What's the best way to drop B+ voltage by 100v or more?

[mort]

I've got a little 5F10 Harvard circuit that was built up from spare parts a while back. The PT puts out a pretty high HT and I'm getting ~450vdc off the rectifier. If I wanted to knock that sucker down to 300-350v, what's the best way to go about it?

Fwiw, the problem I'm having is when the volume is up around 75% or higher, the amp squeals/oscillates/whatever you wanna call it. Sounds fine otherwise. In searching this topic I ran across the advice to open the nfb loop but that didn't resolve it. I'm thinking maybe the voltage is just too much.

[tedmich]

I imagine a MOSFET enhanced version of the common zener voltage drop trick might work. The zener alone dissipates a lot of heat but a high enough wattage version may be available and not too expensive if yours is a little amp.

see here: http://music-electronics-forum.com/t28072/


also RG great heretical paper Mosfet follies:
http://www.geofex.com/Article_Folder...osfetfolly.htm


(of course a nice new Edcor PT won't be too expensive either!)

[J M Fahey]

That said, too high voltage and oscillation are unrelated.

You very probably have a grounding/layout issue.

[tedmich]

oh maybe I should read the BODY of the post nice catch JM!

[Tom Phillips]

I've got a little 5F10 Harvard circuit that was built up from spare parts a while back. The PT puts out a pretty high HT and I'm getting ~450vdc off the rectifier. If I wanted to knock that sucker down to 300-350v, what's the best way to go about it? ...

If you still want to lower the voltage for any reason, consider adding a choke and configure the power supply as a choke input filter. The choke input filter will produce a B+ that is about 100V lower than the stock capacitor input filter. It's another alternative to creating the higher voltage and then dissipating the energy to lower the voltage.

[mort]

That said, too high voltage and oscillation are unrelated.

You very probably have a grounding/layout issue.

Is that the sort of symptom that could arise from having the OT too close to the PT??

[Tom Phillips]

Can you post photos of the build?

[mort]

[mort]

Senor Juan you are correct about the high voltage not having anything to do with the oscillation. Just for kicks and since I'm getting cozy with the Heathkit gear, I pulled the rectifier tube and clipped in B+ from a regulated power supply and fed it 275-400v and it still wanted to oscillate at the top end of the volume control. Maxing out the tone control contributes to the oscillation as well.

As a side note, it was interesting to hear how the amp changed at different supply voltages. Definitely interesting...

[mort]

And here are a couple of odd things I noticed about using the RPS (Heathkit model PS-4)..

I lifted the 6.3v connections from the power indicator lampon the amp and clipped in 6.3vac from the supply tool (as well as still having it tied into the B+) and I would get a high current reading on the tool at idle and when I applied guitar signal the current reading would drop. Is that a false high current(100ma) reading at idle? And that was at only 100vdc B+. I was scared to turn the B+ any higher because the idle current would max out at 150ma before even getting to 150vdc. Am I supposed to use it for only one power source at a time?

Also, if I was using the amp's 6.3v supply and using the RPS for the B+, and if I turned the amp off first, the current would spike out on the RPS' current meter.

What I was trying to do was to run the amp without powering the PT at all. The idea was that if the oscillation was gone while not having the electromagnetic field of the PT, then perhaps the problem IS with the PT & OT being too close to each other. I guess I'll just have to unbolt the OT and physically move it around a little, but that's easier said than done...

[Tom Phillips]

The first thing I would try is adjusting the lead dress.
Specifically, I would set up the controls so that the amp was just starting to break into oscillation and then use a insulated device (chopstick or similar) to push around the leads from the phase inverter output that feed the power tube grids. If that affects the amplitude of the oscillation, especially if you find a position that allows the Amp to be turned up higher before the oscillation/squealing starts then you can explore alternate lead routing.I say this because your grid feed wires look longer than the stock Harvard layout and they look to be routed closer to the power tube plate wiring and the OT, which, in you amp, is a lay down mount protruding through the chassis. Another diagnostic approach would be to route the PI to power tube grid wires temporarily over the parts board in an arc away from all the other wiring. Then do the same test with the volume control setting while moving around the wires. If that cures the problem then you can devise a new lead dress that doesn't cause the oscillation.

OT proximity to the PT and its orientation relative to the PT in your particular amp could be a contributing factor. You could lengthen the OT leads and move the OT around to investigate that possibility. However, I'd try the easier wire lead dress investigation first.

Ground system issues can also contribute to problems like you describe so I'd check the integrity of all the grounds too.

Cheers,
Tom

[J M Fahey]

LOOKS very good but as you know, the devil is in the details.

Maybe you move one of those wires 1/8" and the oscillation stops.

So chopsticks are the tool of the day.

Also grounding there instead of here (a mere inch away) may also mean day-and-night.

Oh well.

[mort]

If I let the amp idle for longer than a few seconds at or over the oscillation point it starts cooking the 470r on the first rc node. Should this resister be a 1w or higher? The schematic only calls for a 1/2 watt...

Anyway, I tried moving wires around with the volume up but not oscillating, one by one, trying to find a trigger and only came up with an OT lead shorting just inside the layers. I can touch it one way and lose 95% of the output, and move it the other way and it comes right back. I don't know if that's related to the issue at hand but it needs replaced regardless. Oh well, at least I can still play it in the meantime. And it's just a little shop beater anyway.

[Enzo]

what is the voltage drop across that 470 ohmer? Then use Ohm's Law to determine the current. Is it excessive? Even if not, calculate the watts dissipated.


My first reaction to the oscillation was a lack of sufficient decoupling on the preamp B+, but the lead dress is surely the best place to start.

Are those two red wires from the white caps looping up and under the board all the way to the power tube sockets? That is a lot of wire ascting as an atenna broadcasting the strong grid drive signal into the rest of the circuits if so.

[eschertron]

Are those two red wires from the white caps looping up and under the board all the way to the power tube sockets? That is a lot of wire ascting as an atenna broadcasting the strong grid drive signal into the rest of the circuits if so.

I don't want to derail the thread, but on the builds I've done, I've twisted the two (out of phase) grid signal wires from the board to the tubes. This, thinking that the fields would cancel. Does it offer any improvement to what might be happening here? Is this technique done by any established builders?