Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on August 27, 2024, 07:17:00 am

Title: Impact 60 rev trem hum issue
Post by: Leevi on August 27, 2024, 07:17:00 am

I have a hum issue with Impact 60 reverb tremolo. The preamp stages are identical if tremolo and reverb are disconnected from the circuit. I get a pretty loud hum (close to 50Hz) from the second channel even if tremolo and reverb are disconnected. If I remove the channel 2 preamp tube the hum drops to an acceptable level and channel 1 works fine. I cannot believe it is not a filtering issue since the voltage supply is common for both channels and channel 1 works alone fine. The hum does not increase with volume. Of course I have tried with several tubes and measured voltages which are ok.


/Leevi

Title: Re: Impact 60 rev trem hum issue
Post by: shooter on August 27, 2024, 08:00:12 am

Quote

I get a pretty loud hum

with NOTHING plugged in?

Quote

If I remove the channel 2 preamp tube the hum drops

I would check the input shorting jacks n the grounds in that area

Title: Re: Impact 60 rev trem hum issue
Post by: Leevi on August 27, 2024, 09:30:32 am

Quote from: shooter on August 27, 2024, 08:00:12 am

Quote

I get a pretty loud hum

with NOTHING plugged in?

Quote

If I remove the channel 2 preamp tube the hum drops

I would check the input shorting jacks n the grounds in that area

It hums nothing plugged in and on zero volume.
I recently changed the tube socked, no progress.


/Leevi

Title: Re: Impact 60 rev trem hum issue
Post by: Latole on August 27, 2024, 03:02:23 pm

Any mods in the amp ?

Can you post pictures of wiring ?

Connect both positive input jacks  to ground ; hum or not ?

Title: Re: Impact 60 rev trem hum issue
Post by: Willabe on August 27, 2024, 04:12:04 pm

Buzz/humm with nothing plugged in;

Quote from: shooter on August 27, 2024, 08:00:12 am

I would check the input shorting jacks n the grounds in that area

Title: Re: Impact 60 rev trem hum issue
Post by: Leevi on August 27, 2024, 11:31:36 pm

The grounds are ok there.


Have attached a picture which is quite difficult to follow.
The most of the filter caps are located under the tag board.
There some small mods.


/Leevi

Title: Re: Impact 60 rev trem hum issue
Post by: Leevi on August 28, 2024, 12:13:43 am

I solved it with a workaround.
The noise comes from the reverb recovery and I wired the reverb pot in another way by swapping the wiper and lug 3.That reduces the noise if the reverb is not fully open.
I also biased the amp cooler which reduced the noise.
/Leevi

Title: Re: Impact 60 rev trem hum issue
Post by: Latole on August 28, 2024, 03:00:14 am

Quote from: Leevi on August 28, 2024, 12:13:43 am

I also biased the amp cooler which reduced the noise.
/Leevi

Power tubes matched help to reduce noise.
Match don't need to be 100 % same.

15mfd bias filter, if bad, will induce hum. I'll put a 25 mfd