Be careful of doobtube videos.

primosounds

primosounds

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I say"doob", as in short for dubious. In this video about rebuilding a Fisher 500b
I was checking out this video since i am working on one of these and was interested to see what kind of mods he was proposing. And then this came up as a "mod" at around the 13 minute mark, the poster talks about adding resistors to the cathodes in part to help avoid tube flashover. The 500b is fixed bias and the cathode is grounded. He installs 10ohm 5 watt wire wound resistors to in part help prevent power tube flashovers. At this point in the video i go WTH, a 5 watt resistor in the cathode will pose no obstacle to a tube flashover at all, besides the fact that you really would not place a 'safety' on the cathode. It seems he is confusing a cathode biased sized resistor with a voltage sensing resistor that the fixed biased tube uses.
No point in trusting anything further after that. It just shows that there are a lot of misconceptions on boobtube and you really need to do some hard research more than just checking out videos.
 
Agreed. I'd definitely be checking with AKers before doing anything I heard about somewhere else. Especially modifying circuitry. :idea:
 
invaderzim said:
I've seen quite a few videos of all subjects where the 'fix' was either the hard way or the wrong way.
Click to expand...
In fact, another video i checked out about "rebuilding" a tube amp was nothing but a video of the posters inexperience with tube equipment.
Per usual, the first thing he did was to check operating condition. Which showed a defect, weak sound in one channel. So he goes through measuring voltages all over the place, and after 10 minutes or so of talking about his process and some theory, he stumbles upon the idea of testing the tubes! WTH?, Anyone who has had more than 1 tube amp would have done tube testing on the tubes of an unknown piece of equipment right off the bat.
2 videos on boobtube, and 2 strikes.
 
I honestly rarely test the tubes unless I have some specific reason for it. More often than not voltage measurements prompt me to do it. I also usually "test" by substitution. If the problem stops I plug it into the tester to confirm. I find it faster and more accurate to just swap a known working tube in to see if the problem stops vs digging out the tester and getting it set up.

About the only time I test tubes first is on something I just bought that is in completely unknown condition.
 
It depends on what the stated purpose of the resistor is and what else is in the circuit for protection. If it's really intended to survive a "flashover", which I take to mean a tube short, where the full B+ is dumped to the cathode, then yes, a 5W might be appropriate. But the issue there is while the resistor might survive, the output transformer may not, unless there's some additional protection in place. I had a commercial amp that used a 20 ohm 5 watt resistor in that location. But it also had a fused plate circuit.
 
I always thought the idea of the 10 ohm 1/4 watt resistor was a place to measure cathode current and if there was a short in a tube or some excessive current flow for what ever the reason the little resistor would smoke and hopefully save the transformer. I am no expert so please correct me if my understanding is misguided.

Gregb
 
the amp I mentioned above needed a large cathode resistor because it was part of the bias comparator detection circuit and I expect they didn't want that resistor to ever blow no matter what happened. So instead it had a plate fuse to handle the case of tube arc or what have you. But lacking a plate fuse, it is reasonable to use a smaller wattage resistor that acts like a fuse if it fed too much current. I've never used a plate fuse. I was just pointing out that there is another way to skin that cat.
 
Greg; Your correct in your Assessment of the job of a 10ohm 1/4 to 1/2 (at most) Watt resistor. 1.) measure the Cathode draw in mv to determine ma, and 2.) to shut down the tube and the 1/2 of the transformer associated with that tube should a short (ARC) occur on the cathode. Some guys are more comfortable with a 1/2w but a unless you're running them at or past the ragged edge, then a 1/4w Metal Oxide is sufficient. When you hear a "Fire Cracker" from the area of the receiver, bets are 1.) a resistor has let loose, and 2.) you'll never forget that sound.

Primo; The Basic Safety/Reliability mods for any 7591/7868 based FISHER receiver are following;
1.) 10 ohm 1/4 to 1/2 w Cathode resistor (replaces ground on each tube)
2.) Screen bus is lifted off the 1st tube pin 4 (7591). Buss wire going thru the tubes at pins 4 and 8 is cut ONLY BTWN the tubes isolating each leaving jumper btwn pins
4&8. Install a 100ohm 1/4 Watt resistor btwn pins 1 and 4. Now Daisy chain a new buss wire from the screen buss wire to each screen resistor pin 1. Attach the 1.2K 7W resistor to the new screen buss at pin 1 of one of the tubes.
3.) Drop the Grid return resistors from 330k to 220K 1/2W. Change the .047uf coupling caps to .068uf 600v or 630V. This maintains as close a R/C constant as you can get to the original R/C constant.

These "mods" help both the Old Stock tubes and New Stock Tubes stay healthy. A 10K pot set up as a variable resistor can go in place of R129/135 and C85 in the BIAS Circuit. This pot is a 1 pot fix, where all BIAS moves at the same time and hopefully at the same rate, based on Matched tubes . If any of them are hot or cold tubes, Best to build an IBAM or Dave G's IBBA board to match the bias to 70% - 80% of max. dissipation.


Larry
 
You fall asleep at the wheel, Primo? Going off the grid for 24 hours or so. The Girl Friend calls..........with a new swimsuit I have to check out! LOL!
 
You know i get the idea about preventing situations but that always is a balancing act between need and complexity. Not many amps but in G2 safety resistors and i have seen amps that use a resistor on the UL tap. Others like Dynaco, which must number close to a million units by now , forego that need. And yet, the oldest ones are still around with the newest models. So, it begs the question, why did the amp have a power tube arc over. I would think most likely causes would be incorrect bias and weak tubes. Both of which are user issues.
 
I had it happen myself. New tubes, freshly rebuilt Pilot SA-260. Its the closest thing to a Dynaco ST-70 I own, push-pull UL EL34 / 6CA7 amp. Different phase inverter design, but the output stage is pretty much the same in terms of voltages and the like. Everything was biased per spec, all voltages where they should be. The first time I wound up the loud knob one of the tubes flashed inside. I put the 100 ohm screen resistors between the tube socket and the UL leads and it has not done it since. I'm convinced they work.

Dave Gillespie actually did a fair bit of research on this. The article is on the Tronola site with a lot more technical detail, but the real short and sweet of it has to do with lower ESR capacitors being able to deliver more current than old ones. Basically it makes for a resonant tank circuit and under the right conditions you can dump a lot of power through the screen grid. Adding some resistance back in to make up for the lower ESR caps stops it.
 
primosounds said:
In fact, another video i checked out about "rebuilding" a tube amp was nothing but a video of the posters inexperience with tube equipment.
Per usual, the first thing he did was to check operating condition. Which showed a defect, weak sound in one channel. So he goes through measuring voltages all over the place, and after 10 minutes or so of talking about his process and some theory, he stumbles upon the idea of testing the tubes! WTH?, Anyone who has had more than 1 tube amp would have done tube testing on the tubes of an unknown piece of equipment right off the bat.
2 videos on boobtube, and 2 strikes.
Click to expand...
There are very few youtubers that actually know what they are doing when it comes to working on tube gear, some I enjoy watching are Blueglow electronics, Mr Carlsons lab, and shango066. Shango is a bit reckless sometimes but great videos, he adds some good humor.
 
Thanx Larry, i have been follow the Dave and Steve show, or is it the Steve and Dave show? before Dave became a fixture here on AK. No doubt they both have contributed mightily to the preservation and advancement of tube gear.
 
Tox, you do realize that all those photos of swim suit models, have pixilated subliminal messages that make you buy stuff you don't need and trust that the government has your best interests in mind.
 
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