Squeal Problem with New MV Build

[Guitar Adjuster]

Just finished this build and I am having some problems with a squealing sound that rides on top of the notes. When playing you can hear the squeal and when you stop playing it gets real loud. Does not squeal using the low input jack. I replaced the can cap under the preamp board, re-flowed all my solder joints, double checked all the grounds, replaced the high input jack and re-shielded--and it is still doing it. Here are some photos's---I also switched out the preamp tubes for known good ones......

[Flames1950]

Your input jacks are so close to the tone control section output that you'll probably have to make a metal shield to go around the jacks to get that to work -- huge out-of-phase signal there coming out right next to those inputs..........

Larry explains it better, if you can find a thread with a Roccaforte amp that has a metal shield pictured -- the poster thought it was a great idea, until Larry explained that it was kind of a band-aid, but I think we've all resorted to it when trying to work with a stock Marshall layout and loads of gain.

[Guitar Adjuster]

Ok, I remember that, it's an L-shaped shield going around the input jack. Hell I use to own a Damn HG-100, you would have thought I knew better I will give it a try tomorrow. I got some shielding metal I can make a shield out of.....Thanks Flames!

Allen

[Flames1950]

I found the thread if anyone else needs to know, nice pics of the shield job:

http://forum.metroamp.com/viewtopic.php ... roccaforte" onclick="window.open(this.href);return false;

[tonepilgrim]

You might also look at pushing the feedback wire going to the presence pot closer to the board (away from the tone pots). That fixed a squeal I had at one time.

[harddriver]

You may want to consider running shielded wire on all your input grids at V2a and V2B possibly V3A as well and maybe shielded wire on your depth mod. Try the shielded wire on your input grids, I didn't have any problem with my input jack being too close but I used the two furthest away from the preamp/volume controls on mine.


It sound like a presence/feedback circuit issue though, you may not want to drape the NFB wire over your power tube input grid wires like that, run it down the back of the chassis and tuck the other one closer to your power tube socket away from V4 PI preamp tube. I once tried putting three values of capacitors on a switch on my depth mod but I keep getting a high pitched oscillation ring with the switch installed, by all rights it should have worked. I ended up just using a single value cap, sounds great, NO RINGING!
Let us know how the troubleshooting goes!

[Guitar Adjuster]

You may want to consider running shielded wire on all your input grids at V2a and V2B possibly V3A as well and maybe shielded wire on your depth mod. Try the shielded wire on your input grids, I didn't have any problem with my input jack being too close but I used the two furthest away from the preamp/volume controls on mine.


It sound like a presence/feedback circuit issue though, you may not want to drape the NFB wire over your power tube input grid wires like that, run it down the back of the chassis and tuck the other one closer to your power tube socket away from V4 PI preamp tube. I once tried putting three values of capacitors on a switch on my depth mod but I keep getting a high pitched oscillation ring with the switch installed, by all rights it should have worked. I ended up just using a single value cap, sounds great, NO RINGING!
Let us know how the troubleshooting goes!

Great looking build Harddriver, I installed a shielding plate around the inputs and it seemed to help but there is still somthing not quite right. I will go back tonight and mess the wiring on the depth mod and also install some more shielded wires. Is that a second master (Jose) on your amp? I need to give that a try!!

[harddriver]

I'm assuming that is a cathode resistor for V2B, what value is it, is it gray/red/red? 8200/8.2K?

What is the black wire with the resistor on the end doing running from the positive side of the V2B cathode resistor? Is it running to ground? It looks like it?

[Guitar Adjuster]

I'm assuming that is a cathode resistor for V2B, what value is it, is it gray/red/red? 8200/8.2K?

What is the black wire with the resistor on the end doing running from the positive side of the V2B cathode resistor? Is it running to ground? It looks like it?

Check it out......

[harddriver]

Ahh! it's a boost cap! Cool! I think when you change out all your input grid wires to shielded wire and reroute your NFB wire for your depth mod you may be fine with the squeal.

Let me know how it goes!

[Guitar Adjuster]

I think I finally whiped this thing, installed additional shielding on the plates and on the NFB, pretty much got rig of the squeal except what would be normal for a higher gain amp. Here is a photo---I am running a signal generator through it into a load box to break in the caps a little quicker and burn in the tubes. BTW---I used a depth mod off a Cemeron amp as I got it to work better then the ones posted here for some reason. I will post some photo's of it a little later after the burn in. Thanks for all the help guys!



Oh, the Red Head Shell is from Jerry at Vintage Head Box, I got it a week before the shit storm started

[harddriver]

Cool! I'm glad you got it all sorted out! I love the RED standby switch BTW, Did you think that detail would get past us here at METRO!

[SDM]

The resonance mod on my site is just a "generic" resonance control, like Soldano and Peavey use. Can be inserted before or after the NFB resistor. Works off of NFB like a presence control does, but for low end frequencies. Lower amount of NFB to start with means lower effect, same as with a presence control. Looks like you have (or had rather) it set up on the 4 ohm tap with a 100K NFB resistor, as low as anyone usually goes with Marshall 100 watters regarding NFB. So it's not going to have as much overall range/effect as it would if it was on a higher impedance tap and/or used in combination with a lower NFB resistor (which is how amps that have resonance controls from the get go are usually set up). Cap value has impact too on which frequencies the resonance control effects. As it states on my site, between .0022uf-.0047uf are the most commonly used values, but preferences may lie outside that range.

So you may have wanted to try using say the 8 ohm tap, then try a 47K-56K NFB resistor for starters on a getting a stronger effect. Maybe a .0022uf cap or such instead of .0047uf as that would extend the range of effected frequencies up higher in the mids. No notable change in control effectiveness, check your pot/pot value, cap/cap value (be sure you didn't use a .047uf or such), connections.

Perhaps similar changes are what you ultimately did comparing to the Cameron control? The basic resonance control will effectively defeat all low end NFB from the NFB loop (when cranked), thus allowing those lower frequencies to pass through the power amp full bore for a low end "boost". So, with all low end NFB already defeated/weeded out of the NFB loop, another version of a traditional style NFB based resonance control cannot add further low end "boost", only appear to do so by either making the control range more broad compared to other frequencies, and/or by extending the effected frequencies up higher via changing the cap value. Again though, the actual low end maximum itself is unaffected by these changes, but the control becomes much more effective overall. Low end boost effect will be stronger, but only relatively so - compared to the mids and highs now (which are now simply further damped down by more NFB in the NFB loop). This is usually the desired effect though, so works for many. This is just what the stuff in the paragraph above would do/does vs. how you originally had things set up. There are some other minor things that can be done, interactions to consider when tweaking, but the general idea on tweaking this kind of NFB resonance control is in the above.

Any way, if there is an actual error on the drawing, some inaccuracy or omission I am missing here regarding this, please let me know. Otherwise, the control (as depicted on the site) may just need some fine tuning around it or cap value tweaking to get the effect you want in a given amp. Still not enough for you, an active resonance boost may be a better approach (but is more difficult to implement).

[Guitar Adjuster]

The resonance mod on my site is just a "generic" resonance control, like Soldano and Peavey use. Can be inserted before or after the NFB resistor. Works off of NFB like a presence control does, but for low end frequencies. Lower amount of NFB to start with means lower effect, same as with a presence control. Looks like you have (or had rather) it set up on the 4 ohm tap with a 100K NFB resistor, as low as anyone usually goes with Marshall 100 watters regarding NFB. So it's not going to have as much overall range/effect as it would if it was on a higher impedance tap and/or used in combination with a lower NFB resistor (which is how amps that have resonance controls from the get go are usually set up). Cap value has impact too on which frequencies the resonance control effects. As it states on my site, between .0022uf-.0047uf are the most commonly used values, but preferences may lie outside that range.

So you may have wanted to try using say the 8 ohm tap, then try a 47K-56K NFB resistor for starters on a getting a stronger effect. Maybe a .0022uf cap or such instead of .0047uf as that would extend the range of effected frequencies up higher in the mids. No notable change in control effectiveness, check your pot/pot value, cap/cap value (be sure you didn't use a .047uf or such), connections.

Perhaps similar changes are what you ultimately did comparing to the Cameron control? The basic resonance control will effectively defeat all low end NFB from the NFB loop (when cranked), thus allowing those lower frequencies to pass through the power amp full bore for a low end "boost". So, with all low end NFB already defeated/weeded out of the NFB loop, another version of a traditional style NFB based resonance control cannot add further low end "boost", only appear to do so by either making the control range more broad compared to other frequencies, and/or by extending the effected frequencies up higher via changing the cap value. Again though, the actual low end maximum itself is unaffected by these changes, but the control becomes much more effective overall. Low end boost effect will be stronger, but only relatively so - compared to the mids and highs now (which are now simply further damped down by more NFB in the NFB loop). This is usually the desired effect though, so works for many. This is just what the stuff in the paragraph above would do/does vs. how you originally had things set up. There are some other minor things that can be done, interactions to consider when tweaking, but the general idea on tweaking this kind of NFB resonance control is in the above.

Any way, if there is an actual error on the drawing, some inaccuracy or omission I am missing here regarding this, please let me know. Otherwise, the control (as depicted on the site) may just need some fine tuning around it or cap value tweaking to get the effect you want in a given amp. Still not enough for you, an active resonance boost may be a better approach (but is more difficult to implement).

No offense Steve, the actual diagram is correct so I made a bad choice of works, what I meant is that it was not very effective. I tried putting it on the selector switch with the same results. On my own SLO it's wired like your diagram but, the twisted wires go directly to the board were one wire hooks up to the NFB resistor and another got to a Cap----see photo below



Here is the photo of the one I used---Not 100% sure it's a Cameron but I was told it was.....

[Guitar Adjuster]

Cool! I'm glad you got it all sorted out! I love the RED standby switch BTW, Did you think that detail would get past us here at METRO!

I knew someone would catch that I ordered 2 red by mistake so that was all I had but I think it looks cool so it's staying!