Hi,
I've been searching through threads and trying to figure something out without posting a new thread, but I'm not finding what I was looking for.
I've got a tube amp with a driver made up of a couple triode stages. The first stage is a 6C5G and I'm using an IXYS 10M90 as a plate load.
I've been using this amp for a number of years now and it works great as far as I can tell, just from listening to it anyways.
I popped it open today though just to confirm some measurements as it's been a while since I've done that. So this is what the circuit looks like:
There's a 500 ohm resistor and a 500 ohm trimpot in series between pin 3 on the 10M90 and the anode of the 6C5G so I can adjust the current where I want it to be.
So currently the trimpot is adjusted such that I read 7V over the cathode resistor, meaning I have about 4.6mA (7 / 1500) flowing through the tube, which is all good, that's about where I want it.
The only thing that's odd is that I'm measuring 85V on the plate of the tube.
If I look at the datasheet https://frank.pocnet.net/sheets/028/6/6C5.pdf
It seems to me from the characteristic curves that with 4.6mA flowing, and the grid biased to -7V effectively I should be measuring more like 190V on the plate.
I think maybe I'm not understanding something about how the 10M90 works in this implementation.. It sounds great as it is, but I just don't understand why the anode voltage is so low. Does anyone have any insights on this matter?
I've been searching through threads and trying to figure something out without posting a new thread, but I'm not finding what I was looking for.
I've got a tube amp with a driver made up of a couple triode stages. The first stage is a 6C5G and I'm using an IXYS 10M90 as a plate load.
I've been using this amp for a number of years now and it works great as far as I can tell, just from listening to it anyways.
I popped it open today though just to confirm some measurements as it's been a while since I've done that. So this is what the circuit looks like:
There's a 500 ohm resistor and a 500 ohm trimpot in series between pin 3 on the 10M90 and the anode of the 6C5G so I can adjust the current where I want it to be.
So currently the trimpot is adjusted such that I read 7V over the cathode resistor, meaning I have about 4.6mA (7 / 1500) flowing through the tube, which is all good, that's about where I want it.
The only thing that's odd is that I'm measuring 85V on the plate of the tube.
If I look at the datasheet https://frank.pocnet.net/sheets/028/6/6C5.pdf
It seems to me from the characteristic curves that with 4.6mA flowing, and the grid biased to -7V effectively I should be measuring more like 190V on the plate.
I think maybe I'm not understanding something about how the 10M90 works in this implementation.. It sounds great as it is, but I just don't understand why the anode voltage is so low. Does anyone have any insights on this matter?
Did both channels fail (85V on both plates)?
Check to see if the circuit is oscillating (easier said than done).
Check that the cathode resistor is 1500 Ohms.
Disconnect the self bias cap, and see if the plate voltage changes.
What is the grid stopper connected to?
What is the plate connected to?
(more schematic details)
Try connecting a 1k resistor between the plate and the bottom of the current source circuit (junction of the 1k stopper and fixed 500 Ohm resistor).
Swap the tubes from left to right channel.
Try another 6C5G tube.
A complete and accurate schematic is worth 1000 Words, and 100 Posts.
Check to see if the circuit is oscillating (easier said than done).
Check that the cathode resistor is 1500 Ohms.
Disconnect the self bias cap, and see if the plate voltage changes.
What is the grid stopper connected to?
What is the plate connected to?
(more schematic details)
Try connecting a 1k resistor between the plate and the bottom of the current source circuit (junction of the 1k stopper and fixed 500 Ohm resistor).
Swap the tubes from left to right channel.
Try another 6C5G tube.
A complete and accurate schematic is worth 1000 Words, and 100 Posts.
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All very good points by @6A3sUMMER.
Just as a very basic sanity check I would also measure the voltage drop across the 500R resistor. At 4.6 mA, it should be 2.3 V.
Just as a very basic sanity check I would also measure the voltage drop across the 500R resistor. At 4.6 mA, it should be 2.3 V.
It'll take a bit of work to dig into making any of those changes.. Until I do that, here is some more information.
This is the entire driver schematic for one channel in the amp. It is 2 stages, 6C5G -> 843, and then this feeds into an 845 power stage.
The weird thing is that both of the 843 stages work exactly as I would expect. The voltages line up with the datasheet on both of them.
And both of the channels for the 6C5G stage act exactly the same, both right at 85V on the plates, with the current set to 4.6mA, so it's not specific to one channel or the other. I've tried a few different sets of 6C5G tubes as well, and they're all acting the same in this circuit.
This is the entire driver schematic for one channel in the amp. It is 2 stages, 6C5G -> 843, and then this feeds into an 845 power stage.
The weird thing is that both of the 843 stages work exactly as I would expect. The voltages line up with the datasheet on both of them.
And both of the channels for the 6C5G stage act exactly the same, both right at 85V on the plates, with the current set to 4.6mA, so it's not specific to one channel or the other. I've tried a few different sets of 6C5G tubes as well, and they're all acting the same in this circuit.
I initially drew the schematic wrong, it's actually a 470 ohm resistor, and I Just measured the voltage over it: 2.17V, so that's right on the money.
I should also mention that I measured the cathode resistors and they both measure within 1% of 1.5K ohm.
If the cathode resistor has wrong value the CCS will force the tube to conduct by setting the plate voltage ...
Must be harmony between the CCS and the tube , unlike the case with plate resistor when only the tube is setting the operating point
Must be harmony between the CCS and the tube , unlike the case with plate resistor when only the tube is setting the operating point
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There is no DC blocking cap
at the input. Make sure the input is properly connected to GND when you do those measurements.
Do yon need a grid leak resistor?
at the input. Make sure the input is properly connected to GND when you do those measurements.
Do yon need a grid leak resistor?
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OMG... I am an idiot.. I was doing these measurements with nothing connected to the inputs.. I grounded them, and now I measure 192V on the plates.. Good lord. Well I feel really dumb, but I really really appreciate you all helping me with this.
Well, don't ask me why I thought of grounding the input. For sure it's not because I am clever. I am the type who learns things the hard way... 😉
Hello guys, sorry for silly question, please let me know, what are benefits of using 10M90 as plate load? I know the device is current regulator - is it only for that? keep plate current steady? what are sonic benefits of this solution over simple plate resistor?
The 10M90 works as a constant-current source (CCS) keeping the current flowing through the tube at a fixed value, even if/when the plate voltage changes. In other words, the (dynamic/AC) plate load resistance is virtually infinite. This (i) allows getting the max. voltage gain out of the anode follower and (ii) (maybe more important) results in better linearity of the gain stage.
Thank you. This makes great sense. Curious why it is not so popular in actually available tube amps? This chip is 4$ at Mouser, just bought some for experimenting. For amplifier producers, cost would be marginal to add....
The vast majority of actually available tube amps came before solid state. Back in the '40s and '50s no amount of money could purchase anything like the 10M90S. Any CCS would have to have been pentode based, and that means another hole in the chassis and the added expense for the extra VDC it would require, so a co$t consideration.
Being a DiYer, I can do things they didn't do back in "the day", like using a cascode LTP for phase splitting and provoding a convenient gNFB summing node. For CCS duty, I favour cascoded BJTs for LTP tail loads and active plate loads. BJTs have way more gain than any VT, and cascoding greatly reduces Early Effect for better "CCS-ness" than you'd get with a pentode.