Pre amp/ amp

[jeff]

I've have an idea to build a preamp with distortion and spring reverb. My B+ is 250V. Can I run the reverb transformer with that low a B+ and what modifications would I have to do to the circuit?
Also I think it would be cool if there was a switching jack to disconnect the 8Ohm tank and connect the OT to an 8 ohm speaker. So you could use it as a reverb preamp to another amp Or plug a speaker in and instaed of having reverb use the reverb Ot as a power amp. Is that possible?
  Any suggestions? 250B+ 29K OT..... 12at7 or 12ax7 or 12au7....?
Thanks

[kagliostro]

Here I think you can stolen some idea

http://home.comcast.net/~seluckey/amps/revibe/revibe.htm

http://home.comcast.net/~seluckey/amps/revibe/revibe.pdf

http://el34world.com/Forum/index.php?topic=17218.msg172218#msg172218

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I think that you can use a 6aq5 (6005) tube (5K OT) as to drive the tank, 6aq5 is a 6V6 but works well with voltage lower than his big  octal sister

http://www.nj7p.org/Tube/SQL/Tube_query.php?Type=6aq5

an alternative may be also to use an ECC99 with triodes paralleled or in PP to drive the tank

https://www.tube-town.net/cms/userfiles/media/low-wild13/low-wild13.pdf

http://tubedata.milbert.com/sheets/163/e/ECC899.pdf

and if you want you can add a dummy load as to have an Herzog style toy in the same chassis

Franco

[jeff]

Thanks for the input. All those look great but I'm trying to build something using only what I already have in parts.
Is it possible to build a SE output section using:
 a reverb Xformer
and 250B+
so that you could use to either drive an 8Ohm speaker(to use as amp) Or an 8 Ohm tank(to use as reverb preamp effect).

[kagliostro]

Yes, is possible, the power when used with a speaker will be low, but is possible

You can arrange a 12at7 or 12au7 tube paralleling the triodes for an SE

that uses your reverb transformer to drive the tank or a speaker

http://el34world.com/charts/Schematics/files/Vox/vox_cambridge_reverb.pdf

http://el34world.com/charts/Schematics/files/fender/Fender_65_super_reverb.pdf

Franco

[PRR]

> using only what I already have in parts.

What parts DO you have?

250 Volts and a 15K OT (where do you see 29K?) points to about 17mA tube current and tube dissipating 4 Watts.

"Ideal" output is almost 2 Watt, but you never get there with a tube.

A Power Pentode will do very much better than a large triode, which does better than a small triode.

6V6 is 3 times bigger than we need, but will do nearly 1.8 Watts output. 6AQ5 will do the same, as will a EL84. These are all over-kill, but super-abundant in tube-amp part-bins.

12AU7, both sections in parallel, will do about 0.7 Watts at best.

Interestingly 6V6/6AQ5 and 12AU7 both need about 10V peak grid input. (You could drop the pentode's G2 voltage for a little better sensitivity.)

The fat side of a 6EM7/13EM7 (dual triode) will make 1 Watt, but needs 50 Volts of peak grid drive. (The small side can give gain of 40, so around 1V peak to the bottle.)

This is assuming the small "onboard reverb" transformer. The Stand-Alone reverb unit is much more like a low-voltage Champ, with big (6F6) bottle and around 7K transformer (Doug tells you to get his Champ OT for this application).

[terminalgs]

250 Volts and a 15K OT (where do you see 29K?) points to about 17mA tube current and tube dissipating 4 Watts.

Jeff,  PRR mentions current draw.  Don't overlook this aspect in the design (I did once in a reverb driver... once!).  make sure your pre-amp 250V transformer can handle the driver, recovery stage,  plus the first pre-amp stage (and anything else you've got in there).


(like others are pointing out) when driving a speaker,  you'll have a very quiet amplifier. Don't expect it to be a dual purpose reverb unit/Champ.  with dual-purpose design in general,  it is a challenge to make a single item that does both purposes exceptionally well.


To attempt to do both purposes well in this case, you'll might end up changing pre-amp gain and driver bias (to get over 2W/3W) when switching from reverb-mode to amp-mode to get more umph (so the amp-mode is something you'd end up using).



If your power supply has lots of current available and you want to shoot for 4W/5W in amp-mode, you could use an el84 driver (250V at plate can get you 5W with el84).

[jeff]

The transformer I have is marked 14500:4. Which is wired because it's labeled as a reverb Xformer but I,ve never heard of a 4 ohm tank. So that works out to be a 29K:8 reflected impedance.
As far as not being that loud, thats ok. I don't need anything loud as a champ because I could always plug it into a champ when used as a preamp and have reverb too. I was thinking much, much quieter than a Champ, like just connect the reverb send to a speaker and get a very quiet playing around volume, like maybe a champ on 2. Just enough that its a little louder than accoustic but so I could get some distiotion. not loud at all. just practicing without disturbing anyone.
I'm heading towards using the 12AU7.
I've been reading up and playing around with load lines for the 12au7, I'm not sure I'm using the right but does one section of a 12AU7 at 250V with 14K5 load make any sence? I'm not sure about bias but is -7V seem right?
When I get time I'll post more, gotta go.
Thanks
 

[DummyLoad]

29K OT will not work well with any tube i can think of @ 250V B+ - won't deliver the power levels you need to drive a reverb tank.

3.5K SE OT with EL84 strapped in triode mode with 250V B+ makes about 2W. Rk = 270R, Ib is ~35mA so SE OT needs to handle that. << this looks like a decent reverb tank driver.
7K SE OT with EL84 in pentode mode with 250V B+ will make about 4W Rk = 230R, Ib is about ~40mA.

--pete

[jeff]

How about this:
The Xformer I have is marked 14K5:4 and I have two of them. Is it possible to wire the primaries in parallel so you have a 7K25 load going to two 4 ohm loads, then wire the secondaries in series for a 7K25:8
 
If so will that work well with a 12AU7 in parallel using the schematic Kagliostro posted for the VOX  http://el34world.com/charts/Schematics/files/Vox/vox_cambridge_reverb.pdf
I called Mercury Magnetics and they said the reverb OTs primary was 7K7(The wierd thing is that they said it was a 7K7:25K8 transformer witch doesn't make sense to me seeing as tanks are usually between 8R to 1450R but they may have been using some wierd tank. does 7K7 seem right for that schematic?)

So if that's possible: 12AU7+Vox schematic+two 14K5:4R Xformers wired as 7K25:8 into either a 8R reverb tank for effect unit or an 8R speaker for tiny, tiny, amp
 
I realize that a 7K load would work well with your(Dummyloads) EL84 example but if this will work with the 12AU7 like the Vox it may be better as far as lower mA consumtion. EL84 40mA vs. Vox 12AU7(10.5/680=) 15.4mA. Again I'm not looking for much power for speaker volume, just a way to run a reverb tank @ 250V, and if plugging a speaker in the reverb tank send makes any sound at all without hurtin anything, great!, but driving the tank @250V is more important to me.

"6V6 is 3 times bigger than we need, but will do nearly 1.8 Watts output. 6AQ5 will do the same, as will a EL84. These are all over-kill, but super-abundant in tube-amp part-bins.

12AU7, both sections in parallel, will do about 0.7 Watts at best."
I guess another good question would be how hard do you need to drive an 8 ohm reverb tank? i.e. if you plugged an 8Ohm tank into a champs 8Ohm speaker jack would you blow the tank? The EL84 @ 4w into a speaker may be good as an amp, but would that be too much for the tank? Is 0.7W out of parallel 12AU7 enough to drive tank and get some sound out of a speaker without wasting power?
 
 
 

[PRR]

> much, much quieter than a Champ

OK. As long as you won't be disappointed.

Let's say 29.5K.... we'll call it 30K.

12AU7 at lower voltage can pull-down about like a 9K resistor.

So 30K+9K is 39K, of which 30K is useful load, so peak voltage is 240V*(30K/39K) or 184 Volts Pk, 130V Sine RMS. 130^2/30K is 0.4 Watts Sine RMS output. 11dB weaker than a Champ. With a typical guitar speaker it won't break 90dB SPL. With a lame hi-fi woofer it may not touch 85dB SPL.

In other words: weaker than mom's kitchen radio. It can still annoy a whole household, but isn't any sort of Party Animal.

The idle current should be around 240V/39K or 6mA. Yes, at 240V plate-cathode this is like 10V of g-k bias (which is why we figured 240V available to the tube). Peak grid signal is 10V positive but about 15V negative, so it will have THD which is cool.

Assuming 30mV peak guitar signal you need gain of 300 to 500 to make full output power. This is more than any one tube will do, so you want two stages. Each needs gain of 20, which is awkward. You go 12AX7 preamp and not-bypass *all* cathode resistors, which gets the right number but will be super-clean (may as well use chips). I'd steal the Champ-AA plan with high gain preamp and a tone-stack in the middle to waste some gain.

[jeff]

Thanks
Just to be sure are you talking one section of a 12AU7 or a 12AU7 wired parallel?
thanks again
Jeff

[jeff]

> much, much quieter than a Champ

OK. As long as you won't be disappointed....
In other words: weaker than mom's kitchen radio. It can still annoy a whole household, but isn't any sort of Party Animal.

That's prefect! Just what I had in mind. At home I can use it with an amp and crank, and when I go to my buddies on the weekend, just bring the preamp(I'll leave a speaker at his house). He's got a 15W amp and we only ever play it below 1, just above the dead spot on the volume knob(as low as you can go and still have sound), as not to annoy his girlfriend. So quiet is good.

 Here's a block diagram and illustration of how it would be used.

(works better if you click blue words not picture)

[jeff]

I'm going to do some bread boarding tonight.
Would 14K5 be a better load than 29K for one 12AU7 section?
Just doing some supposing here
Use the 29K:8ohm as a 14K5:4
Use each 12AU7s section w/ it's own 14K5:4 transformer
instead of using two 4 ohm loads(one for each OT) wire the secondaries of both OTs in series and use a single 8ohm load.

Inotherwords could I use two reverb transformers(29K:8 or 14K5:4) and wire the primaries in parallel(for 7K25) and the secondaries in series(for 8R) to get a 7K25:8?

and would 7K25:8 with a 12AU7 in parallel be better than using the 29K:8?

[PRR]

Triodes (12AU7) are not super-fussy about their load. Neither are you straining to get every last Watt out of your bottle-dollar. "Sound" may be more important than "Power".

I think your questions are best answered with loose wires on the breadboard. (Please do not get killed!)

[jeff]

Ok thank you for the help(I try not to kill myself)
Jeff