Questions on modifying an 5E8A circuit

[baileyblues]

Hello, I'm new at this. However, been studying for over a year.  Looking at schematics, reading everything I can find on the internet.

What I am interested in, is incorporating a pregain, or midboost, as V1.  Also, incorporating a 3 knob Reverb into this unit.

I know, why right?  Well, because I'm a fan of the Clapton Mid boost, and would rather have it or something similar on an amp.  Also, I like having Spring Reverb available when I desire it.

What draws me to the 5E8A, is the sound and feel.  For 20 years or so, I played through a '72 Twin Reverb.  Loved it, but she has moved on.  I replaced her with a Fender Mustang III, and really enjoy the '59 Bassman with a 2x12 simulated Cab.

Live playing, I have played through some Victoria version of the '57 Tweed Twin.  Love it, and it breaks up nicely.  With my Clapton Strat, there about everything I like.

Now I could go further into detail, but I'll stop there before I get off subject to far.

Attached is a schematic.  What I did was place, what I hope and believe, to a be a decent mid (centered around 800 if I figured correctly...math isn't my strong point).  In theory of design, when the pot is set at 0, the signal should pass around the pot to V2. When the pot is raised from 0 to anywhere including 10, it should provide a 'boost' decent enough to overdrive or saturate V2.  V2 on this diagram, is simply copied from schematics, as well is V3, V6, the Power Supply, etc.

I chose to place the Reverb in V1, but prior to anything.  To hopefully tap a clean signal.  Then it comes back up, after V3 and on to the PI.

If I'm thinking correctly, this could have an interesting effect for the multiple styles I play.  Which range from Jazz, to Blues, to 50's through 70's Rock.  Even some Country in the mix.

I tried to put some foot switches in.

Please note, I normally keep the mid boost on the Clapton Strat all the way up, and just use the Volume as my control.  I can go from clean, to full on distorted with my pinky, depending on my mood.  That's part of my theory for the amp.  I really don't want to put a kit in every Strat I own, or purchase down the road.

Anwho, just my thoughts.  Please have a looksee at the schematic.  I'm excited to have come this far, and hope that I've done some things correctly.  If not, then perhaps I learn from the mistakes on paper.  It'll be months before I ever attempt to build this.

Actually, I'd like to build something similar as a preamp for my home studio, before trying a full amp build.

Thanks for any input.

Chuck Bailey

[DummyLoad]

1) there is no way C21,C22,C23,C24 are going to have +300V there. you're going to need a resistor between C23-C24 and/or between C24 & C25 to get B+ to what is indicated. at a glance; 10 triodes at around 1mA each to drop 105V, you'll need around 10.5KΩ with at least a 2W power rating.
2) C22 & C23; how is it that there is 300V on either side of the 10K? - there will be if there is no load on C22. a lot less if midboost is switched on... see next.
3) DANGER will robinson! switching on midboost pedal is going make sparks! well, parts may leak magic smoke anyway; things won't be happy and will get quiet when switched on. put the switch in series with C2 and the midboost pot wiper. tie a 10M resistor to switch terminals.
4) how does a B+ of 235V to V1 & V2 result in 250V at the anode of V1A and 240V at anode of V1B? magic anode amplifying resistors? j/k!
5) are you direct coupling (DC) V1A to V1B (bad idea with midboost pedal as is) & V1B to V2A & V2B? it doesn't seem like with V1B DC to V2A & V2B will be happy.
6) with as much gain as V1-V3 are going to make, you should really consider splitting V1, V2, & V3 power rail. at least V3 on it's own rail.
7) V6B would better drive output stage with a bit more B+.
8) where is B+ of 440V+ for reverb driver coming from?

just what i see at glance... happy solder smokin'

DL

[DummyLoad]

missed another... reverb return summing point is odd. after making all that gain, clean reverb signal may be weak and inconsistent with dry signal. maybe summing wet/dry signals back in front of V1B or V2 may yield better results. just thinking out loud.

respectfully,

--DL

[tubeswell]

Agree with DummyLoad - the wet signal insertion point will mean you won't get any wet signal to speak of, because of the gain imbalance - suggest keeping the wet signal insertion point close to the same point as the reverb driver signal insertion point - like a BF style reverb circuit. Also the CF stage paired to the reverb recovery inverting stage is probably a waste of a triode. This triode could be put to better use boosting the wet signal before reinsertion into the main signal path.

Also the reverb driver stage is likely to need more signal swing at the grid - so suggest a tapping off the main preamp after the 2nd stage.
Apart from that, the dwell pot and the mid control after the 1st stage are in parallel (in terms of a/c loading the 1st stage), and this will reduce the strength of the reverb driver signal - consider ditching the mid control seeing as how you have a tone stack elsewhere.

As to the rest of the amp, the way you have drawn the schematic, the 3rd (parallel) stage does not have a coupling cap in front, so the bias on the grid will be wrong and it won't work. Also it has no grid leak resistor, so you'll have problems.  Also the plate resistor and supply resistor arrangement seems odd.  Why have 2 plate resistors and then mix the signals with mixing resistors? You could just have 1 shared plate resistor instead, and the benefit of the extra gain, if thats what you want. Otherwise consider dropping the parallel stage altogether - its just going to add a whole lot of crunch that will muddy the reverb.

Also consider dropping the 2nd stage altogether and just having the 1st stage going to a vol pot then going to the DC-coupled pair driving the tone stack. You can re-use the triode from that discarded 2nd stage to add extra drive signal in front of the reverb driver.

JM2CW

[baileyblues]

Wowzer.  Thanks for the replies!

I see some mistakes that were pointed out such as C22.  C22, I simply had lost.  It doesn't go there.  C22 should be where C23 is, so and so forth.  That part is copied from a Fender Schematic.  Also, in the PI, labeled as V6 there is a .022uF 630V cap missing that goes between pin 1 and pin 7.  So I'll have to get those on the schematic. 

As far as how I came up with voltages, most were borrowed from their 'inspirations'.  It's clear I need to restudy how those are derived, and re-figure them.

Originally I had the mid in there as the typical 'twee'd circuit.  I had borrowed the first gain stage, moved it down and ran that way.  Then I though, maybe I could get more boost by placing it in more of a BF (for lack of better description) style.  That's where V1 came from.  The pedal additions were a last minute add, and I'll be honest...not sure how those work.  I never use them anywho, might delete them.  I typically set and forget.  When I go to distortion, the reverb disappears to my ears, then when I back off, it comes back.

So, I'll work on tapping from one of the points suggested.

I'm glad I posted this now, cause I figured I had some issues that I needed help with.  It's clear I did.

And I haven't even gotten to my question on the Power Transformer yet!  LOL

OK..so I got some homework to work on.  Will jump on that this evening after the kids are in bed.

Thanks for the help folks!

Chuck Bailey

[HotBluePlates]

V1b needs a coupling cap between it and V2 (both sections).

V2 won't work well as drawn because the cathode resistor is picked as if it were cap-coupled and cathode biased; but the grids are d.c. coupled to V1b and without any other ground reference.

We may need to start this from scratch again, as there are so many minor errors that they're hard to spot.

FWIW, I found from some other posts online that the Fender mid-boost circuit on the Strat is centered at 500Hz. 800Hz is very high-mid in a guitar (highest fundamentals from a guitar are around 1kHz). 250Hz is somewhat the mud-range. Picking the frequency to boost is best selected by ear/trial-n-error, because it will almost never land on the convenient number that seemed like a good idea on paper.

How did you pick your mid-boost circuit? Have you listened to it in an amp and liked it? It is probably worth figuring out if it even works well with your guitar first before building an amp around it.

Just suggestions to help you avoid the head-against-the-wall moments...

[baileyblues]

The mid frequency is a trial and error.  When I record, direct through say my ART Pro Channel, I have the ability to boost and cut frequencies in the mid's.  I typically will boost in that area, and upwards.  Sometimes I'll us the hi and low, and boost at around 500 as well.  That's more of a tailoring that I work with pre tape.

As far as guessing the mid boost, I tried to read at multiple sites on the subject.  Most used inductors to get the Q narrow.  Then I found a site that point to the Ampeg SVT Classic, which has a selectable mid frequency.  It works off an inductor, and comes in after the tone stack, pre PI it appears to me.

My thinking was 'essentially', cause I don't like pedals..and don't want to put a Clapton kit in each guitar I have.  Was to implement essentially, a pedal gain (I guess, long day today).  Much like that of the Clapton Boost, which is active on board the guitar.  I'm not looking necessarily for the 25dB of boost, but something nice that is close.  As far as playing with values, I'm sure that'll just have to happen.

I did notice the missing Coupling Caps.  As well, with the feedback I found some other sites that explained some 'things' I was missing and not understanding fully.

What originally got me interested in this project, was I thought I'd have my '72 Twin around long enough to make the needed repairs to her. She'd never had a Cap job, and some other funny things were happening.  So I essentially started learning the trouble shooting side, and found out real quick it was a good thing to look, listen, learn, so as to live!  I was amazed!!!  Still am.

So, my thinking is to be able to plug in, have a pregain that is essentially a mid boost (maybe not that significant as the EC).  When it's pot is set to Zero, the signal would continue unaffected to the next stage.  When the pregain/boost pot is rotated it would bring in the effect.  Since I usually keep the mid boost on board my Clapton at 10, and use my volume to control clean to 'get some'; it seemed logical this could be done in an amp.  At least, I hoped.

The V1 in this diagram, is just that.  V2 , V3 and V6, V7, V8 all came from existing 5E8 diagrams.  It looked pretty unique, and I do love that Tweed Sound. 

I am fixing, with pencil at present, the error's that have been noticed.  Including a few I thumped my ahead about cause it was so plain as day.

I'll rework and try to clean up the schematic, maybe leave voltages off this next time.  I'm still looking at that.  none the less, get the initial thought better..figure out the nitty gritty after that I guess.

I dunno...I did move the reverb send to V2B, not sure on that..but it sounds like that might be a good spot for it. 

Well, I could go on and on...I really appreciate folks being nice on this.  Like I said, and will elaborate:  I'd like to build this, realistically it is a LONG ways off.  I want it to be as right as it can be, and understand why it is; before I try to build it.  Plus, I'll have to get budgeting correct.

Still have the Power Transformer to work out.  That's for later down the road.

Thanks again,

Chuck Bailey

[HotBluePlates]

Well, you do bring up some fair points.

Using EQ/compression pedals would be very much easier than trying to design these features into the amp.

I have some slight worry that once you incorporate everything else you want, the tweed Twin character will be lost, or the stock tone circuit won't give you the right response. Especially if you're needing to create a lot of preamp gain, and then tailor that newer-than-tweed tone.

Maybe we should start first with drawing the 5E8 circuit, with the added reverb as shown in Hoffman's example. The catch is we're provided the layout he used, and you'll have to go piece by piece to draw your own schematic.

It is interesting in that V1a has its grid connected to V1b, which is the first stage for the reverb circuit. As drawn, it's a 2-knob reverb, but it's simple enough to incorporate a Tone control as shown in the 6G15 Reverb.

Bonus is this is a known, working implementation of adding reverb to a tweed Bassman, so the concepts (and much of the layout approach) should carry over to your project.

As for midrange Q, unless you've found through other experimentation with other EQ devices that a narrow Q works best for you, the broader Q of a cap-only circuit works better for most folks, with less tendency to boost very few notes. Also, you have to be very careful adding coils to low-level signal circuits, as they often easy pick up hum.

[baileyblues]

I hear ya.  You're right.

I got to re-reading your post, and the Clapton probably is around 500.  I tend to EQ when I direct record through the ART Pro Channel (which I've found gives a very nice sound for guitars, in the absence of an amp) I tend to brighten up the character so to speak.

Now on to the redrawing.  I do need to do that.  I'll get to working on that.  Perhaps get back to my original idea.

I actually enjoy recording without a speaker.  Hence, the lineout feature on the amp.  But...my original idea was to just take a 5E8 and plop it into a rack mount PreAmp for that purpose.  I realize, no speaker, it'll sound different.  Then I moved forward with these other idea's. 

I could settle for the basic 5E8 and reverb, even if it's a one knob job. 

So, that's a good plan.  Work on adding the Reverb to existing circuit.  Then decide if it'll be an amp, or a preamp. 

Which, correct me here, but from what I've been reading, the tube rectifier would essentially be out if I did a straight preamp unit.  Get's confusing doesn't it?  Preamp for an amp, or Preamp for Recording...LOL  I think folks get the gist.

I'm a tinkerer...I love innovative idea's and products.  Too bad there wasn't one already made.  =*)

Anywho, got the Hoffman Reverb bookmared for further viewing. 

Thanks again.

Chuck Bailey

[HotBluePlates]

Which, correct me here, but from what I've been reading, the tube rectifier would essentially be out if I did a straight preamp unit.  Get's confusing doesn't it?

Well, it's whatever you want to do.

I tend to not think of the rectifier as a source of compression, though I guess it does contribute some. But the 5Y3 is used in otherwise low-power amps that you crank the snot out of anyway, so is it compressed because it's cranked or because it's cranked with a small rectifier?

But once you put two or more 5Y3's in parallel, it starts getting silly. If you want it to sag, why have the big output stage instead of a small one?

That said, you can tube rectify the power supply for a preamp. And if you want the rectifier to add sag, we'd have to get creative and use a very small rectifier that's overwhelmed by a few mA's.

{PRR Note: I dunno if there's something with less current capacity, but more reverse-voltage rating than a 6AL5, which could fake rectifier sag in a preamp. Guess it's the old series-resistor trick.}

Oh, and if you're gonna have a line out, shouldn't it be a low impedance source? Best place to provide that is the OT secondary, if you're gonna have an OT. Otherwise, add a small line out transformer and a proper circuit to drive it.

[baileyblues]

Yep, you guess it.  Looking for the sag, or compression.  I had thought about implementing a simple (hah hah) tube compressor at the end stages.  Something, from experiment, that has worked out nicely, has been to compress the signal as follows:  Fender Mustang III, speaker emulated line out to ART Pro Channel.  The compressor is after the first gain stage, and before the parametric EQ.  So, what it does, in effect, is give a bit more ring of the reverb (if using) and more sustain either way.

When doing this from the virtual console, I've found I can do neat stuff with reverbs by compressing the wet signal.  So, it does have it's advantages.  To some of us, at any rate.

Now, onwards with the homework.  What I've done is removed the midboost/pre gain.  I took V1A and run it as shown on Fender Schematics.  V1A to V2A (which is a cathode follower set up if I understand correctly).  This comes straight from the schematics found from Fender reissues as well as schematix. 

I have the reverb coming from V1B.  It goes back in, in front of V2A.

After searching this board, and looking at some other approaches, this appears similar.  It's using the 'typical' AT7 that is found in the BF/SF circuits.  I left out some items from U6-U7 (cause I haven't named 'em yet).  However, this is a very rough draft, to see if I'm in the ball park.

I was also wonder if it'd be possible, to put the reverb hook in between V2A and V2B?

Perhaps the mid boost/pre gain feature could be of use off of V1B?

Well, if you get time give it a look over.  Attached is just a preamp section.  There's really not an out on it as of present.  I dropped it to one rectifier tube.  I wasn't clear on the original, but I had considered a 25W max output for amplifier.

I'm still thinking, it'd be better to just build the preamp for recording purposes at present.  Cheaper by a long shot.  I have a nice, deep, and tall PC Rack Mount Case I no longer use, that'd be perfect for such a project.

Just idea's and thoughts, let me know what this rough draft looks like.  I'm going to work on it some more later this evening.

Chuckles

[baileyblues]

After reviewing my rough draft, it occurred to me what folks were saying.  So, I decided to move the reverb, to tap after the coupling cap (.022uf) after V1A and B.  Tapped there, and then returned after the volume before the 270K resistor.

I know the layout is kinda funky, but this should be a bit more valuable than my previous 2 drafts.  I placed notes on the schematic near the points that I have questions.

I also, looked at a BF Style Preamp, Alembic F-2B.  The diagram I found, was different, but showed the output being driven by a 12AX7.  not sure how to link here.  But, lets see worked like a BF circuit.  Say it was one V1.  It came out of V1 to a capacitor, resistor, and jack for a basic output.  No amp involved for the unbalanced output.

The gent whose sight I found this on, had built his own and installed an amp to have both balanced and unbalanced outputs.

So, I placed this unbalanced output at the end of this preamp for time being, with notes.

Here's this schematic, going to bed my head hurts!  LOL  That's always a sign it's time to put something away. 

Thanks again for humoring me.

Chuck Bailey

[tubeswell]

Where you have cascading stages, you really need separate Rk/Ck - otherwise you'll get oscillation

[baileyblues]

The mid boost thingy is out.

I looked around at some options, such as the one that Hoffman offers for the Plexi on this site.  Also, looked at his Reverb Stout and his option of putting a Reverb in a GE Circuit, and read some in the forums.

It appears to me, that the Reverb will run off V1B just fine.  So I've updated the preamp schematic to reflect this.  Hoffman's 'boost' mod utilized the second half of V1.  So that's why I killed the mid boost, and decided to just work with the Reverb.  Would much rather have the reverb, can always put a pedal in front if I desire so.

Also, I'm essentially using the BF Reverb. With exception, that there is a tone control placed within it.  I have seen other designs similar, that even included Bass, Mid, Treble...essentially they just took Channel one from an amp.

I have the Reverb Return as it is on the BF, the Cap/Resistor from pins 3 and 8 on V4 are connected as such, on BF schematics.  I'm using a .1uf Cap to return prior to the 270K Resistor that connects to pin 2 of V2.  On the BF, and the '63 Reverb Unit, a .1uF is shown on the schematic.

I dropped off the extra valve for the output.  I noticed on Hoffman's Stereo Preamps, he comes straight from his tone stack to a 1M Pot and out.  No amplifier or valves.  His recordings sound great to me.  I've seen this set up on other schematics through the net.  Assuming this will give at least a strong enough signal to a, say -10dBU input (instead of a +4, which I have both on my present MAudio Delta 1010).

The Power supply is still being worked over.  It's there for reference.  Voltages are still being worked out.

Hoping this schematic is closer, so can get on to the Power Supply.

I tell ya folks, this is some interesting stuff.  Really needed something new to learn.  Oh yeah, the file is BaileyBluesmasterPreamp07162012.pdf.

Thanks,

Chuck Bailey

[tubenit]

I realize this is a 5E7 instead of a 5E8A.  However, I would be inclined to try something like this regarding adding reverb.

Some information on 1-tube reverb here:  http://www.el34world.com/Forum/index.php?topic=7957.0

IF you don't need "surf" reverb, this may be an easy and reasonable solution to adding reverb.

With respect,  Tubenit

[baileyblues]

@tubenit:

That looks interesting.  If I'm following the schematic correctly, then it looks like there is a reverb 'mix' control prior to RV2.  1 M pot.   That'd work for me.

That also free's up the V1B for use on the  hot mod.  Which is at least thinkable about.

Now, this 5e7 schematic you shared, thank you for that btw...is that something you've built or heard? 

In otherwords how does the Reverb sound on it?

I'll print it off for studying and see how it might be able to put into the 5e8...or maybe just use the 5e7.

Chuck

[tubenit]

Haven't built the 5E7 with reverb.

I have used that one tube reverb numerous times with success and really like it. It has some warmth that I think the Fender reverb will sometimes lack (because the mids are more accented).  However, it is a more subtle reverb.

With respect, Tubenit

[baileyblues]

@tubenit:

I went in and worked on the schematic again.  This time, I implemented the Hoffman Hot Mod on V1B.  If I was able to decipher it correctly, then remove the switch.then it should be good to go.

Then I looked and looked and looked, and studied and researched the single 12AX7 reverb.  On the schematic provided for 5E7, it shows the tone stack and reverb coming off before pin 2 of V2.  V2 pretty much looks obsolete if I'm deciphering this correctly.

now, after going through the tone stack it then connects to the first stage of the reverb.  After going through all of that it comes back up a connects to the PI of the amplifier.  With a 150K resistor between the two stages.

Now, what I've done is come in prior to pin 2 of what is now V3 (on my schematic, and fender schematics).   I go down to the first half of the reverb (V2A), do the reverb magic, come out of V2B as shown on the 5E7 schematic.  I come back up in front of pin 2 of V3.

If you would, when you get some time.  Could you have a looksee.  See if anything I'm doing is making sense.  I might be producing too much in front of V3.  Although, comparing the 5E8 schematic to what I've drawn, I have the same amount of triodes coming in.  Just plotted funny.

Anywho, the B+ is there.  Although, the 5E7 showed big cap's differently than the 5E8 schemes I have.  I s'pose it really doesn't make that big of difference, just depends on where they are gonna go in the chassis.  Hmmm..still working out that part of things.

Well, that's that for the night

It's labeled BaileyBluesmaster07172012.

Chuck Bailey

PS  I listened to some D'Lite amps on Youtube, that s'posedly use this setup.  It was an interesting reverb.  Might be more of what I'm looking for.  Never know until I hear it in person, while playing.

[tubenit]

Reverb will NOT work as you have drawn it.  The inserts of send/return need to be on opposite ends of the mixing resistor (use anything between 100k-220k).  

You do NOT need the 1M resistor on the dwell pot to ground.  Typically I use the dwell at "5" which is significantly less than 500k.

You forgot the input resistor.  Use 33k - 68k.

With respect, Tubenit

[tubenit]

I noted in red some things I want to draw your attention to.  I am advising that you use lower values for the caps (similar to what I indicated)  &  note the mixing resistor for the reverb insertion.

I am going to respectfully suggest that you insert the reverb later in the signal path as I drew in the schematic.  I think you may find that overdrive reverbed sounds better than overdriven reverb.

With respect, Tubenit

[DummyLoad]

c1 and r2 need to be separate for each valve. the stages v1a and v1b are cascaded (one after the other) and this circuit will oscillate. shared cathode r and c will work if gain stages operate in parallel. your two stages (v1a & v1b) at first glance may appear to be in parallel on paper, until you follow thee signal path. cascaded stages are said to be in series.

what is the point of r5 and r6?  attenuation? if yes, then use a pot or a divider AFTER the coupling cap, or use a r divider for the anode resistor and tap there.

unless you're using anode divider resistor tap, the coupling caps (c2 & c3) should be connected directly to the plate of the valve.

reverb insert/recovery point needs work. the same concept of the 270k mixing resistors (r14 & r20) applies. you have r20 but you're missing r14 (in this case it's usually a large resistor; ~10x the value of r12). look at fender ab763 plan - see the 3.3M r and the 470k r? those are the mixing resistors. if you want more drive from the dry signal then use a smaller value than 3.3M, like 2M, etc.. another consideration is the 1M (r7) under pot will give minimum wet signal of 50%. this may not be optimum unless you like a LOT of reverb. lose the 1M r under the pot or consider a smaller value by a magnitude; 1/10, 1/4, 1/2, etc..

respectfully,

--DL

[baileyblues]

@tubenit:

A couple questions for ya.

1.) In V1 of the schematic you shared, does the switch have to be there?

2.)  I see your point about the Reverb.  Never thought about the reverb in terms of distortion.  Ironically, the draft I did previous to the one I sent, Had the reverb coming off prior to pin 2 of the following stage.  Going down, then coming right back up as in the BF/SF Reverb with the mixing resistors.  I took 'em out and well, should've left well enough alone I s'pose. (not really the question though)

3.) Reverb Question:  Can the reverb come off before pin 2 as shown, and come back to pin two of the stage that the tone stack would normally be attached to.

In other words, on the 5E8, the tone stack is after V3, which is the pin 2 I'm referring to. 

The way I read the schematic you provided, is that the reverb comes off prior to pin 2 of this stage, goes around that stage and then goes through the tone stack, then down to reverb, back up then to PI.  If I'm making any sense there.

That's why I have my question as Can the reverb come off before pin 2, go through the reverb stage, then come back into pin 2 of V3 (referencing 5E8 which has the single amplifying stage, with CF if I'm relating that properly) which goes through the tone stack, then on the PI?

If not, then to me..it'd make about as much sense to use a BF circuit, change some values on the tone stack, or even go to a 2 knob tone stack.   

I might be thinking of this all wrong.  And probably am.

Might be time for me to leave this to folks that know way more than I and just be satisfied with the Mustang III.

Well, thanks for humoring me for so long. 

Chuck

[tubenit]

Can the reverb come off before pin 2 as shown, and come back to pin two of the stage that the tone stack would normally be attached to.

The answer is no  (as I understand it).  Sluckey would know for sure.  Why are you wanting to do it that way?

No, you do not need the switch.  However, it's a $4 mod that is VERY worthwhile allowing you two different tone and super easy to install.
Why not do it?

With respect, Jeff

[baileyblues]

One reason I asked on the pin 2 thing, is that'd give an opportunity to EQ the Reverb (which can be neat).  The other is I don't get why the tone stack was moved off of pin 8.

so with that, could the reverb be inserted with the tone stack being left on pin 8, as in the stock set up?  Come off pin 8, go through tone stack, go to reverb, come back up?

I ask this, cause to me with the CF Stage, and the tone stack being moved...and it not 'connecting' anywhere, then it kinda looks to me like some of the mojo of the tweed circuit is lost.  Then again, maybe I'm not understanding fully how that stage works to begin with. As written that is, coming off prior to pin two through 10M Resistor, down to the tonestack and what not. 

For the switch, why is just me. I typically don't use switches for anything.  I set 'em, and forget 'em.  Yet, I was thinking maybe a foot switch not long after I committed to that question.

so, we are getting somewhere here.  That's good.

thanks for your time.

chuck

[baileyblues]

Tubenit and DL:

Here's what I'm trying to describe.  It is essentially the same as what tubenit has provided, I haven't taken the time to draw a proper switch, but got something in there for the time being that shows the switch doing what it does on tubenits schematic.

The reverb is inserted after the tone stack.  However, I kept the tone stack come off of pin 8 of the CF stage, as is on the 5E8. 

Single 12AX7.  I haven't put values in, just rough sketching to perhaps make what I was trying to describe a little more clear earlier.

Some of my confusion comes from that CF to tone stack on the 5E8 vs the way it is on the schematic tubenit has supplied.  I'm not saying there's anything wrong with this, just that I don't understand the change or difference if there is any difference.

Thanks

Chuck

[tubenit]

However, I kept the tone stack come off of pin 8 of the CF stage, as is on the 5E8.

OK, I understand what you're saying now.  I grabbed an old editable schematic & started redrawing it (in something of a hurry)  and forgot to connect the tone stack to the CF.  What you're showing is correct. Good show.

With respect, Tubenit

[baileyblues]

Outstanding.

I'll start working on getting values in over the next few days, power supply and then work on the output.

For now, I'm working on just a preamp that I could build inside an old PC Case I have.  It's rackmountable, heavy too...7 inches tall by 20 inches deep..more than plenty of space to put something in.  vented up front, and with everything out, it is vented out back as well.  =*)

Anywho, I've been studying hoffmans rack mount revibe for ideas on how to mount the reverb inside it. 

After that will be a full amp.  I"m thinking 25W output, but 35 would be good as well.  2 x 12, just cause I like 2 x 12....although the Mustang III is a 1 x 12, and I've played through plenty of 1 x 12's..they're nice as well.

Oops..got off subject.  Renewed energy and excitement.

well, thanks for sticking with me through this.  Really appreciate it.

Chuck

[baileyblues]

OK...so the preamp looks fairly close to being finished now.   I still have some questions for figuring voltages for each stage.  V1 and V2 are pretty easy for the most part, whereas they are known factors from the amp I'm borrowing them from.  With exception of the hot mod...

I'm sure there is no simple way, however I've run into block with web searches on how to figure the voltages for those stages.  I"m sure I need to concern myself with them.

I have not redrawn the power supply for a preamp build, i'll have to work on that.

I figure on using a PT for a 6GC15 standalone reverb.  My calculations are:  Heater current: .9A  (3 12AX7's at .3A each).  B+:  3.6Ma...the 6GC15 more than meats the requirements at:  100Ma, and 3A respectively.

I may look to see if I can find a smaller unit that will suffice, that just something I've been following on this board and looking at as a consideration, or starting point.

I realize it will not use a tube rectifier.  My understanding is Sag can be emulated with some resistors, or using a Copper Cap that Weber sells, including for Bridge Rectifiers.

So, I thought about one of those as options after getting the preamp working.

Any other considerations I should be thinking of that I am not?

BTW...I am working on an amp build as well.  If this works out nicely, then I can transplant the preamp or I can build a speaker iso box (and add the power portion to my unit) or build an cabinet with speakers and new board and all that.  Options, lots of options.

Anywho...that's getting ahead of myself.

Well, updated schematic is attached.  It should match everything we've been talking about, with exception of power supply cause I haven't gotten to drawing up a new one with the bridge circuit in it.

Thanks for taking so much time with me, and answer questions.  Much appreciated, really it is folks.

Chuck

[tubenit]

Looks OK to me with exception of the DPDT hotswitch.  You can also use any value between  500p to .001 for the dwell cap.  I often use .001 because I like the mids reverbed more.

With respect, Tubenit

[sluckey]

My understanding is Sag can be emulated with some resistors

Sag is not a factor with your preamp project. Everything is running class A and the current requirements will be constant. May as well just use ss rectifier diodes.

Another PT to consider would be the Hammond 269JX. It's rated 250-0-250 @ 69ma and will produce 330 to 350vdc. As far as the B+ rail goes, I'd suggest using a parallel node arrangement rather than the more common series node arrangement. Doing so will allow you to change the B+ supply for a particular tube by simply changing one resistor value, and without affecting the voltage for the other tubes. For example, look at the power supply in the schematic for my Warbler project on the bottom of this page...

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

[John]

Those parallel B+ nodes are pretty slick. Yet another thing I learned today.

[baileyblues]

Tubenit:

here's the schematic with the power supply in.  If'n I'm understanding correctly, then this should be about all there is to it.  I took your advice, and broke it down the way you suggested.

Thanks again,

Chuck

[tubenit]

DPDT hot switch is not drawn correct or clearly?

I would NOT use a 25uf on you V2a gain (that is coupled with the cathode follower).  It will sound incredibly MUDDY, IMO.  I would advise starting with 5uf and probably you will need either 2.2uf, 1uf or .68uf.   Try clipping in different values with insulated alligator clipped wires carefully.   I will be quite surprised if you like anything above 5uf.  

With respect, Tubenit

[baileyblues]

Thanks for the advice.  I'm working on counting parts and sourcing them.

It'll take me a while to get everything together.  Will check back in with you once I start putting items together...and will follow up with a sound example.  However, I wouldn't expect a sound example anytime before October.

Lots to do, and lots to purchase.  Like a new printer.  Darn thing shot craps.

Chuck