WAY overpowered PT

[mumford]

I'm working on converting an old Bogen Challenger CH618 (dual 6v6) for guitar. My goal so far has been to get all of the old DC wiring out of there (it was built to take AC or DC input power) and to get it running.

I changed all the caps and rewired the tube rectifier (a 7z4) to SS as a temporary fix because the tube was dead.

Fired up with the lightbulb limiter and it dimmed. I wanted to get true voltage readings, so I removed the limiter and fired it up without tubes. 620vdc coming out of the rectifier even more when the new filter caps violently shot smoke out of them. I had used 450v caps, because the voltages on the schem were lower than 450. Bad idea.

I had heard the bogens ran hot, but this is ridiculous. My initial plan had been to used a string of 15v 5w zeners to lower the voltage from the 430v or so I had expected, but I'm thinking I may just need a different PT--which sort of defeats the point of the conversion (cheap). Any thoughts on how to get the voltage down in the 300-350 range without using 20 zeners? I'm assuming a stud mounted one would be impractical from a heat, cost, and availability standpoint.

[Phil_S]

It's my view that trying to drop that kind of voltage on a PT isn't a good idea. If you use Zener's (or similar tricks), they have to get rid of the extra voltage by generating lots of heat. The more voltage you drop, the more heat. I have no clear idea how much, but it strikes me that it's going to be quite hot and that is a problem for whatever is nearby.

If this were mine, I'd opt for replacing the PT, or I'd start thinking about tubes that can take 600V (KT88, EL34). 620VDC was unloaded. It will drop when you put the tubes in. I'd think a drop of 50-60V is reasonable, but I'm guessing.

Can you tell us, with the PT secondary disconnected, how many Ohms across the high voltage winding, and what is the unloaded VAC from that winding?

Describe your rectification, full wave, full wave bridge? What uF cap do you have there. Is there a choke and if so, where in the power supply ladder? How about drawing this as a partial schematic and posting it?

I'm not convinced that 620VDC is the expected result. I have no doubt you saw that, but I wonder what it is you wired up. What you report is inconsistent with the spec for a 7Z4.

[TheGimp]

Use a choke with no cap on the input of the choke as your firsts filter stage. Choke Input Filter.

You can adjust the voltage out by changing the first cap value after the choke. Follow the first cap with an RC so you have a LCRC filter. You can tune it over quite a wide range.

One advantage of this topology is you can get more current out in exchange for the voltage dropped across the choke, up to about 40% is typical for something like a 10H choke followed by a 47uF cap.

[mumford]

I'll have to do some checking when I get home.

I used 2 1N4007 diodes on each leg of the secondary for the rectifier.

Before I changed it, the secondaries were hitting somewhere around 430v each side.

The original had 2 8uF caps for filtering, I used 10uF.

[mumford]

Ignore the green and red boxes.

[dynaman]

Assuming you wired correctly, a high wall voltage will get you close to 600VDC with that tranny. I think you'd be better of just getting a new PT.

http://audiokarma.org/forums/showthread.php?t=197736

[Phil_S]

Assuming you wired correctly, a high wall voltage will get you close to 600VDC with that tranny. I think you'd be better of just getting a new PT.

FWIW, I agree. 600V is unloaded. I guess 620V is possible. Maybe you lose 4-5% when you plug in the tubes. That will get you down to the high 5's.

It is hard for me to imagine that particular PT sinks down to 420VDC for the 6V6's unless it is a very soft transformer. To answer that question, it might help to know the DCR of the primary winding. If it is in the 5-6 ohm range (or more), it might be possible that it is really that soft.

[mumford]

It's from the 40s, so I suppose the voltages in the schem were based on 110 power. Hmmm.

I'll need to take some measurements, but I'm thinking new PT might be best. Rats.

[David Root]

Even with 110VAC that is too high DC for '40s 6V6s I think. That's why the rectifier tube is there.

I agree if you don't want a tube rectifier then you have to change the PT, even with a 5Y3GT you're still looking at probably 465-475V quiescent. Would be OK with JJ 6V6S, but don't put any '40s or '50s 6V6s on it.

Or ratchet it down to Class A2, that might get you closer to 400V on the plates, with the 5YGT/5V4.