Using a CT PT in place of a non-CT PT

[sluckey]

When i measure the bias winding disconnected from the circuit to ground, it's giving me 70vAC. The only place the secondary HT touches ground is through the " - " end of the bridge rectifier to ground. Would this mean that my ground is not referenced and the 70v is a bogus reading? whenever i attach it to the circuit i read almost 280vAC

That 70V bias tap requires the center tap to be connected to ground. But since you are using a FWB you cannot ground the CT. Therefore, you cannot use the bias tap. You'll have to get the bias source from the HT thru a capacitor. Look at the JCM-900 schematic for reference.

[beasleybodyshop]

Thanks everyone. I think i know what im going to do - Im going to remove the bridge rectifier and drill out some spots to mount iso turrets in the pcb and do a fender style diode setup so i can center tap the damn secondary and be done with this mess lol

[sluckey]

In your original post you said, "Had a center tapped power transformer wound for an amp that it's previous PT didnt have a CT." So, how did that amp get bias with the previous PT? And why can't you get the bias the same way now?

I guess it doesn't really matter because that PT used with a bridge will give you way too much B+, unless you're using some big tubes. Which amp is this? Can you post a schematic?

[beasleybodyshop]

In your original post you said, "Had a center tapped power transformer wound for an amp that it's previous PT didnt have a CT." So, how did that amp get bias with the previous PT? And why can't you get the bias the same way now?

I guess it doesn't really matter because that PT used with a bridge will give you way too much B+, unless you're using some big tubes. Which amp is this? Can you post a schematic?

Its in a Sovtek MIG50 - essentially a 5f6a bassman. I had a transformer wound like a typical fender with a bias tap and center tap. Old transformer had separate windings for bias (0-60) and HT (500-0).

Lesson learned. I will change the rectifier setup this time, but next time i wont make that mistake again.

[rp]

You could have checked w/ New Sensor too, maybe they still have old MIG iron.

[sluckey]

Old transformer had separate windings for bias (0-60) and HT (500-0).

Are you sure the HT was 500-0? That's 700V B+. All the Mig-50 schematics I have are using 5881 tubes. I don't think they would like that much B+.

Using your replacement PT (250-0-250) will only produce 350V B+. Is that enough for you?

[beasleybodyshop]

Old transformer had separate windings for bias (0-60) and HT (500-0).

Are you sure the HT was 500-0? That's 700V B+. All the Mig-50 schematics I have are using 5881 tubes. I don't think they would like that much B+.

Using your replacement PT (250-0-250) will only produce 350V B+. Is that enough for you?

hmm.

maybe. I was going to run some KT66's in this amp. The lower plate voltages might work in my favor, as i was going for the marshall end of things as opposed to the bassman end of things.

This amp had MM iron in it when i got it. i got a spec sheet from them stating that it had a HT winding of 500-0.

[beasleybodyshop]

redid the rectifier to a fender style. Im getting like 350 volts on the plates of the PT, 100v on the plates of V1,V2, and around 200 on the PI.

Really surprised how much of a clean amp this is. Fully cranked it starts to yield some raunch. Ive always understood it that higher plate voltages meant cleaner tone and lower plate voltages give a more saturated tone. im running almost 60-70 volts lower plate voltage than something equivalent (JTM45/50).