As you could already read in previous articles, I prefer TV damper tubes for rectification. My favorite among those is the 6AX4.
The use of TV dampers in power supplies was advocated by JC Morrison in Sound Practices magazine back in the 1990ies. He mostly uses them as a slow turn on device in conjunction with silicon rectifier bridges. TV dampers have been developed for TV service. Their purpose was to dampen oszillations in the deflection system during the fly back period of the electron beam. This requires a very tough diode which can handle very high peak inverse voltages and peak currents. These properties made them perfectly suited for mains rectification as well. However this was not promoted by tube manufactures since they wanted to continue to sell their dedicated rectifier tubes, which where more expensive. There is still a prejudice against TV dampers as rectifiers because of this. Many years of experience with TV dampers in various power supplies proofed that they are extremely reliable and perfectly usable as rectifiers. Another advantage is their extremely low cost.
The 6AX4 shares the same base diagram with other similar types like 6DE4, 6DM4, 6AU4 and many others. It is also available with different heater voltages and for series heater connection as types 12AX4, 17AX4 and 25AX4. A detailed datasheet can be found here. The 6AX4's current rating is on the low side compared to some of the other TV dampers. So why did I chose it as my favorite? The current rating is still way beyond the needs of most amps I build. More than enough to power both channels of a SE power amp. When followed by a choke input filter, a 6AX4 rectifier bridge can deliver more than 300mA continous. The advantage of the 6AX4 is it's lower heater current compared to others. It needs only 1.2A. When 4 of them are used in a bridge, the higher heater current of other types can become a difficulty. Therefor I stick with the 6AX4 for most applications. If higher current is required, I use some of the beefier TV dampers. Another big plus of TV dampers is their good insulation between heater and cathode. Please note that this voltage is quite different depending on the polarity. The cathode can be at substantial higher voltage than the heater, while they don't like the heaters to be too postive against the cathode. For the 6AX4 the cathode can be up to 900V DC above the heater potential. This means that all heaters of the 6AX4s in a Greatz rectifier bridge can be wired in parallel and fed from a secondary winding which is referenced to ground. This is safe for power supplies with choke input filters which deliver up to approximately 650VDC. Higher voltage power supplies like those for 211 amps will require at least 3 different heater windings for a rectifier bridge to ensure cathode to heater voltage is not exceeded. The schematic below illustrates how the heaters need to be supplied and biased in the case of higher voltage PSUs:
The 6AX4 was available from almost any tube manufacturer. Although production of this tube has ceased a long time ago, it is still available in large numbers. Below is a selection of 6AX4 tube boxes of different brands:
The 6AX4 was made in various forms, most with the common large Octal base, in later years it was also broduced with the flat 'coin base':
The last photo shows a 6AX4 Graetz bridge in a power supply:
Do you have a prefered brand or type of 6AX4? I am finding GT / GTA / and GTB versions in many brands. Thx!
No, I am not after specific brands. Brand preferences wouldn't make a lot of sense anyways since a lot of crossbranding was done with tubes. You can never be sure if a tube really got manufactured by the company who's brand name it carries.
GT/GTA/GTB differ in warm up time
can you please explained why you conected the heater with the cathode. As indirect heating valve, you should be able to use one single heating source for all valves. Am I right?ReplyDelete
It is explained in the post. For voltages up to 600-700V the heaters can be supplied from a single source which is referenced to ground. For higher voltages separate heater supplies as shown in the schematic are needed to keep heater-cathode voltage within allowed limits.
Thank you. I thought that the standard schematic is as shown in that figure.Delete
I am trying to build a full wave rectifier for only 170 V so I hope will do well, with one heater.
I'm planning to use 12AX4 to be a slow ramp up device for B+ of 600V 60mA. It will be placed after the solid state rectifier. I think I don't need to tie cathode to heater. So, from your experience, is it safe to do so? Thanks.ReplyDelete
Hi! It would be safer to tie the cathode to the heater. But why not use two 12AX4 with two silicon diodes in a hybrid arrangement. This will give the benefits of a tube rectifier and the slow warm upDelete
Thanks for your advice, Thomas.Delete
how about 6X4 series are smaller??ReplyDelete
The 6X4 can be used in preamps. It can handle only small currentsDelete
we are vacuum tube electrons collector from Jakarta,IndonesiaReplyDelete
wana saying so good for these articles...bravo...
Thanks! I am glad you like themDelete
Hi! Love your website. Your article notes that for power supplies up to about 650 volts "all heaters of the 6AX4s in a Greatz bridge can be wired in parallel and fed from a secondary winding which is referenced to ground". Would an appropriate ground reference be achieved by connecting the heater-transformer center tap to ground? Thanks!ReplyDelete
It doesn't matter if one and of the heater winding is connected to ground or the center tap. Both achieve the same result
Got it, thanks!Delete
Could the heater center tap be connected to a voltage divider somewhere else in the power supply? That would reduce the heater/kathode potential and provide a bit more flexibility (another 50 volts, perhaps).
Carefully examine the heater cathode voltages before you decide anything like that. Be sure you understand what you are doing. It al depends on your exact configuration and voltagesDelete
Is 6AX4 similar to the 6W4? I remember an old Phillips TV that used 2 6W4 as rectifier and another as damper.Delete
Yes they are similar but different ratingsDelete
May I ask why you have chosen a Graetz bridge and not just a normal Fullwave with two diodes instead of 4 ? Only for pratical reasons or as well sound quality difference ?ReplyDelete
The bridge configuration is the better supply. You get better utilisation of the power transformer
Thomas, have you ever tried using a double wound center tapped power transformer with two 6AX4's in a full wave configuration verses 4 of them in full wave bridge configuration? If so, then what did you discover?Delete
I lost interest in center tapped power transformers many years ago. Why don't you simply try out yourself and use what works best for you? If 4 6AX4 tubes are too much you can use two together with silicon diodes in a hybrid arrangementDelete
Thomas, were you seeing specific problems with center tapped power transformers? Thanks!Delete
No specific problems, but I think the bridge configuration is generally betterDelete
When using two 6AX4 with two silicon diodes in a hybrid configuration.
Is it necessary to have a matching pair 6AX4 or just take a two NOS tube of the same brand?
Thanks for the answer.
No watching needed. Not even the same brand is necessaryDelete
I have some questions for you:
1. Why do you take 2 separate windings for 2 tubes 6ax4 that come with anode connected to ground (top and bottom tubes)?
2. Do you compare difference between hybrid and full tubes bridge rectifier in terms of sound quality? Can you share this?
Full tube bridge will make complicated circuit in while hybrid rectifier is very easy.
1. is explained in the text
Hi, I am your fan. I have some question. I am planning PSU for GM70 amp with bridge rectifier with 2 6AX4 and 2 866A.ReplyDelete
Seconday transformer voltages 0~800v
Is it safe if one heater transformer for 6AX4 or do I have to use 2 separate heater transformer for 6AX4.
Thanks in advance.
Hi! The information is actually in the post. At these voltages you want separate windings. Please be careful such voltages are dangerous and it is important to understand the limitations and max ratings given in tube data sheetsDelete
Hi, thanks for your kind reply.Delete
I have thought for a few days to
solve my problem.
Because I have transformer with
separate heater wiring for 866A
and single for 6AX4.
So , my question is if I reverse
866A and 6AX4 position in bridge rectifier
, does it work well?
If I wire like that,does it decrease sound quality ?
Thanks in advance.
Have a nice day.
Hi! I am sorry to be quite direct, but if you do need to ask these questions you might lack the experience to work with such high voltages. You also need to check with your transformer manufacturer if his transformer is unstable (enough insulation) for that. Please understand that I cannot give detailed support as would be necessary. Since more things need to be checkedDelete
Thank you for your most interesting post on the 6AX4, and damper diodes in general.
I have a question for you. You indicated above that, "This means that all heaters of the 6AX4s in a Greatz rectifier bridge can be wired in parallel and fed from a secondary winding which is referenced to ground".
The 6AX4 and all other TV damper diodes that I am familiar with, utilize a heater that is electrically isolated from the cathode of the tube. That said, could you clarify as to why you suggest that the heater be returned to ground, particularly where the heater-to-cathode voltage is well below the rating for the tube? Does returning the heater to ground reduce the possibility of 50-60 Hz hum modulation of the DC output caused by the heater supply?
Is your suggestion that the heater be referenced to ground only necessary when the tube is used in a Graetz full-wave bridge configuration, or even when the tube is used as a soft-start/slow turn-on for the B+, after a solid-state rectifier?
Thanks for your help here.
Thanks & Regards,
Hi! This was meant in the case of the full wave bridge with 4 diodesDelete
Thank you for your very quick reply.
Based upon your comment, is it still necessary to ground one side of the 6AX4 heater circuit when using this tube as the soft-start element at the output of a solid-state rectifier? Is there any advantage or disadvantage to doing this?
And for my own knowledge, why do you suggest grounding one side of the heaters in a Graetz bridge rectifier network? And does this stress the heater-to-cathode voltage rating of the tube?
Hi! Look at the circuit and think about the voltages present at different times. This will help to understand. Always keep the voltage within the max allowed limits. You can also connect the heater to the cathode in your case. But make sure your heater winding is properly isolated.Delete
Hi Thomas, quick question regarding damper diode use. On the 6AX4 datasheet, it says the DC output current is 165mA. Does using 2 6AX4s in a hybrid bridge rectifier double that current capability (165*2 = 330mA)? If that's the case, what would you say is the max output current of a 6AX4 hybrid bridge rectifier with a capacitor input PSU?ReplyDelete
Hi! Read the article carefully it answers your first question already. Note that the data sheet is not giving numbers for rectifier use but only for damper service. If you want to be on the stew side derate the numbers. I never use cap input power supplies so have no practical experience what they can deliver thereDelete