Acoustic Guitar tube PreAmp & FX

[tubenit]

Some of you may remember my experiment to build a D'Mars Overdrive Special PreAmp that would plug into a solid state PA system.

http://el34world.com/Forum/index.php?topic=15921.0

It worked, but I was not happy with the overdrive tone with it.  It did have a very warm transparent tone to it though.  That part, I was very happy with.  Clean tone was great, overdrive was only so-so.  It eventually became an amp with a line out and used a 100w 8 ohm resistor and a Hugh & Kettner Red Box cabinet simulator.  Adding the LTPI and power tubes gave the overdrive tone that was needed.

Now, I have been approached by my great son-in-law about the idea of a tube preamp for his acoustic guitar.  He is a great guitarist and plays in a church band and his signal is sent into a solid state PA system.  We're looking at the idea of a good preamp that gives a warm tone.

There will be a mid boost option using a relay.  And a foot switchable "MUTE" function also.  I choose the tone stack values based on my own D'Mars ODS and playing my Larrivee acoustic thru that amp.  I think the Larrivee sounds fantastic thru the amp with some slight delay.

The FX level will be the master volume.  And the FX will be for a delay pedal or reverb.

I am thinking this will be a great PreAmp idea for an acoustic.  IF this has the same warm transparent tone of my last tube PreAmp, it should sound fantastic.

Any glaring errors on this idea?   The original D'Mars PreAmp had more stuff in it for the overdrive.  I won't need OD in this design.  IF it's used with an electric, then an overdrive pedal can be used.

IF my son-in-law goes with this and wants a non-tweed chassis, then I might try this in one of Doug's new blank chassis.

with respect, Tubenit

[jjasilli]

I think that's a good concept.  Some thoughts:


*  What PU is used on the acoustic guitar.  The point is impedance matching of the guitar output to the impedance input of the amp. 


*  Feedback is a big issue for amplified acoustic guitars.  (BTW:  A mid boost is probably counterproductive for acoustic guitar use.)   Usually the problem feedback frequency is 100 - 150 HZ, but this can vary.  Hence, the better guitar preamps have selectable, narrow frequency bands for attenuation.  If you're interested I can dig out a vintage Bogen schematic with such a circuit: rotary SW & pot.


*  you might also consider a phase reverse SW at the input.

[tubenit]

Yes,  I would definitely like to see what the Bogen was using.  THANKS!

The pickup is this one:   http://jjb-electronics.com/prestige-330.html

Reportedly, the suggested impedence is 1Meg.

I added a switch prior to the volume pot to help shift frequencies.  I'm not sure what values to use, so these are a guess.  Again, the shown values are what is on my D'Mars ODS and I like the tone that I'm getting with my acoustic.  As an FYI,  Larry Carlton plays his acoustic thru his Dumble amp.  I've seen him in concert do that playing live.

Regarding the mid-boost,  I envision that it would probably only be used when my son-in-law plays his thin line Tele with an F-hole thru it.  In series, the capacitance value is 330p.  It might be useful if playing single note lead solo on acoustic, but I've not tried that on my D'Mars?  I'll try that out and see how that works sometime this week.

How would one do a phase reverse switch at the input?  Schematic please?

With respect, Tubenit

[jjasilli]

A phase SW is DPDT arrangement to reverse polarity: I googled Images: "phase reverse switch schematic dpdt" and got this:  https://www.google.com/search?q=phase+reverse+switch+schematic&rlz=1C1RNBN_enUS433US433&espv=2&biw=1280&bih=601&tbm=isch&tbo=u&source=univ&sa=X&ei=Hed9VPj2B-PfsATckIKQDg&ved=0CCAQsAQ#tbm=isch&q=phase+reverse+switch+schematic+dpdt


Low Freq. Filter: Attached is the schematic for the Bogen MX-30a, in 2 pages.  The Low Freq. Filter is cut in half, top middle both pages.


The specific guitar PU will work, but piezzo's would want to see around 10M input impedance; other PU's may be low impedance.  If you want this flexibility it can be done.  Merlin's preamp book has a schematic for hi-meg tube input; for lo impedance an input transformer can be used.


Another factor:  multiband eq, but personally I have not found this to be necessary.  In my experience an acoustic guitar through a tube amp sounds great - w/o all the bells and whistles in SS preamps such as delay, chorus, reverb, mic & guitar simulation circuits, etc., to make-up for the dry SS tone.


Still, another option is a diy Tube Buffer, receiving signal from a simple, quality SS acoustic guitar preamp with vol control, tone or eq controls, low frequency notch filter, phase reversal, and maybe a tuner built-in.

[tubenit]

Jjasilli,

Thanks for the schematic and the information! I appreciate the help.  I found another Bogen schematic that's perhaps easier to read.

As I am understanding this,  there is a pseudo James tone stack going into a "frequency" control which is either a 47p bypassing a 560k .....  OR a Frequency Control ?

I am thinking that capacitors in series after V1a would do a similar thing regarding frequency control?  It might take two spdt switches OR a using a 3-way impedence selector switch to do a variety of capacitors in series?  Again, I might have to experiment with this?  I am thinking the way I am thinking about approaching this may be easier for me then borrowing the Bogen idea?  Thoughts?

IF I am understanding your reply correctly, ........... regarding the pickup impedence,  what I have in mind is OK?

The treble, mid, bass control on my D'Mars seems reasonably useful to me with my Larrivee acoustic  AND if an electric is plugged into this tube preamp, then the tone controls would help with that also.  While primarily thinking acoustic, I'd like it to be useful for an electric that uses pedals for overdrive.

Regarding the reverse polarity,  I understand doing pickups out of phase.  I am not sure how that translates to the guitar PreAmp.  I'm not saying it doesn't apply,  simply stating that it is beyond my ability to figure out how it applies?
 

With respect, Tubenit

[jjasilli]

I found another Bogen schematic that's perhaps easier to read.
For lot's of Bogen Schematics, see:  http://www.makearadio.com/schematics/index.php


there is a pseudo James tone stack going into a "frequency" control. . .
The tonestack feeds into a tone recovery stage.  The Low Filter is immediately after the  tone recovery stage.


IF I am understanding your reply correctly, ........... regarding the pickup impedence,  what I have in mind is OK?
Yes.  (But it is not best for other types of acoustic guitar pickups.)


I understand doing pickups out of phase.  I am not sure how that translates to the guitar PreAmp.
Here, the problem is Mixing -- the acoustic guitar signal is going to get mixed in the PA amp with signal from other devices, such as mic's, other preamps, etc.  These signals may be out-of-phase with one another.  This can cause a weak, nasal tone -- just like el. guitar PU's out-of-phase -- but here, it's undesirable. So, if the acoustic guitar sounds weak & nasally out of the PA, phase reversal is the solution.  Another solution for PA users is to is to always carry an inline phase reverser -- they're made for guitar chords or mic cables.


Today's guitarist usually has a standalone guitar amp, so is personally absolved from the issues arising from mixing his signal with other's.  But if your going into a mix, puff -- like it or not, you're instantly a sound engineer!  All the quality commercial acoustic  guitar preamps have a phase reversal SW. 


BTW:  on the Bogen Schematics page (above) is the Bogen MXM-A Mixer - Preamp.  A great unit.  Here's a project upgrading that device, full of useful ideas:  http://www.clarkhuckaby.com/BogenMod/MxmMain.html


[BTW2:  I forget to mention compression.  SS preamps tend to need it to dress-up lifeless tone.  But compression can also be useful to an acoustic guitarist who wold rather not fiddle with a vol control -  so strumming, finger-picking & flat-picking all comes out at the same vol. ]

[tubenit]

Jjasilli,

Thanks for your continued information and help!

I am now thinking that a DPDT that switches the polarity on the XLR jack would resolve any concerns about phase cancellation?

I'll look a little more at the Low Filter and see what route I want to go. I should be able to draw that up fairly easily on a tagstrip.  Still leaning towards a .01 cap into a multiple impedence switch using caps in series though.

Thanks, Tubenit

[jjasilli]

I am now thinking that a DPDT that switches the polarity on the XLR jack would resolve any concerns about phase cancellation?
Agreed.  (A phase reversal SW could be put anywhere.)


A better view of the Bogen Low Freq Filter is in the Mixer Schematic:  http://www.makearadio.com/schematics/images/bogen-mxm-a-8.jpg

[jjasilli]

    Also, I think if the DPDT phase SW is the Center Off kind,  it would double as a mute SW.  But maybe this would confuse the end user.


BTW: If your Mute SW pops,  a SPST SW under the cathode resistor of the last tube stage might be quieter.

[Jack_Hester]

V1b and V2a have power supply connection 'C' and 'D' tied together. ??

Jack

[tubenit]

V1b and V2a have power supply connection 'C' and 'D' tied together. ??

Not tied together.  Separate.  One of the problems of using "other schematics" to redraw something is errors like that.

With respect, Tubenit

[thermion]

The pa will allow all kinds of eq options, right? I needed something like this and I really like the paralleled 6sl7 pre that I made. Single tube, no eq, premium parts...I perceive it as being very linear. Used with a fishman neo magnetic pickup in my case; seems you could load the front end for whatever imp your pickup wants to see.

[2deaf]

The wiper on the reverb return pot got connected to the top.

The 4.7M on the tone stack got connected to ground instead of the top of the relay.

[tubenit]

The wiper on the reverb return pot got connected to the top.The 4.7M on the tone stack got connected to ground instead of the top of the relay.

Those are done correctly and as I intended.  I do appreciate your desire to help though.

with respect, Tubenit

[2deaf]

You will present the output of an effects device with a dead short to ground when you turn the effects return pot. all the way down. 

 

[DummyLoad]

this is a preamp that RicharD and i designed a few years ago. very much a sweet sounding pre with decent headroom. for more headroom use a 12AU7 and re-bias V1. it was designed for bass guitar, but with some minor TS tweaks, it may make a fine acoustic guitar preamp. it's the "B" channel of an ampeg SVT.


the PT is sourced from tridoelectronics: it's the dynaco PAS2 preamp PT. 


filaments are DC and the filament PS is used for the relays as well. the 12DC also powers the 555C timer IC. the 555C delay triggers after about 30 secs for warm-up: it mutes the output so that there's no preamp noise when powering up it also helps suppress power-off thumps.


i had to reverse engineer the remaining PCB and that is what has taken me so long to jump in: the original schematic was drawn in an application that is long defunct and we both lost the images including the artwork when RicharD experienced HDD crash and i got hit by a java exploit. i managed to save a jpeg of the artwork, but that's about it.


you can sub the edcor 10k:600 OT, however, the altec-lansing peerless and the cinemag iron sound much better than the edcor. if you want to audition the altec iron (15095) i can send you one. insert the FX loop between the HighZ output CF and amp.


just curious, but why do you need an effects loop with acoustic guitar?


--pete




EDIT: PT is part no. PA-211 ignore part no. in schema...

[jjasilli]

I would breadboard it before building it.  To my mind there is the potential for undue insertion loss in the Low Filter in your circuit for the following reasons.  In the MX-30a schematic the Low Filter is driven by the tone recovery stage, which cures any insertion loss issues.  It also buffers the input impedance of the Low Filter from the output impedance of the tonestack.  In the MXM-a schematic the Low Filter is undriven, but its treble pot is 1M, (and there appears to be additional signal additional signal from the balance circuit). The treble pot is in parallel with the 1M Low Freq. pot.  Your treble pot, using a different tonestack, is only 250K -- instead of 1M.  You may have more signal loss to ground, depending on the output of your tonestack.  In addition, you're using a lower mu preamp tube to begin with.  All that being said your schematic might work fine.

I don't know what you mean by "caps in series".  Remember, the Low Freq. control is not a tone-shaping device.  Rather, it is a problem solver for the exact problems that are known to exist with acoustic guitar amplification in live venues.  It's sole purpose is to eliminate boominess & feedback.  Presumably the Bogen circuit accomplishes that.  To do that it uses Notch filters.  A typical notch filter is a cap in parallel with a resistor, which pair are in series with signal -- such as at the top of the Bogen filter.  The component values are chosen to attenuate a certain specific frequency, or a narrow frequency band.   


BTW:  my solution to this problem is a Dano eq pedal.  Everything is SS: ancient Bill Lawrence mag PU > ancient Dean Markely SS preamp > > SS pedal baord >  SS Behringer mixing board > Bogen tube PA amplifier.  But if I were to build an acoustic guitar preamp, I would build-in a low filter, first trying the Bogen schematic.

[jjasilli]

At the bottom of the Bogen Low Filter is a Twin-T notch filter.  See: http://sim.okawa-denshi.jp/en/TwinTCRkeisan.htm


If the Bogen filter failed to accomplish our intended purpose, I would determine the specific frequency at which feedback occurs by sounding specific bass notes on the guitar through the PA system, then build a  Twin-T notch filter to reject (attenuate) it.  For flexibility, more than one such filter can be built, each targeting a narrow frequency band for rejection, to be selected by a SW or even a multi-position rotary SW if desired.  But again, I would first assume the Bogen Low Filter will be good enough.

[tubenit]

Dummyload and Jjasilli & others,

Guys, thanks for the continued information and help!  I will study Pete's schematic and make some mental notes there.  I appreciate your sharing it (especially given that you had to redraw it. Very nice of you) ! 

Pete, I have two Edcor trannies left over from the D'Mars PreAmp project, so I think I'll try those out first.  Thank you for the offer of the trying the Altec.

Jjasilli, that "notch" program is pretty cool.

Perhaps the low freq control would be useful and beneficial in this?  I'll try to read some more on it.

Regarding the FX .....   I have become a real fan of very slight delay in my amps whether playing acoustic or electric.

I have a delay on 90% or the time and 10% use a reverb that has a delay effect in it.   I have yet to find a delay pedal that I like prior to the input jack on an amp.  They just don't sound the same to me and all I've tried are too noisy for me including  Carbon Copy, Wampler Faux Echo and Boss digital delay.  Unless I am playing acoustic unplugged, I always have delay or reverb on.

So given the delay being very important to me,  the FX is also important.  Secondly, the way I am using it makes for an excellent master volume, IMO.  Because of that, I am trying to design a PreAmp around this approach using the FX loop.

With respect, Tubenit

[tubenit]

Pete,  I hope you will be OK with this ......... I redrew the schematic you posted to more easily compare it with the idea I have using a CF then tone stack and recovery stage of an FX. 

So, comparing the two ..................

I think I am headed the "right" direction in what I am wanting to do given 3 specific goals:  1) will sound good with an acoustic guitar,  2) enough switches, etc to be useful to an electric guitar with an overdrive pedal and 3) has an FX that is useful to delay further down into the signal chain and also useful as a master volume.

I think some of the cap & resistor values will need to be tweaked after building it.

With respect, Tubenit

[jjasilli]

Riffing on this thread I've drawn a Variable Twin-T Notch Filter that I think would work in an acoustic guitar preamp to kill feedback at a dial-in frequency in the range from 80 - 150 Hz.


EDIT:  Note that much smaller caps can be used if equally larger resistors are used, which might reduce insertion loss.  E.g.: decrease cap value by a factor of 10, and increase R values by a factor of ten.  The online calculator gives the same frequency rejection results.

[tubenit]

Jjasilli,

OK, I think you've talked me into trying a "low cut frequency filter".  Looking at the PreAmp pedal  Acoustimx and Tonebone Pz-Pre, I found the following ranges:

http://www.bbesound.com/products/instrument-preamps/acoustimax.aspx       70Hz-250Hz
http://www.tonebone.com/pzpre-specs.php                                                    75Hz-220Hz


With respect, Tubenit

[jjasilli]

The process:  The site I posted above has 2 calculators on the twin t-notch page:  1. state the R-C component values, and the calculator gives the rejection frequency; or 2.  state the frequency to be rejected, and the calculator gives the R-C component values.


Research informed me that the most likely offensive feedback frequency is 100 Hz (approx. the open A string).  Or else, about 150 Hz or somewhere in between. 


First I plugged 100 Hz into the online calculator, and got the component values posted on my schematic.  Then I entered other rejection frequencies.  It soon became apparent that the most critical component values were the series resistors R1 & R2, which always equal one another.  Changing them while leaving other values the same yields functioning circuits with different useful rejection frequencies.    Changing R1 & R2, from about 15K each to about 30K each, picked-out various specific rejection frequencies in about the range of the commercial preamps you mention.  I realized that a dual gang pot, wired as 2 variable resistors, could replace R1 & R2.  This makes the Filter variable so that the rejection frequency is selectable, just like the commercial preamps.   And, if a fixed resistor is used with ea. pot, then we can maintain the useful range even if the pot is turned to -0-. 


I then noticed that my schematic uses large caps, 0.1uF, while the Bogen schematic uses small caps, .0022uF.  I then remembered that R-C values are scalable.  Multiply the R value by any number, and divide the cap value by the same number and you get the same frequency response.  This also results in a larger value for the shunt resistor, R3, which should yield less insertion loss.


Bottom line:  use Doug's 250K dual-ganged pot instead of my 20K pot.  Then scale the other component values.  250k/20k = a factor of 12.5; For the fixed resistors R1 & R2 (to be used in conjunction with the 250K dual-ganged pot):  15K X 12.5 = 187K each as their new value (or something commercially close or on-hand).  For R3:  5.6K X 12.5 = 70K.  For the 3 caps: .01 / 12.5 = 0.008uF.  To double check, plug your values into the online calculator.

EDIT:  For the 3 caps: 0.1 / 12.5 = 0.008uF.

OR, use your 1M pot, and do the same math exercise to get the other R & C values.

[tubenit]

Jjasilli,

Incredibly helpful and useful information!  THANKS!  I'll apply that and redraw and repost a schematic with a layout later today or over the wkend. 

I didn't understand how the calculator worked. I appreciate the clarity.

Thank you! With respect, Tubenit

[Jack_Hester]

What if you just paralleled a 25K resistor across each 100K pot?  Would that not give you a linear 20K potentiometer for each?  Then, you could keep your existing component values.

Jack

[tubenit]

Jack,  yes I could do that.  However, if I can use smaller caps and a 250kl dual gang pot, I would prefer that.


Going with Jjasilli's latest revision .......................    I'm revising the low freq filter values.

I'll draw the entire schematic and layout later.

I have tried repeatedly to use the calculator, but I don't know how to read the results that are posted.

Would someone PLEASE double check to see if these actual values will work for the 100Hz "problem area".   Thanks!

With respect, Tubenit

[Ritchie200]

Just doing a quick overview so if I missed this....


If this is to be a dual purpose preamp, why not voice two channels with two inputs?  That way he can leave everything plugged in at a set volume and tone/drive settings with both guitars on the stand and ready to go.  I realize I may have just doubled the work but "jack of all trades, master of none" comes to mind.  Not necessarily saying in tone (because we can obviously effectively switch things in and out), but in playability for the musician.  Plus there is always a backup channel in case something goes wrong.  I'm just thinking out loud about the things I incorporated into my live setups over the years.


Jim

[jjasilli]

Both Dummyload & Ritchie make good, though divergent, points.  Bottom line, Tubenit's plan will probably work.


Issues:  For el guitar, the amp is part of the tone production process.  For acoustic guitar the amp or preamp needs to; a) RE-produce acoustic tone, usually not mess with the core tone of the instrument; and b) eliminate the inherent sound engineering problems.  These design goals are divergent, even incompatible.  This leads to three logical alternatives: 

i) per Dummyload, KISS - do only an acoustic preamp.  Downside: not great for el guitar.

ii) per Tubenit, build a workable hybrid. Downsides: more complex; probably not best for either type of guitar. But very good for one type and good enough for the other. Since the user will play mostly acoustic this seems a good choice.

iii) per Ritchie: go even more complex and build a 2-channel preamp.  Best for both types of guitar, but more complex, larger size, heavier, etc.

[2deaf]

Given that you designed your tone stack to go into a 250K load, you might want to use 220K instead of 68K for your notch filter and adjust the other values to that.

Another thing to consider is that the notch filter turns into a low-pass filter when the relay is in the up position.

[tubenit]

OK, this is the layout and schematic that I am going to go with & will meet the 3 objectives I listed in previous post reply #19.

I plan to build this and report back that it works.  Then tweak it.  Then record both an acoustic and then electric thru it.

THANKS to everyone that helped!  I am genuinely appreciative.  Special thanks to Jjasilli for his patience & perseverance in walking me thru the frequency control idea. 

It'll probably be built before the end of the month, hopefully.  I am anticipating this being a very cool and successful project.

BTW, this project will be built into one of Doug's new blank chassis!   

With respect, Tubenit

EDIT:  changed the switching on the low frequency control to a DPDT to remove it completely from the circuit when playing electric

[jjasilli]

Chassis seems to be missing phase reverse & ground lift SW's.

[tubenit]

Chassis seems to be missing phase reverse & ground lift SW's.

Yeah, I didn't totally revise the original drawing, but they will be included.

Thanks.  With respect, Tubenit

[Jack_Hester]

Tubenit -

I anxiously await your finished build, as I would like to do the same for our Church.  Our music leader builds and uses his own solid state pedals for his acoustic guitar.  He connects it to the Church PA.  I recently gave him one each of Dougs B9 and Pitchfork pedals, for his birthday/Christmas, as both dates are close together. 

The Sunday before Thanksgiving, I carried one of my Guild ThunderStar heads to Church, along with these pedals.  I had asked him previously to bring his Telecaster, but I brought along my '66 Harmony H-19, just in case he didn't bring his electric.  After service, and while the womenfolk were preparing our Thanksgiving meal, I rolled the speaker cab (later Ampeg) and Guild head into the sanctuary and he got out his Tele.  I had already given him the pedals and some patch cables, so after playing a bit through just the amp alone, he got out the pedals.  The Guild has a receptacle on the back, so with Pitchfork connected on the FX loop, he gave it a try.  He only briefly tried the B9, though later at home, he and his son gave them both a thorough tryout.

The Pitchfork was his favorite, and has since given me a report on it with his acoustic.  He uses it primarily with a piezo pickup that he installed, but also has a clip-on magnetic pickup that he will use from time to time.  He likes either with the Pitchfork.  I believe that he has something planned for this upcoming Sunday, with the acoustic and Pitchfork. 

Which brings me back to your design.  I'm anxious to have a tube pre-amp for him to use with his acoustic, in Church.  And, I would rather it be perfected by someone like yourself, before I build.  Our Church is very small, in numbers.  But, the sanctuary is rather high, and the sound carries well with a bit of echo.  A couple just recently started attending, and the lady is an absolutely amazing pianist.  We have an electronic keyboard that rarely gets used, as our primary pianist uses the baby grand piano.  So now, we have a piano and organ, along with Don's acoustic. 

Anyway, I'm following this thread closely.  Thanks for the fine work.

Jack

[tubenit]

I'm starting on it this wkend and will have it finished by end of Christmas week.  Thanks for your interest in this!

I am anticipating success with it and anticipating it needing some tweaking after the initial build is finished.

With respect, Tubenit

[tubenit]

Got the chassis punch out!  Really took some time to think thru the layout as there is a lot going into that small chassis. I think this will be a very quiet preamp with an easy to follow layout.

I really like Doug's "Stout" chassis.  Very nice quality to it & it was very easy to drill and punch out for preamp tubes and the XLR jack.

On all my chassis,  I sand the aluminum with an orbital sander using 120-220 grit paper & then use spray cans of automotive lacquer base coat and then clear coat with about 5-7 coats.  I've been doing this for yrs and yrs.  They hold up really well!  And you can have any color you want using this method. Never had any paint peel off at all or chip off. I just spray the front and back panel.

with respect, Tubenit

[jjasilli]

Are you going to use the Low Filter?


How do you spray? Spat can or Spray Gun?


What do use for lettering to label the controls?

[tubenit]

Yes, I am going to use the Low Filter.  I am calling it a frequency control. So yes that is included. (see schematic above in previous post)

Just inexpensive spray cans from a local auto parts store. They work just fine, IMO.

For lettering, I use decals or labeling on the lower wood edge below the chassis front panel. ON the back, I put labels on the chassis back panel.

With respect, Tubenit

[jjasilli]

Once again the problem is Mixing.  This time in the context of connecting your device (preamp) to the house PA, which may be even further connected to yet other devices like mic's, other preamps, etc.  Hum may arise from pesky ground loops from numerous signal-ground/chassis-ground connections, which are hard to trace.  The best solution is for each device to have its own ground lift switch which can eliminate offending ground loops by trial & error with the simple flicking of a SW. 


Once again, guitarists with their standalone amps & speakers are absolved from these issues.

[tubenit]

Jjasilli,

I did install the ground lift switch as well as the phase reversal switch.  Just wasn't sure the purpose of the ground switch.

You've been remarkably gracious of your time and info helping me with this build!   THANK you!  I am greatly appreciative.

I am probably within 3-4 hrs of being ready to fire this up and try it out

With respect, Tubenit

[jjasilli]

My pleasure!  Hope it works out well.  For a long time I've had these preamp issues simmering on the backburner.  But I haven't had an actual need.  This thread provided me the opportunity to sort some stuff out in more detail.

[tubenit]

Now about an 1.5 hr from firing it up!

I may win an award for crammage into Doug's 12" chassis with this one.  Just hoping it doesn't cause hum?  However, it is amazing what I have gotten away with to end up with very quiet amps!
 

I will admit the layout is not the neatest, but with all the different colored wiring I am using, it's actually quite easy to trace out.  And I think the layout does have some reasonable logic behind it.

I am NOT sure what fuse to use with this thing.  It's only got a preamp and only has two 12AY7's.  So, I am thinking maybe 500ma fuse?
Does that sound right?  Think I need something larger?

with respect, Tubenit

[jjasilli]

Heaters @ 6V draw 300ma ea x 2 = 600mA.  Plate Current  @ 3mA ea section x 4 = 12mA.  Total =  612mA.  Maybe closest is .75A = 750mA.


EDIT:  Fuse @ 2 X 612mA = 1.212A  So use 1A (probably OK at more than 1.5X); or 1.5A .  Slo Blo

[DummyLoad]

Heaters @ 6V draw 300ma ea x 2 = 600mA.  Plate Current  @ 3mA ea section x 4 = 12mA.  Total =  612mA.  Maybe closest is .75A = 750mA.

1.5x to 2x the VA rating of the PT.

[jjasilli]

Agreed.  I corrected my post above.

[sluckey]

Tubenit, I'm using a half amp fuse in my warbler which has 4 small tubes. I think half amp will be fine for your project also.

JJasilli, You don't simply add up the current for all the secondary windings and double that for your fuse size.

For example, I have an 18 watter amp that requires 6.3V @ 4A, 5V @2A, and 350V @ 100mA. Following your suggestion, I would have a total of 6.1A. Now double that and use a 12A fuse. That's a big fuse for this small amp!

The correct method is to sum all the secondary VA requirements. Primary VA = secondary VA in a perfect, zero loss transformer. Then divide the VA sum by 120V to get the primary current needed to fullfill the needs of all the secondaries. Now double this primary current for the fuse size.

So, 6.3V @ 4A is 25.2VA. 5V @ 2A is 10VA. And 350V @ 100mA is 35VA.

Total VA is 70.2VA

70.2/120 = .585A for primary current

2 x .585 = 1.17A fuse requirement. I'd use a more common 1A or 1.5A.

[jjasilli]

Great info thanks! (My approach would apply only to fusing secondary windings.)

[tubenit]

Tubenit, I'm using a half amp fuse in my warbler which has 4 small tubes. I think half amp will be fine for your project also.

Steve,

Man, thanks for the info!  I am always grateful for your excellent help!

Best regards,  Tubenit

[tubenit]

OK, the PreAmp works.  I can get sound out of it and overall the tone is decent EXCEPT I have a terrible hum with the PreAmp.

It seems to be between the FX Level pot and the XLR jack.

EDITED this post as further in the thread shows the solution.

[sluckey]

Disconnect the green wire from the IEC power connector. Does that help?

[tubenit]

Disconnect the green wire from the IEC power connector. Does that help?

I will try that tomorrow or Friday as soon as I get a chance.  That green wire is the ground wire to the IEC of course.

Do you think the Edcor trannie and signal wires (to the XLR socket) running close to the HV are problematic?  Or the Edcor trannie itself being close to the HV is the issue?

Thanks for the suggestion and I will definitely give it a try and report back (hopefully tomorrow)?

The only switch that stops hum is the center off position of the phase reversal.  The only pot that stops hum is the FX level.  No pots increase or decrease hum even the FX level doesn't increase or decrease  (until the FX Level is completely to zero).

With respect, Jeff