V16 Output Tube tries to runaway on startup (sometimes)

[Chetspencer]

Greetings fellow Fisher enthusiast, I own a late model Fisher 400 that's been in my family since it was new.
I've been enjoying it off and on for my entire life.

I've mostly completed refreshing my 400 with the FisherDoc's Resto-Pak kit and a quad of EH 7868's.

It's never been touched before my attempt to bring it back to its former glory. Before I began this project, the output tubes (all tubes 100% original) where getting long in the tooth and there would be a windy noise after a few minutes of running.

I replaced everything in the Resto-Pak kit so far except: the De-emphasis resistors, C221 and C222 caps in the MPX and the silicon bridge rectifier (it has the original Silicon Bridge Rectifier).

After De-oxiting everything and installing the new EH7868's it sounds better than I've ever heard it over the 51 years I've been listening to it.

Now the problem. When I turn on the unit, the tube in V16 wants to run away and tries to start glowing orange slightly.
I've been using a laser thermometer to monitor the temps and the tube in V16 will go north of 400F if I let it, while the other 3 never get over 300F (240F to 280F).

Here's the interesting part. I shut it off and let it rest a minute, turn it back on and the tube in V16 is a happy camper and actually ends up running cooler than the tubes in V14 and V15.

I have swapped tubes and the problem is not the tube. The temperatures of the Power transformer is at 111F and the output transformers are at 101F and 96F after one hour.

Greatly appreciate your feedback.

Chet

 

[larryderouin]

Did you by chance replace/re-stuff ANY of the can caps, including the BIAS Can underneath???? Bets are the BIAS voltage is BORDERLINE low (less than -17vdc (closer to ZERO VOLTS)) causing the output tubes to dissipate more than Max Rating of 19W each and causing redplating. (redplating is very bad JU-JU, replacement of that redplating tube is now mandatory!). Get the voltage back down to -18vdc to -20vdc (the EH 7868's runn better there and aren't overloaded. )

The Resto-kit is a minimal band-aid patch to get it running 1/2 way (like a V8 running on 5 or 6 with clogged Carb Jets, It'll run but not well). The newest 400's are pushing 52 years old. 30+yo can caps usually are out of spec., which makes this one even worse. Do some research on the 400 in the stickies and you'll find a bunch of stabilization, and longevity UPGRADES, that really should be done if you want it to last up until they plant you or your grandkids are in midlife crisis.

Mark the redplating tube, and Pull the tubes, put DeOxit in the socket holes, and scrub with an interdental brush (get-em at Walgreens or Rite Aid). Scrub the tube pins also. ReTension the socket leaves.

Sounds like you have a late model 400. What's the Serial #.

 

[Chetspencer]

I have not touched any of the canned caps. The serial is 57923. I measured the bias voltage at 22v if I recall. Thanks for the input.

 

[Chetspencer]

Christmas Miracle!! So I've been enjoying my Fisher 400 basically day and night since I started this thread. Every morning when I first powered up, I would watch the V16 tube temperature with my laser IR Thermometer. During the first week it would get up to 400F in about 2-3 minutes (if I let it). I would shut it off and wait few minutes to cool off and then it would be fine for the rest of the day. Latter it seemed to calm down slightly but still too hot for comfort. Well yesterday it behaved like an angel and warmed up nice and slow like the other 3 tubes. Try'd again this morning and once again warms up perfectly. I think, If I understand the circuit correctly, that the C60A/B Cap just needed a little love and encouragement to start behaving like it should. So now my Fisher 400 is working perfectly and sounds like a dream even with my ancient Jensen TF-3A Speakers.

My question for you guys: Should I leave it alone or continue to chip away with the simple mods beside's what's in the Resto=Pak I've found here on AK, like removing and or adding certain resistors (not mentioned in the Resto-Pak kit).

Cheers, Chet

 

[audmod01]

Frequent events of red-plating like you have described damage a tube progressively. Each cycle destroys some of the cathode element's ability to emit electrons. Eventually the tube can no longer produce enough current to develop a glowing plate condition. It is possible that the tube socket in that position no longer makes continuous contact withe signal grid, G1, and lets the tube go into runaway conduction. Poor or no contact in the tube socket may be the result of corrosion in the tube socket or bent (spread apart) contacts in the socket. If the socket(s) have not been cleaned yet, do that first. Look down into each socket. Every contact should look exactly the same as all the others. The condition you described may be easily caused by a spread/bent contact from inserting a tube with a bent pin or by plugging a tube in slightly crooked. New production 7868 tubes have their pins in a slightly larger pin size and circle diameter and tend to spread the socket contacts compared to tubes produced in the USA years ago. This has caused considerable trouble for owners of vintage tube equipment.

Equipment is cool at first start. If there is any looseness in a tube socket contact, the warm-up will make the contact expand possibly just enough to make contact with its mating pin (resulting in correct current in the tube) until the unit is turned off and cools down. Drawing excessive current through the output transformer repeatedly may damage the transformer and eventually cause internal shorts or an open winding.

Did you install 10 ohm resistors in the cathode circuit of each output tube? If you did, this will allow you to measure the current in each output tube. They should be substantially equal, that is within a few mA of each other.

Joe

 

[Chetspencer]

Frequent events of red-plating like you have described damage a tube progressively. Each cycle destroys some of the cathode element's ability to emit electrons. Eventually the tube can no longer produce enough current to develop a glowing plate condition. It is possible that the tube socket in that position no longer makes continuous contact withe signal grid, G1, and lets the tube go into runaway conduction. Poor or no contact in the tube socket may be the result of corrosion in the tube socket or bent (spread apart) contacts in the socket. If the socket(s) have not been cleaned yet, do that first. Look down into each socket. Every contact should look exactly the same as all the others. The condition you described may be easily caused by a spread/bent contact from inserting a tube with a bent pin or by plugging a tube in slightly crooked. New production 7868 tubes have their pins in a slightly larger pin size and circle diameter and tend to spread the socket contacts compared to tubes produced in the USA years ago. This has caused considerable trouble for owners of vintage tube equipment.

Equipment is cool at first start. If there is any looseness in a tube socket contact, the warm-up will make the contact expand possibly just enough to make contact with its mating pin (resulting in correct current in the tube) until the unit is turned off and cools down. Drawing excessive current through the output transformer repeatedly may damage the transformer and eventually cause internal shorts or an open winding.

Did you install 10 ohm resistors in the cathode circuit of each output tube? If you did, this will allow you to measure the current in each output tube. They should be substantially equal, that is within a few mA of each other.

Joe

Thank you for the detailed feedback, I have cleaned the socket in question with Deoxit and a dental micro brush. I also installed the cathode resistors and if I recall they were within 5 MA.

At the moment there is about 25F degree different between the 7868 left and right pairs at 265F for V16 & V17 and 290F for V14 & V15 The power transformer will reach a maximum of 118F after many hours of use and the AX OPT's are at about 4F degrees apart 100F and 104F fully warmed up.