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Protection for DC-coupled cathodyne stage?
Protection for DC-coupled cathodyne stage?
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Old 15th September 2018, 03:20 AM   #1
MrSnrub is offline MrSnrub  Australia
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Default Protection for DC-coupled cathodyne stage?

Hello all! I have a Morgan Jones “Bevois Valley” amp which seems to destroy the E88CC valves in the input stage every hundred or so hours of service life.

Symptoms are pops (particularly on startup) and hissing, usually followed by quiet/distorted sound in the affected channel. Measuring between heater and cathode of a failed valve with a megger, the insulation appears intact (well above 20M at 250V applied).

I’ve tried higher-value grid stoppers right on the pin (10k), and replacing the valve sockets, but to no avail. Recently, I’ve elevated the heater supply to 30V or so via a divider off the B+ (330k in the top leg, 33k in the bottom decoupled with 10uF 63V electrolytic). This seemed to fend off the hiss, but I still got pops on startup. I fitted 1N4007 between grid and cathode of the cathodyne sections to protect against possible high positive grid bias and flashover on startup. It’s been a week, and no more pops I’m just waiting on some more valves (Reflector 6N23P) to arrive, and then I’ll see if it’s fixed once and for all.

Just thought I’d share my experience, as it’s been a long-standing issue for me. Has anyone run into similar issues? What were your fixes?
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Old 15th September 2018, 03:38 AM   #2
Eli Duttman is offline Eli Duttman  United States
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A look at the Philips 6922 data sheet suggests you've been exceeding the heater to cathode potential limits, before biasing off B+. A 30 V. bias seems inadequate and 90 V. is what I'd shoot for.

There are several "flavors" of Saratov made tubes tubes you can use. The best is the 6H23Π-EB (6n23p-ev), which is both tough and decent sounding. FWIW, the ElectroHarmonix (EH) 6922 is 6H23Π-EB.
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Old 15th September 2018, 03:59 AM   #3
PRR is online now PRR  United States
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Quote:
Originally Posted by Eli Duttman View Post
A look at the Philips 6922 data sheet suggests you've been exceeding the heater to cathode potential limits...
I read it as 150V cathode positive of heater.

But at start-up, V1a is cold, V1a plate and V1b grid-cathode go to full 285V for some seconds. {EDIT--- no, only 91V due to 47K:22K action.)

I'd be thinking a 150V Zener on V1a plate. No effect once hot, but limits the start-up surge.
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Old 15th September 2018, 04:12 AM   #4
45 is offline 45  Italy
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Yes excessive cathode to heater differential, despite what it says in the datasheet.
This is a common problem with other circuits like SRPP, mu-follower and cascade using E88CC or ECC88. Best way to do it is build a stereo amp and use shared tubes between the two channels: one tube serves as input and one tube as splitter (each for both channels). Each tube has its own heater supply at the right DC voltage. The PCL82 is much better choice for that circuit as is if no triode-sharing between the two channels thanks to 200V limit for Vkf (that was necessary for series operation in televisions). Still getting around 12W power with 300V anode voltage so no noticeable difference respect to 15W....
The diode is necessary or not depending on how the anode supply kicks in. Generally it is recommendable.
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Old 15th September 2018, 07:50 AM   #5
petertub is offline petertub  Sweden
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The problem here is the Vg1k om V1b, when cold the grid of V1b will reach 250V while
cathode is at ground.
A 1n4148 between grid and k on v1b will prevent positive voltage between grid and katode.

Another problem is Vkf, but that would be taken care of by elevating filament to 30-50V
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Old 15th September 2018, 08:03 AM   #6
kokoriantz is offline kokoriantz  Lebanon
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Instead of having the filament supply center pin grounded, tie it at 110v . The EL84 K-F max is 100v will exceed by 10v on start-up, while E88CC by 25v in stead of 135v.
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Old 15th September 2018, 08:31 AM   #7
45 is offline 45  Italy
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Bad idea. It will be even worse. A resistive divider to create an intermediate and convenient voltage reference is almost costless and infinitely more effective and safer.
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Old 15th September 2018, 11:34 AM   #8
cerrem is offline cerrem  United States
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This is the exact reason I insist on STAND-BY switches or B+ delay circuits.... I get bombarded with forum beatings that the tubes don't have cathode stripping issues... It's not about cathode stripping...it's about conducting current when B+ is applied to avoid this heater to cathode arc-over....
Another method to avoid this is to simply DC "float" the heater winding and use a bloody big bypass cap on the Center-Tap to ground....
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Old 15th September 2018, 02:17 PM   #9
DF96 is offline DF96  England
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Nothing inside a valve should be left to float. If the heater must not be too far off the cathode potential, then make sure that one is referenced to the other.
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Old 15th September 2018, 03:12 PM   #10
kokoriantz is offline kokoriantz  Lebanon
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Default 110v

110v can be made by 82k resistor from285v and 2x1N5263b zeners , thus avoids caps . Heath w5m is in same problem ,the filament supply is left floating.
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Last edited by kokoriantz; 15th September 2018 at 03:16 PM.
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