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help troubleshooting gradually distorting tube amp (dropping b+ voltage when hot)


2010-05-07 7:11 pm
i have a tandberg 74b that i just received second hand - everything on the inside is original, excluding previously replaced tubes.

when it first turns on, it sounds impressive / works perfectly. however, once it has been on for 5-10 minutes the sound gradually and evenly fades from nice and clean to distorted (almost as if you are overdriving the tubes) but the signal also gradually fades in amplitude as well. this is consistent across both the right and left channel.

when i was doing some initial readings with a dmm, i noticed that the b+ supply voltage was slowly dropping. taking a reading after the last cap in the power supply, it hovers around 285 v dc when first turned on (as per spec, when cool) and slowly drops to below 240 v dc over the course of 5-10 minutes -- and by this time the audio is very distorted and i just turn it off. if the unit is then quickly powered back up before it has had some time to cool down -- after the initial charging of the system, the voltage drops much quicker this time into low voltages and distorted audio. however, if the unit is allowed to cool for sometime, it will return to working great for the initial couple of minutes before again dropping the b+ voltage and distorting the audio.

the voltage readings slowly and consistently drop when the amp is turned on, all the way back to the ac transformer leads (before the rectifier) while everything is in circuit.

any ideas as to which component(s) might be at fault? i have yet to experience these symptoms with a tube amplifier...


attached is an image of the schematic - any help would be appreciated, thanks!

try pulling the 6BQ5's out and measure the B+ with them out. Also if it is a electrolydic are going shorted you could lift the series resistors in the power supply one at a time and see if the problem goes away. If you leave this on too long it may smoke, or go bang.
Eye protection might me a wise move. Keep up posted.
Check the voltage at the grid of the 6BQ5/EL84, either pin2 or the junction of R205/R206. It should be zero, or no higher than about 0.1-0.2V. If this increases then either the EL84 is gassy, or the coupling capacitor C203 is leaky. Either problem is quite common. It could even be both; leaky C203 causes the valve to overheat and become gassy. Same for other channel.

If OK, check the voltage at EL84 cathode (pin 3). It should not increase, assuming the grid stays constant. If it reduces, then C204 may be leaky. Do the EL84s look discoloured (dark stains in the glass)? Have they recently been replaced? Sometimes output valves overheat, due to the reasons I have mentioned, but instead of solving the problem a repairer might simply change the valve - so the new one cooks too.

Don't run it for longer than is necessary to find the fault. Be aware that a badly leaking electrolytic can heat up and explode.


2010-05-07 7:11 pm
thanks for the replies guys - looks like a cap job is in order

i measured the dc voltage at pin 2 (grid) and did find dc on both channels (varied between startups, but would sit somewhere between 2.1/6.49 v dc on one channel and 4.77 and 14.2 v dc on the other - either channel the voltage kept gradually creeping up)

and just for good measure, i checked pin 3 (cathode) and the voltage on both channels kept going up as well - i'm guessing dumping the excess dc voltage that is coming in the grid? both channels were somewhere in between 9.86 / 11.4 v dc and again gradually rising.

to add one more note, yes the output tubes do have a bit of brownish discoloring, and become quite hot under operation - i bet your thinking is correct, that the last repair person just swapped tubes without checking the health of the coupling caps...


@DF96 i understand the concepts of the leaking capacitors and the problems they can cause, but what do you mean by the tube has become gassy? just curious... do you mean the vacuum has been lost or something else? the color of the tubes appeared normal under operation (orange, not bluish...) i plan on getting replacement tubes once i have recapped the unit.

so in all, looks like the problem is found - i was planning on recapping anyway due to the age of the unit, so glad it is most likely just the coupling capacitors - thanks again so much for the insight, greatly appreciated!


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This deck most likely has a selenium bridge rectifier (Siemens was a common supplier) which absolutely should be replaced. When they fail they emit toxic fumes so it is best to avoid this issue if possible.

It would also be a good idea to replace all of the power supply and cathode bypass electrolytics in this thing as well.

Clearly you also have bad coupling caps as well.
Those 6BQ5s look quite healthy to me, but they won't stay that way for long unless you change the coupling caps. The serious overheating (2 or 3 times normal dissipation) increases the risk of cracking at a pin seal. Before that happens, the residual gas which always gets absorbed by the metal will be driven out by the heat. This creates grid current which makes the valve run even hotter as the bias is shifted. The rise in grid voltage you see could be partly due to this, and partly due to the leaky cap getting worse as it warms up. When this has gone on for a long time the valve develops dirty brown stains in the glass, usually around the bottom and near any holes in the anode metal.

The same effect may be happening for the other valves, so other caps may need to be changed too. This is a common problem, often due to the paper dielectric slowly absorbing water from the atmosphere.

You may find the current valves are OK after recapping. Check the grid voltage again. A small increase (0.1-0.2V) is OK.


2010-05-07 7:11 pm
@kevinkr thanks for the heads up - a guy on a different board told me about the rectifier too - i plan on swapping it with a silicon one with a series resistor (to bring the b+ down to what it is after the selenium rectifier now). seems like a good idea.

@DF96 i agree that the tubes look okay, but there is a little brown residue at the top and bottom that i don't see in any of the other tubes. maybe it is just being caught early enough were it isn't too bad at this point.

i do plan on changing all the electrolytics and the paper capacitors - i rounded up the service manual and already have a list going of all the caps needed.

thanks again for sharing your knowledge and insight with me!
@DF96 i agree that the tubes look okay, but there is a little brown residue at the top and bottom that i don't see in any of the other tubes. maybe it is just being caught early enough were it isn't too bad at this point.

That's normal discoloration, it doesn't indicate anything other than some use and therefore a bit of getter flash erosion. And it's VERY common on Sylvania made tubes, which is what those are.