How simple would it be (or not be) to make a rack mountable preamp from this circuit?

[mort]

I'm pretty good at following a schematic for assembly purposes, but I'm still very weak on principles and theories of audio circuits. Need some thoughts here...


I imagine that I wouldn't need the OT, power tubes, the PT could be smaller/cheaper(use a 5F1 PT??), what other things would I need to take into consideration?

Rumble_Bass_Schematic.pdf

[loudthud]

For just the first page that's 6 dual triodes and a lot of passive components. That will require a very complicated layout. A 5F1 power transformer (assuming it has enough heater current) could work in a 2U high chassis. You might want to only build one channel unless using both at one time gives you your sound.

[mort]

Yeah, this one might be a little ambitious for me, but I think I may go for it anyway. Gotta stretch the ol' brain tissue somehow I guess.

Here's a question, the full amp uses about 1000 mfd of filtering on the supply. How do I determine how much I need for just the preamp section?

And is there a such thing as 'sag'(low filtering) on a preamp section? Or is the current draw so small that it can't really 'sag'?

[loudthud]

Here's a question, the full amp uses about 1000 mfd of filtering on the supply. How do I determine how much I need for just the preamp section?

And is there a such thing as 'sag'(low filtering) on a preamp section? Or is the current draw so small that it can't really 'sag'?

A suitable power supply needs to be designed. One of the hardest things to do is determine what is required in terms of supply noise. Besides a B+ of around 322V, there is a 6.3V DC supply for the tube heater and +24V to switch the relays.

One of the disadvantages of the stand alone preamp is that there is no sag in the power supply. There is not much that can be done about that other than 1) steal B+ from a power amp, 2) build in or use some kind of compressor inline with the signal, 3) build in a small power amp.

[mort]

Hmm... this project would be for a friend of mine who is a pro bass player. The 300W Rumble was one of the best sounding bass amps he had ever played through, but they are few and far between, so he asked me if I could build him up just the preamp section to put into a rack. Sounds like it would probably end up not sounding like what he wants... plus, it's getting over my head to go redesigning parts of it.


Maybe it would be best, if he wants to go through with it anyway, to just do one channel. Simpler, cheaper, and less time lost if it doesn't turn out so well...


just a thought, since alot of transformers don't have 24v, could a second transformer be used just for the 24v? I have plenty of 120/24, 40va xformers that I use for hvac equipment. Of course I wouldn't need it anyway if only doing one channel...

[Tom Phillips]

... this project would be for a friend of mine who is a pro bass player. The 300W Rumble was one of the best sounding bass amps he had ever played through, but they are few and far between, so he asked me if I could build him up just the preamp section to put into a rack. Sounds like it would probably end up not sounding like what he wants...

That’s a good observation and I think you are headed toward the answer to your own question. An amp is a multi-part system. Everything from the pre-amp to the speaker cabinet contributes to the sound. Even if you built him a perfect clone of the pre-amp your friend would still be a long way from being able to duplicate the sound of the 300W Rumble that he liked. Best to get this understood before you spend a lot of time building a stand alone pre-amp.
Regards,
Tom

[J M Fahey]

FWIW, in a similar case I built a couple Rack Tube preamps for some customers

I chose a classic and very popular preamp: a clone of the Ampeg SVT , version 1972:








The blue preamp below is driving a QSC900 amplifier, driving 2 Ampeg 8x10" ; the silver one on top, a 70's Orange clone, is driving an AB1500 , also into 2 8x10" .

The Ampeg head in the middle is the backup/rehearsal amp.