Hi!
I have a SE guitar tube amp that I already changed the stock 6L6GC to a KT66. I'm planing to try other kind of tubes to experiment.
From my research, I found that primary impedance of OT should be (class A, 400V) (correct me if I'm wrong):
8k for 6V6
4k for 6L6, kt66
3k for 6550, KT88
(that's what I found from various specs sheet of various type of tube.)
All I found from the code was this OT replacement:
http://www.angela.com/fendersingleen...ansformer.aspx
It does not specified the primary impedance. It is mainly for Champ, Vibro Champ, Bronco, but says it works also for 6L6GC. They say the secondary can be from 4-16ohm... That makes the primary varying from 3.5k to 14k. Is that selling BS?
I tried to measure the stock OT primary impedance.
With what was said on that site:
http://www.radioremembered.org/outimp.htm
I found at turn ratio of 30:1, that gives an primary impedance of 7.2k.
That makes sense for 6L6, but isn't it high for 6L6?
From:http://tubelab.com/designs/tubelab-sse/applications/
I saw that from 5K to 3K, for 6L6, there is an increase of output power of 30%. So the headroom of the amp could be increased by putting a lower primary impedance? Would it run better with 5K? Any advices?
In my experimentations, I want to try KT88 and 6550. I know it's not a wise choose for energy efficiency and budget, but I don't care... What would be the recommended primary impedance for theses tubes?
Would a primary of 3.5k fit well for kT66,6L6 AND kt88, 6550? Would it be better with 5K for both? Any advices?
Thanks!
Primary impedance question
Moderators: pompeiisneaks, Colossal
Re: Primary impedance question
You already answered your question of whether the OT range of 3.5k to 14K is bs. The 7.2K impedance you came up with a measured 30:1 turns ratio indicates your assuming the secondary load is 8 ohms. Now, assume a 4 ohm speaker load you would have a primary impedance of 3.6K. If a 16 Ohm speaker load is used you have 14.4k primary impedance. There is your range.
Thanks
Mike
Thanks
Mike
Sunnydaze
Re: Primary impedance question
Optimal reflected load is related to the power-handling capability and operating potential of the output tube, and for centre-biased Class A operation the optimal load (Zout) is found with the following:
Zout = Va/(Pa/Va), where
Va = the plate voltage at idle
Pa = the maximum-rated plate dissipation
A 6V6GT is rated for Pa = 12W so, plugging that in:
400V/(12W/400V) = 13,333R (or 13.3k)
You can run it at a lower reflected load (like 8k) but it won't operate as efficiently, and the tube will be passing more current under running conditions than optimal so the tube, (and the speaker) will likely wear out faster - all other things being equal.
(For a 6L6CG idling at 400V, the optimal load is 5k3 using the above formula, and for a Russian 6P3Se ('Russian 5881'), the optimal load would be 7k8).
In tube guitar amps, people break the rules all the time, and there are ways of getting around it. (e.g.; Fender used smaller-cored OTs on many of his amp designs so that the (longer) bass frequencies became saturated in the core (and thus were 'culled' out of the signal going into the speaker coil, helping to preserve speaker longevity.)
Zout = Va/(Pa/Va), where
Va = the plate voltage at idle
Pa = the maximum-rated plate dissipation
A 6V6GT is rated for Pa = 12W so, plugging that in:
400V/(12W/400V) = 13,333R (or 13.3k)
You can run it at a lower reflected load (like 8k) but it won't operate as efficiently, and the tube will be passing more current under running conditions than optimal so the tube, (and the speaker) will likely wear out faster - all other things being equal.
(For a 6L6CG idling at 400V, the optimal load is 5k3 using the above formula, and for a Russian 6P3Se ('Russian 5881'), the optimal load would be 7k8).
In tube guitar amps, people break the rules all the time, and there are ways of getting around it. (e.g.; Fender used smaller-cored OTs on many of his amp designs so that the (longer) bass frequencies became saturated in the core (and thus were 'culled' out of the signal going into the speaker coil, helping to preserve speaker longevity.)
He who dies with the most tubes... wins
Re: Primary impedance question
Great answer thanks!
When you say "people break the rules all the time", because it provides a better tone or it is to get more power out of the tube?
Is there advantages/disadvantages of running the tube at higher impedance than what the equation give?
The primary impedance calculated with the equation give an optimal impedance. Since speaker impedance is varying with frequency, the speaker as 7-8ohm only between 250-600hz.
http://www.eminence.com/pdf/Wizard.pdf
So for all other frenquency the resistance is much higher (200ohm at 100hz, 20ohm at 4khz, etc.) Even a single note is outside the 250-600hz range with all the harmonics. Maybe the equivalent resistance of a 8ohm speaker is 16ohm when doing a chord. So should we guess an average equivalent resistance instead of 8 ohm? Is your calculation is over estimating the optimal impedance? The 8ohm resistance is seen by the speaker only for a small part of the output power...
What do you think?
Why the speaker would wear faster with lower impedance? OT is blocking the useless DC, and if the speaker is 50W for a single 6L6GC, I don't see the problem...
I read somewhere that the smaller OT Fender uses to cut the bass was useful in the time that speaker were a bit to small for the amp. Modern speaker can take much more power now, so I don't think (I may be wrong) that is useful today (except being cheaper to build)...
When you say "people break the rules all the time", because it provides a better tone or it is to get more power out of the tube?
Is there advantages/disadvantages of running the tube at higher impedance than what the equation give?
The primary impedance calculated with the equation give an optimal impedance. Since speaker impedance is varying with frequency, the speaker as 7-8ohm only between 250-600hz.
http://www.eminence.com/pdf/Wizard.pdf
So for all other frenquency the resistance is much higher (200ohm at 100hz, 20ohm at 4khz, etc.) Even a single note is outside the 250-600hz range with all the harmonics. Maybe the equivalent resistance of a 8ohm speaker is 16ohm when doing a chord. So should we guess an average equivalent resistance instead of 8 ohm? Is your calculation is over estimating the optimal impedance? The 8ohm resistance is seen by the speaker only for a small part of the output power...
What do you think?
Why the speaker would wear faster with lower impedance? OT is blocking the useless DC, and if the speaker is 50W for a single 6L6GC, I don't see the problem...
I read somewhere that the smaller OT Fender uses to cut the bass was useful in the time that speaker were a bit to small for the amp. Modern speaker can take much more power now, so I don't think (I may be wrong) that is useful today (except being cheaper to build)...