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Old 24th September 2009, 04:47 AM   #1
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Default Entire Assembly Glowing in 20 Seconds

Hi all, I recently put together a 6V6 amp kit that I ordered, had a few problems (was the first tube amp I had tried at, have since completed a Simple SE) mostly due to stupid mistakes (pretty much my forte, really) but I've finally got it to a point where it makes noise.
I first tried the Shuguang 6V6 tubes that came with it, they quickly buzzed and arced (nice blue zapping inside). I thought it might be faulty tubes (I'm only running one channel now to test) so I put in some Russian 6L6G/B equivalents that I have (6P3S) and they did much the same. In between I also inspected the circuit and checked it over, I couldn't find any stuff ups. As a last ditch effort I put in a set of the Russian Military spec 6P3S-e tubes. These are what I run in my Simple SE and I can throw just about anything at them and they'll take it. This time the tube did not arc, but the entire centre assembly glowed within twenty seconds. I'm not entirely sure of the name of this section, but I think it's the second grid.
Does anyone know what could cause this whole section to glow so quickly or what could cause the arcing? If I have an idea of what causes it then I think I'll have a better chance of tracking down the component/s or wiring that is suspect.
Nic.
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Old 24th September 2009, 04:50 AM   #2
rknize is offline rknize  United States
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Grids and screens swapped? Is this a PCB kit? Schematic would be nice. Measure voltages at the various pins on the tube socket without a tube in there.
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Old 24th September 2009, 09:42 AM   #3
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indeed, the grids 1 glow red hot, while the plates do not get as hot
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Old 24th September 2009, 02:54 PM   #4
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rknize prob correct.
Similiar problem I recently dealt with here.. beginner Sonnyboy wired the sockets the wrong way. 1=8 ;
so the anode pin 6 on an Ioctal would become o/c, g1/2 swapped but heaters still work. Pins 1,8 would work.
Make sure if there is negative bias this appears on pin5.


richy

PS. There's no excuse to get this base wrong.
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Old 24th September 2009, 04:14 PM   #5
Yvesm is offline Yvesm  France
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To do so with a PCB, you have to mount socket on the wrong side !
Check for that.

Yves.
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Old 24th September 2009, 09:15 PM   #6
Arnulf is offline Arnulf  Europe
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Your trully forgot to change heater supply the other day when swapping series heater version of a tube with a parallel one. End result was even more glow (but no arcing inside) and the tube stabilized itself with ungodly high emission and consequently high current due to PTC nature of the filament before I realized that something was wrong and turned it off.

Could it be that your heater supply is waaaaay too high ? This could be due to unsuitable transformer, above avregare line voltage, incorrect wiring, etc.
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Old 25th September 2009, 03:21 AM   #7
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If it's wired OK, it could be oscillating at RF.

Cheers!
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Old 25th September 2009, 04:04 AM   #8
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Or if it uses global NFB,maybe it's out of phase.
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Old 25th September 2009, 10:51 AM   #9
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The kit is a point to point wired amp, which in hindsight was a bad choice for a first amp, as I tend to make many simple mistakes, especially with wiring. Just a few more bits that might help; after power on (once I've turned it off again) the capacitor in parallel with the cathode resistors becomes very hot, doesn't smoke but is too hot to touch; exactly the same thing happens on both channels, so if I have buggered up wiring it is a mirror on both channels; if powered on with the (6SL7) preamp tubes in as well, it WILL make a noise through the speaker, starts with nothing then comes in with a sort of feedback or loud hum, which then fades out once more and comes in even louder.

OK, powered up with no tubes:

P3 = 381v
P4 = 0v
P5 = 379v
P8 = 0v

On one channel, I have just reversed the Ultralinear (23%) and Output taps to check if I had messed that up. On powering up, it did exactly the same as before.

Looking closely at the pins inside, there is obviously a huge current flowing through pin 5 (grid 1) - both rods holding the grid, plus the grid itself are red hot as before.

Arnulf - the heater voltages are right on 6.3 volts + 0.1 volts.

Here are the schematics, it's all point to point wiring:

Click the image to open in full size.
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Old 25th September 2009, 11:30 AM   #10
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What o/p tubes are you actually using.6V6 6P6P ? For problem solving keep to known pairs that work. Don't use fresh tubes when things aren't working properly.My first instinct is to disconnect the global nfb loop,turn vol to min and allow to warm up. . As Geek mentions, if there are oscillations, a cheap AM radio tuned off station MW) put reasonably close by will make noises if circuit oscillates. If the output stage oscillates the speaker cone will usually displace and may emit 100Hz modulation hum.
As the electrolytic cap on the output stage cathodes has got hot, then at some time when all is working it will need replacing. I'm a bit suprised there is no fuse in the B+. A 500mA will do fine in the centre tap o/p tranny.
>Check that phase compensation components R8 C2 are in circuit.
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