Bogen CHB10A conversion

[duce67us]

So I picked up a few of these Bogen PA Amps, and was working on one for a friend...
I Ram across a thread with a link to Platefire's Conversion, and out popped a schematic. 
(Found here https://www.flickr.com/photos/12623503@N04/1328163416/)

I followed it ( Roughly) but have a few interesting problems.

With no signal, and volume at minimum: The B+  is about 327vdc, but has a 120Hz 800mV ripple.
My C+ is cleaner and 288vdc and 35mV of AC  ripple, D+ at the plate of the 12ax7 is 273vdc with 9mV of AC ripple.
I tried to create an artificial center tap with a pair of 100R resistors across the heater to try and eliminate some of the 120Hz Buzz.
The Heater is a little high @ 6.6vac

When I crank up the volume past 80%, the power supply starts a LFO that can be heard in the speaker and effects the + Voltage.
I can't quite catch the waveform on my scope, but the DC voltage drops with the oscillation.

I'm attaching pictures in hopes that someone sees something that could be causing the power supply to destabilize. 
I have tried various output and preamp tubes with the same results. 

Thanks  in advance! You guys ROCK!

[shooter]

couple things that may help;
double up your preamp filter tap cap value
clip your ground buss and run the pre-buss to the ground point for the pre caps
incrementally increase bypass cap in pre.  I "fixed" a LFO by going to about 15uf.

if you have a scope, "look" in the "grass", play with the time-base, I've found a very slow, 3hz-ish, which got fixed with tweaks to the power rail and power grounding.
good luck!

[jjasilli]

I tried to create an artificial center tap with a pair of 100R resistors across the heater to try and eliminate some of the 120Hz Buzz.
Heater hum is 60Hz. B+ hum (fully rectified) is 120 Hz. Wall outlet hum, if your per chord inside the amp is inducing hum in to signal is 60 Hz.  Can't tell if power leads are running near the speaker terminals.

Anyway you need to sort out the specific hum frequency. 

I tried to create an artificial center tap with a pair of 100R resistors across the heater to try and eliminate some of the 120Hz Buzz.
The resistors should not be across the heater.  If you're using twisted pairs, each resistor should go to ground.

The Heater is a little high @ 6.6vac
Loaded, or unloaded?

Schematic:  If you don't have a schematic of your build, you should make one.  That alone may help solve your issue.  It will also enable others to better help you.  Saying that you roughly followed a schematic is not helpful.  Also, it's better to build a known working amp first.  Then if you want to mod, do one mod at a time.  Then if something goes wrong you know to undo, or re-work that mod.

[PRR]

> starts a LFO

Filter caps. Almost certainly. Have you replaced them all? Your D node is critical.

This may go with buzz. 900mV at power stage is "nothing", but 9mV at preamp supply is significant, and suggests you do not have two fully-working filters in there.

[duce67us]

Sorry for the lack of input. 
Here is the current schematic I am working on. 

I rechecked the voltages, 
B+ is 324 vdc - it has a 120hz ripple, of about 370mV
C+ is 285 vdc
D+ is 270 vdc

C+ and D+ both appear much cleaner. 

I changed the initial supply caps from 47uF to 100uF
I also removed the Ground Buss so that the Pre tube is separate from the Output / Primary Filter Caps.

So, The Buzz seems to have gone away, but I still get a tremolo effect when I turn the volume pot past 80%

[jjasilli]

You might try reducing through B+ filtering to the preamp stages.  Stock is only 5uF.  It's often a good idea to stick to the power supply design of a vintage amp.  High filter cap values in early stages can make an amp unstable.

[shooter]

does the "effect" change when you switch off NFB?
you can also experiment with 470pF to maybe .001uF across the plate R, this WILL effect tone though.  You can also try a .01uF 600V from the preamp tap to ground.

[duce67us]

does the "effect" change when you switch off NFB?

The NFB switch has NO Noticeable "effect" on the LFO in any position. 
I can not see the LFO in the signal path, but does show up in the B+

Could this be a bad output transformer?

[shooter]

I can not see the LFO in the signal path, but does show up in the B+

pretty sure what you have is motorboating.  Play with the R values between taps, you already increased the caps.  You "might" find it on the cathodes, in the weeds, doing what trem does, wiggle the tubes bias.  If you inject a 100hz signal, then adjust the time base till it's stretched way out, almost flat, you can probably see that line move up n down, like over a 1 - 2 second sweep.

try a really big Ecap on V1a's cathode, something around 100uF

[Platefire]

I remember that CHB10a Project. I kept it a few years and sold it on e-bay. I've done two amps with a NFB control. The NFB control was a good experiment but I never used it, so IMHO is pretty useless thing. Best option is have the NFB resistor switchable for a on/off setting. Platefire