#1
Hello All,

My buddy has a 3120 and I told him we can swap out the EL34 valves to toy around with the sound if he wants to.
We plan on swapping the valves, setting the bias to approx 37mA, mic the cabinet and record each set.

I have a background in electronics and my hobby is refurbing old tube radios etc...
I purchased all JJ valves, 4 MATCHED of each(the matching option was at no extra cost, so why not).
E34L
6l6GC
KT77

I viewed the block diagram for the head.
My questions are specific to the 3120 itself and I don't have the head in front of me at this time:

1) Has the schematic for this been released? I have searched and found nothing at this point. Is there a schematic for a different head(JSX, XXX) that is similar pertaining to which valve does what in the circuit?

2) What are the functions and designations of the 4 valves? Looking at the layout of the valves, starting from left to right with the bias pot and valve switch to the left. Example: V1 clean, V2 crunch etc...

3) Which valve is the bias read from? Does it matter? I will be using an external bias probe. Looking at the layout of the valves, starting from left to right with the bias pot and valve switch to the left.

4) Mixing valves? Possible? Viable? Example: Having the E34L valves in "V1 and V4" positions and KT77 valves in "V2 and V3" positions. Again, I do not know the function per position and do not have a schematic. I do understand that, depending on the electrical layout of the valves, it may not be a good idea to mix valves do to the difference in electrical characteristics of the valves when it comes to properly biasing.

Thanks in advance!,
Adam
#2
The XXX is the same as the 3120, the only difference being that the XXX comes stock with 6L6's. A schem for the XXX is the last link in this list:
http://www.schematicsunlimited.com/p/peavey/

I really wish I could have been more helpful but this is all I got, man.
Last edited by Invader Jim at Feb 15, 2014,
#3
Quote by Invader Jim
I really wish I could have been more helpful but this is all I got, man.


Jim,
ANY info is a help!!!! If the XXX is the same except for the tubes.... I can answer some of my own questions!!
I'll check it out. Thanks Man!


All other viewers,
Please, still feel free to post your info, thoughts and experiences!

Thanks,
Adam
#4
Don't rely on the bias test point on the back of the amp (it does not read correctly) to accurately set the bias you will need to use a bias probe or test the actual pins of the power tubes. I don't know the technical reason for this, but I have been told by my brother who has been a EE for 30+ yrs and builds amps. And many others here have mentioned the same thing.
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate
#5
Quote by Robbgnarly
Don't rely on the bias test point on the back of the amp (it does not read correctly) to accurately set the bias you will need to use a bias probe or test the actual pins of the power tubes. I don't know the technical reason for this, but I have been told by my brother who has been a EE for 30+ yrs and builds amps. And many others here have mentioned the same thing.


Robb,
My understanding is that those test points read voltage not current.

Voltage readings are inverse of the current and do not change proportionately.

Paying attention to current of the valve is more important.

Someone please correct me if I am wrong on this. Thanks!

Adam
#6
All,

Looking over the scheme from the link Jim sent me... and if it is the same as the 3120....

V5 and V6 need to be matched.
V7 and V8 need to be matched.

Mixing 6L6 with KT77 or EL34 is not a good idea do to the major difference in biasing.

Thanks!
#7
Quote by pudland
Robb,
My understanding is that those test points read voltage not current.

Voltage readings are inverse of the current and do not change proportionately.

Paying attention to current of the valve is more important.

Someone please correct me if I am wrong on this. Thanks!

Adam

That I don't know, I was just letting you know not to try and read mA from them, it will be wrong
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate
#8
Bias jacks on amps usually have you read the voltage across a resistor on the cathodes of the power tubes. Say the resistor is 1 ohm and the voltage across it is 37mV, you can calculate (using I=E/R) that the current through that resistor is 37mA. Using a 1 ohm resistor lets you read the current directly on a digital voltmeter so you don't have to calculate things.
Last edited by Invader Jim at Feb 15, 2014,
#9
So....

Here's what I decided to get to play around with....

Yes, some will say its an excessive waste of money BUT.... We are experimenting.

Power Tubes
JJ-E34L JJ E34L / EL34 Power Vacuum Tube QTY:[4]
JJ-6L6GC JJ 6L6GC Power Vacuum Tube QTY:[4]
JJ-KT77 JJ KT77 Power Vacuum Tube QTY:[4]

Desc
[Matching (up to 9 tubes)]: JJ KT77 Power Vacuum Tube - QTY:[4]
[Matching (up to 9 tubes)]: JJ 6L6GC Power Vacuum Tube - QTY:[4]
[Matching (up to 9 tubes)]: JJ E34L / EL34 Power Vacuum Tube - QTY:[4]

============================================================

Preamp tubes
JJ-ECC832 JJ ECC832 / 12DW7 Preamp Vacuum Tube QTY:[4]
JJ-ECC83 JJ ECC83s / 12AX7 Preamp Vacuum Tube QTY:[4]
SO-12AX7LPS Sovtek 12AX7LPS Preamp Vacuum Tube QTY:[2]
ECC803S Matched and Balanced QTY:[2]

description
[Balanced Triodes]: Sovtek 12AX7LPS Preamp Vacuum Tube - QTY:[1] -- this one is going in the phase inverter spot only --
[Low Noise & Microphonics]: Sovtek 12AX7LPS Preamp Vacuum Tube - QTY:[1]
[High Gain]: Sovtek 12AX7LPS Preamp Vacuum Tube - QTY:[1]
[Low Noise & Microphonics]: Sovtek 12AX7LPS Preamp Vacuum Tube - QTY:[1]
[Low Noise & Microphonics]: JJ ECC83s / 12AX7 Preamp Vacuum Tube - QTY:[2]
[High Gain]: JJ ECC83s / 12AX7 Preamp Vacuum Tube - QTY:[2]
[Low Noise & Microphonics]: JJ ECC832 / 12DW7 Preamp Vacuum Tube - QTY:[2]
[High Gain]: JJ ECC832 / 12DW7 Preamp Vacuum Tube - QTY:[2]


Without going into looooooong explanations of why I got so many and got the same tube with different gain and microphonics etc.... We ALL have different ears. I've scoped out many forums, read many conflicting posts and read many where people were going at each other pretty hard. Well, not in this thread we wont.
Whether it be someones opinion or hard fact. There is no reason for being a dbag.

This will be a long process because:
1) We plan on recording(mic) each time we change a tube(s).
2) Carefully bias each power tube.
3) Same as #1 & #2 after a burn-in period.
4) Call it quits if we feel we had a few too many during the process.
5) We plan on documenting amp setting for each tube to go along with the audio recording.

The power tubes arrived today and the preamp/effects tubes should be here by the weekend.

Please leave your thoughts and/or experiences with the above tubes or combinations of tubes.

Thanks!
Adam