1973 Super bass problem

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Rootz
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1973 Super bass problem

Post by Rootz » Wed Feb 07, 2018 5:34 pm

Hi all,

I'm currently working on a 1973 Super Bass from a band mate. The amp has a strange problem. Sometimes it goes silent all of a sudden and every two seconds some sound can be heard. Crackles a little in the speaker at the same time. I started by replacing the 45 years old Daly filter caps in the amp. Some are standing right next to the output tubes, so it really surprised me that they were working at all. The amp gets played twice a week for a couple of hours, so the 1000h life span at 70 degrees celsius was met a long time ago. I look at it as replacing the timer belt in a car every several years/miles. The new caps did lead to a more powerful sound it seems. More solid bass, although the old caps didn't lose their capacitance. Maybe due to lower ESR? Don't know for sure, but definitely an improvement.

Voltages stayed roughly the same: 460V plates, 458 screens and IIRC 300 or so for PI. My focus was on the power section. Prior to changing the filter caps I did some measurements and when the amp cut out, the screen voltage (and following stages in the filter line) dropped a lot forcing all output tubes into cut off. No wonder there wasn't any sound. Again some sound in a pulsating manner.

Here is what I know:
- Filter caps are new F&T's (50+50uF);
- Equalising resistors over the screen caps both measure 57k;
- Solder of the choke leads was reflown at the PCB;
- Bias is solid, no drift. New caps and fairly new trimmer;
- Tubes are fairly new Tung Sol EL34's, biased at a conservative 60% within 2 mA spread.
- Chopstick metro didn't reveal any obvious problems. One of the preamp tubes is a bit microphonic, don't think that's the problem though.

Just played the amp with the volume on 7 through a 16 Ohm reactive load for an hour. Guess what: solid performance and great sounds! I'm stumped at the moment. The Anode voltage sagged pretty hard to 360-380V, the screens even more off course. As the load as 4 times that of idle that seems normal to me.

Who can help? I definitely need some fresh ideas.

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Re: 1973 Super bass problem

Post by ValveStorm » Fri Feb 09, 2018 5:17 am

Perhaps a flaky impedance selector? Try bypassing it, and see if the problem persists.
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Tazin
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Re: 1973 Super bass problem

Post by Tazin » Fri Feb 09, 2018 9:53 am

Possibly corrosion on the H.T. Fuse or inside the fuse holder. Maybe the Standby switch is starting to loose contact integrity.

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Rootz
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Re: 1973 Super bass problem

Post by Rootz » Fri Feb 09, 2018 10:36 am

Now that I think of it. The last time the amp had the problem was just after I took it out of standby! The standby switch feels loose and has no protection against arcing. Could very well be it.

Fuse holders and many other contacts have been cleaned (also the impedance selector) mainly by scraping away any oxidation. I'm not a fan of sprays.

Impedance selector would lead to sound cutting out, but not to just screen voltage dropping. A bad switch could. Will keep an eye on it. Many thanks!

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