Fender Pro Reverb AA165 distorted Bass Notes

[Platefire]

I just completed replacing all the coupling caps and various resistors. The amp is now dead silent as far as operating noise and the tone sounds better than ever. Only problem is when i hammer down on the bass notes especially openE and A I get this rattling distorted side effect.


The weird thing is it only happens with the reverb on. With the reverb off, I get none of that, crystal clear bass notes.The reverb sounds good to. I've double checked the reverb circuit and can't find anything out of order. The reverb tank is Accutronics 9AB3C1B, 3 springs, Long Delay.


 So before I discovered it only did that with the reverb on, i tried different tubes, cleaned reverb connectors, checked bias, different Speaker cab,  another guitar and cable.

   So once I realized it only rattled with reverb on I  tried hooking up the reverb from my Allen Accomplice(Deluxe Reverb Clone) to it but it wouldn't work with the Pro??? Seemed like it should work, but didn't.I tried another set of RCA reverb cables and still the same. No doubt, the amp sounds its Fendery tone best through the reverb circuit. So any Light you can shed on this is appreciated. Platefire 

 

[tubeswell]

So you tried swapping the reverb tubes?

[Platefire]

Yelp, tried swapping different reverb tubes several times. I'm telling you the amp sounds awesome but it's just when you attack those lower notes with authority  the rattle shows up.

[AlNewman]

Did you do your filter caps as well?

[Platefire]

No----but wouldn't it be making some noise if they were bad When I got it in 2013 it had what looked like brand new Sqrague Atom Filter Caps so I guess I'm thinking that they are still ok. On the other hand, if I can't find
this gremlin, I might have to change them to fine out if that was it! 

[Platefire]

Ok, I hooked the reverb tank from my Peavey Pacer/Deluxe reverb conversion to the Pro and it worked. Did the same old thing when cranked! So it's not a problem with the reverb tank.

Since It don't make the rattle with reverb dialed down to "0", I would think that it's not the filter caps, but in the reverb circuit alone. It don't rattle in the normal channel when cranked and hammering.


However, when I tear it back down to go back over the reverb circuit, it wouldn't hurt nothing but my pocketbook to have a set of filter caps in hand to take care of that too. Those filter caps are 10+ years old.
So I'm looking at them and wondering if Doug's  F&T 100uf/350V would work for those pre-standby switch caps instead of the original 70uf/350V ones? I know the 22uF/450 will be fine for the other 3. Platefire

[sluckey]

So I'm looking at them and wondering if Doug's  F&T 100uf/350V would work for those pre-standby switch caps instead of the original 70uf/350V ones?

They will work just fine.

[AlNewman]

No----but wouldn't it be making some noise if they were bad When I got it in 2013 it had what looked like brand new Sqrague Atom Filter Caps so I guess I'm thinking that they are still ok. On the other hand, if I can't find
this gremlin, I might have to change them to fine out if that was it!

The only reason I mentioned it is because it seems like you can get some funny distortion at low frequencies when the b+ is starving for power.  And in your circuit, one side of the reverb runs off of the preamp node, while the other runs off of the screen node.  So perhaps something you wouldn't hear on the normal channel would rear it's head when coming through reverb...  Just a thought.   Another thing to consider may be the bypass caps on the reverb channel.  I kinda figure that any electrolytic should be suspect, especially when you've already rebuilt the rest of the circuit.

[Platefire]

So I'm looking at them and wondering if Doug's  F&T 100uf/350V would work for those pre-standby switch caps instead of the original 70uf/350V ones?

They will work just fine.

What about the doghouse? at 1" diameter is cutting it pretty close

[sluckey]

Picky, picky!

[Platefire]

Well it's been a while since I re-capped a Dog House. As I remember it had an insulated pad in there between the top of the caps and the chassis---I think? I think the size of the DH is 7 1/8"x 3"x 1 1/4". Length and width is ok, depth is getting close

[mresistor]

I think the middle number is 3 1/2 (width)

[Platefire]

sluckey
Since I will be going back into subject amp for filter caps and what ever it takes to get it up to par.

I was just wondering what adjustment I need to do to my TON circuit to get just a little more intensity?

[sluckey]

I'm surprised you don't have enough trem. Change tube. Maybe try a 250K intensity pot. Maybe replacing filter caps will help.   

[Platefire]

Really I got enough intensity if I turn it to 10. I prefer to have a little over indulgence room but we'll just see what the new filter caps do. At any rate I am getting it to do what I want even now
It's a WIN WIN

[Platefire]

sluckey
Finally got those new filter caps in, no change. That was last night and since I tested it and realized it was still doing the same thing, I've tried everything I know of to do still with no change

[sluckey]

Can you plug into another speaker cab?

[Platefire]

Yes I've done that a couple of times. Tried different pre-tubes several times and several sets ofpower tubes and re-biased them and tried hotter bias, no change. Jumpered in another bias cap.Checked complete circuit with schematic plus voltages. Couldn't find nothing off with that. I've done continuity checks for grounds and connections between components.


It does sound like a busted speaker especially when you hit a power cord on open E. On the higher notes it is like a clean sound with grit.


It seems the reverb circuit accentuates it. I've checked the reverb circuit backwards and forwards but haven't found nothing I could identify. Could the reverb transformer be going bad, but notquit yet? I tried another reverb pan from my Peavey/Deluxe reverb and did the same thing.What ever it is, it's very elusive


I also wondered it that old board that I re-used could be conducting between connections. I've checked that with my MM on continuity between connections but didn't detect any. So I've definitely run out of tricks that I can think of


Edit: Let me add this. I just played in the normal channel
with a delay hooked up. Everything sounds right and normal. Went to the Reverb Channel, used the delay with the reverb knob turn all the way down. Sounds completely
normal. The problem is in the reverb circuit I believe!

[sll]

Have you checked for a leaky cap on either triode in V4?

[Platefire]

Yes, checked those caps on V4. I just replaced them with new about two weeks ago. Thanks, Platefire

[Platefire]

More Info: I can turn the reverb knob all the way down or either turn the reverb off with the switch which eliminates the rattling gritty sound. Have tried another tank which resulted in the same sound.

V3 Voltages:
Into Rev Tran=445(Sch 440), out of Rev Tran to V3 a & b coupled plates=437(Sch 430),
Voltage to V4 Plate Resistors=387(Sch 390), Plate Voltage V4a 254(Sch 260), V4b=357(Sch 360)

BTW-I'm using a 5U4 Rectifier instead of the GZ34 on the schematic. My PT has 3A heater current, so if I understand correctly, that will work.

 

[AlNewman]

It couldn't be a weak solder connection maybe?  Have you done anything in the reverb circuit?

[Platefire]

Well I bought this amp in pieces in a couple of card board boxes. A 1974 Pro Rev, a AA1069. Also who ever had it before me hacked it the circuit with all kinds of mods it looked like they never completed. It's a combo someone cut down to a head. So yeah, I rewired it to AA165 BLackface in 2016. I rebuilt it just enough to get it working back then leaving a lot of old caps and resistors. The amp worked OK but the reverb control has always worked like a gain control. The reverb gets deeper when you turn it up but also adds gain and volume.

So the reverb circuit got new cathode by pass caps and resistors new from 2016. And presently I've been replacing most of the coupling caps and Filter caps. Trying to get it to operate a 100% like it should. So the amp is presently operating quiet as a mouse. Two things that bother me now:


1-Turning the reverb up continues to distort the sound. Turn the reverb off and the sound is pretty close to what it should be.

2-The Overall volume don't seem as loud as I would think it should be. I once owned an early 70's Bandmaster which I would think would be close to what this Pro circuit is. On the Bandmaster the volume was pretty loud on "3". I have to turn this Pro up to about 6 to get the same response and volume.

Other than that everthing is fine

[Platefire]

I've been studying the reverb situation trying figure out what's going on here and one big question I have is what is the main purpose of the 10pF and 3.3M resistor in parallel separating the send and return grid of the reverb circuit.


My guess is that it lets the Vibrator channel use V4b have another gain stage when the reverb is turned off than the normal channel before going to PI. I have noticed that the Vib channel seems stronger than the Norm channel when the reverb is turned off. So it would also be boosting the reverb recovery stage when reverb is on. So this resistor/cap network would let this gain stage serve a dual purpose????


My concern is I have been trying to read the 10pF to determine what's actually in there. This is a DC voltage free grid area, so no way to tell if its doing it's job as far as blocking DC. I have been trying to read the value to make sure the right value is in there with my MM but not getting any reading that makes sense to me like---100nF which is converted to a 100000 pF???


So what I'm getting at is if this cap is bad could it do what I'm experiencing? Everything else seems to check out. Platefire

[sluckey]

The 3.3M is one of the mixing resistors used to combine the huge dry signal with the weak wet reverb signal coming from the reverb pot via the other mixing resistor (470K). At the junction of these two resistors the wet and dry signals are combined together to be sent to V4A gain stage.

The fact that the dry signal is so much larger than the wet signal means that the amplitude of the dry signal must be dropped considerably, otherwise it would overpower the much weaker wet signal and you would have very little reverb effect. That 3.3M works with that 220K resistor to form a voltage divider that knocks the dry signal down by a factor of 3.3M/220K=15. The 10pF cap is a "bright" cap that preserves that bright fender sound. Without that 10pF the amp would lose some of the highs. I doubt my ears could tell the difference. The 10pF does not block any dc because there is no dc present on the cap. DC blocking is performed by the .02 coupling cap just prior to the 3.3M||10pf.

Your MM cannot measure capacitance smaller than some value. Consult your user manual to see what the capacitance range is for your manual. My Fluke can only measure capacitance down to about 500pF.

That 10pF cap is in the signal path even when reverb is off. I don't think this is the rabbit you are chasing.

[Platefire]

OK, the 10 pF is not the rabbit! Thanks so much for the explanation, boy did I ever havethat wrong. I never even considered the amplitude of the two signals and the need to reducethe dry signal way down.


Your explanation of how the mixing network works has raised another question which will show my lack of understanding but that's ok. When I look at the schematic showing the dry signals path from V2. One to the reverb circuit and the other to the mixing network. It seems to me that when the dry signal encountered the 3.3 M resistance it would force most of the dry signal through the 10pF as the path of least resistance and not reduce the amplitude at all. Please tell me what my wrong understanding is. Thanks, Platefire 

[PRR]

> It seems to me that when the dry signal encountered the 3.3 M resistance it would force most of the dry signal through the 10pF as the path of least resistance and not reduce the amplitude at all.

Impedance of 10pFd is greater than 3Meg up to 5kHz.

So 98% of what young folk hear, and 99.9% of what old guys hear, has to come through the 3meg.

[sluckey]

The 10pF cap looks like a resistance to the ac guitar signal. The resistance is actually called reactance. The reactance (in ohms) is inversely propartional to the frequency involved. This means for low frequencies the reactance is high and for higher frequencies, the reactance is lower. A 10pF cap would look like a 160MΩ resistor to a 100Hz signal, or 16MΩ to a 1000Hz signal, or 1.6MΩ to a 10KHz signal. You can see that the higher frequencies will pass through the 10pF with less attenuation but the low frequencies will prefer to pass through the relatively smaller 3.3M resistor. Make sense? It's no different than the bright cap on a volume control.

Look at this page for a better understanding of where my numbers came from...

     https://www.electronics-tutorials.ws/filter/filter_1.html

[Platefire]

sluckey -Well that opens up a whole new way of looking at capacitors, Capacitive Reactance. I read over the tutorial and tried some of the formulas. I came up with the same answers as they did, just couldn't get my decimals to land in the right place. I've always read about different value caps letting certain frequencies pass and blocks others but I never thought of it as a Reactance/Resistance(based on frequency) that would work in the same way a resistor would. Yes it makes sense to me. Thanks for answering my question!

I looked at my Fluke 115 manual and it says on Capacitance Range 1000nF, 10uF, 100uF, 9999uF. A little hard to understand the way they got it written to me


PRR-I got you on the HF hearing loss---Huh? 3m all the way

[Esquirefreak]

I have a Fluke 115 and it's struggling to measure anything lower than 1nF.

/Max

[Platefire]

Yeah I'm beginning to wonder about my 115. Sometimes it struggles to read resistors. I love the Automatic feature on Ohms reading when it's working but sometimes it can't seem to find its way, so I switch over to manual in the K Ohm range on a known ____K resistor and it won't even give me a reading. Somethings either broke or I'm doing something wrong? I may need to re-read my manual again.

[SILVERGUN]

I snagged one of these and I never take it off of the capacitance setting. It comes with some 3" long clip leads (so as not to add much capacitance) and I've successfully measured down to 10pf.It works pretty good for as cheap as it is. I use it mostly to double check cap values as I'm building.https://www.newark.com/tenma/72-8150/capacitance-meter-with-resistance/dp/41M0998

[Platefire]

WOW, I like the price!!!

[sluckey]

I bought this Proster last February. Only got about 45 minutes on the battery.   

Don't know how I managed a 50 year electronics career without needing to check the value of a cap or inductor.   

[Platefire]

Man, I didn't know you had a career in Electronics. What did you end up specializing in?


On the Proster, another good price. Did they have any analog meters 50 years ago to read capacitance? All I ever knew about is those old analog MM--Ohms and voltage

[sluckey]

I attended a 2 year electronics course at a tech school during '68 and '69. Worked for the Air Force from '70 to '76 doing test equipment calibration and Long Range RADAR. Transferred to FAA and worked Enroute and Terminal RADAR. Retired from FAA in 2012.

I calibrated a few of these Sprague Tel-Ohmike TO-5 Capacitor Analyzers while working in the Air Force PMEL at Robins AFB...

&ab_channel=millstap

[Platefire]

Looks like our career time line runs almost neck and neck. I took Drafting at a Trade School in 67 and 68, worked as a draftsman at Nabors Trailers from 69 to 81, started working for the US Army as a Draftsman in 81, then Lead Draftsman, Engineering Tech, Estimator and finally Project Manager when I retired in 2011.

Didn't get into any electronics until I got talked into building my first amp in 2000. I really do love electronics. I wish I had discovered that at a lot younger age
 

Tel-Ohmike with the style buttons, knobs and meters reminds me very much of my Military Tube Tester I use to have. That's a cool capacitor tool that I didn't know existed. I'd say, you need a little bit of training or experience to operate that properly.

I'm glad you decided to apply you electronic knowledge to guitar amps, so you can help us green horns to get our amps to work

[sluckey]

My big city cousin taught me a few chords and Peter Gun intro when I was 13. That summer he got me a job working in a radio/tv shop, cleaning up, toting a tube caddy on service calls, etc. Been hooked on guitars and electronics ever since. 14 years later he introduced me to Carol. We've been married 45 years now.

Here's the first amp I ever built. I was in my second year of tech school. The upholstery department at tech school did the fancy roll/tuck covering. This was a Sunn Sonora bass amp. I was a wannabe rock and roll star, playing bass for a country band on Friday and Saturday nights.   

[Platefire]

Hay, you should be real thankful for your Big City cousin introducing you to so many great things especially your Wife of 45 years. Did you ever learn Chuck Berry's song "Hay Carol"? Trying to remember my first influential song, I'm thinking "Honky Tonk" or "Pipe Line"? I had a friend Charlie, that played lead to the latest hit instrumentals like Tel-Star, Outer Limits and I played the back up part, all the time learning the lead part on my own.
 

That's a nice looking strat and amp. You started Big! I'm trying to figure out on the amp, did you have a Sun Sonora and just reworked it a bit? Looks like a Kustom after the upholstery shop rolled/tucked it. Even a Curly Cue cord. I guess truly we all dream of being a rock star at first and then we appreciate what we have left, joy of playing and making good amps


About the time you was doing your gig with the country band I had got rid of my 1482 and Montgomery Ward Guitar, I got a 2-12 6164 Gretsch Variety Amp and Framus Bass. I got this because the top 40 cover band the "ByGones" needed a bass player and they were allvery good. So I worked all the summer to get the rig. After the ByGone gig was Gone>>>>the Gretsch was a pretty good lead guitar amp. The guitar with the Gretsch is a Kalamazoo by Gibson.

[Latole]

Did you do your filter caps as well?

Good chance  it is the issue.
New filter caps is a must for any old amp.

[sluckey]

I'm trying to figure out on the amp

My Sunn Sonaro was a scratch build into a brand new Bud chassis. While in tech school I replaced the power transformer in a real Sunn Sonaro, the biggest combo amp I had ever seen. When I saw the simple, clean, true point to point wiring inside, I knew I had to build one. Yeah, the cab was an attempt to make it resemble the old Kustoms.

Here's the schematic...   

     https://el34world.com/charts/Schematics/files/Sunn/Sunn_Sonaro.pdf

[Platefire]

Latole
Yes I installed new filter caps but it didn't fix the reverb circuit issues. Also that amp suffered a set back. Not sure what happened but the Vib channel became very weak, vol pot became very scratchy and the plate voltages have dropped.

I kinda got disgusted with the whole thing, buttoned it up and put it up for a while. I will come back to it latter with fresh mind and eyes

[Platefire]

 "scratch build into a brand new Bud chassis"

You lost me on this. What is a Bud chassis?
So yeah, now I understand, your first scratch build. So how did you scratch up the parts in those days? There were hardly any DIY builders in those days, so I know you probably couldn't get everything from one source. Did you take pictures of the Sonaro you worked on to establish your layout and how did that first amp build process go. Last but not least, once you got it fully assembled, how did you bring it up to power for the first time?


I checked out the schematic. Pretty different kind of amp from the normal fender. One thing I couldn't help but noticing is the low "67" plate voltage on the pre Cathodyne PI gain stage, but is also on the PI grid. Kinda different PI arrangement going on there. 

Just one more? Did everything work once you got it powered up

[sluckey]

You lost me on this. What is a Bud chassis?

Didn't even bother to google it did you? 

     Look...     https://www.amazon.com/s?k=bud+chassis

So yeah, now I understand, your first scratch build. So how did you scratch up the parts in those days? There were hardly any DIY builders in those days, so I know you probably couldn't get everything from one source. Did you take pictures of the Sonaro you worked on to establish your layout and how did that first amp build process go. Last but not least, once you got it fully assembled, how did you bring it up to power for the first time?

Parts were easy. I was working part time in the biggest radio/tv/supply house in Andalusia. Everything I needed except the chassis was on the shelves. Chassis came by bus from Grice Electronics in Pensacola. I had no camera back then but I drew up several pages of detailed layout drawings. It's a simple layout. I'll try to find a gut shot on the net. I brought it up in several stages. First power up was for filaments only. Then filaments plus rectifier/filter caps. Finally plug in the tubes. So, three steps, plugged into the wall each time.

I checked out the schematic. Pretty different kind of amp from the normal fender. One thing I couldn't help but noticing is the low "67" plate voltage on the pre Cathodyne PI gain stage, but is also on the PI grid. Kinda different PI arrangement going on there.

Did you also notice the pre cathodyne stage is a pentode? Directly coupled to the cathodyne grid? That cathodyne was a pretty common circuit for hifi amps. A lot of Ampegs used a very similar PI. Did I mention the power amp for these Sunns was a direct copy of the DynaCo Mark III hi-fi power amp (See attachment)? In fact, the very first Sunn amps were an actual Mark III with a separate guitar friendly preamp chassis.

Just one more? Did everything work once you got it powered up

Nope. Failed the filament test. I had correctly put a jumper between pins 4 and 5 of the 12AX7 preamp tube, but I connected the filament string to pins 4 and 5, just like for the 6AN8 PI tube. As soon as I hit the power on switch the entire filament string smoked and melted all the way back to where the PT filament leads attached to the first power tube. No damage to the PT, but I had to redo the entire filament string. Pretty easy fix because the filament circuit was the only thing wired up at that point. Bruised my ego though. I had two classmates watching. By the time I had powered off the whole class was cackling!

I found a gut shot pic of an original. That diagonal cap was not original. Several other components have been changed but you can get an idea how simple this amp was. Lotta emply space under the hood.

[HotBluePlates]

I found a gut shot pic of an original. ...

Haha!!  I'm not the only one that copied Tektronix' layout practice of stringing parts between rows of terminal on either side of the tube socket!

[SoundmasterG]

I checked out the schematic. Pretty different kind of amp from the normal fender. One thing I couldn't help but noticing is the low "67" plate voltage on the pre Cathodyne PI gain stage, but is also on the PI grid. Kinda different PI arrangement going on there.

Did you also notice the pre cathodyne stage is a pentode? Directly coupled to the cathodyne grid? That cathodyne was a pretty common circuit for hifi amps. A lot of Ampegs used a very similar PI. Did I mention the power amp for these Sunns was a direct copy of the DynaCo Mark III hi-fi power amp (See attachment)? In fact, the very first Sunn amps were an actual Mark III with a separate guitar friendly preamp chassis.

Sluckey, I'm friends with Conrad Sundholm, co-founder of Sunn and heard the story from the horse's mouth - The first Sunn amps were the ones with top mount controls and the smiley face logo. These ones used a Dynakit Mk3 or Mk4 depending on if 60 or 40 watts, and they also used a Dynakit PAM-3 preamp. The actual kits from Dynakit/Dynaco. When the Sunn amps went to a single chassis the preamp circuits were slightly redesigned to be better for guitar and bass, and they still bought the transformers from Dynaco for years after this until they switched to Schumacher. There was also a local Portland, Oregon area transformer used in some of the Sunns - Western Transformer. Conrad bought out his brother Norm in the late 60's and then sold Sunn to Hartzell in 1973 or so and then started Bi-Amp wtith a partner, and sold that years later also. The best thing about the Sunn amps aside from the simplicity was the 2x15 cabinet for the 200S as that was anew thing at the time and really put the sound out there in comparison to other things that were out there in 1964. Somewhere I have a LMS plot from the 2x15 200S cabinet, and one for the 2000S cabinet also that we measured at Conrad's. I'll have to see if I can find it and post it.


Greg

[Platefire]

Har! I was thinking you was springing some new acronym I haven't heard of yet with "Bud".
Never heard of Bud chassis and they look like they have a good selection.


Must of been nice having access to all those parts in the golden age of tube/P to P electronics. Everything is nicely laid out. I like those 12 position tag strips. Always liked soldering on those! You certainly didn't take any chances powering up in stages. Worst part of the filament burn out is that you had an audience observing.


No I didn't notice the Pre Cathodyne Stage was a Pentode but I did notice the 7199 designation, which I didn't know what it was but have since looked it up that it was a Pentode/Triode like the 6ANB. Also took a look the DynaCo Mark lll. Got to looking for the preamp/tonestack and realized it was just a power amp. So Sun just added their guitar friendly preamp to it and boom! the "Sonaro".

I keep wondering what kind of speakers you had in that cab back then, I'd would guess 2-15.I also would also guess by your last statement that, some of the components have been changed,that you must still have it right?

[sluckey]

So Sun just added their guitar friendly preamp to it and boom! the "Sonaro".

Not a Sonaro. Their entire line of amps used the same power amp circuit. Slight differences with PT, OT, and power supply, depending on 40W, 60W, and 120W model. Kinda like all the Fender AB763 are the same except PT, OT, and power supply.

There's an interesting story behind the Sunn amp origins. Google "Sunn Shack" if interested. You'll probably want to play Louie Louie.   

I keep wondering what kind of speakers you had in that cab back then, I'd would guess 2-15.

2x15 CTS Bass speakers with big ole square magnets.

I also would also guess by your last statement that, some of the components have been changed,that you must still have it right?

That's not my amp. I just grabbed that chassis pic off the net so you could see how simple the layout was. My clone and the original I copied are long gone.

[judge74]

I did some skimming on this thread… so did you ever fix the problem with the reverb? I just got done with a silver face super reverb that had lots of problems. One of them was with the reverb doing a similar thing. On that one it ended up being the .001 coupling cap between the plate of the reverb recovery and the reverb pot.

[sluckey]

I just got done with a silver face super reverb that had lots of problems. One of them was with the reverb doing a similar thing. On that one it ended up being the .001 coupling cap between the plate of the reverb recovery and the reverb pot.

All the Fender Reverb schematics I've ever seen show a .003µF cap in that position.