Silverface Fender Pro Reverb Problems- Rectifiers- Tubes

[caveing]

okay... so I got the 5u4gb in there. Sounds MUCH better. Replaced the resistors on the input jacks with 47k (that's all I had- is this okay or should I put the stock 68k's in there?), the dirt sounds MUCH better, but it still isnt quite there. I actually like plugging into the 2nd input and cranking it up louder.

My plate voltage is now at 455v (i dont have the best multi meter though) I have an RCA 5751 blackplate triple mica in v2, a NOS 12at7 in reverb driver, 12at7 in PI, mullard RI in reverb recovery... I do have older JJ 6L6GC's- could some newer nicer tubes get me the dirt I need, or is it more likely the voltage, or something else?

Would it be much difference to drop another 20v and get closer to the 437v listed on the schematic? It's still not sounding quite right.

I also still have the crackling static going on in the reverb channel only. It's not as bad as it was with the higher plate voltage- and it even stopped for a while after the amp was on for 20 minutes and then became quieter and sporadic. I really cant find the source of it. Been resoldering joints all over the place.

I feel like lowering the preamp gain somewhere could really help me out. I like the 2nd input much more than the first.

Thanks so much for the help to here- I'm getting really close thanks to you guys!

[Structo]

You mention the power tube plate voltage but not what the amp is biased at.

What are your preamp tube plate voltages?
The AA1069 schematic I have shows 230v on the Normal channel first triode.
I would shoot for around between 200-230v for optimum performance.

Do you have a 12AT7 in the PI socket?

Usually if it isn't a tube making the crackling noise it is a intermittent or poor connection and if it is heat related something may be expanding until it makes better connection.
Check the grounds especially.

Be sure to check the tension of the tube sockets, if a tube pulls very easily out of a socket, the pins may not be making good enough connection.

You can try to re-tension the sockets with a small tool but be sure the caps are discharged first.

What blackface amp did you use your values from?

The amp should not really be gainy, these amps are more known for their clean tones unless you dime it.

[caveing]

So... last night I had the amp opened up and on- poking around with my wooden and rubber stick... heh. And I couldnt not find hardly anything that made noise when touched! I mean some wires made a little noise moving them around, but nothing like a loose component noise... Then I started poking around the back control panel. And I poked the death cap that is still in the amp (have three prong cord)... and the crackling stopped!!! I havent had a chance to leave it on and play with it. But is that possible that that stupid cap was the problem??? I dont see how, but... anyways....

This topic is getting really long so I dont mind repeating this...

this is early SF non master volume pro reverb (1973)... amp has been completely revamped with pure blackface values... all new high voltage caps, prp and dale metal films throughout . The power tx of these early SF amps has the same exact model number as the blackface pro reverb but they are actually different specs. the Sf puts out more voltage (or something, sorry still dont know much about transformers).

So I have the 5u4 in there and the plate voltage is at aibout 445v... Ive tried biasing many times over with different rectifiers, I usually end up a tiny bit over 60% at about 61-63%% i suppose... it gets way too gainy at 70%... right now that means i'm doing about 42ma.

I'm running 5751 in v2, 12at7 in reverb driver and PI, 12ax7's everywhere else. I upped the 470ohm power tube resistors to 1k. I've also changed the bass and mids tone caps to .022uf on the vibrato channel. the input resistors on the vibrato channel are also 47k (i originally had a 2k in there, then i decided to go back up but I didnt have 68k) I am using the 2nd input out of favor for the more padded less preamp gainy higher resistance. I have a very high headroom speaker in there (weber california) and even a 12at7 in v2, i still get some crunch at much higher volumes than typical with these amps.

It's crazy really, back when I first owned this amp (several years ago when I didnt know ANYTHING about amps) it originally had sovtek 5881's and a 5ar4 rectifier tube when I first got it. I couldnt get a smidgen of dirt out of it with a les paul! I'm talking twin headroom. AND, when I replaced all these components I took out 130k's in the plate resistor positions! what the hell! Everything else was stock values as far as I could tell except for a few cathode bypass caps... some were 25uf and one or two were 4.7uf. the slope resistors were 100k IIRC. Could this all be chalked up to dying power tubes? the power tubes and about 5 resistors in the tremolo are the only things that are not brand new in this amp!

I havent measured the preamp voltages since I've installed the 5u4, I'll get back tonight with those numbers.

[caveing]

Ok... I had never tried to measure voltages anywhere but the plates in an amp. Wish I wasnt an idiot and measured them sooner.

So...
V1 -pin1=265v---pin6=277v, pin3=1.86v, pin8=2.2v
v2 pin1=239v, ping6=229v, pin3=2.3v
v3 ppin8=8.2v, pin1= 437v
v4 pin1=266v, pin6=385v
v5 pin1=434v, pin6=239v
PI- pin1=246v, pin6=238v

I also forgot that i have a .022uf capacitor instead of stock .1uf coming off pin6 of the reverb recovery tube (v4)... I dont know why i did this... Or I cant remember at least.

I thought I had the crackle fixed, but its back... well, it's intermittent And my plate voltages are down again

All these measurements were taken with a plate voltage of 442v and bias at 41ma- which is 60%


So it looks like my preamp voltages are still really high in places. I dont know what to do about this at all!

I do only have 400v rated capacitors for the mid and bass tone caps in normal and vibrato channel

anyone got any ideas???
Thanks so much for all the help guys! The amp is sounding better, but I definitely feel like the preamp tubes (as I thought int he beginning- just based on what the amp sounds like) are too dirty for a typical fender amp!

[caveing]

Also... thought it might be worth to note that the amp had some snubbing capacitors in it that i've left because i think the amp is plenty bright as is. There is a cap across the two phase inverter coupling capacitors across the 82k and 100k resistors. it was something like 220pf ceramic and I reduced it to a 100pf polystyrene (i may be wrong on that original value- when I took that off I was getting some extra noise and it was really just shrill souding with the new tantulum caps- those tantulum caps really are EXTREMELY bright and harsh when they're new- they have really cooled down after some burn in time and this has been confirmed by many dudes over at rukind that do this regularly as part of the fender twin reverb jerry garcia mod)



I digress... There is also a .0022uf (i think that's the value it says .002 on it) cap on the reverb output rca jack to ground. If I'm thinking correctly this would take lows out of the reverb sound and act as a hipass filter... am i correct in thinking this? I'd like more lows in my reverb sound, could I raise the cap and let more lows through?

Also about the .022uf cap off pin 6 of v4 that i subbed for a 0.1uf listed by the schematic. I did this because the twin has this value.. I didnt even realize that my pro had a different value listed on the scheme.

What exactly would be the effect of having a .022uf instead of the .1uf?

Thanks again, and sorry it takes me so many words to get my points across!

[Firestorm]

v4 pin1=266v, pin6=385v

Something is strange with the mix amp side of this tube (pins 8,7,6). The plate voltage should be nearly identical to the other side, so either the tube is shot or you have a load resistor considerably smaller than the stock 100K. It's also slightly possible that you're getting DC on the grid. Might check for that.

So it looks like my preamp voltages are still really high in places. I dont know what to do about this at all!

V4 pin 6 is the one that looks out of line; the others seem like reasonable Fender numbers. High voltages on preamp plates won't make the amp any dirtier, per se (the opposite, actually), but something screwy is going on in V4.

[Firestorm]

There is also a .0022uf (i think that's the value it says .002 on it) cap on the reverb output rca jack to ground. If I'm thinking correctly this would take lows out of the reverb sound and act as a hipass filter... am i correct in thinking this? I'd like more lows in my reverb sound, could I raise the cap and let more lows through?

No, this cap bleeds off highs (and upper mids). If anything, I'd make it smaller (like 250-500pF). There's a .003 you could raise to push the frequency a little lower, but if you go much above .005, it will stop sounding like reverb and become something else.

Also about the .022uf cap off pin 6 of v4 that i subbed for a 0.1uf listed by the schematic. I did this because the twin has this value.. I didnt even realize that my pro had a different value listed on the scheme.

What exactly would be the effect of having a .022uf instead of the .1uf?

I've never seen a Fender reverb amp use anything but .1 here (unless maybe the "evil Twin"?). .02 will just kill a little of the bass response.

[caveing]

Yeh i'm digging the sound of the bass on this amp... super spanky... cuts through the mix very nicely. But i'm gonna go ahead and put the .1uf in there just cuz

that's crazy that the .002 cap on the reverb jack is cutting lows... my reverb sounds pretty trebley.

i've also always thought my reverb recovery tube was getting hit too hard... doing the microphonic tube "tap test" you can hear the tapping very easily in v4... it used to not be like that iirc.

so could this one voltage be the source of my super gainy pro reverb? I've measured all the resistors, and they all seem perfect! I have even taken quite a few out to get correct out of circuit measurments!

I'm sure this isnt the problem- but i've heard decoupling the shared cathode bypass cap between v1 and v2 is a good idea on these amps. should i try that for the helluvit?

Thanks guys!

[caveing]

v4 pin1=266v, pin6=385v

Something is strange with the mix amp side of this tube (pins 8,7,6). The plate voltage should be nearly identical to the other side, so either the tube is shot or you have a load resistor considerably smaller than the stock 100K. It's also slightly possible that you're getting DC on the grid. Might check for that.

So it looks like my preamp voltages are still really high in places. I dont know what to do about this at all!

V4 pin 6 is the one that looks out of line; the others seem like reasonable Fender numbers. High voltages on preamp plates won't make the amp any dirtier, per se (the opposite, actually), but something screwy is going on in V4.

what about the 2.2v and 2.3v arent considered high?

All my plate resistors are fine... and if the tube was shot shouldnt my reverb not be working?

I ended up punching the amp last night a few times before i turned it off. after 20 minutes of playing with the crackling just getting unbearably loud. I just slapped it on the top a few times... and then, today I come home from work and flip it on.... DEAD QUIET- I couldnt tell a difference between the hum cancelling positions on my strat and the single pickup positions! seriously. I just played the amp for an hour straight... no crackling, no noise, quiet as could possibly be! I had it on 5 and it would just slightly breakup when I switched my strat to series combos. loud as hell at 5 too, but not cold at all. Just a beautiful sound... exactly everything I've wanted! out of nowhere..... I ended up turning the switch to take the tremolo completely out of the circuit and plugging into my second input (which is between a normal fender 1st and 2nd input)- damn good stuff.
I actually had one of those glorious moments of guitar and amp connection that happens only every so often!

I turned off the amp... and DAMN! that switch was hot!!! The chassis was hot enough to burn you anywhere near the power side. I've never felt it this hot... I mean I put my fingers on there for a couple seconds and i didnt get like actual burns, but it hurt and I have calloused cooking hands and fingerpicking right hand finger tips.

I've noticed its been hotter since the 5u4 install. I guess that's a byproduct of it absorbing all that voltage? Is this something I should be worried about? Maybe a sign that my power tranny is going out?

Today was a beautiful playing day... but damn this thing is driving me crazy! I'm almost positive that if I turn it on again soon it will crackle and pop like a mofo

[Gibsonman63]

A few post ago you mentioned that the death cap was giving you some grief. Why not just clip it out?

This may be completely unrelated, but I now carry an outlet checker in my gig bag. You would be suprised at how many outlets are wired incorrectly.

[NickC]

.............. I now carry an outlet checker in my gig bag. You would be surprised at how many outlets are wired incorrectly.

+1 Me too, for decades now.

Excellent advice! It's shocking how many outlets are dangerously wired. I have repaired quite a few in my time. I make sure the club-owner knows I made it right. Just a little "value added" service that more-often-than-not will get you hired again.

[Firestorm]

what about the 2.2v and 2.3v arent considered high?

Most schematics show 1.9, but that's not intended to be precise. Cathode voltages are affected by how much current the tube wants to draw and tubes are all different. You could sort though two dozen before you found one that produced 1.9. Not critical, especially since none of these gain stages clip.

All my plate resistors are fine... and if the tube was shot shouldnt my reverb not be working?

Reverb recovery is the other side. 12AX7 is two tubes in one envelope. One side can work perfectly while the other side is completely dead. Try swapping a known good tube in V4 and recheck the voltages there. Also check for DC on the grids.

I turned off the amp... and DAMN! that switch was hot!!! The chassis was hot enough to burn you anywhere near the power side. I've never felt it this hot... I mean I put my fingers on there for a couple seconds and i didnt get like actual burns, but it hurt and I have calloused cooking hands and fingerpicking right hand finger tips.

I've noticed its been hotter since the 5u4 install. I guess that's a byproduct of it absorbing all that voltage? Is this something I should be worried about? Maybe a sign that my power tranny is going out?

Was the switch hotter than the rest of the chassis? Or the same. Heat on that side is almost all tubes; just heat rising up from the tube and heating the chassis. 5U4 is just adding to it. But now that you mention the switch, these things do get dirty and don't make perfect contact internally. Standby switch especially is under a lot of stress. When that one is dirty, it can make crackling noises. You can sometimes get contact cleaner inside (amp off and drained), but replacing the switch is easy enough.

[Structo]

Yes, Firestorm touched on something important.

Toggle switches don't like to switch high DC voltage.

Eventually the contacts arc and even when the switch is closed it can do some funky things if it is damaged.
I would replace that with a new Carling switch.

[caveing]

Yes, Firestorm touched on something important.

Toggle switches don't like to switch high DC voltage.

Eventually the contacts arc and even when the switch is closed it can do some funky things if it is damaged.
I would replace that with a new Carling switch.

Man... I remember reading this a while back... I'm almost sure this is it now... It pretty much has to be... I tried cleaning it a few weeks ago and forgot about it. Thanks for the reminder- Let's cross some fingers.

Should I just replace the standby? would the power switch possibly be the problem?

And from above... how do you correct a miswired outlet? Having a 3 prong cord installed... isnt my death cap useless at this point?

This may not be a problem just something of note... since installing the 5u4 rectifier I've noticed the amp turns on differently. Notably- more noisy and intensely... it makes a much louder sound when i flip the standby on. the power tubes also get brighter and bluer when i flip the standby on. I always let the amp warm up at least a minute for the hell of it.

I'll try some new tubes on this reverb recovery spot tomorrow. hopefully it's just a tube

[caveing]

and i completely forgot...
the normal channel doesnt crackle and pop at all! could the standby switch cause the vibrato channel to crackle while leaving the normal channel unscathed...

and when i say the normal channel doesnt crackle and pop... i am talking about when i take out all the preamp tubes but v1 and the PI- then the crackle disappears. I dont really know how arcing works, but that would prevent the standby switch from being the source of the problem, right?