Fender Pro Reverb- not enough clean headroom!

[caveing]

When I first got this amp it had wayyyy too much clean headroom. Like, I couldnt even get it to distort with a les paul without cranking the hell out of it, and then it sounded too sterile and stale.

The amp has recently seemed like the headroom has been steadily decreasing. I've done a lot of work on it, but none of it has seemed to directly affect the amount of breakup. I've replaced nearly every single component in the amp to higher quality stuff with realtively minimal modifcations. It's so weird, I actually did a hot rod mod to the normal channel today- which involved a lot of component swaps and whatnot- inlcuding 180k plates and lowered slope- I even have the normal channel wired into the reverb circuit for the extra gain stage. But there is not a noticeable difference in gain change between the mostly stock vibrato channel adn the modded normal channel. I even have a 5751 tube in the vibrato channel. I'm not saying the normal channel has tons more headroom when i have stock values in it either.

The headroom on this amp is pretty minimal now- t starts breaking up at 3 on the dial which I would say is barely loud enough to piss off neighbors in an apartment. maybe theyd get pissed if it was the middle of the night.

Could this just be power tubes? They are old... but they dont sound bad- like the dirt on this amp soudns pretty darn good after the work I've done.

No matter where I set the bias it breaks up early... and I even feel like the bias doesnt affect my overall tone THAT much... I've started thinking the bias circuit could be messed up. But then again the power tubes have lasted fine for a year and a half.

I have an old set of sovtek 5881 wxts I installed to see if that would change anything. And nope... They broke up sooner as expected. I actually got the amp with these tubes in it a few years ago when the amp had mind blowing amounts of headroom.

I did change the resistors in the filter cap tank to blackface values (from silverface values) I've tried replacing tubes all over the place. It definitely sounds like power tube crunch to me too.

The voltages all seem fine, i've checked resistor values, checked all over for connections an everything- i spent a long time hunting down plate resistors that were casuing a lot of popping noise- and thanks to this forum found em and put new ones in and fixed the problem... Now today I am getting very low volumed popping sounds JUST like the old ones, but quieter- it sounds just like my popping plate resistors sounded before they got really loud! maybe my amp is turning plate resistors microphonic?

I've gone over the amp a billion times and it all checks out-- And the thing is that the amp sounds just fine! it just doesnt have the headroom a pro reverb should have from waht i've experienced in the past. It's got less headroom than princetons I've played.

Could a dying transformer cause this?

Some other things i've noticed
the reverb recovery tube is very sensitive to microphonic tubes. My bias settings dont seem to change the way the amp soudns that much. I cant tell a difference in headroom with a cold bias vs a hot bias. A gz34 rectifier in place of the 5u4 only gives a tiny bit more headroom.

I do have .022uf tone caps and some .022uf coupling caps in the reverb channel.

I was also thinking that maybe one of my cathode capacitors went bad... maybe a coupling cap? i dont know!

any ideas would be greatly appreciated!

[caveing]

and I just measured v5 (the tremolo tube) and i'm getting super high voltages.

pin1 is at 460v
and pin 6 is at 412v

i tried 3 different tubes there and they all came out nearly the same

my voltages coming off the transformers are all normal as far as i can tell- 370v AC which is what the early silverface pro's are supposed to have- the blackfaces had 340V

My plate voltage is still high as well at 460v... which is more than 20v higher than the schematic reads with my amp (5u4g rectifier) I also have 1k resistors on the power tubes instead of the stock 470ohm

and i do believe these plate resistors are beginning to head towards rice krispies town all over again!

Thing about these high voltages on the tremolo tube is that I have a switch that takes the tremolo circuit out of the path (for a gain increase) I dont know whether that takes the tube out of the circuit or not though.

I'd really like to get some of my headroom back! anyone!

[Milkmansound]

sometimes parasitic oscillations can sound like distortion - check lead dress, especially if its a silver face Fender and you were moving stuff around or removing ceramic caps etc

[caveing]

sometimes parasitic oscillations can sound like distortion - check lead dress, especially if its a silver face Fender and you were moving stuff around or removing ceramic caps etc

ill try to move wires around- i've done tons of this, but have only found mild changes in noise levels and whatnot.

but i very sincerely doubt that. unless parasitic oscillations make great sounding highly harmonic distortion. as i said this dirt sounds good. and i am all too familiar with crappy sounding fender dirt.

it might work, but i'm looking for something else hear i'm 99% sure.

[caveing]

sometimes parasitic oscillations can sound like distortion - check lead dress, especially if its a silver face Fender and you were moving stuff around or removing ceramic caps etc

sorry if I came off like a dick there. Could the parasitic oscillation sound like halfway decent overdrive? I mean it really doesnt sound that bad. Though my high notes do have a weird reproduction on them... kinda sitar like... not too bad, wouldnt notice it too much in a full band situation, but if you're listening carefulyl its pretty annoying.

[abelljo]

sometimes parasitic oscillations can sound like distortion - check lead dress, especially if its a silver face Fender and you were moving stuff around or removing ceramic caps etc

sorry if I came off like a dick there. Could the parasitic oscillation sound like halfway decent overdrive? I mean it really doesnt sound that bad. Though my high notes do have a weird reproduction on them... kinda sitar like... not too bad, wouldnt notice it too much in a full band situation, but if you're listening carefulyl its pretty annoying.

It sounds like we might have similar situations. I posted the same thing about my 70's Vibrolux. I've been going crazy trying to find the source of the problem. I posted a sound clip vid on my thread. See if yours sounds like mine and let me know.

https://tubeamparchive.com/viewtopic.php?t=18101

[Colossal]

Though my high notes do have a weird reproduction on them... kinda sitar like... not too bad, wouldnt notice it too much in a full band situation, but if you're listening carefulyl its pretty annoying.

This sounds like "ghost noting".

If you look at the AA763 Vibrolux schematic (and most Fenders), there are four triodes being fed B+ off the last filter cap in the chain. This is marked as +300V on the drawing and both the Normal and Vibrato channel stages are fed from this cap. If this problem existed before you modded the plates, but got worse after the mod, it might be that the issue is with the cap or grounding for that node in the first place. If you read Blencowe for example, he mentions that it is a general no-no to feed that many plates off a single node. The reason is that it can create feedback loops when one triode is pulling current, other nodes in the loop can suffer. The direct result of this can be oscillation caused by a reduction in signal and increase in noise. Your mod to the plate on the normal channel changed its bias point (and the current demand for that node) so I'm wondering if the asymmetrical current demand finally tipped the scales in favor of making the oscillation very noticeable.

The solution would be to set up a separate B+ node of 10k/16uF in parallel with , branching from the node before. Put a diode where each node branches to isolate each. This would give you Bsub1 and Bsub2 B+nodes, each at 300V but with a path to ground for noise, breaking the positive feedback loop. There might be room in the doghouse for piggybacking that extra 10k/16uF. Also, and this is personal opinion, I think the long wire runs to and from the four triodes to the B+ doghouse is suboptimal to begin with, so increases in gain could lead to more noise in the affected nodes.

It's early in the day and I've had one cup of coffee so YMMV, caveat emptor, over and over, amen.

[caveing]

Though my high notes do have a weird reproduction on them... kinda sitar like... not too bad, wouldnt notice it too much in a full band situation, but if you're listening carefulyl its pretty annoying.

This sounds like "ghost noting".

If you look at the AA763 Vibrolux schematic (and most Fenders), there are four triodes being fed B+ off the last filter cap in the chain. This is marked as +300V on the drawing and both the Normal and Vibrato channel stages are fed from this cap. If this problem existed before you modded the plates, but got worse after the mod, it might be that the issue is with the cap or grounding for that node in the first place. If you read Blencowe for example, he mentions that it is a general no-no to feed that many plates off a single node. The reason is that it can create feedback loops when one triode is pulling current, other nodes in the loop can suffer. The direct result of this can be oscillation caused by a reduction in signal and increase in noise. Your mod to the plate on the normal channel changed its bias point (and the current demand for that node) so I'm wondering if the asymmetrical current demand finally tipped the scales in favor of making the oscillation very noticeable.

The solution would be to set up a separate B+ node of 10k/16uF in parallel with , branching from the node before. Put a diode where each node branches to isolate each. This would give you Bsub1 and Bsub2 B+nodes, each at 300V but with a path to ground for noise, breaking the positive feedback loop. There might be room in the doghouse for piggybacking that extra 10k/16uF. Also, and this is personal opinion, I think the long wire runs to and from the four triodes to the B+ doghouse is suboptimal to begin with, so increases in gain could lead to more noise in the affected nodes.

It's early in the day and I've had one cup of coffee so YMMV, caveat emptor, over and over, amen.

Id try this if I could understand exactly what I'm supposed to do here. I think I got it, but not sure about the diode.

It was like this before the mods... just a bit worse with them.
I could just revert back to stock normal channel if that would help? How about disconnectting the norma channel

Its weird because the normal channel with the higher gain mods is actually really close to sounding pretty bad ass

[martin manning]

Caveing, if you want to give this a shot here's an easy mod to test the theory of what Colossal is saying. This splits the power supply for the reverb recover and mixing stage from the preamp gain stages without getting into the doghouse. The electrolytic I show as 22uF, it could be 16, or even 10.

[jjman]

The voltage on the plate of the 1st half of the trem tube should be "high" when the trem is in its default mode (off.) This is because, with no pedal switching it on, the negative bias circuit has its 1st triode turned off. The 2nd triode should be "high" as shown in the schematic.

Sounds like you have preamp distortion which resulted from the changes to the circuit. I don't really notice significant changes in tone/breakup from bias adjustments either.

You would really need a scope to see what's going on. Do you have a scheme showing the changes?

[caveing]

The voltage on the plate of the 1st half of the trem tube should be "high" when the trem is in its default mode (off.) This is because, with no pedal switching it on, the negative bias circuit has its 1st triode turned off. The 2nd triode should be "high" as shown in the schematic.

Sounds like you have preamp distortion which resulted from the changes to the circuit. I don't really notice significant changes in tone/breakup from bias adjustments either.

You would really need a scope to see what's going on. Do you have a scheme showing the changes?

This problem started before I did the mods to the normal channel. Also the normal channel doesnt have the shared cathode anymore. i also get the problem when I have the normal channel preamp tube removed. the mod is just 180k plates, a 2.7k .68uf cathode bypass (instead of shared one) 820ohm shared ressitor was increased to 1.5k, 500pf treble cap, .022uf bass and mids, .022uf coupling cap.

I feel like the problem has continually gotten worse over time... its not as bad as "abelljo's" vibrolux- its cleaner and distortion sounds better. Could this signify a dying output or power transformer?

Also the lead dress is obviously MUCH better on my early silverface amp than abelljo's amp. Most of the wiring is twisted up and wrapped with a ground wire and shoved under the pots. The amp is QUIET. I mean dead quiet too. like as long as i'm standing 10ft away you cannot tell a difference between hum cancelling and non humcancelling positions on my strat.

and "martinmanning"- did you mean to post a link or a graphic with your post?

thanks for the responses guys- I'm gonna revert back to stock or very similar to stock on my normal channel.

As i've said it definitely seems like it's continually getting more and more distorted. like 6 months ago, i couldnt get dirt out of it without the amp being played at ear splitting volumes above 8- and it was terrible sounding clipping. now the clipping sounds much better less fizzy, less farty, nire sustain and roar- i think anyone would agree it sounds better... like very obviously better.

But i dont know- if i could just get a little more info about the mod posted by the guys above I would love to try anything out.

I do have 1k 2 watt resistors on the power tubes (instead of the stock 470ohm) and I did remove the snubbing caps... except for one I left between the phase inverter caps at the junction of each .1uf cap and the 82k and 100k resistor, respectively.

I've had some trouble with these PRP metal film resistors I installed in this amp, maybe some are starting to go bad?

Thanks so much for all the help!

[martin manning]

...and "martinmanning"- did you mean to post a link or a graphic with your post?

It's there, you have to be signed in to see it. Maybe you should post some good pics of the insides of your amp?

[caveing]

...and "martinmanning"- did you mean to post a link or a graphic with your post?

It's there, you have to be signed in to see it. Maybe you should post some good pics of the insides of your amp?

Got it! thanks. Can i try that with a 22uf 350v? I have one of those on hand. or does it have to be 450v? and what about the diode mentioned by collosal?

And what exactly will that do for me? or prove?

Ill post some pics as soon as I can! I could do a youtube video or something too if that would help anyone.

Thanks again!

[martin manning]

A 350V cap should be okay for a quick test, and the diode isn't absolutely necessary. This will test the theory that there is an oscillation caused by feedback through the power supply.

Pictures will give folks here an idea of the state of the lead dress. High-resolution stills are needed; video is probably not going to be much help.

[caveing]

here are some pics of the amp
sorry, dont have a good camera or a very steady hand (it requires more steadiness than anything i have ever attempted- samsung and sprint suck!!!!)

I'll hopefully have a chance to try the stuff above tonight.