'72 Bassman High Frequency Oscillation

[StonesFan]

Hi all. I removed the 2000pf suppressor caps from the output tubes of my ’72 Fender Bassman and now I’m getting oscillation when the amp is cranked. I know this is probably due to sloppy lead dress. I was going to just try a lower value (1000pf) but I would really like to implement something along the lines of the 5F6-A Bassman by using a 47pf across the plates of the phase inverter. The tweed Bassman uses a 12AX7 for the phase inverter where the silverface uses a 12AT7. Because the plate resistance of a 12AX7 is so much higher than the 12AT7 (62K vs. 11K), would this mean that I would need to use a much higher value cap than 47pf to get the desired effect? I was thinking somewhere around 250pf. Does this make sense or am I way off target? Thanks,

-Brian

[Chuck H]

Sharp consideration. And what you propose seems right. Of course if the large-ish value bothers you any smaller value that stops oscillation if fine. Ideally the lead dress issue could be dealt with too. If you're ever going to run pedals for overdriving the input you may find that just using that one PI shunt isn't enough. The Bassman isn't worst offender in this regard. Not as bad as the guitar amps anyway. Make sure the preamp leads aren't sharing proximity with output leads. Pay special attention to the NFB circuit lead and keep it away from the preamp. Also make sure preamp leads are moved away from the NFB circuit node. Make sure OT leads inside the amp are twisted together and no longer than necessary, etc. There may be one or two bundles of leads wrapped with lead wire. You may need to undo this in order to minimize proximity of later circuits with earlier ones.

[jpfamps]

What plate load values do you have on the PI? Are they the standard SF 47k?

These lower the output impedance of the PI and hence increase the chance of oscillation.

You could try increasing these to the BF values.

[StonesFan]

What plate load values do you have on the PI? Are they the standard SF 47k?

These lower the output impedance of the PI and hence increase the chance of oscillation.

You could try increasing these to the BF values.

Yes, 47K plate resistor, 330K grid resistors. I'm going to see if I can get rid of the oscillation with better lead dress and then I might experiment with the blackface values. Thanks for the suggestion.

[StonesFan]

Sharp consideration. And what you propose seems right. Of course if the large-ish value bothers you any smaller value that stops oscillation if fine. Ideally the lead dress issue could be dealt with too. If you're ever going to run pedals for overdriving the input you may find that just using that one PI shunt isn't enough. The Bassman isn't worst offender in this regard. Not as bad as the guitar amps anyway. Make sure the preamp leads aren't sharing proximity with output leads. Pay special attention to the NFB circuit lead and keep it away from the preamp. Also make sure preamp leads are moved away from the NFB circuit node. Make sure OT leads inside the amp are twisted together and no longer than necessary, etc. There may be one or two bundles of leads wrapped with lead wire. You may need to undo this in order to minimize proximity of later circuits with earlier ones.

Hi. Thanks for the response. I ended up clipping a 250pf across the plates of the PI and it did stop the oscillation. More importantly I didn't hear any roll off in the highs so this does appear to be a valid solution. Ideally I would like to get rid of the suppression caps so I will experiment with your suggestions on lead dress.

[TimmyP1955]

What circuit designation is that one?

Make sure it has the 1k5 grid stoppers, and that they are mounted on the sockets.