ending a preamp

[snowballfight]

I'm building a preamp, which I'm planning to patch into my bass amp head to give just that looovely tube tone lol. I've gotten to the end of the design, but I don't know what I have to do to get a line-out that works and yet doesn't blow up/not work with my bass head. Below is the third and probably final valve schematic, comes just after the tone stack. Haven't put in the biasing figures yet cause I'm constantly trying to squeeze out a little more power from my transformer, but I'm just wondering what things I need to get a viable line out, as I say.

Anyone can help/tell me?

Oooooh also, what kindof resistance should I be using for the grid stopper here? is 220k after the tone stack too much?

Thanks!

[J M Fahey]

Is this (as shown) a single triode gain stage, which you will patch (presumably) into the out/in loop of a regular SS amp, or the last stage of a full preamp which you will use on its own, just driving the SS power stage?
If so, post the full schematic.
Anyway, just splicing a tube into the signal path , all by itself, won't provide much of a "warm tube amp" sound, no matter what the marketing department usually says.
That warm tube sound comes from slightly overdriven power pentodes, driving a big, underdamped speaker box.

[snowballfight]

Hi J,

This is one of three tube stages I will have in my preamp unit, it comes at the end of the chain (including a big muff tone stack varient). I understand only the one valve will not do much for the tone. I am actually either going to patch this whole preamp section into the effect loop in of my SS amp (skipping theirs), plug it in as a effectively a tube pedal (attenuated accordingly) or design in a power section myself. This is not the problem. I was wondering what I would have to have after this final tube stage in order to have a useable line out jack. For instance, as I have here, would this give a useable output? Do you know how I could match it to the usual 2/4/8 ohm amp standards?

Sorry if I was unclear in my original post!

Yours,

[snowballfight]

Aaaah I see the confusion here!

By the third and final I mean the third and final valve in the chain :P Sorry about that!

[J M Fahey]

Hi snowball.
You first must define the resistors used in the last gain stage (I know nothing of the others, for obvious reasons).
I'd suggest Rk=820r and Ra 47K.
Then a coupling capacitor of .1uFx400V and an attenuator to ground, composed of a 100K resistor and a 10K pot (Log preferred), as a final level adjuster. This will give you voltage and impedance levels compatible with an effects loop and allow you to overdrive slightly the tube to get a raunchier sound, yet don't blow your SS power amp.

[snowballfight]

Thanks, yea that sounds good - could I trade the 47k Ra for a hgiher one, say around 100k if I also beef up the attenuation? Of course, I see I've forgotten to ground reference the attenuation here! Pah.
I would like the warmth of a lower Ra, but might this give too little drive? I'm not entirely sure these are related, other than lower Ra s give better harmonics.

If the bypass capacitor is to be a gain control is 22 microfarads too large?

[J M Fahey]

The Ra suggested *is* 100k, but since you use two triodes in parallel, everything gets halved.
The "warmth with lower Ra/better harmonics/etc." is forum babble, not scientific fact.
Skip the cathode bypass gain control, it won't work, specially because it's bypassed by a capacitor !!!
22uF are fine there; only id the overdrive sounds farty you might lower it to, say, 1uF; but I repeat, let your ears be the judge.
The ¿bright? control switched by sw01 will also be almost inaudible (maybe 1 dB boost).

[Merlinb]

Here is an alternative line out arrangement which is low impedance and entirely safe for use with SS devices.

[snowballfight]

And the halving of the anode resistor is because the preamp tubes are a double triode within the same envelope, even if the 1, 2 and 3 ports are hardwired to 6, 7 and 8? I didn't know that before! I haven't read that so thanks again :S

The new arrangement looks good but how would the signal differ from the corrected first one? Would it be in the opposite phase?

[Merlinb]

Would it be in the opposite phase?

Yes opposite phase.

[Guitarist]

Being "opposite phase", what would be a good way to flip that? Free up that triode and make it close to unity gain, somehow. How about a 12au7 with a low plate R, say 33K 2 watt, and a 6.8 K cathode resistor? Or add a small transformer out of mic for example? (Those parallel loops are plagued by this issue, even a BadCat I had open here a while back).
Snowballer, that's gonna be a nice pre for sure; an over designed Alembic!

[Merlinb]

Being "opposite phase", what would be a good way to flip that? Free up that triode and make it close to unity gain, somehow.

Why not free up the extra triode and just add an extra gain stage before the line out? Nothing wrong with having mo' gain on tap! Or even use the two triodes for a gain stage >DC-coupled cathode follower line out.

[snowballfight]

Ha well this is my first build so I'm expecting it to have waaaay too many variables and switches etc! I'll lower the complexity after I've got it sounding niiiiice