telefunken: 55 Export HiFi-System; Opus

I have a problem with my 55 opus TS in that under certain conditions (while using external input) the speakers vibrate excessively at very low( including sub audio) frequencies. This can happen even at a very low volume setting.  Reducing the bass setting helps however also cuts the good sounding audible bass frequencies. I was wondering if anyone else has experienced this or can offer advice on the best way to fix it. 

thanks Stuart Matousek

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> speakers vibrate excessively at very low (including sub audio) frequencies. This can happen even at a very low volume setting.  Reducing the bass setting helps

Probably "motorboating".

Large signal from the power output stage "sneaks back" through the power supply system to an earlier stage. Because there are multiple bass phase-shifts in coupling capacitors, this tends to form an oscillator.

The main reason that tube amps DON'T motorboat is ample power system filtering. However this uses capacitors, usually electrolytic, which tend to weaken with age.

So you should consider replacing the power supply capacitors which feed the audio stages.

There is one other possibility. Less filtering is needed if the amplifier has push-pull stages. However if the push-pull is NOT well balanced, the original filtering may not be sufficient. On an ArKay, I found two "47K" resistors, one had drifted to over 100K. Replacing both (just to be thorough) fixed the motorboating. A very sick (or dead) power tube could also do this.

I don't see a schematic for your specific model. I have presumed that a 54 Opus will be very similar, and have marked-up the critical power supply capacitors and the push-pull driver's resistors. See attached.

I do think the 10K 2uFd to the audio amplifier tube is under-sized. I would not use less than 10uFd here. At today's prices and sizes, 20uFd-40uFd is not unreasonable.

> I was wondering if anyone else has experienced this

The term is at LEAST 84 years old.

The techniques advocated in that article are not adopted to modern (post 1930s) designs. Our primary defense is BIG electrolytics (not available in 1927). 

Attachments:

• TF-54-motorboating (28 KB)

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Thanks for your advice Paul it is very helpful. As it turns out I have not yet replaced the filtering capacitors but will do so now and increase the value of the audio filter. I would have checked the resistors but perhaps ill replace to ensure balance. Im not sure there is actual oscillation ocurring rather than excessive response at certain low frequencies. Anyway i shall let you know the results after I have made these modifications.

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