Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Oddvar on November 19, 2016, 06:43:43 am
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Hi. I just bought a set of nos Sylvania EL84 for my AC30 Hoffman build. For some strange reason they only work in V4. In all other positions the make a terrible, crackling nois. My original tubes, reissue Mullard, JJ. RCA an GT tubes work just fine. I wonder if there's a problem with the voltage in V5/6/7?
I had 4 Shuguang tubes, they all burnt out, in all positions.
Any suggestions?
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I wonder if there's a problem with the voltage in V5/6/7?
might be, what ARE the volts at those plates?
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Is pin 1 connected to pin 2 inside those Sylvania EL84s? Use your ohm meter to verify.
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Did your Shugang tubes "burn out" after a normal life (at minimum, many months) or did they burn out immediately or super fast after you installed them?
There are some amps that run EL84 really really hot so nobody expects decade-long life out of them, I'm just trying to get an idea if your Shugangs are reasonable quality tubes in a general sense.
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The shuguangs didn't make it through 15 min. But other tubes works very well?
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The plate voltage on the v4 and v5 is 326, v6 and v7 is 327/8. Should it be a bit lower?
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Should it be a bit lower?
max Plate is 300ish so I'd say yup. For high gain-ish I like around 180ish at plate, low gain-ish I like 100 to 120ish at plate.
EDIT: by the time I got to the schematic and typing, V 567 became V 123 :cussing: :BangHead:
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Is pin 1 connected to pin 2 inside those Sylvania EL84s? Use your ohm meter to verify.
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When I measure pin 1 and 2, it's moving around a bit, but in the area of 0.4/8 ohm?
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When I measure pin 1 and 2, it's moving around a bit, but in the area of 0.4/8 ohm?
That means they are connected together inside the tube. Do the bad shuguangs have low resistance between pin 1 and 2 also?
Look at this RCA 6BQ5. You can see pin 1 and 2 tied together. I'll tell you why this may be important shortly...
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On the Shuguangs there are no reactions, some are visibly connected at two, some aren't.
The Sylvania tubes have no connection on two, just a loose pin sticking up.
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Seems like the NOS EL84s have internal connections between pins 1 and 2 where modern EL84s do not. Since the resistor goes from pin 1 to pin 2, that might be a problem? Any solutions?
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Seems like the NOS EL84s have internal connections between pins 1 and 2 where modern EL84s do not. Since the resistor goes from pin 1 to pin 2, that might be a problem? Any solutions?
It's not an issue between NOS and modern tubes. It's just an inconsistency between tube manufacturers. You will find some NOS tubes that don't have pin 1 and 2 connected, others that do. Same with modern tubes also.
Just don't mount the resistors between pins 1 and 2. The easy thing to do is connect one end of the resistor to pin 2 and connect the wire from the board to the other end of the resistor. Do the same for all 4 resistors.
I don't know that this will cure your odd problem but it's worth a try. Sometimes odd problems need odd fixes. One thing for sure though, get that resistor off pin 1 and then you can use any EL84/6BQ5 without shorting out the grid stopper.
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The board wire goes to pin 9, should I connect taht to the resistor so it uses pin 2? Or?
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The board wire goes to pin 9, should I connect taht to the resistor so it uses pin 2? Or?
No it doesn't. LEAVE PIN 9 ALONE! We're talking about the board wire that goes to pin 1.
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ok, get it, got it, good, as Danny Kaye would say.
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I really don't like to have wires just connected to the unsupported end of a resistor. This is how I'd do it...
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Thanks, great way.