TW Type 2 MV changing bias voltage

[v00d00blues79]

Hi All,

I have a weird one I’d like to see if the collective might have some insight for me on.

I’m finishing up a small combo based on the Diaz CD-30/100 amps, and the master volume is giving me a bit of trouble. It’s a train wreck type 2 master volume, and I’ve already built this circuit once before and have fogged that amp for over a decade, but this one is altering the bias voltage when the MV is adjusted. When I turn the MV to 10 my bias voltage shoots up incredibly high… almost 100mV.

I’ve gone through and cross checked this one’s wiring with my original, and everything appears to be the exact same. I get ~70mV from the bias circuit to the right hand lug of the bias pot, the middle lug is wired to the grid leak resistor junction, and the left hand lug has a 15k tail resistor wired to ground.

The grid leak resistor junction has a feed to the left most lugs of the dual 100kL pot with the middle two going to pins 5 of the output tubes, and the right most are going to the grid caps on the board.

My original is wired the same exact way yet it’s master doesn’t change the bias voltage at all when adjusted. My question is has anyone seen this type of because before? And if so, what was it? I’m stumped on this one. I’d like to keep the MV in there, but I’m about to go ahead and wire up the grid leak resistors like a normal non-MV and come back to it later…

Any ideas are appreciated.

Thanks,

Andy

[martin manning]

It's likely you have an oscillation starting up at high volume settings. Lead dress is critical. Post a picture of your MV wiring.

[Stevem]

Got a schematic?

[sluckey]

I get ~70mV from the bias circuit to the right hand lug of the bias pot, the middle lug is wired to the grid leak resistor junction, and the left hand lug has a 15k tail resistor wired to ground.

70mV is wrong. You should have many negative volts at the bias pot. Depending on the tubes it may be -25v, or -40v, or even -60v. But it should never be a tiny 70mV.

Pull the output tubes and recheck.

[v00d00blues79]

It's likely you have an oscillation starting up at high volume settings. Lead dress is critical. Post a picture of your MV wiring.

Martin,

That's what it was. I switched over to shielded cable for the runs back to the grids and now it behaves as expected. I don't know why I didn't just do that in the first place, but I'm going to chalk it up to its been a while.

Thanks again!

Andy

[v00d00blues79]

I get ~70mV from the bias circuit to the right hand lug of the bias pot, the middle lug is wired to the grid leak resistor junction, and the left hand lug has a 15k tail resistor wired to ground.

70mV is wrong. You should have many negative volts at the bias pot. Depending on the tubes it may be -25v, or -40v, or even -60v. But it should never be a tiny 70mV.

Pull the output tubes and recheck.

I get -70V coming off the bias supply. I was typing this out on my phone and hit the wrong key. Apologies for the confusion.

Thanks,

Andy

[sluckey]

Did you remove the original grid leak resistors as you should?

[v00d00blues79]

Did you remove the original grid leak resistors as you should?

Yessir, I've built this circuit before so I had everything the way it was supposed to be. This time I just tried to cram it into a smaller chassis so the lead dress was playing hell with me.

Thanks!

Andy

[sluckey]

Pull the output tubes and monitor the negative bias voltage on pin 5 of the tube socket. Does that voltage change as you rotate the MV pot? If so, what is the minimum and maximum voltage?

If the voltage on pin 5 remains constant then put the tubes back in and disconnect the NFB circuit if there is one. Does the MV still affect the bias voltage?

[v00d00blues79]

Pull the output tubes and monitor the negative bias voltage on pin 5 of the tube socket. Does that voltage change as you rotate the MV pot? If so, what is the minimum and maximum voltage?

If the voltage on pin 5 remains constant then put the tubes back in and disconnect the NFB circuit if there is one. Does the MV still affect the bias voltage?

Right now with the MV at 0 I get -50.00 on pins 5 of the output tubes. When I adjust the MV to 10 the voltage drops by .5V to -49.50V. I've attached some pics. It's definitely a tighter fit, but I'm doing my best to keep the signal and power runs away from each other as best I can.

I'm getting ready to use a short run of shielded cable to the grid caps to the MV pot. The 100kL dual I have in there now is the second pot I've tried. I've also tried a 250kA with 3M3 resistors ala Lar/Mar and had the same issue.

Thanks,

Andy

[sluckey]

The bias circuit is working properly so disconnect the NFB. IE, remove the yellow wire that connects the speaker jack to the 820Ω on the board. Any joy?

If the voltage on pin 5 remains constant then put the tubes back in and disconnect the NFB circuit if there is one. Does the MV still affect the bias voltage?

[v00d00blues79]

The bias circuit is working properly so disconnect the NFB. IE, remove the yellow wire that connects the speaker jack to the 820Ω on the board. Any joy?

If the voltage on pin 5 remains constant then put the tubes back in and disconnect the NFB circuit if there is one. Does the MV still affect the bias voltage?

No joy. I pulled the NFB feed to the output jack and I still get the bias voltage moving with the MV. I'm wiring in a shielded run from the MV to the grid caps now to see if that helps.

[v00d00blues79]

Its in the preamp somewhere. I pulled all of the preamp tubes and left the power tubes in. I was able to get the bias voltage stable throughout the range of the pot. I'm working my way back starting with the PI and will put each tube back in one at a time and see when the fun begins. Will report back.

Thanks!

Andy

[v00d00blues79]

Well... you know what really helps keep a preamp under control? a connection to the Volume pot wiper to the grid of V1B... I've been chasing my tail since last night on this, and that's what it was. I don't know how I could have missed that. I'm going to chalk it up to it was late and I was tired... The missing Vol pot was causing the preamp to go batshit and oscillate causing the rest of the amp to misbehave. At least I went ahead and put in shielded cable to the connections of the MV. That should at least keep it shielded from any power supply noise... I'm going to run one more shielded run from the inputs to the V1A grid to help quiet things down a bit.

Now to get the combo cab ordered...

Thanks again everyone, especially you Sluckey, for assisting on this!

Thanks,

Andy