Bogen CHB-50 Rebuild

[physics]

Hi Guys

These extra resistances we add to limit surge currents are also there to enforce a high-frequency filter effect, but...

In the context of surge current limiting, you are forgetting the dynamic resistance of the diode itself, plus the PT winding resistance, plus the connecting wiring itself. All of this adds up to the fact that standard 1N4007s can be used almost universally in guitar amps <100W.

Yep, that fixed the numbers. The bogen chb50 transformer's plate winding is about 7.5 ohm, taking that as the resistance of the 272JX plate winding and plugging that into the formula I get about 7A RMS for The Standard's surge current. Knew I must have been missing something. Winding resistance and inductance were left out originally to simplify things and get a worst-case value, but it looks like I got too far from reality. I hadn't thought about the diode's dynamic resistance or the wiring resistance, good point. Re-doing my calculations for the Bogen's surge with the plate winding resistance accounted for, it looks like I can add an extra 100u filter cap in parallel with the doubler and still keep nice comfy margins. Great! I had wanted to add extra filtering or have the option to, but up 'till now I thought I might overheat the diodes on startup.

Thanks for clearing that point up!

For essentially no mark-up, you can use the faster UF4007. Or, if you stick with the 1N-series, use 1N4004s in series as those are as fast as the UFs.

What's the benefit of faster diodes? Better efficiency of the supply?

If you are contemplating going to 3A diodes, think twice. They have huge fat leads since they are meant to cool the semiconductor die. You will find more amenable easy-to-solder and easy-to-place into eyelet leads on most integrated bridges.

Yup, that was my main hesitation with the 3A diodes. I can just barely cram two of their leads into the Keystone 44 eyelets, which hardly leaves any room for whatever else needs to go in.

If you use a bridge with a CTed plate winding, be certain to add a resistor to ground off the "unused" negative terminal. Of course, there should be one off the positive terminal, as standard practice. These will prevent damage to bridge.

I assume by "negative terminal" you mean the unused outer tap on the transformer, assuming I hook up the bridge between the center tap and one outer tap? How do these help protect the bridge? As a guess, maybe there are inductive spikes associated with diodes turning off and on, and the resistors help damp that?

Your comment about transformer winding resistances unearthed a few other problems with my bias supply design. After you mentioned the importance of transformer winding resistance as it relates to surge current, I went and measured the transformer I was planning on using for the bias supply. The line-side winding that I was going to use as the high-voltage source for the bias supply is 126 ohms! So my RC constant for the bias supply is an order of magnitude higher than the B+'s RC constant, and high-voltage will be applied to the tubes before they're properly biased. That's inconvenient. So looks like I either need to find a different transformer or maybe work out a soft-start circuit.

The other problem is that the line-side of the transformer is rated for 80mA max. Fine during normal use of the bias supply, however at turn-on I calculate around 200mA RMS leaving out the diode dynamic resistance and inter-component winding, but including the transformer winding resistance. Is this going to be an issue? I'll dig around in my books and the internet to see what I can find quantitatively, but I figured I'd ask.

The voltage drop of a solid-state diode in a tube plate supply is immaterial and need not complicate the calculations. Besides, varying mains voltages and loading conditions will make an attempt at precision pointless. Another significant point is that there is only about a 15% chance of turning the amp 'on' at the mains peak voltage.

Right, precision is a losing battle. I'm instead trying to get reasonable upper bounds on things so that I know in the worst-case I should be fine. I figured there's a small chance of turning on the mains at/near peak, but it seemed large enough that it'd happen more than once in the lifetime of the amp so I didn't think I could ignore it.

Thanks for your input! It definitely helped clear up stuff with ratings, though I suppose now I have new problems to deal with. The hamster wheel spins on...