Bogen CHB-50 amp question

[PSUorganguy]

Hey everyone,

I purchased a Bogen CHB-50 amp for my custom leslie project. The first order of business is recapping, however, I have a question about what I'm seeing. On the schematic (attached) there are two capacitors listed as .1 mfd one off of each 6L6 (I circled them in red). It is not given a voltage on the schematic. When looking at my amp (picture attached) those capacitors are labeled .1 mfd 1000 WVDC. I'm confused about what type of capacitor to replace these with. I'm not thinking these should be electrolytic, maybe something like a mallory 150? Does it need to be something rated for 1000V? I'm not thinking I will find that if it does.

I know several people here have used Bogen amps for leslie projects so I hoping someone else has come across this!

Thanks!

[David Anderson]

The general rule for output stage coupling capacitor voltage ratings in fixed-bias amps is the B+ voltage plus the C- bias voltage, plus some safety margin for power supply voltage overshoot at turn-on before the tubes begin to conduct. This would be 445V + 45V = 490V + overshoot.

In this case, a modern 600V-630V film capacitor should be fine. I might suggest Mojo Dijon since they are axial polypropylene film capacitors with long leads that fit nicely in point-to-point chassis, but you could use Mallory 150s. There's no "magic" to either type. Guitar amp types tend to fetishize particular brands, but then, they fetishize everything... Personally, I think polypropylene is slightly closer to the voicing of paper capacitors than Mylar, but that's just me.

The original 0.1uF 1000V capacitors here were high-reliability paper capacitors sealed in white ceramic tubes. Sealing paper capacitors this way actually made them pretty reliable, albeit more expensive, and the 1000V rating added an extra safety margin, so it looks like Bogen was trying to make this build as reliable as possible given the technology of ~1959. This is not as much a concern with modern film capacitors. The other coupling caps I can see in the amp are ceramic disc, another reliable type, but they were not usually available in values as large as 0.1uF back then.

This amp uses a voltage-doubler B+ power supply, so use the highest-quality electrolytic capacitors you can find for the 100uF caps in the doubler circuit. They get charged and discharged more on each rectification cycle than standard reservoir capacitors.

Interestingly, this amp uses a hybrid bias circuit: a -45V fixed-bias supply combined with a cathode resistor balance pot for fine-tuning, somewhat unusual, but not unheard-of.

The phase inverter is a "cathodyne," which provides good signal balance, but it can sometimes sound unpleasant when overdriven. This looks like a mono HiFi amp not intended to be driven into distortion. There are a few ways to fix an ugly overdrive problem that you can find in online discussions of the Fender 5E3 circuit. It can be tweaked.

[picothinker]

Bogen

Many use the little Bogen Challenger tube heads for Leslies. I use a mono solid-state Bogen PA amp for my FrankenGigger Model 51. I think mine is 60 watts, never turn it up beyond 50%. The cab is internally miced, I don't need it any louder than it is, and obviously no tube distortion. I use it on a clonewheel, the amp had an RCA jack for the line-in channel, works perfect for what I want. I've slowed down this last year, but I gigged it about 450 times in 6 years, it worked well.