• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

24v aikido distortion

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
you know when you shut off a tube pre while the music is still playing and the music starts to died down and distort? Well, that is what my 24v aikido is doing all the time. I was barely getting anything out of it until I recalculated the voltage divider at 115k instead of the recommended 130k on broskie's site. Now I hear some music behind this scratchy noise. It sounded like it is just running out of steam.

I am running 2 6922 and 2 7DJ8 . The tubes work fine in a buffer that I use on my dac. I don't know much about the 78j8 other than it can take a higher heater voltage.

power supply is per schematic on the sitewith a 24v, 50VA toroidal tranny, 60mh choke, regulated to 24.9v. Originally it was down a few volts until i dialed in the regulator. It plays better with the extra couple of volts.

any thoughts
 

Attachments

  • 6gm8_aikido_schematic.png
    6gm8_aikido_schematic.png
    13.3 KB · Views: 530
I am sorry but the problem is really obvious:
For starters, why did you 'wishful think' instead of actually read the 24V aikido documents?
12V across cathode-plate is really too low for a 6922 type tube. The 7Dj8 is a 300mA series chain version of the 6DJ8, which means if you connected all the heaters in series, you likely have the 6922 underheated, which just adds to the problem. Brosikie clearly says you need to use 6GM8/6N27P/ECC86, designed for the job, and you are not - it's as simple as that.
 
mmm, I'll have to check my aikido stuff, but I think you can do a 6DJ8 in a 24V aikido.

Have a look at the booklet and see. I can say I am running a 24V one with 6GM8 and it sounds bloody great (despite a small bit of hum left).


Check the heaters at the pins of the sockets and do a few measurements is all I can suggest and post back here with them. JB himself is helpful but can be slow in replying - I'm sure he gets huge volumes of mail.

Fran
 
cherhit, thanks, I had been looking at the design posted in may of last May when he first posted the 24v project and didn't see this.

ilimzn, yes, I didn't really look too carefully at the specs for the 7dj8 and just assumed they were compatible in this set up. The voltage drop across across each of the heaters in series is as specified in the old write up, but the heaters heat up at considerably different speeds. This should have been a tip off that something was off with one of the pairs. I guess I need another pair of 6922s- 6GM8s are out of my price range. Thanks.
 
It seems that the ECC88 can be used down at 12V plate to cathode per valve, but it seems a totally stupid idea. The 24V Aikido was mean't to use the ECC86 and thats what I would use and nothing else. These are not an expensive valve and should be cheaper than any ECC88.

Shoog
 
woodturner-fran said:
mmm, I'll have to check my aikido stuff, but I think you can do a 6DJ8 in a 24V aikido.

There are a number of issues when using 6DJ8 and similar at such low plate voltages. In theory, you should be able to get some 2.5mA with zero bias at 12V, which means that for a realistic situation you should be running the tubes with really very low current, perhaps 1-1.5mA. This is only part of the problem, at such low bias voltages this tube family can exhibit non-trivial grid current.

You CAN use the 24V Aikide BOARD with 6DJ8 (etc) but certainly at higher supply voltages, which also means you whould be supplying the heaters with a separate power supply.
 
poolorpond said:
ilimzn, yes, I didn't really look too carefully at the specs for the 7dj8 and just assumed they were compatible in this set up. The voltage drop across across each of the heaters in series is as specified in the old write up, but the heaters heat up at considerably different speeds. This should have been a tip off that something was off with one of the pairs. I guess I need another pair of 6922s- 6GM8s are out of my price range. Thanks.

Either that or you can use 2 pairs of 7DJ8. In fact, this could help your cause as you could increase the power supply voltage to approximately 28-29V, and for these tubes at these voltages, every volt will help. You should also take care to check that the heater chain is drawing 300mA rather than that the voltage is 7V, as the heaters are sopecified for a target current, the 7V spec is only approximate for that current, and in fact, can vary quite a bit depending on the manufacturer and version of the tube.

Regarding different heat-up times, this is normal because series heater chain tubes have a different heater characteristic, designed to heat up so that neither tube in the chain develops either a too high or too low voltage during heat-up (because it would either overhat one tube or momentairly increase current for the other tubes). Because of this reason, you can use tubes designed for series heater chains in a parallel connection, but you run into problems when mixing paralel and series chain spec'd tubes in series. IF connected in series, they either all have to be spec'd for it, or all have to be the same type for parallel connection.

Finally, 7DJ8=PCC88, are often said to be drop-in replacements for 6DJ8 (etc), but this is not always a given. Some 7DJ8 are more like 8DJ8 and thay will be underheated with a 6.3V heater voltage. Others use about 7V or some fraction of a V less, and end up being heated with about 10% less voltage, which is within tolerance, but can still affect how the tube sounds - sometimes for better, sometimes for worse. Series heater chain spec heaters, having a so called 'controled heater characteristic' have such a heater voltage to current dependency (this is not linear for any type heater, BTW), that they tend to keep the heater at a pretty well constant temperature for a fairly large variation in heater voltage, so this will help when replasing a 6DJ8 by a 7DJ8. BUT as I said, this may not always be enough.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.