Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: isaac_teller on December 21, 2020, 01:10:13 pm

Title: buzz on volume control from about 0 to about 2-3, quiet when higher
Post by: isaac_teller on December 21, 2020, 01:10:13 pm
What would cause this? Thank you.
Title: Re: buzz on volume control from about 0 to about 2-3, quiet when higher
Post by: Latole on December 21, 2020, 02:59:59 pm
Buzz or scratchy volume ?

Wich amps ?

I had a scratchy volume pot on vintage amp ; answer : bad filter caps
A DC leaky signal cap could be a issue too
Title: Re: buzz on volume control from about 0 to about 2-3, quiet when higher
Post by: sluckey on December 21, 2020, 03:20:49 pm
Do the simple stuff first. Test the pot. Ohm meter connected to ground and wiper. Set the pot to zero. Resistance should be close to zero ohms. Now turn the pot up slowly. Resistance should smoothly increase. If you don't have an analog meter you will have to turn up very slowly so the dmm display can do it's sample/update.

Or you could just replace the pot and re-evaluate.
Title: Re: buzz on volume control from about 0 to about 2-3, quiet when higher
Post by: PRR on December 21, 2020, 04:46:56 pm
Buzz on zero?

Pot "ground end" is not at same potential as next-stage's ground reference.
Title: Re: buzz on volume control from about 0 to about 2-3, quiet when higher
Post by: pdf64 on December 22, 2020, 06:05:31 am
It’s unhelpful to ask technical generic questions, as generic answers are impossible. Or at least are rather more complex than necessary.
As the particulars of any scenario are critical facts.
Quote from: PRR on December 21, 2020, 04:46:56 pm
Buzz on zero?

Pot "ground end" is not at same potential as next-stage's ground reference.
Need the schematic or at least some specifics on the amp in question.
As with some, eg 5E3, the volume control isn’t wired as a pot, so the issue could be due to poor lead dress or a hummy tube with unbypassed cathode.
Title: Re: buzz on volume control from about 0 to about 2-3, quiet when higher
Post by: isaac_teller on December 22, 2020, 06:29:15 am
The amp is a 50 watt Marshall JCM800 style with reverb.
Title: Re: buzz on volume control from about 0 to about 2-3, quiet when higher
Post by: Latole on December 22, 2020, 07:59:03 am
Buzz or scratchy volume ?
Title: Re: buzz on volume control from about 0 to about 2-3, quiet when higher
Post by: isaac_teller on December 22, 2020, 09:38:01 am
Definitely a buzz(50 Hz)
Title: Re: buzz on volume control from about 0 to about 2-3, quiet when higher
Post by: pdf64 on December 22, 2020, 01:56:20 pm
Schematic - there lottsa jcm800 models?
Is it a DIY?
Was it ever buzz free?
What controls affect the buzz? Has a tube issue been eliminated?
What’s the resistance between any part of the heater circuit and chassis 0V?
Title: Re: buzz on volume control from about 0 to about 2-3, quiet when higher
Post by: Latole on December 22, 2020, 02:37:53 pm
Quote from: pdf64 on December 22, 2020, 01:56:20 pm
Schematic - there lottsa jcm800 models?
Is it a DIY?
Was it ever buzz free?
What controls affect the buzz? Has a tube issue been eliminated?
What’s the resistance between any part of the heater circuit and chassis 0V?

IMO, with reverb, it is a DIY 

I built from scratch a Marshall Master volume  model 2204 with reverb "a la Fender"
Work great
Title: Re: buzz on volume control from about 0 to about 2-3, quiet when higher
Post by: 2deaf on December 22, 2020, 03:30:27 pm
Quote from: Latole on December 22, 2020, 02:37:53 pm
IMO, with reverb, it is a DIY 

I have two Marshall JCM800's with reverb, so reverb is not a distinguishing feature of DIY.
Title: Re: buzz on volume control from about 0 to about 2-3, quiet when higher
Post by: Latole on December 23, 2020, 03:13:54 am
Quote from: 2deaf on December 22, 2020, 03:30:27 pm
Quote from: Latole on December 22, 2020, 02:37:53 pm
IMO, with reverb, it is a DIY 

I have two Marshall JCM800's with reverb, so reverb is not a distinguishing feature of DIY.

Always something to learn here, thank you
Title: Re: buzz on volume control from about 0 to about 2-3, quiet when higher
Post by: aabbs20 on December 23, 2020, 09:02:06 pm
Quote from: Latole on December 22, 2020, 02:37:53 pm
Quote from: pdf64 on December 22, 2020, 01:56:20 pm
Schematic - there lottsa jcm800 models?
Is it a DIY?
Was it ever buzz free?
What controls affect the buzz? Has a tube issue been eliminated?
What’s the resistance between any part of the heater circuit and chassis 0V?

I don't mean to hijack this thread, but this is interesting to me. Where did you insert the reverb?

IMO, with reverb, it is a DIY 

I built from scratch a Marshall Master volume  model 2204 with reverb "a la Fender"
Work great

{EDIT -- untangled quote -- PRR}
Title: Re: buzz on volume control from about 0 to about 2-3, quiet when higher
Post by: Leevi on December 23, 2020, 10:00:21 pm
Based on the similar cases I have had it can also be a wiring issue.


Try to move the signal wires and listen if there is any change.

/Leevi