vdc drop

[dobias]

The DIY PS I assembled has a 437-0-437vac power transformer& a choke filtered supply of 350-0-350vdc, no load.
I've finally debugged one channel* of the separate DIY SE 2A3 amp to operate & find the voltage has dropped to 86VDC. The volume is low, about what I expected but wonder if I shouldn't have 350vdc operational & much more volume. The speaker is a 95dB Hemptone FR8.
Is there normally such a wild difference between no load & loaded?

*It's a bitch to read the fine print on the resistors with failing cloudy eyesight.

[tomchr]

It is common for the output voltage of an un-regulated power supply to drop a little bit (maybe a few percent) when the load is applied. But a drop from 350 V to 86 V is not acceptable.

As you hint at, some resistor in the supply would likely be of the wrong value. OR the amplifier draws too much current.

~Tom

[M Gregg]

[QUOTE=dobias;2606869]The DIY PS I assembled has a 437-0-437vac power transformer& a choke filtered supply of 350-0-350vdc, no load.
I've finally debugged one channel* of the separate DIY SE 2A3 amp to operate & find the voltage has dropped to 86VDC. The volume is low, about what I expected but wonder if I shouldn't have 350vdc operational & much more volume. The speaker is a 95dB Hemptone FR8.
Is there normally such a wild difference between no load & loaded?

QUOTE]

Thats a huge drop! Can you post a schematic?

Unless you do not have enough current. What is the current available on the PSU.
What is the bias current on the power tubes?

If you struggle to read the resistor values ..measure with a digital multi meter!

Regards
M. Gregg

[dobias]

M Gregg,
"measure with a digital multi meter"
That's how I found my mistakes.

The Power transformer is 250mA at 437-0-437. The output is 350vdc. I thought that would be way adequate for a Moondog 2A3 amp.

How should I read the bias current on the 2A3's?

Thanks for responding,
Frank

[M Gregg]

Originally Posted by dobias

M Gregg,
"measure with a digital multi meter"
That's how I found my mistakes.

The Power transformer is 250mA at 437-0-437. The output is 350vdc. I thought that would be way adequate for a Moondog 2A3 amp.

How should I read the bias current on the 2A3's?

Thanks for responding,
Frank

Is this the schematic for your amp?

Welborne Labs 2A3 Moondog Monoblocks

Or are these values close?
http://home.earthlink.net/~aeholsten...ondog_info.htm

If this is correct R10 is 2K with 50 Volts across it using Ohms law v/r = current 25mA

Regards
M. Gregg

[boywonder]

Frank: One quick check with the mulitmeter is to disconnect the power supply from the amp circuit, and check the resistance of the B+ into the amp circuit to ground, to determine if you have a short or incorrect R value in the amp circuitry somewhere. Obviously, do this with everything powered down.

You can also check the DC voltage coming out of the PS with the amp circuitry disconnected, assuming that your caps are rated for the unloaded B+ voltage.

[dobias]

No short found in the B+ of disconnected amp.
353.7-0-353.8vdc of disconnected PS with no load.
I've been losing the previous replies & am having trouble sending a schematic sketch of my PS.
I'll send this bit and keep trying.
As for the Moondog, I followed the original plans. I need to know what items (ID'd on the Moondog parts list)that are being replaced with the recommended parts, such as the chokes.

My PS has :
-A 250mA 436-0-436v power trans.
-Independent 5v 4A filament trans.
-Paralleled 5R4gya rectifiers
-15-20H swinging choke
-25K dropping resistor
-12K5 bleeder resistor
-Two series 8uF caps for 4uF
PS splits into two independent LC's of 4.5H smoothing chokes & two series 20uF caps for 10uF.
I hope my description is sufficient. I'll keep trying to upload the schematic.
Frank

[dobias]

With the 25K dropping resistor jumpered out, the PS output under amp load is 384vdc & only a volt or two higher when disconnected from the amp.
However, it isn't playing. Sigh!
I'll have to wait for better light tomorrow or risk getting a shock.
Frank

[boywonder]

Originally Posted by dobias

-12K5 bleeder resistor

That seems a bit low........Is the bleeder still in the PS circuit when you got the 354V at no load?

If it's a 12K5 resistor, it's bleeding 28ma and dissipating 10W!

[dobias]

boywonder,
Yes, the 12K5 bleeder has been in the circuit with the 25K dropping resistor when I had a no load of 354vdc & the huge drop under load. Jumpering out the dropping resistor gave a 385vdc under load and not much change without a load.
It looks like the bleeder drop of 10w plus the dropping resistor was too much. Since the 12K5 bleeder is two 25k resistors in parallel, I can rewire them in series to drop only 2.5w & then play with the dropping resistor to get the design 350vdc.