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Fixed bias problem, and no, it is not the tubes.

141.239.172.61

Posted on January 11, 2017 at 18:00:49
DAK
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In my newest amp which is SE with fixed bias. The power tubes kt88, kt120 and even kt66, the bias starts drifting after the tubes warm up after 5 minutes. I know it is not the tubes because i use the exact same tubes in another fixed bias SE amp with roughly the same design and they were stable after many hours of use and at the same bias currents.
Both amps are designed for kt120 outputs but with fixed bias i can simply dial down the current for the lower power output tubes. They both B+ voltage of around 350vdc. They both use 6sn7 drivers, but one has cathode resistors while the other uses LEDs to set the driver tube bias. The bias voltage is obtained from a reversed 6.3v filament trans of about 1.0A rating. the AC is full wave bridge rectified with Schottky diodes and filtered by a PI filter for 1 amp and a choke input filter for the other. Both circuits have roughly -60vdc feeding the adjustment pots. Weird thing is i had it playing at the shop with 6l6gc for an hour with no bias drift. I brought it home and problem showed up.
Any help or suggestions are appreciated, thanks in advance, Dak

 

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RE: Fixed bias problem, and no, it is not the tubes., posted on January 11, 2017 at 19:30:09
sony6060
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Bad resistors in the bias supply series (dropping voltage) circuit. Have a schematic or picture?

 

RE: Fixed bias problem, and no, it is not the tubes., posted on January 11, 2017 at 19:55:11
Jim McShane
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What value are the grid resistors? If they are too high that can cause problems. For the KT-120 with fixed bias the max is 51K - so if you have higher than that it could be the problem.

 

RE: Fixed bias problem, and no, it is not the tubes., posted on January 11, 2017 at 20:17:20
DAK
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Hi Jim, they are 50K and the adjusting pot is also 50k. thanks for replying

 

I think i rechecked them, posted on January 11, 2017 at 20:19:17
DAK
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But only in circuit. They don't appear abused but i will check them again to be sure. And both sides have the exact same problem. I can bias them correctly at the start and then they both drift equally on both sides. thanx

 

RE:Here is a photo of the bottom amp, posted on January 11, 2017 at 21:18:48
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The bias circuit consists of the 6.3vac filament trans in the center, then to the choke about 800 ohms followed by 50uf then to 5K w/ 50uf to 10k w/100uf 50k pot.

 

RE:Here is a photo of the bottom amp, posted on January 11, 2017 at 22:32:44
sony6060
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My bias supplies use a three section filter and a filter capacitor on the output voltage adjustment pot. It produces less than .001 volt AC ripple. I use 3K dropping resistors. At 140-160 volts initial bias voltage I would use 15K dropping resistors if it has a three section filter. 22K if a two section filter. Your transformer will supply 50ma current.

That is a estimated guess on resistor values, but supplies a stabile and stiff bias supply. I really need to see the amplifier schematic.

 

RE:Here is a photo of the bottom amp, posted on January 12, 2017 at 00:44:40
DAK
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Right, i measured the residual AC on my bias supply and it is around there .002volts. The only thing that is out of the ordinary is the filament transformer which got hot after 45 minutes. It is a used transformer so maybe a question mark as to its reliabillity? This trans is connected to a .8A 6.3v tap. It is not unbelievable that previously it may have be over worked. If it was actually abused could that cause it to not maintain it voltage regulation?

 

RE: Fixed bias problem, and no, it is not the tubes., posted on January 12, 2017 at 00:50:04
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The bias transformer should not get hot. It has to deliver only very low power.

 

I monitored the bias voltage over time, posted on January 12, 2017 at 02:25:27
DAK
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And it slowly declined after about 5 minutes. The coupling caps are 2 brand new Mundorf Silver and Gold in oil. The voltage rating on those are 800vdc so i seriously doubt that it is defective. The 10R resistor is a 2 watt unit and i did check it at the same time and it was not even warm, so again, doesnt appear to be a problem.

 

RE: I monitored the bias voltage over time, posted on January 12, 2017 at 02:30:19
Acoustic-Dimension
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When I read the other answers you gave in this thread, I realized that it could not be the things I mentioned, because both sides (left and right) behave the same.

So it has to be something different.

Then I read your answer that the bias transformers gets hot
And that should not happen.

So I would replace the transformer.

 

RE: I monitored the bias voltage over time, posted on January 12, 2017 at 03:23:31
Uncle Mike
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AD has a point. As the transformer heats up the DCR is drifting. Perhaps check your connections first as you said it was fine in the shop.

 

RE: I monitored the bias voltage over time, posted on January 12, 2017 at 04:28:00
Acoustic-Dimension
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That the bias transformer heats up can be a problem of the transformer, but could also bet hat one of the capacitors in the bias supply is not good (internal leak) so the bias transformers has to deliver some current.

 

RE: I monitored the bias voltage over time And it slowly declined after about 5 minutes., posted on January 12, 2017 at 04:45:31
Chip647
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There is your problem. You have to check the bias voltage not only at the end. Where you find the decline, there is your bad part. Never assume good parts cannot be bad.

 

RE: Fixed bias problem, and no, it is not the tubes., posted on January 12, 2017 at 05:17:23
Jim McShane
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So the "leak" path is 50K plus some portion of the pot element resistance. That should be okay but if you can't find anything else causing the problemkeep in mind the total grid circuit resistance is still about 75K or so.

Best of luck with your detective work!

 

RE: It is the same circuit as on the other amp, posted on January 12, 2017 at 09:26:57
DAK
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And that amp works fine with the same tubes. All the resistors are 1 watt except the grid which is 1/2watt carbon comp.

 

RE: I monitored the bias voltage over time, posted on January 12, 2017 at 09:35:23
DAK
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Hi AD, i have 2 tx in the circuit, the filament trans and the choke. The filament tx gets warm and hot at about 30 minutes of use (i noticed the heat during the first few times i powered up the amp and now i notice that it gets warm after about 5 minutes) the choke is cool. My feeling is i should try a different fil trans.

 

RE: I changed a 10K 1/2wCC and it seems to be ok., posted on January 12, 2017 at 16:05:46
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I put in a 10K 3/4W DALE rn65 resistor and i monitored the amp closely for over an hour and the bias voltage at the new resistor and at the power tube cathode both stayed unchanged over that time. I am going to pop in some good tubes and check the voltages again. Thanks for all the help guys. regards, Dak

 

Now it's back to its bad ways., posted on January 12, 2017 at 16:41:41
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Well, i put in some KT90s that test really strong and that i have used many hours with no problems, and the bias current started to run away again. The bias voltage also was dropping so there is definitely something going on. The problem is affecting both channels so it must be something from the filament trans to the 10K resistor, since both circuits at the pot seem to be affected equally. I think i will try a new filament trans.
I was checking my filament transformers and i have one that is 6.3v @ .5A. Is this trans ok for my circuit? Which would require enough current to bias 2 x kt150 tubes.

 

enough current to bias 2 x kt150 tubes You don't need current, posted on January 12, 2017 at 16:50:03
Chip647
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You only need voltage. You can bias these tubes with less than 5mA. Sounds like you have a bad cap in the bias circuit that is drawing current. That or a bad transformer.

 

RE: Now it's back to its bad ways., posted on January 12, 2017 at 16:52:27
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Can you post a schematic of the bias supply including adjustment pots and grid resistors?

Thanks

Tre'
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RE: enough current to bias 2 x kt150 tubes , posted on January 12, 2017 at 17:08:46
DAK
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Ok, so the .5A 6.3vac filament trans should be perfectly fine for the job?
I still don't understand why it worked in my shop for over an hour with solid cathode current on the output tubes of 75ma and the bias voltage was rock steady as well at -52vdc measured at the last dropping resistor. I took out the "test tubes" which were a mismatched duo of a GE 6550A and a Russian kt88. Plugged in the kt90s and after about a 5 minutes the current started to steadily climb while bias voltage steadily lowered. I turned it off, put in the old testing tubes and this time they could not hold bias worth a darn.

 

Unstable amp, posted on January 12, 2017 at 17:43:26
FenderLover
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Schematic will help. Almost sounds like you have an induced instability or parasitic oscillations.

What kind of grid stopper and screen resistors are you using?

 

RE: Unstable amp, posted on January 12, 2017 at 19:37:39
DAK
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I really don't see that as i have made at least 3 of these amps. Here is the basic outline http://rh-amps.blogspot.com/2013/05/rh-universal-v2-totally-universal.html
My amp differs with a 6sn7 driver tube, and fixed bias. I have made the fixed bias version 2 other times with no problems at all.

 

RE: enough current to bias 2 x kt150 tubes , posted on January 12, 2017 at 23:10:22
6AS7_6SN7
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A faulty diode (it may become 'open circuit' or a sort of non linear resistance) in the full wave rectifier of the bias supply might turn it into a half wave rectifier.
As a consequence DC current arises that might overheat the bias transformer.
If KT90 tubes are brand new I would also put a temporary cathode resistor durung the initial 200-300 hours ageing (to be eventually removed later on).
During the initial burn in some excess air molecules may be trapped inside the tube, excessive grid current results and getter action takes a significant time to clean up them.

"In bocca al lupo!"

ecc230

 

RE: enough current to bias 2 x kt150 tubes , posted on January 13, 2017 at 09:56:50
DAK
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The bias voltage starts out normally and at the shop it was running normally for over an hour. That doesn't seem to indicate a problem in the fwbr. I even changed the battery on my Fluke meter in case that was the problem. I am going to test again now that it has been sitting for over 12 hours.
The KT90 are used but i will remember your tip about burning in new ones. thank you for the tips.

 

Maybe the problem is my first filter caps., posted on January 13, 2017 at 10:58:35
DAK
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Hi Tre, i could not find the schematic in my files but it is a very basic type. The filament tx is reversed wired to a .8A 6.3v tap, the 120v is FWBR and filtered by a choke about 25H x 715dcr. this voltage is filtered by 50uf x 120v then dropped by 5K 3/4w mf then 50uf x 120v then dropped by 10k 3/4w mf then 100uf x 100v to the 50k AB 2W carbon pot. The wiper is 50k 1/2w cc connected to the power tube grid stopper. The wiper also has a 50uf x 63v connected to it.
Well, i did not have a single 50uf x 120v cap so i series connected 2 63v x 100uf caps, and not remembering that the metal can is "hot" sometimes they were sitting on the chassis. I guess i better change them out. thanks for the help! cheers, Dak

 

RE: Maybe the problem is my first filter caps., posted on January 13, 2017 at 11:20:44
Tre'
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"The wiper is 50k 1/2w cc connected to the power tube grid stopper"

Did you mean the wiper of the pot connects to the bottom of the grid resistor?

BTW I think you said there is -60 volts feeding the pots?

two 50k ohm pots in parallel to ground is a 25k ohm load.

60/25000=2.4ma. of current. That should be the total load current on the bias supply as long as the output tubes are never driven to grid current (overload).

Tre'
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the problem is NOT the first filter caps., posted on January 13, 2017 at 17:52:56
DAK
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I changed them out and the amp worked fine at the shop for almost an hour everything stable with nice audio. Brought the amp home hooked it up was stable for about 10 minutes then the kt90 bias current started to climb and i shut it down when the meter was at 2.0.
So i unhooked the amp and put my 2 chassis amp in place with the same kt90 power tubes. this amp is the same circuit with fixed bias. I hooked everything up and turned it on. Adjusted the bias to .90vdc or 90ma and everything is good. No bias drift the meter is showing no problems. The sound is very nice.
Regarding you picture. That is exactly how i have it with the grid resistor being 50k. The actual bias voltage before the pot is -51vdc. The bad amp is SEUL with Edcor outputs the 2 chassis amp is SE pentode with Electraprint outputs.
I dunno, the fact that it seems to work at the shop and not at home is really pissing me off. cheers.

 

RE: Hi FL. can you contact me offline? , posted on January 13, 2017 at 19:23:34
DAK
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I am beginning to believe that with nothing else left to rule out besides the transformers, it might be an oscillation that is inaudible. It is probably easier to add some resistors to tame this but I would like to discuss off forum for now. cheers. Dak

 

RE: Unstable amp, posted on January 14, 2017 at 04:38:44
PakProtector
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well...disconnect the finals from the rest of the amp. If it misbehaves still...

cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE:You mean run the amp w/out the driver tube?, posted on January 14, 2017 at 08:47:42
DAK
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Since the amp seems to be "unstable" only at home seems meaningful. At home i have wifi, several computers hooked up to it and also a cellphone signal booster all in the same room.
The amp is a simple 2 stage SEUL, 6SN7 to KT90/120. in the design there is no grid stopper on the input/driver tube and the grid stopper for the kt90 is 240R. I have made 4 of these amps with no problems with instability. In this version the 6sn7 is biased with 2 LEDs instead of a 380 ohm resistor. This increased the gain of the tube because it made the amp louder compared to the carbon comp resistor.

 

Powered it up WITHOUT the driver tubes...still misbehaving, posted on January 14, 2017 at 13:09:40
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It took roughly the same amount of time for the power tubes to start drawing excess current. Does that rule out the driver tube circuit as the source of the problem? I guess i should replace the bias power trans?.

 

RE: Powered it up WITHOUT the driver tubes...still misbehaving, posted on January 14, 2017 at 16:20:34
Tre'
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It doesn't rule out the coupling caps.

Not that I think that's your problem.

I still think it's filter caps in the negative supply.


Tre'
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Just for a chance.Disconnect the coupling caps, posted on January 14, 2017 at 23:59:45
Michael Samra
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You may have one that is leaky.. If you tried everything else,you may have some DC on G1.

"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: Powered it up WITHOUT the driver tubes...still misbehaving, posted on January 15, 2017 at 03:34:18
PakProtector
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take the coupling cap off at the power tube's grid. You'll need a couple meters with their leads clipped to a few places. The power tubes current sense, and some of the bias supply. Then fire it and you should find the problem.
cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: Fixed bias problem, and no, it is not the tubes., posted on January 15, 2017 at 07:06:48
sony6060
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I would rebuild the entire bias supply and use a new 6.3 volt @ 1 amp transformer as well. Those tiny caps in the picture look cheap anyways. Use 250 volt rated capacitors.

 

RE: Fixed bias problem, and no, it is not the tubes., posted on January 15, 2017 at 07:12:40
sony6060
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My bias supplies. Note the poly type capacitors that last forever. I use two bias supplies. -19 volts for power 6V6GT tubes & -140 volts for the phase inverters. With bias the phase inverters do not need a large capacitor to ground. Hard to see, but the rectifier bridge is at the bottom. I used 4 diodes for each supply. The silver caps at bottom are .01uF @ 1000 volt capacitors across the bridges to shunt any possible noise. The bias supplies have less than .001 volts ripple.

 

Residual AC in the bias supply is .002v, posted on January 15, 2017 at 15:22:48
DAK
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That would seem to indicate that the bias supply caps are good.

 

RE: Fixed bias problem, and no, it is not the tubes., posted on January 15, 2017 at 16:03:18
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Nice job Jim.

"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: Residual AC in the bias supply is .002v, posted on January 15, 2017 at 16:06:40
Michael Samra
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DAK
Just disconnect the coupling cap going to output tube and run it.You only have to disconnect the cap on one side and you can't play music but you can run the amp at idle to see if it drifts.It may not be your problem but it's quick way to find out.

"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: Residual AC in the bias supply is .002v, posted on January 15, 2017 at 16:07:07
Tre'
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I guess you're down to environment induced oscillation.

Grid stoppers and ferrite beads?

Maybe a bad PT.

Tre'
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Where would i put the ferrite beads?, posted on January 15, 2017 at 22:52:30
DAK
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I can put a grid stopper no problem but where would i put the ferrite beads?

 

Both coupling caps are new Mundorf Silver/gold and oil, posted on January 15, 2017 at 23:08:58
DAK
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If one is bad i can see that as a possibility but both new caps being bad does seem unlikely. Even if 1 cap is bad can it pull the bias voltage down equally in both power tubes?

 

RE:What would i be looking for in the meter readings? nt, posted on January 15, 2017 at 23:27:00
DAK
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.

 

RE:What would i be looking for in the meter readings? nt, posted on January 16, 2017 at 03:48:33
PakProtector
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I offer an example; hook at least one to the bias supply. Absolutely have one on the final's cathode sense resistor; that is your primary indicator, yes? So, if voltage on the bias supply remains constant, and cathode current climbs, what does that tell you?
cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE:What would i be looking for in the meter readings? nt, posted on January 16, 2017 at 08:08:19
DAK
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Right, thanks, i will do those checks.

 

RE: Where would i put the ferrite beads?, posted on January 16, 2017 at 08:27:00
Tre'
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I would start with the large snap on type on the power cord and audio input cable.

Then the tiny donut type around the wires leading to the grids.


Tre'
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"Still Working the Problem"

 

RE:What would i be looking for in the meter readings? nt, posted on January 16, 2017 at 08:38:46
PakProtector
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"Right" is not usually an acceptable answer to an essay question...:)
cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

I was not being facetious or disrespectful. , posted on January 16, 2017 at 11:44:11
DAK
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I had just crawled out of bed, and these days, i mean crawled, and dashed off an answer. Nowadays, dash is just a concept. I will give an update after i get to the shop and try some of the suggestions.

 

RE: Where would i put the ferrite beads?, posted on January 16, 2017 at 11:50:08
DAK
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I got a snap on the power cord already. But i can put the bead type on the inputs. Do you think a grid stop resistor is warranted on the input/driver tube? The amount of gain from the driver tube is already on the low side since the amp is only 2 stages. thanx tre.

 

RE: Both coupling caps are new Mundorf Silver/gold and oil, posted on January 16, 2017 at 12:06:23
Michael Samra
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Even if 1 cap is bad can it pull the bias voltage down equally in both power tubes?

Manufacturers do things in runs.If there is an issue with those caps,being they came from the same run no doubt,that can be an issue for both.
It's very simple to do that test and get it out of way.That may not be your problem but newer oil caps have had issues at times,including the Jensens.There may be cases where the oil has become conductive for whatever reason. You have built these amps several times and this is the amp you are having trouble with. I would try the caps. Just disconnect one side of each cap and run it and that way you will know.

"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: I was not being facetious or disrespectful. , posted on January 16, 2017 at 16:41:14
PakProtector
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What I was hoping to see was an answer to the last question. Not jacking you up( or at least trying not to). Attempting to gage level of understanding of a proposed scenario.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: Where would i put the ferrite beads?, posted on January 16, 2017 at 19:14:58
Tre'
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If the amp behaves in a different location then we are assuming that you have a lot of RF at home so, yes. A grid stop resistor on the input tube might be just what you need.


It's easy enough to try.

Tre'
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It helped a little., posted on January 16, 2017 at 20:47:41
DAK
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The amp was stable for about 10 minutes then slowly the cathode current of the 6550 started to tick up, and then it picked up speed and i shut it down. I also put in a grid stopper on the input/driver tube. I put the ferrite bead on the wire lead to the grid stopper. it is short not even 2". Maybe i should go with a shielded input wire? The run is short as i said.

 

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