Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on October 15, 2014, 02:38:27 am

Title: Has anyone ever tried this type standby ?
Post by: kagliostro on October 15, 2014, 02:38:27 am

I was wondering if someone has ever tied this particular standby circuit

 I'm curious about performances

also I don't understand which is the purpose of the DPST relay (if it isn't part of the standby)


(http://www.chambonino.com/construct/constwire1c.jpg)

(https://i.imgur.com/4esZlP9.png)

K

Title: Re: Has anyone ever tried this type standby ?
Post by: John on October 15, 2014, 06:43:33 am

I've used a 47k/3W resistor across a standby switch before,and it worked fine. I put mine in before the PT, that way it lowered the voltages to everything. Haven't bothered with it again since the consensus seems to be that, for lower powered amps it's not necessary.


The DPDT seems to be part of the standby circuit (lifts the ground to the top 2 windings) but I also think the dotted rectangle is drawn incorrectly. It shouldn't include the resistor--- unless that's not the resistor but the coil for the relay. Same thing with the other rectangle, not sure what's going on there.

Title: Re: Has anyone ever tried this type standby ?
Post by: kagliostro on October 15, 2014, 07:10:31 am

Thanks for the answer John

Within the rectangle there is the relay coil, and the standby isn't there (at least so it seems to me)

Look to the right in the schematic, it is labeled "Standby switch SPST"

it acts disconnecting grids from power and connecting the grids to the negative voltage of the bias supply

a bit odd to me (I haven't seen one other circuit do in that way) but intriguing and maybe high-performance

Franco

Title: Re: Has anyone ever tried this type standby ?
Post by: sluckey on October 15, 2014, 08:06:15 am

Quote

also I don't understand which is the purpose of the DPDT relay (if it isn't part of the standby)

It's a safety interlock. The bias supply must be working to energize the relay. If for some reason the bias supply fails the B+ and screen supplies will be switched off.

Title: Re: Has anyone ever tried this type standby ?
Post by: kagliostro on October 15, 2014, 10:01:57 am

Many Thanks Steve

Franco

Title: Re: Has anyone ever tried this type standby ?
Post by: darryl on October 15, 2014, 06:11:54 pm

The amplifier in my avatar ( and all the other amps I built in the 1970's ) uses a simplified version of that standby. It disconnects the output valve screen supply, but does not connect the screens to the bias supply.

Using the bias to apply a negative voltage to the screens is a good idea. It would ensure the output valves were completely cut off in standby. My amplifiers would bleed a tiny, distorted signal if a large input signal was applied to the amp whilst in standby.

These days I don't even bother with a standby switch.

Title: Re: Has anyone ever tried this type standby ?
Post by: kagliostro on October 16, 2014, 01:39:30 am

Many Thanks for the info Darryl

Franco