The rack is Back?! Maybe not for everyone, but at least for me !

[fred.violleau]

Fellow ampgarage members,

I am thinking of putting together a class AB tube power amp (2 x 20 watts) in a rack and would probably then derive a 2x50 watts version.
My objective is to avoid building over and over the same power section for all the amps I have in mind, but also to allow swapping preamps and power amps.

I checked some Mesa Boogie schematics stereo tube power amps (2x20, 2x50, 2x90, 2x100) and besides the usual PI and power tubes, they all seem to have another gain stage in front. I suppose it's here to set the operation point at a 100k impedance so it works with valve preamp or SS preamps ?

I am planning to use these power amps with other tube preamps, and would like to keep the gain stages as close to what an amp offers : many preamp use 2 preamp tubes and then PI + Power tubes;
I want to build Dumble preamps with the output taken at the preamp out jack; That would be the level used to feed the power amp;
One of the potential drawback maybe the interactivity on the B+line between power and preamp.
What is your opinion and/or experience with these?
So far, in my little tube amp experience,I have not seen projects that allow to tap B+ current from a separate power amp to feed a preamp.
I know that commercially does not make any sense, but does it musically ?
Besides safety and security which are obvious issues, has it been done before ?

Thanks again for your valuable opinions!

Fred.

[bepone]

been there..add one small triode in front of PI, or small pentode triode connected.. you will miss some volume otherwise if run only from multi-effects board

[Reeltarded]

This ^^^


You are best of forgetting how pre works with power and using big headroom. Get everything out of the preamps.

This is another case of anything less than AC30 is a toy. 20w works but 50w works! There really has to be some way of making the things feel reactive like a normal amp.. that extra stage is the best you can do, probably. But it isn't bad. Honestly we are back to why complicate ot when you can use class D power..

Do not hate me.

[Reeltarded]

This ^^^


You are best of forgetting how pre works with power and using big headroom. Get everything out of the preamps.

This is another case of anything less than AC30 is a toy. 20w works but 50w works! There really has to be some way of making the things feel reactive like a normal amp.. that extra stage is the best you can do, probably. But it isn't bad. Honestly we are back to why complicate ot when you can use class D power..

Do not hate me.

I agree with Reeltarded. Ok, so the other thing is making a small power+/load that the preamps drive a small power section that is fully loaded and then to class D power.

[fred.violleau]

been there..add one small triode in front of PI, or small pentode triode connected.. you will miss some volume otherwise if run only from multi-effects board

Hey Bepone,

Thanks for the clarification. My plan is not to drive the amp with a multi effect, but rather to have a lot of flexibility between preamps and power amps coupling. I have a musicman PT and OT that I would like to couple them with any preamp to get the feel of those 700V without building 3-4 different amps (lots of tubes, and redondant parts, lots of wasted space as well).

I realized that Soldano SLO Power amp is the same as TR John Mayers and many others, so why should I bother build full amps, when I can have preamps on one side and different power amps on the other side that would allow flexibility on the output and configuration? I could have single 20 watts (6V6s, el84s etc..) and multiple rendition of 40-50 watters.

This is another case of anything less than AC30 is a toy. 20w works but 50w works! There really has to be some way of making the things feel reactive like a normal amp.. that extra stage is the best you can do, probably. But it isn't bad. Honestly we are back to why complicate ot when you can use class D power..

Do not hate me.

I love 20 watt for the right room, and the right feel (amp slightly pushed) to get on the edge of breaking up the bigger bottles. Class D is probably the most efficient way to squeeze the juice out of the preamp, but that would only account for half of the part in a tube amp. I know some power amp designs try to keep the output stage clean (Two Rock John Mayer, Soldano, ODS and more), but I like the softness of the power tubes. Switching JJ 6L6s to TAD WGC STR 6L6 affects the sound in a big way. I don<t want to loose that extra mile.

I have stumbled upon Synergy amps, and they seem to do in an amp format what I am trying to achieve in a rack version :

SYN50-4.jpeg

This one is a 50 watts amp with the ability to drive two different preamps.
Has anyone been into one of these ? I like the concept, and would be curious to understand the biasing of the preamps;

Again, thanks for your opinions !

Fred.

[fred.violleau]

Found this gutshot that seems to show a tube on the left 12A_7 (PI? or preamp?)

SYN2-Midi-fix_start_sm.jpeg

And a thread that talks about the modules and found this schematic :

SYN module Structure.jpg

So they get 320V B+ DC from the power module (midi or amp I suppose). I like the old fashion eyelet board, not a fan of the PCB board yet, even though I know there are quality ones. What would you suggest as connector to get the B+ power from the power amp to the racked preamp?

Fred.

[ElectronAvalanche]

Found this gutshot that seems to show a tube on the left 12A_7 (PI? or preamp?)

SYN2-Midi-fix_start_sm.jpeg

And a thread that talks about the modules and found this schematic :
SYN module Structure.jpg

So they get 320V B+ DC from the power module (midi or amp I suppose). I like the old fashion eyelet board, not a fan of the PCB board yet, even though I know there are quality ones. What would you suggest as connector to get the B+ power from the power amp to the racked preamp?

Fred.

Hi Fred,

maybe I can chime in with my experience regarding stand-alone preamps, power amps and Synergy SYN-2 systems.

You said: "My objective is to avoid building over and over the same power section for all the amps I have in mind, but also to allow swapping preamps and power amps.".

I also am going that route. I built a couple of Dumble-style preamps (#183 /HRM, Pentode Amp, SSS double preamp) and also have the Synergy Syn-2 (of which you posted a pic of the insides. BTW: the tube is an input tube, not a PI tube).

With my setup I can use whatever preamp I like into whatever dedicated power amp or power amp section I like.

I either use the Power Amp in of my Dirty Shirley clone, SLO clone, SSS 002 amp or any other amp having an FX-loop or Power amp in or my Hughes&Kettner VS250 stereo 2x50W power amp. Keep in mind that I do use this system only in my home-studio. If using a preamp - power amp combo live I would go for at least 2x50W or better 2x100W power amps. Or again simply using an existing amps power amp in (ODS with Dumbleator e.g.). The Synergy system is very nice since you also can use the preamp section of your amp with sort of a 4 cable method. With that you can switch between the Synergy preamp modules and your preamp. I also have an old Randall RM100 (100W beast) head that can be used with three MTS Randall modules or with the Synergy modules (I installed a mod to be able to use both channels of the Synergy modules).

I sometimes also use the above mentioned Dumble style preamps directly into the UA OX line in (another mod, the UA usually does not have a line in) and this also sounds fantastic for studio use. The major problem I have with rack power amps is the need for a fan which is always noisy in a home-studio setting (so I modded my VS250 to allow the fan to be turned off for short times and when only powering one side of the amp). I considered building a clone of Dumbles Odyssey amp which in essence is simply a stand-alone Power Amp (with CF drivers) to be used e.g. with preamps (search for the Eric Johnson rack in the forum).

Oh and btw: the MTS Randall modules (i.e. the predecessor of the Synergy modules) are extremely nice modding platforms, I modded a few of them so now I have a Marshall, hot-rod Marshall, VOX (with Cut), Ampeg B15N (for bass), Tweed style Fender, Blackface Deluxe style). So ultimately all the classic Rock sounds in a tiny format.

I never missed any interaction between the power section and preamp section in regards to a share power supply. I would also not go for having the B+ be supplied by the power amp since you loose a lot of options. E.g. you can not use the preamps with other power amps, you have the hassle of having a cable with B+ running between the preamps and power amp. So all in all I would say the alleged "magical" interplay between preamp section and power amp section via shared power supply (aka SAG) is only coming into play at very high volumes I think and all the hassle is not worth the effort usually (then simply use a non-MV Marshall and not use preamps). Btw the Synergy SYN-2 offers some sort of SAG emulation which gives a tiny bit of compression and change in sound. Maybe we need to find out how this is implemented. I never looked in to this.

Hope my ramblings are somehow helpful.

Electron

[fred.violleau]

Hey Electron,

Thanks for the valuable feedback, much appreciated! In essence, I am chasing the same dragon.

For the B+ sharing, an easy way to keep the interaction and the autonomy would be to put the external feed on a switch. I would use a power transformer to provide all required voltages so the preamp would work on its own on one side, and the other side of the switch would rely on the external feed to get the power. The external source would be only for me to use with my custom power amps.

I noticed the synergy SAG option. Have not found any more detail onto it yet. I remember on rob Robinette's website a way to emulate SAG on the B+ when using diode rectifier bridge instead of a tube. He was using some resistors o the B+ line, quiet simple in fact. I am curious to see how synergy is dealing with it.

Funny enough, Andy just posted on Facebook his take on a racked preamp, I suppose it's a sign of the times

FB_IMG_1715175679664.jpg

Thanks again for all the info, will give a look at the Eric Johnson thread on the forum;

Cheers;

Fred.