tube PreAmp & FX into SS power PA

[tubenit]

I've posted this once before but couldn't find my original post doing a "Search" so I'll post this again.

I am talking to someone who is interested in the idea of me building them a PreAmp that can be plugged straight into a PA system.

So, I like the D'Mars ODS clean & overdrive. And I like the FX onboard. My thinking is to build this with the hopes that it will work reasonably well into a solid state PA system.  I certainly would anticipate a warmer sound with the tube then just plugging straight into pedals and the PA which is what the individual is currently doing.

My main questions are:

1)  Would this work plugging into a PA system?

2)  Would the output jack be the same as the input or FX jacks? (shortening)

3)  I am presuming that with NO power tubes that running this preamp without
     it being plugged into the PA or a speaker will not be a problem (compared
     to running an amp without plugging into a speaker)

4)  Would it be OK to just ignore nodes A and B and simply use nodes C and
     D for the preamp setup?  (In other words no problems with not using A and B
     connected to anything, but have them there for filtering?)

With respect, Tubenit

I am also thinking that IF somewhere down the line this person wants an entire amp with power section and speaker that I could do what HBP and Timbo have done with an upstairs & downstairs chassis.  I would use a separate PT & OT for a lower chassis and build an LTPI that is point to point and into a push/pull power section.  

I'm thinking that instead of B+ umbilical cord, I could have a preamp head sitting on top of a speaker cab with a power amp.  And the only umbiical cord would be the signal instead of voltages.  

In fact, one could build this preamp and then build a power amp section using various tube types like GlennJeff has done. Have a power amp with the capability of EL84's or 6V6/6L6.

[rzenc]

Hi Jeff.

Plans looks interesting. I built some guitar preamps and they sounded remarkably well when pushing SS power amps.

About your questions:

1) Yes;

2) Not necessarily;

3) No problem that I can report at all. I tested them on the bench with no load attached to output expect scope probes and it did not catch fire, unlike amps when pushed without speaker cab/proper load;

4) I would keep the filtering and possibly add some capacitance to it. I believe you are also aiming for low noise power supply.


IMHO, I would employ a CF stage after your FX level instead of running it straight from the plate. 12AU7/12AT7/12BH7/ECC88 are triodes worth experimenting with CF and maybe some pentodes too.

I like your idea of preamp having it's own PT. As you said, later on you can build a poweramp to suit your friend's needs.

I attached an AC CF output stage that I used on one of the pres.

Let me know whether I can help you further.

Happy hunting!!


Best Regards

R.

[tubenit]

Rzenc,

Hey, THANKS so much for the information and help!  I truly appreciate it and it's nice to hear from someone with actual experience trying to do this.

If you would explain the CF idea after the FX. It has a CF going into the FX almost exactly like the one you've drawn.  All three of my amps have this FX on board and it seems to work flawlessly, IMO.

Is the CF needed there for going out to the SS power amp?  Is that the reasoning behind it?

Thanks and with respect, Tubenit

[rzenc]

Is the CF needed there for going out to the SS power amp?  Is that the reasoning behind ?

Check out input impedance on SS PA amps. Some have it ~10K. IMHO, you will be better providing about 10x less that, say ~1K to minimize inserction loss. That ckt should be around 600ohms Z.
Also, the lower out Z is supposed to handle noise around cable connection and 'shield' it against external interference.

Also, the last stage is a gain stage with high amp factor and output impedance around 40K. The CF would isolate it from PA input stage and help to better transfer the integrity of your processed guitar signal.

The main idea behind it is to buffer the preamp output stage. Thus helping to isolate from noises and improve signal transfer.

Edit: CF stages may introduce some compression effects too. Which may or not help.

Hope this helps!

Best Regards

R.

[sluckey]

You're saying to add a CF at the end of the FX signal chain, so that the CF is just prior to going into the ss PA system?

I agree with rzenc about using a cathode follower (or source follower as in your drawing). A solid state amplifier will likely have a low input impedance that will load your hi Z tube signal considerably.

Doing it this way will improve the tone into the PA?  Is that correct?

I think the follower will have minimal impact on the tone.

Something else to consider... Rzenc is discussing sending your preamp signal to a POWER AMP. The signal level from your preamp will likely be a good match for a power amp input. But... Your preamp signal level will likely be much too hot to send to a mic input on a PA system. If that's the case, you will need a voltage divider at the output of your preamp. Maybe even use a switch at the output to select high level or low level. However, if your PA accepts line level inputs, no need.

[tubenit]

Steve,

Man, I always am grateful for your help!  Thanks!  

One thing that I was not thinking thru correctly was noting that the Power Amp and a PA system are not the same thing. I was erroneously not making the distinction that needed to be noted.

I am thinking that the FX level pot should function as the voltage divider that is needed prior to the PA system?

Assuming this guy wants this built, I'll definitely include the CF at the "end" of the FX prior to the PA.

Thanks and with respect,  Tubenit

[tubenit]

I have an impedence adaptor but I think it's the "wrong" kind & would need more adaptors to make it work.  It's lo-Z to hi-Z & I'm understanding that I want the opposite.

http://www.sweetwater.com/store/detail/LittleIMP/

Tubenit

[DummyLoad]

cathode or MOSFET follower will drive the best and what you need with either is reduction in signal (line level) to the SS amp. you can do this with a Cc 10k-15K : 150/600 ohm transformer or a divider network. the source follower or cathode follower's output signal will likely be over 100Vpk - that'll kill most SS amp inputs. the advantage with the transformer approach is the line-side load can be configured balanced or un-balanced. you'll want to design so that the full volume output to the SS amp is around 2Vpk. it really helps to have an o-scope with this kind of work.   

--pete

[jjasilli]

Related thoughts - how long is the cable fun from the preamp to the power amp.  If more than 10' you should consider a balanced line out.  That would be a 3 conductor cable terminating in either TRS plugs, or XLR.  Lundhal has schematics on its website for the things I mention in this post.

As Dummyload says impedance matching from the hi-impedance output of the preamp to the low-impedance (hi-Z) SS input is a concern.  This can be addressed with a matching tranny inside the preamp to drive a balanced line-out.  You may consider 2 lines out:  one hi-impedance for a tube power amp, for which your CF schematic is fine; the other low-impedance to feed a SS amp or effects loop.

For the fx return, as rzenc says: impedance matching may again be a concern.  A CF input is one alternative; also, see Merlin's book for a tube CF input circuit. "Designing Tube Preamps for Guitar & Bass".  Another alternative for the fx return, again, is a matching transformer, this time from low to high impedance. You can build both TRS and/or XLR jacks into the preamp chassis  - and maybe hi-lo impedance SW's - to dispense with the need for adapter plugs which may vanish when needed.

Impedance matching is a PITA.  I feel your pain.  I'm grappling with the same issues, having managed to avoid them for years.

[tubenit]

THANKS guys!  I'll study your responses and help and figure out what is a reasonable risk approach and go from there.

Always appreciate the input and responses!

With respect, Tubenit

[tubenit]

Pete,  

Thinking about what you said while mowing the yard. .............  

Any chance you can take my schematic and modify it to show the transformer placement.  I'm thinking this might be between the CF and the FX level pot?   I attached the SCH schematic to be changed to show your idea.

2nd question:  Will this change the preamp tube tone?

3rd question:  Any chance you already have a source for the trannie that you're mentioning?  IF not, I can look for Hammond or Edcor.

4th question:  Is it reasonable to build this first with out the transformer & then try it and IF it's not what is wanted then add the trannie?

With respect, Tubenit

[terminalgs]

I attached an AC CF output stage that I used

Regarding the CF schematic, R101, the grid leak, should be connected to the grid of the Triode.

[Glennjeff]

This is a bit longwinded, sorry, just trying to clarify what others have already stated. Hope I don't create more confusion.

If your adapter is a transformer it may work OK wired the opposite way tubenit, just don't drive too much signal into it. Here is a schem, may need tweeking with regards to levels. Low Z to Hi Z / Line to Mic / Unbalanced or Balanced transformers are a very standard component at any electronics store that caters to audio guys. You get the quality that you pay for often. Your looking for 10 K (to 100 K) unbalanced Line to 600 Ohm balanced Line to drive a long cable run to a mixing desk or a typical balanced amp input.

If you use a CF or MOSFET follower this "Hammond 140QEX CHASSIS MOUNT" should do the job. (http://www.hammondmfg.com/pdf/140QEX.pdf) but you may need to lower the value of the output pot a little or a lot.

Here is another possibility http://www.tubesandmore.com/products/P-T42TM022


Two issues really:
1)  providing the correct signal voltage for the actual input that he is plugging into. (Is he plugging into a MIC input - need very low signal. If plugging into LINE input need about 1/2 to 2 V peak signal.)
2) Preventing signal degradation if the signal is to be transmitted over a very long cable, a long cable looks like a capacitor to ground and will lose lots of high frequencies if not driven by a low impedance source, such as a CF or MOSFET FOLLOWER. Low impedance balanced cable also prevents HUM from being picked up over long cable runs.

You could also just use a resistive divider to get the right signal for your clients specific application but best to provide a more general solution in case his circumstances change. The potentiometer after the MOSFET follower should provide all the adjustment needed, a transformer adapter will take care of signal degradation and noise pickup (impedance matching).

The inclusion of a transformer also helps prevent strange ground loops between guitar amp, PA and microphone and provides a small increase to the electrical isolation between the various components of the "live gig" sound system thereby improving electrical safety to a small degree.

All the best.

[DummyLoad]

Any chance you can take my schematic and modify it to show the transformer placement.  I'm thinking this might be between the CF and the FX level pot?   I attached the SCH schematic to be changed to show your idea.

i attached a schematic of something RicharD and i built about 4 years ago. it's the output section of a bass preamp we built. ours had a NE555 one-shot delay timer coupled to a relay that is a mute feature to avoid power on/off thumps noise and other weirdness as the tubes warm up or fade out.

your plan seems like it will work. my only concern is the output level may be too high and could damage downstream equipment. you really should get an oscilloscope to aid with testing and confirmation this design. if the SS amp you're using doesn't have input over-volt protection you could easily fry it.

2nd question:  Will this change the preamp tube tone?

probably. how much? unknown.

3rd question:  Any chance you already have a source for the trannie that you're mentioning?  IF not, I can look for Hammond or Edcor.

http://www.edcorusa.com/p/159/xsm10k-150

http://www.edcorusa.com/p/160/xsm10k-600

http://cinemag.biz/output/PDF/CM-2810.pdf

http://www.ebay.com/itm/Altec-Peerless-Transformer-15095a-/190883089186

the edcor's are a bargain i've heard and read good things about them but have no experience with the small signal stuff; of the two i'm most familiar with, the altec is best sounding overall; the cinemag is the most neutral sounding. we have tested the altec and cinemag in our bass pre-amp. we liked the altec best, but they are drying up, hence why we trialed the cinemag. the cinemag will run about $70.00/ea last time i priced them. hammond has some stuff. i've never bothered with it.

4th question:  Is it reasonable to build this first with out the transformer & then try it and IF it's not what is wanted then add the trannie?

yes. good small signal transformers are costly.

[tubenit]

Pete,

Oh man, I hate revealing how ignorant I am about this stuff.  

I have had ZERO formal training in electronics. Not even one hour's worth in a classroom ever. I don't know if I've ever actually seen an oscilloscope in person? And would have no idea how to use it at all. Almost everything I've learned about electronics has been from this forum and reading some of Kevin O'Connor's stuff.  
 

Due to my lack of knowledge, I was thinking I could just build this thing, plug it into a PA system with a guitar cord and have it work just fine?  I didn't realize that there would impedance issues and that it could actually "fry" something down stream.

So, in light of that, I want to explore options and try to come up with an idea that is suitable for my lack of knowledge & within my reach.

How about the idea of building this with the mosfet CF at the end of the active FX and then plugging into a "device" that then plugs into the solid state PA?   That would allow me to have a tube preamp set up that could then also be plugged straight into an upstairs/downstairs arrangement using another chassis for an LTPI and tube power amp.

Would this device accomplish what is needed for the concerns you have expressed?   IF you can  bear with me a little longer, I sure would appreciate the help and advice.

http://www.musiciansfriend.com/pro-audio/motu-zbox-guitar-impedence-adapter

OR maybe this direct box:

http://www.musiciansfriend.com/pro-audio/whirlwind-imp-2-standard-direct-box

In other words, I'd like to be able to build the design with the schematic I posted (which I can do successfully),  plug into some reasonably inexpensive device that addresses the impedance issue and then plug straight into the PA system.

With respect, Tubenit

[tubenit]

The more I am thinking about this ........... the more I am thinking I'd like some external device that is between the tube preamp and the solid state PA .

OK, I found an article "how to convert high Z microphone to low Z".   I don't know if this would apply or not?

You don't need an Op-Amp, you don't need a transformer.  A simple JFET like a J310 will work perfectly.

Use a 9V battery to power the JFET drain. Connect a resistor equal to the element impedance from gate to ground. Put a 270 ohm resistor from source to ground (battery negative and mike ground). Pick the audio off the source with a 1uF or so capacitor. Feed the mic into the gate via a .1uF cap.

You have an instant $1.00 conversion. I use this with D104's on my ICOMs and Yaesu's.

http://www.eham.net/ehamforum/smf/index.php?topic=26560.0

With respect, Tubenit

[Glennjeff]

tubenit,

Sorry we frightened you with "geek speak". In practice it should be very simple to accomplish, especially for someone of your experience.

How about, I go down to the audio shop tomorrow, pick up a transformer and wire it into my D'Mars and then send you some pics, schematics and photo's. I've got mixing desks and SS PA amps here to test it all and make sure it's peachy.

The jfet solution is for microphones which have a very small signal to start off with. You are needing to adapt a rather large signal in such a way that it will be adjustable for different situations. You basically have your FET there already, it's just a MOSFET, and with a few extra resistors around that output pot could be made to work fine under most circumstances. The transformer idea is  "the more professional" solution. The chances of blowing something up downstream are zero if you start with the volume turned down and then work it up slowly.

All the best.

[tubenit]

How about, I go down to the audio shop tomorrow, pick up a transformer and wire it into my D'Mars and then send you some pics, schematics and photo's. I've got mixing desks and SS PA amps here to test it all and make sure it's peachy.

Wow!  That is remarkably generous and kind of you!  IF ......... you have time, convenience and some interest, that'd be great.

However, if you're reasonably confident that this approach will work and I'm over complicating it in my thinking and response, .......... then maybe just confirm what Pete has posted in the schematic with trannie?  I'd hate for you to go to all that effort if the schematic is already answering the needed questions?

If that schematic will work as drawn, then I think maybe I'd be good with it.
If you'd alter the schematic in some fashion, then I'd be interested in hearing about it.

I probably will not start on this build for about a month from now, cause I want the player to come over and try my 3 amps out first & listen to his playing and response to what tone he is looking for.

You guys are great!  I sure appreciate the help with this type of a challenge!

With respect, Tubenit

[Glennjeff]

Well,

Pete's schem looks like he's running the transformer from a normal high plate gain stage to give a lo z out and using a second cathode follower to give a hi z out. Sure it works fine but need to get his input on that.

I'm talking about running a transformer off of your last MOSFET follower, as per the diagrams I included, which also should work fine.

I did want a studio line out facility on my amp so I'll do it both ways and get back here with the outcome.

All the best.

[jjasilli]

Whoops - it was Jensen that has the schematics for their trannies.  Here's a tube driven, line level output circuit:  http://www.jensen-transformers.com/datashts/10k611m.pdf

[tubenit]

GlennJeff and Jjasilli,

THANKS guys!  Appreciate the help alot. Once I get a schematic clearly defined, I think I can figure out how to make this work OK.

With respect, Tubenit

[DummyLoad]

Whoops - it was Jensen that has the schematics for their trannies.  Here's a tube driven, line level output circuit:  http://www.jensen-transformers.com/datashts/10k611m.pdf

thanks, jj for that link. now we have an alternative to the cinemag and altec.

--pete

[DummyLoad]

...normal high plate gain stage to give a lo z out and using a second cathode follower to give a hi z out. Sure it works fine but need to get his input on that...

it's actually a plate follower. it works very well.

the hard work is already done. on our bass pre those two 12AU7 are fed by a bax-stack and what is basically the it's second channel of the ampeg SVT bass head with a minor tweak to the cut filter.

a later mod was to add a 1M level control to the plate follower; it replaced the 1M grid-leak/FB of VXb; it uses the wiper as the input. this mod allowed RicharD to adjust the level being fed to the mixing board independently of what's he's feeding the stage amp.

the schematic i posted is the output stages of our pre in the links below. 

http://sotxampco.com/Images/KB02.jpg
http://sotxampco.com/Images/Kelsey-Board.jpg

--pete

[jjasilli]

It seems to me you could put a SPDT SW on the Jensen CF output: then one leg to the line driving tranny per the schematic; the other leg  to a phone jack to connect to a tube power amp, like you usually do.  

Also, you can put a ground lift SW in the XLR connection.  This is a handy option that might eliminate hum in case of ground loops at the venue.  

[DummyLoad]

in jj's plan you need to add a coupling cap between the wiper of 250K (FX out pot) and pin 7 of V4b. 

--pete

[DummyLoad]

in the "pete" plan, delete the 250K pot and make it fixed resistor value experiment with 100K-1M. increase/decrease .033 coupling cap value accordingly.

--pete 

[Tone Junkie]

Funny thing is its cheaper to buy the whirlwind DI box that the jenson trans thats inside it.
Bill

[PRR]

> I have an impedence adaptor but I think it's the "wrong" kind

You can turn it around.

BUT it is for MIKE level. Much lower Power than Line level. It will distort bass.

I'm not sure why there are two "return" jacks on your first drawing?

I'll assume V3a carries a strong guitar-level signal, suitable to go to an effects chain. Then V3b should have a gain of about 10, to bring 200mV up to 2V to drive a big power-amp's input.

This must be LOW impedance. It may face 10K or less. 2.8V into 10K is 3mA, so the tube doing this work should be flowing 6mA. A MOSFET may be convenient. Alternately use one of the small power pentodes (or triode-pentode {6BM8?}) and a Reverb Transformer so you have a Watt output which will drive any Line input or a speaker for the bedroom.

> something RicharD and i built

That's fine also.

There should also be a 10:1 divider to output 200mV into a PA board's line input (they can usually handle such a level; if the sound dude is a fool, he can take the "power amp" tap instead).

For total flexibility, you want to drop below 100mV and then feed a 7:1 transformer. That part you have will work but the connectors are wrong, pull the guts out.

[DummyLoad]

That part you have will work but the connectors are wrong, pull the guts out.

what part???

--pete

[Glennjeff]

Well, for first experiment, made it as simple as it gets, and it worked surprisingly well.
This was using a el cheapo 600 Ohm to 10kOhm audio matching transformer, $5 from the local audio hobby shop.
(SPECS http://www.altronics.com.au/index.asp?area=item&id=M0712 )

Needs a small filter cap across the primary of the transformer to roll off some high frequencies as it will be running into a PA, effectively a very large mid quality hi fi, no roll off like with a guitar speaker.

Bear in mind that FX return pot gives you a volume control.

Changing source resistor ( much smaller, WAG 10k Ohm, matching impedance of transformer primary) and coupling cap (bigger, like .47uF, Xc= 3k3 at 50 Hz) may be a way to optimise.

[Glennjeff]

By all means leave the source resistor as 100k, I tried it at 22k with 1uF bypass and got a lot more signal but it might have been a bit "compressed" and grittier.

It depends on how much signal you actually need for the final application. I was running this into a mixing desk, the desk has plenty of adjustment for 100k source resistor. You will need to increase the coupling cap to prevent bass roll off.

Your schematic looks great for adjusting treble roll off as well just need to tweek values till you get it right. Maybe 50k - 250k pot rather than 1 Meg, season with capacitor to taste (WAG .01uF).

Pete's idea of using a tube, well, tubes are always better. Depends on real estate, cost considerations and other dull worldly constraints. I'll have a go at Pete's over the next day or so. I also have a somewhat better quality transformer to try out as well.

[Glennjeff]

tubenit,

What I'm seeing on Pete's schematic is different to your take on it (I think).

[tubenit]

What I'm seeing on Pete's schematic is different to your take on it (I think).

I don't know what that means?  

Pete's idea of using a tube, well, tubes are always better.

OK, then let's work with Pete's plan.  

My main goal is to preserve the D'Mars OD  clean, OD & FX tone.  And then send it effectively into a PA system without losing the integrity of the tone.

in the "pete" plan, delete the 250K pot and make it fixed resistor value experiment with 100K-1M. increase/decrease .033 coupling cap value accordingly.

There is a point with the D'Mars that lowering the FX level below "4" begins to lose some of the tone I like.  

So what I want and need to do is make it so I can keep that tone going into Pete's impedance topology.

So my main concern is that I want to keep the FX Level pot at around 100k and then use the 220k to drop some signal if needed?  I'm concerned a 25ka pot for the FX level is going to mess with the tone in a way that I won't like.

Q1:  Is that a reasonable plan?  

Q2:  Is the DPDT needed after the iso trannie?  Can I just eliminate that?

Q3:  Any drawing errors that I've done with Pete's impedance topology that you can see?

Q4:  No ground is needed on the primary side of the iso trannie?  Correct?

Thanks for the help guys!!!  I'll be building this for someone else so I want to nail this down as close as possible to a reasonable DRAFT.

I've got both 12AU7 and 12AV7 tubes available for this.

With respect, Tubenit

[Glennjeff]

tubenit,

Q1.
It will be somewhat different as it's no longer driving a PI, you are the Tsar of Tweak however, so go with your plan.

Q2.
The DPDT is there for convenience and flexibility, but not absolutely necessary.

Q3 & Q4.
Go back to reply 14 and re examine DL's schems, I think you may have slipped up. The OT does need a ground. (As always I may be wrong). I believe his plan is two independent circuits, one for Balanced Send to Desk/PA (long cable run), and one for Hi Gain / Hi Z unbalanced send to stage amp / SS Power Amp (short cable run). They are kind of drawn around a central ground wire so it's a bit of an optical illusion.


ALSO, the MOSFET "MAY" or "MAY NOT" be needed if going with pete's  (DL's) topology.

[tubenit]

GlennJeff,

THANKS for the response and clarification!

With  respect, Tubenit

[tubenit]

OK, I want to toss out an alternative plan.


The unit will probably not be plugged into a tube power amp and only be used with a solid state PA.

I want to try to keep the FX level pot with a high enough value (100k-250k) to preserve tone.  #1

Assuming that the signal eventually needs to be lowered ........... how about using a 100k into the iso trannie?   #3
(in other words, I want to preserve the D' Mars OD tone going into the impedance topology & lower the gain coming out of the impedance topology instead of compromising the D'Mars OD tone.)

IF I use this approach then I can eliminate the mosfet CF and use the first triode of a 12AU7 for the CF at the end of the FX.

I am thinking I can use a simple spdt to switch between high and low Z?  #2

I have now grounded the lower part of the iso trannie primary  Is this correct now?  # 4

I have also eliminated the dpdt into the lo-Z jack. #5

How about this alternative plan?   Any comments, corrections, edits, errors ................. ???

Can you guys give a thumbs up with this being a reasonable plan with my objective to preserve the D'Mars OD tone as best I can into a ss PA system?

In other words, can you name each idea #1, #2, etc........  and OK or edit it when you give your response.

THANKS for the help!!

With respect, Tubenit

EDIT:  Left schematic to explain questions but the schematic was NOT used

[Glennjeff]

Hope others with more experience can reply also but,

#0: I think you need to be clear about what exactly it is being plugged into, I'm assuming some MIXER DESK portion of a small club PA from your description.

#1 DL's plan will probably work OK with 250k input pot.

I guess what I was saying was "If it's different circuitry after the FX return then it may sound different, it may be an insignificant difference however."

#2 Couldn't say. (Will that CF alter your tone? You probably know that from experience).
     The Hi-Z branch may need to be field tested (Looks good)?

#3 Should work OK
#4 Correct
#5 Not a problem if the unit is only plugged into one plug at any one time.


You are the one with the really sharp ears, maybe breadboard a few ideas and make a decision based on results.

[tubenit]

GlennJeff,

Excellent!  Thanks for the replies on each of those items!  Very helpful & encouraging to me.

I can always put a resistor across the 250ka FX level pot to lower the value if needed.

The CF does alter the tone but not necessarily in a bad way. Just different.  I think it's a little more compressed?

Not exactly sure what type of unit it's going to be plugged into but will find out within a week or so.

After Pete (DL) looks at this, and hopefully gives similar "thumbs up" responses.  I will presume it is a reasonable risk and will post a layout for it for others to see.

Ultimately, I will just have to build it, try it in "that"  PA system and then tweak from there. I've always presumed that I will need or want to tweak something.

With respect, Tubenit

[jjasilli]

Am out of town.  But briefly for a level out control I think you can put a pot in place of the grid leak resistor  on the input feed to the Jensen  CF schematic.  (That prior part of the circuit is not shown in that schematic. 

[jjasilli]

I still think that a pot before the transformer is not only superfluous, but may screw with impedance.  The pot before the driving tube should be fine.

[tubenit]

I still think that a pot before the transformer is not only superfluous, but may screw with impedance.  The pot before the driving tube should be fine.

Think I can get away with a 100k or 250k pot prior to the driving tube?  Reason I ask this is because I lose an important piece of the D'Mars ODS tone  when I dial a 250k pot down below around "4'?

So I was wanting to preserve the D'Mars tone as best as possible and then find another place to drop gain/volume levels.

IF I can get away with at least a 100k pot prior to the driving tube, then I don't mind losing the "superfluous" pot before the trannie.

with respect, Tubenit

[jjasilli]

I mean the pot before the 12au7 tube that drives the fx loop

EDIT.  GE data sheet for 12au7 shows typical circuits for a grid leak resistor from 100K to 500K.  So you're fine.

Again, the issue with a pot before the edcor or Jensen tranny is that the resistance of the pot will be in parallel with the tubes internal plate resistance, the plate load resistor, and the primary winding of the tranny.  OTOH the pot before the tube will control vol w/o introducing new issues.

[tubenit]

jjasilli,

THANKS that's useful info.

With respect, Tubenit

[jjasilli]

Ok.  A pot or fixed resistor before the edcor tranny was bothering me in regard to impedance matching the output of the 12au7 to primary of the edcor.  That tube's plate resistance is about 8K in parallel with the plate resistor is about 7K, in parallel with the 100K shunt resistor is about 6K.  This is feeding a 10K primary.  This will work but probably cause frequency loss which I don't know how to calculate. 

I don't understand the purpose of this resistor.

I'm guessing the Jensen schematic will give hi-fi results with no frequency loss.  OTOH, using the Rule of Ten's in your schematic, a 100K plate resistor and no shunt resistor should keep you within about 20% of the edcor's primary impedance. 

[tubenit]

I don't understand the purpose of this resistor.

Pete had it in his schematic, so I put it in.  I am presuming that it's simply dropping some gain/volume?

I can always bypass it and compare.

with respect, Tubenit

[Glennjeff]

I think you need a low impedance source driving a high impedance load to prevent frequency loss / loading effects jj.

That circuit, as it is drawn,  looks like it needs to be built so it can be assessed, and should get you pretty close to where you want to go.

I would build it like this, but I'm a bit strange.

All the best.

[jjasilli]

That's impedance bridging.  But preferably the output impedance of the driving tube circuit is designed to match the input impedance of a tranny's primary coil. 

I agree that tubenit's plan should work.

[Glennjeff]

Let's agree to agree that we are not necessarily disagreeing.

But preferably the output impedance of the driving tube circuit is designed to match the input impedance of a tranny's primary coil. 

Yep.

6K source : 10k load - good enough for rock and roll.
Lose the 100K and up plate resistor if you like it a bit more spicy (7K : 10K).

All the best

[tubenit]

Guys,

I really truly appreciate the help!  I will go back thru the thread and delete previous schematics that I posted.

This will be my prototype with the understanding that things will need to be tweaked. Pete suggested getting two of the Edcor trannies so that if the signal is too hot, I can change to a more suitable trannie.

Unless anyone sees any significant errors, I'll use this.

Many thanks to all of you!

With respect, Tubenit

[jjasilli]

Per a previous post I too am grappling with these issues, and this thread is very helpful.  My understanding is that the 12au7 's output is  not supposed to have a load within the fx driving circuit, including the edcor tranny.  Rather the load should be presented by the receiving device, in this case an SS power amp.  Modern SS devices, at their input, want to see a source impedance of 200R - 2000R.    The edcor tranny satisfies the source impedance requirement of the receiving device.  But, again, the load on the fx output circuit in the preamp comes not from within, but from the outside device it is driving.

This is just like power tubes matched to the primary of the OT, driving a speaker load.  The plate impedance of the power tube ideally is matched to the impedance of the OT primary.  Though a load is needed, there is no load within that circuit.  The load comes from the outside - from the device that is driven, in this case the speaker.  The OT matches the output impedance of the power tube(s) to the input impedance of the speakers.  We are all familiar with that.

Same thing applies to an fx driving circuit.  So, the primary impedance of the edcor ideally should match the source impedance presented to it by the output circuitry of the 12au7 which I roughly estimated based on the GE data sheet and the resistor values in Tubenit's schematic.