Silverface Fender Pro Reverb Problems- Rectifiers- Tubes

[caveing]

Hey guys. New to the forum and have been loving it so far! Such a great collective here.

I have a silverface fender pro reverb- I've had it for years and have recently decided to start souping it up a bit. It has been modded to blackface specs. I have taken out every electrolytic cap, every tone and coupling cap, tons of resistors- went for the jerry garcia modded twin thing- hi-fi, punchy, clean, spanky, etc. Mild mods- more about component choice. PRP metal film resistors in plates, slope, PI, orange drop 225's everywhere else, I did the filter caps to 500v and 450v F&T's, resistors in the can too... power tube grids and screens, weber california paper dome, polystryrene treble tone caps, foil positioning... etc etc etc etc.

So here's my problem. My amp has a rather high plate voltage. The amp has always had a 5ar4 rectifier tube- even though it is a silverface with a 5u4 on the chart- I changed it to a SS rec to go for more spank and "quickness" dyanmic punch, chime and brilliant cutting tone is my goal here. The SS rectifier only raised the plate voltage from about 480v with 5ar4 or GZ34 to about 490-500v. I've heard fender's can handle this voltage, but I'm thinking this particular amp isnt liking it. My caps are all rated to handle it (well my tone stack caps are only 400v). I like the sound with the SS rectifier- it just has that quicker reactiveness to it that I love... However it sounds like doo-doo when i crank it up. When I install the 5ar4 and drop the voltage a little bit I get a little bit better sounding overdrive. The overdrive sounds crappy because it is metallic, crossovery (? haha), ring modulated at times- especially when playing high on the neck on the g, b, and high e strings.

I FEEL (not know) like the amp has too much power in the preamp. I know the high plate voltage can indicate higher voltages throughout the amp- and running the preamp tubes too hot (right?) I have a 5751 in the v2 now... but I actually really like a 12at7 in the first gain stage even more. And I like a 12ax7 in the PI. I bought some NOS 5751's just because people say 12at7's wont sound as good as 5751's because 12at7's arent made for v1 v2 spots in this amp. i've got a 5u4 on order right now just to try it out... but really I dont even like the small amount of sag the gz34 adds in place of the diode rec. the 5u4 is gonna be MUCH more saggy right?

I was also thinking maybe I should try a 12u7 in the reverb driver position. It seems like my reverb pan is getting hit a lot harder than it used to. the reverb is more metallic and sensitive to knob adjustments. The reverb recovery tube seems to run hot as hell. The amp did burn out the 2.2k resistor in paralell with the reverb driver cathode bypass cap. I replaced it with a 1 watter and it's been fine.

and another MAJOR thing- I've got low volume snap crackle and pops... sometimes they're there, sometimes they're not, most of the time theyll go away after 20 minutes or at least get lower in volume. When I first turn on the amp it's annoying as hell, but I can play over it without much craziness. I've tried fixing my lead dress (well I'm thinking about rewiring the damn thing) It could probably use a couple new tube sockets too.

I've gone on much too long here, and i could go on longer with the symptonms. but I just cannot figure this thing out. Dont get me wrong, the cleans sound great! Maybe they could sound better, but with the NOS glass and all these recent component upgrades that are just getting settled these days... the cleans are great. The amp just doesnt sound like it should when I crank it up... no matter what I bias at (It definitely stays more stabile at 60% than 70%... should I try going below 60%)

The amp is completely blackfaced and has the original power tx, choke and reverb driver. the output is newer, but its got the same number that both the balckface and silverface schemes have.

Is there anywhere I should start checking voltages? Should I just wait for the 5u4? The problem with that is that I dont want the saggy 5u4 sound- I want that punchy SS rectifier sound. Could this be something besides voltage?

If anyone could point me in the right direction to start working I would greatly appreciate it! Let me know if you'd like me to take pictures... I could even make a sound demo

Thanks!!!

[caveing]

One more thing... could low resistor wattages be a problem... I mean I've used half watts and 1 watts depending on the location... 2 watts some places I believe. I did use 1/2watts in the plate and slope resistor spots. could that be causing crackle? I thought I'd be fine as they are somewhat nice american made metal film PRP resistors

[martin manning]

Which circuit is it?

Generally half-watt resistors are large enough (except in the power supply), and if they are good quality metal film they should be quiet. The crackle could be poor contact at the tube pins, so clean the sockets if you haven't done so.

Re the voltages, get a set of measurements from the power supply nodes and the preamp tube plates (pins 1 and 6) to see where it is now. I'm thinking that perhaps increasing the size of the dropping resistor following the choke to bring the preamp voltages down might improve the tone. The reverb driver runs off the screen node, though, so it would need it's own (additional) R-C to lower the voltage there.

Where have you got the power tubes biased?

[caveing]

Which circuit is it?

Generally half-watt resistors are large enough (except in the power supply), and if they are good quality metal film they should be quiet. The crackle could be poor contact at the tube pins, so clean the sockets if you haven't done so.

Re the voltages, get a set of measurements from the power supply nodes and the preamp tube plates (pins 1 and 6) to see where it is now. I'm thinking that perhaps increasing the size of the dropping resistor following the choke to bring the preamp voltages down might improve the tone. The reverb driver runs off the screen node, though, so it would need it's own (additional) R-C to lower the voltage there.

Where have you got the power tubes biased?

I believe it's the aa1069. 73 silverface non master with the 5u4 rectifier no mid control or anything like the later model ultra linears.

I do have 1 watts in the filter tank and on the power tubes. the phase inverter just has half watts though

I just realized somthing that seems really obvious, but it was never a problem before. it has shielded wiring where the insulation is kinda coming off... it looks like wire shielded but its not fraying out. And it's only grounded at the input jack and the volume pots. it's not grounded on the tube side... It has shielded cable going from input jack to 1st gain stage and back to tone stack.

Something else I did was replace the 68k resistors with 4.7k resistors on the vibrato channel.

Also my bright switch was acting up for a while- when I would turn it on it would make this horrible squeal. I resoldered it and it seems to be working fine... maybea little microphonic though?

how would I go about increasing that dropping resistor? I can get voltages later today.

Thanks!

[martin manning]

Those shielded runs should only be grounded at one end or the other, so they are ok. 4k7 is pretty small for a grid resistor, and if you replaced both of them the low input will have 4k7 to ground when you plug into it. I'd keep thos at 68k. Are you sure they're not 47k? The dropping resistor I'm talking about is in the filter "doghouse" at the return from the choke.

[billyz]

In my experience it will sound much better with the 5U4 in there. Too high voltages sound exactly like you are experiencing. If you really think you need the SS rectifier then use some zener diodes to lower the b+ to 450 or less. It will sound much better .

[billyz]

Also try some different tubes. Lately I have found the Tungsol RI to sound very warm , rich with vintage goodness.

[Firestorm]

What value caps did you use for the main filter? Stock Fender Pros are significantly underfiltered and will exhibit all kinds of non-musical sounds when you push them to the point where power supply ripple starts modulating the notes. If you stick with SS rectification, you should try to get the first filter up around 100uF.

What kind of 6L6s are you running? There's a huge difference in how well tubes handle high B+ voltages, especially the screen voltage. The screen voltage rating for a 6L6GC is only 450V and the choke drops almost nothing. The data sheets didn't say so, but NOS GE 6L6GCs don't mind 500V screens. The only current production 6L6 with 500V-rated screens is the Svetlana/Winged C. It's a nice, but comparatively expensive tube.

Andy Le Blanc has a long thread (https://tubeamparchive.com/viewtopic.ph ... highlight=) discussing the implications of running screens at higher potential than the plate, which definitely will happen in a Pro when pushed hard. So that's another thought.

I usually prefer 12AU7s a reverb driver in Fenders. They give you slightly higher voltage ratings and way higher current ratings. Worth a try, IMO.

[caveing]

okay lots of good stuff there guys. Firestorm- I used 100uf 450v caps... someone just told me those were in parallel- giving me 50uf at 900v... but it looks to me that they're in series 220k resistors in parallel... right? that is an up from the stock 70uf 350v- but down in voltage a bit from the stock 20uf 525v

I did use blackface resistor values in the can too. So those were 220k 1w (same as SF) in parallel with 100uf caps. and 4.7k 1watt (instead of 1k 1 watt in SF schematic this is the one that connects to choke) and 10k 1 watt (instead of 4.7k 1w in SF scheme)

So unless you're saying I need 220uf caps in my filter can. I guess I'm good there, maybe try 2 watts or even high value resistors?? Or can we rule out that stuff... unless I have a malfunctioning cap- which would probably be more obvious or kill my amp if it were a filter cap right?

I'm using mostly NOS preamp tubes- good ones. but I am using JJ 6l6GC reissue power tubes. I've been told the TAD 6l6str's or whatever and one of the tung sol sets can handle high plate voltage as well as the SED's. My power tubes definitely need to be replaced soon... I've been using them for more than a year. But I think just recently something caused my plate voltage to go up... When I first got this amp, I couldn't get close to making it distort unless I was using my danelectro and was able to turn it up to 8 without breaking glass... my les paul could make it distort too, but it would have been so freaking loud the room would be vibrating around me. It's really strange... I've triple checked all my resistor values in the amp... and really i've replaced a lot of em with PRP's or Dale RN70's just for the helluva it. The amp is supremely quiet, besides the crackle and pops.

Something that might help me out- seeing the lead dress of a silverface amp that is in great health- maybe one that's been partially rewired?

Should I further up anything in the filtering?
Should I just wait for the 5u4? if that fixes it, try a zener diode if I wanna go back to SS rectifier?
Would anyone still like to see voltage measurements? Sorry to ask, but which ones would you like to see?

Thanks so much!

[caveing]

oh and about the input resistors... really? It's a pretty common modification to take those out. I guess it would be causing me to get more preamp gain though right?

I dont use the 2nd input at all and it's still at 68k.

[Firestorm]

I used 100uf 450v caps... someone just told me those were in parallel- giving me 50uf at 900v... but it looks to me that they're in series 220k resistors in parallel... right?

100uF in series (+ end of one to - end of the other) will give you 50uF at 900V. The 220Ks go across each cap. Make sure they're not actually in parallel because they won't be able to handle the B+.

But I think just recently something caused my plate voltage to go up... When I first got this amp, I couldn't get close to making it distort unless I was using my danelectro and was able to turn it up to 8 without breaking glass... my les paul could make it distort too, but it would have been so freaking loud the room would be vibrating around me.

You probably wouldn't notice a small change in plate voltage (unless the tubes really hated it). But you would hear a change in bias; that does need to be adjusted from time to time as tubes age. What is your bias at now, and how did you set it?

Something that might help me out- seeing the lead dress of a silverface amp that is in great health- maybe one that's been partially rewired?

There's nothing to be gained from looking at the lead dress of a silverface amp. But if you can find pics of a blackface, that might help. But even the blackface ones have tone stack leads that are too long and are frequently a source of problems.

You could try a 30V/50W zener on the center tap to knock the voltage back a little bit. You could also try changing the screen resistors to 1K to get the screens under better control. They will sag a little, but nothing compared to a 5U4. Makes the amp a little more touch sensitive.

[caveing]

The improvement from the 10-15v drop from the SS rectifier to the 5ar4 is pretty noticeable when the amp is cranked. That crappy overdrive sound is a little less crappy. But I can hear the difference in sag between the two and that's not what i'm liking- i like the stiffer SS rectifier better

the resistors are correct in the filter can- they are right next to the 100uf caps like you said. should I get larger capacitance caps to make the total closer to 100uf in series? or did you just mean 100uf a piece?

I feel like its the voltage that's making the amp sound crappy... I'm not positive of course. Do you think the 1k's will make that much of a difference?
another thought is that I just dont like the way an SS rectifier breaks up... but i'm 80% sure it's not how its supposed to sound.

the crackle noise is a different issue I guess, but is it possible that it might be healed by lowering the voltage?

[caveing]

So I just noticed the variable voltage drop systems around. I thought these were only really popular in cathode biased amps like ac30's. But apparently they can be installed in fixed bias amps as well.

Is this an option for me to drop my voltage? Or will this not work quite the same? i've tried searching for comparisons of the two methods but cannot find much info... apparently some people dont like vvr's (as with everything out there of course)

So people use the vvr more for a nicer master volume control right? will the weber vdump dropping 50v drop my headroom considerably?

Could I use the VVR and an SS rectifier instead of just throwing in the 5u4?

[billyz]

What bias current are you running ? try running it up to 70% , may drop your voltage and sound better too boot.

Never mind I see you have at 60% , what do you mean more stable than at 70% ?

I still think your voltages are on the high side, But I have seen 550 + on later silver face fender and they managed to sound decent, Just not my kind of tone.

If you don't like sag, I doubt you will like bigger screen resistors either.
And after all the capacitor changes you have made it may take a few hours to sound it's best any way. like 20 or more.

Talk to Dana about his vvr in your amp, also check to see how hot your PT is running, you'll need to keep the vvr cool to handle that power.

I think you need to drop the voltage with a zener diode string ( it will get very hot too). Or try a new transformer with lower B+ .

[caveing]

Yeh I'm running at 60%... 70% seemed worse when I adjusted it. I didn't realize raising bias % decreased voltage much.

I can see the ultra linears running 550v... but not this one. Remember it is setup exactly like a blackface, but it apparetly has a different power tx with the same model #.

I was originally thinking getting a new power transformer made for blackface amps would drop voltage and allow me to run a gz34 or better yet even an SS rectifier. Am I correct in this? My power transformer is all brown and looks like it has been burnt up... it has always looked that way though.

So a 5u4 will drop voltage about 30 to 40v from a 5ar4 right?
Anyone have any recommendations for power transformers t insure a good voltage?

I do like amps tha have a lot of sag... but not with this speaker. I was going for a tight amp with this one. I like saggy amps with loose sounding speakers like the oxfords. Totally differentsound altogether... should I just go for that or wil a new power tx fix this?

Is there any way I can find out for sure without a variac and scope?

Thanks again! This has been informative. I've got quite the handful or recommendations now. Can y'all agree that a power tx will lower this if that's in fact the case... or should I just throw a 5u4in it and call it a day? Keep in mind my goals