Effects of filter caps and chokes

dynaman · Post by **dynaman** » Tue Sep 14, 2004 5:52 pm

I wanna build a 50W based around the "Plexi" circuit. I recall reading that that a lower plate voltage can result in earlier power tube distortion. Would lowering the usual values of the filter caps and choke achieve this? How about using a power tranny with lower B+? Wouldn't this give the same effect?

Post by **VelvetGeorge** » Tue Sep 14, 2004 6:42 pm

Using a lower voltage PT would accomplish exactly what you're talking about.

Using lower value filter caps and choke will have some of the same effects, but not exactly. Their effect is more frequency dependant, and will change the feel of the amp more. They will make it seem more loose, less punchy.

George

dynaman · Post by **dynaman** » Wed Sep 15, 2004 8:51 am

I have access to a pair of original 50W Marshall (Drake) trannies. Do you know what their specs are? How low can I go with plate voltage (I'll be using EL34's)?

I bought some of Kevin O'connor's books and he demonstrates how to add a tube rectifier that can be bypassed with solid state diodes. I can't resist the urge to try this. Yes. I realize I'll have to add a 5VAC transformer for the filaments. I understand that a 5AR4 will drop plate voltage about 30 - 50 volts. Again I ask you, how low can I go with plate voltage? Thanks for the help!

Post by **VelvetGeorge** » Wed Sep 15, 2004 12:46 pm

I've never seen a pair of EL34's in push-pull with less than 300 volts on the plates. You could go lower, as described in Kevin's books. But it seems to me that things would get more non-linear the lower you go.

Not that that's a bad thing necessarily, especially in guitar amps.

George

rjgtr · Post by **rjgtr** » Thu Sep 16, 2004 11:18 am

The only problem I would see with going too low is that the amp won't feel tight. The lower you go, generally, the looser the amp is going to feel. This isn't necessarily bad, just a different sound.

I think plate voltage is why some of the Fenders (and Music Mans) of the 70s sound so stiff, because the plate voltage is extremely high.

Post by **VelvetGeorge** » Thu Sep 16, 2004 2:20 pm

rjgtr wrote: I think plate voltage is why some of the Fenders (and Music Mans) of the 70s sound so stiff, because the plate voltage is extremely high.

And the screen voltage is overly regulated. You've got to let those things breathe.

George

rjgtr · Post by **rjgtr** » Thu Sep 16, 2004 10:10 pm

Yeah, I don't like over regulated amps.

There's a guy in town here who adds caps to the power supplies of every Marshall he services, to "fix" them. It doesn't help that he doesn't always clip the slack on the ends either!

He makes a plexi sound like an 800 (nothing wrong with 800s, but PLEASE!). Leave perfection alone!

You know this is one of the interesting things to me about the evolution of the 45s to the 800s, is the tightening of the power supply. That and the gradual brighting of the amps, which some have said is due to higher and higher output pickups.

One thing I love about old Marshalls is that the differences in parts are so small, but one change can make such a big difference. That even just changing a bright cap makes a huge difference is so cool.

Flames1950 · Post by **Flames1950** » Thu Sep 16, 2004 10:38 pm

To be honest, I've been tempted to up the filtering in both my 50's. They ghost somewhat noticeably to my ears, but I get the impression not everyone hears the ghosting effects to the same degree. I'm trying hard not to do it.
All three of my sixties Fender heads have had to have extra filtering put in them though. Crank any of them more than halfway with the stock filtering and the ghosts were almost as loud as the actual note you were playing. The two Bassman heads each got an extra 100uF, the Bandmaster got 80uF. I was surprised that they didn't tighten up near as much as I thought they would. The Bassman heads sound great now, the cleaner Bandmaster is pushing how tight I'd like it to sound.

5150loveeddie · Post by **5150loveeddie** » Sun Sep 19, 2004 2:36 pm

Excuse my ignorance Flames but what do you mean by "the amp is ghosting..."

Flames1950 · Post by **Flames1950** » Sun Sep 19, 2004 2:53 pm

I'm referring to ghost notes, where the note you play has an out-of-tune harmonic under it caused by an under-filtered power supply's current ripple. (You can get a similar effect called "cone-cry" from some speakers, which doping helps.)
Both my Marshalls have ghost notes that are noticeable to my ears, even after replacing all the filter caps. The Fender Bassman heads I've got were the worst offenders, they were damn near unplayable to my ears, they drove me nuts.

jcourtjr · Post by **jcourtjr** » Sun Jan 09, 2005 11:21 am

Flames1950 wrote:I'm referring to ghost notes, where the note you play has an out-of-tune harmonic under it caused by an under-filtered power supply's current ripple. (You can get a similar effect called "cone-cry" from some speakers, which doping helps.)
Both my Marshalls have ghost notes that are noticeable to my ears, even after replacing all the filter caps. The Fender Bassman heads I've got were the worst offenders, they were damn near unplayable to my ears, they drove me nuts.

i think i notice this in my reissue. its only a coupe of notes that i have notice on the g string. it is definatley lower than the played note. is this something that can be fixed. its not bad but it is noticable.

myker · Post by **myker** » Sun Jan 09, 2005 12:19 pm

BANG! i got it! thanks flames! what you said just triggered a solution to a problem i have been having with my 800. i added a couple of gain stages and increased the filtering by 50uf. THATS WHAT IT IS. I hadn't thought of that. there is this noise, phasing sort of underneath the notes i am playing. could not get rid of it! must increase filtering.
mike

myker · Post by **myker** » Sun Jan 09, 2005 12:23 pm

also flames, i know what you mean, once you hear something in the amp, your ears get used to listening for it and you get really good at hearing it. and you start trying to get rid of it, and your listening to it, then you get even better at hearing it. funny how your ears can develop so fast to hearing things that you dont like and dont want to hear...
mike

Zoso · Post by **Zoso** » Mon Jan 10, 2005 1:26 am

So to increase the filtering do you just use higher value caps? I suppose this tightens up the amp as well, right? Any other effects of doing this(other than removing unwanted ghost notes)?

5150loveeddie · Post by **5150loveeddie** » Mon Jan 10, 2005 2:50 am

remember that ghosting comes from the screens basicaly so increase there and if you have to high filtering the amp may go into motorboating so go up gently and increase it till the ghosting is off not more and yeah your amp will be tighter on the bottom ends and in general