C+ screen grid resistors--UPDATE

[Elpelotero]

My amp has blown some tubes in the past (and the fuse), but each time, the tubes do not appear to be blown. They look normal. I have reused the tubes again with no problems on my recitifiers, but on the C+ they blow on occasion.

I'm afraid the screen grid resistors may be messed up...I'm not a technical guy, so I'll describe what I've tried doing to test them and hopefully a techie can put together the pieces.

I borrowed a friends voltmeter to check the resistors. At first, we had no reading, even after turning the amp on with tubes in place. We then got a digital voltmeter and the resistors only measured .2 on the 6l6's and .8 on the el34's. From what I've read online, the measurements are supposed to be in the deep negatives. I don't know if I was supposed to be playing guitar to send a signal through the tubes or not...

Furthermore, logic tells me that if the resistors were truly bad, they would have blown up the tubes almost immediately. The tubes I used to test all this with today are different than the ones that typically blow.

As for the resistors themselves, they appear normal. They don't look "burned" or "open," but then again I don't know what a burned resistor looks like.


Am I reading too much into this resistor thing? Can a tube be bad even if it doesn't look blown? Can a blown tube still work for a long period of time and then suddenly blow the fuse (on repeated occasions?

 

[bunyok23]

are you sure you set the voltmeter to read resistance? in ohms?

 

[AdmiralB]

Measure the resistance. If it's proper, you're OK.

Beam tubes (6L6, KT-series, 6550s) draw almost zero screen current, so you would expect to see little to no voltage across the resistors when these are the tubes in use.

Pentodes (EL34s) can draw significant screen current, but you're measuring them at idle, where again you would expect to see less than a volt or two.

As far as your problem...some of the Mark series were very high-voltage. The Mark III Coliseum I had was about 560V, rectifiers are about 100V below that. Could be that the tubes are OK in one environment and not in the other.

 

[Elpelotero]

Admiral, that would make sense. Think of how the Sylvania 415's were special because they could withhold the 105 transformer better than current day production tubes. The tubes that kept blowing the fuse on me were 420's, which are known for not being a long lasting tube.

I guess I would have to measure the resistors when actually playing guitar.

 

[Elpelotero]

are you sure you set the voltmeter to read resistance? in ohms?

we tried various settings until we got some numbers showing up on the screen.

 

[AdmiralB]

The Sovtek 5881 is a good choice when you need something that'll take really high voltages. Mesa calls them 425s, IIRC.

 

[Elpelotero]

I really wanted to check out the GT clones of the 6l6ge and 6ca7. Would these take high voltages well?

 

[AdmiralB]

I would NOT use the 6CA7s. They were never designed to be shadow-grid tube (that is, where the screen voltage is at or near the plate voltage). The original Sylvanias and GEs did work OK like this, but they were really designed for voltage-doubler circuits like the old Music Man amps, where the plates were way up there (around 700V) but the screens were half that.

GT isn't importing the 6CA7 anymore due to quality issues, so I'd definitely avoid them in amps "on the edge".

I have used the EH 6CA7 with excellent results, though not at anything over 500V.

 

[Elpelotero]

****...i really wanted to finally test some quality el34's in the outer sockets without going NOS. To further exacerbate the problem, I'd need to do a rush delivery of tubes for a show this weekend. I'm just looking for a tried and true el34 tube that will give me a sweet brown sound.

 

[AdmiralB]

JJs will work (get them from someone who tests them), SED, Svetlana...all good.

 

[JOEY B.]

I had my experience with screen grid resistors in my C+ head. Tubes that lit up like lightning in a bottle, blown fuses and SMOKE bellowing out of the the amp. I finally gave up and sent the amp to Mike. Like yours, it would not blow the tubes, just an arcing symptom that was bought on by a master vol=5 and lead master=3 setting. After weighing the options, I chose the NOS power tube route. See if this thread sounds like the problem that you're having.

http://forum.grailtone.com/viewtopic.php?t=15935&postdays=0&postorder=asc&highlight=str447&start=0

 

[Boogiebabies]

I had my experience with screen grid resistors in my C+ head. Tubes that lit up like lightning in a bottle, blown fuses and SMOKE bellowing out of the the amp. I finally gave up and sent the amp to Mike. Like yours, it would not blow the tubes, just an arcing symptom that was bought on by a master vol=5 and lead master=3 setting. After weighing the options, I chose the NOS power tube route. See if this thread sounds like the problem that you're having.

http://forum.grailtone.com/viewtopic.php?t=15935&postdays=0&postorder=asc&highlight=str447&start=0

Don't forget to connect pins 1 and 8 with a buss wire.

 

[Elpelotero]

**** Joey, that was a heck of a thread...You experimented like mad!! What was the final outcome?

I'll have to do the same experiments as you are doing to see if I am having the same problems...I'm "skeert" to fire things up though. **** these things!

 

[JOEY B.]

**** Joey, that was a heck of a thread...You experimented like mad!! What was the final outcome?!

I gave up and sent the amp to Petaluma. It received a checkup per Mike B. with NOS STR-415 and STR-416 tubes installed in the power section. The amp has been fine since its return. At this point, I don't even care what was wrong. I just know that it sounds sweet these days. Check your pin 1 to ground, just to see.

 

[Elpelotero]

with those tubes, it better sound sweet!!

 

[Elpelotero]

Today I experimented a little like Joey with the tubes in a dark room. All 6l6's run at low volume because my neighbor was next door. One tube in particular started lighting up like a red/orange christmas tree when in the el34 position next to the 105. I switched it out and tested the other tubes in the same spot and it was only that particular tube that ran hot. When in a 6l6 position, it did not seem to light up too much.

Also, when placed in the other el34 position, it did light up, but not nearly as much as before...

So to recap, one tube is definitely funky and must be used in a 6l6 position or it fries. Something is wrong with that particular el34 position to make it get soo hot.

Do you think bad things will happen if I play at loud volumes for an hour? What would be the best course of action if you were me?
thanks

 

[Boogiebabies]

Today I experimented a little like Joey with the tubes in a dark room. All 6l6's run at low volume because my neighbor was next door. One tube in particular started lighting up like a red/orange christmas tree when in the el34 position next to the 105. I switched it out and tested the other tubes in the same spot and it was only that particular tube that ran hot. When in a 6l6 position, it did not seem to light up too much.

Also, when placed in the other el34 position, it did light up, but not nearly as much as before...

So to recap, one tube is definitely funky and must be used in a 6l6 position or it fries. Something is wrong with that particular el34 position to make it get soo hot.

Do you think bad things will happen if I play at loud volumes for an hour? What would be the best course of action if you were me?
thanks

I would connect pins 1 and 8 with a buss wire and then you can carry about your day with normal paranoia.

 

[JOEY B.]

Also, when placed in the other el34 position, it did light up, but not nearly as much as before...

So to recap, one tube is definitely funky and must be used in a 6l6 position or it fries. Something is wrong with that particular el34 position to make it get soo hot.

Set your meter to kohms and check across the resistor coming from pin 5 of your Class-A sockets (with the power off). I think it should read 220 kohms. I would be more concerned if one Class-A socket varied much from the other at this resistor.

 

[Elpelotero]

thanks for the advice...i'll test it out tomorrow!