MarkIIb could be better

[Cheapshoes]

Hi all, I've been absent for a little while. I bought a 60W MarkIIb that's lived a hard life, recently brought back into shape. I have a few questions:
(1) Compared to other amps I have, it is far more "dimentional' - that is it can do about anything, and do it in spades. However, I can't get a smooth compression or smooth overdrive at usable volume levels. The OD just jumps right in, either full on-or-off, and can't get that just on the edge of breakup that I love so much --- at least can't at anything approaching indoor levels. -- are there some simple things to change to get there?

(2) When refurbishng, I did not touch the large carbon screen resistors as they appear to be holding their own fine and .... of course the original Mo-Jo factor... However, I've read reference to raising these values from 470 Ohm up to 1-2K and using higher wattage like 5-Watt. The touted benefits are a little less power, more compression, and protecting the tranny if/when a tube blows. Well... it seems a little more compression/smoother tone would be a really nice thing on this amp on the clearner settings (can get harsh and uncompressed as-is). On the downside, I sure don't want to fix what ain't broke, but these resistors are 30 years old and carbon, but both look and read good as new. Thoughts?

(3) I sometimes get a nasty cracking for a few seconds when switching off the standby to play. This seems to only happen when the amp has been sitting for several weeks. After warming up or playing within a day or two it has no problems. Could there be a moisture or other transient problem in an old amp like this? Cures?

Thanks for putting up with my long-ish message. :mrgreen: CS

 

[>Photi G<]

Have you had the Electrolytics replaced recently? Their typical lifetime is about 20 years, and if the caps are original, they may be way past their expiration date.

 

[Cheapshoes]

Photi: the Electrolytics were replaced six months ago. Has no hum problems and a tight bass response. The originals were in bad shape and bulging when I got the amp.

 

[Neptical]

Gonzo ( a forum member here ) gets a really, really sweet tone with his Mark IIB. You can check out some of his work down in the 'Rigs and Tones' forum. I dig that tone alot.

~Nep~

 

[>Photi G<]

Well, that's good. Maybe you should talk to Mike B. at Mesa about the crackling sounds, and any other problems you're having. Could be the switches, as those are pretty cheaply made. And +1 on Gonzo's tone. Really sweet.

 

[>Photi G<]

Oops, double post.

 

[Cheapshoes]

Well, since I've been playing every few days, no cracking to deal with. I also tried something else and very, very pleased wih the result. I put a 12AT7 in V2, instead of the 12AX7. Now the "gain-boost" setting on the clean channel is purrrrect, and usable for both a Schecter C-1 and a Telecaster. The problem before, was I had to set the master on extremely low, on the very edge of cutoff, because the volume was too great. Couldn't even take it to practice without harshing everyone out (this has the original EV speaker). The lead channel wasn't nearly as intense, but more suited for midrangy Santana sounds then for the Stones or Eagles, etc. The unboosted clean channel was decent, but could not make it into slight OD territory at any setting below ear-spitting. After changing the tube however, the boosted clean channel can at least have the master on 2-3 (where the contacts are properly engaging), set the preamp betwen 5 and 9, and use the guitar volume to gently slide between clean and nasty. Very nice. 8)

Other hint: The type/quality of V2 affected the tone more than I'd guessed. I tried four AT7 tubes: 1.) new Chinese-made Mesa, 2.) 1982 Indian-made NOS, 3.) USA-made old used RCA, and 4.) old used Tung-Sol.

The RCA is the best for sweetness, harmonics, and Hi-Fi, but has some occasional minor microphony off-and-on. It's staying in there for now. Next, I'd go with the Indian-made one or the Tung-Sol. They were decent, harmonic, but a little midrangy (less-hi-Fi). If I had to guess, the 1982 tube might be what was originally used since this amp was built in 1980. The Mesa tube was more Hi-Fi than the Indian or Tung-Sol, but to my ears was just plain brittle and not harmonic in the amp. I think these would see better use for a stereo hi-fi system. Sorry Mesa. :?