Fender twin reverb (ab763) PI trouble

[Lifto]

Hi all!
After lurking around for a while I finally decided to register myself. I've been learning my way around tube amps for a little while now. I'm not very experienced yet but I really enjoy the marvelous world of tube amps. I started with building some simple guitareffects and started to proceed towards tube amps (don't worry, I've learned about the high voltages and I know how to properly drain the caps and how to operate a DVM). It seems now however that I'm a little bit lost with my last project.

I bought a '73 Twin Reverb which had a non-original circuit board installed. I ordered a AB763 build and bought all the parts. I've finished the build and hooked everything up. I do have some sound from both channels but it's very distorted and has a very low output (I checked the polarity of the speaker). I' m running the amp on 2 JJ6l6's with an 80hm speaker load.
I' ve measured all the pins on the tube. I noted them down here:
Valve Data Recording Program

Altough almost all tubes show some deviation of the layout it seems to me that the biggest problem is with the Phase Inverter (v6). P1 measures 406v, P6 393. The wire going from the caps to the 100k and 82k resistors measures approximately 440v. I checked the solder joints and the resistors and every things checks out fine.

Does anyone have an idea what could be the culprit?
Thanks

[Enzo]

Clearly the phase inverter is not right. It is not conducting much. Because you built it and bought all the parts, my first thought is a wrong part. Believe me, we have all done that. It is so easy to put a 470k rsistor in place of a 470 ohm resistor. And as one person found out, it is also easy to transpose a couple parts on the eyelet board and have it still "look" OK.

On the PI tube, pins 3 and 8 are the cathodes tied together, they ought to have 100 volts DC on them. You get 12. So power down and discharge. Now verify each resistor in the PI stage. And we cannot forget the wiring itself. You could have a bit of excess wire lead length touching something.

But it is important not just to have the right parts, but also the right circuit. SO look at the AB763, from pin 3/8 on the PI tube I see the path to ground through a 470 ohm, a 22k ohm, and a 100 ohm resistor. That totals 22,570 ohms from pin 8 to ground. What reading do you get?

measure directly from pins 2 and 7 to 3 or 8. Do you get about 1 meg?

Pins 1 and 6 are likely wired right, but lets check. Measure directly from pin 1 to pin 6 for resistance. Do you get about 182k?

[Lifto]

Wow what a great advice, thanks Enzo.
You made a good guess. I did indeed mix the 470 ohm resistor up with a 470k resistor. I just did a quick check before I was off to work and the amp actually sounds good! The readings from pins 2 and 7 are correct now, so is the resistance from pin 3/8 to ground. There was still some humming going on, but the volume was way up and so was the dsitortion. I didn't have the time to check what voltages are on the PI now but I guess it'll be a lot closer to the voltages on the layout. Too bad I will be gone for a few days but when I return I will recheck the voltages and the resistors to see if they're correct.
Thanks for the help (made my day!). I'll keep you updated

[überfuzz]

There was still some humming going on, but the volume was way up and so was the dsitortion.

I've played some Twin Reverbs that were dead silent and some with a, well, lets put it this way, a pleasant hum. I'd say you just need to find your level of hum tolerance. :-)

[Lifto]

Today I finally had the time to do some more checks! At this moment, I have clean sounds from both channels so you would think so far so good!
I still need to install the connectors for the reverb and vibrato footswitch, so I don't have reverb or vibrato yet.
Today I measured the voltages at the preamp tubes again and found some interesting results:

Valve Data Recording Program

On V3, pins 3 and 8 were 8V but now read 0.
On V5 pins 3 and 8 rwere 2V but now read 0.
More interesting however are the voltages on pins 2 and 7 of V5, these are reading -17V.
The PI now read good.

I guess I'll have to check for some resistors with the wrong values and bad connections.

I sometimes get a loud hum, the weird thing is it can come back or dissapear by itself while the touching the amp. It's on both channels. When I was checking the voltages the hum was there but I noticed that when I measured volts on pin 2 or 7 of V6, the hum dissapeared. Does this mean there's a grounding issue on V6?

Lots of question but I'm really satisfied the amp produces sounds, now I need to tweak it to make it perfect!

[Lifto]

Hi all,

I need some more advice to complete my build. I still have a loud hum hum sometimes suddenly goes away, but can also come back. I isolated the problem to V4 (reverb). When I pull this tube the hum stopts. I resoldered the tube socket and the other connections but haven't found the cause of the problem yet. The pitch/intensity of the seems to be affected by the volume on channel 2. When I increase the volume a little bit to (+-) 3 the hum softens.
Is there anyone that recognises this problem and show the direction to look at?

Thanks alot for the help so far! Lots of good reads on this forum

[überfuzz]

If you kill the hum while touching the amp, you probably just ground something. Did you check the grounding scheme in the amp?

[Lifto]

I think my post was a little vague, sorry!
The hum doesn't stop when I touch the chassis. Today I resoldered the grounds (also the ones in the cap can) but the hum is still around. I'm sure the problem is in the preamp since pulling the PI stops the noise. When I turn one of the volumes to 3 the noise fades a little bit (seems like the volume in the vibrato channel has more influence on the noise). I also checked for bad tubes.
Since today another problem has arisen. When I turn the standby off there's a loud crackling sound coming from the speaker. I think this problem is related to the reverb/vibrato since I was working on that yesterday. I checked for dirt in the reverb input and output.

I'm getting a little bit frustrated but I'm sure that in the end it'll be worth the hassle, I loved the clean tones during the moment it was working properly.

[lowell]

do you have a reverb tank plugged in? If you do not, and the reverb level is turned up you will get hum.

regarding V5: If the vibrato switch is set OFF, the grid pins will have a little more than your negative bias voltage, which can be read at the juntion of the 220k resistors feeding the power tube grids (pin5). I THINK that V5 grids should be 0vDC with the vibrato switch ON.

[Chuck H]

+1 on the reverb settings and hum. All Fender "in amp" reverbs hum because they didn't use isolated jacks. This causes a ground loop. More hum than seems normal though may just be a result of the reverb not being hooked up.

It IS interesting though that the hum is reduced when you read on pins 2 and 7 of V6. I'll assume this is the PI tube? The actual Fender schem has no V#s to reference. Since the PI grids (pins 2 and 7) have a positive voltage for bias and the PI input impedance is very high, even the high impedance of your meter offers some path to 0V reference (chassis) and this shifts the PI bias cold (less output) This is what may be percieved as a hum reduction. But, IMHE, the phenomenon is normal behavior.

Your doing great so far. Tearing an amp down and rebuilding it as a novice can result in several snarky problems that need to be hunted down without the benefit of experience. Your ahead of the game.

[Lifto]

Thanks Chuck for the kind words! This afternoon I will continue my investigation. I'll focus on the reverb connections.

[Lifto]

So yesterday I spent some more time on my amp. I cleaned up te wires going from the pots and reflowed the reverb connections and vibrato/reverb cinch connectors. I managed to solve the crackling but still the hum remains.
Today I was comparing the AB763 schematic with the AA270 schematic (which my amp originally was). I noticed that on the AA270 schematic ther're 2 100ohm resistors going from the pilot light to ground. These resistors aren't included on the AB763 schematic. After some more reading I found out that the ab763 transformer had a CT connected to ground and there the 2 100ohm resistors aren't installed. Am I correct assuming that the AA270 transformer doesn't have a CT wired to ground and therefore I should install the 100ohm resistors?

[lowell]

You still haven't reported back on turning the reverb knob down. I'm assuming you tried this and it didn't fix the hum?

If your green heater wires on the PT secondary don't have a 3rd green/yellow (typically this is the CT color) going to the chassis then yes it'd be wise to install the 2 100ohm resistors.

[Chuck H]

If your 6.3V PT secondary DOESN'T have a center tap then absolutely install two 100R resistors. One from each end of the 6.3V wind to ground.

[Lifto]

You still haven't reported back on turning the reverb knob down. I'm assuming you tried this and it didn't fix the hum?

If your green heater wires on the PT secondary don't have a 3rd green/yellow (typically this is the CT color) going to the chassis then yes it'd be wise to install the 2 100ohm resistors.

Sorry I didn't report back on this question yet. The reverb did increase the noise when turned up but wasn't the cause of te problem.
Today I installed the two 100r resistors and now the amp is dead quiet! Very happy the amp is working properly now and looking forward on testing the amp in a band situation.
Thanks you all for your input and suggestions. I' ve enjoyed working on this project and will continue learning about (and building) tube amps. I'm already thinking about a next project. Seems like I'm addicted! I'll definatively will hang around on this forum!