Silverface Champ mods?

[porge]

Hey Guys,

I'm thinking about modding a silverface champ (I have 3) for a bit more gain, and have heard that lifting the resistor to ground on the bass pot will effectively take the tone stack out of the circuit, and result in such an increase. Is this the case?

Also - If I do this "mod", do I need to beef anything up, or change any components?

Thanks

Andrew

[bob p]

Originally Posted by porge

I'm thinking about modding a silverface champ (I have 3) for a bit more gain, and have heard that lifting the resistor to ground on the bass pot will effectively take the tone stack out of the circuit, and result in such an increase. Is this the case?

that's a valid mod. you mention the bass pot -- IIRC you're actually lifting the midrange resistor's connection to ground when you do that mod. it doesn't completely remove the tonestack from the circuit (the treble control should still work) but you will notice a lot more gain. IIRC that exact mod was done in one of hte early Mesa Mark amps. personally, i like to just bypass the entire tonestack with a coupling cap. you're on the right track -- that's a huge bang forthe buck mod for a SFC.

[porge]

Cheers for that Bob.

I'm guessing I could whack a push-pull pot in to control this. I like the stock sound as well - I think if they are running right, the clean tone is great.

With your bypass cap, where do you locate it/what value? I know the hot end of a soldering iron, but I consider myself very much the newbie

[bob p]

here you go.

[porge]

Champion!

I'll grab a p/p on teh way home. Nice one indeed!

[Rob Mercure]

Hey Bob,

Why add the standy switch? Just seems like a bit overkill for a Champ with the front panel power switch.

Rob

[porge]

Hey Bob,

did the mod last night on one of the champs and thinking about doing this for the lot of them! I like the fact that it is relatively discrete. I used a 250K P/P on the treble control to activate.

Pardon my ignorance, but what will dropping the negative loop do for my tone?

[chocopower]

Look at this......

http://www.schematicheaven.com/mods/...ifying_mod.pdf

[bob p]

Originally Posted by Rob Mercure

Why add the standy switch? Just seems like a bit overkill for a Champ with the front panel power switch.

Well, I guess its a matter of style. "Good engineering practices" would recommend using a standby switch all the time to protect the amp from the sudden jolt of B+ when the heaters are cold. But being a budget amp, Champs never seem to get the royal treatment and there are a fair number of shortcuts in them. I'll sheepishly admit that I haven't done the standby switch mod on my Vibrochamp. I use a delayed warm-up rectifier.

[Rob Mercure]

Bob,

Yeah, but I also guess whose "GEP" one chooses to utilize. If you remember about 7-8 years ago on Ampage we whipped this horse pretty good and the consensus seems to be that this/these particular principle/s had resulted from large industrial power handling tubes and that no one seemed to be able to come up with any hard evidence that cathode stripping - the effect that delaying B+ should prevent - really was a proven problem with "consumer sized" output tubes. And there are numerous quite well designed HiFi amps - Scotts, Macs, etc. - that produced lotsa power while not experiencing output tube distress from the lack of a standby switch. So the champ - with the rectifier heatup "delay" probably doesn't "deserve" <grin> such "royal" treatment.

Now this doesn't mean that I don't like and use the standby switch on the 67 Vibrolux that is my main stage amp - but it's probably more of a psychological reassurance that I'm only adding the B+ to an already operating warmed up amp once I return to stage after a set break. Not having the energy to lug around a spare in case the 'lux takes a dive during a performance the "never turning it off" seems to be more comforting. And while I'm sure some HiFi folks would suffer mentally if their amp didn't fire up I suspect that the embarassment factor for a stage musician is somewhat greater even if my equipment is more "primative" <'nother grin>.

I've never added a standy switch to any of my homebuilts but I did add a time delay relay to a Bogen M60A that I converted into a "acoustic" guitar amp - SS rectos - only cuz it uses 8417s which seem to have been made, as is commonly stated these days, of "unobtainium." And tis a shame (have I stated this far too often?) but I really love these tubes which combine high power output, low drive signal, and a wonderful sound that the 6550 derived tubes just can't produce for my tin ears.

Rob

[Tim Hall]

Bob, Porge,

I hope you guys are still keeping an eye on this thread. I’ve had this idea (to bypass the tonestack for more gain) in mind for some time, and being completely new to electronics I didn’t know if it was even possible. I only understand amps in basic conceptual terms, and I was excited to discover here that it can be done.

This is my first post here, and I hope you guys will entertain my question. Since I’m new to electronics don’t hesitate to tell me my idea is impossible or just plain silly.

Assuming you had no reservation in making a less-than-discrete mod: Instead of using a switch to fully bypass the tone circuit, could you use a pot to dial out the tone control, simultaneously dialing in the gain? This way you would have some mix of the two in between?

Thanks

[Sweetfinger]

Your idea will work great. It is called a midrange control

That resistor in any similar tone stack is essentially a mid control set at about half up. Wire a 10 K pot in place of that resistor and you'll have your basic Fender tone stack. Raise that pot value to something like 25K or 50K and you'll have a mid control that will appear to add gain and raunch in the upper part of the rotation.

Knowing how that tone stack works is one of the keys to getting good overdrive sounds out of BF/SF Fenders. In a stock Twin for example, to get the tightest, raunchiest overdriven sound, I turn the mid all the way up, the treble and bass ALL the way down, and the volume all the way up. I'll just ever so slightly crack open the bass knob for extra low, but any more will make the amp farty or tubby. Same thing applies to a two knob stack such as the Bassman. Turn the bass and treble all the way down, but internally, you know the mid is permanently set at halfway up. Crank the volume, and you've got a really tight good ol' fashioned rock-n-roll distortion. Add a boost of some kind to the front end for even more fun.

[Tim Hall]

Sweetfinger,

Thanks for the reply...I need to study up on tone circuit design a little more.

[Bruce / Mission Amps]

Here is an old mod I've done for so many years I can't rememebr.
Replace the 1m audio volume pot with one that has a push pull dpdt switch behind it.
Wire a .022uF cap from the plate (lug #1) of the 12AX7 which is connected to the tone caps and that 100K resistor in the tone stack.
Use this switch to bypass, but still in parallel, the tone caps right to the audio high lug of the volume control, this is the same as the treble wiper lug.
Pull the volume pot knob to engage the .022uF cap. 90% of the raw signal will now bypass the tone stack.
Use a 68K to 470K resistor in series with the .022uF cap to tame some of it down for taste.

[bob p]

Bruce, I like that idea -- with that mod do you ever get run into problems with switching pops?

on a related note, does anyone know whether the Alpha switched pots that Mouser carries have the switch at the beginning of their rotation, or are they push-pull switches?

http://www.mouser.com/catalog/629/539.pdf

[Bruce / Mission Amps]

Hmmm... not really... I have used larger values like 100nF and then run a 220K resistor to ground using the other side of the DPDT... that would be inbetween the and before the volume pot end of the cap so it acts like a voltage divider when engaged. It can get pretty wild with the many extra dB of signal bypassing the tone controls.
I think those pots have the on off switch at the begining of the pot rotation as view clockwise turning the volume up.
They work good as an on-off switch volume control like and Old Champ would have with no seperate on-off toggle.

[Tim Hall]

Bruce,

Do you think subbing the 12AX7 for a 5751 would help tame it, or does the drop in gain defeat the purpose? Also (referring back to my earlier question in this thread), could you replace the fixed resistor in your mod with a no-load pot to have variable gain? Sorry if these are dumb questions - like I said, I'm new to this.

[Bruce / Mission Amps]

Originally Posted by Tim Hall

Bruce,

Do you think subbing the 12AX7 for a 5751 would help tame it, or does the drop in gain defeat the purpose? Also (referring back to my earlier question in this thread), could you replace the fixed resistor in your mod with a no-load pot to have variable gain? Sorry if these are dumb questions - like I said, I'm new to this.

Sure you could use a pot but that is what the volume control is already doing.
The resistor I mention is to create a filter, to cut some of the harshness and the other resistor, (which would be grounded) will form an AC voltage divider, stearing some of the raw audio off to ground to reduce signal level.

I'd experiment with a 12AY7 and a 5751.

[Tim Hall]

Bruce,

Thanks for the reply. I understand all this better now. I suppose what I’m aiming at is being able to shape the tone somewhat while mixing in a little bit of the raw signal.

I‘m imagining (with my non-technical imagination) something like a 1Meg pot effectively acting as a kill switch (for the bypass) if turned all the way up, preserving 100% of the signal path through the tone stack. When turned down I imagine it dialing out your ability to shape the tone as it dials in more raw signal.

Maybe the ability to do this isn’t worth the effort or extra parts.

[Alex R]

...as some of the links in the posts have mentioned, removing or reducing the negative feedback around the OT makes a big difference to these amps - seems to bring them alive, and delivers a nice gain increase. Generally there are no problems if you completely remove the resistor (the 2K7 wired from the speaker jack). Simpler.

A.

[bob p]

Originally Posted by Bruce / Mission Amps

I think those pots have the on off switch at the begining of the pot rotation as view clockwise turning the volume up.
They work good as an on-off switch volume control like and Old Champ would have with no seperate on-off toggle.

I thought the Log Taper/SPST pot/switches were rotary on/off switches, like those you described for the champ.

What threw me for a loop is the Linear Taper/SPDT pot/switches. I can't understand the reason for an SPDT switch if its a rotary on/off type. To me, it would make sense if those were push-pull, so I just thought I'd ask if anyone was familiar with them.