Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Joel in Texas on May 20, 2019, 03:51:37 pm

Title: Help modding standby switch implementation in an Ampeg B25
Post by: Joel in Texas on May 20, 2019, 03:51:37 pm: Hi all. Working on an Ampeg B25.

Here is a link to the schematic:

https://irationaudio.files.wordpress.com/2015/03/ampeg-b-25-schematic1.jpg

I'm thinking of moving the standby switch to a more typical setup - away from the center tap of the high voltage winding of the PT secondary (currently functioning as a ground / ground lift), to somewhere on the B+ line (typical open / closed implementation). Why? The stock implementation makes an audible pop when used; also, it just seems weird I guess? Anyway if I move it to the B+ line, should I put it before or after the first power supply filter cap (marked C25, first 30uf in the schematic after the recto tube)? Or somewhere else? Where would you put it? I have seen in different schematics where it was either before or after the first filter cap(s), and don't know exactly what the pros / cons would be, or what would be best here in this circuit. Any opinions or advice?

Thanks!
Title: Re: Help modding standby switch implementation in an Ampeg B25
Post by: vampwizzard on May 20, 2019, 08:50:39 pm: You can get rid of the pop by adding a surge/inrush resistor across the switch terminals.. theres no choke so youre not storing EMF to flyback to the PT and F4 is a diode preventing flyback from the OT. Honestly? Anywhere before F4 diode in the power section should do you ok. It depends on what you want to use the standby for. If its just to silence the amp and do no diagnostics you could do before the first filtering cap.

Edit: Turns out you shouldnt cut the B+ when they dont have a dedicated PT secondary providing the bias voltage. So dont take the advice ive said here. Strikethrough so others dont replicate my mistakes.

just my two cents. I haven't put standby switches in my amps.. I mute it at my tuner.
Title: Re: Help modding standby switch implementation in an Ampeg B25
Post by: pdf64 on May 21, 2019, 05:15:40 am: Quote from: vampwizzard on May 20, 2019, 08:50:39 pm
You can get rid of the pop by adding a surge/inrush resistor across the switch terminals.. theres no choke so youre not storing EMF to flyback to the PT and F4 is a diode preventing flyback from the OT. Honestly? Anywhere before F4 diode in the power section should do you ok. It depends on what you want to use the standby for. If its just to silence the amp and do no diagnostics you could do before the first filtering cap.

just my two cents. I haven't put standby switches in my amps.. I mute it at my tuner.

F4 is the bias supply rectifier, a switch there would be a bad move :huh:

Standby has no place in an amp with a GZ34 rectifier, as it provides a slow, smooth ramp up of the HT supply.
The stock standby is a hot switching type, horrid. See Merlin for perhaps the best way to implement standby, if a standby is essential http://www.valvewizard.co.uk/standby.html
Title: Re: Help modding standby switch implementation in an Ampeg B25
Post by: vampwizzard on May 21, 2019, 05:03:29 pm: OH SNAP.

Yeah dont do what i suggested. Youll stress the hell out of C22? voltage in the bias circuit will find its way back through the B+ circuit. Ill edit my post
Title: Re: Help modding standby switch implementation in an Ampeg B25
Post by: PRR on May 21, 2019, 08:18:27 pm: > The stock implementation makes an audible pop

Any scheme which cuts all power from the audio stages is gonna make some plop/thump.

The electrons do not know where "ground" is. You can cut a chain at any link and drop the load; likewise a circuit can be broken anywhere and the power is broken.