I have a pair Transcendar 10K/600R output transformers that I plan to use for a 45 preamp. Normally people use 5K ones for 45 tubes. My question is would there be any disadvantage of using 10K ones?
Also have a Harman Kardon XT-3 Microphone transformers that I am thinking of using as stepup input transformers. Could anyone suggest the best configuration to make the best use of these transformers that I currently have?
Cheers
Also have a Harman Kardon XT-3 Microphone transformers that I am thinking of using as stepup input transformers. Could anyone suggest the best configuration to make the best use of these transformers that I currently have?
Cheers
two completely different questions-
first question is very much answered. I would prefer an OPT with lower primary impedance - you might find 10k weak on the Bass with 45 (like I did).
for the Harmon Kardon XT-3 do you mean step-up for MC phono? That is what I would use them for, depending on the cartridge...
first question is very much answered. I would prefer an OPT with lower primary impedance - you might find 10k weak on the Bass with 45 (like I did).
for the Harmon Kardon XT-3 do you mean step-up for MC phono? That is what I would use them for, depending on the cartridge...
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10k to 600 ohm is 4.08 : 1 (winding/voltage ratio)
5k to 600 ohm is 2.89 : 1.
So not twice lower gain.
Actually the difference is only 3 dB.
More important with these transformers to have enough primary inductance.
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Turns/voltage ratio of transformer in question says nothing about Ri.
You need to know what the transformer's secondary is hooked up to in order to know Ri. As most of us know it's not a good idea to run a tube amp with no load on the secondary, with high enough B+ you will see some very large voltage swings on the primary.
So an impedance ratio of 16.64:1
If this preamp was hooked up to a power amp with an input Z of 100k, we would have an Ri of 1,664,640 for the 45 tube.
You need to know what the transformer's secondary is hooked up to in order to know Ri. As most of us know it's not a good idea to run a tube amp with no load on the secondary, with high enough B+ you will see some very large voltage swings on the primary.
So an impedance ratio of 16.64:1
If this preamp was hooked up to a power amp with an input Z of 100k, we would have an Ri of 1,664,640 for the 45 tube.
The gain of the tube depends on the load. It will become its mu if load is infinite. Going from 5K to 10K the gain of the 45 will be some 15-16% higher.
In both cases however there is no real overall gain if the 45 is the only tube in the preamplifier circuit. To get some real voltage gain it has to be a two stages preamplifier or it has to have a step-up input transformer.
I would use another tube with more gain like a triode connected 4P1L (or the 6H30 if more gain is needed) with 10K.
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Agree about the gain.
I wonder why a transformer coupled tube preamp must be "loaded" sothat the tube sees a 5k or 10k primary impedance.
It does not just cost gain but I always found not loading (merely reflecting the input impedance of the power amp to get "infinite" load and gain like mu) to sound best.
Start backwards. What are you trying to drive? Headphones? Power amp?
If it's the latter then you only need a couple volts which most sources have. Multiply that by the mu of a 45 which is 3.5. Now divide by the loss of the transformer 4:1 and you are right back to where you started but with more distortion.
If it's the latter then you only need a couple volts which most sources have. Multiply that by the mu of a 45 which is 3.5. Now divide by the loss of the transformer 4:1 and you are right back to where you started but with more distortion.
I guess with Ri you mean load impedance (normally called Rl)?
Ri is also used for Rp (plate impedance).
No sorry, I usually call it Rp, which is the plate load resistance
rp = internal plate impedance
The primary impedance of signal transformers is generally referred to the source impedance used to test the transformer and gives also a hint of what the primary inductance might be. In other words it is a recommended source impedance to be used that gives a best balance between low frequency and high frequency response. If you reduce the source impedance low frequency response will be better but some peaking might appear. If you increase the source impedance high frequency roll-off will be smoother but low frequency response worse.
Of course a custom transformer that has high primary inductance and behaves fine with low plate resistance can be made but it is not standard because most signal tubes that are suitable for transformer coupling have plate resistance between 4K and 11K. If the transformer has enough inductance there is no harm in using higher source impedance unless lower Zout is required.
The 45, 71a, 4P1L, 31, 46 etc are all power tubes.
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