really wierd voltages...need help

[Chuck H]

TIA... I'm glad your reading this.

This is a brand new build. 2 x EL84 cathode biased. I haven't been able to check voltages everywhere because I don't have much time with the amp off standby. It's eating filters for lunch. Just the main filter. The rest are still O.K.

270-0-270 PT. All voltages from the PT are fine (with the exception of modern wall voltages making it a 310-0-310) Tubes glow and 570 volts at the top of the bridge with the filters and OT disconnected. With everything hooked up I get 20 volts on the top of the cathode resistor and the plates fluctuate between 560 and 600V With voltage on top of the cathode resistor (and having blown two 6A fuses) I have to assume its drawing current. So why are the plates so high??? Needless to say, the amp sounds like it's cranked through the speakers and there is a little hum. Nothing I wouldn't expect with the tubes and power supply slowly incinerating. I did try grounding the grids on the power tubes just for laughs but this changed nothing except the output at the speaker. It goes like this: Switch off standby-amp sounds like its cranked (plus some)-Then the main filter starts to make an arcing sound - Then it blows a 6A fuse and / or smokes the main filter entirely. I also disconnected the OT primary and still got arcing on the main filter.

I Quadruple checked my design and checked all grounds etc... I can't find a thing out of place after about 5 hours.

4 x 1n4007 bridge rect.- 40uf@500v- 2.2k- 20uf@450- 5k- 20uf@450- 5k- etc...

Any help is welcome

Chuck

[Enzo]

Uh, you list 4 diodes, but you also list a center tapped PT. What rectification circuit are you using? 310v should yield about 440VDC, not 560. 560v will explode a 500v cap, so if you are actually getting that voltage, I don't doubt you aer arcing your caps.

Is your center tap grounded?

You ought to have the center tap to ground, then a diode from each 310VAC to the first filter. That shoould provide about 440VDC there. Then the following B+ nodes should flow from there.

If you make a full bridge with four diodes and wire is across the two 310VACs, then the net result is 620VAC rectified. That will set about 875v across the filter cap, which will load it way back down as it dies.

[Chuck H]

Thanks

Enzo

Thanks for the reply. I am not using the CT on the PT right now. I will try a regular full wave rect.

FWIW the duncan PSU shows identical voltages for both types of rectifiers.?.

I was never surprised that the caps were unable to handle the voltages.

AAAARG, even after I get through this problem I'll still have ALOT more volts than I wanted. May have to go with a tube rect anyway.

Chuck

[Chuck H]

OOOH...oops

O.K.

Duncan PSU uses the total voltage for a bridge and one side to ctr for a full wave, so...

Oh well. thats a mistake I won't make twice. Thanks Enzo.

Chuck

[Enzo]

Yes, you can get the same voltage using either methid, but in the full bridge type the entire winding is the voltage you neeed, while in the two diode version, each galf of the winding is the voltage you need.

Read around this forum - search for zener and voltage reduction and similar words. A lot of guys run a big power zener in the center tap leg and it chops out some volts.

ANother trick would be to add a resistor after the first filter. Then take the output center tap supply from that second node rather than the first. That would even give you some life like sag without a recto tube. You'd have to determine the value resistor that got you where you wanted.

[Chuck H]

Thanks Enzo

I"ve only built two amps from scratch really. The rest were all bastardized rebuilt Traynors, Marshalls, Fenders, Boogies etc... I even did an old VOM phonograph once. It's actually a great amp. I think after 15 years as a hobbyist I'm getting pretty good at this stuff but very low tech. I design nearly all my own circuits and people love my amps. Of all the amps I've built I only have one. The rest found homes with people who "had to have them" so thats really cool. But as a consequence of using existing power supplies I made a really foolish error. I'm really quite embarrased.

As it turns out... With a full wave bridge and 78ma of idle current (2xEL84 cathode biased) I have 392Vp. Just what I was aiming for. I know thats more wattage at idle than these tubes should have, but the tubes don't seem to mind. I've done it before too.

I don't care much for the zener on the ctr tap 'cause it seems to stiffen the envalope. And as hot as these tubes are running, too much sag in the power supply (adding resistance before the plates) would loosen things up too much when cranked.

Yeh, a pair of EL84s as close as possible to 400Vp without going over. Cathode bias around 35ma each. Run the screens lowish (by comparison) at 355 volts. No choke, no tube rectifier. Works and sounds just great, is cheap to build and, with an efficient speaker, puts you right at club volume requirements.

Unfortunately I hate the preamp I built in this one, so I'm rebuilding it now.

Thanks again

Chuck