Princeton RI Plate voltage

[ziggy007]

Gday Guys,
Just interested if anyone has come across lower than 'spec' plate voltage on one of these Ri amps. First RI i've worked on. Its at 375 (spec shows 440) when biased at 55% dissipation. New 6v6s and GZ34 tried for same result. Obviously if i try and bias it higher towards 60% it drops.
No real issue, It sounds fine, just curious.
Cheers
Gav

[m1989jmp]

Gday Guys,
Just interested if anyone has come across lower than 'spec' plate voltage on one of these Ri amps. First RI i've worked on. Its at 375 (spec shows 440) when biased at 55% dissipation. New 6v6s and GZ34 tried for same result. Obviously if i try and bias it higher towards 60% it drops.
No real issue, It sounds fine, just curious.
Cheers
Gav

What's your mains voltage?

Edit: just curious; are both sets from the same company?

[mozz]

Try tacking a 22 or 47uf cap across what is already there. Maybe one of those Illinios caps are going south.

[g1]

Agree with Mozz, at least check the AC volts (ripple) where you have 375V coming out of the rectifier at F3 or R58/C28 junction.
That seems quite a bit lower than the 445V specified there.

[ziggy007]

Agree with Mozz, at least check the AC volts (ripple) where you have 375V coming out of the rectifier at F3 or R58/C28 junction.
That seems quite a bit lower than the 445V specified there.

Thx for reply guys. Yeah it just seemed ‘too’ low for me.
Wall voltage 240 +-1v. So that’s cool. Original rectifier is a fender badged GT. I chucked a bran new JJ in. Exact same voltages in and out.
Will try some more investigating tonight.
Cheers
Gav

[SoulFetish]

Thx for reply guys. Yeah it just seemed ‘too’ low for me.
Wall voltage 240 +-1v. So that’s cool. Original rectifier is a fender badged GT. I chucked a bran new JJ in. Exact same voltages in and out.
Will try some more investigating tonight.
Cheers
Gav

what's your secondary AC voltage before rectification? Can you take measurements across both phases (leg to leg), as well as from center tap to each leg?
Whats the peak to peak voltage? any imbalance from CT to each phase?

[ziggy007]

what's your secondary AC voltage before rectification? Can you take measurements across both phases (leg to leg), as well as from center tap to each leg?
Whats the peak to peak voltage? any imbalance from CT to each phase?

Fired up and r58 (first PS node R) started to fry after 5min. Replaced and all now seems ok.
All voltages in spec. A bit over actually.

Lesson learnt as an aside. Wall voltage, normally pretty good here in Sydney. Huge difference when I’ve measured over different times last 3 days. Today it’s 247. Yesterday 241 midday, 232 during peak times.

Thanks as always guys.
Gav.

[g1]

R58 has to burn for some reason. And even if not, then the B+ out of the rectifier would not have dropped to below spec.
New tubes are not always good, that resistor feeds the power tube screen grids. Also, C26 could be giving you intermittent problems.
For now I would definitely be keeping an eye on those power tubes.

[ziggy007]

Yep, Thanks G1. one of the new JJ 6v6 redplated in company with R58 frying.
Im thinking two different issues, wildly variable wall voltage at different times and a faulty valve.

[Randall]

I'm thinking the red plating tube took out the resistor.