Hi -- I'm trying to figure out the source of a strange hiss in amps that I have.
Schematic is here.
They are AKAI/Roberts conversions (
more info on the page I made here). I have received lots of help re: these amps here in the past. (Thanks!)
Executive summary: they have an EF86 stage, an 12AX7 stage, and a power tube stage (6BQ5). There is an XLR input to the EF86, an EF86 output, a 12AX7 input, a 12AX7 output, and an internal connection from the 12AX7 to the power tube with an associated speaker output.
I've replaced a bunch of caps already, to no apparent benefit. I was going to start replacing resistors, but I noticed an odd pattern when trying to isolate which stage might be causing the hiss, and so I post here. Most routes through the amp are totally quiet (obviously some noise somewhere, but basically "silent" by old tube amp standards.)
EF86-in ->
EF86-out: totally quiet
12AX7-in ->
12AX7-out: totally quiet
EF86-in ->
EF86-out ->
12AX7-in >
12AX7-out: totally quiet
12AX7-in ->
12AX7-out ->
power tube ->
speaker out: totally quiet
(for the EF86 and 12AX7 outputs, I monitor via audio interface, for the speaker out I monitor via a speaker)
It's
only when I do:
EF86-in ->
EF86-out ->
12AX7-in ->
12AX7-out ->
power tube ->
speaker out
...that I hear the hiss (in the speaker),
even if the EF86 pot is turned all the way down. Simply making the "tip" connection between the EF86-out and the 12AX7-in there is hiss, regardless of the pot level. It increases with the pot up, of course, but it feels like there is a constant "floor" of hiss regardless of the pot position (the pot doesn't increase hiss until well into its travel), and only when that particular connection is made. (This isn't a subtle increase in hiss: i'm not just talking about a few dB.) The hiss happens even with nothing plugged into the EF86 input.
I have four of these amps, and all of them do it, to slightly varying degrees, but it's always the same pattern. In one amp I have replaced C25, C26, C7, C2, C10, C15, C16, C9, and C11. I have tried replacing all tubes with newly-made tubes. I have squirted contact cleaner in all tube sockets and jacks. All to no effect.
This happens whether the EF86-out in the above-described chain is from the same amp, or from any of the other amps. When using a different amp as the EF86-out source, the other amp must be turned on and warmed up for the hiss to appear.
I conclude from all this that it's likely not a defect in a particular amp but due to the nature of the circuit; but I'm not clear why that would be, since some of the routes described above would seem to have the same gain but don't exhibit the hiss, and since it seems partially independent of the EF86 pot or anything being plugged in to the EF86, and so it makes me wonder if there is some way to avoid it. Any insight? Thanks so much for your time!