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Old 31st March 2013, 07:40 PM   #1
6J7 is offline 6J7
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Default blue flash inside 5Y3GT

Hello,

Since few days I have a nice little amp (Raphaelite DP84).

Having some 5Y3GT, I tried an RCA instead of Chinese 5Z2P.

At startup there was a blue flash in the tube and noise in the speakers, then OK.

Looking at the schematic, is it not too hard for the 5Y3 to "load" capacitor 270uF (C9)?

Should I put a 100 ohm resistor between the rectifier bridge and the plates of the 5Y3? something else?
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Old 31st March 2013, 08:11 PM   #2
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"At startup there was a blue flash in the tube and noise in the speakers, then OK".

Not for long. 270uF as a reservoir capacitor with hollow state diodes is way, way, way too much. Look at the spec for the 5Y3: Isurge= 2.5A/plate. Even though that looks like a lot, it's really nothing as compared to the Isurge of even small Si diodes.

You can use these enormous reservoir capacitors with solid state diodes, but not for hollow state. Hollow state remains high voltage, low current, and whoever designed that forgot that basic difference. Even back in "the day", it was sometimes done, but was always a compromise between cost and reliability. Then, it didn't matter because the largely atechnologically illiterate PaL didn't realize that having to replace the 5Y3s every six months was abnormal. Nor did it matter so much when you could get new ones at nearly every pharmacy, 7-11, grocery store, etc -- where ever you found those "You Test 'Em" kiosks. Even then, the biggest reservoir capacitors hung off the cathodes of a 5Y3 were 100uF -- less than half the 270uF that schemo calls for.

It's a fundamentally bad design. Either replace the hollow state diodes with Si units that can stand up to the excessive Isurge those 270uF capacitors produce, or redesign the PS reservoir capacitors sized to keep Isurge in spec, and lose the AC ripple with the appropriately designed LPF.

Otherwise, expect to be replacing 5Y3s frequently.
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Old 31st March 2013, 08:37 PM   #3
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Thank you for your reply.

So the first cap - C8 on schematic - already charged by the rectifier bridge before the 5Y3 conducts does not help the 5Y3 ?
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Old 31st March 2013, 08:43 PM   #4
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The Chinese are not the only folks who ignore cap. size limits, when working with vacuum rectifiers. Cary is another example. Grrr!!!

Look at the 5Y3 data sheet. 20 μF. is given as typical in a cap. I/P filter. Miles is quite correct in stating that unless the amp is modified, it will "eat" 5Y3s.

Eko2, please post a scan of the amp's schematic and top and bottom photos. One solution to the problem is installing a small cap. and a choke between the rectifier and that 270 μF. monstrosity. We need to see how much room is available.

A possible quick fix is the installation of a CL-90 inrush current limiting thermistor in the line between the rectifier and the big cap., but I don't know if the CL-90's cold resistance of 120 Ω is sufficient to tame the turn on surge and stop the destructive arcing.
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Old 31st March 2013, 08:51 PM   #5
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what some do is take a big resistor say 5 k and then after some time say 20-30 secs bridge that / with a relais that picks up. It will give s slow start.
But it might still give a spark, depending on the total resistance (trannie+limiting resistor).
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Old 31st March 2013, 08:54 PM   #6
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Quote:
Originally Posted by triode_al View Post
It will give s slow start.
The irony is that the tube rectifier is there for no other reason than slow start / voltage sag. A solid state bridge rectifier does the rectification (see schematic)...

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Old 31st March 2013, 09:04 PM   #7
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I missed the schematic. It seems the "designer" is using the vacuum rectifier to delay B+ rise. The Chinese 5Z2P appears to have a cathode sleeve, making it a 6087/5Y3WGTB "equivalent". However U.S. made 5Y3s are filamentary and turn on almost as fast as the "sand" diodes.

The low competence level of the designing person(s) is illustrated by the fact that 2 of the SS diodes in the rectifier bridge are unnecessary. The vacuum rectifier can serve as the bridge's "hot" side.

Cheap Chinese amp's are usually built on PCBs. Maneuvering room for design defect correction may be badly limited.
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Last edited by Eli Duttman; 31st March 2013 at 09:05 PM. Reason: typo
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Old 31st March 2013, 09:09 PM   #8
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I think the main problem here is C8. This will be fully charged before the 5Y3 has chance to warm up and conduct. C8 will hold an enormous charge and if the anode/cathode voltage breaks down then one hell of a current will flow. I suggest putting a 1k resistor across the anode / cathode of the 5Y3 or remove C8.

Last edited by payitforwardeddie; 31st March 2013 at 09:14 PM.
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Old 31st March 2013, 09:18 PM   #9
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eddie,

that could be the crucial point. The large C afterwards might be safe due to the 100R series resistor limiting the current.

Most vacuum rectifier manufacturers state a minimum resistance in the anode circuits, usually created by the power transformer impedance and additional series resistors, if necessary. In the schematic above, the anode circuit resistance is close to zero due to the charged C8.

Remove C8 and see what happens.

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Old 31st March 2013, 09:19 PM   #10
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The big 270 is not charged by the valve,it's in front of it.So nothing wrong here,schematic ok.For a decent delay of the powervoltage the valve has to be indirectly heated.
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