Tube/Valve selection

[eenaoo]

Hi,

I usually use my AVO valve tester to measure the transconductance/mutual conductance for each tube I buy new. They vary a lot and are in the 1.4-2.2 mA/V range. Do any of you characterise your tubes and select the high/low gm tubes for certain position in the 6100? Or select the best balanced ones, 12AX7 with dual anondes, for certain positions?

I do however use the valve tester to sort the EL34 output tubes. I sort the 4 "matched" tubes in 2 pairs with similar anode/plate current. I use the voltage drop across the output transformer (anode to centre tap) to check the bias for each pair.

Regards Arne

[surfnorthwest]

Man you could write a book on what tubes to use in the 6100.

I find it best to simply make sure V1 has a uber quite tube in that slot. I also look for low microphonic tubes before I look to high gain tubes, the 6100 has more than enough gain.

[SIKSTRINGKING]

I never realized how many different combinations of tubes were possible for the 6100. I would really enjoy chatting with one the experts on this matter. If anyone has time for a crash course in what is what, I would be forever in your debt.

[SIKSTRINGKING]

I have the 6100 le by the way. Half stack with an orange bottom cab with v 30’s.

[Anitoli]

For a 6L6 head the 5881 Sovtek is best bang for the buck.
Preamp tubes i am liking the EHX 12ax7's best right now followed by Tung Sols.
Went through a stage of using pulls that tested good and these higher gain amps they don't cut it, the CP tubes are much better.

[eenaoo]

Then we are 2 Silkstringking. I also have a LE that I bought new with two matching cabs. I had a shootout regarding tubes a while ago. I tried different preamp tubes in the amp. These were mainly current productions tubes and a few original BRIMAR tubes I had in 1967 Super PA. The current production tubes I tried were:
- Shuguang AX7B (ECC83) from Aliexpress/ebay
- Shuguang 7025 from Aliexpress/ebay
- JJ 12AX7 (ECC83S) from Tube-Town
- Sovtek 12AX7 (ECC83 LPS) fromTube-Town
- Tung-Sol 12AX7 (ECC83) normal selection from Tube-Town store

To be honest I did not hear that much difference. It is difficult to do a very good comparison test and also not to be biased since you know which tube you are testing and what other have written on the forum. I ended up with this following configuration, but this was very biased on what others have written

7025 for V1, the tube for CH1
Tung-Sol for V2, the tube for CH2/C3 first stage
Sovtek for V7, phase splitter
Shuguang AX7B for V3-V5, CH3, and V6,CH2/CH3. Here is used my AVO MKIV tube tester and picked the AXB with the highest transconductance for V6. Since CH2 is what I use.

These are the specifications on the tubes that I measured. Note that the solvtek is the one with highest "gain". Also some of these tubes are poorly matches, i.e. each anode transconductance differ. Old NOS tubes like BRIMARS where spot on acording to AX7 specifications.

Tube spesification preamp
Position Type A1 gm [mA/V] A2 gm [mA/V]
V1 Shuguang 7025 0.6 0.9
V2 Tung-Sol 12AX7 1.8 2.0
V3 Shuguang 12AX7B 1.5 1.7
V4 Shuguang 12AX7B 1.5 1.6
V5 Shuguang 12AX7B 1.5 1.6
V6 Shuguang 12AX7B 1.9 2
V7 Sovtek 12AX7LPS 2.1 2.08

For the powertubes I went with Shuhuang EL-34M from Aliexpress/ebay and biased them to 60%. Here I paired two two tubes that had the lowest and the higest bias current together to even it out, since it is not possible to bias them individually. I always set the bias by calculating the anode/plate current from the OT voltage and the OT resistance, i.e. the voltage and reistance from the anode to the centre tap on the transformer. These are the power amp tubes biasing measurments for my amp:

Valve v1 v2 v3 v4
Plate voltage 469 469 469 469
OT resistance 14.2 14.2 14.9 14.9 Ohm
OT voltage 950 950 955 955 mV
Plate current 33.5 33.5 32.0 32.0 mA

Dissipated power 15.7 15.7 15.0 15.0 W
% of maximum 25W 62.8 62.8 60.1 60.1 %

Tube pair matching
% difference to mean 4.3 %

Note that V1 and V2 is a pair and V3 and V4 is a pair with the same part of the OT etc. i.e. the some OT voltage and OT resistance. But each tube will draw slighlty different bias current.
I made an excel sheet calculating the values above if anyone wnat a copy (That is if you are not familiar with Ohms law)
This is what I measured for the power tubes in my tube tester:

Tube spesification power amp
Position Type A gm [mA/V] Plate current [mA]
V1 Shuguang EL34M 10 69
V2 Shuguang EL34M 9 64
V3 Shuguang EL34M 9.5 66
V4 Shuguang EL34M 10 68

Hence I paired the 69mA with 64mA and the 66mA with the 68mW to get an even current draw between each pair.

Arne

PS! I added my Excel sheet wiht the caluclation that I use when I bias my amps