Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: glassesnvests on October 17, 2013, 09:45:34 am
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Just finished up a Hoffman AC30 build, and when i turnit on, it was silent at first, but then as it warmed upit makes a deafening hum and V6 and V7 start red plating. I've swapped out tubes and it's still doing it. Anybody have any help of thoughts? I can get voltages and pics up later today. Thanks!
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Check first the wiring of the OT. Make sure that the center tap is connected to B+.
/Leevi
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If it's a new build you would be best served by powering it up on a lamp limiter. It could be that you reversed the plate wires so you have loud oscillation feedback? But it also sounds like you miswired something somewhere? Double check everything/all wiring & connections from pi to output jack.
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Severe redplating is usually accompanied by a loud hum (yep, I've done it a few times).
I'd look for how you might have biasing messed up. Maybe miswired sockets for the EL84's, cathode bypass cap is installed backwards or is shorted, control grid connected to a positive voltage instead of resistor to ground, etc.
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Alright, So I've checked the Plate voltage (I think? that;s the voltage that runs to pin(s) 9 on the El84's right?) and it's at 415 at each Pin 9. The tubes aren't in cause they were plating and I couldn't leave it on long enough to get a reading for fear of killing the tubes. This seems extremely high, right? Or is it high because the tubes aren't in?
Pic links :
http://i840.photobucket.com/albums/zz325/Adam_McGoon/IMG_20131017_122045_196_zps17428ced.jpg (http://i840.photobucket.com/albums/zz325/Adam_McGoon/IMG_20131017_122045_196_zps17428ced.jpg)"
http://i840.photobucket.com/albums/zz325/Adam_McGoon/IMG_20131017_122343_449_zps18787872.jpg (http://i840.photobucket.com/albums/zz325/Adam_McGoon/IMG_20131017_122343_449_zps18787872.jpg)
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More:
http://i840.photobucket.com/albums/zz325/Adam_McGoon/IMG_20131017_122309_682_zps09dea0c5.jpg (http://i840.photobucket.com/albums/zz325/Adam_McGoon/IMG_20131017_122309_682_zps09dea0c5.jpg)
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Assuming there are no short circuits which are not obvious in your pictures, you seem to have things wired correctly for the EL84's.
Are you certain the Cut control is wired correctly? And there's no way your 0.1uF caps from the phase inverter to EL84 grids are shorted?
... The tubes aren't in cause they were plating and I couldn't leave it on long enough to get a reading for fear of killing the tubes. ...
Does your meter have leads which can be clipped in place in the circuit? If not, you might need to buy some.
Here's what I would do:
- With tubes out, measure from EL84 grids to ground. This should be 0vdc. If no, check the wiring of the Cut control and 0.1uF coupling caps.
- If previous step is good, clip black meter lead to ground and red meter lead to non-grounded side of the EL84 cathode resistor. With power off, put the EL84's in their sockets. Turn power on and monitor d.c. volts across the cathode resistor. As tube current climbs, this should go up to some value (like ~10-11vdc) then stabilize. If it keeps climbing, tubes hum or redplate, shut off power.
- If you get redplating in the previous step, unsolder the cap bypassing the EL84 cathode resistor, and repeat the previous step. If redplating stops, the cap is faulty, shorted or its polarity is reversed.
Runaway redplating should be impossible with cathode-biased EL84's. That means there has to be a wiring error in the EL84's. Look hard for solder bridges, melted wire insulation, strands of wire sticking out and contacting other socket pins, etc.
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Alright, So I've checked the Plate voltage (I think? that;s the voltage that runs to pin(s) 9 on the El84's right?) and it's at 415 at each Pin 9
NO!!! Pin 9 = G2 = Screen, Pin 7 = Plates
luckily in your pic you have it wired correctly for those! What you don't have correct is having the grids going to pins 1 on each tube. Don't wire anything to pin 1, 6, or 8 on an EL84 as they are internally connected. That very likely is your problem. But, 415v is very high for EL84s and 50ohm pretty low for bias resistor on those at that voltage. First take care of grid wiring then power up again and check bias afterwards.
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I would check pin 2 and make sure there is not positive voltage there. Would not worry to much about the high plate voltage. Peavey runs the classic 50 right at 400 vdc.
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... What you don't have correct is having the grids going to pins 1 on each tube. ...
He has the grid wires running there as mounting points, with a grid stopper between pins 1 & 2. This matches Hoffman's AC 30 layout.
Unless the specific EL84 has a connection at that pin, it should be okay.
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The hoffman schematic uses pin 1 as a lug strip basically. for the resistor and connection to other screens.
Sorry! I meant screen not plates.
HotBluePlates, You're a saver! It was the Cut control. I missed the connection from the 220k resistor of the cut to the .1uf cap that goes to pin 2. Thanks so much!! Still has some hum, but it's mostly the normal kind. Maybe a little more than I'd like, but hey! I have sound! Thanks again!
Now onto the problems I', having with my Tweed princeton build... the 10k 2watt resistor between the 16uf and 8uf filter caps is glowing and smoking ha.
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... What you don't have correct is having the grids going to pins 1 on each tube. ...
He has the grid wires running there as mounting points, with a grid stopper between pins 1 & 2. This matches Hoffman's AC 30 layout.
Unless the specific EL84 has a connection at that pin, it should be okay.
I saw how it was used but checked my Amperex data sheet & it listed the three pins as internally connected. I never connect to those pins because you can never be sure what tubes are going to be used. Better to be safe then sorry.
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I saw how it was used but checked my Amperex data sheet & it listed the three pins as internally connected. I never connect to those pins because you can never be sure what tubes are going to be used. Better to be safe then sorry.
This has come up before here and I think your right Jojo. You never know what tube will have the internal connection(s) or not.
Brad :icon_biggrin:
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Besides Amperex, other datasheets I could find for EL84/6BQ5 are RCA, Mullard, Phillips, Mazda, STC, & LaRadiotechnique - ALL list pin 1 as internally connected. Only JJ's doesn't. The interesting thing is look at RCA's below. It actually says, "DO NOT USE" for pin 1.
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Unless the specific EL84 has a connection at that pin, it should be okay.
I have a pair of old RCA 6BQ5s in my AC-15 Lite. Pin I and pin 2 are connected together inside the tube. So, mounting grid stopper between pin 1 and 2 would be useless for these tubes. OTOH, my JJ EL84s do not have pins 1 and 2 connected.
I highly recommend not using pin 1, 6, or 8 on any 6BQ5/EL84 because there is some inconsistency between manufacturers about these three pins.
Do a google search on the phrase "use pin 1 on el84" and you will get plenty of discussion about this issue. Some variations even have pin 6 connected to pin 9. That's a possible disaster!