Problems - 30watt JTM45 kit

[Jalamaroz]

Need some advice please.
I just finished my 30watt jtm45 kit. Some problems I'm having are:
Channel 2 (both ch 2 inputs) work fine. Sounds great.
Now it gets confusing :-/
Channel 2 is fine if the channel 1, volume is on zero. If im playing through channel 2 and adjust channel 1's volume (greater than 0), channel 2 will not work. Also there is a noticeable hum when I adjust channel 1's volume (as well as channel two going silent).
Now more confusing: these symptoms seem to only be present when bass, treble and middle are not on zero. Seems most effected by treble tho.
Last problem: my presence pot is microphonic - it can be lightly heard amplified through the speaker.
In general terms, I can play through channel 2 with no problem....sounds great. I can adjust base, treble, mid and be fine....presence is consistently microphonic though.
When I adjust channel 1's volume (with treble, mid, bass greater than zero), channel 2 inputs do not work.
Some things I've done:
Changed out presence pot and 100nF cap
Changed out volume 1 pot and 250pF cap.
Soldered grounds again
Changed out preamp tubes
Ohm and wire checks, several times.
Rewired and soldered input jacks.
Nothing has helped.
Later today: I'm planning to take out the turret board to inspect underneath. It ohm checks fine though. I even veriy shorts to ground.
Any suggestions???

[pdf64]

My guess is that it's oscillating; does lifting one leg of the 470pF (across the 270k) and 100pF (across the volume control) affect the issue? That should make both channels the same. A grid stopper on V2 socket terminal 2 may help, eg 22k.
What do you mean by the presence pot being microphonic?
It's normal / expected for it to make a rustling noise as it is being adjusted, due to the Vdc across the track.

[The Dude]

......or, is it possible the OT primary is reversed/out of phase?

[pdf64]

That's a very tidy build, well done!
I've found it can be helpful to put a twist or 2 on the plate and grid wires for each triode, where feasible.
And keep plenty of separation between the pre-amp signal wiring and the speaker sockets.

The standby arrangement is very poor on these amps, best not to use it.

[Jalamaroz]

My guess is that it's oscillating; does lifting one leg of the 470pF (across the 270k) and 100pF (across the volume control) affect the issue? That should make both channels the same. A grid stopper on V2 socket terminal 2 may help, eg 22k.
What do you mean by the presence pot being microphonic?
It's normal / expected for it to make a rustling noise as it is being adjusted, due to the Vdc across the track.

Hey pdf64,
Thanks for the suggestion!
I was inspired by your advice and decided to disconnect:
Volume 2 from the 27k and disconnected V1 from the 27k||500pF. When I disconnected Volume 1 and Volume 2, I'm referring to the center terminal of the pot, only. The other connections remained.
I then attached "alligator clip" style jumpers to conveniently switch configuration...trying to isolate the problem. Here's what I found:

• With Volume 2 disconnected from the 270k, I connected Volume 1 to the 270k: problem seemed to go away
• With Volume 2 disconnected from the 270k, I connected Volume 1 to the 270k||500pF: the original problem came back, seems to respond to the treble adjustment (with treble on "0" the problem goes away)
• With Volume 1 disconnected from the 270k||500pF, I connected Volume 2 to the 270k: problem seemed to go away
• With Volume 1 disconnected from the 270k||500pF, I connected Volume 2 to the 270k||500pF: problem seemed to go away

Now a different configuration:
With volume 2 connected to 270k||500pF and volume 1 connected to 270k: I was hoping the problem would flip-flop but it actually seemed over all better but a few things I noted was:

• guitar plugged into Input 1, I noticed a slight squeal in when adjusting the presence pot.
• guitar plugged into Input 2, I noticed a slight squeal in when adjusting volume 1. Again, this squeal didn't occur when treble adjustment was on "0".

I apologize for not following your suggestion exactly (disconnecting the 470pF cap). I was thinking this method would offer similar results. If you still feel I should try disconnecting the cap, I'll give it a shot.
This is my first amp build so I'm very new to this stuff and terms....What exactly does a grid stop do? valve 2's input 2 should be a very high resistance to ground already right?

What I mean by microphonic: (again, I'm new to building tube amps so I'm still learning the basics and lingo) is that when you're adjusting the presence adjustment, you can hear the pot being adjustment being amplified through the speaker of the amp. this sound I'm referring to isn't scratchy or and anything that sounds bad....you can just hear the pot being adjusted CW and/or CWW. It's a smooth audible sound of the pot being adjusted. Do you say this may be normal when the pot has DC voltage on it? You can see in the schematic that, typically, it will have voltage on this pot.

Thanks for the compliment with the neatness of my work. This was my first amp but I've had a lot of experience in electrical work, in general....also have my EE degree.
Also, I did watch a few of your band's videos on YouTube. you guys rock man! Your band sounds great. I really want to thank you for taking the time to help me with this.

[MarkusBass]

What I mean by microphonic: (again, I'm new to building tube amps so I'm still learning the basics and lingo) is that when you're adjusting the presence adjustment, you can hear the pot being adjustment being amplified through the speaker of the amp. this sound I'm referring to isn't scratchy or and anything that sounds bad....you can just hear the pot being adjusted CW and/or CWW. It's a smooth audible sound of the pot being adjusted. Do you say this may be normal when the pot has DC voltage on it? You can see in the schematic that, typically, it will have voltage on this pot.

The problem has nothing to do with "microphonic" and yes, it's normal when the pot has DC voltage on it. In other amps the PRESENCE control is wired differently so there is no DC voltage on the pot.

Mark

[Jalamaroz]

That makes sense. Thanks Mark

[pdf64]

Regarding grid stoppers, see Grid Resistors - Why Are They Used?

I think the issue is ultrasonic oscillation, probably due to stray capacitance creating positive feedback paths.
So there's nothing wrong with anything you've done, it's just unfortunate but fairly common.

Many JTM45 don't have that 470pF cap bypassing the 270k (the JTM45 based amp I use that doesn't).

In your amp, I think that cap is acting to-
1/ increase the high frequency gain.
2/ disable any grid stopper effect that the 270k resistor would otherwise have (it will be reduced due to being mounted remotely to the tube, ideally they would be mounted at the tube socket terminal).

So it's having a double edged action on reducing stability.

The slight squeal indicates that the change-around you did may have affected things (ie the positive feedback paths) sufficiently to bring the frequency of instability down into the audio region.
Also that your amp is only marginally stable now; to get the best out of it, my view is that it should have a high margin of stability.

A good way to test for this is with a signal generator set to a square wave and use an oscilloscope to view the waveform, as a square wave makes resonant (ie due to positive feedback) visually apparent, eg peaks on the leading/trailing edge, aka ringing; use a resistive dummy load.
Whilst viewing the wave, moving sensitive wires around with a non conductive probe (eg chopstick, plastic pen) will affect the degree of coupling due to positive feedback and alter the ringing.

Thanks for the kind words about my band!

[Chuck H]

Also, don't simply disconnect the volume pot CT's for any operation or testing. It leaves the associated triode without bias and can cause damage. The grids must have a 0vdc relationship. A connection to ground directly or through a resistor.

[Jalamaroz]

Also, don't simply disconnect the volume pot CT's for any operation or testing. It leaves the associated triode without bias and can cause damage. The grids must have a 0vdc relationship. A connection to ground directly or through a resistor.

Thanks for the good advice......this is my first build and I'm learning a wealth of information.

[Jalamaroz]

Regarding grid stoppers, see Grid Resistors - Why Are They Used?

I think the issue is ultrasonic oscillation, probably due to stray capacitance creating positive feedback paths.
So there's nothing wrong with anything you've done, it's just unfortunate but fairly common.

Many JTM45 don't have that 470pF cap bypassing the 270k (the JTM45 based amp I use that doesn't).

In your amp, I think that cap is acting to-
1/ increase the high frequency gain.
2/ disable any grid stopper effect that the 270k resistor would otherwise have (it will be reduced due to being mounted remotely to the tube, ideally they would be mounted at the tube socket terminal).

So it's having a double edged action on reducing stability.

The slight squeal indicates that the change-around you did may have affected things (ie the positive feedback paths) sufficiently to bring the frequency of instability down into the audio region.
Also that your amp is only marginally stable now; to get the best out of it, my view is that it should have a high margin of stability.

A good way to test for this is with a signal generator set to a square wave and use an oscilloscope to view the waveform, as a square wave makes resonant (ie due to positive feedback) visually apparent, eg peaks on the leading/trailing edge, aka ringing; use a resistive dummy load.
Whilst viewing the wave, moving sensitive wires around with a non conductive probe (eg chopstick, plastic pen) will affect the degree of coupling due to positive feedback and alter the ringing.

Thanks for the kind words about my band!

Hey for my future builds, how can I minimize the possibility of this unwanted oscillation??? I'm guessing something you touched base on previously about smarter wire routing??? eg shorter wife connections to gates/plates.....
Seems like running the output transformer wires the way I did was a rookie mistake.

I'm gonna install a 15kohm resistor to the gate of V2, as you previously suggested. I'll let u know how that plays out....
Let me also ask you this: if I need to I will probably try one of your previous suggestions....to remove the 470pF cap from channel 1. You mentioned that your jtm style amp, didn't have this cap....it sort of makes me think I might could just leave it out??? I'd still hole the channel 1 would keep its high end attributes. Guess there's only one way to tell....what are your thoughts???

[Chuck H]

FWIW, I said:

Also, don't simply disconnect the volume pot CT's for any operation or testing.

That should have better clarified that you can't disconnect both volume controls. As long as one or the other is connected at any time that the amp is on you will still have a ground reference and safe operating conditions.

Thanks for the good advice......this is my first build and I'm learning a wealth of information.

Just paying it forward. I learned most of what I know here over the course of decades. Stick around and you could be as messed up as I am

[Jalamaroz]

FWIW, I said:



That should have better clarified that you can't disconnect both volume controls. As long as one or the other is connected at any time that the amp is on you will still have a ground reference and safe operating conditions.



Just paying it forward. I learned most of what I know here over the course of decades. Stick around and you could be as messed up as I am

Cool. Again I appreciate all the wise words.....
Building these amps is def something I enjoy doing and already looking fwd to my next project, after I debug this one of course....lol

[pdf64]

Yes, my preference is to leave that 470pF bypass cap off the 270k mixer resistor; whilst making one channel brighter, by the same token it acts to make the other channel muffled.
Rather I prefer to fit each channel with different value bright caps ( ie between hot track end and wiper), eg 470pF and 220pF.
It seems to be fairly random as to what value caps if any got fitted originally by Marshall as bright (volume pot cap) and mixer resistor cap, see the Marstran database Marshall Specs
Here's a schematic of a JTM45 based amp incorporating my preferred mods of the time; I've since come down against the boostrapped cathode follower The Classic JTM45 But With A Few Amendments gif by pdf64 | Photobucket

[Chuck H]

I have a question and a couple of observations.

In the picture below, what is the wire with the red arrows on it doing and what is it?

It's a nice tidy build. But that's not always the best layout. You really do need to keep any leads attached to grids as short as practical. Even if it means NOT looping them to the back or front into another wire bundle. I can't see where your NFB lead for the presence control is, but I think it's under the tone stack. Not a good place. You want that lead as far from any preamp leads and circuits as practical. Running leads on the face and rear panels and bundling looks tidy, but it shouldn't be done in lieu of good layout practices. Keep output leads away from preamp leads. The multi point ground you're using is ok, but it's best not to daisy chain grounds. You definitely want all the preamp grounds in the same place and separate from power amp grounds.

Even factory Marshalls have suffered oscillation problems. Veering away from the normal layout is really taking a chance unless any differences are planned to improve stability. You'll probably need to do a lot of poking and moving leads around. Perhaps even relocate or separate some grounds.

Also, I've used normal pots for bias circuits too. I don't do it anymore though. The quality of currently made panel pots is nothing you want to trust your bias to. Try to use multi turn cermet pots if you can. Or at least a very high reliability part that is clearly better than a typical panel mount pot.

Now, about that lead with the arrows on it?