Adding a Pre-amp out to Sunn 100s circuit

[nateflanigan]

Hey everyone, long time no see.  
I'm doing some tweaking to my Sunn 100s/bassman build, I'd like to add a pre-amp out.  I'm thinking between the two halves of the 6AN8 would be a good spot.  What I'm unsure of is what to do wiring wise so that I can run the amp solely using the pre-amp out with out having a cab plugged in to protect the power amp.  Do I just wire up a shorting jack so that Pin 2 (of the 6AN8) shorts to ground?

http://www.el34world.com/charts/Schematics/files/sunn/sunn_100s.pdf

Thanks!

[PRR]

> between the two halves of the 6AN8

High impedance, HIGH signal voltage.

> Pin 2 (of the 6AN8) shorts to ground?

That will make a hellofa thump/crack; and if OT is unloaded, the kind of surge that zaps OTs.

BTW: that drawing is missing the connection between 6AN8 pin 6 pin 2.

All things considered:

1) carry a 200 Watt dummy-load, take a tap off that.

2) break connection at 6AN8 pin 8. Ground pin 8. Take the treb+reverb mix to a unity gain buffer. You have a low-volt supply at "D" which would power a TL071.

[nateflanigan]

Oh, sorry my bad, if you scroll down there's a second rather different schematic in that PDF, that would be the one for my amp.  Thanks for the reply though.


I built this as a guitar amp, I followed the Sunn schematic up through the PI, but the output uses a bassman OT and 6L6's. and is not wired UL.  It's great for guitar, which is just fine, but I also really like the way it sounds with my p-bass only I can't turn the amp up past 2 before the low end gets too distorted.  There's two experiments things I'd like to try...

1) Running the pre-amp of the "Sunn" into some other power amp.
2) Using the "Sunn" pre for DI recording.

Out of curiosity, I have an unused edcor 10k:600r transformer in my basement, what if I ran the pre amp out through that? 

[sluckey]

I'd like to add a pre-amp out.... What I'm unsure of is what to do wiring wise so that I can run the amp solely using the pre-amp out with out having a cab plugged in to protect the power amp.

This simple but versatile alternative gives you a Hi Z preamp output and a power amp input. Send your preamp signal to another device OR connect an external preamp (or other line level signal source) to the Sunn power amp.

Always use a dummy load when not connected to a speaker cab. Or you can kill the output tubes by putting a switch between the output tubes cathodes and ground.

[terminalgs]

Did the idea to add a pre-amp out come up only after you found the amp doesn't sound good with a bass?    If so, that issue is worth pursuing on its own.  (maybe the speakers can't handle the low freq? )

Also, perhaps you are speculating that the "post-2" bass distortion is coming from something after the pre-amp, and that by tapping into the signal before the PI will provide a super clean bass signal.  perhaps so, but it could also be coming from the first gain stage. That double gain stage is popular with guitar amps, but  I'm not sure it was widely used for bass. Sunn being only exception that comes to mind.  that said, the solos, 100S, 1000S, 1200S, and spectrums were all marketed as guitar amps;  the 200S, 2000S, and Sonics were bass amps.  On the bass amps, they did that same two gain stage, but with a different coupling cap and that 'lo boost' switch, no doubt, intended for bass:



Regarding the 10K:600: you'd need to use a 12AU7, 12AT7, or another 6AN8 to drive it.   the transformer may or may not have the freq. response at either (or both) high freq. or low freq. to get the perfect signal facsimile you are looking for. easier to use the tube for a unity gain buffered out like PRR suggested (unless you just want to fiddle with that OT).

If you add a preamp out, and run the OT into a dummy load,  another tweak would be to replace that 1M grid leak in front of the 6AN8 pentode with a 1M pot (master volume), so if you are running a dummy load, you can turn the MV down to 0 to prolong the life of the 6L6's and the 6AN8.  maybe that's over-kill/over-engineering.  but it give you a master volume, which you might dig for guitar (other Sunn guitar amps, like the Model-T had an MV.  The Model-T is more or less the same tone stack you've got, but with a Vox style "top boost" 12ax7 Gain+CF driving the tone stack....).   

[terminalgs]

On second thought, that 'low boost' switch is probably not the switch to focus on with the 200s, perhaps the the 'hi boost' off position (conveniently not pictured in my posted schematic clip).

[nateflanigan]

Sluckey: Something like that would be groovy, seems like having a good dummy load on hand would be really useful regardless.

Terminalgs: Speaker's fine, I was running it into an ampeg b-15 cabinet.  When I built the amp I first had the 200s tone stack going but I didn't love it with guitar so I changed it to the 100s.  Which is really great, I use a 12ay7 in V1 though, the 12ax7 was just too hot.  A master volume could be interesting on this amp, it sounds pretty unreal cranked but is also very very loud.  Back to the problem at hand, I'm mostly just looking to grab the signal I've got going and feed it into a more powerful power amp.  If that experiment really panned out I'd make myself a little stand alone pre/DI perhaps.

[terminalgs]

Here is a way to integrate a CF buffered 'out'. The gain stage isn't really needed for sending the output to a PA or recording console.  It might provide make up gain for powering a power amp, if you need to keep the preamps volume low for clean signal.  You can leave the gain stage out and couple the CF with a cap.

R1 can be chosen based on desired output impedance, then select C1 for a desired low end bass roll off.

The 6an8's grid leak is bumped from 1M to 2M. The parallel 'tap' 1M R and 2M pot give you 3M or 2M, so you still have 1M to 1.25M for a grid leak.

[nateflanigan]

Looks pretty interesting, I'll have to tack that together.  I can certainly follow the drawing, but on paper it's not clear to me what that would do that the 6an8 isn't already doing.  Since I have the amp open and on my bench it'll be nice to try out a few different ideas.  Thanks

[terminalgs]

Looks pretty interesting, I'll have to tack that together.  I can certainly follow the drawing, but on paper it's not clear to me what that would do that the 6an8 isn't already doing.  Since I have the amp open and on my bench it'll be nice to try out a few different ideas.  Thanks

To power a power amp, use sluckey's suggestion.  the input impedance depends on the power amp,  but 1M input impedance is likely (like the input imp of the 6an8 pentode in your circuit).
Voltage might be up to 30v or 40v p-p (peak to peak).

But a recording console, PA, or anything that says 'line' or 'mic' won't be happy with and input providing  30v @ 100k output impedance.

For those you should use lower signal  voltage and lower impedance.

For some 'pro audio' 'line'',  input is +10dBu nominal, +25dBu max, -40dBu min. @ 10k input impedance. For that, you want an output imp. of 1k and a signal voltage of maybe 5v p-p.

If you want unbalanced to plug into a DI box to convert to balanced 'mic line', you need to plan for more along the lines of  0.7v p-p @ 200ohms.

So what's your signal voltage before and after the 6an8?  Too high for the recording console out, maybe too high for the power amp too.  Certainly, it'll make your volume knob super sensitive.  So you can put the voltage divider on the plate side of the pentode, but you will have more attenuation to do, plus it might make your volume knob too sensitive.

[nateflanigan]

Ok that's very helpful, I hadn't considered the z of the signal at that stage in the amp.  Merlin's got a great page on designing cathode followers as well.