Reverb - what is wrong?

[txstrat]

Hi folks,

this is my first post so hi everybody.
I recently finished my second amp with reverb but the reverb doesn't work as it's supposed to.
It's a standard reverb circuit like in every other Fender amp (actually it's the Vibroverb 6G16 circuit).
At time I'm trying to locate the problem and want to post what I've already done to locate it.
If my assumptions are wrong I would appreciate any answer to get me right.

When I turn up the reverb pot I can hear slightly more hum increasing in the speakers.
I can hear the reverb (springs) in the speakers when I nudge the reverb pan. That should indicate the recovery circuit as working.
To make sure it's got nothing to do with the reverb tank or cables, I hooked the unit to another amp and it worked fine. The other amps unit did not work on the new amp.
I swapped the tubes (they are brandnew) with two different ones known to be ok. Still no change.

Then I got back into the circuit. I measured the voltages of the reverb transformer. Red wire reads 300V blue wire reads 296V. Seems OK for me. Haven't measured the cathodes voltages yet. When I touched the annodes of Tube1 (driver tube) with the tip of the multimeter I could hear a krackle in the speakers mixed with reverb. That should indicate the driver circuit and the transformer as working too.
So far I have a working reverb unit(?), only the guitar signal doesn't go through. I suspect the 500pF in front of the grid of Tube1 to be broken but haven't changed it yet. (see attached schematic)
Any other suggestions?

Thanks

Matt

[Enzo]

OK, so the recovery works and the pan is OK.

The drive end has the problem. I will assume you have tested the cables. If not, do. The tube drives the transformer, so indeed, check the cathode voltage. Since ther is some voltage drop across the transformer primary, I will think the tube is conducting until you tell me different. If tehre is no cathode voltage, then the tube ain't doing its job.


Touching the plate makes a little pop and it reverbs some. OK, but now test the tube. Touch the grid. Now the tube should amplify whatever pop your meter represents. Check the grid for DC voltage. SHould be none, or maybe a very small negative amount. If so, that is good. Now touch your meter probe to the grid. REverb pop louder now? SHould. Flip your meter to ohms and touch it to the grid. Ground the black probe. Now your meter ohms test voltage will hit the grid. That should make a real solid pop through the reverb.

The amp works fine otherwise, right? That means the signal should be on the outer end of the 500pf cap. It should pass through the cap, losing some bottom end as it does, and feed the grid of the reverb drive tube. So sure, suspect that cap. This is a go or no go situation, so clip ANY little cap across the 500pf and see if the reverb wakes up. We don't care about tone, just does it work. If that wakes it, then replace the 500pf. Use whatever you have, not electrolytic, like .01uf, .047uf, .001, etc doesn't matter.

OK? We made sure the tube was conducting, tested the tube for amplification, and bypassed the input cap. That should find it.

[txstrat]

Hi Enzo,

yes, I used different cables with the second reverb pan. I'll check the voltages as you supposed. A failure then might be in the circuit (bad parts?), cause I already swapped the tubes with two different ones. That should tell if one of the others was bad. When I noticed the anode popping sound for the first time I think I tried the grid too and there was no sound at all while touching it. But this is only an assumption from my memory. I can tell exactly when I tested it again.

[Alex/Tubewonder]

Swap the 47k and 470k resistors on the last triode and it should be working ok.

[txstrat]

How come you think it will work when I swap the resitors? I used exactly this configuration in my other amp and it works fine (it's from the Fender Deluxe Reverb II schematic).

[Alex/Tubewonder]

470k and 47k grid leak resistor as per schematic form a 1/11 (ca 20db) voltage divider/attenuator, even with reverb pot at max the delayed signal is attenuated by 20db after the recovery amp stage.
Your recovery amp has voltage gain ca 45 (33db), with 5mV signal from tank you will have max 5mV x 45 / 11 = 20mV at the grid of dry/wet mix triode with reverb pot at max. Voltage gain from tank to mix point is thus max x4 i.e about 12-13db. It's too low.
If resistors are swapped the attenuation wil be 1.1 instead of 11, total gain through recovery circuit to the mix point will be 45 / 0.91 = 41 i.e. 10 times more, your max recovery signal will go up to 200mV at mix point.

[Bruce / Mission Amps]

470k and 47k grid leak resistor as per schematic form a 1/11 (ca 20db) voltage divider/attenuator, even with reverb pot at max the delayed signal is attenuated by 20db after the recovery amp stage.
Your recovery amp has voltage gain ca 45 (33db), with 5mV signal from tank you will have max 5mV x 45 / 11 = 20mV at the grid of dry/wet mix triode with reverb pot at max. Voltage gain from tank to mix point is thus max x4 i.e about 12-13db. It's too low.
If resistors are swapped the attenuation wil be 1.1 instead of 11, total gain through recovery circuit to the mix point will be 45 / 0.91 = 41 i.e. 10 times more, your max recovery signal will go up to 200mV at mix point.

And to add to that obervation, you are also taking signal to the reverb driver from a VERY low point, the wiper of a volume control through a tiny 500pF cap. That is a weak point to start from anyhow.
The raw dry signal is usually taken from an unattenuated place such as directly from the coupling cap of a previous gain stage!
Even though it probably works anyhow, you are using a 12AX7 to drive the reverb OT (maybe 15K-22K primary?) it is probably not the right tube for a reverb OT like you are using. Try the much lower impedance of a 12AT7....
With respect to your mixer stage, you might find it better to use a smaller value cathode biasing resistor with a bypass cap on it.
That could be an issue for gain later.
One more thing... hum from the reverb level, like this one, is usually a function of a ground loop in the audio chain with respect to the dry and wet gain stages, volume or level pots, reverb amp filtering sections and the dry signal triode.
It is tricky!

[txstrat]

Thanks alot.
I didn't know that much about the reverb stage. Anyway.
I fixed the problem basically with the help of Enzo. I checked all the points on the driver tube and found out it makes a popping noise on the anodes and the cathodes.
A slight popping sound could be heard when I touched the cathodes and afterwards the grids. When I touched the grids again right after the first time there was no sound at all. Touched the cathodes again and then the grids - slight pop. I thought the popping sound when touching the grid was the unloading bypass capacitor on the cathodes. That would mean the cathode resistor was bad - and it was. It was open. Swapped it for a new one and - dramatical pause - REVERB.
Thank you Enzo and everybody who posted here. And yes, the transformer is 14.5 K primary. I'll follow you and try a 12AT7. (by the way, I use that one already in my other amp)
The reverb stage of a tube amp can be improved in many ways as I learned from you. I for myself use the reverb at around 9 o'clock on the dial. That means no audible hum to my ears. I'm happy with this one now. Thank you all again.