'61 Reverberocket-- Odd Subharmonics

[nihonlabtiger]

So I've put my H400 on the back burner for now; at least until the new 50C5s come in.

Recently I brought an early 6V6 Reverberocket back to life- it needed all new capacitors, power tubes, and plate resistors for the reverb driver and recovery.  It sounds absolutely killer, other than maybe lacking a little in bass.  I'm thinking that's just the nature of the amp, especially with the original Jensen Syntox 6.

Yesterday I pegged the volume knob for the first time and noticed something strange:  when I play an Eb in a bassier setting on the guitar (ie w/ neck p/u switched on, I get a subharmonic that seems to be down an octave.  If I let the note decay long enough, it will drop another octave.

Anybody know what could cause this?  I've never even heard of such a thing happening.

[shooter]

using this schematic as ref;

https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_reverb_rocket_12r.pdf

you're sure the echo unit isn't messing with you?
try n disconnect the NFB

[nihonlabtiger]

sorry, I don't follow.  How would the echo be messing with me?  If you're asking if it might be slop from the springs, def not.  This sounds like a bass synth, square wave kind of sound.  Much quieter than the guitar signal but still noticeable.

I will try disconnecting the NFB.

BTW, now that I'm looking at the schematic again, I realize I was mistaken about the plate resistors I replaced.  They were the two 100ks to V2.  Not to the reverb tube.

[Backwoods Joe]

Try another speaker. Rule that out first.

[shooter]

You used the phrase;

I get a subharmonic

the Echo twin I worked on did some crazy scope features when I sampled it.

[jjasilli]

https://www.thegearpage.net/board/index.php?threads/damn-sub-harmonics.199571/

[shooter]

Thanks JJ, nice find 

I pulled this;

Lower gain tubes really helped though.

I build with AU7 s

[nihonlabtiger]

good info guys, thanks.

disconnecting NFB didn't help, unfortunately.  Different speaker didn't either.

I ordered an assortment of 6SJ7s and 6SN7s that should be here today, so after work I'll try swapping tubes around to see if that helps.  Also wondering about power filtering, the thread mentioned that and it sounds plausible to me.  I recall reading somewhere (can't recall where) that these amps really weren't designed to be pushed hard anyway.  Supposedly the owners' manual says not to turn the volume past half way.

Just to be clear about the sound I'm hearing, I really don't think it's acoustic in nature.  It sounds exactly like someone plugged a synth into one of the inputs and is playing a square wave at half the volume of the guitar, one octave down and then two octaves down.  It's really bizarre.

[shooter]

It's really bizarre.

If it's the "same" problem;
I build mostly self biased xSE, when I 1st started my guitar guys complained about a "tremolo" effect.
took some time, with a scope I was able to find a coherent signal, below the noise level in the PS (both ground and HT, or either) .  Beefing up the caps in the 1st and 2nd PS nodes pretty much killed it.
you can do a quick n dirty by doubling the PS caps IF your rectifiers will allow (I use SS so nbd)
IF that's not an option, try gatorclipping more cap to the 1st gain stage cathode bypass (Ck) as experiments

[nihonlabtiger]

hmm interesting

this uses a 5Y3... I'm gonna have to look into whether or not it'll support higher capacitance.  But that idea def sounds like it's worth a shot

[shooter]

IIRC a 5Y3 should be able to handle something in the 40uF range.