5U4GB tube rectifier flash

[nsubulysses]

Yeah they should be 90 degrees to the right to be correct.

Anyway, my numbers in post #12 are wrong. The amp was biased much colder than I posted because I used the transformer shunt method (forgot bias probe at home) and forgot to divide by 2. Wonder if I couldn't tell soundwise because I am using a Strat as a test guitar, or because Mesa's are biased pretty damn cold anyway. I installed a trimpot.

Here is what I got today, with driven voltages as well as per pdf64's request. These bias settings are dumb.....or I am

In tube rectifier mode:
27mA at 412Vp. A weak ass bias of 37%. When driven to clipping the plate voltage dropped to 332V

In SS rectifier mode:
47mA at 448Vp. Biased at about 70%. When driven to clipping the plate voltage dropped to 414V

Wonder if this is normal. I will search the board and web. feel free to chime in if you know I've never worked on a Dual Rec before.

[g1]

I don't get why the bias is changing so much between rectifier settings. It's only a difference of 35V at the plate. Is there a big change in the DC at the power tube grids when you switch over?
And being biased so cold in tube rect. mode, I don't see why the plate voltage is dropping so much under load. Does it drop even more with one of the rectifier tubes pulled?

[nsubulysses]

Tube rectifier grid voltages:
screen: 412
control: -45

SS rectifier grid voltages:
screen: 452
control: -45

I will try one of the rectifier tubes pulled later. DOn't have time right now. Thanks for the input!


Also I searched and foudn this post by redcobra. He seemed to be experiencing the same, so I was about to think this was normal and finished. See his first post -- http://music-electronics-forum.com/t36276/

[g1]

Also, if this has the bold/sponge switch for the PT, make sure it is set to bold position.

[nsubulysses]

It is in bold position. My reasoning was that bold and silicone rectifier would yield hottest bias, so set that around 70% dissipation and let all the other settings settle at their colder idles. Seems similar to red cobras findings in the other thread?

[g1]

Yes, I see redcobra had similar readings. Yours seem a bit wider in range due to the hotter bias settings than he used. You seem concerned about it being biased too "cold" (like stock -51V setting). Do you notice a bad sound when stock settings are used? If not, bias cold like stock and save tube life. Or start cold and bring it up to where it sounds good, it may be able to be biased colder than you have it now.
Otherwise, if you are confident both rectifiers are working right (trying one at a time would verify), then you are probably done.

[nsubulysses]

Thanks g1. I am just a little concerned because I've never encountered this amp and all it's weird settings before. I am unfortunately rushing too because a friend is trying to use it to record at 5:30pm today.

The amp sounds normal. Rectifiers seem fine to me. I'll pull one as you say to 100% confirm.

[okcrum]

Sounds like the new 5U4 wasn't completely flashed at the factory. It should be fine from here out.