Questions about modifying 79’ Twin Reverb amp.

[chikov]

Thank you, that helps a lot. I will see to it that the new tank will be ordered. I do have a silly question, should the reverb pedal be closed or open if you want to hear the Reverb effect?
I found another reverb tank that has the proper model number ( off another twin amp), but the effect is so quet... is it normal for Fender Twin reverb to be so quiet?

[sluckey]

open

[chikov]

I found another reverb tank that has the proper model number ( off another twin amp), but the effect is so quet... is it normal for Fender Twin reverb to be so quiet?

[sluckey]

no

[Stevem]

The 3rd preamp tube needs to be a strong 12AT7 to drive the pan.
A 12AX7 does not have enough drive current and will not live long besides!

A foot switch is only needed to turn off the reverb or activate the Tremolo.

[chikov]

I have one good news. I found that the reverb output RCA plug was not making a good connection. I replaced it and now I have a strong reverb when you use the Vibrato inputs. Is it normal for this amp not to have the reverb in the Normal input?
Everything else seems to be working now but I have a strange issue with Vibrato. I replaced RCA socket with 1/4 jack and now, when I plug in a cable and short the hot to the ground the vibrato comes on for a second and then dies down. I can even see the diode blinking at first and then goes dim. Does anyone know what is caiusing it? I tried a different tube in V5 already and it did not help ( it was working ok a couple of days ago...).

[Stevem]

The normal channel is called such because it does no have reverb, however it is possible to rewire the amp to have reverb on both channels if you like.
In regards to your Tremolo issue please take the voltage readings on the tube for when it's flashing and when it stops and report back please.

[chikov]

Thanks again for your help. When I measure the voltage on the pin 1 of a V5 tube It shows 365 V. When I ground the vibrato foot switch it drops to 220 V. The voltage on the pin six is 313 V and when I ground the foot switch it goes down to 308.

[chikov]

At some point the vibrato effect worked, But now it’s back to slowly dying off after I ground the foot switch. As soon as I grounded the octocoupler oscillates But slowly dies down and the voltage on the pin the one of the V5 tube Starts at 365 then isolates and stops at 212 or so. I replaced .01 caps already and bypass capacitors too... The tube is 7025 and it’s strong. Does anyone have any ideas what else can cause it? Thanks

[Stevem]

Replace both 25uf 25 volt cathode bypass caps on that 12AX7.
The common 22 UF caps found today are fine for this useage.

[chikov]

Sinse I already replaced both bypass caps with the new ones, i decided to inspect the solder joints above and below, but did not see any problems there. Now, though the vibrato intensity pot completely turns off the signal when I turn it all the way to the right ( works like reverce volume pot). I am completely lost at this point and will have to walk away, do some more reading on it and wait, see if anybody here has any idea what in the world going on here ...
PS. On V5B the voltage on cathode ( when the effect is on) is about 3.75V instead if 13B per schematic

[chikov]

I just found that both of my phase shift capacitors (.01) were shorted. I wonder what voltage rating should they have?
Update. After the replacement of the failed capacitor is everything started working. Like they say, if the vibrato is not working it’s best to replace bypass capacitor and phase shift capacitors and make sure that the tube Is strong

[Stevem]

Yes, those .01 caps need to be rated for that voltage seen on the plate when the Tremolo is not on.