Help needed for preamp valve positions and gain sequences.

[OryCheyne]

Hi everyone
I have a problem with my MkV channel 2 sounding weak as if there is a second note breaking up in there. Most obvious when playing single notes in Edge and Crunch.
To recount, the amp sounded great like a normal V got awesome tone in every channel. Then developed a buzz in one of the power tubes, replaced sus tube with a matched tube same colour. Sounded great for a around 4 hours then one time channel 2 developed the stated prob. Swapped good power tubes through each posi with no improvement, back to original power tubes.
I cant actually play at the moment on account of an injury, but I can now look at swapping preamp tubes to get it back into shape. I guess my first and most obvious step is to change V2.... and I know I should just get in there and swap these out and any others required to get it sorted....but here's my question

Page 62 of the MkV manual shows each preamp tube having two different gain stage functions for the various channels.
Do 12ax7 tubes have two signal path inputs/outputs, or just one input one output?
If only one input and output, does this mean that the signal gets reintroduced in with the previous gain stage? hope this makes sense. For example V1A and V1B (and V3A and V3B) states each are gain stages for channel 1 twice???

Reason for asking is that I have a dozen various 12ax7's which I will be testing in my studio 22, before applying to my MkV. If the MkV has two functions for each valve I don't know if the studio 22 will test these tubes out properly.

Cheers.

 

[elvis]

Yes, a 12AX7 has two independent amplifiers in it, A and B. It is a dual 6AX7.

 

[OryCheyne]

Thanks for the response elvis.
Every day something new. I can't believe I never knew this before.

Is there a best method for testing both A and B paths in 12AX7's?
Given each amp is different, I assume the method would be unique to each amp type.
Given this is a MkV thread, I guess we should focus on a method using a MkV.

For example, placing preamp tube in pos V1 would expect to easily hear whether the clean is good, which would confirm V1A. But then what would be the next step to confirm if V1B is also good, And obviously the tube requirements would differ for each position.

Is this a valid question? Has this already been discussed to death previously? If so how long ago, as I'm not having much success describing the search parameters.
Screamingdaisy - feel free to chime in and tell me to just swap tubes and trust my ears!

Cheers
Ory

 

[bandit2013]

The tube function chart may help but there are other things occurring in the Mark V that are not clear in the manual. Each channel and voice will have an effect on the preamp tubes that are common to all channels. The obvious is V2 which is only used with CH2. What is not obvious, change in operating characteristics of the preamp tubes by change in plate resistance and bypass caps on the control grid of the preamp tubes may cause a weak tube to create noise in certain channel modes vs others.

Preamp tube chain: (based on schematic of MKV) Excluded reverb circuit and FX loop.
CH2:
V1A -> V2A -> [CH2 tone controls] -> V1B -> V3A -> [CH2 master] -> 5BEQ -> V7a/b

CH3:
V1A -> [CH3 tone controls] ->V1B -> V5A -> V4B -> V3A -> V6A -> [CH3 master] ->5BEQ ->V7a/b

CH1:
V1A -> [CH1 tone controls] ->V1B -> V3A -> [CH1 master] -> V3B -> 5BEQ -> V7a/b

 

[OryCheyne]

That info is golden. Explains why some preamp tubes are better suited to specific positions. I hadn't considered that each channel mode put different stresses on its corresponding tubes. Only makes sense.
I got Ch.2 sounding better but not fixed. I ended up buying two new 12AX as all my old ones had different looking internal parts (I know that shouldn't be an issue, ok I'm just anally retentive - different story). Plus had been too busy learning new songs for a mates party - heaps of fun - but that's a different story too. I tried both new tubes in v2, one was crap straight up (labeled it for later stage only), the other was better but the sounds still weird. Basically I avoid using Edge or Crunch, when I do I need to dial the volume back and tone down. Same result with each guitar, single or dual coil.
I've decided to try some SP 12AX, I haven't ordered any just yet. If these don't work I figure it's probably a cap.
Has anyone had any experience with the SPs, they should be top spec right?

Really appreciate Bandit's explanation. Thanks to you.

 

[bandit2013]

I have done many tube rolls in the Mark V. This amp does respond well to different preamp tubes. There are however some tube positions that are susceptible to supply noise (sometimes caused by AC voltage heater circuits). Positions that use AC voltage heater circuits are V3, V4, V6, and V7. The most sensitive being V3 as it is situated early in the gain stage chain. This may explain why there is more hum in the clean channel vs the others as it uses both circuits of V3. Tubes that I found to be noisy in V3 (just about every tube made by Reflektor [Tung Sol, Mullard (12ax7 and CV4004) EH and Sovtek.] They may start out sounding great when brand new but will not take long before the hum invades the signal. So far the best results I have had in the preamp of the Mark V were the stock tubes. For starters, I would get a complete set of the Mesa Stock preamp tubes for you amp so you can hear it as it was intended to sound. I also prefer the Mesa SPAX7 in V1. It is the same tube as the Mesa 12AX7A but selected for certain characteristics and gain structure. As for power tubes, I have had great success with Ruby 6L6GCMSTR (almost same tube that Mesa uses) that were matched and selected for the Mark V (when ordering tubes, specify "for Fixed Bias Mesa, Mark V Combo" It should not matter if your amp is a Combo or Head, you should get tubes that do not rattle as much. I prefer to hear my signal at the speaker vs singing tubes that vibrate based on applied frequency. The last set of SED 6L6GC that I purchased a while ago were not best quality as they produce mechanical noise that can get annoying. I have yet to find a set of EL34 that are quiet (except for the Mullard EL34 and the TungSol EL34B [probably not suitable for Mark V in 10W mode due to low impedance] and the Gold Lion KT77.) The TAD6L6GC-STR sounded really good in the Mark V but the key post can break off the base easily and if the tube gets indexed in the socket improperly, watch the light show and poof... The same would apply to the power tubes and rectifier. I would consider replacing them too with Mesa. Note: the Mesa Rectifier 5U4GB is the same as the EH 5U4GB. Nothing special about the rectifier tube as it will not affect your tone as is the case with preamp or power tube.


I would look into the forum for settings. CH2 crunch is one of my favorites if set up just right.

 

[OryCheyne]

Thanks again for the great advice Bandit.
I had been holding off responding until my issue was resolved, but no luck so far.
I've tried swapping preamp tubes in positions 1 through 3 using different combinations with 2 new SPAX and one new stock, but I still haven't shaken the funny sound out from ch2 crunch and edge. I've tried dropping the height of my pickups, used different guitars, no change as yet.

I've yet to roll through the remaining tubes. Which I will do this weekend sometime.

 

[BlackBoxy]

Thanks again for the great advice Bandit.
I had been holding off responding until my issue was resolved, but no luck so far.
I've tried swapping preamp tubes in positions 1 through 3 using different combinations with 2 new SPAX and one new stock, but I still haven't shaken the funny sound out from ch2 crunch and edge. I've tried dropping the height of my pickups, used different guitars, no change as yet.

I've yet to roll through the remaining tubes. Which I will do this weekend sometime.

on the schematics , saturation stage of the ch2 is v2a triode , the follower stage is V1b !

ch 3 saturation triode is v5a !

on my mark IV i use NOS GE long plate for this break up fonction , ressued tung sol do the job, but other new productions tubes are awefull at this place!

for crunch sylvania old long plate have a nice breakup too!

good luck

 

[OryCheyne]

I'd hung this thread out to dry for a while.
Just wanted to close off, the problem appears to be due to my guitars specifically SSS Strats. But not all of them. My Tokai and Ibanez do make the sound, even after a setup and new strings. SRV American Strat no noise!

Really loving my TeleDelux through it at present, using extension 2x12 8ohm dual C90's plus the combo C90, sounds massive with the ported cab. Crunch is back but I'm enjoying it with gain down, inverse V eq., Mk1 & Extreme full gain the hotter the chassis the better.
Life is good.

 

[ATR1]

Hi everyone
I have a problem with my MkV channel 2 sounding weak as if there is a second note breaking up in there. Most obvious when playing single notes in Edge and Crunch.

Does this sound like a warble? Almost like an effect on it.
This is the issue I had in Dec. 2015 Drove me nuts sounded like a keyboard synth. guitar distortion. It took me awhile to figure out it was my amp. I have had issues in the past with my Ibanez guitars g string having a slight warble when playing single notes up around the 15-17 frets. I took it to a guitar tech. he told me the neck was bad on my JEM. Then I found a seasoned lutherier who found the problem. Once you hear it you can not unhear it. That ended up being a crack in the block that holds the string on trem. (For you Ibanez guys.) I play most of the time in channel #2 crunch, sound fantastic with a KOKO Boost hitting the front. I ended up re-tubing my amp with tubes from Mesa Boogie store. That fixed my issue. I should of replaced one tube at a time isolate which V slot that was because the issue came back yesterday. I am assuming it is a preamp tube. I bought a couple of new ones (tung.sol) just swap out and locate the v slot. I hope to do this today swapping one by one. I will post results. It has to be in the 1st 3 V slots because this where channel #2 sits.

 

[OryCheyne]

Hi ATR1,
Great description of the sound. Funny that it only occurs only on the g and that it came back. I also got i a bit on the b string. I think it's a mix between the frequency, break up of the channel, fact these are not wound strings, and pickups.
I'll be interested to see how your tube swap goes, because at present I have two guitars that need to avoid channel 2.
At the moment I'm too busy to be changing pickups as a solution... but one day.

 

[ATR1]

I ordered a tube tester the other day. I will test all my tubes. I did swap out each tube with no success at the moment. I am sure it is a pre-amp tube. I will keep you posted..

EDIT: I tested all my tubes with VT-1000. Appears all tube are good. Then a day later I plugged in and noticed a volume loss. I started swapping out tubes. V6 Loop went bad, and only 7months old. Replaced all good now. I am thinking that problem I had was from V6.

 

[ATR1]

Hi ATR1,
Great description of the sound. Funny that it only occurs only on the g and that it came back. I also got i a bit on the b string. I think it's a mix between the frequency, break up of the channel, fact these are not wound strings, and pickups.
I'll be interested to see how your tube swap goes, because at present I have two guitars that need to avoid channel 2.
At the moment I'm too busy to be changing pickups as a solution... but one day.

That problems (G-string) for sure was the crack in the block that holds the string in bridge on my Ibanez. Now a days I am in love with my Les Paul's Stds. so much I bought (2) The 2016 Std models are off the hook good. Going from a Ibanez to a Gibson LP definitely a learning curve and dedication.

 

[Thaymz]

Hi OryCheyne / ATR1

I am having this exact same problem with Edge / Crunch modes.

Unfortunately I haven't played too many other Mark V's to confirm whether its just character of the modes circuit, an issue with my amp(tube or otherwise) or something with my guitars. So figured it might be an amp issue given it happens on all of my guitars (4 of the 5 are ibanez with 5th being fender with floyd rose installed). So odd that it might be a guitar issue but not occur in channel 1 / 3 or mark 1 mode of channel 2?

I also thought it might be v2 but haven't gotten around to replacing any tubes.

I've noticed the problem is less noticeable when I raise the channel 2 master past 12oclock.

Are you convinced it was a problem with the string block?

 

[ATR1]

Yes on the G-string block. I saw it was cracked when I replaced it. I will plug in one of my Ibanez and check. I have not played any of IB in a while. I sit mostly on the crunch channel with a boost and do not notice any warble going on.
The weird nose I noticed ended up coming from V6 Loop. I would tube swap V1-7 and have only GTR > front of amp only. Maybe turn loop off and start swapping with a new tube.