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I added (kind of) the last components to my take on the KT88 build and brought it up on my variac tonight.

Nothing exploded! :banana:

Compared to Marks (Blueglow Electronics) measured voltages I get some 10-15 V lower values all around, except for at the plates (pin 1 & 6) of the ECC85 where I have 27 V HIGHER value - 256 V compared to 229 V.
Should I be worried about this?

I have only tested the amp without a signal or load on the output yet, didn't have time tonight for a more thorough test.
 
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Is your u.l. triode switch rated for 450+ volts? Any chance it could be shorting to ground?

The switch says 6 amp 125v, pretty cheap as I recall, actually I had a hard time time finding the switch, guess it could be the problem. Thanks for suggesting it.
 
View attachment 1446436 View attachment 1446437

I added (kind of) the last components to my take on the KT88 build and brought it up on my variac tonight.

Nothing exploded! :banana:

Compared to Marks (Blueglow Electronics) measured voltages I get some 10-15 V lower values all around, except for at the plates (pin 1 & 6) of the ECC85 where I have 27 V HIGHER value - 256 V compared to 229 V.
Should I be worried about this?

I have only tested the amp without a signal or load on the output yet, didn't have time tonight for a more thorough test.

Nice job, my voltages actually measured close to Marks B+ was a few volts higher, then it blows the fuse.
 
Nice job @Heffa. I think the reason your voltages are different is because you are using a 5U4? which has a higher voltage drop and more current draw than the GZ34 in the original.
 
Nice job @Heffa. I think the reason your voltages are different is because you are using a 5U4? which has a higher voltage drop and more current draw than the GZ34 in the original.
Thanks!
Oh, and sorry for uploading the wrong picture - I actually tried a GZ34 too, and then got the values mentioned.
With the 5U4 I got some 30V lower values, as expected.
Still confused about the higher plate voltage though o_O
 
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I would think the higher voltages on ecc85 plates is fine. Mine varied quite a bit depending on what ecc85 tube I installed. Some running in the 250 range. I think Mark was using a 6n1p with a max plate voltage of 250. The ecc85 is 300.
 
Is your u.l. triode switch rated for 450+ volts? Any chance it could be shorting to ground?

Ordered a heavy duty ul/triode switch, replaced it and fired the amp up, variac all ok, brought it up to full ac, got the 1 khz tone, all appears normal, b+ at 455v. Turned it off, watched until the b+ dropped down and hit the switch, arcing within the GZ34, blew the fuse. Haven't replaced the tranformer yet, will be a while until it comes in, thought I'd try it with the new switch.
 
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You said in an earlier post that the GZ34 was humming. A bad rectifier tube would explain a few things. Mark the current one as suspect and try another.
 
Tube rectifiers do not like a short off then on cycle. I agree, rectifier tube could very well be compromised. Thermistor would not be doing its job either with fast cycle.
 
So, took another step, disconnected the b+ to the left channel,
You said in an earlier post that the GZ34 was humming. A bad rectifier tube would explain a few things. Mark the current one as suspect and try another.

10-15 seconds, got up this morning and decided to see if switching it to triode mode made any difference, now the tube is arching at 100 v, switch position doesn't seem to make much difference, BTW I did try a second tube.

At 80 volts in, I have about 300 v b+, after the b+ drops I can turn it back on without an issue.
, no arcing.

At 100 v my b+ is 365v and I don't hear any humming.
 
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Man do I feel stupid, looks like I found the problem, somewhere during all my messing with this thing, the chassis ground had broken off and was laying along side the the lead on the thermistor. re soldered the wire and success, I have started it on full wall ac several times, I do have a hum through the speakers but all my wiring is from the transformer is untwisted so probably the cause. Nothing like being such a noob!

Tubetime & JohnT123, you guys are great for putting up with me.
 
That's not enough cool down time if the B+ has drained off. The heaters and cathodes are still warm enough to pass current into empty filters and you get a high current draw through the rectifier and outputs. You should give your amp at least a minute of cool down after turn off. The 5AR4 is a low internal resistance rectifier so it could need even more time to cool down depending on the load it gets from downstream. Amp to amp is different. 1 minute is minimum.

Thanks, I am giving it 10 minutes or so now while testing it.
 
Good that you finally found the issue Bill, great work!

I guess I'm just one of those guys that doesn't see the obvious, don't know how long that ground wire was over on the the thermistor lead, looked good, wasn't until the cautions about quick re-starting I was going to see if the thermistor was getting warm to the touch I noticed it, it had broken off and just bent over for the contact, looked completely intact, it's still playing, Dire Straits, sounds great, puts out more bass than my ST120, playing with the sub settings. BTW, my LSA-1's are only 88 db and it drives the heck out of them.
 
BTW, the one tube brighter than the other is the tube, switched sockets and the brighter tube moved. Concerning the GZ34 I did ruin one of my three, tested very low so I tossed it, of the other two one tests higher than the other so put that in the amp.
 
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