Alembic F2-B Pre-Amp Restoration project

[RicharD]

I like the chassis layout.  If you're going to add a tone stack, at that time you'll probably need to eliminate Rfb & change R2 to a 22 or 33k.  W/O the feedback, gain will increase probably enough to compensate for the losses associated with the tone stack.  Also w/o the feedback, R2 will affect the hi end.  I would suspect that 47k will cut into the range of hearing but it might be OK. 

The way I showed the output, if the 1/4" jack is plugged in, then the XLR output and it's associated transformer are disconnected, otherwise your output transformer primary is in parallel with your hiZ output and it'll sound like there's a sock stuffed in it.  It's an either or output situation but..... Since the input jacks are wired such that when using the (left) mono jack, sends signal to both halves of the unit, you can use the hiZ out on 1 channel and the loZ out on the other.  You'll have separate level controls (& tone stacks) if you go this way.  If you want hiZ and loZ simultaneously on each channel, you'll need to add another stage which blows the whole power supply since we maxed out that tiny cheapo power xfmr. 

I finished the revised schematic.

[oatmeal769]

... Also w/o the feedback, R2 will affect the hi end.  I would suspect that 47k will cut into the range of hearing but it might be OK.

So, using a lower value resistor (like 22k or 33k) will not affect high frequencies, but the 47k possibly might??

The way I showed the output, if the 1/4" jack is plugged in, then the XLR output and it's associated transformer are disconnected, otherwise your output transformer primary is in parallel with your hiZ output and it'll sound like there's a sock stuffed in it.  It's an either or output situation but..... Since the input jacks are wired such that when using the (left) mono jack, sends signal to both halves of the unit, you can use the hiZ out on 1 channel and the loZ out on the other.  You'll have separate level controls (& tone stacks) if you go this way.

I'd never use both Hi-Z and Lo-Z out's simultaneously.  I'll only ever use a single Lo-Z output, and only one input at a time.
Since that is the case, could I simply wire channel 1 direct, so that is NOT feeding channel 2, and then sum the two outputs to a single OPT?  This way, no matter what, I get one Lo-Z output, no matter which input I use.  (not to mention save some $)

I finished the revised schematic.

Wow!  I'm even learning how to read schematics here, this actually (magically) makes total sense to me now.

[oatmeal769]

I'd never use both Hi-Z and Lo-Z out's simultaneously.  I'll only ever use a single Lo-Z output, and only one input at a time.
Since that is the case, could I simply wire channel 1 direct, so that is NOT feeding channel 2, and then sum the two outputs to a single OPT?  This way, no matter what, I get one Lo-Z output, no matter which input I use.  (not to mention save some $)

 
In other words, could I do this? :  (see attachment)

[RicharD]

I don't think you can sum the output as shown.  Obviously you could simply delete 1 channel all together and recalculate the power supply, or you could make a 2 in 1 out mixer as shown below.  Personally I'd stick with the 1.21 version.  With a mono input you get dual outputs with separate level controls, a glorified active direct box if you will, or a straight up stereo in/stereo out.

[oatmeal769]

Dang!
6 week wait for the Edcor output transformer(s).

Looking at the Alembic f2-B circuit, (and my unit) it sums the two outputs using a 47k resistor on each.  I actually got rid of the resistors for more output, and it's working fine so far...  I think it's worth a shot. 
If it doesn't work though, I will stick with 1.21 version.  I'll have to run around back when I want to switch channels though. - Only one input to the compressor...

Chassis holes drilled/stamped!

[oatmeal769]

Schematic question, and picture.  

In the oval, there is an arrow pointing to the pot [EDIT: NEVERMIND, FIGURED IT OUT WHEN DRAWING THE WIRING SCHEME] C4/C104 cap.  

Also, I'm guessing the dashed line means that the two pot 'cans' are connected via a grounding bus?

Here's a 'so far' pic.  Power supply and power wiring done. Heaters and B+ wired.  

All that's left to do is the audio path.  Oh, and wait 6 weeks for the blasted output transformer(s).

[RicharD]

The dotted line represents a "ganged" pot but isn't necessary or even preferable if used as a dual mono.  Looking good so far.

[oatmeal769]

Hey B,

Got channel 'A' all wired.  All I get though is a very small and very distorted signal.  What I'm wondering is if I have the two 'stages' or units of the tube crossed. 

In the schematics I've studied, the first unit corresponds to pins 4,5 and 6.  The second unit corresponds to pins 1,2 and 3.  On this Oatmeal schem, the input signal goes into the second unit first, 1,2,3) and then to the first unit.  Is there a difference in gain etc. between the two stages?  Or are they identical and it doesn't matter which is first? 

It would make total sense if I had the wiring exactly backwards - giving me a small, distorted sound...

[RicharD]

The 2 triodes match.  It does not matter which 1/2 you use.  Double check your wiring.  Make sure filaments light up on both tubes.  Even if you don't have the 2nd bottle wired for signal, you still need the filaments wired in series since you're using a 12V supply for 2 - 6V tubes.
OK, so check some DC voltages now. 
First look at point A.  Is it 250VDC (or close +/- 15%)?
Now look at the cathode of the first stage.(pin 3 or 6 depending which 1/2 you used).  It should be in the neighborhood of 1.5VDC.  The plate (pin 2 or 5) should be.... call it 155VDC.
Next look at the 2nd stage.  Obviously the plate is 250V.  The cathode should be 100ish VDC.

[oatmeal769]

The 2 triodes match.  It does not matter which 1/2 you use.  Double check your wiring.  Make sure filaments light up on both tubes.  Even if you don't have the 2nd bottle wired for signal, you still need the filaments wired in series since you're using a 12V supply for 2 - 6V tubes.

All good -

OK, so check some DC voltages now.
First look at point A.  Is it 250VDC (or close +/- 15%)?

Not even close.  It fluctuates widely from millivolts to about 80 volts, depending on how hot the X4mr is.  I double checked the power supply wiring, I think it's right.  The transformer gets too hot to keep a finger on for more than a few seconds.  The heater voltage is only about 4V and the DC out from the primary leg is only about 40 volts.  Looks like I have a dud?  I think I should be seeing about 125VDC entering my power supply  (?)

[RicharD]

Sounds like you have a shorted or reversed filter cap, or you miswired the voltage doubler power supply. 

Disconnect everything from the PT secondary and measure it's AC voltage.  Filament voltage should be right now too if the transformer isn't injured.

Next check your orientation (wiring) of D1 & D2, C5, C6 & C7.  Reconnect the PT secondary but disconnect the amp circuit from the power supply at point A.  Check voltage now.  W/O load it should be something around 265ish VDC. 

[oatmeal769]

Sounds like you have a shorted or reversed filter cap, or you miswired the voltage doubler power supply.

  One diode - D2 - was backwards I think, which I repaired.  The stripe side is negative - cathode, and the plain side is positive-anode, right?  In the schematic,  the arrow points toward the positive end, right?

Disconnect everything from the PT secondary and measure it's AC voltage.

If I place my Multi-meter contacts one on each of the two leads, I get 120VAC  (there is nothing DC.)  I've looked over and over, it must be that I can't read schematics.  The + is the positive side of the cap, and the U is the negative right? 

Filament voltage should be right now too if the transformer isn't injured.

it is, about 5.5 - 5.8 VAC.

Next check your orientation (wiring) of D1 & D2, C5, C6 & C7.  Reconnect the PT secondary but disconnect the amp circuit from the power supply at point A.  Check voltage now.  W/O load it should be something around 265ish VDC.

Nothing I can find, I'm stumped.  Have a look.  I had one of the red leads between the 2 diodes, and the other at the +/- junction of C5 and C6

[DummyLoad]

something is sucking too much current - perhaps there is a short under the board? based on your description of where the PT leads went, it seems to be wired correctly. does the PT have a CT? if yes, is it grounded? it shouldn't be. confirm both diodes and the caps are good.

http://www.allaboutcircuits.com/vol_3/chpt_3/2.html

[RicharD]

>The stripe side is negative

Stripe is +.

[oatmeal769]

No transformer CT, and no shorts.  The diodes seem to be good, I don't have the means to test capacitance.
One thing I did do, is to cut the bus between c6 and c5.  With that done, I do get about 60VDC at B+, but that's it.

 Also, the grounding signs in the schematic... - doesn't the AC stiff have to go back to neutral, (like the AC heaters, and AC ground from the power supply before it's converted to DC?)  and only the DC stuff gets grounded to chassis/safety, right?  I played around a little with this, but nothing conclusive.

I'm pretty sure the transformer is fine.  Taking it away from any connections causes it to measure fine.  When connected though, it's all the same as I've described.

[DummyLoad]

any chance of getting pics of the entire build?

 

[oatmeal769]

ISO -
Yep, here ya go, thanks for any help!

Two alternate views of the PS.  The cap in the middle of the pics is C5.  C6's NEG was attached to C6's POS side, and one of two leads from the transformer was attached to this.  I have snipped the bus between them, and that's how I got the 60VDC on B+. 

Also the rest of the build.  Some is messed up as I've pulled apart stuff to test...

[RicharD]

I think you have a bad diode.  Your layout looks correct, but just in case, attached is a redraw of the power supply.

How to check a diode:

Set your VOM to ohms.  With at lest 1 end of each diode out of circuit,  connect your meter across the diode, then reverse your meter leads.  1 way you will get almost a short reading, the other way you'll get an open circuit. 

>One thing I did do, is to cut the bus between c6 and c5.  With that done, I do get about 60VDC at B+, but that's it.
When you did this, which cap (c6 or c7) did you leave the PT secondary connected to?

I think the PT is fine, but something is obviously shorted.

[oatmeal769]

I think you have a bad diode.  1 way you will get almost a short reading, the other way you'll get an open circuit.

D1 = .6 M Ohms one way, Open circuit (no connection) the other.  D2 = .7 MOhms one way, open circuit the other.  Both = bad, right?
EDIT:  JUST TESTED A NEW ONE AND GOT THE SAME READING.  ALSO .5V FORWARD VOLTAGE WITH THE DIODE CHECKER FUNCTION ON ALL.

When you did this, which cap (c6 or c7) did you leave the PT secondary connected to?

I left power between the two diodes, and to the + of C6 (C5's Neg was no longer connected).

[RicharD]

Diodes are good.  C5 looks burnt on the end.  It's really too close to that turret IMO.  Measure with VOM.  It should NOT read short.

>I left power between the two diodes, and to the + of C6
So C5 is now out of circuit.  Is the transformer still getting hot?  What do filaments read?

SAFETY NOTE:
You do know those caps will store a charge w/o a load connected right?  Be careful and bleed the caps before touching.

[oatmeal769]

Diodes are good.  C5 looks burnt on the end.  It's really too close to that turret IMO.

Yeah, that's me soldering and unsoldering the power lead for tests. 

Measure with VOM.  It should NOT read short.

C7 and C5 have varying resistance BUT - C6 has none!  That would then short, right?

>I left power between the two diodes, and to the + of C6
So C5 is now out of circuit.  Is the transformer still getting hot?

yes

What do filaments read?

About 4.5VAC or so when lit.

SAFETY NOTE:
You do know those caps will store a charge w/o a load connected right?  Be careful and bleed the caps before touching.

Yes, but thanks for the heads up.  To bleed, I'd connect a grounded alligator to the +, right?  Either way, I've been zapped before, it makes you very conscious of where your hands go!

[RicharD]

Checking an electrolytic cap for shorts:

Take the cap out of circuit.  Put you VOM on ohms, Black to - Red to +.  This will put a tiny charge on the cap.  You meter should read greater than 1 mega ohm and slowly climb.  This would be a non-shorted cap.  If you're reading anything substantially less than 1M, it's trashed.

 >About 4.5VAC or so when lit.
Is that across each tube or off the PT filament secondary?
You did wire the filaments in series right?
Pull the tubes.  What do the voltages do now?
(Power supply point A is disconnected from the tubes right?)

[oatmeal769]

  > About 4.5VAC or so when lit.
Is that across each tube or off the PT filament secondary?
The 4.5 -5 is when measuring lead to ground on a single tube with the tubes in the sockets. 
With the tubes out I get about 40VAC measuring from one tube's lead to ground.

Pull the tubes.  What do the voltages do now?
I get 12VAC measuring both leads. 

You did wire the filaments in series right?
Yes in series - One lead goes to one tube's 7 with 8 grounded, the other lead goes to the second tube's pin 8 with pin 7 grounded.

(Power supply point A is disconnected from the tubes right?)
Power supply point A has never been disconnected, no.  It runs to each tube's pin 5

If you're reading anything substantially less than 1M, it's trashed.
There is no connection when I test the cap - OL is all I get on the meter.  (in other words, infinite ohms)

[RicharD]

OH Kay.  Isolate the power supply from both pin 5's at point A.  We're trying to get the PSU up with no load to make sure it's OK.
Put the tubes back in.  Disconnect the tubes from point A.  Put the PSU back together (D1 &2, C5 6 & 7, and R10). Recheck voltages.

[DummyLoad]

Yes in series - One lead goes to one tube's 7 with 8 grounded, the other lead goes to the second tube's pin 8 with pin 7 grounded.


what you describe is not a series heater circuit... wire transformer lead 1 to pin 8 of V1 - pin 7 of V1 wired to pin 7 of V2 - pin 8 of V2 to the second filament lead of transformer.

  transformer(1)  ---- 8 V1 7 ---- 7 V2 8 ---- transformer(2)
                                     

[RicharD]

Yes in series - One lead goes to one tube's 7 with 8 grounded, the other lead goes to the second tube's pin 8 with pin 7 grounded.


what you describe is not a series heater circuit... wire transformer lead 1 to pin 8 of V1 - pin 7 of V1 wired to pin 7 of V2 - pin 8 of V2 to the second filament lead of transformer.

  transformer(1)  ---- 8 V1 7 ---- 7 V2 8 ---- transformer(2)
                                     

Yes it is.  Virtual center tap between the 2 tubes.

[DummyLoad]

Yes in series - One lead goes to one tube's 7 with 8 grounded, the other lead goes to the second tube's pin 8 with pin 7 grounded.


what you describe is not a series heater circuit... wire transformer lead 1 to pin 8 of V1 - pin 7 of V1 wired to pin 7 of V2 - pin 8 of V2 to the second filament lead of transformer.

  transformer(1)  ---- 8 V1 7 ---- 7 V2 8 ---- transformer(2)
                                     

Yes it is.  Virtual center tap between the 2 tubes.

yes, you are correct - why V tap?  how do you balance? i guess it really doesn't matter on w/ class A.

[RicharD]

You wanna ground the filament circuit somewhere.  Seems ideal for both tubes to get grounded.  Balance from = filament current.

[oatmeal769]

Re-assembled the power supply as before - with C6 cap (?).

Removed B+ from both pin 5's.  X4mr still hot, AC measures about 40 VAC from point A to ground.  DC voltage is negligible (Less than a volt)

Heater filament voltage is at about 5.5 to 5.8 VAC

[oatmeal769]

When I remove the two leads from the transformer and test them, the transformer is still outputting 120 VAC.
  
I can't see that there would be any shorts, There is nothing on the underside of the PS except the turret backs, and the two leads from the transformer.  I've even removed the point A feed entirely...

It's now GOT to be something in the PS itself, it isn't connected to anything else.  The diodes I think are okay, and the resistor is ok.  It has to be one or more of the caps (?)

 Maybe I should just order all new parts and make a new one from scratch...

[RicharD]

Something is shorted, caps or diodes or just a plain hard wired shorted.

>Maybe I should just order all new parts and make a new one from scratch...

I'm going to say yes.  Even if you get this up and running, you're going to have hum issues.  I'm certain I see a ground loop and your layout has problems.  I'm gonna sketch up a layout real quick. I would suggest you use these

Cheap and effective.  Easier than drilling a turret board.  I love em.

[oatmeal769]

I've seen those in a lot of high quality, old style point to point builds, which I guess I'm trying to emulate.   Good idea, only I just ordered new parts from Hoff (I'm impatient) .  And, Dang him, he's so fast, he had them shipped out an hour later so I can't change it.  I can probably pick some up at my local store.

Thanks for doing the layout, it helps guys like me who are schema-tarded.




As a side note to the 'original' topic of this thread:
I just did a quick session today of a couple cover tunes, one with a fretless, and one with a fretted bass.  The engineer was blown away by how quiet my F2-B was, and also how punchy and phat.  I just plugged in to his feed, and the tone was there instantly.  You really get to hear what your gear sounds like when you record it.

[RicharD]

I recognize the style you're emulating.  It begs for trouble.  Have a look at the attachment.  I tried to accommodate your existing chassis config, but you're gonna hafta move the tubes.  I didn't draw the PT or line supply.  What you got is fine. Separate chassis grounding point for the power cord.  Keep the filament wires as far away from everything else as humanly possible.  I didn't draw a balanced output.  Get a couple of extra tag strips for that.  Pay close attention to the grounding scheme.  No extra wires.  No need for a #12 ground bus. 

[oatmeal769]

That certainly looks a WHOLE lot easier...

I'll see if I can pick some strips up tomorrow.  If not, I'll get some from Hoff.

Other than to mount the tubes outside the chassis, (Thus defeating the single rack space) I don't know where else to put them.  I need the clearance above them to remove/replace.  I can't really go to one side or the other, there's the PS on one side, and the upcoming OT on the other.  I think I could replace the tube mount board and get the sockets all the way on the back side of it and still have just enough clearance.  I can also run longer filament leads, maybe along the back wall and then straight in.

I think if I get rid of my 10 GA. ground thingie, I could mount a row of tag strips, and go directly to the pots / input jacks from there?

You know, I'm having so much fun with this... ... Do you still have the circuit boards you had made for the bass pre-amp you designed?  I think after this torture, I'll be ready for something even more painful, LOL.  If so,  how much would you want, and do you know an approximate component cost?

[RicharD]

You could prolly turn the audio section 90 degrees and make it it.  The tag strip area is roughly 2.25" x 6".

I have 1 unpopulated board left.  I can't remember what the BOM priced out at but it wasn't cheap.

[PRR]

> doesn't matter on w/ class A.

Yes it does.   

> Balance from = filament current.

I don't see that. What we mostly want is voltage cancellation, a "virtual Neutral" in electrician's terms, AT each socket.

That silly AC is always wobbling. "Freeze" it at a peak. With the conventional guitar-amp heater wiring, we have +9V on one lead and -9V on the other lead. A point in the middle, or a point at a distance, feels the difference: Zero volts.

With your wiring, V1 feels the difference of +9V and zero, or 4-some V; V2 feels the difference of -9V and zero or negative 4-some V. It is in fact equivalent in hum-effect to grounding one side of the heater, as the oldest amps did.

There isn't a clean answer for 12V power and 6V preamp tubes.

If you could stand 12SL7, you would wire normal with two 100r resistors. And 12SL7 are still a buck cheaper than 6SL7. And they may not be making any new 12SL7. But I dunno if the karma is identical.

Does the PT have 12VAC at well over 0.6A? While I say DC heat is a frill, and that good twisted 6V is usually penty fine, a bass front end for studio work might get my blessing on DC heat.

[oatmeal769]

I've seen that 12SL7 has identical ratings except for the heater voltage, and it'd be easy to wire a switch or something to use both types...  But what does the tube sound like?  Is it identical in sound?
There do seem to be quite a few on ebay.

I'll either pick that stuff up tomorrow, or order it.  I want to do this at least semi-well.  Even if it is my first learning project...

[oatmeal769]

Oh yeah, what's involved in changing an AC heater to DC?  Does that open a whole new can of worms?
There's AC in my F2-B as well....

[oatmeal769]

Success!   

Power supply rebuilt, about 200V to point A. I changed out everything except the original PT.  There had to have been a short somewhere, maybe under the board.  The PT is nice and cool now, and everything is fine.
To increase to the specified 250V range, I can lessen R-10's value a bit, I assume.  Is it crucial that I hit as close to 250V as possible? 

So, then I totally re-did the circuit using your layout - except I couldn't fit both terminal blocks, so I used one, and changed a couple things.  (I'll take a pic tomorrow)  I re-did the routing for the heaters as you suggested, as well as the whole grounding scheme.   - Several other things too.  I didn't move the tubes - Took the risk on hum, but so far it's dead quiet. 

You were right, those tubes are markedly different than the 12AX-7.  I was instantly reminded of an old Portaflex.  Big fat whompin' tube-ness.  It makes me want to slap the black right out of my bass 

Channel 2 isn't right yet.  It has about half the gain, and my 3-way switch for R102 (between 0, 33 and 68K) doesn't function either, but I think this is minor.  I'll bet the switch is broken, or the feedback cap is wrong, I'll try to trace it down tomorrow....

Channel 1 however is perfect!  I might mess with the R2 resistors a bit, but it already goes clean to overdriven just with the gain knob.

I swapped several tubes, and the actual tube used in the circuit affects the sound much more than in my Alembic.  The Alembic is subtle at best between tubes - this is not at all.

Tone stack?  I dunno yet, I want to play this tomorrow through my rig before I make that decision, but so far so good!
 

[RicharD]

Rock on!  No, it's not critical you hit 250V, but you have the right solution.  You'll probably get more "punch" with a voltage increase up to a point where distortion will take over.  Go too far and you cook the tube.  I'm working on another project for Kagliostro and right now I wanna rip the tone controls out and throw the parts back to the fishes.  If you like it w/o tone controls, don't mess with it. 

[oatmeal769]

How high can I go with it?  No such thing as too much punch, but yeah, don't want distortion, nor do I want to blow up the tubes.

Pix tomorrow.

[DummyLoad]

How high can I go with it?  No such thing as too much punch, but yeah, don't want distortion, nor do I want to blow up the tubes.

Pix tomorrow.

experiment... find the sweet spot you like. 

[oatmeal769]

I'm reading that 300 volts is the max, is that right?  Will this PS do it, or will I need some bigger / more caps?

And yeah, I think regardless, this pre-amp will be tone stack free.  That is what this whole concept was about, and I must say I'm excited about the results I'm just starting to get.  Now I want to build another - like on the PCB you have left.  Money's tight, but I could just do it little by little.  Do you have a parts list?

Oh, and check this out - I went back and re-read some of the more technical posts made earlier in this thread - it makes SO much more sense to me now!  I think I'm learning!

[PRR]

> 300 volts is the max, is that right?

On the tube. (And 330V is factory-safe if measured; we know these tubes will stand more.)

But the supply voltage SPLITS between tube and 100K resistor. 500V maybe 600V will not leave 300V on the tube, nor will it cause excess dissipation.

One other detail. The "a" section of each tube runs with cathode near ground, the "b" section of each tube runs with cathode at high voltage, nearly half of supply voltage. The heaters inside both cathodes are connected together. The heater-cathode insulation has a voltage rating. G.E. says 6SL7 heater-cathode voltage should not exceed 90V.

If heaters are grounded, as B's plan shows, the "b" section's cathode should not be over 90V. That suggests the supply voltage must be less than about 200V.

Actually the "b" section could be biased for a lower cathode voltage. Change its 100K to 47K. No real change in audio performance but significantly lower cathode DC voltage and lower heater-cathode insulation stress. Now over 300V supply should not be a problem. And I don't see a reason to need over about 250V supply, for this gain and normal instrument pickup output. Which is a very traditional B+ level, and what Butterylicious suggested.

[oatmeal769]

Most interesting.  I'm kind of understanding these talks a bit more now. 

Basically, I could lower R8/108 to 47k, (or maybe 68k?) and drop the R10 resistor a bit allowing more flow, but less voltage.  Interesting based on the second part of what you said - normal instrument pickup output.  Mine's anything but normal.  I use active EMG's and like to thwack the heck out of my bass - I'm not an easy player, I really dig into it.  Dynamic range and headroom - while keeping that transient 'punch' has always been what I'm after.  So, giving the tube plenty of reserve to keep up, sounds like it's worth experimenting with. 

I do think the 6SL is a better deal for me than the 12AX-7  It still breaks up, but it sounds a lot nicer (to me) doing it.  The I don't like how the 12A breaks up on bass.  It's great on guitar though.

The big point is that by changing these, I'm not going to blow anything up, right?  Is there a point at which I'll need to add more / bigger caps, or is there 'plenty'?  Maybe I should put in a couple switches.

As ISO said, Play around until you get the sound you like.

[RicharD]

>I don't see a reason to need over about 250V supply, for this gain and normal instrument pickup output. Which is a very traditional B+ level, and what Butterylicious suggested.

Right on.  I picked 250V for several reasons.  1st it is a traditional value which means as you learn, and start reading tube data sheets you can find information at this voltage w/o having to recalculate or estimate for a different voltage.  250V is relatively a "safe" voltage.  If you get across this B+ it will certainly get your attention but it probably won't send you screaming from the room.  It shouldn't explode if you wire something up wrong.  It'll probably just make the PT get real hot.     Finally because I'm lazy.  Working at typical voltages are quicker easier to design.

>The big point is that by changing these, I'm not going to blow anything up, right?
You've got the unit working correctly, so you're pretty much past the point of doing any real damage.  I assume you've gotten the 2nd channel to work right.  Please do this before you get into experimenting with the circuit, ie. keep the horse in front of the cart.  If 1 channel isn't conducting properly, you probably are drawing the correct current, so you're shooting yourself in the foot to be messing with the power supply voltage before you take care of this. 
  The power supply cannot possibly make anything greater than 350VDC.  I doubt it's actually making 350V since the PT is so small.  I didn't have one of these on hand so it was all a guesstiment.  Here's what you need to do.  Measure the voltage at the point where D1, C5, & R10 come together.  This is your actual max B+.  I normally call this point A, but since there's no tap here, I didn't want to confuse you (until now)     Let's call this point "X" so it has a name without a + symbol.  Now measure the voltage at point "A".  Subtract the "A" voltage from the "X" voltage.  This is the voltage dropped across R10.  Now take that voltage drop and divide it by the value of R10.  This is your total current.  My estimate looks like this:  350/250=100V  100/25000=.004 or 4mA.  You mileage may vary.  Ok, now take your voltage drop and multiply it by the calculated current.  100*.004 = .4 watts  Using a 1/2 watt resistor would be foolish and destine for failure, a 1 watt would be safe, and a 2 watt will never get beyond a little warm.
  OK, now you're armed with all the info you need to recalculate R10 for a higher voltage.  Pick the voltage you want to run and subtract this from the measured voltage "X".  Take this calculated voltage drop and divide it by the current you calculated.  This is your new R10 value.  250Vshould be optimal.  300V [/i]should[/i] safe.  Given the limits of this power supply, as long as you've wired correctly, it's gonna be hard to toast a tube.

>I could lower R8/108 to 47k, (or maybe 68k?)
Could and probably should.  Of course you'll be flowing a little more current which affects everything I just talked about in the power supply.  Welcome to the merry go round.  It's not that drastic of a change but this is typical of playing with a circuit.  Fix the power supply, jack with the circuit, & refix the power supply.  (repeat if necessary)

[oatmeal769]

Buttery - Thanks again for all this, you've definitely gone way beyond.

I actually understand - everything - you just said, and it's a great working order for today's experimenting!

Here are some pix, there's extra room over on the left for the OT(s) when they get here.  In 5 weeks.  Still, since the output is lower in impedance than my Alembic, I'm thinking I'll already have better luck with it.
I'll report back when I do some tweeking.

[RicharD]

>Buttery - Thanks again for all this, you've definitely gone way beyond.

You are most welcome.  This has been fun for me.  There are 3 basic levels of learning, what you read, what you do, and what you teach.  Typically one retains very little of what they read, they retain more of what they do, but almost everything they teach they retain so this is most beneficial for me.  I'm glad your happy with the project.  Your rebuild look +100% better.  If you get hooked on this stuff (which BTW it IS addictive), you'll get better and better.

-Richard

[oatmeal769]

You ar 100% correct.  Not blowing my horn, but I'm a supercalifragialisticexpialidocious diver.  Mixed gas, deep decompression, wrecks, the works.  I remember learning to dive involved reading which helped get the ideas, but it was immediately (same day) followed by doing. - And then repetition.  That's where the 'learning' really happened for me.

Once I had some salt under my weight belt, I learned and got certified to teach diving, and over time became able to teach all the way up into the technical diving realm.  Going back and teaching the very first open water course, and the 'second level' stuff, even though I've now taught it maybe 40 times, is STILL beneficial to me.  It's funny how you re-connect those basic blocks with some of the most abstract idiosyncrasies, once you're looking top down.

There you have it.  A 'how-to' on connecting tube pre-amps to scuba diving.

[oatmeal769]

Hey guys, this is great fun!  

Fixed channel 2, it was a wrong value at R103  (note to self - watch that decimal!  1K is a far cry from 100K.  
I'm wondering if, say, a 150K @ R3/R103 might increase clean output a bit more?  The amp is clean at a gain setting of about 2, but not enough output.  Setting of 5 (50%) is about where I need to be for gain, but it gets a little too 'fuzzy' at 5 as well.

Then I did as instructed RE: power supply / cathode ground and settled with a 6.8K on the PS, and a 47K on the c. ground.  The results are 248 V at B+, 293V at point A.  Today I think I will shoot for 250V with a resistor of 68, maybe higher, to combat some of the distortion.  

So, I played at length with it in an actual band situation last night, as well as solo - loud through my amp...  and here's what I took for notes...

After playing 'for real' my notes were:

1)  Very Ampeg-y, perhaps too much.  It is a little too warm (i.e. distorted/over-driven) for my tastes, and a little too 'clanky' in the mid-range.  Some of this may be resolved in changing tubes.

2)  Even more punchy, close and 'present' than my Alembic  (Quite a feat actually, we're now approaching the punch-iness of Solid State - without Solid State sterility and 'gack')

3)  Wouldn't want to mess with overall 'tone' too much, but would like a 'DEEP' and a 'BRIGHT' switch.  A sweepable midrange at about 6dB cut would be the bees knees, but now I'm dreaming I think.  

EDIT:  One other thing,  I'm noticing that my heater voltage is about 5.3V each tube.  I'm also noticing that when I swap out a tube, the voltage on the non-swapped tube stays higher, like around 6.5V until the swapped tube heats up.  During this time, the tube with the higher voltage sounds MARKEDLY better - Much less distortion, more clarity, etc.  It's not subtle, the improvement is huge.  I can then noticeably hear it fade away though as it's heater voltage re-stabilizes at 5.3V.
This is the same transformer I was using when I had the short for two days, and it got extremely hot.  Is it possible I cooked it?  What can I do to get that voltage back up in the 6 to 6.3 specified voltage range?  (I have a new transformer ready to go, but was going to return it - should I try it instead??)

Comments / suggestions?