Pedal Amp Design

[SilverFox]

What sort of topology would be a good design for a preamp used primarily with amp emulators? My thoughts would be perhaps an input tube followed by parallel 12AX7 with a tone stack afterwards and a cathode follower. From that point the signal would go to the effects loop. It would be desirable to gain slight distortion to add an edge to the emulation, which would already be distorted..

Would there be any value in having two different bias values for the parallel tubes in order to voice the signal?

The power amp is already done.

Any suggestions?

Silverfox.

[Stevem]

What the heck type of pedal, analog?

[JMFahey]

In general lines, if you already have a "cooked, ready to serve" signal, and a clean and loud power amp (so it has no trouble handling what you feed it), the preamp between them shoud be simple and flat.
Even a single triode, a classic "12AX7/100k/1k5" a.k.a. "Fender stage" would do.
All you need is to rise preamp out level to whatever's needed to comfortably drive the power amp.

Since you have more gain than needed (around 50X instead of, say, 5X) you may add a tone control between them.

I suggest a passive HiFi/"Ampeg" type one, for the simple reason that it can be easily set "flat" and just tweak sound a little, if needed, while classic Fender or Marshall are harder to set flat.

Hi Fi:
[img:700:315]http://www.next.gr/uploads/11-91134af9be.gif[/img]

Ampeg:
[img:634:654]http://www.vintageampeg.com/wp-content/ ... 4tone1.png[/img]

remember both require Log/Audio pots, but the beauty is that "setting them to 5" guarantees flat sound, also both provide smooth and symmetrical bass or theble boost and cut ... which simply does not happen with Msh/Fen/Vox tone control types.

The idea is to start flat, set playing level with the band, and only then make minor tweaks if you need a little more or less bite or thump.

IF you need extreme settings, then you have a speaker/cabinet problem (trying to extract 4 x 12" thump from a tiny open back cabinet or bite/sparkle out of a really dull speaker) .

[stretch2011]

Look at the matchless hotbox.

[SilverFox]

stevem- " a preamp used primarily with amp emulators?

JMFahey- Thanks for the detailed response. Do you have any thoughts on a JTM45 as the basic preamp topology? That amp doesn't distort much and takes overdrive pedals well. Perhaps using the tone stack you suggested would be viable? As I mentioned in the OP I think I want just a little hair to grit up the tones from the AdrenaLinn 3 emulator I run.

As for the power amp- This forum contributed towards my Sunn Sceptre power amp section and so far that is working out just fantastic.

silverfox.

[Mat Cortex]

hi,

i've used a white cathode follower (WFC) ( see attached) to make myself a recording/practicing preamp with line out output. mine is a 2rock JM Sig preamp with a WFC.

it Still under troubleshooting but it works so far.

i've used EDCOR XSM10K/600 for OT.

Regards
Mat

[SilverFox]

Thanks for all the posts so far. I'm really leaning towards putting a JTM-45 in front of the power amp at this point. Unless there is some reason not to... In fact I'll also take the opportunity to screw up what is currently working by rewiring the power amp build I posted here under another topic. I will be posting that project as soon as I get ready to start.

silverfox.

[Mat Cortex]

Hi Silverfox

Here's what inspired me for my stand alone preamp.

http://www.frontiernet.net/~jff/SonOfAl ... icF2B.html

search on TAG for JM signature recording preamp, you'll find my complete schematic. Still troubleshooting the reverb squeal but it works well nonetheless.

Regards
Mat

[tele_player]

remember both require Log/Audio pots, but the beauty is that "setting them to 5" guarantees flat sound, also both provide smooth and symmetrical bass or theble boost and cut ... which simply does not happen with Msh/Fen/Vox tone control types.

The idea is to start flat, set playing level with the band, and only then make minor tweaks if you need a little more or less bite or thump.

IF you need extreme settings, then you have a speaker/cabinet problem (trying to extract 4 x 12" thump from a tiny open back cabinet or bite/sparkle out of a really dull speaker) .

Audio taper required, but what audio taper? 10% ? 30%? Seems that would be important to get flat response with both set to 5.

-Robert

[SilverFox]

Yes that is very nicely done Mat. Just out of curiosity- I've got several 600 ohm mic matching transformers from a Bogen type PA. I wonder if those would work for creating a balanced line out?

Please keep in mind I want to be able to go from clean to a "Haired Up" tone coming from an amp emulator. I already have 40 amp models to play with. I really only use about 5 of them and it is really getting down to the Ecstasy and the JTM 45 models. I think too if the JTM tube preamp is over-driven with the emulator it should take on a classic Marshall tone.

Another reason for JTM prejudice is, if I just want to play with no emulator I like the JTM sound and the Sceptre power amp reproduces low frequencies very well.

Presently clearing out the space for the amp project and it's about time to start the build thread...

silverfox.

[Mat Cortex]

I wonder if those would work for creating a balanced line out?

Hi,
I'm not the good guy to ask this question. Not enough knowledge. This was my first tube amp building beside some minor mods to an existing amp.

You can still compare your "OT" to other line matching OT datasheet.

A friend of mine test the WFC by highjacking signal from a jcm800 then get throught the WFC then to a "standard OT" and speaker. As him, you can build some kind of circuit to test the XFo you already have.

The signal at the treble pot in a complete amp feeds the phase inverter (PI). PI’s are, for the most part, unity gain devices. The PI output drives the power tube grids which are biased at ~-35 VDC. So…the voltage at the treble pot has to be at least 35 VAC p-p (I think) to drive the power tube grids enough to obtain full output power., so you'll need to calm down the voltages to 0,7@1,25 volts for line level via a matching XFo.

There's plenty of guru here that will help you as they helped me! Thanks everybody for the help you've been and the one to come ! Amp designing is a fun journey.

Have fun !
Mat

[SilverFox]

How would a Dumble Loop circuit compare with all these discussions? With the addition of a tone control network inside the loop?

This is very similar to the Ampeg Tone stack suggested by JMFahey.

silverfox.