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1973 Marshall super bass problem
1973 Marshall super bass problem
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Old 7th February 2018, 10:33 PM   #1
Rootz is offline Rootz
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Join Date: Jan 2010
Default 1973 Marshall super bass problem

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. Your help is appreciated!
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Old 9th February 2018, 11:21 PM   #2
danFrank is offline danFrank  United States
diyAudio Member
Join Date: May 2005
Location: California
One of the solder connections on your choke (choke itself or choke lead soldered to board) has gone bad (high resistance/intermittent) when amp warms up. You may have fixed it by resoldering choke leads to PC board. 40+ year old solder connections on a tube amp are bound to go bad from the heat and/or vibrations involved in the amp.

Other cause may be the 56K resistors across screen caps; each one has 230+ volts across them and each dissipated about 1 watt. They get hot. I would replace with 100K 2 watt metal oxide resistors. Metal oxide resistors can take the high voltage better than the carbon resistors in place.

Don't worry about the voltage sag when playing hard. The early SL/SB power transformer HV winding is only rated for 300ma. This is part of what makes a SL/SB sound the way they do.

Last edited by danFrank; 9th February 2018 at 11:23 PM.
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Old 10th February 2018, 01:49 AM   #3
nigelwright7557 is offline nigelwright7557  United Kingdom
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Location: Carlisle, England
I would be inclined to apply a small sine wave then follow it through the amp until it disappears.
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Old 10th February 2018, 05:26 PM   #4
Rootz is offline Rootz
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Join Date: Jan 2010
I redid the choke leads in the pcb side. Old solder sucked away, applied new. Are there tabs behind the endbells of the choke? If so, connection could be bad too.

56k resistors measure fine and get a bit warm, not hot. Can still tough them (after turning the amp of and draining the caps). Those are 2 or 3 Watt Pihers right? MOX would be better, but I try to keep the amp as original as reasonable.

After some more thinking. Problem could be related to the standby switch or ht fuse too. I cleaned the fuse holder. Switch not, and there is no protection against arcing between the contacts. Would need to investigate more, but the amp is currently with its owner again. Last rehearsal went flawless.

300mA makes sense. I have a diy power amp with an equally large pt, but that’s a 50 Watter.

Currently without a scope here. New one should arrive in 2 weeks. Will investigate further then.
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