Lower gain substitute for a 6sj7?

[bluzmn]

I recently bought a Masco ME-18 to play harp through, but I can't even turn the volume up to 2 without the amp feeding back. Is there a lower gain octal tube substitute for a 6sj7?

[vintagekiki]

About 6sj7 substitute

http://www.oddmix.com/tubes/tsub_06s.html
Substitute Table Type 6S Replacement (6S0-6ZX)

http://www.vintageradio.com/history/RiderCaddySub.pdf
Tube substitution 'guidebook

http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=3757
6SJ7 replacement

https://el34world.com/Forum/index.php?topic=23506.0
6SJ7 vs 6SH7

https://www.tdpri.com/threads/1946-princeton-woodie-preamp-tube-substitution.100398/
1946 Princeton Woodie Preamp Tube Substitution

https://www.thetubestore.com/rca-5693-special-red-tubes

Replaces 5693, 6SJ7, and 6SJ7GT tube types. The RCA "Special Red" tubes (5690, 5691, 5692, 5693) were designed for 10,000 hours of use. RCA claimed the 5693 tube could withstand shocks of 100 G's for extended periods of time!

These should be considered a top pick for old amps like Gibson, Danelectro, and any other vintage amp requiring a 6SJ7 tube. Also audiophiles with the Musical Paradise MP-301 can also upgrade their amps from the Chinese 6SJ7 input tube to the 5693.

[mozz]

You could probably change a few resistors to knock the gain down.

[vintagekiki]

If you already have an idea, put it on paper and upload schematic, to help bluzmn.

[vintagekiki]

... can't even turn the volume up to 2 without the amp feeding back ...

Not problem in Masco ME-18. At acoustic instruments, acoustic feedback occurs.
https://ampgarage.com/forum/viewtopic.php?t=32990
Masco ME 18 Mod
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I recently rebuilt a Masco ME 18, and while it sounds really great with my guitar, it isn't working with a harmonica. This amp is supposed to be a harp monster, but I am getting feedback almost instantly.

https://geoffthegreygeek.com/understanding-acoustic-feedback/
Understanding Acoustic Feedback

[Leo_Gnardo]

Although it won't make an overall difference in the gain structure, consider the volume pot - is it possibly a linear taper control? If so, that may make it difficult to find the "sweet spot" if it's all the way down near 2 out of 10. If it is linear, swap it out for an audio taper pot before doing anything else, which will expand the control zone over a wider range of the pot turn. Push comes to shove, you could pad the pot with a fixed resistor so you have to turn it way up to get to the feedback zone.

[Enzo]

But first... Feedback happens when the speaker signal gets back into the microphone. SO spend some time putting the speaker ahead of you are aiming somewhere other than at your head.

[J M Fahey]

^^^^^^^^ THAT.

What matters is TOTAL loop gain.
A lower gain tube solves NOTHING that just lowering "volume" a little can´t do.

As said above, your problem is Acoustic, not Electric.

[bluzmn]

Wow! Thanks for all the replies, gentlemen. I see that I should've asked a more complicated question (or series of questions). I was hoping that if I asked a simple question, there would be a simple solution. That's alright, though, because you all have touched on some of the further questions I have. But first, I want to say that I have been playing amplified harmonica for about 30 years, and I am very well acquainted with the monster that is acoustic feedback. So when trying out this amp (or any other) in my apartment, I try to position it as much as possible as I would in a club; way off to the side of me, in the back line, while I stand toward the front off the stage. I have several other amps with similar wattage, and while they all have a point at which they'll feed back, it isn't as low as 2 on the dial. That's why I think the problem is electronic, not acoustic. Plus, I found other instances of people encountering feedback with an ME-18. You see, the first thing I do when I have a problem like this is turn to the computer. So when I searched for "ME-18 feedback" the first pertinent response I found was the one vintagekiki referred to, where it was suggested that the value of R6 should be reduced. Then I searched on the moderbluesharmonica.com forum, and there was a guy who had built an ME-18 replica who mentioned that he was having a problem with feedback, which he thought was due to too much voltage on the plate of the preamp tube, which once again implicates R6. I'll change that if I have to, but first I want to see if there's a simpler solution. A common "hack" that blues harp players use when dealing with an amp with too much preamp gain is to just swap in a lower gain preamp tube. That's easy with 12a?7 tubes because they all have the same pin-outs; it's trickier with octal tubes. When you install a lower gain preamp tube, you get a notch or two more on the volume knob; I realize that you may not get any more volume than you did with the higher gain tube at the point where it fed back, but you since you're driving the amp harder, you get more (or at least some) distortion. I want to thank vintagekiki for his first reply; I must have searched at least an hour on Google and Duckduckgo for "lower gain substitute for 6sj7". All I found was a sentence on thetubestore.com website - "Types 6sd7, 6sk7, 6ss7 or 6137 may be substituted in an emergency, but will give less gain and more distortion." That sounds like what I'm looking for, but what I want to ask you all is: will using any of those tubes harm the amp in any way (the 6sk7 looks like the most likely candidate)? Sorry for the long post, but you can probably see why I didn't lay this all out first.

[Mick Bailey]

It may be worthwhile checking the amp voltages as your mains voltage could be much higher than when the amp was made, resulting in increased heater and plate voltages. Also check to see if any of the tubes are microphonic - particularly the 6SJ7. Just tap them lightly with a chopstick or pencil to see if this gives rise to any excessive noise. Sometimes acoustic feedback can be triggered by an input signal and sustained within the amp itself if a tube is microphonic.

[vintagekiki]

Interventions on 6SJ7 cannot harm to amplifier.
If soldering iron is your friend, surest way to reduce the gain of input stage is to rewire 6SJ7 to triode.

Another solution is instead of 6SJ7 use 6SN7 dual octal medium gain triode. It is necessary schematic rewired to some vintage Champ versions.
https://en.wikipedia.org/wiki/6SN7
https://www.thetubestore.com/early-fender-guitar-amp-schematics

[Leo_Gnardo]

I'll pitch in a couple more ideas, here goes.

A) Swap out the 1.5 meg input load / grid leak resistor for a potentiometer. 1 meg, 500K, even 250K. Knock down some signal from the harmonica mic before it gets to the 6SJ7. Or use a voltage divider, two resistors, to do the same, though in a non adjustable manner.

B) Turn the 390K plate resistor into a "split load" pair of resistors so only part of the signal is sent on to the next stage.

C) Again, use an audio taper volume pot if the one now in the amp is linear.

and finally

D) Insert a fixed tone stack designed to minimize gain at the main offending frequency of feedback between mic & speaker. Some version of the old standard Fender stack (I'm thinking mid 60's Twin Reverb) ought to work. They have a midrange "scoop" that you can tune by swapping parts so it will work for your mic/speaker combination.

OK I"m out of ideas now. But I'll be watching to see what develops. Good luck!

[vintagekiki]

A) Swap out the 1.5 meg input load / grid leak resistor for a potentiometer. 1 meg, 500K, even 250K.

A small correction
6SJ7 cathode (pin 5) is on ground.
R4 (15M) is bias resistor and it provides grid leak bias (0.8V) for 6SJ7.

https://www.diyaudio.com/forums/tubes-valves/272689-grid-leak-bias.html
Grid leak bias, how does it work?

https://electriciantraining.tpub.com/14178/css/Grid-Leak-Biasing-45.htm
Grid-Leak Biasing (page 33 - 35)

[g1]

check to see if any of the tubes are microphonic - particularly the 6SJ7.

Very important to do this before making any modifications to the circuit. I find the metal can type octal preamp tubes especially prone to microphonics.

[mozz]

It may be worthwhile checking the amp voltages as your mains voltage could be much higher than when the amp was made, resulting in increased heater and plate voltages. Also check to see if any of the tubes are microphonic - particularly the 6SJ7. Just tap them lightly with a chopstick or pencil to see if this gives rise to any excessive noise. Sometimes acoustic feedback can be triggered by an input signal and sustained within the amp itself if a tube is microphonic.

Exactly, they are known to be excessive in this area, so it might be the lesser of 2 evils in your search.

Here are tubes with the same pinout, i didn't look them up for gain though so you need to try or research them.

6AB7, 6AC7, 6AJ7, 6SD7, 6SE7,6SJ7, 6SK7,6SS7

[35L6]

I have SK7's in 2 of my amps , so they will work . They are variable mu / remote cutoff so you can bias them colder and reduce gain .