A few random observations...

[splawndude]

So I went to open up my amp the other day for several reasons and thought I'd share a few observations.


tl;dr version
1.) I think my bias probe is borked so if I get desperate, I will buy the Weber 2 socket version.
2.) The amp runs fine (on the gain channel) with V1 pulled FYI
3.) I have a small crack in my plexiglass


Firstly, I wanted to check the bias of my amp. Not because it sounded bad or 'off', it has just been awhile and I figured I'd go ahead and at least check the bias while it was open. Well, either my bias probe is bad or I have some other issue because I was getting funky numbers. I know the multimeter is good because I tested it against some known sources. If anyone reads this and knows how to test a bias probe for functionality please let me know. Anyway, amp sounds fine so no worries there.

Second observation is that I remembered I had all JJ 12AX7s in the preamp (and PI). I had forgotten I put the Phillips 5751 in the PI of my Rectoverb. I remember doing that thinking it would tighten things up in the Mesa a tad and provide a bit more clarity in an amp that already has enough gain to share with 3 other amps. I do believe I will leave it there. It did get me thinking that I need to go ahead and get some more pre amp tubes so maybe a New Sensor something and a Telefunken. I've heard so many good things about the Telefunkens. That would go in the Splawn though. Also the Mesa is running EL34s as well. Old new stock Mullards.

OH SO....I was doing some general tube reviewing and I remembered that other thread here where we were talking about preamp tube placement and purpose etc etc. So I pulled the V1 tube and it made NO difference in the tone (I have a 4 pre QR). I was pretty sure that pulling V2 would kill the signal chain and it did. What I was not 100% sure on is what would happen if I pulled V3. Well, as you'd suspect, it killed the signal. Didn't bother doing this little experiment on V4 (I have a 4 pre QR) because that is the phase inverter and its job is to feed the signal to the power tubes. Not only would it not work, but could potentially do damage (although I don't know what that would be). This was all relating to the gain channel. Clean channel would not work pulling V1 as you'd guess.

Lastly, and the reason I took the amp out to begin with, is this weird 'crack' in the plexiglass front panel. It is not very long (well it is long to me), but I meant it doesn't run the length of the chassis. I still can't figure out what it is or how it got there. I take very good care of my amps and my amp has been sitting on top of its cab unmoved for maybe a year. See below.

[gtrjunior]

When we were having the preamp tine discussion it that other thread a while back I remember one of the points was how V1 would affect the gain channel.
I’m not sure if I had posted it, but I emailed Scott about it. His response went something like this:
“Your Quickrod is a 2016 model. The circuitry for the clean channel and gain channel are independent of one another. The tube in V1 has no tonal affect on he gain channel”.
From what I have read, at one point the circuitry was not independent. I believe that is when the owners manuals were made. Iirc, the manual says something about one side of the V2 tube being part of the clean channel’s gain staging. BUT...I’m not exactly sure when Scott made those circuitry changes.

Can you describe any better exactly what your bias probe is doing? Or not doing? What probe do you have?

[briman]

That looks like a scratch? No? How the heck did that crack or scratch get there if you never move you amp? Bizarre...

[splawndude]

Yeah, tomorrow I will re-review the whole V1a, V1b, V2a, V2b thing.

I 'think' that at least in my amp, 2008 I believe, along with the way that tube description in the owners manual was saying that for the clean channel the signal would go in V2 and then go to V1 and then on to the phase inverter so that was 'separate' from the gain channel in some regard.

With the original 3 pre amps you only have 2 preamp tubes creating gain right, because the 3rd is your phase inverter and the phase inverter is technically part of the power section. So in those amps, everything was coming in V1 (our V2) and therefore, not as 'separate'. The idea of the 4 pre QR was for a better clean channel, not more gain.

As far as the meter goes gtr - I don't know. Hard to explain. I mean it worked, but the numbers didn't look anything like I'd expect. Like for plate voltage I would it expect it to start in at like 300mv and slowly climb up and then stabilize. It was something like 025. Made no sense. Milliamps on the cathode was 4 or 5. But things jumped around or gave inconsistent measurements which led me to believe it was the probe. The meter is fine. If I put it on a 9v battery is jumps to 8.5v (for example) and sticks hard.

My probe is a Weber single probe. $60 job. Never played the guitar while the probe was connected. They are such simple devices I can't imagine there is much to go wrong. I also seriously doubt it is the amp.

Brian - no idea. It doesn't feel like a scratch. It is smooth. It is almost like the 'scratch' is on the backside of clear coated front. I'm wondering if maybe the last time I put the amp back in the shell (over a hear go) it got scrunched or crammed against the headshell. You'd think I would have noticed though.

IIRC - Jim Marshall started this plexiglass trick because it was a great way to put printed indicators and such on the front of an amp with the added protection of a non conductive material to protect the player. (Plexi)Glass does not conduct electricity. At least that was what I thought it was all about.

[briman]

My multi meter has a ton of settings and sometimes i put it on the wrong one. Maybe you had it on wrong setting?

[gtrjunior]

What I was driving at in my earlier post concerning the manual’s description of the gain staging, is I think that info is outdated.
I’m guessing that at some point scott made the independent circuitry changes but the manual wasn’t updated to reflect that.
I could be wrong but if the clean channel is separate from the gain channel (which yours seems to be) then the manual is incorrect.

[splawndude]

FYI - I think I've got everything back to normal for the most part

I'll report back tomorrow on my more recent observations.


GTR - I may be missing something but I feel like the manual and what Scott are saying are the same. At least for the 4 pre QR on up. For the 3 pre QR - I don't know...wouldn't make sense.

Scott was saying the circuit updates on the 4 pre gave the clean and gain channel their own separate paths. From what I can tell, that may be true. I mean, they share V2a. They obviously share the Phase Inverter. What happens in between would require someone who knows amp building and how to read schematics.

The 4 pre QR as we know added a new V1 and bumped everything else down the line.




So signal goes into V2 first regardless of what channel you are on. For the clean channel your signal then goes to V1 and then on the PI. A bit counter intuitive but that's the way I'm reading it. For the gain channel you signal starts at V2a, then V2b, then V3a, then V3b and then the PI. Remember guys, every preamp tube in a Class A/B amp like this has 2 sides. 'A' and 'B'. They are called stages and you'll often hear the term 'cascading preamp'.

[splawndude]

Another way to say it would be like this (for the gain channel)

V2a - 1st gain stage
V2b - 2nd gain stage
V3a - 3rd gain stage
V3b - 4th gain stage
PI


The clean channel goes

V2a - 1st gain stage
V1a - 2nd gain stage
PI

That's why when I pull the V1 tube while on the gain channel nothing happens. What this means as far as tube rolling is important because if you want really really clean cleans then maybe something like a 5751, 12AT7 or even the cleaner 12AY7 type tube would be best for V1. Probably something like a Tung Sol or EHX for some bright clean cleans. If you want your clean channel to have more grit - maybe get JJ or NOS something. But also for a really clean clean your V2 tube is going to affect things somewhat as well as the PI. So it's a balance there. I rarely play cleans and when I do it is going to be on my RectoVerb - because let's face it, they are super nice there. Fender Bell like.

What's really interesting though is that when I looked up the Competition and Pro Stock (single channel gain only amps) - the tube layout shown in those manuals is exactly the same as the Quick Rod. So. There's that.



Also, why no use V1b?

[gtrjunior]

So what happens if you are on the clean channel and pull V2? According to the manual it shouldn’t work since the path starts at V2.
If it does work then I’d surmise that the manual is incorrect (?)

When I had asked scott about it a while back I had been rolling tubes at that time. My question to him was something about the tube in V2 affecting the tone on the clean channel and vise versa....that’s when he told me they were independent.
But, like you said...we aren’t techs so I suppose we are just guessing!! Lol

[splawndude]

gtr - I think we are on same page. And the owner's manual may indeed be wrong or misleading.

I think I see what you are trying to say now. I think maybe we were just talking past each other. Yes, if you pull V2 you will get no sound regardless of which channel you are on. V2 is a very important tube in this amp because that's where your tone shaping begins (outside of the power section, choke, filter caps, and the rest of the preamp etc).

For this reason - I'd consider the signal 'shared' up until this point. From V2 on - I'm guessing the signal takes a different path depending on what channel it is in. Then, those signal paths come back together at the Phase Inverter V4 stage.

So - they are only separate for a little while (not uncommon). Now you are deep inside the amp with your tone on a printed circuit board (also not uncommon) passing through resistors and capacitors etc. A combination of resistors and capacitor types I might add that Scott has selected to give his amps his signature. Splawns are not direct copys of Marshalls.


In the OP, I said this (regarding the Overdrive Channel):

"So I pulled the V1 tube and it made NO difference in the tone (I have a 4 pre QR). I was pretty sure that pulling V2 would kill the signal chain and it did. "

So to me, that is pretty consistent with the manual.

There really is no way to make completely separate channels. I believe it has been done though but the amp would be very heavy because it would have 2 power transformers and 2 output transformers in addition to completely separate everything else including knobs, dressing, wiring, tubes, sockets, blah bla blah



Sorry for the long post

tl;dr

This depends on your definition of 'independent circuits'. I don't think the manual is wrong necessarily for a 4 pre QR. Misleading maybe. V2 is super important and it needs to be there. It, along with the PI, are shared by both channels. But, there are two sides of each preamp tube so the signal never really crosses to the other side. If you were to stomp on the channel switcher as fast as possible while playing a note, your signal would jump from one side of V2 to the other, but never cross (V2a and V2b).

The manual for the Competition is def wrong.

[gtrjunior]

Ok, I understand now...lol

[splawndude]

Hope I didn't over do it LOL


I'm not an amp tech - I know just enough to be dangerous.

I'm going to upload some Pics a bit later since I tore everything down and built it back up again.

[gtrjunior]

No...the way you explained it made it sink in. I really do understand what you’re saying.
Besides, full explanations are how we all learn!!!

[splawndude]

That's cool. Glad it helped.

As far as the tube probe goes I don't know what happened there. I'm guessing I had a dirty pin socket or something was not in all the way. I did learn that my amp is biased low regardless of what power tubes I put in. I could not get much above 26mA. I also discovered that some of my old Mullard EL34s are microphonic so they have finally bit the dust I'm afraid. They still sound really good though so I'm just going to ride them into the sunset.

And actually, the tube I have in V1 of the Splawn is a Russian made 12AX7b that is Mesa branded and came with my Rectoverb. The getter (top of glass) is all messed up on that one and I think it has some serious issues. Probably why I put it in V1 of the Splawn. I remember the day now. I figured I'd get more bang for my buck by putting the Phillips 5751 in the PI of the Rectoberb. So probably did that and then put the Mesa tube in the Splawn after putting a regular JJ 12AX7 in the Splawn's PI.

What I have left now is a no-label Chinese 12AX7b and a Groove Tubes 12AX7C (also Chinese).

I need to mess around one more time with the 12AX7C but it is clear to me that I need to do some tube purchasing. You know I will keep you posted.

Also - that 'scratch' is still there but much more faint now. I think I 'rubbed it out'. It looked more like a crimp on the backside of the plexiglass maybe and I literally rubbed on it real hard. You literally cannot see it if at the right angle.


Bottom line. As I preach all the time - when you are in doubt tear everything down and build it back up again. I like to do this anyway about once a year. It is good for my brain and my amp. I clean up all the dust and find all the picks that my cat knocked behind the amp wall LOL. I also put my volume knob back in the FX loop because you guys have been raving/talking about it a lot.

Now for a the wall of pics

[gtrjunior]

Is that the Carl what’s-his-face volume box I see there?

Also....the power tubes. Next time you open the amp up, locate the R27 resistor and take a bunch of clear pics of it. You’ll want to have the pics clear enough to make out the color coding on it.
Then email Scott and tell him you’re having a hard time gettin the bias high enough and have him send you a set of his tubes that he would have installed during the build. This way you’ll be able to get the bias up a little more.
When I recently blew out that tube I asked scott about the various posts here that I saw regarding people not being able to bias up without swapping that resistor.
In my case I have the larger size resistor so that bias issue didn’t apply to me. But it sounds like it does apply to your amp.
I know that Chinese Splawn tubes aren’t your favorite but at least you’ll bias up and get the optimum tone. I’d also ask Scott if he could help you determine what aftermarket tube(s) might be suitable in your amp. The Valve Queen might be helpful here too. The key is that resistor value. It’s going to determine what bias range aftermarket tube you’ll need. Then you’ll know each time you order from somewhere.
“I’d like a quad of matched Tung-Sol EL-34 in the 24-26 range” for example.

Try cleaning your probe and seeing if that helps. From what I’ve read, it is basically a 1 ohm(?) resistor and that’s about it. I suppose that resistor may be no good.