120 Hz intermodulation

[MikeMcK]

My Hayseed seems to have a power inter-mod issue. I started noticing phantom notes at high volume, so I "earballed" the pitches, worked out the frequencies of the notes I was playing and what I was hearing, and found consistent differences of ~120 Hz.

Most likely just a filtering thing or are there other common causes?

 

[Cirrus]

Vintage AC30 style amps have low filtering and ghost notes, it's just the nature of the beast to some degree. Lead dress has an effect - my AC30hw is terrible at vintage values (8uf pres, 16uf first filter). You've identified it's 120hz so it's in the B+ line. Is it something that's started recently? If so might be tube related, try swapping new ones in starting with the rectifier. if not that, then maybe a lead has shifted inside or a filter cap is failing (would be surprised on a recent amp though). If it's always been that way you could try moving leads with a chopstick or upping the filter cap values. Assuming you've got vintage values you could go up to say 47uf main filter. It'll change the feel of the amp though - tighter, quicker bass response and it'll hold itself together more at volume.

 

[Roe]

ordinary problem with ac30s. improved grounding and filtering usually solves this problem. I've installed a big choke (CLC or Pi filter) in mine

 

[teefus]

did you contact rob? he is very good at taking care of his customers.

 

[MikeMcK]

did you contact rob? he is very good at taking care of his customers.

I did not yet on this question, but you're right. Last time I asked him about one of his design tweaks, he sent instructions and the parts... all for free.

 

[teefus]

he really knows his stuff and is super nice to boot.