Fender amp Volume Bleed at "0"

Hiro Keitora · From: New York, New York

Folks, I just got this 70's silverface Princeton reverb( no- master Vol. No pull-gain-), and everything seems quiet, except when the Volume knob is turned all the way down to "0" ( on fender knob, it's "1", but it gets confusing so I call it "0" ), guitar sound bleed through with distorted fuzzy sound. It's actually loud enough, you really hear it.

Once you start to turn the volume up, it clears up the fuzz, and typical Fender sound comes out.

It seems like when the volume knob is shot, it's not completely grounding the signal.

Which section should I check out?? Like, sometimes, scratchy pots are caused by bypass caps leaking,and this one seems scratch free.
Sad

Brad Sarno · From: St. Louis, MO USA

You probably could use some fresh filter cap's. It's possible that the B+ supply is not perfectly clean at the preamp tubes. When you turn down the volume knob to zero (or 1 on a Fender knob), that's muting the signal to stage 2, but stage 1 is still amplifying. Since it's amplifying stage 1 AND stage 2 and 3 also share the same B+ power source, that means that there could be some tiny bit of audio rippling thru the B+ supply and getting into the 3rd tube stage, literally jumping past the muted stage 2. If you have good, clean filter cap's, this may just clean up and go away. If it's an old Princeton with stock filter cap can, it's probably time anyway to put fresh filter cap's in there.

Brad

Hiro Keitora · From: New York, New York

Ok, Brad ! Very Happy

I'll give it a complete cap job with power filters and all!! It needs it anyway, so I'll do that.

But it's very quiet, and no hum or anything thou---
Not that, that doesn't mean it doesn't need new cap job, but since 70's Fender had lots of lead dress issue, I thought it may be one of those issue.

Thanks Brad, and I'll keep everyone posted.

<H>

Brad Sarno · From: St. Louis, MO USA

Hiro.

It's still possibly the filter cap's. They may still be doing a good job of filtering out the supply ripple, 60Hz, 120Hz, etc. but they also may have very poor high frequency performance. Lead dress may also be a culprit, but until you've replaced the cap's it's gonna be hard to know.

Brad

Hiro Keitora · From: New York, New York

Hi Brad, I don't have Multi-cap in a can( will order tomorrow), but have few 25uf/25V Sprague, so I just did bypass caps. ( and one cap by diode) So far, it's same.

Nothing to do with anything, but I must say everyone uses Sprague but they are so hit or miss, brand new and already leaking... On this Princeton, for x4 bypass caps, I had to consume x6 caps to find x4 good ones--that's silly. Confused

Ken Fox · From: Nashville GA USA

Make sure the ground palne for the pot is clean and the nut is tight.

Hiro Keitora · From: New York, New York

Ken, so I should make sure losen it and tighten it on ALL pots??

Ken Fox · From: Nashville GA USA

At least the volume pot for sure. Also clean the pot as well with a good contact cleamer with a lubricant in it.

If the pot is good, clean and tight no signal should get around it, I would not think.

Without testing the amp on the bench I really do not have any other ideas that come to mind.

Hiro Keitora · From: New York, New York

I thought it can be a faulty pot, but then it doesn't explain why "fuzzy" sound at 0 volume...

I ordered can filter, it'll be in next week, and when I open up the chasis again, I may replace the 1meg pot just to see what happens.

I'll post the result in next week.

<H>

Brad Sarno · From: St. Louis, MO USA

The distorted character you're hearing is an indicator that it's bleeding around that 2nd gain stage where the volume pot mutes the signal when turned all the way down and the activity exists in the B+ supply string, not in the main signal path. The idea is that since stage 1 is still amplifying and it shares the B+ power source that feeds all 3 preamp tube stages, the "slow" and old filter cap's don't react quickly enough to keep the power supply to stage 3's plate resistor perfectly clean and is polluted with some of the audio activity at stage 1. The fact that it's distorted implies that you may be hearing the high frequency performance of the filter cap itself at work, which typically isn't very good in an old electrolytic. Generally a faster, healthier supply cap will eliminate this form of crosstalk. Keep us posted.

Brad

Ken Fox · From: Nashville GA USA

What Brad is referring to is called interstage coupling. The filter provide an AC resistance to ground for the signal at the plate, preventing it from coupling back into a previous stage (often refereed to as decoupling caps). When they get really bad an amp can howl like a wild banchee! Had that on an old Fender amp I worked on, As the treble control was turn up to a certain point the amp would go into oscillation. A filter cap fixed it just fine. That was a long time ago and took me a while to figure that out. A lesson I never forgot for sure.

Hiro Keitora · From: New York, New York

My, amn't I a fortunate one, two experts in this field are holding my hand and guiding me through! Very Happy

Can't wait to change the big filter caps!!

By the way, direct replacement of FP type can caps from Antique Electronics Supply are back ordered. They told me April sometime. So I ordered JJ 40/20/20/20 @500 with clamp. I used this cap on my other Princetons years ago and so far, still no problems.

<H>

Hiro Keitora · From: New York, New York

JJ filter cap came in today, and right away, I installed it... well, it's didn't cure. Sad

Of course, amp sounds puncher, but the fuzzy low volume guitar signal is still heard at volume "0".

I replaced 1meg pot, and it didn't make any difference.

Now what?? Confused

Ray Minich · From: Bradford, Pa. Frozen Tundra

Gently, politely, carefully, and tastefully reflow all of the solder joints.
_________________
Lawyers are done: Emmons SD-10, 3 Dekleys including a D10, NV400, and lots of effects units to cover my clams...

Hiro Keitora · From: New York, New York

Ray, I'll poke around to make sure no "half lose- half soldered" wire connection...initially, I thought those shielded wires off of the Vol. pot and input jack that go to the first tube were the suspect.( I had this before~ A tiny wire from braided shield was missed and soldered together at the hot lug. ) but they seem ok, but I'll check them again.

In reality, this is negligable because once you pass the Vol. over 2, real, clean signal kicks in, but it's bothering the helmet out of me... Oh Well

Hiro Keitora · From: New York, New York

Folks, I found a culprit, it was V1 Groove Tube !!
I never thought tube could cause this, but this was it. Just to prove it, I installed this Groove tube in different Fender Princeton Reverb at V1, and it causes perfect Princeton breeds at "0". and made the Volume pot scratchy.

I installed USA 12AX7 in that troubled Princeton, and breed is gone. Surprised

Now, the mystery still goes on----I happen to have x5 brand new JJ ECC83s laying around, and I installed this to the Princeton at v1, then, the amp hums and squeal at vol.6 Whoa!

What's up with that? I reinstalled USA 12AX7 back at V1, amp is clean and loud and punchy all the way up.

I tried rest of the x4 JJ ECC83s at V1, and it behaved same. So I thought maybe this particular batch of x5 brand new JJ are faulty.

Just to see what happens, I installed these JJ to another Princeton at V1, and it works fine. Confused

Back to the troubled Princeton, I tried other brand of tube like Sovtek at V1, it works fine. Even a cheap Chinese one works fine. For some reason, this Princeton's v1 doesn't like JJ !! Confused

I installed this same JJ at Phase/tremolo(V4) and it works fine, tried at reverb recovery(V3) and it works fine. It just doesn't like to be at V1. Hmmmm---

Has anyone experienced anything like this!? Shocked

J Fletcher · From: London,Ont,Canada

Just goes to show you. I'll have to remember this! Sounds like there is some coupling going on from one triode section to the next.

Hiro Keitora · From: New York, New York

yeah, you ain't kiddin' !!

I swear all those soldering joints at tube socket and board were untouched before. (you can usually find some "trace" if someone was there before.)

Although, this particular Princeton works perfectly fine with any brand of 12AX7 except JJ, is acceptable, but it's really is not science. I can understand "compatibility" but that's usually be able to back up with logic and science.

it's really a mystery to me... Oh Well

Hiro Keitora · From: New York, New York

here's something I found others are talking about:

http://www.thegearpage.net/board/archive/index.php/t-695357.html

Tubes to avoid in cathode follower positions in amp.

I don't know '73 Princeton Reverb falls in to this, but it looks like I'm not the only one who's having a ptoblem...

<H>

Hiro Keitora · From: New York, New York

This morning, I gathered old salvage of more 12AX7/7025 and tried them all in V1. Most of these tubes are old USA except x2 Sovtek mixed in( one was wxt+ and the other is WA) They all function fine.
No hum, no squeal, clean and loud even at Vol.3

Now, when I installed JJ tube in V1, and turn the Vol. from "0" to "2", significant "Pop" noise comes out, and there's constant hum behind, made the Vol. pot scratcy and it does not give "loudness" of volume it suppose to give.
at Vol. 3, it's muddy and somewhat distorted. when the volume turns up beyond 5, it squeal in high pitch.

(it kind of reminds me of the symptom of Output Tranny leads were flipped, but then it doesn't explain why it works perect with USA tubes---)

Of course old USA tubes are far superior in quality and it can tolerate a lot, but it can be a drag if it doesn't take current production tube...

Anyone can help me on this please??? Crying or Very sad

J Fletcher · From: London,Ont,Canada

Hiro, sounds like you have the problem sorted out. Some tubes work in that slot, some don't.
I was thinking of your problem and was reminded of the fact that all components have their ideal, desirable qualities, and then there are those things that are side effects of the components physical make up and structure. Like stray capacitance, stray resistance, stray conduction, stray inductance, etc.
So a tube socket, or connecting wire, or the board the components are mounted on are all contributing these qualities to the circuit. Sometimes they add up in a good direction, sometimes in a bad way. Maybe why some amps sound better than others...Jerry

Brad Sarno · From: St. Louis, MO USA

The pop and scratchy pot from the JJ indicates some type of tube failure because it seems you're getting DC on the grid of the tube, which goes directly to the volume pot's wiper. I had a few JJ's with internal shorts, maybe some particles inside creating a short circuit, maybe a bad vacuum, not sure. But I had enough of these issues that I stopped using JJ. When they work, they're nice sounding tubes, but I had too many with strange problems and microphonic issues.

In your case, you're not dealing with a cathode follower, but that again is one reason I can't use JJ's in my Black Boxes because one of the two stages IS a cathode follower. The JJ's don't hold up under those conditions well where the cathode voltage is quite high. The cathode is very near the heater, and when you have a high cathode to heater voltage, some tubes fail to be reliable, like JJ's. The best new tube that holds up well to that heater/cathode voltage spec happens to be the Shuguang 9th generation (aka; china 12ax7b). Most old tubes hold up to this spec just fine. Any tube with a spiral filament is a BAD candidate for a cathode follower. But that's not the issue here.

That JJ that's popping and scratching should probably go in the garbage.

Brad

Hiro Keitora · From: New York, New York

Thanks guys! I'll tell you these brand new JJs are very anoying thou... Mad

The story still goes on~

So I'm still tinkering with it this morning, right? then all the sudden I get this distortion at vol.3 with USA tube in V1. I checked around all over the place and can't find where it's coming from. Then I remembered that as experimentation, I put JJ in V3 and V4 a few days ago. It seemed like it was working fine, but as soon as I replaced them with old USA 12AX7, distortion is gone, and clean and punchy all the way up in volume.

So, Brad, as you suggest, I'll throw these JJs out in gabage!!

I'm curious to see if new chinese tube will work in this Princeton. I'll keep everyone posted how it behaves.

John Billings · From: Ohio, USA

I have this same amp, except mine has been modded to 20 watts. My tech built several of these modded PRs for guys around town. I watched him work on several. He always moved one of the wires at the volume pot. He said, "They didn't run this wire in the right place."

Ken Fox · From: Nashville GA USA

I used close to a 1000 JJ tubes last year. All our steel amps use an ECC83S in the cathode follower part of the circuit. Never had an issue. You might just have got a bad batch of tubes. I can say again I have never had that as an issue with JJ tubes in that application. We have had a few fail right out of the box, but very few indeed considering the number of tubes we use in the shop every year.

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