Questions about modifying 79’ Twin Reverb amp.

[sluckey]

six-pack of 1N5408s

[Stevem]

You really don't need 6 5408s as each has a reverse voltage rating of 1 kv which is a large amount above the original ones used in the AB763 amps .
Two on each leg is fine!

[TUBEDUDE]

Be sure to check the bias caps aren't installed backwards. You have enough going on. One of the best twins i've heard i blackfaced, reduced gain and removed feedback to the P.I.

[Stevem]

And get rid of that dam .02 cent cost ceramic cap that feeds the PI section and put a good one in.
Also with the cost cutting way these era amps where wired doing the Kendrick mod of moving all of each channels tone caps and resistors onto the tone pots themselves is very worth the time spend to do so!

[chikov]

I have good news. After I modified the Rectifier diodes and negative voltage bias supply and replaced the rest of the old electrolitic caps ( and yes, ceramic ones too), the amp came to life! I adjusted the bias by measuring the resistance of the OT primery and the voltage drop, then dividing one into another and then multiplying by the plate voltage. I have about -53V bias, and about 442V on the plates. The problem is that i have about 443V on the screens... my understanding the screen voltages should be about 5-7 volts lower. Does anyone know the good way to lower those or why are they almost the same?

[sluckey]

The problem is that i have about 443V on the screens

That's perfectly OK for an UL amp.

Good job on the power supply!

[chikov]

Another strange thing, i can not hear the vibrato. The V5 tube has 360V on plate 1 and 6 and V3 has 360 on pin1 and 312v on 6. Does it mean the tube is not conducting? Should I check the bypass caps and grounding?

[sluckey]

V3 is the reverb driver and has nothing to do with tremolo. Even so, pins 1 and 6 are tied together, so how can they measure different voltages?

V5 is the tremolo tube. You must have the footswitch plugged in and turned on in order for the tremolo to operate. You can enable the tremolo by temporarily grounding the vibrator footswitch jack. If you have done this and still have no tremolo, peek inside the roach to see if it is flashing. If it is flashing, replace the roach. If not flashing, change V5. If still not flashing measure and post voltages for V5 pins 1, 2, 3, 6, 7, 8.

[Stevem]

Just make note that if you have the original chrome duel foot switch for the amp that theTremolo button gets it ground from the rca jack for off of the reverb cable inside the foot switch.
This means the if the reverb can not be turned on and off by the foot switch then neither will the Tremolo be able to be turned on by the foot switch.
This assumes that the neon bulb in the optocoupler talked about above is working.

[chikov]

Thank you again for all your help. Last night I was tired and made a mistake. I have the Reverb tube which is V3 All good and 360 V voltage is normal according to my schematic. However it’s on V5 tube that pin 1 has 360 V .It seems a little high and the schematic calls for 250v voltage on pin1.. the bias resistor seems to be ok and the cap is new. I tried to swap the tube but it is still the same high voltage. When I take out any other preamp tube the plate voltages go up, but on V5 they remain the same. Does anyone know why it might be this way?
BTW, after i shorted the tremolo plug it started working wonderfully. Thank you for the info.

[chikov]

I just sold my own problem. The VE five a is not conducting the current until the vibrato pedal foot switch is closed. That gives the grid ground reference and then the triode begins to work. Smart design

[sluckey]

I just sold my own problem. The VE five a is not conducting the current until the vibrato pedal foot switch is closed. That gives the grid ground reference and then the triode begins to work. Smart design

If you look closely at the schematic you will see that -58vdc from the bias supply is applied through a 2.2M resistor and a 1M resistor to the grids of both triodes of V5. This ensures that both triodes are biased into hard cutoff when the footswitch is off (or unplugged). This is done so the oscillator will be 'shocked' into quick conduction when the footswitch is turned on. This oscillator circuit can be slow to start without this negative bias. It's aggravating to stomp the vibrator pedal and have to wait a couple seconds for the tremolo to start. This quick start circuit overcomes that issue.

[chikov]

Thank you for clarifying the physics behind this tremolo circuit. I was wondering about it but did not have enough info.
I do have one more issue - i have no reverb in my mix. When i rattle the springs in my reverb tank i can hear them loud and clear... when i inject 500 hz signal and probe it with my signal tracer, i hear a weak tone on the reverb input plug, but no reverb in the mix... can anyone give me a hand and see what else I can check to nail the problem?
On the output coil of the reverb tank I reed 166ohms and on the input it is 167 ohms... aren’t they should be further apart?

[sluckey]

The input dcr should be about 2Ω. Your tank is either bad or an incorrect tank. The part number you need for that Fender circuit is 4AB3C1B.

[Stevem]

If the input side of the tank does indeed read 166 ohms, then for sure you have the wrong tank, and your buddy that your doing this work for got ripped off as for sure the seller new the tank was wrong and was just in there for show!

It sounds like the resistance of a Peavey or Kustom reverb tank you have, is the rca input jack on the tank grounded to the tank?

As posted the right tank would read less resistance test wise then it's 8 ohms rated impedance.

What this means is that to test the drive circuit you can simply hook up a 8 ohm speaker if you had to , but just setting a voltmeter for AC volts will prove out drive signal also.