200W Power Supply and Power Amp

[Gaz]

Hi,

Just wanted some experienced eyes to take a look at my Hiwatt inspired 200W power amp and supply. I tried to add some better fusing, switching, and flyback protection. Most of the ideas came from user Trobbins. I mainly would like feedback on my fuse values, which I high-lighted in red. I jacked a couple values from other designs and tried my best to calculate the others. Any advice those or the design in general would be really helpful. Thanks!


http://i43.tinypic.com/3500yev.jpg

[J M Fahey]

Sorry but your secondary is floating, no ground connection.

[Gaz]

Sorry but your secondary is floating, no ground connection.

Thanks for the reply, but don't worry I won't be floating any secondaries! Regardless, I actually don't see what you're referring to, as all secondaries are grounded. What am I missing?

[Steve Conner]

No, the two high voltage secondaries are floating. The good news is, with bridge rectifiers they're supposed to be floating!

I can't see any obvious errors in the circuit. The feedback arrangement from the OT secondary looks a bit funky, but maybe that's how a real Hiwatt is? (edit: nope, the OT secondary is hooked up completely wrong)

Fuse values in these circuits are usually "calculated" by trying bigger values until they don't fail at switch-on. If the fuse value is marginal, the failure can happen only after a couple of hundred power cycles. Not a problem in DIY builds, but could be bad in commercial production.

[Gaz]

Steve, thanks for looking at the drawing. Ok, I see what he meant, but not sure why it was mentioned...

I don't see any errors in the OT secondary either. I'm using this variant: http://hiwatt.org/Schematics/DR_Pre4Input_v1a.gif

That's good advice about 'calculating' the fuses. I was just wondering if they looked like they were grossly under/overrated. Thing is, I'd really like to have the face plates made up first to use a drilling template for the chassis, which requires me to know what to write on the face plates. I guess I can always use a label-maker incase I need to make them bigger. Maybe I'll get lucky with my educated guesses.

[J M Fahey]

I mean the Output Transformer secondary(es).
No current will flow through the voice coils.

Edit:
There's also an error with the fuses.
The 280VAC secondary needs the same as the 170VAC one, because both secondaries are connected in series and by definition carry the same current.
In fact, the 280VAC one carries a little more, because it *also* feeds preamps and screens.
Although the 50 to 100 mA increase means they will both use the same, because that small difference will be absorbed by the fuse tolerance anyway.
Quite precise values may be calculated using the tube datasheets, but a rough seat-of-the-pants estimate can be: (RMS Power x 2)/(+V).
This also is useful for the cathode protection fuses.
The 6.3A Primary fuse looks fine, for an estimated max power consumption (including the many losses) of 400VA (which would mean around 4A) and multiplying it by 1.5X safety margin.
Please keep us informed along the way, this one looks like a mighty project.
Good luck

[Gaz]

JM, that's an extremely helpful observation about the fuses. I'd been treating the HV windings as if they were independent out of my own ignorance. I think I could try either T1A or T1.5A fuses in both windings (this is something Trobbins actually suggested before, but I just understand what he meant at the time).

The Hiwatt design uses a high value fast-blow fuse in the bottom winding's negative side of the bridge, however both similar Sound City and Carlsbro designs use a 2A SLO-BLO there. Would a 2A SLO-BLO in that location be equivalent to two 1A's in the positions I have in the drawing I posted?

As far the equation (RMS Power x 2)/(+V), does "+V" stand for idle DC B+? I was also wondering if I would divide the answer by two for the cathode fuses because each pair has it's own fuse.

Thanks again for the help!

[J M Fahey]

I'd been treating the HV windings as if they were independent out of my own ignorance.

Don't worry, it does not *look* they are.
In fact, I type faster than I think; what I should have said is not that "the secondaries" are in series, but "both DC supplies are in series".
Anyway, as far as the fuses needed, it means the same.

I think I could try either T1A or T1.5A fuses in both windings (this is something Trobbins actually suggested before, but I just understand what he meant at the time).

Probably. Start with 1A ones and if they blow "on their own" go to 1.5A ones.
But you are close enough.

The Hiwatt design uses a high value fast-blow fuse in the bottom winding's negative side of the bridge, however both similar Sound City and Carlsbro designs use a 2A SLO-BLO there. Would a 2A SLO-BLO in that location be equivalent to two 1A's in the positions I have in the drawing I posted?

No, they are in series, so the 1 or 1.5A value still holds, they do not add up.
Anyway modern safety rules demand that *every* secondary is fused, so your additions are fine.

As far the equation (RMS Power x 2)/(+V), does "+V" stand for idle DC B+? I was also wondering if I would divide the answer by two for the cathode fuses because each pair has it's own fuse.

Yes on both counts.
In theory you should use the +V under load value, but the difference is small and this is an estimation anyway, so use the easier to measure value.
Good luck.

[Gaz]

Thanks a lot for clearing all that up - it's been very very helpful!