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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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26th Mar 2023, 5:21 pm | #1 |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
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Gone to air?
Hi folks, have these old RCA 6550 gone to air? I ask because the emission tests ok, well the emission is down as one would expect on 50 year old valves from a guitar amp?
I should say the bloom looks worse in the pics than to the naked eye |
26th Mar 2023, 5:25 pm | #2 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,785
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Re: Gone to air?
They might be a bit gassy. They'd be whiter than that if they'd lost vacuum completely.
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26th Mar 2023, 6:04 pm | #3 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
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Re: Gone to air?
If you've got any kind of reasonable emission then there can't be a lot of air in the valve. It chemically changes the cathode coating as well as causing the heater to run cooler. The former really hurts the emission and the latter doesn't help.
If you just power the heater and then measure the grid-cathode voltage with a high impedance meter (a 10Mohm ordinary multimeter will do) you should find the grid becomes hundreds of millivolts or even a volt negative. It's a quick and dirty check for thermionic emission from the cathode. Cheers, GJ
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26th Mar 2023, 6:30 pm | #4 |
Heptode
Join Date: Dec 2020
Location: Swaffham, Norfolk, UK.
Posts: 582
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Re: Gone to air?
Perhaps they've been red plated at some time and that's caused some out gassing, consuming some of the getter. I would continue to use them if they check out OK.
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26th Mar 2023, 7:05 pm | #5 |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
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Re: Gone to air?
Many thanks for the replies chaps. I wasn't sure about the effects of a gassy valve, confusingly I have a 6L6 with apparently no getter but that still works?
GJ, thanks for the tip Last edited by ITAM805; 26th Mar 2023 at 7:10 pm. |
26th Mar 2023, 8:54 pm | #6 |
Dekatron
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 4,184
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Re: Gone to air?
If there's not even a white trace of where the getter deposits once used to be, it likely has a solid state getter. Often a pill mounted on a tab somewhere.
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26th Mar 2023, 11:04 pm | #7 |
Dekatron
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
Posts: 5,263
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Re: Gone to air?
I would agree with G Castle post 4, probably been used with glowing anodes which have outgassed and stressed the getter somewhat. Hopefully if you fix that little problem they will live a bit longer.
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27th Mar 2023, 5:00 pm | #8 |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
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Re: Gone to air?
Thanks guys. Umm, this isn't a nice job, no diagram and a 680V HT It's an AIMS, an early 1970's guitar amp from the US, 105W RMS so it says on the back panel.
It came with a popped T2.5A mains fuse (no HT fuse), it says it requires 117V on the back but the converter tx that the owner gave me is kicking out 130V, so I'm wondering if it's just being over-powered? Our mains down here is usually 250V Last edited by ITAM805; 27th Mar 2023 at 5:28 pm. |
27th Mar 2023, 6:00 pm | #9 | |
Heptode
Join Date: Mar 2017
Location: Cedar Grove, Wisconsin, USA.
Posts: 823
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Re: Gone to air?
Quote:
It seems that the Russians are making the latest issue valves. Dave, USradcoll1, as usual. |
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27th Mar 2023, 11:07 pm | #10 | |
Dekatron
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 4,184
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Re: Gone to air?
Quote:
As a rule of thumb for an optimal valve life, heater voltage should be within 5% of nominal, or if that's not possible within 10% where -10% is usually better than +10% (not applicable here, but the exception to that rule are series string heaters, where current instead of voltage should be within 5 to 10%). In any case, since the mains fuse popped, there's probably more important problems than just a somewhat high supply voltage. Maybe leaky capacitors, leaky valve holders, leaky valves, another leak or short circuit. Last edited by Maarten; 27th Mar 2023 at 11:15 pm. |
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28th Mar 2023, 8:51 am | #11 |
Octode
Join Date: Dec 2017
Location: Morden, Surrey, UK.
Posts: 1,552
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Re: Gone to air?
If the correct fuse for 117 volts 60Hz pops regularly it could well be the very significant additional surge from running it on 130 volts 50Hz.
As above, check the heater volts. |
28th Mar 2023, 10:02 am | #12 |
Heptode
Join Date: Sep 2010
Location: Hythe, Hampshire, UK.
Posts: 626
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Re: Gone to air?
Given the comments about the possibility of the valve anodes being run red at some point I would strongly suggest you check the bias on the 6550's. Does the fuse blow immediately on switch on or is it after the valves warm up?
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28th Mar 2023, 10:09 am | #13 |
Heptode
Join Date: Sep 2010
Location: Hythe, Hampshire, UK.
Posts: 626
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Re: Gone to air?
Dave are you saying Tungsol made them for RCA? Just that I'm looking at an original RCA datasheet for the 6550....
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28th Mar 2023, 1:47 pm | #14 | |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
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Re: Gone to air?
Quote:
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28th Mar 2023, 4:11 pm | #15 | |
Heptode
Join Date: Mar 2017
Location: Cedar Grove, Wisconsin, USA.
Posts: 823
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Re: Gone to air?
Quote:
When looking at an RCA valve, there was a dot next to the warranty code, which is two letters meaning week and year the valve was made. It was usually guaranteed for one year. Dave, USradcoll1 |
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30th Mar 2023, 4:24 pm | #16 |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
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Re: Gone to air?
I inserted a variac before the converter so as to get 117V supply and the quiescent current is about 1A at the transformer primary, using a clamp meter. Winding the amp up with a sine wave this goes up to 2.2A with the output heavily clipping, so we're well into in fuse popping country if the amp was running at 130V mains.
The valves appear to be well under biased at 29mA at the anodes with a 568V* HT, according to online bias calculation it should be nearer 50mA at that voltage. This may be due to the valves being somewhat tired perhaps? ps: my earlier report (#8) of a 680V* HT was with the 6550s out of circuit, so not a valid measurement |