The Amp Garage

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......

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 bac...

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 b...

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 ...

sluckey wrote: ↑Tue Jul 18, 2023 2:09 pm 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

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 -2...

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 t...

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 befo...

M Fowler wrote:I have two Dumble/Two Rock style oval open back 212 cabs one loaded with WGS ET65 and the other with EVM12L both work well for SSS, ODS and JM sig Reverb amps.

I had my cab like that loaded with 12L's. It required a fridge mover to lug it around.

Andy

http://www.youtube.com/watch?v=u79FFU6l98Y

There were some extra tags at the end of the URL.

Andy

IC,

You sir are an animal! To take it from some of the fuzziest pictures I've seen in a long time to an amp that sounds that good is some serious talent!

Thanks,

Andy

Emailed you!

Thanks!

Andy

Does anyone happen to still have copies of the dxf chassis drawings from Shad's site they could post or send me? I thought I had grabbed these, but I cannot find them and the link doesn't work anymore.

Thanks!

Andy

I saw that thread, but it didn't strike me as useful since the pictures of the Borderline didn't show an opto in them. I've been looking more at them today and it looks awfully close to the tremolux's bias modulating trem with some different values.

Thanks,

Andy

Thanks for the tip. I'd searched the forum briefly, but I didn't see anything about this one. I'll go digging some more!

Thanks,

Andy