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 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 3k resistor the voltage is dropping to 271v, then to 257v after the 10k.

On my actual Goodsell its the same rectifier, same filter caps, but a 3.5k resistor and it starts at 368v, then drops to 320V, then too 310V after the 10k.

I've tried a couple different resistors bridging the filter caps and measured both while disconnected from the amp and their fine, but voltage drop stays the same. I've also double checked my soldering to make sure there isn't extra resistance from a cold solder or something. I'm sure the voltage isn't right, but am not sure why...

Any suggestions?

Thanks!

[ToneMerc]

I would change that first 3K to 1K to bring the voltages up.

TM

[surfsup]

Youre dropping 10v over the 10k on both so it seems lots of current through the 3k to get a larger drop. The tubes are the same in both amps?

[tubeswell]

Hey all,

The question I have is: I'm seeing 362v at the 1st cap, but then after the 3k resistor the voltage is dropping to 271v, then to 257v after the 10k.

On my actual Goodsell its the same rectifier, same filter caps, but a 3.5k resistor and it starts at 368v, then drops to 320V, then too 310V after the 10k.

The B+ of 362 vs 368 at the reservoir cap is much of a muchness, might as well call it the same. So...

either one of those '3k' resistors isn't '3k' (you really need to yank them out of the circuit and check them with your R-meter to tell for sure), or

you have different tube loads in the different amps (which is affecting the current draw through the supply resistors, and hence different voltage drops), or similarly your gain stages are loaded differently with different value plate resistors and/or cathode resistors, or

you have bleeder resistors in one amp but not the other (or some other voltage divider or resistance between the B+ and the ground) which is loading the supply in one of the amps some more, or...

[ninjadave7]

Tubes are the same in both amps, but I'll try it actually using the goodsell tubes in the build. I DID pull both the 3k resistors and test them, both were right on. It's a part for part build, maybe it's as simple as a bad tube?

[jelle]

This is what I would do:

Pull the tubes and measure. If the difference is still there without tubes, investigate the filter caps.

If the difference is gone, test both amps with the same set of tubes, to see if the amps are the same under load.

If so, then compare both sets of tubes in in one amp to see if the tubes are different.

Hope this helps,

Jelle

[ninjadave7]

Good call, I'll try that right now, I'm at the bench.

[ninjadave7]

ok, voltages were the same without tubes: 420v at all nodes on the Goodsell, on the build voltages went down 1v at every node.

popped the tubes in from the Goodsell. sound is very bad, measured coupling caps off the PI and one had 35ma on backside, also the wire from the pot to input cap on PI is oscillating...

[tubeswell]

measured coupling caps off the PI and one had 35ma on backside, ...

So you have now replaced that bad cap? (which was evidently partially shorted and therefore loading the B+ down)

[ninjadave7]

Yes, I replaced the cap. the new one (actually new, not used) still is letting 25ma through. Their both Mallory 150's, I think i have one more to try, but isn't Mallory normally better about that?

[tubeswell]

The cap shouldn't be leaking any DC. Are you talking about those newish mallorys that are polystyrene caps? Polystyrene caps are bastids to solder in because they melt easily.

Got any other type of cap handy that you can use?.

[ninjadave7]

Maybe? they're white tubes? I haven't left the heat on long, but maybe they did melt a bit, who knows. I might have another lying around, or can just change the value a bit. I was told anything under 10ma isn't really a problem.

[surfsup]

Just gator clip it in temporarily instead of soldering....?

I do that all the time.

[Structo]

I thought the new 150's were polyester?

[tubeswell]

The new 150s are polyesters, but even they tend to be of variable quality. I got some in a Weber kit a while back and several of them were leaky (and new to boot! )