Low B+ Voltage problem

[ninjadave7]

Hey all,
I'm working on a small wattage amp with old Hammond iron from an M-100 organ, based on a Goodsell Super-17.

The secondary from the PT is running through a solid state rectifier (2 diodes) to a 50uf, thru a 3.3k/50uf R-C, then thru a 10k/22uf R-C.

The question I have is: I'm seeing 362v at the 1st cap, but then after the 3.3k resistor the voltage is dropping to 250v. On my actual Goodsell its the same rectifier, same filter caps, but a 3.5k resistor and it only drops to 320V, then too 300V after the 10k. I've tried a couple different resistors bridging the filter caps, but results are the same. I'm sure that isn't right, but am not sure why...

Any suggestions?

Thanks!

[loudthud]

Have you measured the 3.5K resistor? If your meter won't give a steady reading, you'll have to disconnect one end of the resistor. What is the voltage at the first cap in the Goodsell?

[ninjadave7]

I have, and I subbed in a 3.3k too. Both check at the labeled value on my meter when removed from the circuit, as does the 10k at the 2nd node.

Updated (just checked) information:
On the Goodsell: getting 368v ->3.6k ->320v -> 10k ->310v.
On the build: 361v -> 3k ->271v -> 10k -> 257v

[Enzo]

Link your schematic.

SO are all the filter caps fresh?

In the last set of numbers, 90v drops across the 3.3k resistor. Ohms law tells us there is roughly 30ma of current flowing through it, and under 2ma to the last node. SO something is drawing that current. Either a leaky cap or a tube circuit. Are the tubes good? What tubes are running off that second node? Check the preamp tubes powered by it, any have rather high cathode voltage or getting very hot? Are power tube screens off that second node? Excess screen current at all?

[ninjadave7]

I drew one up, I'll have to find a scanner or something. It's very similar to a spitfire but I used the dc30 ef86 preamp, if that helps. The filter cap is brand new, everything is but the irons.

the power tubes are coming off that 2nd node, the wire goes to a turret, then through 470R resistors to G2 (pin 9) on the 2 EL84s. I've pulled the 470s and they meter at the correct values, although they are old CC resistors off the original Hammond.

[Enzo]

Measure the voltage drop across the two 470 ohm screen resistors then. Use Ohm's Law to determine current through them and add those together. Add the 1 and a half milliamp through the 10k, then whatever is left would be going through any other tubes on that node. Are the screens the ONLY thing on that node, or does a phase inverter or some other preamp stage also live there? Your circuit may be based on the other, but it is possible the voltage drop across that resistor is normal for what you built.

[ninjadave7]

Alright I'll try that. i found a bad coupling cap (leaking 35ma from PI to EL84 grid resistor) and replaced the tubes with ones that worked in the Goodsell. Doesn't sound great, but voltage drop changed. What's weird though, is the JJ's EL84's I was using that I think caused the drop came right from a DC30 that worked fine, straight from one amp to another.