *how much* current will that triode pass?
No.

Unless some lab starts measuring and publishing such data.

Will it have *some* voltage present?
You bet.

To begin with, and to be pedantically precise, we should talk about an *electrical field* there (go Google that difficult term ) which is what will actually control the electron flow.

Of course, applying an external voltage there (yes, grounding, a.k.a. "0 volts" is *also* a voltage) *will* establish an electrical field.

In the absence of a purposelly added voltage or reference, THERE WILL *STILL* BE THERE *SOME* ELECTRICAL FIELD , which by definition will control electron flow.


Go back to study Electron Tube Theory, it was demonstrated over 100 years ago.






Yes, I will be sure to call Fender, and tell them you don't approve of the 1/2 triodes in their amplifiers either.
Fender has been using 1/2 triodes for decades...
and they leave the other half UN-terminated, just like I do...and just like a lot of other builders. do...

But since this is such an egregious offense...
Maybe you should call Fender, and get their designs into compliance....to satisfy YOUR requirements.

Because after all, we are here to satisfy YOU. We are not allowed to use our own ideas, without your approval, and neither should Fender.

We don't want any UN-terminated triodes picking up Mars, especially since these 1/2 triodes are built into 120,000 Fender amplifiers....

You no like my English?
The guy from South America doesn't approve my English either.
I made a note of that.

When you pick up Mars with an un terminated triode, what message are you receiving?
What will our Martian Overlords convey to us on this day?


" Earthlings, your Les Pauls are jamming my illudium Q-36 explosive space modulator.
Cease 1/2 triodes immediately........... or I will be verrrrrry angry !!!
Prepare to be vap---or---ized!"


Guitar instead of sine wave? We can't have that either.
We don't build guitar amps for guitar players. My mistake, sorry.

[big_teee]

I just read through the thread?
I don't see what the amp is used for?
Bass or Guitar?
If Bass there are better options.
If Guitar it seems to be a waste of good hardware to me.
If Guitar use the other preamp stage and make a Cascade 2204.
That still only uses 2 jacks, and you have a High and low output side.
Just my 2 Cents.
T

[g1]

Fender has been using 1/2 triodes for decades...
and they leave the other half UN-terminated, just like I do...and just like a lot of other builders. do...

We were not talking about un-terminated triodes. In the particular case we were talking about a triode that has plate and cathode connected but the grid is un-terminated. So JM was 100% correct. Try it for yourself, disconnect only the grid of a preamp tube in an amp and see what happens.

Once again you jump into a thread without reading what is going on or paying attention. No wonder you end up being wrong most of the time, half the time what you say is completely out of context to what is being discussed.

[Justin Thomas]

Okay, so... Juan is talking about the unterminated (GRID) triode in the schem that nashvillebill posted in post #6, which I've never tried. SGM is talking about the unconnected triodes that just sit there, a la Fender, which I know about cuz I have one unused triode each in my Concert & Bassman. Funny what happens when you're talking about 2 different things... Damn, now I have something new to try. I always though SETI and inter-species communicatiion was pretty cool.

Justin

And I see g-one beat me to it.

[g1]

You mean inter-species communication like us trying to discuss things here with the "great gazoo" ?