ab763 reverb squeal and more

[nateflanigan]

Hey everyone, I fired up my DRRI to "super-vibro-man" build this weekend.  For the most part things are good, but their are a few issues I could use a hand with.

1) If I turn the reverb knob past about 3/4 the amp kicks into what I'm going to call self oscillation.  It's a high pitched squeal that over powers the guitar signal.  I'll I'll quadruple check the wiring and parts placement in that section, but in the mean time assuming all that is correct, what might be the cause and how might I fix it.

2) I've got some hum.  Not over the top, even what I'd consider acceptable for jamming with friends but a bit much for studio use.  Before I go at it with chopsticks if we could narrow the field a little bit that would help. 
     -with the volume down all the way on both channels the hum doesn't change.
     -If I put the amp in stand by the hum goes away.
     -I followed Doug's grounding scheme, I think I got it all correct, but I'll check again.
     -It's hum not hiss

Thanks!

[Tiny_Daddy]

Does it squeal with the reverb tank disconnected?

[Fresh_Start]

Nate asks a good question.  If it squeals with the tank disconnected, the problem is in or near the recovery stage.

I had greater or lesser versions of the reverb squealing with all three Blackface amps I've built.  Here are some suggestions:
1 - shielded cables going to and from the Reverb pot
2 - carefully review every mm of wire going to and from the reverb driver tube and reverb recovery tube.  Keep plate leads flat on chassis and sensitive grid leads (and reverb return cable) away from everything else.  Remember you have 3 dimensions to work with
3 - make sure that the "return" end of the reverb tank is far away from the PT
4 - experiment with reverb cable placement.  Inside the tank bag, I run one cable on each side so they aren't touching.  I use one or more pairs of cable anchors on the side of the cab to keep the cables parallel but separated as they go up toward the chassis
5 - some guys report success insulating one or both of the reverb jacks (SEARCH here to find recent discussion)
6 - find a quiet tube for the reverb recovery stage.  New EH 12AX7s have worked for me.  YMMV
7 - try different 12AT7 reverb driver tubes.  In one amp, it was the 4th 12AT7 that worked quietly
8 - if you have more than enough reverb at "5" on the dial, think about reducing gain from the recovery stage by removing the cathode bypass cap

None of these may solve the problem but IMHO they're worth trying, one at a time.

HTH

Chip

[nateflanigan]

Does it squeal with the reverb tank disconnected?

Yes, much worse.  It went from a low squeal to a freak out squeal.

[sluckey]

Do you have a 220K resistor between grid and ground for the recovery tube? Usually mounted on the reverb return or footswitch jack?

[nateflanigan]

Do you have a 220K resistor between grid and ground for the recovery tube? Usually mounted on the reverb return or footswitch jack?

Yes, I do.

[sluckey]

Does the squeal go away if you put a ground on the reverb footswitch jack?

[nateflanigan]

Does the squeal go away if you put a ground on the reverb footswitch jack?

I'm not sure what you mean exactly, so I turned up the verb until it squealed then alligator clipped from the terminal on the footswitch jack that controls the reverb to ground and the squeal stopped.  Of course the reverb did as well.  What does that mean?

[nateflanigan]

Fresh Start, I've been moving around some wires and have been able to get another notch or two out of it.  Implementing the shielded cables is going to take a bit more time.  Thank you for the tips!

[sluckey]

Does the squeal go away if you put a ground on the reverb footswitch jack?

I'm not sure what you mean exactly, so I turned up the verb until it squealed then alligator clipped from the terminal on the footswitch jack that controls the reverb to ground and the squeal stopped.  Of course the reverb did as well.  What does that mean?

Yes, that's what I meant. May be a clue. Do you have a footswitch plugged into the jacks? If so, unplug. Still got squeal?

If you still get squeal, then use your ohmeter to measure resistance between the recovery tube grid (directly on the tube socket pin) and chassis ground with the reverb pan disconnected. Is it 220KΩ?

[nateflanigan]

I haven't been using the foot switch.  Measuring from grid to ground I'm getting a solid 229k. 

Thank you for the help! 

[nateflanigan]

another clue...

I sat down to do some playing and forgot I had the reverb cables unplugged, when I turned the amp back on it was super hot, the reverb channel stated squealing instantly.  Even after I turned the reverb down both vibrato and normal channels were really hot, meaning louder than I left them and really gainy.  If I picked with any strength it was like distortion pedal saturated.  That's gotta be a clue to something, right?

[sluckey]

Disconnect the NFB and see how it sounds.

[phsyconoodler]

I just had a silverface super in with the same issue.I tried everything including shielded cables,ne reverb transformer,cables,tank,lead dress and replaced every reverb component on the board and socket,tubes,etc..
  It still squeals.
 Now sluckey mentions the negative feedback and that gets me thinking.Hmmmmmm........
Why would that affect the reverb,unless somehow the phase issue is causing the oscillation?

[sluckey]

Why would that affect the reverb,unless somehow the phase issue is causing the oscillation?

Notice that we now have more symptoms to go by. Both channels are very gainy. Disconnecting the NFB is easy to do and will quickly tell if that is the problem.

My thoughts about the squeal showing up in the reverb channel first is that channel has an extra gain stage and might get upset quicker than the normal channel. It's also possible that Nate jumped on the first symptom he noticed. Since both channels now show symptoms that 'could' be NFB related, I think disconnecting the NFB is a good next step.

On any new build involving NFB, especially when working with an unknown OT, it's a good idea to eliminate wrong phase NFB early on, unless the amp just sounds perfect.

[nateflanigan]

Well doctors, I set the vib channel to a reasonable volume, turned up the verb to squeal then backed of a hair.  As soon as I disconnected the NFB the squeal kicked in.  I had to turn the reverb down quite a bit to stop the squeal.  Another thing I noticed, with the amp set on the verge of squealing if I turn DOWN the treble it brings about that wretched noise.

Also, to clarify, the normal channel never squealed, it was just really gainy and freaked out sounding when I disconnected the reverb cables.

Incidentally, aside from the reverb issue, disconnecting the NFB is the best boost pedal I've ever heard.  Once I get this all sorted I'll have to work on a footswitch for that.

I'm going to go put shielded cables on the verb pot now, aside from that, what's my next move?

[nateflanigan]

Almost forgot...

I'm using mojo "super reverb" transformers.  The plate voltage is 450v and the bias is at 31ma.

[sluckey]

OK, it's not a NFB issue.

[nateflanigan]

PROBLEM SOLVED! 

In addition to the shielded cables on the reverb pot, I used shielded cable from the return jack to the recovery tube.  I can turn the reverb all the way up without any problems. 

Thank you so much guys

Now, I swear to god the amp got hairier.  Not noisier, but it's like crunchier, and there's some distortion as a chord rings out.  Not something I'm going to get involved in tonight but maybe I'll try some different guitars and speakers and tweak with the bias a bit.

again, thank you.

[Fresh_Start]

Nate - are the V1b and V2b triodes sharing a cathode resistor?  The DRRI uses a shared 820 ohm cathode resistor for those two triodes just like Leo did.  However, if you have a separate cathode resistor for each triode, they need to be 1.5K to get vintage levels of gain.

Only mention it because it came up in another recent discussion about converting DRRI amps.

Cheers,

Chip

[nateflanigan]

separate 1.5k's a la the hoffman layout. 

[Fresh_Start]

separate 1.5k's a la the hoffman layout. 

Time to check resistor values!  3.3 meg reverb mix resistor, preamp plate resistors, cathode resistors, etc.  The bad part is that sometimes it's really hard to figure out what your DMM should show for resistance in a circuit.  Look at color bands first.  Tedious?  Sure!  But it's a brute force way to make sure everything's what it's supposed to be.

Chip

[nateflanigan]

Ok, got some changes to report, and a few more questions.

I put in a 50w speaker instead of the 25w I had, definitely an improvement.  Then cooled of the bias a bit more, that really cleaned up the hairiness.  The normal channel sounds just right now.  Although I noticed one tube is reading 21v and the other is 14.5v, is that a problem?  It sounds great so I'm not super worried.

The reverb channel is still too gainy/crunchy for my tastes.  Not bad sounding, but I can't get a good clean sound at a volume that would work with a band.

I checked every resistor and cap.  Everything's where it should be.  Tomorrow I'll double check the wiring. 
I'm thinking I should try some different tubes.  Right now there's tung-sol's in the preamps, jj ecc81's in the verb diver and PI, EH 12ax7's in the other slots.  I don't have any spare 12at7's on hand so I'll have to order some. 

[Fresh_Start]

Don't understand what you mean by tube voltages above.

I've found a couple of really hot Tung Sol 12AX7s - fun in the right place but maybe you've got one where you don't want it.

Chip

[nateflanigan]

I noticed one tube is reading 21v and the other is 14.5v

my mistake I meant miliamps, not volts, for the bias readings. 

[Fresh_Start]

I noticed one tube is reading 21v and the other is 14.5v

my mistake I meant miliamps, not volts, for the bias readings. 

Even 21ma is running a bit cool IMHO for a 6V6.  14.5 is really cold.  Neither channel should be "gainy/crunchy", especially with the bias set that cold. 

I'm not a fan of tube matching, but you've got a 30% differential there.  Have you tried swapping the power tube spots to see if the difference is the tubes or something else?

Chip

[nateflanigan]

Good call, I didn't think of that.  They're new Sovtek tubes from the tubestore that we're sold as a matched pair.  Not something I would usually care about but the difference seems unusual.  I'll swap the tubes around tonight and report back.

 

[nateflanigan]

Swapped the tubes, the bias followed the tubes.  I don't have any other 6l6's around but I do have some 6v6's I can try, just to see what kind of numbers show up.

[Fresh_Start]

Swapped the tubes, the bias followed the tubes.  I don't have any other 6l6's around but I do have some 6v6's I can try, just to see what kind of numbers show up.

Nate - those cathode current measures were really low for 6V6s.  No wonder the amp sounds funky with 6L6s!

6L6 data sheet:  http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6L6GC.pdf

Cathode current * Plate voltage = (70% * 30 Watts)

Assuming plate voltage of 400:  0.021 volts * 400 = 8.4 watts

(70% * 30 watts) / Plate Voltage = cathode current goal

21 / 400 = 52.5ma (just an example)

Your goal for cathode current is a function of plate voltage, and that will change every time you adjust the bias.  My guess is that you started chasing it and just went the wrong direction.  Easy enough to do!  Either your bias voltage is way to high (in absolute value; IOW bigger negative number) or something else is wrong.

Also, you can't just plug 6V6s in there.  The maximum plate dissipation drops to 12 watts (or arguably 14 watts).  The lower current draw of the tubes will let B+ jump upward.  Whole new set of calculations.

This may help:  http://www.geofex.com/tubeampfaq/taffram.htm

Cheers,

Chip

[nateflanigan]

Thanks for the heads up. 

I started somewhere around 35ma, and brought it down from there.  There was another thread about getting a deeper tremelo, some one was talking about reducing the bias to 17ma working for them, I was futzing around with that.

I added the mid pot to the reverb channel, that cleared up my crunch issues.   

Tomorrow I'll bring the bias up to the normal range, and see how things sound.

[Fresh_Start]

Thanks for the heads up. 

I started somewhere around 35ma, and brought it down from there.  There was another thread about getting a deeper tremelo, some one was talking about reducing the bias to 17ma working for them, I was futzing around with that.

I added the mid pot to the reverb channel, that cleared up my crunch issues.   

Tomorrow I'll bring the bias up to the normal range, and see how things sound.

I'll bet you grounded the Vibrato channel's tone stack for the first time when you added the Mid pot.  It may be compulsive of me, but one of the last things I do before first power-up is to go through the schematic and check every single ground connection with the DMM.  It won't necessarily turn up a cold solder joint, but it's saved me some time more than once.

The discussion about reducing bias to 17ma for the tremolo had to be in the context of 6V6 power tubes.  My AB763 build with 6L6s settled in happily with 43-48ma on the power tube cathodes plus 470 volts on the plates +/- and there's more than enough tremolo.  (Voltages HERE)  I DID make a couple of the changes recommended in the Archives for the bias vary tremolo with 6L6s though.

Chip

[nateflanigan]

Thanks for the chart, I'll make my own for comparison. 

I have some bad news (for me).  I brought my amp over to a friends studio to compare my amp to his stock DRRI.  The results were really weird...

I suppose I should focus on the negative...

-My amp was much quieter than the original, which is really weird since I added 6l6's and the mojo OT for super/vibro/bassman
-My amp is still pretty dirty sounding in comparison, maybe it's a taste thing, doesn't seem right.
-My amp has a low frequency problem, I can't turn the bass up on either channel (especially the reverb) past 3.  It just get's really dark, muddy, and distorted. 

It's not like the amp is horrible, it makes some nice sounds.  I was pretty pleased with myself, it's just that in comparison there's definitely some issues.  We tried another set of tubes from a bandmaster he had, tried my amp into his deluxe's speaker as well as the 212 bandmaster cab.  Those experiments changed the tone a bit but not the nature of the problems.

I've gone over the circuit a skillion times and I'll do it a skillion more, but I must be overlooking the same things every time.  So maybe if I take a step back, I'm using the ab763 board, with two 6l6's, and super reverb transformers, into a 50w weber 12, in a deluxe cab, any red flags in that set up?

thanks for your time!
 

[Geezer]

any red flags in that set up?

No, just the old Hoffman axiom..... "If were wired right, it would be working"

Double check every resistor (like a 470k where a 470R should be), have you swapped the OT primary wires(?) (disconnect the NFB resistor on one end....any improvement?) You're going to have to divide & conquer

Maybe with some good hi-res pics of the guts, someone here might be able to spot something.....

[nateflanigan]

Here's a link to my voltage chart

http://www.el34world.com/forms/valvedata/looks%20ok.htm

also I have 429v coming off the rectifier

went through everything again, sadly I found nothing.  Playing the amp at home it didn't seem as bassy, but it's still really dirty.  Which isn't all bad, it's a really cranked rock out sound at reasonable volumes but it's also not what I was going for. 

I'm going to take a break for a while, then I'll go through and check for continuity at every connection, and take some photo's.

(disconnect the NFB resistor on one end....any improvement?) You're going to have to divide & conquer

I have the nfb on a switch right now taking it out is a cool effect for more distortion but doesn't help anything.

[nateflanigan]

Not much has changed but since I've tried a few things and made some observations I thought I'd post ,maybe something will click with some one.

-I've added the mid pot in place of the #2 jack, if I set it so that I'm measuring 6.8k from wiper to ground that's when the amp is really dirty, if I back it off to like 1.5k or less the amp cleans up.  This is all with the volume around 3-4, it's a decent volume for playing at home but that's about it.

-I swapped out the 6l6's for 6v6's and rebiased, the bias difference between the tubes was less extreme (one at 17ma one at 16ma).  The volume of the amp is about the same, still really early break up though.  However the noise floor of the amp is much quieter.  I decided to try the swap based on...

Q:  You said in your book The Ultimate Tone that you can put 6V6s in a Twin. Are you crazy?

A: No, I'm not crazy. If you carefully study the conditions for this substitution, you will realize how safe this swap is. In TUT our advice was that if the plate voltage is less that 450V, the 6V6 would be okay. We revised this in TUT3, to a limit of 500V, since the 6V6 is rated for 1200V environments in TV circuits. The flash-over point for the tube is just above this.

The 'V' has just over half the transconductance of the 'L', so it tends to draw a proportionate idle current, and yield about half as much power. Any 80W or 100W Twin will easily accept 6V6s. Note that the Fender Deluxe Reverb has a V+ of 420V and bias level of -37V, the same as most 100W Twins.

I'm confused about the -37 bias level so I stuck with the 17ma I've seen elsewhere, at least that number makes sense with the bias equations I'm familiar with.

[Fresh_Start]

I'm confused about the -37 bias level so I stuck with the 17ma I've seen elsewhere, at least that number makes sense with the bias equations I'm familiar with.

-37 is the bias voltage measured on pin 5 of each power tube.

I don't know where the 17ma figure comes from but it does not compute for me.  Conservatively:
6V6 max plate dissipation = 12 Watts
70% = 8.4 watts
Plate voltage * cathode current = 8.4 watts
440 * .019ma = 8.4

Now, assume 14 Watts rating on a 6V6:
http://www.mif.pg.gda.pl/homepages/frank/sheets/135/6/6V6GTA.pdf

14 watts * 70% = 9.8 watts plate dissipation
Add one watt for the screen grid - its current is part of your total cathode current and max dissipation is 2 watts => 10.9 watts
440 plate voltage * 24.7ma = 10.9 watts

You are measuring millivolts dropping across a 1 ohm resistor on the cathode.  In this special case millivolts = milliamps. Ohms Law:
I = V / R

Sorry for beating a dead horse, but this amp seems to be biased too cold to me - both with the 6L6s earlier and now with the 6V6s.  That's math, although my 24.7ma for 6V6s might be considered a little bit aggressive since I adjusted for screen grid current too.  However, that's also about the same plate dissipation that sounded good to me (and some other guys) in my Princeton Reverb builds.

Have you double checked the value of each of the pots in the Vibrato channel tone stack?  Just a wild guess, but your voltages look OK.  Something has to be off though.  Have you measured the value of the 3.3meg reverb mixing resistor?

Argh!  Wish I could help here,

Chip

[nateflanigan]

it came from this...
http://www.diyguitarist.com/Images/BiasChart-6V6.jpg

But your right, I rebiased for 25ma.  It sounds better but that's not what's causing the problems.

Argh!  Wish I could help here,

You are, A LOT!  Even if you don't have the magic answer, just being able to get this stuff out of my head and bounce around ideas with some one is a huge help.  I really think I have the board populated correctly, I've been over it so many times, I think the issue might be in something I'm miss understanding with the wiring.  Being that the tone stack seems to have a big effect on the distortion levels I did go over the pots, I can't see the designations so I measured across the outside lugs.  Everything looked ok.

I have another friend with another studio who has a SF deluxe and SF bassman, Is there a danger in pulling out the chassis from say his bassman and jumpering the output from my PI to his power amp section and vice versa to try and isolate which sections are problematic? 

[Danskman]

 jumpering the output from my PI to his power amp section and vice versa  

I would never do this. But you can open the SF Deluxe and get an idea about how to wire a Fender amp to get it working flawlessly. OK, you can find it can appear as a mess, but these amps are working for years. You didn't gave us pics of your chassis's guts, so if you are SURE about components placement and value, tubes characteristics, tubes sockets, voltage, pots and inputs wiring, it's becoming harder and harder to help you further... you got very good advices from other forum members; I cannot see anything to add to help you, unfortunately 
Hope you'll get it soon!
BR,
Danskman

[sluckey]

RE your voltage chart. V4-7 voltage can't be right. V4-6 voltage doesn't look right either. Recheck both of those. Also check that the plate resistor connected to V4-6 is really 100K. I'm guessing that V4-3 and V4-8 are tied together and have a 820 ohm resistor to ground?

At this point, we really need to see some hi rez pics of your build. We need to be able to read component values and trace wiring. Your exact schematic and/or layout would be helpful too.

Jumpering the preamp of one amp to the power amp of another amp can be very useful. A better way to do this would be to jump the preamp to the INPUT CAP for the PI using shielded wire. Be careful and understand what you're doing.

[nateflanigan]

Thank you. 

I cannot see anything to add to help you, unfortunately  sad

It's not unfortunate, really, I'm totally grateful just to be working through this as a conversation.  I'm trying to borrow a better camera, mine is so so.  I don't really have much time the next few days.  But I'll confirm Sluckey's observations and get some photo's up as soon as I can

Your exact schematic and/or layout would be helpful too.

The hoffman ab763 layout.

[nateflanigan]

RE your voltage chart. V4-7 voltage can't be right. V4-6 voltage doesn't look right either. Recheck both of those

My mistake, v4-6 is 268v and v4-7 is .1 mv.

Also check that the plate resistor connected to V4-6 is really 100K.

Check

I'm guessing that V4-3 and V4-8 are tied together and have a 820 ohm resistor to ground?

No, they have separate 1.5k resistors to ground, bypassed by a 22uf cap

At this point, we really need to see some hi rez pics of your build

http://picasaweb.google.com/109914091355686420139/Ab763#

The pictures starts at the normal channel looking at the front of the chassis and move to the right.  Then the board is shot from the back of the chassis starting with the normal channel and moving left towards the bias section.  Then it's the tubes in numerical order.  I don't know if this covers everything well enough or not.  If there's something specific any one would like to see, just ask.

Thanks for taking the time to help me with this!

[nateflanigan]

thought I had it...

I got some new 12at7's (one of them didn't even work) from banjoworld, put a mesa 12at7 in the PI things sounded clean, it was late, my girlfriend was annoyed so I figured I'd live to fight another day.  My replacement 6l6 from thetubestore showed up.  So, I put those in, rebiased and plugged in, sounds like a distortion pedal again.  I switched back to 6v6's and I've tried every tube I have in every position. 

I got nothing.

I think I'll pick up another new 12at7 and see what happens.  After that I'm in the mindset to tear it all down and start over.

bah-humbug

[Fresh_Start]

Dear Mr. Scrooge,

Please do not give up!  You need to figure out what is wrong with this build before tackling another build.  

Divide and Conquer! You may end up re-building the amp one section at a time, but that's OK.  This is NOT a trivial circuit. 

Figure out what IS working first.  Disconnect the tremolo from the bias supply.  Disconnect the negative feedback.  Pull the reverb driver tube, disconnect the reverb tank, and ground the grid of the reverb recovery triode.

Do you have another tube amp?  Find 4 12AX7s that you KNOW work and sound ok.  Grab a set of 6L6s (preferably Svetlana Winged C, but whatever) because that's what you want in the amp, right?  The voltages looked OK in your chart, except I still think you're biasing it too cold (cathode ma * plate voltage = plate dissipation with target of 21-24 Watts).  Find a 12AT7 that works in the PI position.

What works now?

I didn't want to be critical, but your photos show room for improvement in soldering technique.  You can see that several caps have been melted by the side of the soldering iron.  Some places the tops of the turrets don't look like there's enough solder.  There's a section on soldering in the "References" sticky.  Look at Hoffman's board building instruction pages:  http://www.el34world.com/boardmaker/BOARD3.htm

I am SURE that I re-soldered every single solder joint in my first Princeton Reverb build.  Just because it shows continuity doesn't mean it's a good solder joint.  De-soldering braid is your friend - get some and use it.  Also, carbon comp resistors are more sensitive to over-heating than metal film.  When in doubt, take it out of the circuit (one leg anyway) and measure the resistor's value.

Finally, I still think that jumble of high current wires around the impedance selector shouldn't be anywhere near the reverb jacks and especially the reverb recovery triode.

Sorry if this post is too critical.  It's late and I'm tempted to delete the whole thing, but maybe there's something useful here.

Chip

[nateflanigan]

Sorry if this post is too critical.  It's late and I'm tempted to delete the whole thing, but maybe there's something useful here.

Help is help, it's not like you're calling me a jerk, if something can be done better it's a fact, and a result of my own actions.  I'm cool with that. 

Figure out what IS working first.  Disconnect the tremolo from the bias supply.  Disconnect the negative feedback.  Pull the reverb driver tube, disconnect the reverb tank, and ground the grid of the reverb recovery triode.

will do.  NFB is fine, I'll report back once I've tested the other stuff.  I got roped into helping a friend re-model his kitchen this weekend, so I' don't think I'll have much time.

[John]

Hi Nate, first understand that I know maybe 1% of what most of these guys do. But I do see problems with the soldering also, which might be a big part of your problem? Be sure that you have the iron against the lead and the turret at the same time, and then when you have the joint, remove the iron and the roll of solder at the same time. Should give you a nice peaked, shiny solder joint. Hope this helps! 

[nateflanigan]

After back to back weekends of weddings and house guests I finally got some time to sit down and solder tonight.  I revisited every turret on the board, nothing changed.  I'm guessing you kind folks have done about all you can for me with the information available.  The onlly way I can think of to proceed is to start isolating the different stages by jumpering my pre-amp stages to a different power amp and/or vice versa to try and determine which section isn't working.  I brought this up earlier and I think there was enough support for the idea that it's not out of the question.  Originally I thought I'd borrow a friends SF bassman, but I really don't want to bitch up some one else's amp.  There's an unused PA power amp at my practice space, what issues might there be in wiring that up?  I'd have to double check, but I assume that power amp is expecting to see a line level signal at 600 ohms.

[Fresh_Start]

You really don't need to chop up either amp.  You can build a very simple box that let's you listen to your amp's signal at different points in the circuit.  Scroll down to the bottom here for Hoffman's listening amp box:
http://www.el34world.com/Hoffman/tools.htm

Here's my take on the listening box:
http://www.el34world.com/Forum/index.php?topic=3950.0
I just plug it into a small, solid state amp I don't use anymore and keep in the shop.  Make sure you always turn the volume down to "0" on the box before turning things on though!

Here are Hoffman's tips on tracking down weird noises:
http://www.el34world.com/charts/fenderservice6.htm

Hope that helps some,

Chip

[nateflanigan]

Awesome, that's really helpful just when I thought I'd explored every corner of the hoffman site.  Your box is super slick, that's a really intelligent idea!  I was figuring I'd have to buy a SF bassman or something, do my futzing then sell it once I got my amp working. 

[Fresh_Start]

just when I thought I'd explored every corner of the hoffman site

There's a ton of great stuff tucked away in little corners here and there.  I didn't find the best part of the "Board Manufacturing instructions" for at least a year or two:  http://www.el34world.com/boardmaker/BOARD2.htm

Don't neglect the "Archives" either.  For example, everything you ever wanted to know about fine tuning bias vary tremolo is in there... somewhere.

Just a couple of examples,

Chip

[nateflanigan]

Alrighty, finally got around to setting up the hoffman/fresh_start listening box.  I really can't thank you enough!  It was a very interesting experience tracing the signal through the amp.  Here's what I found...

Normal Channel

Signal sounds great all through the preamp, including the mixing resistor and coupling cap to the PI. 

Vibrato Channel

Lot's of weird issues but let's focus the other stuff first. 

Phase Inverter

Pin 1 clear with some background crackle, kind of like a crappy cable or input jack, really not too bad at all though.
Pin 2 clear
Pin 3 distortion
Pin 6 like pin 1 but louder, meaning both the signal and the crackle are louder.
Pin 7 distortion, an overall more quiet signal than pin 3/8

1st Power Tube

Pin 3 very distorted
Pin 4 very distorted
Pin 5/6 clean with minimal crackle

2nd Power Tube

Pin 3 very distorted but much louder than pin 3 on the first tube with some clean signal mixed in
Pin 4 very distorted
Pin 5/6 clean with minimal crackle

So, I'm a little confused about the distortion around the PI but since there's a clean signal going into the power tubes maybe that's normal?  Seems like the problem is in the power amp section, first order of business is obviously going over that section with a fine tooth comb.  Which I'll do tomorrow, but in the mean time does anything jump out at any one?  Any voltages I should be revisiting?

If the parts placement and wiring are all ok that would leave a faulty or damaged component, seems difficult to track down, any pointers?