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B+ too high in converted Bogen PA

2/3/2003 6:58 PM
Dave Culgan
B+ too high in converted Bogen PA
I've been working on converting an early Bogen PA amp to a guitar amp with a schematic I drew from some of the Fender tweeds. I used the original Bogen P.T. and biased the two 6L6 tubes to 35mA using the cathode resistor. The original circuit has the output of the 5U4 recitifer in series fist with a 100 ohm power resistor, then in series with a discrete secondary winding on the P.T. (then it goes to the first filter section). The problem is the B+ voltage is right around 500V, already over the rating of the filter caps I bought (450V). Any thoughts about this? Increase the value of the 100 ohm resistor? What is purpose of running the circuit through the PT winding? Inductive filtering?  
I hope I've explained my question and supplied enough info, although I've a good bit of electronics background, none of it is in audio technology, si I've been trying to get an education here. FWIW - the amp powered right up (only one missed solder connection!) and I thought, sounded great.
2/4/2003 1:52 PM

disconnect all secondary windings, and measure the ac voltage on each while the primary is energized. that should give us a better idea of what's going on.  
that's pretty strange having the rectified output for the b+ go BACK through a PT secondary!  
2/6/2003 2:24 PM
Dave Culgan

Disregard what I said about B+ going through a secondary winding, today I learned about tube rectifier 5 volt filament windings :)  
But - I energized the primary and measured the secondary unloaded. I got 872 VAC (not true-RMS DVM)or 436 VAC from each winding to the center-tap. As I mentioned this is giving me 500 VDC when the P.T. is loaded by the bias current. I guess I'd better replace my 450 volt filter caps with 500V or better 600V ones (the original filter caps were rated at 500V). Can 6L6EH's handle a 500V B+ supply?  
Dave Culgan
2/8/2003 8:14 AM
You could use a totem pole design for the first filter stage. This would give you 900v capacity with a pair of 450v caps. After the first stage the voltage is probably low enough for the other caps to survive.  
"Totem pole" is a term I've heard used to describe this design: Two caps in series (+ to the filter node then - to the + on the next cap, then the - of the second cap goes to ground.) I usually see a 220k 1watt resistor bridged across each cap to help equalize the voltage.  
Of coarse, two caps in series of equal values will cut the capacitance in half. ie: Two 350v 100uf caps in a totem pole arrangement will have 50uf capacitance and will handle 700v.  
I hope this helps. I know that caps bigger than 450v are impossible to find in your average store or catalog.  
2/10/2003 7:14 PM
Dave Culgan

Thanks for your reply Chuck. I'd thought about putting caps in series to spread the potential out, but thanks for the reminder about the equalizing resistors, I'd forgtotten about that.  
The Bogen power supply circuit did have a 5 or 10W 100 ohm series resistor in the circuit, right after the rectifier tube and before the first filters. I noticed that by increasing this to 300 ohm (using some resistors left over from the not-used Bogen circuit) I was able to reduce the B+ at the filter caps to 470 VDC, with 426VDC across the 6L6s. I'm going to change my first filter caps (probably the others as well) to 500V Sprague Atoms, looks like Mouser stocks these.  
Other than these minor issues the amp works great. I'm also going to replace the audio taper 1M volume and tone pots with linear as from the way the controls work on my amp, they don't need to be audio taper. Next step - build a simple open back combo cabinet.  
Dave Culgan
2/4/2003 6:52 PM
That isn't by any chance the HV CT wire, is it?  

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