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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 15th June 2018, 07:30 PM   #1
Medisinmannen is offline Medisinmannen  Norway
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Default Filament voltage divider burned up

My tubelab sse have been working perfectly for a year, playing el34 and 6l6 My friend has kept it for some time, and the other day there were no more music. Went out quietly while playing, fuse poped and there was smoke!

R3 and R4 were toast. This is the voltage divider for tube heaters. Im not familiar with tubes and how they fail, could an failed tube explain this?
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Old 15th June 2018, 07:41 PM   #2
JMFahey is offline JMFahey  Argentina
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What does R3 and R4 mean?

I dont see them on the invisible schematic.
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Old 15th June 2018, 08:03 PM   #3
Medisinmannen is offline Medisinmannen  Norway
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Click the image to open in full size.

Sorry about that
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Old 16th June 2018, 05:29 AM   #4
stratus46 is offline stratus46  United States
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Quote:
Originally Posted by Medisinmannen View Post
Click the image to open in full size.

Sorry about that
I don't see why a failure of R3 & R4 would cause it not to play. It looks to me like all they do is elevate the DC of the filaments to 1/16 of the plate Voltage -- which is what? A failure of the transformer or a heater - cathode short could be a different story. If the resistors failed so that they shorted out the plate supply would do as you say and probably ruin the rectifier.

Resistors failing to lower ohms was a common failure with carbon composition resistors. I replaced several in the Ampex VPR-2 videotape machines AST board (several hundred Volts). The resistor values were similar to yours both resistance and 2 Watt.

G
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Old 16th June 2018, 06:03 AM   #5
KatieandDad is offline KatieandDad  United Kingdom
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There should be no current from the divider into the heaters.

Either one of the resistors has failed or one of the valves has failed.

The last resort is a short inside the mains transformer.
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Old 16th June 2018, 12:44 PM   #6
Tubelab_com is offline Tubelab_com  United States
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Quote:
It looks to me like all they do is elevate the DC of the filaments to 1/16 of the plate Voltage -- which is what?
The plate voltage in an SSE can be anywhere from 250 volts to about 450 volts depending on the builder's choice of power transformer. With EL34 or 6L6 the B+ is likely around 430 volts.

R3 and R4 serve two functions. They elevate the heater as stated to reduce the possibility of hum. They are also redundant bleeder resistors in the B+ supply. There is a resistor directly across C1 to discharge it, but C2 could remain charged if an external choke was used and it becomes disconnected. Since these boards are intended for DIY, and disconnecting a choke is a real possibility, I wanted C2 to have it's own bleeder resistor, so the heater elevation pair serves this purpose.

I have never seen these resistors fail, but I have seen a similar failure in a guitar amp, and it was not pretty. If a tube amp is operated at a high volume level and the speaker is suddenly disconnected the voltage on the plates of the output tubes can rise into the several kilovolt region usually causing an arc somewhere. The arc is usually inside the OPT usually destroying it. If the OPT is built with good enough insulation to withstand the several kilovolts generated by a no load situation the arc may occur somewhere else.

The second most likely place for the arc is from pin 3 (plate) to pin 2 (heater) of the output tube. This arc usually occurs inside the base of the tube, or inside the tube socket. This puts a whole bunch of voltage into the tube heater circuit causing something to fail. Usually it's the heater bypass cap (C3 in the SSE), but it could fry the resistors. This kind of stuff happens often in a 50 watt guitar amp cranked to extreme when the speaker blows, but should not happen in an SSE operating at normal levels even if the speaker is unhooked.

Something caused a high current to flow through these parts, and there is no obvious source for that to occur. As stated, a short inside the power transformer could put a high voltage on the heater winding, thus frying these parts. It could be a bad output tube, but it would take some sort of strange heater short to do this. A short from pin 2 to pin 3 in one of the output tubes could do it, but might damage some other parts beside the resistors. C3 or the 12AT7 tube would be the likely victims.

The amp will play with R3, R4 and C3 removed IF ALL THE OTHER PARTS ARE GOOD. I would remove these parts replace the fuse and try the amp to see if it works. Use a spare set of tubes if you have them.
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Old 20th June 2018, 04:07 PM   #7
Medisinmannen is offline Medisinmannen  Norway
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Finally got around to testing. It plays music again, just by removing the damaged resistors and c3. All tubes actually works, but im no wiser on why this happened. When new parts arrive, ill see if its something weird with heaters.

One of the resistors were a carbon comp, but something must have caused them to go bad.
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Old 21st June 2018, 05:13 PM   #8
Tubelab_com is offline Tubelab_com  United States
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Resistors do have a maximum voltage rating. It is often ignored without issue although I have seen flames and even parts explosions from seriously violating this rating (600 volts on a 300 volt part). R4 does have about 400 volts across it which could have caused it to fry leading to the death of R3 and C3.

Most 2 watt parts are good for 400 volts, but maybe yours weren't, particularly if R4 was the carbon comp. Some of them were not meant for 400 volts, and even if it was fated for 400 volts 50 years ago, it might not be now.

Connect a voltmeter where C3 was, then turn on the amp. Measure the voltage with the meter set for DC volts, then measure with it set for AC volts.

With no parts there the voltage will be a random number depending on leakage currents inside the tubes, but a voltage less than 75 volts or so would rule out a short inside a tube or transformer. If that's the case new parts (R3, R4, C3) should fix the amp.
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