Hoffman Amplifiers Tube Amplifier Forum

Other Stuff => Solid State => Topic started by: kagliostro on October 07, 2013, 02:51:59 am

Title: Vox VBM1 PT voltage question - Schematic Here
Post by: kagliostro on October 07, 2013, 02:51:59 am

I need help about PT voltage identification

Here is the schematic of the Vox VBM1

as you can see the secondary of the PT has no voltage labels

The first electrolytic caps are rated for 25v DC and the max DC voltage seems to be 15v

so I think the DC voltage may be around 18v-20v and the AC around 13v-15v

looking to the schematic can someone say which will be a correct voltage for the circuit ?

Many Thanks

K

Title: Re: Vox VBM1 PT voltage question - Schematic Here
Post by: sluckey on October 07, 2013, 08:55:19 am

A 24VAC (12-0-12) with center tap would work. Your highest unregulated voltages are +VCC and -VCC and are not specified. The other lower voltages are regulated down by zeners.

A 24V PT would give unregulated +17V for +VCC and -VCC.

Title: Re: Vox VBM1 PT voltage question - Schematic Here
Post by: kagliostro on October 07, 2013, 01:51:08 pm

Many Thanks Steve

Franco

Title: Re: Vox VBM1 PT voltage question - Schematic Here
Post by: blackcorvo on September 13, 2015, 04:14:08 pm

Hello, I prefered to revive this old topic instead of making a new one since it has the schematic already there, so it'll be simpler to explain the problem I encountered.


I recently got one of these little amps (used), and I love the dirty sound from it! However, when you try cleaning up the sound (aka turning the guitar volume down), or just let a note ring out, there's this weird "cutting" effect that happens in the point of transition from clean to dirty.
It sounds like bad bias on the "Deacy power transistors" (Q4 and Q5) or something around that. I ruled out the booster because it sounds fine on other amps, and I also ruled out anything after Q4 and Q5, because the tone control doesn't affect the amount of cutting that happens (it does change the tone as it's supposed to do, tho), so I assume it's between these two blocks on the circuit.


I also noticed a "pop" that happens about 1 or 2 seconds after turning the amp off. It happens regardless of the booster switch being on or off. Voltages on the PSU seem fine (+/-18v, +/-15v and +/-9v all measured perfectly fine).


Is that cutting sound I'm experiencing just something the amp is supposed to do (perhaps to reduce background noise), and I'm just being picky?


Thanks ahead for any insight in this situation.