Mark IIB preamp tubes, reverb driver tube in V4

[Tommy]

I bought this amp three days ago and absolutely love it. 8) It sounds great but I think there is a small amount of reverb so I usually play with reverb control all the way up. I checked preamp tubes and noticed that there is one chinesse 12AX7A (with "Audio Classic, the tube the sound" on it) in V1 slot and four unknown (red logo) 12AX7A's in V2,V3,V4,V5 slots. I checked "maintenance & repair" and it seems that there should be 12AT7 in V4 slot (it's reverb driver tube). Should I replace this tube,or maybe replace all preamp tubes with JJ or Svetlanas?

tubes:

 

[eldi]

I tried replacing the 12AT7 with a new 12AX7 (Groove tube) in my Mk IIb, and i thought it didn't sound as clean, so I put the 12 AT7 back in (it was an older tube, it had RCA's in a couple of the other slots..)

The tube on the right in your picture has the Yamaha corporate logo on it, so I would assume it's some sort of Yamaha-branded tube....

My MK IIB's reverb sound's good on the clean channel, but if i don't turn it down before using the lead channel it can be obnoxious & drown the sound out. Is it still quiet when you switch to the lead channel on yours?

 

[dancullinane]

There is a simple reverb-mod which will increase the reverb level. I don't know the details but involves adding a resistor. Try a search or maybe one of the gurus will have the answer.

Definately try a new tube to make sure that isn't the issue. I didn't notice much change in my IIB when I switched from an AX7 to an AT&

DC

 

[Boogiebabies]

Those are both mid to late 80's Chinese.
Back in the day, they were about all you could get other than Ei's which were poorly made or NOS. Even Marshall was putting them in labeled ECC83. I have a box full of old pulls with Soldano, Mesa, National and a host of other names. They are not audibly bad tubes, but they were hit and miss with quality as well. The ones I preffered the most in the 80's were the ones with the silver mica spacer on top, also known as Silver Specials. I don't know where this Svetlana preamp tube phase came in, but they suck now and always have, even the EF-86. The JJ's, even with a bright amp can be a dark sounding tube. It's a crap shoot to find the perfect tube for you amp and the tone you are looking for. I usually keep between 6 to 12 preamp tubes from all brands to try out, no Svetlana's of course.
Oddly, when I bought a MK III GS, I left the mish mosh of old Mesa Chinese, EH and GT's in with the new Svetlana 6L6 and EH EL-34's.
The amp sounded so good I did not want to, as we say queer it. The V4 on the IIB schematic is a 12AT7, but increaseing the gain 20% with a 12AX7 should technically boost the reverb. I think you may want to give the amp a spring cleaning if you have to run the reverb on 10. It could be due more to dirty pots, contacts or a spring in the tank coming off. I had one with sticky springs from the glue evaporation on the foam. They felt like they were dipped in maple syrup.

Good Luck.

 

[Shep]

Hey BB What's the 3.3k 1/2 watt resistor mod for the mk III.. I am needing that extra reverb..where about's does it goes.. Fx loop return ?

 

[Shep]

Thanks GTS