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Here's a dumb question...

On the schematic, it calls for 2 68K input resistors....and according to most layouts, they are hooked to the input lead in a triangular fashion...leading to v1.

I've read alot of mention that these resistors at this point, really don't have the effect intended...and to really prevent any radio interference, you have to have them at the tube socket.

Which begs the question...why not have two input leads coming from the input jacks...and both 68K resistors sitting on the tube socket?

-F

My personal opinion is that it is 6 of 1 and a half dozen of the other. I think the more important thing is to run shielded cables to the tube socket. Any possible RFI can be circumvented with a small value cap to ground right after the input cap. A 47pF should work.

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PJM

http://www.diyguitarist.com

Frank
I do this with all my builds, mods, and repairs. But if you use shielded wire from the 68k's at the input jacks w/ the shield tied to chassis at one end only(input jack end), you really will not have an RF problem. That's only IF you're having a problem.
That all being said, I've heard of some Marshalls that weren't ever able to be RF free, no matter what was done.
I've read the same thing about the grid resistors having to be right at the socket pins in OLD literature from the 50's. I have some old electronics publications from WWII and it's all tubes, but a friend is borrowing them at the moment.
I do it just for insurance. No problems so far.
FWIW

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the bastard son of Jack Parsons
http://www.rotten.com/library/bio/mad-s ... k-parsons/

That's exactly what I do.

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PJM

http://www.diyguitarist.com

That is true. If you use shielded wire, it doesn't matter as much. If not, those grid stoppers really need to be at the tube socket.

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--s2
www.s2amps.com
www.category5amps.com

Believe it or not, I have never had a prob with RF and Never use sheilded cable. A friends 80's Deluxe Reverb is terrible about RF. Generally when using a Str@. He won't let me fix it though. Sheeeeeeeeeesh!!!

CR ><>

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WoodCross Amps

http://www.geocities.com/woodcrossamps/

Me too. Just to be safe. It's a lot easier to do when you're building rather than as a retrofit.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Yep.

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PJM

http://www.diyguitarist.com

I did an experiment with the 68k resistors in my Spitfire.

First, I ran one shielded cable from the tube with the 68k's splitting off to the input jacks, and grounded to the preamp ground. There was no 60Hz hum using this configuration.

Second, I put the resistors at the tube and attached two shielded cable, which ran to the input; cables were grounded at the tube socket. There was no 60Hz hum using this method.

I never tried running bare wires with the 68k's at the tube, nor did I try running only shielded cable from the tube without the 68k's. From what I've read here, both methods should also be 60Hz free.

Now, if only there were a way to get rid of that 120Hz hum...!

Two words: Lead Dress.

When I first built my Spitfire, I had a major hum problem. After many hours of pulling my hair out, changing grounding schemes, etc., I traced the problem to one wire from the OT needing to be moved 1/8" from a wire running parallel to it. Now it's pretty much my quietest amp!

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PJM

http://www.diyguitarist.com

I guess it must be time to break out the wooden toothpicks. This amp presents an interesting challenge - Spitfire in an 18w chassis.

Now that's tight! Did you start with a blank chassis and cut it out your self?

You're getting 120 Hz hum? Could be a bad filter cap.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Stephen,

I worked with Nik @ Ceri@tone, and we came up with the layout. Everything fit as I would have expected, except the board was 1/4" wider than I anticipated - which made it a little tighter.

All the filaments are twisted pairs, the OT leads are twisted, there are 2 star grounds - one for preamp, and one for power, shielded wire for all wires leading to pots (grounded at one end only and grounded at preamp star ground)... Should I use shielded cable going to the sockets?

I'd like to think it was a filter cap, because that would make it so much easier to correct - vs. the long process of lead dress checking. I did read on a forum (either here, or 18watt) that Matchless amps are prone to 120 cycle hum, and though I could live with it in a live setting, I wouldn't want to record it that way. I am relatively certain it is 120Hz, because it is about an octave higher than single coil hum - doing a quick A/B between the inbetween setting (with RW middle pickup) on a Strat. and one single coil.

Have you any thoughts as to which cap might be a likely culprit?

Are you using the pot cases as a ground? I had a similar problem on my first 18 watt, and I removed the ground buss from the back of the pots and used a separate ground bus that is grounded at one point by the PT and it cured most of my noise issues. I think you can sometimes get a ground loop going if you are using the ground on the back of the pots for signal grounds. Here is a pic of what I ended up with, and I used the same layout on my amps ever since.

Eddie

I always use shielded cable going from the input to the sockets and put the grid resistor right on the input pin of V1. The shielded cable is grounded only at the input end. I've never actually used shielded cable to the pots, nor grounded the pots with a bus bar either.

You can test the caps by clipping another similar sized cap in parallel with one and see if the sound goes away. Start with the first cap in the chain.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

That picture was taken before I removed the buss from the pot cases, as I thought it might be the source of some noise. Incidentally, it made no discernable difference in the noise, which is definitely not 60Hz...

I rewired the input to the sockets after the pic was taken, and instead of running shielded cable from the socket to the two resistors at the input jacks (shield grounded at the input), I put the 68ks at the tube, and grounded the shield at the socket. This made no difference to the 120Hz hum I am getting.

I will definitely troubleshoot the filter caps, though. The hum is absent when the Volume and Master are both at 0. Turning up the Master brings the hum, even if the Volume is at 0. Hmmmm....

hello there, it's hard to tell from the pic but if you have the power section components and preamp amp components grounded at the same place this can cause hum. the filter caps for the plates and screens, the h.v. ct. and the power tubes cathode resistor/cap combo should be grounded over close to the power trannie, and likewise the preamp filter caps and gain stage cathode resistors/caps should be grounded to the chassis over close to the input area of the amp. at the very least, take the power section grounds off the pot buss (if that's where they are). hope this helps. rh

hello again, i did just notice on your pic that all the filter caps are grounded together on the board. snip the jumper between the power section filter caps and the preamp caps and reground the power amp stuff to a lug over at the power trannie side---away from your preamp grounds.
i bet this helps the situation. good luck rh

Yes, I lay out my amps that way. Power ground at PT , Pre at input side. Seems to work for me (although I have done 18s at one ground point) but now do them with two. Same layout as my Trinity 15.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps