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Help...Bassman Bias Supply Question

5/31/2004 6:51 PM
Bill Harbour
Help...Bassman Bias Supply Question
I've got 47v ac out of the bias supply from the PT. After going through the rectifier diode, I'm getting somewhere in the neighborhood of -23v dc. I notice from the schematic that I've got to supply -48v dc to the amp. How am I going to get that? Will it be close to -48v dc by the time it gets through the bias supply circuit?  
Thanks for the help...  
Bill Harbour
5/31/2004 10:19 PM
Bruce /Mission Amps

Tweed or modern Bassman?  
6/1/2004 7:24 AM
Bill Harbour's a 5F6A.  
Thanks for the help.  
6/1/2004 11:16 PM
Bruce /Mission Amps

Well there is a series of drops there.  
Does the PT have a single wire bias AC tap?  
Is it wired just like the stock layout?  
Diode, filter cap, 15K, 56K bypassed by another filter cap?  
I've done quite a few of these and not had this happen so it something simple.  
If the bias supply parts values are all correct or very close to spec, my first guess is one of your filter caps is installed backwards or blown up.  
Center tap is grounded on the PT right?  
6/2/2004 5:41 AM
Bill Harbour

Yes..single wire bias tap. Stock Bassman bias circuitry. The 23vdc reading is taken on the negative side of the rectifier diode. The bias supply isn't yet connected to the board. I'm going to temporarily hook the bias supply up to the board and get a reading after the bias supply has gone through the bias circuitry.  
6/2/2004 11:43 AM
Bruce /Mission Amps

You aren't using a center tap standby switch are you?  
And, you have just tried a new or different diode...  
6/2/2004 12:07 PM
Bill Harbour

no...not using a center tap standby switch...just the normal standby switch. Keep in mind that the board isn't in the amp yet...the bias supply is hooked up to the bias rectifier only. I'm reading the 23vdc from the (-) side of the bias supply rectifier. I tried a full wave rectifier (just + and - hooked up) and a 1kV diode...same result.  
I know that I'm only reading half of the wave which explains the 23vdc...but my question is this...will the voltage be higher after going through the bias circuitry?  
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