At higher plate current.
For middle-high freq duty or with feedback?
Try to avoid those with uranium glass seal (the greenish one), they tend to be gassy.
You need to think about doing work with NON-linear devices.
rp is not a tube constant. It will be low at high current and high at low current.
500V 10mA, 5W, is "small" for a 25 Watt tube.
rp may be 10K at 200V 100mA or 500V 40mA. Note that this is the Positive Grid region-- it takes real work to drive the grid. We don't usually go here for "preamps".
And you specifically asked "10mA". Why? It just so happens that the 3C24 has a very odd "flat line" at 10mA. This implies a very high *dynamic* plate resistance, for small signals which do not vary from 10mA.
This is not wrong for a Power Amplifier. Transistors driving 8 Ohm loads will have small-signal collector impedance maybe 8,000 Ohms.
Pentodes same thing. 6V6 drives 5K load with 50K rp from 80V to 600V. What happened to "matching"?? Well, the pentode curves have a sharp knee-bend. The LARGE-signal swing possible is greater than you would guess from a single-point "rp" number.
I should have mentioned that my op points were approx 400 vpg at 8 mA; G(v) was about -3 v.
Thanks. It was not clear that HK24 is "same" as 3C24, but on overlay it does seem to be the same blueprint built and measured a different day. And the HK24 plot does not have that very odd flat-line at 10mA from 200V to 700V.
So 10K, somewhere in there, probably VERY variable because the tube is scaled for large power in tuned circuits, not small linear power.
Attachments
Ok, my 3c24 arrived...and I guess it is one of those cases:
Heated it up for a couple of minutes, started with a small 600 and raised slowly manually the -Ug from -100 to 0v...no current flow...suddenly a white light/flash, current goes through the roof... and the protection circuit of my tester stopped measurement.
Ok, next tried this with 300V...same behavior, at -15V white light, but now 3mA current...but only small change to 0V.
Next raise to 600V at 0V...only 2-3 mA.
I guess this is on of those many broken copies, right ? So, no curves...
Heated it up for a couple of minutes, started with a small 600 and raised slowly manually the -Ug from -100 to 0v...no current flow...suddenly a white light/flash, current goes through the roof... and the protection circuit of my tester stopped measurement.
Ok, next tried this with 300V...same behavior, at -15V white light, but now 3mA current...but only small change to 0V.
Next raise to 600V at 0V...only 2-3 mA.
I guess this is on of those many broken copies, right ? So, no curves...
A senior member of Diyaudio offered me to send some of his samples to measure them...how great is this ??? I am really impressed.
While waiting for them...I am looking into alternatives ( I hope this is ok even if the topic here is the 3c24), here is potentially one: The 841. It is the sister tube of the 801A.
I measure it through:
datasheet:
https://frank.pocnet.net/sheets/049/8/841.pdf
Looks promising...12W. Mu of 30. rp between 19k-23k (measured) Thoriated tungsten as well.
The only thing which makes me scratch my head: the max. plate voltage of only 425V.
I ask my self if this is really the max. rating or if we have to read the data sheet the right way, sometimes you need to set stuff into a context before understanding the real story.
What makes me wonder:
- The 801A is the same tube....with a differently winded grid only. Look here: VinylSavor: Tube of the Month: The 841
- The 801A is speced to 600V.
- The HV-Supply-Voltage is given with 1000V using an anode resistor, so the tube can stand in operation much, much higher voltages.
- You do not really reach the 12W when you want to keep the grid voltage negative and can go up only to 425V. Ideally, you should be able to use it at 9W (75% of 12W) at 600V with -5V Ug and 15mA...which will drop rp to 17-18k, give you nice headroom etcetc.
- So, I understand when these tubes has been intended for positive grid-operation (as well as the 801A). But when we want to stay negative at the the grid and have no grid-current flowing: What can go wrong when using 600V and stay below 9W?
While waiting for them...I am looking into alternatives ( I hope this is ok even if the topic here is the 3c24), here is potentially one: The 841. It is the sister tube of the 801A.
I measure it through:
datasheet:
https://frank.pocnet.net/sheets/049/8/841.pdf
Looks promising...12W. Mu of 30. rp between 19k-23k (measured) Thoriated tungsten as well.
The only thing which makes me scratch my head: the max. plate voltage of only 425V.
I ask my self if this is really the max. rating or if we have to read the data sheet the right way, sometimes you need to set stuff into a context before understanding the real story.
What makes me wonder:
- The 801A is the same tube....with a differently winded grid only. Look here: VinylSavor: Tube of the Month: The 841
- The 801A is speced to 600V.
- The HV-Supply-Voltage is given with 1000V using an anode resistor, so the tube can stand in operation much, much higher voltages.
- You do not really reach the 12W when you want to keep the grid voltage negative and can go up only to 425V. Ideally, you should be able to use it at 9W (75% of 12W) at 600V with -5V Ug and 15mA...which will drop rp to 17-18k, give you nice headroom etcetc.
- So, I understand when these tubes has been intended for positive grid-operation (as well as the 801A). But when we want to stay negative at the the grid and have no grid-current flowing: What can go wrong when using 600V and stay below 9W?
Last edited:
Happy new year to all of you,
During the holiday I received a nice parcel with four 3c24...as promised as a present from a senior member of diyaudio...Thanks a lot !!! I was really impressed.
Now, I tried to be smarter than last time in making this tube work. So, this time I applied filament power to it for 5 hours to burn any potential gas in there as suggested by some folks.
Second, I gave it a much softer HV to start with and used the manual mode of my tube tester (Rohrentestv9 Röhrenprüfgerät Röhrenmessgerät RoeTest ) to see how current flows with which Anode-HV and grid). So I started with 300V and -20V. No current. Ok, so I raised VERY slowly the -Ug and at approx. -2V it started to show a white bright corona and from NO current at all it jumped up to 50mA or more. Like a mercury tube which just switched on.
OK, As this jump in current is not controllable, I did it the other way around after that: 0V grid voltage and slowly raise of Anode voltage to see when it starts to conduct. It start at 26V on the Anode !!! Yes, 26V. this time it was much better controllable at currrents of aroung 8-10mA and it raised its current flow, so you needed to raise -Ug fast with it. You got the white corona when having currents of more than 20mA or so.
Ok, this is where I stand. I can play further with it and have another three to try...but before doing more strange stuff:
What are you guys who used this tubes in your amps before say ? Forget that copy, its another bad one ? Or is there a reliable procedure to wake up these tubes ?
During the holiday I received a nice parcel with four 3c24...as promised as a present from a senior member of diyaudio...Thanks a lot !!! I was really impressed.
Now, I tried to be smarter than last time in making this tube work. So, this time I applied filament power to it for 5 hours to burn any potential gas in there as suggested by some folks.
Second, I gave it a much softer HV to start with and used the manual mode of my tube tester (Rohrentestv9 Röhrenprüfgerät Röhrenmessgerät RoeTest ) to see how current flows with which Anode-HV and grid). So I started with 300V and -20V. No current. Ok, so I raised VERY slowly the -Ug and at approx. -2V it started to show a white bright corona and from NO current at all it jumped up to 50mA or more. Like a mercury tube which just switched on.
OK, As this jump in current is not controllable, I did it the other way around after that: 0V grid voltage and slowly raise of Anode voltage to see when it starts to conduct. It start at 26V on the Anode !!! Yes, 26V. this time it was much better controllable at currrents of aroung 8-10mA and it raised its current flow, so you needed to raise -Ug fast with it. You got the white corona when having currents of more than 20mA or so.
Ok, this is where I stand. I can play further with it and have another three to try...but before doing more strange stuff:
What are you guys who used this tubes in your amps before say ? Forget that copy, its another bad one ? Or is there a reliable procedure to wake up these tubes ?
Hi Blitz,
I regularly use 841 tubes as VAS device (for example first tube of my PSE 801a amp) over 500V anode voltage (but about 9mA current at -3.8V bias).
The only one disadvantage is in these operating points the horrible load (over 200k - I use another 10Y CF tube stage).
Do you know Steve (now Stephany) Bench article of A2 capable 841 amplifier?
420V, 36mA, 10k load, 5W output, 1% THD.
Thanks for the link, I need to study this a bit further...I thought of the 841 always only as a driver...this is very interesting...
Ok, folks. I have no idea if I have now woken up the 3c24 correctly or not. After various tries, I followed now simply the approach to make its anode red quickly and let it than raise current itself at a fixed bias, which it did from 8mA to 25mA at 600V and -3,15V. No idea if this is a normal bias point for this tube and if this emission equal 100%. But I have few more to play with and see.
The curves of this one:
Some data measured at 600V, -3,15V, 25mA:
S=2,38mA/V
mu=27
Rp=12,5K
Ok, folks. I have no idea if I have now woken up the 3c24 correctly or not. After various tries, I followed now simply the approach to make its anode red quickly and let it than raise current itself at a fixed bias, which it did from 8mA to 25mA at 600V and -3,15V. No idea if this is a normal bias point for this tube and if this emission equal 100%. But I have few more to play with and see.
The curves of this one:
Some data measured at 600V, -3,15V, 25mA:
S=2,38mA/V
mu=27
Rp=12,5K
Ok, I tried another two...one got a broken filament (ouch), but the next one is now the first out of four which simply works without any hassle from the first second on. No flashes, white coronas etc. So, it is possible to get a flawless copy, hurray !!!
Data and curves are very near to the first one, not fully identical, but closeby:
S=1,96mA/V
mu=25,3
Rp=13,4
------------------------
Euro21,
I am not sure if I got your comment right about the 200K load...when I put a good (104%) 841 on 500V and -3.8V, I get about 11.4mA and with that an rp of 19.2k and a mu of 30 at S=1.61mA/V, far away from 200K
Data and curves are very near to the first one, not fully identical, but closeby:
S=1,96mA/V
mu=25,3
Rp=13,4
------------------------
Euro21,
I am not sure if I got your comment right about the 200K load...when I put a good (104%) 841 on 500V and -3.8V, I get about 11.4mA and with that an rp of 19.2k and a mu of 30 at S=1.61mA/V, far away from 200K
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.