Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plumcrazyfx on July 30, 2018, 08:26:04 am

Title: Adding a pre filter stage - AB165 Bassman
Post by: plumcrazyfx on July 30, 2018, 08:26:04 am

So, I have a 68 AB165 Bassman with ghost note issues above the 12th fret. Really bad ghost notes like a low octave pedal.  All electrolytic caps were replaced, different tubes tried, different speakers tried.  Even changed a bias ballance to ballance + bias and finally to the bias circuit from the AA165.  Still there.


I'm thinking of adding a pre filter stage inside the chassis and am wondering if I have it right or if this is the right direction to go.  Seems like this circuit is prone to these issues.  I wanted to keep it close to original but it's unplayable right now.


I'm thinking of placing two 100uf/450v caps in series with 220k resistor in parallel with the Caps and adding a 100ohm resistor between it and the original first stage.  Do I have that right?

Title: Re: Adding a pre filter stage - AB165 Bassman
Post by: ginger on July 31, 2018, 06:53:31 am

The AB165 has a stupid - low value cap feeding the pre-amp.... 8UF . I raised mine to 16 UF . It stopped the ghost notes . You could try a 22 UF also . This circuit has no shielded grid wire ... I would do that too .

Title: Re: Adding a pre filter stage - AB165 Bassman
Post by: plumcrazyfx on August 01, 2018, 07:11:17 pm

I haven't added the filter stage yet but I clipped the 220k resistors from the 6L6 plates to the PI plates and it seems much better.