Bogen CHB 20 conversion questions

[jewishjay]

I want to build a 5E3 with Robs cascade mod, and I have questions about the rectification in the old circuit. Is this half wave? Full wave? Is the diode circled in red part of the HT rectifier, or could I delete everything from the red square on?

Would it be better? Safer? Quieter? to make a full wave bridge from 4 diodes?

[sluckey]

The circuit past the red square is a fixed bias supply and can be deleted. It's not useful to a 5E3 amp.

There's nothing wrong with Bogen's voltage doubler circuit. But measure the ac voltage between the red and red/yel wire to determine if a FWB can be used.

[jewishjay]

voltage doubler circuit

ah ha. ok. so, do the stacked 40mf caps of the doubler "count" as my first filter stage? if so, would they benefit from 220k strapping Rs?

[thetragichero]

i would leave the voltage doubler part of the circuit alone
a bleeder resistor to ground on the second filter cap stage would be useful

[PRR]

... stacked 40mf caps of the doubler "count" as my first filter stage? if so, would they benefit from 220k strapping Rs? ...

No. Voltage balance is already enforced by the way this rectifier works.

It is an excellent plan just the way it is. If you needed *half* the voltage at twice the current, you could go FWB. But that's real unlikely in a tube amp.

[jewishjay]

ok, i'm having some trouble with this voltage doubler.  maybe a short somewhere? it wants to arc and spark and burst in to flames. i tried removing the diodes to test them out of circuit and they test good. and without the diodes i get 180v from red to red yellow. but when i connect it (in what i think is the right way) it arcs and sparks. i tested it through the variac and problems start around 80v input.

could it be the caps are too small? or one is bad? the bogen used 40uf caps but i was trying to use what i had on hand... 33uf. am i a dum dum? could the too small caps cause a short?

[astronomicum]

You need two 2 independant caps in a doubler. A can won't work. Check your schematic again and rewire your rectifier circuit.

[sluckey]

You need two 2 independant caps in a doubler. A can won't work. Check your schematic again and rewire your rectifier circuit.

You only need one individual cap. A multi-can cap can be used for the second cap in that doubler. Just like is shown in that Bogen schematic.

[2deaf]

ok, i'm having some trouble with this voltage doubler.  maybe a short somewhere? it wants to arc and spark and burst in to flames. i tried removing the diodes to test them out of circuit and they test good. and without the diodes i get 180v from red to red yellow. but when i connect it (in what i think is the right way) it arcs and sparks. i tested it through the variac and problems start around 80v input.

could it be the caps are too small? or one is bad? the bogen used 40uf caps but i was trying to use what i had on hand... 33uf. am i a dum dum? could the too small caps cause a short?

Maybe try individual diodes instead of that VB500?

[jewishjay]

Yea, all I can think to try is bigger caps and new diodes. I was just hoping someone more experienced might see an wiring mistake. This is my first voltage doubler. Maybe I did something wrong? I'm really sure it's just like the schematic, with the exception of the size of the caps, so maybe thats it....

[astronomicum]

You only need one individual cap. A multi-can cap can be used for the second cap in that doubler. Just like is shown in that Bogen schematic.
[/quote]

I see. Thank you.

[jewishjay]

Ok. Rectifier sorted. A couple of 3A diodes and 100uf caps, now the voltage doubler works. Next problem is bias...

Although the 5E3 original called for 250 ohm cathode resistor, with todays higher voltages I figured 500 was a safer place to start. But it was dropping 56v across 492 ohms at 295 plate volts...so like 16w of dissipation?

So I swapped out for a 1k cathode R. But that drops a massive 131v across 978 ohms at 385 pv....25w ??? and the 1k 5w R started smoking so I shut it down.

I dont know where to go, please help...

[sluckey]

Just use a 250Ω and repost your plate and cathode voltages.

[jewishjay]

Just use a 250Ω and repost your plate and cathode voltages.

Drops 45v across 246ohms with 354 at the plate...32watts? Red plate....

[sluckey]

You have something very wrong with your circuit. You are following a 5E3 schematic, right? What voltage do you measure on pin 5 of the 6V6s?

[jewishjay]

You have something very wrong with your circuit.

Oh fudge....I had the pair of 220k to ground on the wrong side of the coupling caps after the PI. So after correcting that. I think i have at least 2 leaky coupling caps. Here are my unloaded voltages....

12ax7
1 445
2 3.6
3 0
6 448
7 1.5
8 0

12ax7
1 445
2 0
3 0
6 458
7 .1
8 .01

6v6
3 461
4 459
5 3.6
8 0

6v6
3 461
4 459
5 .05
8 0

[sluckey]

Voltage on pin 5 of those 6V6s must be ZERO! Change those coupling caps. You show 3.6V on the grid of one 6V6. That's causing the excessive tube current and super high dissipation numbers.

[jewishjay]

Whoa. Youre not gonna believe this. It wasnt a leaky cap. That voltage is leaking across a terminal strip from one contact point to the next. I thought phenolic was non conductive? Something has leeched into it, or theres gunk on it, and even after scrubbing with alcohol on a toothbrush theres 2 to 3 megs of resistance across the brown plastic terminal strip. EVER SEEN THAT BEFORE?