Voltage problem in new build

[tubedogsmith]

I'm using a known good Marshall style 100wt power transformer. 116vAC out of the wall, heaters are right at 6.3v, bias seems good, it's at -37v now. the problem is I'm only getting 380vAC across the HT secondary at the rectifier and rectified voltage is at 242vdc. Transformer grounds are good and transformer is wired correctly. I'm thinking this is somewhere in the power supply but I can't see the forest for the trees here. Any ideas??

[tubeswell]

What sort of rectifier is it (Valve or diode? halfwave/fullwave? - bridged?)? And is the HT wired up to it properly? (Does the HT have a center tap that should be grounded? If a bridged diode setup, is the rectifier grounded?)

Is (one of?) the reservoir cap(s) malfunctioning/not soldered up properly?

(Just guessing in the absence of more detailed info)

[Robert M. Martinelli]

Hi,
TW is right, we need more details; e.g. are you taking your measurements under loaded / unloaded conditions ? ( If you' re reading 6,3 VAC with the tubes on, ok, if you have 6,3 VAC with no load that' s a bit low IMHO ) - as to the bias, it' s not necessarily good because the bias voltage looks OK, you should check idle currents ( either using the shunt method or the 1 Ohm cathode resistor method ) - As to the +B ( HT ) voltage, which seems way on the low side ( I would say about half of the expected voltage, Marshalls usually run at about 490-500 VDC ), you could have one section of the rectifier ( which should be of the full wave type, again, more detail needed ) not working, and don' t overlook wiring also, even if your wiring seems correct, it could be wrong ( not necessarily because of a mistake of yours, but e.g. if the winding info provided by the transformer' s manufacturer is wrong, this has actually happened to me once )

Just thought it was good to ring some bells....

Hope this helps

Best regards

Bob

[tubedogsmith]

Sorry on the short info, it's a fullwave rectifer with a center tap on the transformer. No tubes. I've tested all the diodes in the rectifier and they seem fine. I mentioned the heater and bias voltages because they "seem" ok as the HT voltage is roughly half what it should be. All caps seem fine as well.

[MWJB]

-37v seems low for the bias supply.

Don't a lot of Marshalls use a bridge rectifier? What is the rated B+ voltage, compared to actual voltage, at the B+ secondaries before the rectifier?

[tubedogsmith]

The transformer should be 350-0-350

[MWJB]

Not if it's expecting a bridge rectifier, then it would be around half that (350VAC end to end, as opposed to 700VAC end to end in your example).

[Steve Conner]

Indeed. I've seen Marshall schematics where the rectifier is a bridge, but the transformer still has a centre tap. The tap is used for balancing the voltage across the two series-connected filter capacitors.

So, your transformer probably came out of one of these, and you need to change to a bridge rectifier.

[tubedogsmith]

The HT secondary has a CT which is connected. I'm running a fullwave rectifer.

[tubedogsmith]

Indeed. I've seen Marshall schematics where the rectifier is a bridge, but the transformer still has a centre tap. The tap is used for balancing the voltage across the two series-connected filter capacitors.

So, your transformer probably came out of one of these, and you need to change to a bridge rectifier.

I believe it did, Thanks!!

[tubedogsmith]

Well, I'm back. Hooking this up with a full wave bridge rectifier got it working better but something is still pulling the voltage down. B+ should be around 465v or maybe more and it's at 408v with the tubes biased cold at 25mA. I"m really stuck on this one.

[Robert M. Martinelli]

Well, I'm back. Hooking this up with a full wave bridge rectifier got it working better but something is still pulling the voltage down. B+ should be around 465v or maybe more and it's at 408v with the tubes biased cold at 25mA. I"m really stuck on this one.

Hi,
Have you measured how much +B voltage do you get with the tubes out ? You stated before you have 380 VAC out of the HT secondary, if memory serves me well 365 VAC are enough on Marshalls to get the correct +B voltage ; so I would expect you to find about 500 VDC out of the rectifier/filter cap section ( with the tubes out ) . To better sort the problem out try to do some loaded-not loaded comparative measurements...

Regards

Bob

[tubedogsmith]

With the tubes out I get 460v b+. I tried disconnecting the b+ wire from the ps and only got 408 volts on the wire. After measuring this and turning the power off I found the caps were discharging voltage even though the voltage was not connected, weird. The only voltage still connected was bias but the tubes were not in the amp.

[tubedogsmith]

The caps all read a little over the markings but they all read when I measrured them individually. I started reflowing all the solder joints on the power supply board as I thought the problem could be there. One of the negative leads on one of the caps just fell out of the cap when I hit it with the solder iron. I'd been all through this thing with a chop stick and not found anything lose but there it was. I replaced it and it seems to be rocking. The wall voltage is back down to 114v but I'm getting a b+ now of 444v and all the weird tone artifacts are gone. thanks for the help everyone.