120 Hz intermodulation

MikeMcK

Gold Supporting Member
Messages
5,566
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

Member
Messages
2,429
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

Member
Messages
8,051
ordinary problem with ac30s. improved grounding and filtering usually solves this problem. I've installed a big choke (CLC or Pi filter) in mine
 

MikeMcK

Gold Supporting Member
Messages
5,566
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.
 




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