Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Paulycoo on October 05, 2017, 02:24:47 pm

Title: Hoffman Princeton reverb crackles at high volume
Post by: Paulycoo on October 05, 2017, 02:24:47 pm

Hello again all!
Thanks to sluckey et al I have a beautiful Princeton reverb build. It sounds gorgeous including reverb and tremolo.
Apart from when I crank it up over 7.
I get this crackling sound when I play chords.
I removed the chassis and put electrical tape all around the edge that may be touching the copper shielding sheet stuck onto the cabinet top.
Replaced chassis.
Still the same.
Anyone help where to look first?
Thanks for helping
Paul McNamara

Title: Re: Hoffman Princeton reverb crackles at high volume
Post by: SILVERGUN on October 05, 2017, 03:19:59 pm

Have you /can you try a different speaker?

Title: Re: Hoffman Princeton reverb crackles at high volume
Post by: Paulycoo on October 05, 2017, 04:49:46 pm

Yeah good idea will do first thing tomorrow...its a brand new jupiter 10"
Thanks Paul McNamara

Title: Re: Hoffman Princeton reverb crackles at high volume
Post by: pompeiisneaks on October 05, 2017, 05:30:24 pm

Some pictures of the guts may help.  If you did the stock hoffman PR, links to the schematic and layout makes it quicker for people to review.  Crackling could be arcing somewhere and at high volumes the highest current is being pushed through, makes me suspicious of a few things:

1. dry solder joint not being perfect for transmission somewhere
2. tube socket connections not tight enough causing arcing

etc.

You could also just try turning it to that volume level, and open the chassis up and tap around with a chopstick and see if you  hear the noise getting worse or disappearing.  That may focus your efforts on the 'where' of the issue.

Are the tubes all new?  If not maybe one of them is having issues?

~Phil

Title: Re: Hoffman Princeton reverb crackles at high volume
Post by: PRR on October 05, 2017, 07:13:13 pm

> chassis and put electrical tape all around the edge that may be touching the copper shielding sheet

These should touch. And electrical tape won't last long.

I'd be thinking bad joint, not arcing; though arcing is possible.

Title: Re: Hoffman Princeton reverb crackles at high volume
Post by: bnwitt on October 05, 2017, 09:47:19 pm

What PRR said.  I'd check solder joints.

Title: Re: Hoffman Princeton reverb crackles at high volume
Post by: Ed_Chambley on October 06, 2017, 04:33:05 pm

Had a phase inverter unbalanced once enough to cause crossover which sounded awful at higher volumes. Also I had artificial center taps drift twice and got worse at more volume.


I would check PRR comments first tho. Makes sense vibration at louder volume.

Title: Re: Hoffman Princeton reverb crackles at high volume
Post by: Paulycoo on October 07, 2017, 01:11:15 am

Thanks to everyone, much appreciated.
I opened it up. Turned everything up.
Tapped with chopstick and found a 0.047uf cap attached to the bass pot crackles upon being tapped.
New one on order.
Brilliant advice all thanks.
I read on the net about shielding on cab not touching properly all around the chassis lip causing crackles when vibrating...
Should I remove the electrical tape stopping them touching?
I presume that the shielding stops rf? Does it have to make a complete circuit like a faraday cage?

Thanks all!
Best regards Paul McNamara

Title: Re: Hoffman Princeton reverb crackles at high volume
Post by: davidwpack on October 07, 2017, 01:15:46 am

I would try reflowing your solder first before replacing it. Crackles are indicative of bad solder joints.

Title: Re: Hoffman Princeton reverb crackles at high volume
Post by: Paulycoo on October 07, 2017, 07:06:04 am

Ahhh...never thought of that.
Nice one thanks.

Title: Re: Hoffman Princeton reverb crackles at high volume
Post by: bnwitt on October 07, 2017, 07:27:04 am

Amp start up and testing should always include chop sticking all components and tube pins in order to check for bad solder joints.

Title: Re: Hoffman Princeton reverb crackles at high volume
Post by: Willabe on October 07, 2017, 03:49:42 pm

Quote from: Paulycoo on October 07, 2017, 01:11:15 am

I read on the net about shielding on cab not touching properly all around the chassis lip causing crackles when vibrating...
Should I remove the electrical tape stopping them touching?
I presume that the shielding stops rf? Does it have to make a complete circuit like a faraday cage?

Yes, I would take the tape back off.

Title: Re: Hoffman Princeton reverb crackles at high volume
Post by: Paulycoo on October 09, 2017, 03:14:13 am

Thanks guys!!!!
I realised the joint.
Perfect.
Will take the tape off as advised.
I've learnt so much from troubleshooting
Chapeau to all you who helped!!!
Amp sounds incredible.
Cheers Paul McNamara