Sound City B 150-why is this cap here?

[01redcobra]

Can anyone give me some insight as to why this .022mf cap was added between pin 2 and pin 6 on V2 of this SCB150? I'm kinda lost on this one. The amp plays, but sounds like dirt. Also the plate and screen voltage on the KT88's seems a bit high to me at 557vdc plate and 558vdc screen. The OT has been replaced but appears to be a correct replacement.
sound-city-bass-150-amp-schematic-diagram.pdf

[Tom Phillips]

Nice job with the photos but can you mark up the schematic? That would be the best way to help us evaluate the situation.

[MarkusBass]

This is C20 on the schematic. This is a connection between the anode of V2 (second half) and the grid of the next stage (first half of V2). On the schematic there is additional 2.2k grid resistor which is omitted here.
It seems that the problem is somewhere else. You can try replacing the tubes. If you don't have new tubes, you can check the amp with signal generator and oscilloscope.

Mark

[Steve A.]

As MarcusBass said, C20 is the coupling cap going from the plate/anode of V2A to the input grid of V2B. The way it is drawn makes it a bit confusing.

Steve Ahola

[g1]

I think what they are saying is that it is between pins 1 and 7, not 2 and 6.

[MarkusBass]

You may be right. It is very difficult to read this schematic. What if they simply changed V2A triode with V2B? The schematic looks OK so you may verify if it matches the layout.



Mark

[g1]

Markus is right, they have switched triode sides. So where the schematic calls out pins 1,2,3, substitute 6,7,8, and vice versa.
Even so, there are problems with that socket wiring, pin8 appears to be missing the cathode resistor, pin2 wiring doesn't seem right either (labelled as pin7 on schematic).

[01redcobra]

Thanks to all. I didn't notice the flip-flop on the triodes. Mark, you are right about the 2k2, but it is there, just not in series with the cap. It's actually running off of pin 2 to the board. Definitely gotta change that. G-one, that cathode resistor and cap are there, but in the pic, the resistor is hidden by the cap. Bad camera angle. I cropped the schematic to just show that area and attached it to this post. Looks like I'm going to have to spend a bit more time to make sure everything is where it's supposed to be. Leave it to the Sound city boys to mislabel those pins. SS150CropV2.pdf

[g1]

Ok, if the cathode resistor is there, all you should have to do is move one end of the .02uF. Where it is connected to pin2, disconnect it and connect to the other side of the 2K2, at the 470K.

Edit: scratch that, all the pics I can see show it wired up like yours, so that is probably the way they are.
Bottom of this page shows it:
http://forums.vintageamps.com/viewtopic.php?f=9&t=99318

[01redcobra]

Sorry it took so long to get back to this. I've had to work doubles at my day job. G-one, I see that cap wired the way this one is in other photos too. The cap is stopping the dc, but would the 2k2 to the pin reduce the ac signal if wired like the schematic?