AB165 mod help please!

[pochie45566]

Hello! This is my first post on the amp garage so I would just like to say thank you all for being here!

I am very new to working on amplifiers, I picked up a silverface Ab165 as my first amplifier to mod, I was researching different things you could do to the circuit and I found a common thing to do was convert the power amp to be a spec much more similar to the Pre CBS bassman amps. I have attached the schematic below of the mod that I did.

I turned on the amp, and I have very little power. It is super quiet, distorts very easily, like 1-2 on the volume knob. it doesnt seem to get much louder past that. I have checked my wiring a dozen times and I cant seem to find any errors. I have checked voltage on the tubes and put them in my tester and all checked out okay. I am just wondering where I should go from here in the process of debugging what I did wrong. It isnt a very complicated Mod so I am not sure where I could have gone wrong!

please excuse the sloppy soldering job, I have bee racking my brain trying to fix this issue, swapping leads around and checking. I will surely clean it all up before I am done

If I can upload anything else let me know.

[Stevem]

First off was the amp working normal before you made the mod's?

Second thing is someone new to working on and learning to mod amps should not be starting out with a amp that has so many gain stages!

You may have bitten off more then you can chew here for a newbie!

It's real hard to tell from the photo, but it looks like that negative lead of that right leaning 22uf 35 volt cap is shorted / sitting on the lead of that 1.5k Cathode resistor.

The other thing about these 2 black electrolytic caps if these are part of your mod / changes, then they're the wrong caps ( you have .022 uf caps circled in red) to change out, you have the phase inverter plate to output tube coupling caps circled in a Red and those are not them!

Also you can't just remove those 220k 1/2 watt resistors that go from pin 3 of each output tube back to the phase inverter plates without making a bunch more needed changes since the circuit gain changes greatly without them in circuit.

One of the most important things to confirm so that you don't blow anything thing up or fry your output tubes is that on pin 5 of each output tube you have at least a -47 volts DC reading on your meter there, so check that .

[Phil_S]

...please excuse the sloppy soldering job...

There is no excuse for sloppy work. Do things the right way. Making a good workman-like solder joint is important for many reasons. The main reason is that you really need that joint to perform correctly. For example, a cold solder joint may act like there is no connection at all. I am attaching the best thing I've seen on how to solder correctly. It may take a little practice. Anyone can get this right.

Good luck with your "mod." As Steve notes, this was a lot to tackle for a first time project.

[pochie45566]

First off was the amp working normal before you made the mod's?

Second thing is someone new to working on and learning to mod amps should not be starting out with a amp that has so many gain stages!

You may have bitten off more then you can chew here for a newbie!

It's real hard to tell from the photo, but it looks like that negative lead of that right leaning 22uf 35 volt cap is shorted / sitting on the lead of that 1.5k Cathode resistor.

The other thing about these 2 black electrolytic caps if these are part of your mod / changes, then they're the wrong caps ( you have .022 uf caps circled in red) to change out, you have the phase inverter plate to output tube coupling caps circled in a Red and those are not them!

Also you can't just remove those 220k 1/2 watt resistors that go from pin 3 of each output tube back to the phase inverter plates without making a bunch more needed changes since the circuit gain changes greatly without them in circuit.

One of the most important things to confirm so that you don't blow anything thing up or fry your output tubes is that on pin 5 of each output tube you have at least a -47 volts DC reading on your meter there, so check that .

Thanks for chiming in.

Yes, the amp was working before I dove into it lol. I know I probably bit off too much to chew but I am also really excited to learn what's up here.
I just checked that 22uf cap, it was not touching that lead and being shorted out. Just a camera trick. Those black caps are not a mod, the previous owner just replaced old caps that were leaky.

The schematic I got was off of this website http://fenderguru.com/amps/bassman/ so I figured it would be a well-documented mod. I thought that the removal of the 220k resistors was counteracted by the increase of negative feedback and the slight reduction of gain in the PI. Or that's what the website leads me to believe. Mind you I have not completed all the changes in the schematic, I did not change the coupling caps going to the power tubes. And yes, I have -47 volts on pin 5 and have tested the tubes.

There is no excuse for sloppy work. Do things the right way. Making a good workman-like solder joint is important for many reasons.

I completely understand and agree. I have leads i need to cut down and solder joints to go over properly.

Thanks for the help so far guys.

[pochie45566]

Anyone else have any more ideas on where to go from here? maybe restore the circuit to stock then try again?

[sluckey]

Anyone else have any more ideas on where to go from here? maybe restore the circuit to stock then try again?

That's a good idea. You may even discover your gremlin(s) in the process. But start with the original and verify it is working properly. Then do small incremental steps and verify the amp still works after each step. When you finally finish you should have a working amp (providing the mod schematic you are working from is proven to work).

[Stevem]

Even without a O -scope if that happens to be the case there are simple ways to check signal flow through a amp with just your voltmeter, some clip leads and your guitar or something like your phone to pump out a test signal.

What you want to do is set your meter for AC volts and then check the down stream end of any gain stage coupling cap to test for audio output while pumping signal into to the amp, this will show up as a AC voltage on your meter.

If we assume that all the voltage supplies in the amp are feeding each gain stage as needed then here's what to do.

To see if the phase inverter section is working and feeding your output tubes hook you meter set for AC volts across pin 1 of each output tube socket and play/ apply signal to the amp and you should read atleast 15 volts of signal as you open up the volume control of whichever channel your in.

If this output side test of the phase inverter fails then move to the input side of that what should be a 12AT7 tube.
Now hook your meter up from pin 2 to ground and make another test and you should see atleast 5 volts of signal there.

It's important that you only test for signal with a meter on the down stream end of any coupling cap because if you do not you will not be seing an AC voltage , but a D.C. Voltage.
Please do these test and report back, but first before doing these signal test confirm that you have plate voltage on every preamp tube as is shown on the Fender schematic for that amp, and also that on any preamp tubes Cathode pin ( 3 or 8 ) you should be reading a at least 1 volt.

These plate and Cathode test should be done with the meter set for DC volts, not AC.

Report back please.

[pochie45566]

Even without a O -scope if that happens to be the case there are simple ways to check signal flow through a amp with just your voltmeter, some clip leads and your guitar or something like your phone to pump out a test signal.

What you want to do is set your meter for AC volts and then check the down stream end of any gain stage coupling cap to test for audio output while pumping signal into to the amp, this will show up as a AC voltage on your meter.

If we assume that all the voltage supplies in the amp are feeding each gain stage as needed then here's what to do.

To see if the phase inverter section is working and feeding your output tubes hook you meter set for AC volts across pin 1 of each output tube socket and play/ apply signal to the amp and you should read atleast 15 volts of signal as you open up the volume control of whichever channel your in.

If this output side test of the phase inverter fails then move to the input side of that what should be a 12AT7 tube.
Now hook your meter up from pin 2 to ground and make another test and you should see atleast 5 volts of signal there.

It's important that you only test for signal with a meter on the down stream end of any coupling cap because if you do not you will not be seing an AC voltage , but a D.C. Voltage.
Please do these test and report back, but first before doing these signal test confirm that you have plate voltage on every preamp tube as is shown on the Fender schematic for that amp, and also that on any preamp tubes Cathode pin ( 3 or 8 ) you should be reading a at least 1 volt.

These plate and Cathode test should be done with the meter set for DC volts, not AC.

Report back please.

So, I converted the circuit back to stock, turned it on, and it plays. But it crackles. and crackles. when i play loud the crackling gets worse. I went through the circuit tapping with a pencil to ensure there wasn't any loose components, swapped all preamp tubes, checked my output tubes in a tube tester, and nothing.

here are the results from the tests i did: pin 1 on both power tubes gave me about 11-14 volts, and pin 2 to ground gave me 3 volts. Which is too low in respect to the 5 you said I should have, What does this mean?

[pompeiisneaks]

First off, be very careful with a pencil, graphite is conductive and can kill you, use a wooden chopstick to poke around.

When you mention your readings you should mention what they were AC or DC. Pin 1 should be DC readings, Pin 2 is a heater pin, so it should be AC. 3 volts AC should be about right on pins 2 and 7. (the heaters). you should get DC on all the other pins, of some kind, depending.

11 VDC seems possibly okay on the power tubes pin 1, that's the ground connection. (I just don't know for sure on this amp what it should be.)

~Phil

[sluckey]

here are the results from the tests i did: pin 1 on both power tubes gave me about 11-14 volts, and pin 2 to ground gave me 3 volts. Which is too low in respect to the 5 you said I should have, What does this mean?

Pin 1 should be about -45VDC, same as pin 5. Pin 2 is filament and should be approx 3.15VAC. It means you're in the deep end of the pool.

[pompeiisneaks]

here are the results from the tests i did: pin 1 on both power tubes gave me about 11-14 volts, and pin 2 to ground gave me 3 volts. Which is too low in respect to the 5 you said I should have, What does this mean?

Pin 1 should be about -45VDC, same as pin 5. Pin 2 is filament and should be approx 3.15VAC. It means you're in the deep end of the pool.

Wait, I'm confused?

Pin 1 on a 6L6 or 6V6 is the ground pin no? Why would it be -45VDC?

isn't the negative bias on the grids like pin 5?

~Phil

[pochie45566]

First off, be very careful with a pencil, graphite is conductive and can kill you, use a wooden chopstick to poke around.

I actually used a paintbrush

I think my voltage readings are all okay from what I can tell on the schematic. The amp was working just fine before I tried to change the circuit and now that I have put it back to stock, theres just this nasty crackling sound when i turn up the amp. It gets worse as it gets louder until it just cuts out. Too much gain right? It sounds ok ish at like 1 but once it is at 2 it gets messed up...

Do you guys I made a mistake in the rewiring that is leading to just too much gain? It happens on both channels so I think it is either something involving the PI or power section. I put the 220k's back on the grid of the 6l6's and put the negative feedback back to stock, which I guess are both things relating to gain. But what else could be causing this? Maybe I damaged a component when i dug into the amp? I've been going over my work following the schematic and layout diagrams. I think I am capable of fixing this issue I just do not have enough experience in troubleshooting procedures to know where to go.

and it was 11 VAC on pin 1 to see if signal was even getting there

[sluckey]

here are the results from the tests i did: pin 1 on both power tubes gave me about 11-14 volts, and pin 2 to ground gave me 3 volts. Which is too low in respect to the 5 you said I should have, What does this mean?

Pin 1 should be about -45VDC, same as pin 5. Pin 2 is filament and should be approx 3.15VAC. It means you're in the deep end of the pool.

Wait, I'm confused?

Pin 1 on a 6L6 or 6V6 is the ground pin no? Why would it be -45VDC?

isn't the negative bias on the grids like pin 5?

~Phil

Look at the fender wiring diagram and it will all make sense.

[pompeiisneaks]

This one?

http://www.prowessamplifiers.com/schema ... _schem.pdf

it doesn't show pin 1 on the schematic at all from the horrible quality that I can tell?

~Phil

[sluckey]

Look at the fender wiring diagram and it will all make sense.