MkIIB - pull gain works in reverse...???

[McBarry]

OK, so I'm comparing my IIB's, and notice one of them DROPS volume when the gain boost on m/vol is pulled..

I check the other one, and then check the IIA and yep, they both INCREASE gain when pulled..

Hmmnn, what gives..

Discounted the switch as culprit and also checked shorting between contacts, not the gain f/sw jack, not the 33k resistor or the .003 cap..
"What the hell is it", I'm thinkin'...

Finally desolder the switch on 2 amps and confirm the switch is backwards internally, ie: pushed in should be closed and pulled should be open.
BUT on this little sucker it is CLOSED when pulled, and open when pushed.

Thus when knob is in, there's max gain and virtually no tone controls, when pulled the amp is quieter and the tone controls work nomally..

How bizzare.....

 

[McBarry]

just for closure, the pot + switch look incorrect when finally removed today.
Pot is 250k, not 1meg and switch is open when pushed in - it should be closed.

The pot/sw is old and the install is old, so I wondered if it was factory original, BUT I can't see why they'd put a 250k pot there, and even more can't believe a reversed switch would be installed.

I suspect just another crazy mod which was done for whatever personal taste and tone..
DB

 

[6br]

My IIa is the same as yours, not sure if its a 250k /1meg pot. Built 10/79
It dosn't appear to have been changed but who knows.

DI volume on the back was changed to a master volume along with a tube serial fx loop at some point.
Normal channel is pretty much spot on to a Fender TR, close enough that I wound up selling the TR :lol:

Give these a try, not your run of the mill MKII settings.
With the boost / mid shift engaged (lead channel) its similar to a 2204.

V 4.5, T 8 (Pull), B 10, M 10, MV 10(push...lol), LD 6(pull), LM 2.5
R 5, Pre 8, MV2 3-5

Cheers



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[McBarry]

Beejeesarse..
T8 B10 M10 R5- I could kill small animals at twenty paces with mine!!
And if I wound the Presence up to 8, I'd likely kill myself in the process!!

How's the bias with those GT's in there?

I've just spent a whole day using the IIA to check current draw for my (growing) collection of 6L6's.
Some real suprises there, not the least were then STR 415's and the STR 387's..

And an annoying hum had me stumped - culprit was V2 - a Russian Mesa 12ax7a - I read a thread talking about these but I think it was pertaining to Rectifiers from memory..

Tomorrow it's tone-chasing time with preamp tubes..

 

[6br]

With the mid shift engaged it drops the bass / treble. It's the only way I could run the EQ wide open and still get a killer crunch with no flub. Its a fine line...lol. I'm sure the 2nd MV helps as well.

I don't use either(boost/ms) on the clean channel.

I set it at 52V as per the owners manual with the factory pot. I tried it higher, but it didn't really make much difference. If it needs to be warmer I just turn up v1.

For tubes I have RCA's in 1/5, chinese in 2/3, JAN 12at7 in 4....sounds great.

What is the difference in tone between the IIA / B...... since you have both?

 

[McBarry]

Tone diff between IIA /B.. I'll try, but beware it's gets subjective very quick.. and I'm not such a good wordsmith.. but here goes..
Irrespective of tubes, and I've tried about a billion of 'em, both pre and power..
I find the IIA cleans more "crisp", and pick attack is definitely faster.
Lead channel on IIA is more "in your face" - it's a fast response, has a cutting edge, BUT has only a fine line before it starts to sound unnatural and raspy, if that makes sense.
It's VERY responsive to volume ie: mine needs tweaking when volume changes considerably - more so than either of my IIB's. Maybe it's the EVL 12L.
The IIB's are definitely a "warmer" sound - maybe more compressed, and are not as finiky to dial specific sounds.
The IIA has a god-awful pop when channel switching - well documented and as yet not completely diagnosed or resolved.

Being super-subjective, the IIA seems more like an unrefined beast - it'll punish you quickly if poorly set up, but when the "sweet spots" are found it's like your best friend.

Sure the IIB's also can emit some truly awful sounds if provoked, but I find both of them wider in their scope / range of "friendly" zones.

Hope that makes sense to you, regs Dave

 

[6br]

Thats my experience as well with the A. Big Fendery cleans are easy, the lead on the other hand is very interactive depending on the volume your playing. V1 adds body, while the LD adds topend. Volume adds more of everything...lol. Definately a tweeker but worth the effort. :wink:

 

[McBarry]

And also I forgot, my IIA doesn't like hyper-gain tubes in V1. That is, it muddies up the rest of the food chain, dep on how uber-gain I go. EG, older RFT's are rich and silky, but anything super new and I gotta keep input gain around 4 otherwise it can get edgy.
Esp if bigger stage and I'm ramping up the vol, and hey, who doesn't..LOL.
Whereas I can put a nuclear device in V1 of the IIB's and they seems to cope better.
It's also highly possible the IIB's have tiring coupling caps, and so are naturally attenuating some signal.. Another rainy day job with the sig gen and the CRO..

 

[McBarry]

Also a question to IIB owners, hopefully with the RP-8B preamp board.
How is your lead drive pot connected?????
This one was strange - top lug - to earth.
-middle lug - incoming signal from board AND shielded wire going straight off to V3 pin7.
- bottom lug - not connected. And also weird, looks to have had no solder on it ever.

But how is the pot working if the supply also goes off to the evential recipient..
I don't follow this..
Dave