|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
64.12.116.131
I had taken bias and other voltages yesterday and today decided to put in a matched set oh EH 7951s in my Fisher X-101-C and the one channels volume output was out and the one tube had a scary huge red glow, it burnt the side of the glass and left a big mark that looks like getter. It was on 15 seconds and the glow was getting worse.I had a real sick feeling and play time was over real quick and I took a nap. Caps are all new and voltages were perfect? Cheap tube? I used two other quads with no problems in the 101c. Original Fisher black bases and RCA's. I used the EH's for 3 hundred hours in a 500c and had no issues until tonight. Guess I'll have to use the variac and plunge in tomorrow. Too much dramaAny opinons appreciated
Follow Ups:
No problems with the RCA tubes. All sockets were cleaned. I guess the EH didn't like the 101C. Too cheap? There were supposedly a matched pair and I used them in a 500C with no issues. Is the tube bad now? It has a burn mark that looks like getter on the side of the glass. What is this? Guess it's time to invest in NOS outputs.Have a great weekend
He has talked about this before. I had another asylum guy that had problems like this in his Fisher. I had him go over his bias supply so many times he must have hated me. Jim set me on the right track and as far as I know the amp has been trouble free for years. Or at least he hasn't emailed me for anymore help and was satisfied enough to send me a fisher tuner as a gift:)Don't forget to change the coupling caps either as the grid leak resistor works with them to create a "filter".
I have to say I am not a fan of stringing the heaters under the cathode this way and injecting positive voltage to the grids. I'd get a transformer and build a DC supply and go with straight cathode bias. I'd hate to think what would happen if a small signal tube ever had an internal short. Besides the tube heater's cold resistance is way different than its hot resistance and they could very well inject some noise into the output tube's grids.
I would discard the tube. If you sent Jim your other tubes he could see if they match and maybe supply one tube that matched up with them.
"If you sent Jim your other tubes he could see if they match and maybe supply one tube that matched up with them"
Some tubes have voltage sensitive cathodes that will not maintain a correct bias and will quickly fail. Most likely it was simply a crappy tube. However, it's always a good idea to slowly bring new tubes up to the correct bias over a brief period to keep from running them too hot right out of the box. If your amp has manual bias adjustments and the tubes you were replacing have needed increases in negative bias, then the newer tubes were probably biased too hot...or at least that's a thought.
There are a number of possibilities:1. Was the bias properly adjusted?
2. Were the tubes properly matched? (it would seem not)
3. Was one tube defective or damaged in transit?Since the old tubes are undoubtedly worn, they likely would not show the same symptoms. But if you set the bias to the schematic voltages you have only 40 minus 28.5 or 11.5 volts of negative bias. If the new tubes you had are toward the "hot" end of the normal range then they could easily be overstressed.
Finally, any of the vintage pieces that use 330K resistors for the output tube grids are at some risk when using either the JJ or EH tubes. In the RCA tube manual the maximum grid resistance was spec'd at 300K. Plus those 330Ks typically have drifted up about 10% over the years to the 360K range. You can get away with it with the old stock tubes, but the new production isn't as tolerant.
I recommend you change the grid resistors to 180K or so an chnage the coupling caps to .1 uf. Then you can use the old and the current tubes safely - as long as the tubes are well matched and not defective/damaged.
I know Mr. sgmlaw doesn't agree, he's a purist and doesn't want to make the changes. He advocates tracking down NOS or good UOS tubes instead. That's well and good, but the 7591 old stock supply is finite - and it's quite thin already. So it seems that at some point even the purists will have to use current tubes.
The current tubes are nice sounding, and very reliable with one exception - they are not tolerant of the real high grid resistor values in many vintage amps. It was the exceptionally low grid leakage in the old tubes that allowed the makers back in the day to exceed the tube grid resistor spec by 10% right out of the box!
The X-101-C is a cathode biased amp...and the RCA tube manual states that the 7591 maximum grid-1 resistance for cathode biased operation is 1 megaohm vs .3 megaohm for fixed biased operation.Another thing to check for is rather simple, a dirty or defective tube socket.
"The X-101-C is a cathode biased amp..."Go here and look at this schematic:
http://oldtech.net/Fisher/X-101-C/page5.gif
The amp is partially cathode biased unless the schema is incorrect... There is a bias adjustment pot present that changes the grid voltage.
Hi Jim,Yeah, I think you're right, I think it's biasing. I think everyone talking about a defective tube are probably barking up the wrong tree.
I have made a bit of a specialty around modding AA-151's, I have a small herd of them now, and I have witnessed one el84 going cherry in two of them that were otherwise OK - that is, the iron was OK.
I changed el84 makes, and it didn't make a bit of difference, went right to cherry on the same socket.
My fix was I changed the circuit from a single cathode bias resistor for all four to a separate resistor for each pair, and raised the value a little, and got rid of the bypass cap (with a separate cathode resistor for each push-pull pair, I could tell absolutely NO difference with it in or out, so out it went), and that fixed it.
HOWEVER, I think your suggestion about lowering the grid resistors is a great one, and I shall do that mod forthwith!!
best, charles
"It ain't a comeback until it's left the shop" Jimmy Dunne, the first man to drive a VW Beetle faster than 200 mph, and he has the forehead scar to prove it; I will always honor him for taking a chance on me when I wanted to be an engine mechanic.
Sounds like the tube had a miss-aligned grid structure or it was damaged in transport. If this was a matched quad, somebody may have matched them at low voltage/current.With any luck you did not fry any other components. Just put your old tubes back in and go. Try to return all of your new tubes.
_______________________________
Long Live Dr.Gizmo
Did it work ok with the old tubes? If unit is in good working order otherwise it cold be a defective tube or a bad pin/socket connection. EH 7591's at one time could be problematic. I use JJ7591S in my Scott 340B and they work well but supposedly the latest production JJ's have pin diameter problems.
This post is made possible by the generous support of people like you and our sponsors: