Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: paddydukes on June 28, 2019, 02:44:54 pm
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long time listener first time caller...
Looking for advice on what might be going on. I just "completed" a PR Single ended build and seem to be having a strange issue and am looking for advice that seems to be above my current pay grade.
using a light bulb limiter and a variac for initial power up i was unable to get the lamp to come on, so i followed all of Rob Robinettes trouble shooting PT secondaries and watching the light bulb limiter to glow bright, All was good with the high voltage secondary, the 6.3 was indeed the problem so I was able to isolate the problem to the lamp. it seemed to be in the bulb itself, upon putting in a new bulb, I powered amp up slowly and sound was coming out, all was seemingly good after the new bulb. Voltages seemed appropriate enough for the amount of voltage i was giving it so i decided to remove the light bulb from the circuit and power it up slowly again with the variac looking for smoke and things, however the lamp again would not light, I found this odd and plugged the limiter back in and variac'd it up again, and it was working again. The part for me that was the most strange is i turned OFF the variac, drained the caps, and started checking continuity making sure i wasn't grounded where i shouldn't be. I had continuity from my fuse/switch PT primary to ground, and to top it off it popped the GFI on the circuit in my house...
I am using an IEC power connection and normal fender type fuse holder with a carling switch, and all of that seems to not be showing continuity with ground at the moment, i checked the cord going from the IEC to the variac and it also is showing no continuity with ground where it shouldn't be.
any advice would be greatly appreciated, and thanks for being such a wealth of knowledge on this forum! I'll be spending my evening checking my work again to make sure nothing is blatantly wrong, but hot to ground intermittently? hmm..
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Lack of response indicates that your problem is not understood.
I'll say this. Robby has a great site, but I disagree that a variac and bulb limiter should be used together. The sole purpose of a bulb limiter is to detect a short. IMHO combining it with a variac will confuse things.
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Thanks for the reply, the long and short of the problem is that after the amp was on for a few minutes at reduced power there seems to be a short that developed between "hot in" and ground, must be a short somewhere, i'll keep looking around in there and see if i can find something, but thus far, nothing is blatantly wired in such a way to cause this...hence my confusion/concern!
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Are you testing with tubes IN?
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When posting a trouble shooting question, it often helps to provide:
1) a schematic,
2) some good resolution photos of the build
3) voltages if possible
With respect, Tubenit
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Hey guys, thanks for the responses, i seem to have sorted out the issue. I think the shorting issue was more of a fluke than anything else, it has not re-emerged...
The real issue was i forgot the grid leak 220k resistor...this was discovered after going over the schematic with the highlighter, like Doug insists we all do before posting on this very forum with problems...i hang my head in shame for jumping the gun.
The amp was based off the SE PR schematic that was posted perhaps on here(?) that was mainly a deletion of the PI and the second power tube, but then i decided to get wild and wire in the second tube in parallel in the idea that i would run 2 6v6's, but i have it set up with just one 6L6 and it sounds pretty great. I also decided to delete the tremolo portion of the schematic.
I went point to point without a board in there just to see what would happen, it turned out to be remarkably quite, way more quiet than i expected.
Apologies for posting too soon!
I'll see if i can find a time to toss up a picture and maybe even a sound clip if i get ambitious
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i forgot the grid leak 220k resistor...
I've done that!