techie guys: My idea for an amp...

[Hot _Grits]

Hi all,

I've been busy trying amps and reading up about amp building, and I have an idea for what I think would be a killer amp for my rock band.

at this stage, I'm at least a year off getting this to happen myself, so I'm toying with getting one made, possibly based off a ceriatone kit. But eventually I'd like to make my own stuff.

Basically I want to take a 50w plexi clone and add:

-a post phase inverter master volume (and possibly London Power's power scaling at a later date)
-switchable tube and SS rectifiers
-a switchable negative feedback loop.
-I'd also like to take a leaf out of my Blue Angel's book and get it to run EL34 and 6l6/KT tubes at the same time, so you can switch between them (though not with the option of adding them together). Basically the idea is to be able to choose any combination of rectifier and output tube without opening up the guts. In performance would be cool, but I've never seen an amp where you could switch the rectifier on the fly.

now, the first two additions should be easy. But I'm not sure if the NFB thing is going to have a similar effect to the very cool switchable NFB on my Vibrolux, seeing as it's basically the presence control. And I'm also unsure about the practical requirements of getting two tube sets to run together.

So whaddaya think? Practical or hare-brained?

My other idea is a true two amp in one head: basically one channel (with bright switch) from a pre-blackface tremolux or vibrolux (minus trem) and another from a plexi (switchable rect again), both with completely separate paths until they hit the OT. Is it possible to get that to happen?

 

[LJ King]

Re: techie guys: My idea for an amp...

I personally don't like post PI masters. For one reason, all it takes is a speck of dust to get inbetween the pot's wipers and your bias will disappear sending the amp towards meltdown. And, if you are going to be switching around with the feedback loop, keep in mind that since the post PI is in the middle of the power amp, when you turn down the volume you will also be reducing the level of feedback around the power amp at the same time.

Switching rectifiers during a performance is usually not provided for because of the high voltages involved. Also, depending on the condition of your caps and the current draw of the amp, it will take some amount of time, a delay, for the higher SS voltage to reduce to the level of the tube rectifier voltage and the other way is pretty much instant.

If you want to switch output tube types, then the easiest way is to switch the cathodes. The blue angel is a self or cathode biased amp and this is much easier to switch due to the self bias. The plexi 50 is a fixed bias amp so if you decide to keep it that way, you might want to set up separate bias circuits.

Keeping totally separate paths and then join them at the output transformer? Depends I guess - the problem would be that the two separate feedback paths would be impossible to maintain. It would be simpler just to have a single power amp.

 

[Scott_F]

Re: techie guys: My idea for an amp...

Instead of putting an on/off switch with the NFB, use a pot so you can dial in the right amount.

The concept of dual power pairs is a fun one. Mesa has done it, so has Carr. Mixed results from the reviews I've read. I'd think maybe running 4 6V6 and make a pair of them drop out with a half power switch might be an angle I'd look at.

The ss rect/ tube rect is not difficult to do at all. That custom shop Fender 64 Vibroverb has that option on it. THe only down side was a noticeble Popping sound when you switch it. But, if you go standby first and then switch it, it's a fix. You'd kinda have to find a good break even point for your bias with tube and bias with SS. A bias that won't leave the tube rectified too cold or the SS too hot.

And, I'm not that fond of the master volume mods on that design, as LJ pointed out.

 

[Pierre]

Re: techie guys: My idea for an amp...

Are 6V6s any close to EL34s?
You could use EL84s if they're more British and have kind of a hybrid Vox/Marshall. I always wanted to try that... Half power switch to switch out a pair, a pentode/triode switch, the standard Marshall 4 input type (maybe with a couple switchable caps for each for added versatility), three band standard EQ, switchable rect... Mhmmm... 2 gain stages only but a boost on each 12AX7. So a lot of switches but a lot of tones

 

[Hot _Grits]

Re: techie guys: My idea for an amp...

Thanks for the comments, guys.

The problem I find with half power switches is the half power setting never sounds as good as the full power setting. My idea is to get different tonal flavours rather than any power reduction.

As far as i can tell, a pot-style NFB would essentially be the presence control on a plexi circuit...

 

[Scott_F]

Re: techie guys: My idea for an amp...

Thanks for the comments, guys.

The problem I find with half power switches is the half power setting never sounds as good as the full power setting. My idea is to get different tonal flavours rather than any power reduction.

As far as i can tell, a pot-style NFB would essentially be the presence control on a plexi circuit...

Ooops! I see that. I was thinking Fender I guess.

 

[lastwinj]

Re: techie guys: My idea for an amp...

I personally don't like post PI masters. For one reason, all it takes is a speck of dust to get inbetween the pot's wipers and your bias will disappear sending the amp towards meltdown.

never a problem if sealed PEC or Clarostat pots are used.

germ

 

[lastwinj]

Re: techie guys: My idea for an amp...

all of the above could be done. question comes down to how much you are willing to pay. it wont be inexpensive. switching the rectifier on the fly would probably be the kicker. you would need to switch in both types in parallel, then switch either off a split second after this, so that you neve rlost power to the rail. not usually a good idea to run different rectifiers in parallel either, so proto circuitry would have to be done. R&D costs money.