THE HW Marshall thread. Tweaking it step by step.

rockstah · Post by **rockstah** » Wed May 10, 2006 5:21 pm

Bainzy wrote:Well all the grids are shielded, and I've got (apparently) 5.6k grid resistors on each power tube.

How will adding an input grid resistor to V2a affect the sound?

high freqs do not like resitence.

of course your lead dress, grounding scheme can be looked at as well to address PO issues.

i edited my last post.

Bainzy · Post by **Bainzy** » Wed May 10, 2006 5:37 pm

but would it affect the gain of the rest of the signal? I'dve thought resistance would reduce current flow and thus gain.

rockstah · Post by **rockstah** » Wed May 10, 2006 5:38 pm

Bainzy wrote:but would it affect the gain of the rest of the signal? I'dve thought resistance would reduce current flow and thus gain.

one way to find out - me thinks it makes the sound browner. and not effect gain so much

Bainzy · Post by **Bainzy** » Wed May 10, 2006 5:39 pm

ok thanks Mark - first I'll try the plate resistor bypass cap; then changing the screens; then adding a V2a input grid resistor. I'm really dying to see if replacing the mains filter caps will have any effect though aswell. If anyone has any spare working dual 50uF's going for free I'd be forever indebted to them, lol.

rockstah · Post by **rockstah** » Wed May 10, 2006 5:41 pm

start low. what r u replacing the mains to from what? or r they just bad at this point?

novosibir · Post by **novosibir** » Wed May 10, 2006 5:58 pm

rockstah wrote:i have also read of instead of grounding the shielded cable to ground you would connect the shield to the plate( pin3)- something about freqs 180 degrees out of phase will cancel the oscilation happening.

That is acting like a local FB loop, where the huge plate level in the shield is capacitive dampening the signal in the grid wire. The higher the frequency, the higher the dampening factor.

But I wouldn't recommend this method! Not only that it's killing the nice sparkling highs, it's dulling the amp's sound!

If that would be the only solution, to solve the PO problem, then already BEFORE in the amp's layout much was done wrong.

Larry

Bainzy · Post by **Bainzy** » Wed May 10, 2006 6:00 pm

Well the two mains caps are the only ones I haven't replaced so far on the amp. A while back I noticed 1 mains and 1 screen cap had large bubbles in the top, so I took those two out, bought F&T dual 32u's for the screens, and took the dual 50uF I used to have on the screens that didn't have a bubble, and put it in the other mains position. I've also replaced the PI cap which was leaking, as replacing it (with the same value, spare of same brand I had) cleared up some of the noise, and I've changed the preamp cap a few months ago to a dual 16u F&T for preference.

So they're still one of the possible problem areas with the amp - I've effectively eliminated all the filter caps except the mains ones. Then other areas could be a bad resistor somewhere on the board, a bad socket, a bad jack etc. Funnily enough, I need to replace the power tube sockets anyway as the JJ E34L's don't sit right in them - sometimes I have to wiggle the tubes around in the socket just to get them to light up.

Post by **VelvetGeorge** » Wed May 10, 2006 6:11 pm

I threw down some clips this afternoon. If you can get past the bad playing and out of tune les paul- there's a good sounding amp in there.

I made sure nothing moved (mic, preamp settings ect). The harmonics now jump out in a nice musical way. And it does sound close to my 12 series.

George

BashCoder · Post by **BashCoder** » Wed May 10, 2006 6:19 pm

If you don't mind explaining to me: With parasitic oscillation, which signal is interfering with which other one? Is this generally a preamp signal affecting the final amplification, modulating it in some undesirable way?

rockstah · Post by **rockstah** » Wed May 10, 2006 6:19 pm

novosibir wrote:
rockstah wrote:i have also read of instead of grounding the shielded cable to ground you would connect the shield to the plate( pin3)- something about freqs 180 degrees out of phase will cancel the oscilation happening.
That is acting like a local FB loop, where the huge plate level in the shield is capacitive dampening the signal in the grid wire. The higher the frequency, the higher the dampening factor.

But I wouldn't recommend this method! Not only that it's killing the nice sparkling highs, it's dulling the amp's sound!

If that would be the only solution, to solve the PO problem, then already BEFORE in the amp's layout much was done wrong.

Larry

agreed!

rockstah · Post by **rockstah** » Wed May 10, 2006 6:23 pm

VelvetGeorge wrote:I threw down some clips this afternoon. If you can get past the bad playing and out of tune les paul- there's a good sounding amp in there.

I made sure nothing moved (mic, preamp settings ect). The harmonics now jump out in a nice musical way. And it does sound close to my 12 series.

George

George,

i think it sounds killer! nice! i thought the playing was good also!

Mark

Flames1950 · Post by **Flames1950** » Wed May 10, 2006 10:24 pm

That amp's finally got back.

LeftyStrat · Post by **LeftyStrat** » Thu May 11, 2006 12:30 am

The stock amp sounded good, but seemed slightly sterile. Now it has what I can only describe as a "singing" quality. It sounds much more harmonically rich now. Just my humble .02
Great job George, on the sound and the playing. Very cool

-pk-

Ricky Lee · Post by **Ricky Lee** » Thu May 11, 2006 9:02 am

What a shame, pay thousands of dollars for an amp from a so called reputable manufacture "Marshall"and then have to spend another load on getting the amp to sound the way it should sound. What are the Marshall techs thinking when they build these things, it's as if they've never heard vintage sound before. If people can get past this name thing "Marshall" they'll get a much better and more vintage sounding amp by purchasing a ready built from George, or even building one from scratch themselves. If your one who needs the "Marshall" name on your amp, they sell the logos on EBAY for about $20.00, which is much cheaper than an amp overhaul!

LeftyStrat · Post by **LeftyStrat** » Thu May 11, 2006 9:43 am

Actually, George has them in 6" script for $15.00. I've gotta get one for my '74..

-pk-