Tube measurement question...

I recently upgraded the 300b tubes in my Manley 300b pre-amp.  I was running the Shuguang 300b-z treasures and on the recommendation of a friend I decided to try the PSVANE ACME 300b.  As many have already reported, my results were similarly positive, with the PSVANE tubes providing more punch, better dynamics and generally an overall better listening experience.  My question is regarding the tube measurements.  And I think plate current in particular...which I believe is what they are measuring and notating on the outside of the boxes and designating as Ia = xx mA.  The box on my Shuguang tubes says 65 mA.  My PSVANES measure at 80 mA.  I guess I have several questions...first, what does it mean exactly?  Is it telling me what the tube is doing or is it describing a limit?  Then I guess I'm wondering if the PSVANES, measuring at 80 vs I believe their spec of 60 that good/bad/or a no care?  I'm sure there will be variance in all the tubes produced within a certain tolerance...just wondering at what point, if any, it starts to matter, and in what way...

Apologies if I'm not asking the question properly or if I've left out some other key data needed to respond.  Pretty new to tube rolling and since this caught my eye I thought I'd try and learn something...



I believe that spec is idle current draw when biased to spec.   IP on the tube box is the current draw.  Gm is the transconductance of the tube 

The Psvane's higher plate current probably explains what difference you are hearing 

Matched pairs or quads will have those specs identical or as close as practically possible. 

the higher current is a function of the tube design.  So long as your power supply can handle the extra 20mA (it can) there is no issue.


Thanks, guys, so if I understand correctly then first it's not an issue because my preamp can handle the extra current.  I imagine that if my gear couldn't handle the extra current it would probably blow the fuse.  Also, it sounds like the plate current may be one of the factors influencing sound, and the higher current may be partially responsible for greater dynamics and fuller sound.  

@bh123 I have a different take on what those numbers mean. If they are handwritten, I would expect them to be a reading taken from a tester, which suggests that the Psvane tube is a bit stronger.

All tubes are built to a tube spec, so that if bias is adjustable, they would all be set to the same idle current. The idea of that number meaning anything else from what I'm saying here is a bit terrifying as a manufacturer. It would mean that there's no standard that the tube is built to meet.

Thanks...yes, the numbers are handwritten.  The spec sheet that I can view from the PSVANE site says the tubes are 60 mA and the handwritten numbers on the outside of the box say they are actually 80 mA.  But perhaps measuring the tubes under different conditions?

Those are testing numbers. The bias should be set based on the amp manufacturer’s recommendations. General rule of thumb is you shouldn’t exceed the recommendation, but going lower is OK and prolongs tube life. I checked my manual and (copied below) are their notes on calibration...basically no bias to set.  I don't hear any volume difference between left and right channel...everything sounds great.  I'm still confused now on what these meansurements mean.  Since they are handwritten it seems reasonable to assume that they are actual tested measurements of these specific tubes.  But what is is measuring?  Is it telling me how much current the tubes pull during operation?  Or is it telling me something about the limit of the tubes?  Is it good or bad that these are measured at 80 mA vs. the tube spec sheet (from PSVANE site) that shows a spec measurement of 60 mA?


CALIBRATION: Each channel is factory-set for 12 dB of voltage gain (1V in = 3.2V out ) via a pair of trimpots located on the main printed circuit board near the headphone jacks. These pots adjust the amount of negative feedback in order to exactly match the gain be�tween both channels. Only 8.5dB of Global Negative Feedback is used which can be con�sidered minimal. Unless you hear a channel-to-channel imbalance after say, changing tubes, you probably do not have to re-trim the levels. There is no other adjustment that needs to be made. There is no bias to have to set

FWIW here are the specs from the PSVANE ACME 300b site for this tube.  I was looking at the below spec for Ia...which is 60 mA...and then looking at the handwritten measurements on the box which said Ia = 80 mA and trying to figure out what that means about my set of tubes...


Uf(DC)………………           5               V 
If(DC) ……………                1.2            A 
Maximum Rating 
Ua……………                        480           V 
Pa………                                 40             W 
with cathode bias………     100          mA 
with fixed bias……………  70            mA
Direct Interelectrode Capacitances
input……………                   9              PF 
output……………                 4.3          PF 
grid to plate……………      15            PF

Static parameter
Ua……………… 300      V 
-Ug……………    60        V 
Ia………………   60        mA 
Gm……………    5.5       mA/V 
ri………………   0.7       KΩ 
u ………………   3.85 
Pout……………  6.6       W 

@bh123 I just finished writing a long explanation but an errant finger swipe wiped it out. (I hate that when it happens.) Let offer my non-expert interpretation of the numbers. The tube spec of 60 mA describes the current that should be found at the cathode of each 300B after the amplifier is well warmed up, but not playing music, it is the ‘idle current.’ When Atmasphere builds his amps, he designs his circuits to be capable of delivering 60 mA to the cathode of each 300B in order to bias it for proper operation. You can thinking of it as adjusting the trigger pull of a gun. The tube needs the proper bias current in order to be ‘ready to go’ whenever it senses the incoming signal.
Conditions can vary at the cathode, tubes have a break-in period, and during this time the bias current must be closely monitored. The cathode slowly erodes from electrons flying off the cathode to the plate all the time the tube is in use, so the current to the cathode needs to be adjusted occasionally (every month or two) in order to keep the amp working in top form. Many manufacturers offer individual potentiometers at each tube, with switches and a meter so that the user can check and adjust the bias current at any time. Your manufacturer has included an auto bias circuit, so you don’t have to check such things, the circuit takes care of it for you. The correct bias current for a 300B tube, regardless of who manufactured it is given in the ‘Static Parameters’ you list: it is 60 mA as specified.
Once the tube is operating, music or test signals passing through, the current will begin to vary in response to (1) volume knob or magnitude of the input signal, the frequency of the signal which may trigger, (3) different impedance, or AC resistance, at the load. The current must not get too high or the tube is damaged. This is where the ‘Maximum Ratings’ you list above the Static Parameters come in. Your tube should never exceed 100 mA of ‘Ki’or Kathode current if is is in a circuit employing Cathode bias, as yours does (and never more than 70 mA if the circuit is designed with fixed bias).
Due to the nature of tube manufacture, age of the tube, etc. there may be slight differences in the way to different tubes perform. The best way to grade tubes and provide a means for ‘matching’ pairs or quads, is to test them. A speaker is replaced with a dummy load, a constant frequency signal (often 1000 Hz) is input, ‘volume’ is set to a specified voltage range (voltage output as viewed on an oscilloscope), and the tube is warmed up and tested for the current at the cathode. Under test conditions, your tube measured 80 mA, that is why ‘80 mA’ is marked on the tube. It could then be matched with other tubes which also tested out to 80 mA and thus pairs and quads could be formed with some assurance that they would perform is a ‘balanced’ fashion upon installation.

I hope this is enough information to ease your anxiety. Remember, I’m no expert. I think I know how you feel. I’ve been there, I expect we all have been at one time or another. Your system and your tube is fine. If the tube starts glowing red or white hot, shut it down, if you hear popping sounds, or smoke starts rolling out from under the chassis, pull the plug pronto, otherwise sit back and enjoy the music.

Edit: The manufacturer may specify a different bias for the tube in his or her design than that specified by the tube manufacturer; follow the manufacturer’s recommendations (this is not a concern for you, @bh123 ).



I hate it when that happens. I quickly post… then do corrections… to make losses like that as infrequent as possible. 

Thanks for the great explanation.  Makes perfect sense...really appreciate it.

@bh123 Glad to know that I have been of service to you. As someone here says, enjoy the music and don’t worry about milliAmps, cathodes, or impedances for awhile.