Hello guys,
I wanted to share a little tube experiment I am doing at the moment with my ha-6a.
disclaimer:
First please, this my personal experience with my amp, please do not try to reproduce what I am doing here unless you know what you are doing, and then you are doing it at your own risk
What I verified in my amp, may not hold true for yours.
SOME MORE TUBE ROLLING MADNESS :
I have some 6sn7 lying around from my Freya preamp, and knowing that the 6sn7 is one , if not the best preamp tube available, I wanted to experiment a bit....
The 6sn7 have the same electrical parameters as a 12au7. BUT, it is octal based, and draws twice the heater current, and the heaters runs only in parallel.
So... not quite the drop in replacement right ?
Right, but not too complicated either !
To use a 6sn7 in place of a 12au7, one must first know if the 12au7 are used with heater in series (12.6V) or in parallel (6.3V).
the 12au 7 is a double triode, that means, two triodes in the same tube. The 12a7 family has the heater center tap on pin 9, and heaters on pins 4&5.
To Know if the tube is used with triodes in parallel or in series, you have to look inside the amp: if pins 4 and 5 are connected and pin 9 left unconnected, that means it is wired in series so 12.6v is applied.
If pin 4&5 are wired together, and pin 9 connected as well ( center tap ) that means a parallel connection is made and the heater voltage is therefore 6.3 V.
On inspection , I have noted that
MY HA-6A is wired in 6.3V !!! so that means even more tube rolling possibilities are opening !!
second disclaimer : the correct adapter (6.3V) must be used, never ever do this with a 12.6V adapter or fire will happen !
be absolutely sure you are using the correct adapter on the correct socket !
with this out of my chest :
12bh7,6sn7,6cg7,6fq7..... are possible candidates !
so just put in an adapter and all is good right ?
wait my friend not so fast !
second we have to know if the power transformer can give us enough power without burning itself trying...
the 6SN7,12bh7,6cg7,6fq7... wants 0.6A of current for it's heater.
the 12au7 wants 0.3A.
that is twice the current ! and the power and rectifier tubes still needs to be powered as well...
So how one can do this safely ?
ask the manufacturer of the amp if the power transformer is overbuilt enough to withstand the load ?
I am therefore asking Mr
@Andykong : Does the power transformer of the ha6a has enough current capacity to withstand the added total load of 0.6A when used with compatible tubes ? Even better would be to know exactly what is the rated amperage on the power transformer used in the ha 6a ?
lets do some calculation here on what is the amperage consumed from the stock tubes : based on the tubes datasheets.
12au7*2 : 0.6A
kt88*2 : 3.2A
22de4*2 : 0.9A
total is 4.7 A.
putting 6sn7 in place of the 12au7 would take us to 5.3 A.
Problem is, we don't know if that is safe , as we don't know the rating from the power tranny.
But, what if we use a less current hungry power tube ? the el34 draw 1.5A per tube, so the problem is the same...
Here comes the 6l6 tube, with it's current draw of 0.9A only !
Those of you who read that thread since last year, knows that it is one of my favorite tubes in this amp
so with the 6l6 installed we have :
12au7*2 : 0.6A
6l6*2 : 1.8A
22de4*2 : 0.9A
Total : 3.3A
so know adding 0.6 A to the total for the 6sn7 tubes will get us to 3.9A only... even less than stock with the kt88 !
so here you go : ( I know It looks kinda silly.... )
I had to put an extra socket saver between the adapter and the socket, as the adapter would not fit in the small recessed cutout in the amp chassis.
So this looks like crap right... but let me tell you, Holy mama ! the sound is know nothing short than amazing !!!
The noise floor is lower, less microphonic, hiss is almost completely gone and the music is just superb.
With my utopia, which is very sensitive , the hiss can only be heard a tiny bit with the highest impedance setting.
The bass is tighter, more full bodied, soundstage is wider, highs are super smooth, music flows freely with an added sense of fluidity.
The tubes here are nothing really special, new production Tung sols 6sn7 GTB pulled from my main system Freya preamp.
I am now eager to try it with kt88 also, And waiting eagerly for
@Andykong to respond
The tube preamp hierarchy is respected the 6sn7 are far better than any 12au7 Nos or new production.
Now this opens even more possibilities with that .6A heater :
the 6cg7 : a 9pin 6.3V tube ( adapter still needed ) that is basically a 6sn7 in smaller envelope. with an internal shield that reduce noise and increase separation.
the 6fq7 : Same as 6Cg7, only without the shield. A lot of great NOS options available for cheap
the 12bh7 : direct drop in replacement ! no adapter needed, only the heater at .6A as the on above.
I have some of those coming on the way.... will be reporting here
Yes I know I am crazy, In France we call it not an audiophile, but an audiopath....
see you next time fellas !