Hi , I have to buy the OT for 2x6V6 amp :
my indecision is between these:
125A1A with 6,7K prim. imp.
1839 with 8,1K prim. imp.
some suggestion ?
Thanks
R.
trafo for 6V6 choice
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Stevem
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Re: trafo for 6V6 choice
The lower impedance one will give a tad more output and hence shorter tube life, and the opposite for the other one.
If you are after real nice tight bottom end on the low E string and had the Bucks than a Hammond 1650F would be my pick!
If you are after real nice tight bottom end on the low E string and had the Bucks than a Hammond 1650F would be my pick!
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Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: trafo for 6V6 choice
What type of circuit is it in, eg 5E3 or Deluxe Reverb, fixed or cathode bias?
What VB+, ideally at full load, or if not the static VB+, rectifier type (Si or tube, if tube which type), and reservoir cap value?
Have a play with the loadlines yourself http://bmamps.com/Tech_tds.html
What VB+, ideally at full load, or if not the static VB+, rectifier type (Si or tube, if tube which type), and reservoir cap value?
Have a play with the loadlines yourself http://bmamps.com/Tech_tds.html
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Re: trafo for 6V6 choice
What VB+? Every supplier of 5E3 stuff has their own take on that (key) parameter.
Whatever, my take on the loadlines is that 6k6 has less power output and higher plate dissipation than 8k.
That backs up my experience of a DR 6k6 OT in a 5E3 type circuit having a lower power output than an 8k OT intended for 5E3.
Whatever, my take on the loadlines is that 6k6 has less power output and higher plate dissipation than 8k.
That backs up my experience of a DR 6k6 OT in a 5E3 type circuit having a lower power output than an 8k OT intended for 5E3.
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Re: trafo for 6V6 choice
The basics for calculating optimum centre-biased Class A reflected load for 2 P-P 6V6 is the same as for 1 x 6V6 in Class A.
Zout = Va/(Pa/Va)
Where:
Va = Plate voltage.
Pa = Maximum plate dissipation.
So you ideally want to know what plate voltage you'll be running those 6V6s at.
Assuming it is 340V and JJ6V6S then
340(14/340) = 8k2
If the tubes are biased on the cool side of 14W, then you'll get Class AB1 operation as the output signal from the PI gets bigger. Mind you, all this is fine in theory.
If you go for a lower Zout (like 6k6) the tubes will run hotter and wear out a bit sooner (as the others have said). If they're good 6V6s they should take a bit of abuse anyhow. RCA 6V6GTA is excellent in this regard. (And you should be alright with JJ6V6S because I reckon they're more like a 16W tube anyhow.)
Zout = Va/(Pa/Va)
Where:
Va = Plate voltage.
Pa = Maximum plate dissipation.
So you ideally want to know what plate voltage you'll be running those 6V6s at.
Assuming it is 340V and JJ6V6S then
340(14/340) = 8k2
If the tubes are biased on the cool side of 14W, then you'll get Class AB1 operation as the output signal from the PI gets bigger. Mind you, all this is fine in theory.
If you go for a lower Zout (like 6k6) the tubes will run hotter and wear out a bit sooner (as the others have said). If they're good 6V6s they should take a bit of abuse anyhow. RCA 6V6GTA is excellent in this regard. (And you should be alright with JJ6V6S because I reckon they're more like a 16W tube anyhow.)
He who dies with the most tubes... wins
Re: trafo for 6V6 choice
Thanks tubeswell!!!
R.
R.
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martin manning
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Re: trafo for 6V6 choice
Tubeswell's calculation limits the output tubes to 100% of max dissipation along the load line, and does not consider the screen voltage. In push-pull, up to around 200% of max dissipation can be tolerated, and without knowing the screen voltage the load line cannot be placed optimally.
As an example, if you use that method to find the Zaa for a pair of 6V6's at 400V (as in a Deluxe Reverb), the result is 13k3, which is twice the standard 6k6 (i.e. the Fender 125A1A you mentioned). Looking at the plate curves, I might prefer 8k.
To pdf64's point, what circuit? what voltages?
As an example, if you use that method to find the Zaa for a pair of 6V6's at 400V (as in a Deluxe Reverb), the result is 13k3, which is twice the standard 6k6 (i.e. the Fender 125A1A you mentioned). Looking at the plate curves, I might prefer 8k.
To pdf64's point, what circuit? what voltages?