Myles...
How do the Brimars differ from the stock Svetlanas?
Prowler,
I cannot really do a true comparison because I only have three of each tube and the Brimars are borrowed ... and ... they all spec and trace totally differently.
The Svets have a slower rise time and require more input signal to reach an expected output ... in easy terms ... the Svets are not as strong as the Brimars.
The Brimars I have are all over 1.2 milliamps at test spec and over 1580 on transconductance.
TC 1580 1.3 iK
GP 0.017 A side
GAIN 92.9 Gain
This above is the LOW Brimar ....
TC 1460 1.1 iK
GP 0.018 B side
GAIN 81.1 Gain
The above is the HIGH Svetlana
As you can probably see, the gain of the Svet is 10 points down from the Brimar. If these were used in an amp with a complex front end such as a Mesa Recto or Bogner the difference in 0.2mA in current drive would be the kiss of death in the Mesa or Boger as a second aspect.
So, I am not able to compare apples to apples.
When I test tubes I like to have different tubes at the same test spec and measurement otherwise you might as well compare a boat to a piece of pizza.
Sonically .... the Svet has less mids than the Brimar. The Brimar has much of the mid bump of the classic short plate ECC83s from England that were originally used in the Vox, Marshall, Selmer, Orange, etc., amps in the past.
The Svet is a nice tube and smoother in the midrange than the Brimar. It is a pretty articulate tube and nicer than the 12AX7EH that has the same numbers as an example. It is (the Svet), more linear than a 12AX7R2 (Sovtek 12AX7LPS) when used in the first gain stage but this is on test junk ... playing in an actual amp using actual ears can yield some different results when personal taste is tossed into the mix.
I currenty have three hand traced Ei long smooth plate 7025s (gray, not silver, plates). I had to go through 60 tubes to find three that I liked. I normally find these way too bright in a "Marshall" but the tone stack of the SRZ-65 lets me use these along with the presence control to get some really articulate tones and the front end compression can be held down. When these are pushed in a Marshall Plexi, JTM-50, or JCM 800 they normally squash really quickly. In the SRZ-65 or 6545 they seem to me much happier for longer.
TC 1460 1.0 iK
GP 0.014 A side
GAIN 104.3 Gain
This is the figure for the Ei in V1. The currrent is down 20% from spec but the front end of the SRZ-65 is very clean and I can give up a bit of current push (think of current as torque and gain as horsepower) because these have very low plate resistance and their gain can be higher than the expected 12AX7 "100" target.
This is the Ei tube currently in V2:
TC 1510 1.1 iK
GP 0.015 A side
GAIN 100.7 Gain
In V3 I am using an Ei also at the moment.
In the phase inverter I am also using a 7025 that was trace matched. In trace matching the two curves have to line up exactly like this:
Not like this:
These are photos of older tests ... I did not take any scope shots for these tubes. In the Not like this example you see the traces at lower signal input levels not well matched. In the first photo that is matched, I moved the camera ... so the line is not sharp but both traces appear as a single line without the bogus camera work.
The snapshot of the two sides of the tube used in V3, the phase inverter is:
TC 1470 1.0 iK
GP 0.015 A side
GAIN 98.0 Gain
TC 1460 1.1 iK
GP 0.0148 B side
GAIN 98.6 Gain
We are off 10% or so on current and 0.6 on gain. This is a pretty decent match static match wise but on the curve tracer when the tube is running it is matched over the entire operating range.
To get these three tubes (Ei) I had to go through 60+ tubes as a side note.