preamp tube question

[John]

I picked up some old mono "hi-fi" amps from Ebay for cheap, and a couple of them used a 6SL7 for the preamp tube (and 6V6 for power tube). Anyone have any experience with these, good or bad? Just a sometime project that I might turn into a little SE amp. Some day. 

Also, I was swapping out the V2 tubes on my build, a 12aY7 there sounds great, but was going to pop in a 12aT7 as well for fun. However, all 3 tubes I tried squealed badly. I can't believe all three are bad (they are JAN NOS) so I'm thinking I need to change either a few component values or lead dress, although the amp is dead quiet with the 5879/12a_7/12a_7 tubes that are in it now. I was only tinkering with the tubes out of curiosity.. you know, "gee, I wonder what happens if..."

Thanks in advance.

[tubeswell]

...Also, I was swapping out the V2 tubes on my build, a 12aY7 there sounds great, but was going to pop in a 12aT7 as well for fun. However, all 3 tubes I tried squealed badly. I can't believe all three are bad (they are JAN NOS) so I'm thinking I need to change either a few component values or lead dress, although the amp is dead quiet with the 5879/12a_7/12a_7 tubes that are in it now. I was only tinkering with the tubes out of curiosity.. you know, "gee, I wonder what happens if..."

Thanks in advance.

What build was it?

Could be that if it has global NFB* (supplied from the OT), that you have the NFB wired to the wrong polarity, in which case you should swap the OT primaries around.

* Like many amps do

[John]

Tubeswell, it is the HoSo amp by Geezer and Tubenit, no GFB (unless of course, I'm too dim to know it's there (always possible)). But without the pseudo channel switching. Schematic attached. Thanks!

[HotBluePlates]

I picked up some old mono "hi-fi" amps from Ebay for cheap, and a couple of them used a 6SL7 for the preamp tube (and 6V6 for power tube). Anyone have any experience with these, good or bad?

I like em. They're slightly less gain than a 12AX7. All the 6S_7 series could be more microphonic than the 12A_7 series, simply due to the larger size. I have not run into that issue myself.

I used to have a '54 tweed Princeton that used a 6SL7, and I thought it sounded nice. At least one memeber here has built a 5F6A Bassman with 6SL7 preamp tubes rather than the stock 9-pin tubes.

... However, all 3 tubes I tried squealed badly. I can't believe all three are bad (they are JAN NOS) ...

Maybe.

I tried out some 9-pin tubes once that looked like they could be a good sub for 12AY7's. I'm 90% they were 5965's.

Anyway, they're *very* cheap to get, even for 50's NOS samples. I got maybe 12 for next to nothing, and all were military, still in the old military bulk packaging. All but maybe 1 were horribly microphonic. I mean, I turned the amp volume to zero, slowly turned it up, and by the time I hit 3-4, the amp was howling loudly as though I swapped OT wires in an amp with feedback. Almost 100% unusable.

I thought, "what a bunch of crap!" Until I looked again at what a 5965 is supposed to do: operate as a switch in tube digital computers. Since they only needed to be ON or OFF to work properly in a digital circuit, microphonics were a non-issue.

Your tubes might be bad. Or they might ring a little. Or, they might be a circuit with more gain/bandwidth than what they can comfortably handle. They might work great as-is in another amp.

But I don't know how microphonic yours are... My situation was with no guitar, no background noise, and the tubes simply howled (louder than a guitar would play) on their own from positive acoustic feedback. Do your do that? Or do they just "ring" a little?

[John]

HPB, thanks for the info. Mine would start squealing really bad as soon as I got the volume knob close to halfway. IIRC, one would wait till I started playing guitar, the other 2 didn't need that encouragement. Your explanation of what they were intended for does make sense. Thanks!

[HotBluePlates]

12AY7's were always intended for low-level audio stages, where hum and microphonics might be a problem. It says so right on every data sheet. However, this is not necessarily true of 12AT7's.

All the 9-pin tubes seemed to evolve over time to reduce microphonics. Often, this was done by shortening the plate structure, sometimes in conjunction with adding a 3rd mica spacer, and possibly extra support rods. The idea was to stiffen the structure so microphonics didn't happen.

I haven't seen as many 12AT7's where this extra care was taken. That's because their characteristics aren't especially suited for an input stage (doesn't mean YOU can't use them that way). You might have to select tubes for low microphonics. The most likely candidates for 3 spacers will come from late-50's to early-60's. Fortunately, old 12AT7's are cheap compared to 12AX7's.

[tubeswell]

Mine would start squealing really bad as soon as I got the volume knob close to halfway.

Well sounds like either a microphonic tube, or possibly oscillation from lead dress (related to signal strength). Have you tried holding the pre-amp tube(s) with a (dry) towel when the noise happens to see if that dampens it? (the right-sized O-ring(s) rolled over the tube's envelope also can be helpful here, as can 'shock-mounting' the tube socket onto the chassis. I have used both of these techniques on EF86 combos to keep microphonics in check).

[John]

Tubeswell, I didn't try any of that, no. When I get to messing about with it again I will just for fun, which is why I was trying it anyway. I was concerned about the 5879 when reading about some builds with it, but haven't had a bit of problem with that one. And really, the tube lineup right now sounds really nice, and none of them are expensive tubes... Sovtek and/or EH, I believe.

HBP, thanks for that info also. Makes perfect sense not to re-design a tube for something it's not normally used or intended for anyway.