Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on February 20, 2011, 03:17:25 am

Title: PSU HELP
Post by: TIMBO on February 20, 2011, 03:17:25 am

Hi guys, Just sorting out the PSU for the Silver Jubilee.I'm wanting to add a VVR to the circuit.Is this what it will look like? Also i've changed a couple of the power resistors just to bump up the volts as the PT i have is a few volts short than the stock PT. Thanks

Title: Re: PSU HELP
Post by: LooseChange on February 20, 2011, 05:38:30 am

Yes. That will work well.  One thing to note, and I've never actually done anything about this, is that as you reduce the voltage to the power amp using the VVR, the preamp voltages will go up. Possibly more than you really want.

Title: Re: PSU HELP
Post by: TIMBO on February 20, 2011, 01:23:33 pm

Thanks LC, So i'l keep the 10ks in and adjust when built. Thanks

Title: Re: PSU HELP
Post by: TIMBO on March 28, 2011, 03:38:59 am

Hi guy's, I've had some thoughts about the PSU and i've came up with the attached schem and i think this will work. As i said before i was wanting to add the VVR but the one i've got will only work with a Cathode Bias.I like the idea of a Fixed Bias amp and trying to keep the Silver Jubilee as close to the original and the use of the VVR in the Cathode mode will allow me to crank it up at low levels :icon_biggrin: So half the DPDT switch will switch the power tubes between Cathode and Fixed (as per Sluckeys november project) and the other half switches the VVR in and out. So please confirm the schem is right. Thanks

Title: Re: PSU HELP
Post by: sluckey on March 28, 2011, 06:52:36 am

The bias switching is not right (see attachment). Also, if you want to use separate cathode resistors/caps you'll need a 3 pole switch.

Title: Re: PSU HELP
Post by: TIMBO on March 28, 2011, 02:28:49 pm

Thanks Sluckey,I was following the Matchless Chieftan schem as there does not seem to be too many cathode bias EL34 amps around.It is not a must have to have separate cathodes. I was just looking for away to add the VVR i have.Can an amp be cathode bias to sound similar to a fixed bias amp? Thanks

Title: Re: PSU HELP
Post by: jojokeo on March 29, 2011, 10:06:13 pm

Hi Timbo,

Why the cap across the LED/DIODE clipping section? This will attenuate highs even more and you likely won't want that. The square wave that's created already does this by clipping off the tops/bottoms of the sine wave which removes highs.

Title: Re: PSU HELP
Post by: TIMBO on March 30, 2011, 12:05:34 am

Hi jojokeo,  :dontknow: You could and have answered that question  :worthy1:I just followed the schem but i will keep it in mind when i get it up and running.You guys are far more advanced with this stuff than i ever be :w2:Thanks for then input.

Title: Re: PSU HELP
Post by: jojokeo on March 30, 2011, 12:34:39 am

No worries mate, if it sounds a bit dark you may like it but at least you will know to change or tweak it better. With a really high gain signal this also functions similarly as a smoothing cap would in this regard, meaning it helps eliminate any parasitics at this stage.