1978 SFDR boost switch

[C Moore]

Can you guys explain to me how this works? With the switch closed, is there a voltage divider with the 220k and 1k resistor. If so, what does that accomplish?
Thank You

http://www.ampwares.com/schematics/delu ... _boost.pdf

[XgamerGt03]

It looks somewhat like a negative feedback switch to me.

From what I've heard about the amp, the boost function is a waste.

[David Root]

Not negative feedback, unless I misread which crossed lines are joints and which are crossovers.

Looks like it would dump most of what it takes off the transformer secondary to ground thru the 1K, not much making it thru the 220K. It is coming from a very low (8 ohm) impedance, so that would maximize what does go thru the 220K.

Tonewise it's a clean boost with the bass wiped out by the 500 pF cap, which makes sense.

[marcoloco961]

Hmmm. Primary of trans. is in the 400V range. Signal bypasses the reverb tank. and is re-inserted through a voltage divider back into the signal chain coming from the reverb tank. Looks like signal is 180 out of phase from signal coming from reverb tank.

My guess is a boost for the reverb stage through negative feedback, while bypassing the tank. The 500pf cap seems to be in the signal path regardless of switch position.

This is a best guess. I am curious to know for sure myself.

[C Moore]

As always......Thanks
The amp belongs to an acquaintance. I have disabled the push-pull switch, and returned the reverb driver cathode to "normal" BF/SF specs. He asked what was going on there, and I did not really know what to tell him.
So I asked you guys at TAG....
Thank You

[C Moore]

......

[David Root]

I see what you mean. I was looking at V3 not V4B. Seems to be a combination of "feedback" to V4B plate, which is 180 deg out of phase, and boost going forward.

[marcoloco961]

I see what you mean. I was looking at V3 not V4B. Seems to be a combination of "feedback" to V4B plate, which is 180 deg out of phase, and boost going forward.

Sometimes I just browse random schematics and try and decipher what the hell just happened there?????? I know, I'm a sick, sick man. LOL Maybe I'm just extremely bored.

[C Moore]

I guess I am still confused....
With the switch closed, does all the reverb signal go through the switch? Doesn't that rob all of the signal before it can go through the tank? Or is that where the voltage divider comes into play?
Sorry, this kind of taxes the limits of my knowledge.
Thanks

[marcoloco961]

I guess I am still confused....
With the switch closed, does all the reverb signal go through the switch? Doesn't that rob all of the signal before it can go through the tank? Or is that where the voltage divider comes into play?
Sorry, this kind of taxes the limits of my knowledge.
Thanks

I don't think the switch would completely cancel the signal going through the reverb tank, it looks like it adds some "dry" signal back into the mix after the reverb is added and attenuated. A boost, and a "mix" effect at the same time kinda?

I'm trying to help ya here, but I'm still in the learning process myself. I keep checking back looking to see if anyone a little more knowledgeable will confirm what it's exact purpose is.

Could you hear a difference between the two settings? If so, what kind of sonic difference did it seem to make, if any?

[C Moore]

I never heard the switch. It came to me with a new tank to install, and he asked that I disable the switch, he said it sounded "awful". So I do not really know.
PLUS......I am one of those drummers that took to amp repair because all the guitar players I knew never could/wanted to work on their gear. So, as embarrassing as it may be, I can only play enough to see if any signal passes through an amp. I have a decent ear, but I do not play guitar. I realize it must seem crazy to you guys, but I had the interest and really wanted to learn/gain the knowledge.
Anyway..... I would love to read anything further regarding this little boost circuit.
Thank You

[David Root]

marcoloco, you definitely need more blank chassis to think about new builds with!

[mike9]

I have one of those on my '80 SFPR - I just leave the damned thing alone.

[marcoloco961]

I never heard the switch. It came to me with a new tank to install, and he asked that I disable the switch, he said it sounded "awful". So I do not really know.
PLUS......I am one of those drummers that took to amp repair because all the guitar players I knew never could/wanted to work on their gear. So, as embarrassing as it may be, I can only play enough to see if any signal passes through an amp. I have a decent ear, but I do not play guitar. I realize it must seem crazy to you guys, but I had the interest and really wanted to learn/gain the knowledge.
Anyway..... I would love to read anything further regarding this little boost circuit.
Thank You

Not crazy at all HH. I am a no guitar playing, bass dude. Most of the amps I build get tested with a bass It sounds weird, but I am starting to be able to tell if the sound will be decent for guitar by what the bass sounds like. Then of course, I have a real guitarist try it out and offer their opinions for the tweaking.

This amp building thing is as addictive as tattoo's , I swear! Thank god I never got that first tattoo. Within the last several weeks, I have started to have small moments of clarity, where alot of this stuff that was just mumbo jumbo previously has started to really sink in (bout time). I am still at odds with the whole feedback thing, as there is so many ways to do it and so many different places to re-insert the feedback. I have to admit it gets the best of me at times. The Impedance issue is also a gray area for me along with some of the more elementary things like calculating loads, tone stack loads etc. I just haven't gotten to those areas well enough yet. I am starting to understand the circuits better and can pick out different circuits within the circuit quickly now. I can also troubleshoot the simpler things and am starting to understand where to look for trouble just by the symptoms. Not by any means do I pretend to know that much overall. I kinda started in the middle and have been working towards both ends.

Regardless, I have enjoyed the ride. I never would have started this journey without the support and encouragement of some of the folks here, and I cannot begin to say enough nice things about all the talented individuals here who are willing to take the time to help those who are just beginning and eager to learn. I always try to pay it forward by helping others when I can, it's just that my knowledge is limited at this point. I just keep reading more and brushing through the random threads here, because I learn a lot from reading the posts where they troubleshoot the circuits having issues. All I can tell you is keep at it. It seems like it takes forever sometimes, but when it all starts making sense, it pays high dividends.

[marcoloco961]

marcoloco, you definitely need more blank chassis to think about new builds with!

WORD! I have too much time on my hands bro..........................LOL. If I had some $$$$ I'd be an amp buildin' mug!