SC20c feedback without input

Beccaio

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Greetings,

Ive had an SC20c for about 3 months which has been outstanding. This week during rehearsal and set at:

- power low (5 watt)
- preamp 10
- master volume 10
- guitar into high input

after about 30 mins started hearing a feedback high pitched hum. At first we thought microphone feedback and or speaker placement feedback. Put amp on standby and the hum stopped. So obviously my amp. After cooling down the hum was gone until a few minutes after restarting play. So after removing signal chain (in front of amp) and reproducing the issue a few times including going direct into the high input jack from guitar and after a few minutes the hum would return.

at this point I switched over to 20 watt mode and decreased the gain and master. Problem solved

so my question is…any known issue with the SC20c in 5 watt mode and gain plus master at 10? Would welcome opinions and insights.

B
 

South Park

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Back off the gain the tube is oscillating . Use a pedal for the gain
 

Pete Farrington

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It may be a microphonic valve, most likely an early stage preamp.
As a valve amp user, be sure to have ‘known good’ spare valves handy :)
Is the amp a combo? A speaker in close proximity to an input stage valve, together with a mountain of gain, is a pretty good recipe to bring valve microphonic to the surface.
 

Beccaio

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Pete. Yes it’s a combo. I did place an order for a full set of ecc83 preamp and el34 power tubes. Gonna try playing a few more times at drum set volumes and see what she does at the 20 watt setting. If it goes into hum mode will start replacing tubes. Have been using the 5 watt setting for recording with great satisfaction. Sounds so great at around master volume midway. This week was the first time I dime’d it on 5 watt to get the higher volume.
 

Pete Farrington

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Try swapping the ECC83 around. A valve that’s too microphonic for high gain V1 may be fine for V2 or whatever, where there’s less gain after it.
 

Chris-in-LA

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Put it in 20 watt mode, preamp at 10 and work the master for the volume you want. I don’t understand the 5 watt mode on those. You can run the 50 watt version at television volume all day long. Also, how are you keeping up with a drummer in 5 watt mode?
 

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