potentially reversed OT primaries? Now Express trouble shooting

[aceofbones]

Ok
1) use that 1 meg to parallel that 220k.

2) confirm that you can set the bias voltage on pins 5 to -33 volts and leave it set there,

3) hook up a voltmeter set for AC volts across your load resistor.

4) pop in the output tubes and power on, if you read anything above 1 volt on the meter then the amp is oscillating and we need to help you track down where and why!

Alright, I've added the 1M and now have a bias range of -25.3v to -35.5v. I've left it maxed at 35.5 for now.

Sorry, I'm not sure which load resistor I'm supposed to be testing.

[bepone]

I don't want to be critical, but this recommendation about the heaters isn't right.
[/quote]
ok,

I agree that the original method of wiring the Express' heaters is a little suspect, but nevertheless, it does result in a very quiet amp. On the build Ace is referencing, I copied Ken Fischer's wiring from Francesca.

ok guys agree it is not so important for the noise explained in the subject..my next concern is oscillating of VVR. i had a lot of problems with that before..i would remove it tottaly and test the amp without VVR and see.
p.s a ctually the best VVR what i saw working well is in the Marshall AFD 100, really like how is working , smooth and not changing the character

[aceofbones]

Ok, just did a voltage check at full power with all tubes installed. I've included a filled in voltage chart. Something seems messed up. Cathode and grid numbers in the preamp are wayyy off, unless I somehow measured those incorrectly. There was a pretty high pitch sound the whole time. I thought this was because of swapping primaries, so I swapped them back. That made it wayyyy worse, so I've swapped them back, again.

IMG_8003.JPG

Also, the bias range I wrote down was without tubes but the actual bias on the tubes was with them in. The -35.5v changed to -31.8v once the tubes were in.

I've also included transformer info in case it's needed.

[sluckey]

All little tube cathodes are reading waaay high, as if the cathodes are not connected to ground. Measure resistance from V1 pin 3 to chassis. One probe directly on the socket pin 3 and the other probe on the chassis. What have you?

[aceofbones]

All little tube cathodes are reading waaay high, as if the cathodes are not connected to ground. Measure resistance from V1 pin 3 to chassis. One probe directly on the socket pin 3 and the other probe on the chassis. What have you?

106k ohms

[sluckey]

BINGO! Should be 1.5K. Now use a gator clip lead to connect that buss bar on the back of your pots to chassis.

[aceofbones]

BINGO! Should be 1.5K. Now use a gator clip lead to connect that buss bar on the back of your pots to chassis.

Ok, I've done that and now read 2.68K.

[sluckey]

BINGO! Should be 1.5K. Now use a gator clip lead to connect that buss bar on the back of your pots to chassis.

Ok, I've done that and now read 2.68K.

Better, but still not good enough. The problem is that copper buss bar soldered to the back of the pots. That buss bar is not providing a solid ground. All 3 little tubes have their cathodes connected to that buss bar. You do not have a good electrical connection at the backs of those pots. You must scrape the coating off the back of those pots before even trying to solder to them. Sandpaper or a dremel works well. Then put a little flux on the scraped area and the copper buss bar and finally use sufficient heat to make the actual solder connection. Most pencil irons cannot do this properly. But I would not bother. Solder a short #18 wire to the end of the buss bar and bolt the other end to the chassis near the input jack.

Once you have the buss bar connected to the chassis properly, you will have zero ohms between the buss bar and chassis. And V1 and V2 cathodes will measure the correct resistance to chassis ground. And then your cathode voltages will be correct.

[aceofbones]

Better, but still not good enough. The problem is that copper buss bar soldered to the back of the pots. That buss bar is not providing a solid ground. All 3 little tubes have their cathodes connected to that buss bar. You do not have a good electrical connection at the backs of those pots. You must scrape the coating off the back of those pots before even trying to solder to them. Sandpaper or a dremel works well. Then put a little flux on the scraped area and the copper buss bar and finally use sufficient heat to make the actual solder connection. Most pencil irons cannot do this properly. But I would not bother. Solder a short #18 wire to the end of the buss bar and bolt the other end to the chassis near the input jack.

Once you have the buss bar connected to the chassis properly, you will have zero ohms between the buss bar and chassis. And V1 and V2 cathodes will measure the correct resistance to chassis ground. And then your cathode voltages will be correct.

Damn, that's disappointing. After all the reading I did about what a poor design choice the buss bar was, I decided to still give it a try. I did indeed sand down the back of the pots, cleaned with isopropyl alcohol, fluxed them, then hit them with the heat. I have a hakko soldering station that seemed to have no problem making the connection. Guess it wasn't good enough. I'll add another ground where you've said and connect them up. I'll report back after the fix. Thanks very much sluckey!

[bepone]

very good,
other side of the pots/ touching the chassis are also the problem..

chassis here unfortunatelly is return path for the currents from the minus pole of the electrolytic cap to the first stages and pots are on the way , making the contact...usually on the amps one end of buss bar is connected to the chassis directly, to prevent open contact..and C is gnd'ed to the beggining of the buss bar

[aceofbones]

I've now added a wire at the input end of the buss to a chassis ground I put below the input jack. The buss bar now reads 0 ohms resistance, however pin 3 on V1 still reads 2.68K.

[sluckey]

I've now added a wire at the input end of the buss to a chassis ground I put below the input jack. The buss bar now reads 0 ohms resistance, however pin 3 on V1 still reads 2.68K.

Haha. I'm cross-eyed! I was looking at the 1.5K resistor on the end of the board and thinking that was connected to pin 3. Duh! Pin 3 connects to a 2.7K so your resistance reading is just fine. Redo the voltage readings for V1, V2, and V3 and I bet they look much better. The amp may even play now if you don't have more gremlins hiding.

[aceofbones]

Haha. I'm cross-eyed! I was looking at the 1.5K resistor on the end of the board and thinking that was connected to pin 3. Duh! Pin 3 connects to a 2.7K so your resistance reading is just fine. Redo the voltage readings for V1, V2, and V3 and I bet they look much better. The amp may even play now if you don't have more gremlins hiding.

Ah good. I wondered because the other pins I checked all read the same as the resistor in line with them. That makes sense now then.

I fired it up, but as soon as I take it out of standby the high pitch noise of doom appears. The kids were all sleeping so I had to shut it down. I may have to lay my 2x12 face down and cover the thing with blankets to try and knock the sound down so I can continue working on this thing at night if it's going to shriek at me.

Clearly I got the gremlin wet at some point and it has multiplied!

[Stevem]

Now try swapping the OT primary's again since the preamp section now has its needed ground reference.

[sluckey]

Unplug the power tubes to avoid the loud noise and recheck preamp voltages. What are the voltages on V1, V2, and V3 now? I know they will be a bit high but just want to confirm the tubes are behaving as expected.

There are two changes I'd like for you to do. One permanent change is to move a couple ground wires. The other change is to disable the VVR temporarily while troubleshooting. See attachment.