Maybe you can help me Figure this one out?

[Platefire]

Got a 1974 Fender Pro Reverb Head that I bought way back in 2013 that was in pieces in boxes. I re-built the circuit to be stock AA165. It works good but I've been noticing something I can't quite figure out. When I turn the reverb control up it increases gain in the Vibrato channel. I've been studying the schematic to try to figure what may would allow that. I checked the input driver(V3) thinking I may have installed a 12AX7 instead of a 12AT7, but it's got a 12AT7 in there. I'm thinking the problem has to be somewhere between the 100K Reverb Pot and where the signal grid re-enters the pre-Phase Inverter circuit. Another clue is it's got a slight buzz that increases as you turn the reverb up.

Truth is I kind of like the affect, sounds and feels pretty cool. But I am now curious of what's causing it. I know the next move would be to pull the chassis, start comparing what there with the schematic and check the condition of components.
Anybody ever had this issue/effect before? Platefire 

[nandrewjackson]

I could envision that if the 220K to ground , that's after the reverb pot , were missing, then that pot would act as a gain control.


??

[pdf64]

That’s all I can think of too, but then a Princeton Reverb seems to manage without that 220k, and not exhibit that behaviour.
https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_reverb_sf_aa1164.pdf

[sluckey]

I've never thought about it but I bet that if you connect a proper sine wave test signal to the input jack and connect a scope probe to V4-7 you will see a small change in signal amplitude as you turn the reverb pot. What you are experiencing may not be a problem at all.

Didn't you put a Trem-O-Nator mod in this amp? Should not affect the reverb unless you didn't follow instructions closely.

[pdf64]

Does it still do it if the valve in V3 is removed?

[Platefire]

Hay Thanks for the Replies! I've been reading a thread that went from 2011 ro 2018 on another Forum that had the same problem on a Silverface vibrolux that turned out to be a faulty ground on the return cable.
https://www.tdpri.com/threads/overall-volume-increase-when-i-turn-up-the-reverb-sfvr.497178/
The Trem-O-Nator works fine and I don't think would have any effect on this?
I put new reverb send and return cables when I overhauled this amp. I just checked them and seem to still be working ok.
I haven't tried pulling V3 yet. Thanks, Platefire



 

[sluckey]

So, tell us more. Is the gain increase barely noticable, very obvious, or even irritating? If you kill the reverb with the footswitch does the reverb pot still increase the gain?

The Trem-O-Nator works fine and I don't think would have any effect on this?

The TON circuit has nothing to do with the reverb circuit on paper. But some of the TON components share the same ground lug as the 220K resistor mentioned above. Worth checking. The 220K mentioned is the blue resistor in the bottom right corner of this pic. It has been mucked with.

[Platefire]

The volume increase in minimal yet noticeable. For example, I usually run my reverb on 3 and the volume on about 2(Just jamming by myself). So if I cut my reverb off, I would have to raise my volume back up to 2.5 to compensate for the loss. So minimal! But---along with the volume bump from the reverb being on 3,  I get a boost in compression. Also the compression effect increases gradually as you turn the volume up with reverb remaining at 3. I like the effect but the other Fenders I've had never did this. When you turn the reverb up on other Vibrato channel fenders, it had no effect on volume or any compression. Like I said in the original post, I not necessarily  wanting to correct this, I was just hoping to find out why the different behavior. Hay I might want to put a switch on it and make it switchable:>) I'm thinking this sound would be fun to play with in a band. I should probably say at this point the AA165 circuit is stock but I am running and 5U4 rectifier instead of a GZ34 and have it biased at about 60%+ Dissipation. So I'm putting everything out on the table so you get the full picture. So I may want to fix it at some point if I latter determine this oddity is not usable.

BTW--what did you mean by the 220K resistor being mucked with. Not soldered properly? in the wrong location??
Both? Thanks, Platefire
 

[Platefire]

So your saying the 220k resistor to ground that you pointed out as others have already pointed out is the main suspected culprit?

[sluckey]

If you want me to play you will have to answer my questions.

[Platefire]

OK sluckey, let try this again:
So, tell us more. Is the gain increase barely noticable, It's not huge but noticeable

very obvious, or even irritating?  If you turn the amp on with the reverb already on it is obvious that theamp is louder at very low volumes like 1.5 with a touch of compression like effect. You can really tell the difference if you then turn the reverb to "0" then you can really tell the volume and gain loss to a sound you would normally expect at a volume of 1.5. The sound is kinda cool if your going after that kind of driven tone but you might not want that all the time.

If you kill the reverb with the footswitch does the reverb pot still increase the gain? No! If you kill the reverb with the foot-switch there is no boost and the sound returns to normal
But some of the TON components share the same ground lug as the 220K resistor mentioned above. Worth checking. In checking should I disconnect or clip the grounded leg of the resistor and maybe alligator clip it to another chassis ground to see if it makes a difference to return things to normal?

[sluckey]

If you kill the reverb with the footswitch does the reverb pot still increase the gain? No! If you kill the reverb with the foot-switch there is no boost and the sound returns to normal

That's the one! 
No need to mess with the 220K. I don't think you really have an issue.

[Platefire]

OK Thanks sluckey! I reckon I'll just go ahead and enjoy it

[murrayatuptown]

Not much help, but I occasionally remind myself when I can't see why a symptom might be occurring that the engineers never updated the drawing to include component failures and someone else's handiwork efforts.

Sometimes that's just commiseration with my own reflection, but sometimes it helps me look beyond the self-installed blinders I might be wearing.

[Platefire]

Hi Murry
I guess I'm a little more sensitive about this amp than usual. The reason being that after I got the tube amp bug around 2000 I was constantly hunting the pawn shops, flea markets, Garage sales and thrift shops looking for a vintage Fender amp. I ran across some and made offers, couldn't make a deal. I found and bought a lot of other amps that wasn't fenders. Hi-fi amps, reel to reels and such as that. So in about 15 years of being on the hunt I finally was able to pick up this 1974 Pro Reverb. I really don't consider 1974 all that vintage but it was close as I have got:>) So this is my complete Vintage Fender collection.

Yeah I hear you about updates and changes sometimes not getting included on schematics and layouts until maybe years later. Everybody is scrutinizing the Fender info now days and corrections and improvements are coming forth. Still with technology being limited for a long time with Fender, they manage to produce some great sounding amps with what they had.

Regarding taking off the blinders, that's where this forum helps a bunch. You get different ideas coming from different experiences and directions. The Bible says, it is Wisdom in a multitude of counselors and I've found that to be very true. Platefire