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Old 28th July 2006, 05:55 AM   #1
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Default Confused by the numbers

Hello! I re-wired my El34 SE amp for Triode mode (it just sounds better!) and I took some readings after the amp had been going somewhat loudly for 40 minutes or so. the problem is that I don't quite understand if these numbers are normal, or ab-normal

after 40 minutes of playing music:

Supply Voltage 420v
Voltage across plate 414v R-ch 415v L-ch
Load Resistance 2.5Kohm primary 8ohm Secondary

Cathode Bias resistor 500ohm
Voltage across Cathode Bias resistor 13v
Current ~26ma?

Both El34s has Blue Glow on the glass

Do these numbers seem out of whack? or is my Amplifier on the Road to glory?
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Old 28th July 2006, 01:36 PM   #2
kevinkr is offline kevinkr  United States
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You need to measure the plate current in UL or pentode mode, whichever mode it initially used.

Your operating point seems to have shifted towards cut off - I would normally expect there to be somewhere around 30V on the cathode resistors. Where did you connect the screen grid to? (They should be connected to the plate through a small resistor say 100 - 1K)

IMO I have never observed a large shift in operating point with EL34 switching from UL mode to triode mode, however I have never run them with cathode bias.. So I am puzzled.. What value are your grid resistors (large??) and is there a voltage developing across them - aka grid leak bias? Could be the case if they are gassy. Some dull blue glow on the glass and around the screen grid is normal with modern relatively soft vacuum EL34, bright blue glow that fills the space between the electrodes and generally around the envelope is a sign of excessive gas.

The amp probably isn't going to blow up, but the operating point sounds wrong. You need to investigate further and how about posting a schematic.
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Old 28th July 2006, 01:50 PM   #3
Sherman is offline Sherman  United States
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The voltage seems high for an EL34 SE. The numbers I have around here indicate a more "normal" value is 350V on the plate.

Also, 26mA seems almost loafing for an EL34. In SE class A they can go to about 70mA. Lowering the B+ to 350 or so and changing Rk to 360 ohms would be more in line with hi-fi use.

What you have might work great and last a long time, EL34s are pretty solid tubes in general I think. I've been thinking of building an EL34 SE since I have all the pieces and the numbers I suggest are just the values I came up with.

I only have experience with new, Russian EL34 tubes and only a very small sampling of those (4) but they have all had a blue glow near the base. I think that is just normal for those tubes.
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Old 28th July 2006, 04:32 PM   #4
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kevin is right... something is abnormal. everything is fine except the 13V bias and corresponding current. way too low for that plate voltage. according to the curves, that would be the expected bias at 150V on the plate. screen gets connected to the anode, not the cathode
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Old 28th July 2006, 04:37 PM   #5
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The Blue glow is the normal kind, it moves with the music sometimes, and it doesn't look like gassy glow. Here is a schematic.

The Amplifier sounds pretty amazing, but it really has trouble with mid-range, and it doesn't sound very good with Rock music
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File Type: jpg el34 set.jpg (42.6 KB, 373 views)
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Old 28th July 2006, 04:55 PM   #6
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Hi Alex,
Nothing appears to be amiss in that schematic, are you sure that the 13V reading you got was correct. I see nothing in the design that would inherently account for the weird reading you mentioned.

Dumb question - meter set to AC?

Also with the triode connection you probably don't need to connect the cathode bypass to the top of the secondary for the little bit of feedback it provides. And are you sure the feedback is not positive?? I'd just ground the cap directly. I'd definitely add some smaller film bypass caps or replace with 51uF Solens.

250K grid resistor seems reasonable for an EL34 - are you sure that the output stage is not oscillating? I recommend adding a 1K grid stopper right at pin 5 of the EL34. I have had these oscillate in the VHF region at a time when I didn't have a scope that could "see" it.
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Old 28th July 2006, 07:08 PM   #7
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It was most definatly set on DC,

Which do you recomend I try first?

I think I am going to ground the cap before I put in a stopped resistor purely because its a very easy modification

Is there anyway I could see if the amp is ocsilating? I don't have a scope, I do have a radio that can do AM/FM/LW/SW1-9 and a tv could I use these to check?

for the grid stopper method, just put a 1k resistor in between the Coupling cap and pin 5? or from pin 5 to the ground?
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Old 28th July 2006, 07:56 PM   #8
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Start with the grid stopper which is 1K right at the EL34 socket to the coupling capacitor. It should be the very first thing connected to the grid, everything else on the other side.

Cathode bypass to ground will not affect VHF oscillations. TV receiver or FM receiver might pick up the oscillation if present. 1K resistor will almost definitely eliminate it if present.

More thoughts later..
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Old 28th July 2006, 09:18 PM   #9
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Your schematic is very similar to mine except that I am using a different input tube. I have tried a bunch of different output transformers including the 125CSE. I find that an EL34 performs better with a 5 K load if run over 400 volts. This is very speaker dependent.

I also find that cathode feedback works wonders for the 125CSE. You must make sure that the phase is correct however, because the amp sounds bad if it is wrong. I don't remember which way worked right on the 125CSE, but I can check it this weekend. I agree with Kevin, to disconnect it until everything else is working correctly (ground the 1000uF cap).

Neither of these things will cause the cathode voltage to be off though. With a 500 ohm resistor and 430 volts you should have plenty of current. I had to go up to 560 ohms, but I am running 485 volts. I was running the amp with a 510 ohm and 450 volts and it worked good. It plays loud rock quite well.

Measure the resistor with an ohmeter to make sure it is really 500 ohms.

I have found oscillating amps with a TV set. If you have cable, unscrew the cable enough so that only the center conductor is making contact. This should result in slightly fuzzy pictures. Place the amp near the TV and play music through it. Try all of the lower numbered channels (up to 30 or so) and look for any interference that changes with the music. if you find anything suspicious, turn the amp off and see if it goes away. This doesn't catch every oscillating amp but I have found several this way. Some have been so bad that channel 6 was gone when the amp was on.
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Old 28th July 2006, 09:30 PM   #10
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I did measure it, it measures are around 498 ohms which is well within the standard limits

Unfortunatly, I don't have Cable (I have DirecTV), is it possible to notice this with just static? I might be able to get a station well enough to tell, but there is no garentee it will be one one of those stations.

I'm starting to think it'd be easier just to put the darn resistor in, just take a guess, and see what happens
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