Latest JJ 803S info and test results

[Myles]

Just letting folks know that the current run on the JJ 803S long plate tested a bit differently than the last batch.

Expected Plate current 1.2mA
Expected Transconductance 1600
Expected Plate reistance 62.5K ohms
Expected gain 100

The above figures are design spec by RCA, Sylvania, RFT, Mullard, Telefunken, GE, etc. Tested at 250 plate volts as per spec with a 2 volt bias.

The actual test results were as follows:

Average plate current 2.2mA (it will run hotter than expected)

Average transconductance 2090 (more abrupt tone, faster reacting so to speak) Less input signal required for a given output voltage is another way of expressing this.

Plate resistance 39.2K. This is very low and the reason for the high plate current and low gain. This low plate resistance can also contribute to crosstalk between the two sides of a tube where the two sides are used for different purposes (which is generally the case) in some amps.

Gain on average is 82, about average today for production 12AX7 tubes. In past runs the gain was at least five points higher on average with many in the 90+ to 95 range.

Hopefully JJ will resolve this in future runs.

 

[jeremy]

Re: Latest JJ 803S info and test results

thanks for the update myles. you are the man when it comes to the stuff.

is there any position that these tubes could be good in? long tail pi or something?

 

[Myles]

Re: Latest JJ 803S info and test results

thanks for the update myles. you are the man when it comes to the stuff.

is there any position that these tubes could be good in? long tail pi or something?

This is very much personal taste but I do like them in V1 or the first gain stage of a lot of amps, they are not too bright. They also make nice phase inverters for those that like the long plate feel vs a short plate PI feel.

 

[Guitar Guy]

Re: Latest JJ 803S info and test results

This is very much personal taste but I do like them in V1 or the first gain stage of a lot of amps, they are not too bright. They also make nice phase inverters for those that like the long plate feel vs a short plate PI feel.

What is the difference in the feel of long plate vs short plate for phase inverters? I've only used short plate, but long plate might be interesting.

 

[Myles]

Re: Latest JJ 803S info and test results

What is the difference in the feel of long plate vs short plate for phase inverters? I've only used short plate, but long plate might be interesting.

A LOT of this has been written up on my GAB website in detail.

Here is a link to one piece of info. I also wrote a lot about this in the deluxe edition of The Tube Amp Book that was from GT.

http://www.guitaramplifierblueprinting.com/files/Phaseinverter.pdf

 

[CaughtLikeFire]

Re: Latest JJ 803S info and test results

thanks for the info.

I just picked up a few longplate 12AX7s to use as PIs - glad I went with Sovtek LPS this time since the latest JJ are so far out of spec.

 

[Myles]

Re: Latest JJ 803S info and test results

thanks for the info.

I just picked up a few longplate 12AX7s to use as PIs - glad I went with Sovtek LPS this time since the latest JJ are so far out of spec.

The LPS were more inconsistent than the current 803S and varied 75% from one another. But in the end they are closer to 12AX7 design spec than the current 803S run.

Average TC was 1505 which is much closer to spec than the current 803S but they ranged from 0.6mA to 1.5mA in plate current, a wide spread. The again, 1.5 is much nicer than 2.0mA when you are expecting 1.2mA.

55k ohms plate resistance on average is better than the 803S by quite a bit and this helped keep the TC and plate current closer to spec. Spec is 62.5K at 250 B+ and 2 volt bias.

Average gain was 82.69 which is in line with the current 803S but it gets this in a smoother way, smoother curve traces and less heat.

 

[scottish]

Re: Latest JJ 803S info and test results

hey myles

i know this is slightly off topic for this thread but since you are a bit of an amp guru i wanted to get your take. Blues jnrs and the factory hot bias vs Billm mods to cool the bias. Ive heard lots of arguments on either side...im just wondering what your view of it is, if you have one of course.

 

[Myles]

Re: Latest JJ 803S info and test results

hey myles

i know this is slightly off topic for this thread but since you are a bit of an amp guru i wanted to get your take. Blues jnrs and the factory hot bias vs Billm mods to cool the bias. Ive heard lots of arguments on either side...im just wondering what your view of it is, if you have one of course.

I have no thought on this. These are fixed bias amps rather than cathode biased so if I had one I would just put an adjustable pot in there and adjust to about 60% ID depending on plate voltage and leave it around there.

I am sure there are camps on both sides of the debate but in the end trust your own ears and what you prefer. There is really no right or wrong when it comes to music or tone for the most part. I say, for the most part as I played accordian from the age of 5-8 and tone wise it never made me smile. Laugh yes. Smile in pleasure ... no.

 

[scottish]

Re: Latest JJ 803S info and test results

I have no thought on this. These are fixed bias amps rather than cathode biased so if I had one I would just put an adjustable pot in there and adjust to about 60% ID depending on plate voltage and leave it around there.

I am sure there are camps on both sides of the debate but in the end trust your own ears and what you prefer. There is really no right or wrong when it comes to music or tone for the most part. I say, for the most part as I played accordian from the age of 5-8 and tone wise it never made me smile. Laugh yes. Smile in pleasure ... no.

Thats actually what i did. I didnt measure anything, i just put in a pot that would give some range and turned it until i heard something i liked, lol. I was just wondering if there was any technical reasons behind either side of the argument. I know the guy from eurotubes is adamant that those amps have to run hot to sound good.

 

[Myles]

Re: Latest JJ 803S info and test results

Thats actually what i did. I didnt measure anything, i just put in a pot that would give some range and turned it until i heard something i liked, lol. I was just wondering if there was any technical reasons behind either side of the argument. I know the guy from eurotubes is adamant that those amps have to run hot to sound good.

1. I disagree that these have to run hot to sound good but that is subjective.

2. I do agree with you turning things until you like them ... and as long as the output tubes don't redplate you are fine and dandy.