Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: billcreller on June 28, 2012, 11:57:36 pm
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I'm fixing (!) a Valco clone amp I gave away in 2006, which needed a PI tube socket changed. This amp wont have any output for about two minutes after it's turned on. So I measured the B+ when I turned it today, and it was around 445 VDC, on the first node when it came on, and after the longer than usual wait, the B+ (first node) was around 340VDC, which was normal, according to the record I keep on a build. All the heater voltages are normal when powered on, so the that seems normal, but it has the impression that it takes a long time for heaters to come up to temp etc.....
Any ideas why it's a long warm-up time? Otherwise it works fine, and has zero hum, even wide open.
It was built with a 6J7 pre-amp tube (grid cap ) , and I rewired it for a 6SJ7, to get rid of the grid cap & shielded wire to it.
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In my experience, problems that don't start until the amp has been on for a while are thermally induced, which means maybe there's something that's expanding when it gets warm. Did you check all the solder joints? And did you try different tubes? And while nobody likes to contemplate it, it's also possible that you have two different problems here - the wait for output might be separate from the low voltage.
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Do you have a tube rectifier ?
If yes, what happen if you swap it with one other ?
K
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... This amp wont have any output for about two minutes after it's turned on. So I measured the B+ when I turned it today, and it was around 445 VDC, on the first node when it came on, and after the longer than usual wait, the B+ (first node) was around 340VDC, which was normal ...
What happens when you measure, say, that input 6SJ7's voltages? I'm thinking especially B+ node, plate voltage and screen voltage.
Does everything stay high for the full two minutes, then drop to proper voltages? Or does it start high and drift downward steadily to proper operating voltage?
An analog VOM or VTVM may help see when/how fast the B+ settle to where it needs to be.
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In my experience, problems that don't start until the amp has been on for a while are thermally induced, which means maybe there's something that's expanding when it gets warm. Did you check all the solder joints? And did you try different tubes? And while nobody likes to contemplate it, it's also possible that you have two different problems here - the wait for output might be separate from the low voltage.
+1
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... it's also possible that you have two different problems here - the wait for output might be separate from the low voltage.
Bill seemed to be saying that 340vdc is the typical, loaded, B+ voltage. From that I gather that 445v is high because it's as though there's no current draw; i.e., the unloaded rectified voltage.
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Yeah, the 340 voltage is normal. I tried measuring the B+ while it was warming up, and the transition from high to normal is quite quick. The high voltage readings are on all the B+ nodes, like pre-amp etc. I suppose it could be a tube, but it doesn't seem like one tube could do it.
I don't have an extra rectifier tube handy. It's an EZ80, with the 6 volt heater, to match the PT winding. The PT is from an old Sony reel-to-reel player. But like mentioned, the power is there, I don't suspect that tube.
It is getting interesting though ! :icon_biggrin:
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I know that there are lot of people on this forum that do not share my view that a standby switch needs to be installed. Looking at the low amps required for the heater 0.3 amps, this application may be one where a standby switch to allow the heaters to preheat the tubes, before power up.e
On the otherside, I looked at the spec sheets for the 6J7 and the 6SJ7, and did not see where GE recommended preheating the tubes. :w2:
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This evening I replaced the pair of 6V6 output tubes, and the amp came to life quicker than it did previously. I'm still playing around with it, mostly for curiosity.
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I like standby switch .
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I tried measuring the B+ while it was warming up, and the transition from high to normal is quite quick. The high voltage readings are on all the B+ nodes, like pre-amp etc.
That sure sounds normal to me with an indirectly-heated rectifier, like your EZ80.
I have an amp with a 5U4 (also indirectly heated). On switch-on, the B+ in my amp rises to near-500vdc. It drops back down to a bit over 400vdc.
The rest of the amp gets B+ voltage, but not current, until that rectifier's cathode heats up.
This evening I replaced the pair of 6V6 output tubes, and the amp came to life quicker than it did previously.
How much more quickly? Is it down to normal tube warm-up time? If so, I'd wonder if the older 6V6's were about to lose all emission...
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> The rest of the amp gets B+ voltage, but not current, until that rectifier's cathode heats up.
I would say the B+ voltage comes up with the rectifier cathode heat, and the rest of the amp does not _draw_ current until their cathodes heat.
Since rectifiers typically heat faster than other tubes, this leads to the 500V...400V bump.
Not sure why or _if_ Bill has a problem.
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Maybe not a problem at all. It warms up quicker now than it did. Maybe those old 6V6s are junk ! I have a big box of them, so it's working more normal with a different pair.
Thanks all, for the responses :icon_biggrin:
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It's so dam hot here this week that I'm building another Valco clone......in my basement, where it's cooler.
I have enough parts around for another one :icon_biggrin:
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> so dam hot here this week that I'm building another Valco clone......in my basement, where it's cooler.
Same here, except I am re-building the really disgusting cellar stairs. They seem to be leftover wood from a collapsed house and a failed sawmill. They been under a leaky Bilco for 30 years. They wobble, and stuff got down under: plastic french-fry, once-nice carving knife, and stuff I dunno WHAT it is, all damp and filthy.
But it's 89 outside and 68 down cellar.