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Old 27th June 2004, 04:17 AM   #1
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Question Had a strange problem today...

I went back into my amp (SE 2A3 with C4S'd ECC99 driver, 5AR4 rectifier) after a long time to make some changes. I finally found some 1uF 400V film caps that I wanted to add as bypass to the PS caps, and I decided to try paralleling the ECC99 sections to see how that would sound. I got all the soldering done OK, though on some of the tube pins I was soldering pretty close to (and sometimes touching) some heatshrink. Did a resistance/continuity check on all the important points, and everything looked fine. -ve end of the PS caps were still at ground, +ve end wasn't shorted to ground, none of the tube pins were shorted to each other, and all other circuit and PS grounds were fine. Put all the tubes in and plugged it in. Transformer started humming as usual, everything good so far. The amp was upside down so I could check voltages, propped up on a stack of CDs. I was on my hands and knees watching the tube heaters come on.

The 5AR4 flashed. It was a whitish/bluish light inside the tube. I immediately turned the amp off. Took the fuse out, and it was blown. Unplugged it from the wall, checked all grounds again, put a new fuse in, and it blew immediately (didn't hear the transformer start humming). Took out all tubes including the rectifier, tried it again, and another fuse blew instantly. Decided that 1A fast action fuses weren't going to work, went and got some 2A and 3A slo-blos (not sure what I had in there before, but it was definitely a slo-blo).

This time, the fuse stayed. Transformer secondary showed about ~600V, which was good. Plugged in the 5AR4, hooked up the VM to the first PS cap, turned it back on. Looks like it's working so far. The voltage got up to about 400V (this cap wasn't where I added the film caps, but since there was no current draw, I think the other caps would have been at the same voltage) and I was about to turn it off, when I heard this crackling noise. So I turned it off. Checked all grounds and the film caps again, nothing shorted, nothing looking burnt. I thought maybe I crossed 400V on the caps and they didn't like it, so I added the 2A3s back in and moved the VM to the film caps. Tried it again, and the crackling started at about 250V on the film caps. Since I was turning it off the moment I heard it, I couldn't tell what exactly was doing it, but it was around the front half of the amp.

So I figured it wasn't the caps, which left my paralleled ECC99s. Checked the pinouts to make sure I hadn't mixed up plate and grid or something, checked for shorts across pins again, yanked some dangling heatshrink and insulation off some pins. And then, no crackling. I left it that way for a while, then put the amp back in my system and played it for about 30 minutes, and everything seems fine.

So... any ideas on what might have happened? When the amp got hot it smelt like hot/burning insulation, and I haven't noticed it do that before (which doesn't mean that that's not how it smells usually). So is it possible that I had solder around some heatshrink and that was burning through or something like that? I'm a little nervous about just forgetting about it, since the problem seemed to go away without me really doing anything.

Also, a question about the paralleled triode halves. I just connected the pins on the tubes to parallel them. So my C4S is now shared across both tube halves, which means each triode is now getting half the current it was before (which is about 11mA each now, so that's still OK for an ECC99). So the total current that can be delivered to the 2A3 is still the same, right. But the output impedance that the current is being delivered from should be halved, right? The Miller C of the 2A3 will interact with the output Z of the driver to set the HF rolloff, and I know the current delivery controls the slew rate which also controls the HF behavior. Are those two independent parameters, or are they just different ways of looking at the same thing? I think the amp sounds a little more dynamic now, and more extended in the treble, but I could be imagining it since that's what I think this change should do.

Thanks for looking,

Saurav
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Old 27th June 2004, 06:31 AM   #2
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When you said it was crackling without the rectifer that's a red flag it was the transformer or something. But removing the heatshrink fixed it you say?

Quote:
I'm a little nervous about just forgetting about it, since the problem seemed to go away without me really doing anything.
Yeah, I hate it when that happens!!

I don't know what you put the heatshrink on, if it was some filament pins, at the line or the HV secondary, it could've been the culprit. Pulling the rectifier culls a lot of suspects, for it to still do that without it really narrows down the circuit.

As for the ECC99, how did you have it before? Just one triode? Did you rebias (change cathode resistor) after paralleling?

The typical curve of Rp vs. Ip is an inverse one, going towards infinity at zero current to some definite level at higher current. Depending on where you are on the curve, your plate resistance will go up or down. If it's in a flat portion of the curve, you'll get slightly more than half the resistance. If it's in the low current portion, you'll easily double it.

Yes, it will change the HF pole with the miller capacitance, but if it sucks down 22mA, chances are it's already well above 100kHz. You'll never notice it.

Tim
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Old 27th June 2004, 06:45 AM   #3
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Thanks a lot for the help.

Quote:
When you said it was crackling without the rectifer
Oops... did I say that? I know my post was long winded enough to be confusing and maybe I got some of it wrong. No, the crackling only started after I put the rectifier in. The only thing that went wrong with even the rectifier out was that the 1A fast-action fuses kept blowing, but that's to be expected I suppose.

Quote:
I don't know what you put the heatshrink on, if it was some filament pins
The one I cleaned up was a filament pin. Also, since both filaments are on now, there's twice as much current going through there as before.

Quote:
As for the ECC99, how did you have it before? Just one triode? Did you rebias (change cathode resistor) after paralleling?
Just one triode, different triodes on each channel and I'd swap the tubes every couple of months or so. I have a NiMH battery under the cathode, so that holds at 4V no matter what, and I did the math back then and picked something that I thought could handle the ~ 20mA DC bias current.

Quote:
Depending on where you are on the curve
Here's the plate curve:

Click the image to open in full size.

The battery under the cathode biases it to 4V, and I measured about 150V from plate to ground, which puts me right at about 10 - 12mA. I guess I'm on the borderline of being OK with this change. Some day I could get another pair of CCS boards and put 20mA into both triodes. I don't think this one board will handle 40mA very well.

So the HF pole probably hasn't changed because of this. Assuming I have actually reduced my effective output resistance, is there any other advantage to having done that?

And once again, thanks for the help.
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