Lead 100 Watt started

[angelodp]

She lives, only minor issues with one cold solder on V2 Heater, otherwise all good. Needs some tweaks for Tone. Waiting for owner to chime in.

Many thanks and appreciate your help and patience.

Have to say that although the Mojotone basic parts are a beginning, this amp (for me) was only possible with the Metro guide and the good people here at TAG. The amp is very quiet, thanks Aaron and Dave for the prompts, Sluckey/Steve props for the various heads up, Mark, Roe all here at the font of amp knowledge. The kid came over to play with his Van Halen chops and guitar and was beaming. I will keep it for a bit to further test and document but its pretty much good to go. I would say if I was doing this again it would be Valvestorm all the way.



Ange

[Roe]

break it in before you tweak it

[angelodp]

Roe, will do. I found this note by Jana. My speaker jacks are isolated but the ground would seem to be not correct per this instruction. I will try.

BTW I initially had the amp at a very cold bias setting, until I checked levels. I then put it at 50% dissipation (23ma 470vdc B+) per Aaron and I do favor that setting. When I had the amp at 32ma there was a hum/vibration on the chassis.... when you touch the chassis it was a low level vibration and hum?? This was decreased markedly with the colder bias setting. Is this hum/vibration common with these amps? I do not recall feeling that sort of hum/vibration on my other amps, but I don't have any 100watters.

Oh, and wondering what recommendations you might have for the bright channel, its verging on Fender ice-pick. Pull the bright cap (500pf), lower it...other.

When all other avenues are exhausted, PI hum is almost always caused by a ground problem. The speaker jacks need to be isolated and the common from the OT secondary needs to be grounded at the same place as the PI--most commonly at the same place that the presence pot is grounded if using a 5K pot or the 4K7 resistor if using the other style presence. The OT secondary common is connected to the speaker jacks but a ground wire is run to the aforementioned location.

This layout represents the marriage between a Mojo kit and Metro layout and approach....and TAG special sauce.

[angelodp]

BTW, you all know this... but I cannot emphasis enough the need to check over your work. The early Marshall's had that red dot on the solder joints as an indicator of a serious double check. I just found two sub-par solder joints in the B+ while chop-sticking..... little tiny sparks. Reflowed and the hum vibration thing went away completely...it was mild, but that was enough to cause issues. Now can easily vary the bias from 50% to 65% dissipation and its all good. Had a blues player buddy by and we cranked her up full blast, no red plating, no hiss just Marshall bliss, even jumped the channels to ease the treble thing. I want one.

[xtian]

Man, the pressure is much higher when you're building for a client!

[angelodp]

My hats off to those, like you Aaron who are creating for clients. This would be my first build for someone else and I was really quite nervous.... Read more, test more, read more.

[Roe]

Roe, will do. I found this note by Jana. My speaker jacks are isolated but the ground would seem to be not correct per this instruction. I will try.

BTW I initially had the amp at a very cold bias setting, until I checked levels. I then put it at 50% dissipation (23ma 470vdc B+) per Aaron and I do favor that setting. When I had the amp at 32ma there was a hum/vibration on the chassis.... when you touch the chassis it was a low level vibration and hum?? This was decreased markedly with the colder bias setting. Is this hum/vibration common with these amps? I do not recall feeling that sort of hum/vibration on my other amps, but I don't have any 100watters.

Oh, and wondering what recommendations you might have for the bright channel, its verging on Fender ice-pick. Pull the bright cap (500pf), lower it...other.

When all other avenues are exhausted, PI hum is almost always caused by a ground problem. The speaker jacks need to be isolated and the common from the OT secondary needs to be grounded at the same place as the PI--most commonly at the same place that the presence pot is grounded if using a 5K pot or the 4K7 resistor if using the other style presence. The OT secondary common is connected to the speaker jacks but a ground wire is run to the aforementioned location.

This layout represents the marriage between a Mojo kit and Metro layout and approach....and TAG special sauce.

500pf sounds bright at lowervolume. 100pf is better then. But 5000pf is great at higher volumes

[angelodp]

How active is the presence pot on these amps? I have lost a lot of the highs above 6000hz. Does the PPIMV kill some the functionality fo the presence pot?

A

[Colossal]

Does the PPIMV kill some the functionality fo the presence pot?

Yes, because the Presence control is inside the NFB loop. If the PPIMV is turned down, then there is a smaller signal appearing at the output transformer secondary that is getting fed back in, and so the Presence control becomes less effective at low levels of PPIMV. You can compensate (somewhat) by decreasing negative feedback by putting a 100-250k pot in series with your feedback resistor and other trickery.

[martin manning]

Does the PPIMV kill some the functionality fo the presence pot?

Yes, because the Presence control is inside the NFB loop. If the PPIMV is turned down, then there is a smaller signal appearing at the output transformer secondary that is getting fed back in, and so the Presence control becomes less effective at low levels of PPIMV. You can compensate (somewhat) by decreasing negative feedback by putting a 100-250k pot in series with your feedback resistor and other trickery.

To compensate I believe you need to increase the FB signal by decreasing the FB resistor value by about as much as the PPIMV reduces the open loop gain.

[angelodp]

My FB R is the standard 47k, would halving that be adequate. If you can share how that calcs out.

A

[martin manning]

That’s probably not a bad place to be if you are going with a fixed resistor. That would line up with the PPIMV cutting the signal out of the PI in half.

[angelodp]

I think quoting you here/below. I do not hear much from the presence pot except for the final 20 degrees. So how does not deal with this conundrum when using a PPIMV.... not much can be done?

A NFB pot on the rear?




Usually there is more than enough range on the presence pot- the presence effect doesn't usually come on until the pot is nearly all the way up.

[Colossal]

Does the PPIMV kill some the functionality fo the presence pot?

Yes, because the Presence control is inside the NFB loop. If the PPIMV is turned down, then there is a smaller signal appearing at the output transformer secondary that is getting fed back in, and so the Presence control becomes less effective at low levels of PPIMV. You can compensate (somewhat) by decreasing negative feedback by putting a 100-250k pot in series with your feedback resistor and other trickery.

To compensate I believe you need to increase the FB signal by decreasing the FB resistor value by about as much as the PPIMV reduces the open loop gain.

Thanks Martin, as always. Bloody dyslexia

That said, you do gain some flexibility with having a largish value pot in series with your NFB resistor as a means to get a little more hair at low settings of PPIMV. It's a very useful mod if you are doing a kitchen sink Plexi build.

[martin manning]

I think quoting you here/below. I do not hear much from the presence pot except for the final 20 degrees. So how does not deal with this conundrum when using a PPIMV.... not much can be done?

A NFB pot on the rear?

Usually there is more than enough range on the presence pot- the presence effect doesn't usually come on until the pot is nearly all the way up.

This is a different problem. A reverse log presence pot would help, but most people just live with it.