Postby eenaoo » 29 Oct 2020, 14:52
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
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Attachments
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- 202007161 6100 Bias.xlsx
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