MV-55: bias issue with hum appearing

[RJKflyer]

I have been using a set of Tung-Sol EL34Bs in my MV-55 since 2018. I today noticed a slight hum and decided to rebias.

On one channel the bias LEDs could not be illuminated for either valve even at FSD, and on the other it was near FSD.

I changed the valves for a set of Watford Valves EL34Bs and the bias was now settable on both channels, albeit still towards FSD.

However, I noted that when approaching the bias point a hum appeared in each channel.The appearance of the hum is distinct as the bias is turned clockwise. At 'correct' bias (LED just on then back off say 1/4 turn) there is still a slight hum. Present even with pre-amp (a PV10AL) disconnected.

I've never had this issue previously.

At this stage I've not elected to replace the 6SN7s or the ECC83s (which were also replaced back in 2018).

Any thoughts please?

EDIT: I fortunately have a set of new big bottle EH 6CA7s and have tried those - problem resolved - significantly reduced settings to bias.

I would still appreciate an understanding of what has happened here - the Tung-Sols were initially similarly biased easily and through age appear to have drifted out of bias range (I can appreciate this may happen), but the brand new EL34Bs evidently were too 'hot' for the amp?

[admin]

I think both set of tubes were going bad.

[RJKflyer]

I think both set of tubes were going bad.

The Watford Valve ones were brand new (not NOS) unused - is that likely?

What causes the hum in this circuit if the bias is too high?

[admin]

Tubes can be bad from the manufacturer or they could have been damaged in shipping or handling. I'm not sure if it's even about the bias circuit (although I suppose that could be going bad as well), but I have had multiple bad tubes go bad with increased hum and biasing issues.

[RJKflyer]

OK, so hum is not necessarily a consequence of something suddenly going bad (in both channels in this case and simultaneously!), but most likely the tubes?

BTW thanks for your very quick responses!

[admin]

If a new set of tubes fixed the problem, I think it is most likely the tube. And at the end of the day, increased hum in my experience has in most cases (but not all) been tube related.

[RJKflyer]

Sadly, all is not well. I still have an audible hum with correct bias - pots set about half way.

Where would you recommend I start looking? I have the schematics...

Intuitively I'd suspect power supply as it sounds like 100Hz (UK).

[admin]

Do you mean 50 hz? If so, most likely it's a ground loop issue. Easiest way to diagnose that is to use an RCA to RCA ground loop isolator. If the hum goes away, it confirms it.

[RJKflyer]

No, there's no ground loop - nothing plugged into amp still the same.

It seems to be 100Hz - i.e. rectified mains. I might try to see if I can detect it on the speaker output with the 'scope.

[admin]

To be clear, you hear the hum coming from the speakers, not the actual unit (ie vibrations coming from the MV-55)?

[RJKflyer]

Correct, and very distinctly appears as I reach the bias point on the pots. I no longer believe the tubes are relevant - just the first set I tried were at extreme of bias travel. Third set (and also the original set I replaced as suspect) bias about half way and hum appears.

[admin]

Are your speakers very sensitive? Does it happen when a preamp/source is connected to the amp?

[RJKflyer]

Yes, Cabasse Farella 401.

But only ever had minor tube hiss with ear close to speaker which is expected.

This hum is a new phenomenon, and remains as I said with preamp disconnected, and/or MV-55 inputs shorted.

[admin]

If it's not the tubes, and not a ground loop, and not normal tube hiss, and in both channels, I guess it makes a power supply issue more likely. It's unfortunate as that is typically harder and more expensive to fix.

[RJKflyer]

OK, had the bottom off and seem to have only 3.6V DC on the heaters of the signal tubes (same across the smoothing cap), plus 1.3V AC.

Thus it 'works' but hum is getting fed right into the signal stages, and on both channels equally as witnessed.

Output tubes have 12.5V (AC) on heaters as expected.

Initial suspicion is the 4700µ 35V smoothing electrolytic has gone bad. Sound reasonable?

[RJKflyer]

Replaced the cap, but still a problem:

The heater rail is now 4V DC but still with 1.1V AC RMS on it at 50 Hz.

Doesn't this suggest a problem in the bridge? I should be seeing 100Hz across the cap, not 50Hz?

[admin]

The MV-55 schematic I have is really blurry and with hand written values. Are there any markings on the original component that may give a hint, can you post a picture maybe?

[RJKflyer]

Would need to get board out which is not a small job. However, looking through the EL34 socket with tube removed, almost certain that they are UF5401.

Can't measure FWD voltages across each as in circuit the transformer shorts the AC ends.

(Edit of above post crossed - agree re the 50Hz issue?)