Fender Pro Reverb AA165 distorted Bass Notes

[judge74]

It’s the 79 ultralinear. The schematic is fuzzy it could be a .001 or a .003 looking at it. However, the cap in the circuit that I pulled out and replaced was .001.

Maybe that explains something about the sound of my amp. I can turn the reverb up really high like 7 or 8 on the knob without getting muddy. The reverb just kind of sits like a shimmer above the note. It is different and I like it…

https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Super-Reverb-Ultralinear-1978-1981-Schematic.pdf

[judge74]

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...

Interesting, I went to tech school for avionics + an A&P license in 1999/2000. First amp I ever rebuilt was in tech school. It was an old Ampeg reverborocket with the octal preamp tubes. My teacher thought it was weird I wanted to work on tube stuff but he helped me anyway. Even though I took all the electronics stuff in tech school, all the jobs I got were always airframe related. I worked for Delta Airlines and Lockheed Martin. I am a data analyst/programmer for Boeing on the 787 now. I have lived all over Georgia an Alabama…

[Platefire]

Hi Judge74
This project is presently on hold until I feel like messing with it again. Refer to post #41 for an explanation

[Platefire]

Hay sluckey
Ok, so yeah that was a Sunn Gut shot off the net. I was thinking that was your original---butI'm pretty sure yours was pretty close or better as meticulous as you are on your layouts and lead dress.

I did go to the Sunn Shack and read some of the history. The Kingsmen's Bass player needed a bigger more powerful amp. Because of the rest of the band playing so loud and the crowd hollering and screaming. That's exactly why the Beatles got Vox to build the Super Beatle but I think that SS amp was a dud.

My old high school picking buddy Charlie I previously mentioned has a big Sunn head in his shop.We had talked about getting together and see what we could get it going. Now I'm wondering what model it is? 

[sluckey]

Now I'm wondering what model it is?

I'm curious too. Ask him. It's written on the faceplate. If it's one of the '60s or early '70s it's worth restoring to it's former glory.

This is my Sceptre that I restored back in '06...



And my Solarus 2x12 cab...

[Platefire]

That's some cool Sunns sluckey. I visited your site and seen where you installed new 6550's, twin bias pots and test points in your Sceptre. I bet you could get some great loud clean Ventures/Surf tones out of that. Thought it was neat that the Sunn your repaired in School was the one you ended up playing on with the C & W band. You got to test it out real good!!!!

I got my Pro Reverb back out and started doing an all points checkup. Turns out I have several leaky coupling caps in the preamp section. Those I just installed that I thought was good, were apparently shot. So I'm ordering some new from Doug. Hope that does the trick

[Platefire]

I got kind of a dumb question? Just wondering on this 74 Pro why is the 250pf's in the tone stack, the 500pf to the reverb driver, 120pf bright caps and 10pf on the reverb mixer are all ceramic caps instead of Silver Mica?

The reason for my question is because I bought all Silver Mica for all the ceramic replacements and then got to wondering if there was a reason other than economics for them being in there in the first place.

One More?? Is the "P" following the value on Silver Mica's
same as pf?

[sluckey]

Ceramics were cheaper and readily available when that amp was manufactured. You may find them to be very reliable too. I can't remember ever having to replace a small value ceramic.

Is the "P" following the value on Silver Mica's same as pf?

yes

[Platefire]

Thanks sluckey
That clears my mind up about that. I've never known the ceramics to fail either, to the pointI'm hesitant to change them out. On the other hand, never recall a Silver Mica failing either

[PRR]

I've heard of "modern" SilverMica failing. Also of SM rated 50V, when all my life 500V was the least you would get.

Also Mica is dug-up by child labor. Not that hundred pF caps are any large part of the abuse, but why buy?

Ceramics to at least 1,000pFd, maybe more, are as perfect as caps get. (Very large ceramics do distort, but do we care?)

[Platefire]

Well for this amp I'm mostly interested in getting good old classic Fender clean with that deep reverb in the background. Nothing quite like it IMHO. If I want dirt, I got plenty of dirt boxes.

Thanks for this discussion on Ceramic and Silver Mica pf caps. I guess I was wanting permission to feel good about ceramic caps and now I think I can I guess they are like the ugly duckling that is beautiful inside(for the pf range)

[AlNewman]

I believe mica caps are very heat resistant, and are used where there can be little to no variation in capacitance.  Ceramics can sway a lot with heat.  So in radios, mica caps are used exclusively.  In guitar amps, ceramics work well.  Although I've heard that ceramics require a minimum of a 1kv rating, otherwise they can be microphonic.  That came from BillM's page, so I have no reason not to believe it.

[Platefire]

OK, I replaced all my leaky coupling caps last night. Looking over all the voltages, everything looks very close within reason of all the voltages except on the PI grid. I would think the PI grids should have 0 DCV but on the one with the .001 cap I got about 65DCV and the one with .1 cap about 69DCV. Don't see anything on the schematic or layout that this should be?

So its coming though the the 1 meg resistors from the cathode voltage of about 100VDC. So based on the PI network of resistors with the 1 Megs behind the coupling caps to the grid, I'm guessing this is normal?

[sluckey]

I would think the PI grids should have 0 DCV but

You would be wrong. Shirley we've talked about this before?

[Platefire]

I hate to admit it, but I didn't realize that
We probably have talked about it over the years but apparently I didn't retain it

[sluckey]

So its coming though the the 1 meg resistors from the cathode voltage of about 100VDC. So based on the PI network of resistors with the 1 Megs behind the coupling caps to the grid, I'm guessing this is normal?

Good guess. This is called bootstrap biasing. In reality, the grid voltage will be only a couple volts less than the cathode voltage. But because of the loading effect of your meter on this high impedance grid, the meter will always show less voltage than the actual voltage. I don't pay much attention to the voltage reading at the grid because I know it's bogus. Instead, I measure the voltage at the junction of the two 1M resistors. That is the same voltage that will be on the grid.

For example look at my Phoenix schematic Same PI as your Pro). Notice there's 78V on the cathode? And 76.4V on the junction of the 1M resistors? That means the bias on this tube is -1.6V. This is my preferred method of checking voltages on a LTP PI tube.

[Platefire]

Thanks for explaining the Bootstrap Biasing. I think I'm guilty of building amps by the recipe/schematic but not thinking to much of how it all works. If you put it together right with the right parts it's going to sound great just like the original did, but that don't mean you know Jack about whats going on electronically. I'm trying to get better at that!

I was surprised about your Phoenix because I built a Phoenix once but mine was a reverse engineered Dr. Z Carman Ghia. I think Dr. Z was being real secretive about his schematics and there was none to be had, so somebody reverse engineered it, called it "Phoenix" and posted it on the internet. I got a request to build one, so I found and used that.

I noticed in your in Phoenix for your first preamp section you used the Ampeg Gemini ll preamp/tone stack. If you recall, you recommended that preamp for my Bogen CHB-20 conversion and helped me with that. That's a good one, been real happy with that one. I don't think I will forget Bootstrap Biasing anytime soon. Thanks, Platefire


 

[Platefire]

  Latest Development on the Snake-head Pro. I have gone over and over the circuit and everything seems to be in order to the best of my ability to tell. So I have a new set of JJ 6L6GC's that I dropped in there. Well the bias went hay wire. With the prior set of 6L6GC's I had plenty of play in my bias control to get up to 50 mA or more. Now the setting is topping out at bout 37mA and that's the highest it will go.


I usually try to bias between 60 and 70% dissipation. Attached is a sketch of my bias circuit.I need it to adjust out to about 47mA to get around 70% with the plates at 453DCV.

I think I need to change out the resistor on the bias pot from existing 22K to about 15K? but I'm not totally sure. Is that right?     

[tubeswell]

Yep decreasing the range resistor on the pot tail should get you into a warmer bias range. if 15k doesn't do it, 10k should. (All talk, no responsibility)

[Platefire]

Thanks Tubeswell

This gremlin has been a hard one to chase down. I love a good challenge, but this is ridiculous

[Platefire]

What's happened to sluckey. He must of took a break?

Well I think I finally may can do a victory lap on this Pro. I changed the bias range resistor from 22K to 10K and boy, that gives me a wide range of adjustment. I biased it up to 65% dissipation and I don't detect any of the former rattling. The thing is solid as rock with absolutely no flab.So that what I was trying obtain. 

  The only thing is last night when i was Jamming on it with that old 22K bias range resistor that wouldn't adjust any higher that 37mA's it had such nice harmonics and it was so touch responsive and I was really getting into that.

So when I biased up to 65% with the new 10K resistor, the response was little stiff and hard. So now that I know it's acting good like a Fender should, I adjusted it back down to 37mA and liking it!!  The clean tone is still there, the harmonics are nice and the feel is just all right. So right now it's about at 55% dissipation(I think, plate voltage 443). I'll hang there a while.  Platefire
 

[sluckey]

What's happened to sluckey. He must of took a break?

Aww, you missed me! That's so sweet.   

But there was really nothing to say except, YOU GOT THIS!   

[tubeswell]

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…

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.

So it had parts from a 74 pro reverb? Including the main board? Did you keep the .002 (2000pF) cap across the reverb transformer secondary? (On the 70s CBS amps, it’s a disc ceramic cap on the main board)

[tdvt]

Well I think I finally may can do a victory lap on this Pro.

Good the hear!

So it had parts from a 74 pro reverb? Including the main board? Did you keep the .002 (2000pF) cap across the reverb transformer secondary? (On the 70s CBS amps, it’s a disc ceramic cap on the main board)

I was just looking at that cap & the other "new" cap between the plate & cathode on the reverb driver tube. Were these both added as part of the CBS noise-reduction effort?

[Platefire]

Hay sluckey, welcome back. It's just when your so active answering everybody's questions andthen disappear, makes you wonder You need to put it a "Request for Leave Slip a34jub-00##" for approval before you go!!!

tubeswell----I don't recall ever seeing that cap across the reverb transformer secondary. I basically removed all the non-fender mod stuff someone had added to the circuit. Then I rebuilt what was there to AA165 schematic. I see it is on the AA1069 schematic. Anyway it's not there now


tdvt------I looked at the AA165, AA1069 & AB668 Pro Schematics and don't see that cap between the plate and cathode on the reverb driver tube. Can't say for sure I never removed it, that was 2014. This circuit was seriously hacked before I got it, so I built it back AA165 so I would know what I got. So it may have been there previously but I don't see it now. Yes I know CBS did a several circuit additions to combat previous noise issues in the 70's but like I said, it's all AA165 now with the exception of a Bassman OT, sluckeys Tremonator, bias bal pot to bias adjustment pot mod and am using a 5U4 rectifier instead of a GZ34.

So much for the cool bias noodling. I tried my hotter humbucker guitars on 37mA bias setting and they was distorting a relatively low volumes. So I went ahead a jacked it up to 43mA to get a balance between tele to LP. Platefire 

[PRR]

> You need to put it a "Request for Leave Slip a34jub-00##" for approval before you go!!!

So dock his pay.

[Platefire]

Yes 50% of his next paycheck and plus, shame on him for at least 30 minuets or even longer!

[sluckey]

Nothing from nothing leaves nothing.

I took permanent leave on 12/31/2012.   

[Platefire]

Ain't it Nice

[Platefire]

Is it possible to Ohm test an OT without pulling the chassis? I did try by going through the Rec socket pin 8 to center tap(standby off) and through the 6L6 sockets pins 3(plates).

I got  38 Ohms between center tap and on one socket and 53 Ohms on the other. Got 90 Ohms between plates. .5 Ohms between Primary leads and .4 Ohms between secondaries.

I figure that this is not too good being also connected to the dog house caps through the choke and also to the Reservoir caps. I just hate to pull that chassis on this head converted from a combo, because it's so hard to get it out and back in.

Anyway I'm still getting low power and grit along with the clean sound. It sounds pretty good up to 3 and the louder you get past that the grittier rattling gets worst especially on the low note cords.


I'm now suspecting the OT of being bad. It came with the amp. Its a New Sensor NSCO18343 Rev B 238-9544 Bassman type OT. I got it with the amp parts and it looked brand new plus the leads looked like they had never been hooked up. So if I have to pull the chassis again to test it properly, I will. Platefire

[sluckey]

The primary plate leads looked OK... until you said this...

.5 Ohms between Primary leads

Please clarify. Exactly where was each meter lead?

[Platefire]

Ok, I went back and re-took those measurements just to be sure.

The measurements between the primaries were correct.

The measurements between secondaries was just between the green 8 Ohm speaker tap andthe black ground-.5 Ohms

Also measured between primary and secondaries. CT to 8 Ohm green speaker tap started at1.3M descending toward "0"

CT to black speaker ground started at 20M descending toward "0"

[Platefire]

Well, kind of embarrassing to think you got your problem fixed and it's not. I think I can  safely say that the OT is good.

One thing I'm pretty sure of is the rattling gritty sound is related to the reverb because I can remove the cables either or just turn the reverb off with the knob or foot switch and the rattling is gone.

One potential problem is the fact that there is only about 1/8 to 1/4" separation between the bottom of the OT and reverb tank. I did try to stick some aluminum foil in between the OT and R tank but no change. Of course it's so close that the foil was touching the tank and OT, so with that I don't know if that shielded anything.

I know grounding of the reverb cables to the amp chassis and reverb chassis can be a problem.I checked that and the reverb in cable is grounded to amp chassis but is not grounded to pan chassis. The reverb out cable is grounded to both amp chassis and to pan chassis. To my understanding, that's the way is should be?!?

As far as the amp chassis reverb circuit, I installed a rca jumper from in to out reverb jacks on amp chassis, bypassing the reverb tank. The sound with the reverb turned on adds gain and a treble boost but none of the rattling. The more the reverb is turned up creates distortion but in even harmonics, not gritty rattling. So considering the very small reverb signal that requires a lot of boost, I would think this is normal.

So something is this reverb loop is amiss but so far I can't lay a finger on it. If you see something I stated wrong or I missed, please let me know. Thanks, Platefire

[sluckey]

The combo cab has been cut down to a head unit, right? Pull the reverb tank and use some longer cables so you can move the tank around on the bench top. Any joy?

[Platefire]

yelp, already done that with plenty of separation between tank and amp. Tried it with three different reverbs with the chassis out---the reverb in the Pro head cab, the reverb in my Accomplice and in my Pacer/DR conversion---all no change.


It's in the reverb loop circuit. Turn the reverb off it's gone. It's a odd harmonics distorted rattle and the more reverb you dial in, the worst-er it gets. The reverb sounds good, it just brings with it this odd distorted rattle especially accentuated on the bass notes.


Got me


 

[sluckey]

But you bypassed the tank and had no rattle. That seems to indicate the circuit is OK.

[Platefire]

Yes bypassing it with a rca patch cable it dialed in even distortion. I played around with it at different settings. Produces some very good OD except it is a little too bright for me.

So that seems to indicate that the problem is the tank! but doing the same thing with other tanks seems to eliminate that idea?

Last thought I had is since the reverb sounds normal and the noise gets worst as the reverb is dialed up, makes me wonder if the problem is after the reverb control pot on the return. Could a bad cathode bypass cap or resistor do that on V4 shared 820 cathode resistor/25uf cap? I've tried several different good tubes in that location with no change.

[sluckey]

A lot of people use separate resistor/cap on V4 cathodes. Need two 1500Ω and two 25µF.

[Platefire]

One thing I'm realizing about this amp is it only has about 50 to 75 % of the volume it should have at 10. That's what made me suspect the OT. Double checked power tube screen and plate voltages and they are good. I've gone over PI wiring and voltages and haven't found any problems. Checked for incorrect component values, but haven't found any. Done continuity checks all across the circuit. 
This amp seems to be dodging every punch I make at it. I don't find anything really off but the overall power is not what it should be and the noise in the reverb persist. 


This is one of those amps that test your endurance to the max

[Platefire]

I think I've found the low volume problem but it wasn't what I was thinking. I was out tending to some appointments and put my hearing aids in. When I got back home before I took my hearing aids out I picked up my strat and plugged it into the pro. Wow! with my hearing aids the amp sounds pretty much as loud as it should be I knew I had some hearing loss, but didn't think it to be that bad. I haven't been using the hearing aids only occasionally but I  I've got to! I will just have to get use to playing with hearing aids. So I'm going to declare I think this is the low volume problem pending further testing, with the hearing aid of course!


Reckon if I cranked up that hearing aid amp, I could get a little saturation??
 

[PRR]

....I knew I had some hearing loss, but didn't think it to be that bad.....

It gets more every year.

I should call for an appointment to tune-up or upgrade my aid.

The usual advice in the aid-using community is to wear the aids "all the time", except sleeping or mowing. (I assume you have good muffs for when you mow, hammer, etc.) Hearing is tricky and your brain needs that much practice.

> The amp is now dead silent as far as operating noise

That's another clue. I noted that one side of my stereo monitor hissed. No, actually the other side did not hiss. But did if I used my other ear. I lost my "hiss frequencies", one ear before the other. (The other was not that far behind). Now my "dead silent" is moderately loud to a "normal" ear.

And like my grandfather, I can hear thunder on the other side of the county better than anybody. (And UPS trucks before the dog, but I think she is losing it too.)

[tubeswell]

couldn't get my decimals to land in the right place.

1uF is a millionth of a Farad 0.000001F
1nF is a thousand-millionth 0.000000001F
1pF is a million-millionth 0.000000000001F

[Platefire]

PRR---Second thing I noticed after the extra volume I was missing is I had to turn down the treble---way too bright. Also another sign, I use to never use the bright switch, but lately I've been using it.

One good thing though, even with my hearing aid in, the amp is still real quiet. So it's either one of two things, my hearing is too far gone for even a hearing aid to help or either the amp is actually quiet---ehh


Tubeswell----Thanks for the actual breakdown of decimal places. Something I can refer back to. I also refer to a "Capacitor Conversion Chart" that has side by side comparisons of the same value for uF, pF, nF and K
               

[PRR]

> my hearing is too far gone for even a hearing aid to help

They can help even profound hearing loss. Find a good audiologist.

(I don't think much of the current trend to self-prescribed aids. Even as a lifelong soundman, the adjustments for hearing are not easy/simple.)

[Platefire]

Yes I think my insurance covers a Preferred Provider audiologist and prescribed hearing aid, I need to check into that. Right now the wife and I in the process of Cataract Surgery in both our eyes.

On the amp, if I can figure out the reverb circuit Rattling odd distortion, then that amp will be up to snuff!

 

[Platefire]

This is a you tube video but there is no video, only audio. This is the amp noise my Pro Reverb is making with the volume set on 4 and Reverb set on 3. It's a rattley fuzz like distortion as I strike it on the bass strings in open E. It should be clean tone. I've done a lot of research lately on the net to find a similar noise problem on a fender reverb and have not found another.

It only occurs when the reverb is on. With the reverb turned off with switch or by knob, the amp is clean, as it should be.

If you got any ideas, let me know. I'll be diving in soon to find it. Platefire

eature=share

[tdvt]

Dredging up this old thread as I was wondering if you ever found the culprit?

In the end, I decided to keep my UL Pro Reverb project in a stock configuration channel-wise & while checking things out before it went back in the cabinet, I realized I have a very similar rattle/distortion as you posted in your YT video. Low E/open E chord break up more than I would expect, upper register sounds normal.

I am not sure my issue is reverb circuit-related, but need to go back though it now that I have re-read your thread.

[Platefire]

Yeah, I changed out everything with new that was questionable and did everything I was suggested to do or could think of and the rattling persisted. I finally just had to give it a rest because it became to much of an obsession to me. So I put it up and moved onto other things.

About a month later I took it back out and tried it and low and behold, it was working fine and still does to this day

[tdvt]

About a month later I took it back out and tried it and low and behold, it was working fine and still does to this day

I don't usually have that kind of luck.

Sounds like the shoemaker's elves got to it before you did.

[bmccowan]

About a month later I took it back out and tried it and low and behold, it was working fine and still does to this day

I don't usually have that kind of luck.

Sounds like the shoemaker's elves got to it before you did.

I once took a fine Brownface Concert to a friend's gig to let him borrow it. Unloaded it, plugged in - very low volume. I was running the soundboard, so only had a few minutes - could not find the problem. Took it home and the next day removed the chassis to troubleshoot. It worked fine While I had it open I cleaned up some sketchy solders. I never did figure out what the issue was, but hope that cleaning up those solders fixed the unknown.
BTW - don't ever volunteer to run the sound. Every band member wants it different.