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OTs in parrallel without one tube

6V6dude

Member
2014-09-09 11:07 am
I've got an amp that uses parallel OTs. Removing one tube lessens it's output but I wonder what actually happens to the OTs since one remains powered and the other does not.
So with only one powered OT, the secondary is feeding not only the speaker but also the un-powered OT. Both OTs are 8ohm so I am thinking the un-powered one would be seen as an 8ohm like the 8ohm speaker, and this changes the load to 4ohm?
Or even feeding the un-powered OT back to secondary is just plain bad for it?
 
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6V6dude said:
So with only one powered OT, the secondary is feeding not only the speaker but also the un-powered OT.
The 'unpowered' OPT will present a high impedance load so will not consume much output except at low frequencies.

Both OTs are 8ohm so I am thinking the un-powered one would be seen as an 8ohm like the 8ohm speaker, and this changes the load to 4ohm?
No. You need to do some reading on transformers.

With only one transformer driven, the load on that valve will double. Two 8ohm secondaries in parallel (both driven) will match a 4 ohm speaker. With one OPT not dirven the one which is driven will see the 4 ohm load unaided. Output power will be approximately halved.
 

20to20

Member
2010-06-23 9:25 pm
With 2 tubes driven you will have a 4 ohm output. But with one tube pulled you will have an open transformer primary circuit. The secondary on that transformer will have a very high AC impedance and look like an open secondary to AC signals. So the output impedance will go back to something closer to 8 ohms to your speakers since you have 8 ohms and a high impedance in parallel.
 

6V6dude

Member
2014-09-09 11:07 am
Right, thought that with one tube out, one OT would see the other OT as 8 ohm and together with 8 ohm speaker the output will drop to 4 ohm. Ok, so it remains at 8 ohm with one tube out and both together do 4 ohm.
This is just an experimental amp I've build from parts laying around. It also uses 150V TV power trafo with doubler to give me 300V. Fairly small trafo but is pulling 4x 6V6 and 3x preamp tubes and hardly gets warm. Probably could run lot more tubes :)
 
You will have an unloaded high-impedance transformer primary that might be at risk of failure due to high voltages induced from the audio. This could be similar to running a tube amp with no load, which is a definite no-no. I suggest disconnecting the unused transformer if it isn't going to be driven by a tube. All other comments about load impedance will apply here as well.
 
Post 1:
6V6dude said:
Removing one tube lessens it's output
Post 10:
6V6dude said:
No I don't run it with the tube out
Post 1 says 'I have removed a tube and it reduces the output'.
Post 10 says 'I have not removed a tube'.

But it's good to be assured it might damage the OTs
Perhaps you meant to say 'it's good to warned that it might damage the OPTs' or 'it's helpful to have confirmed that it might damage the OPTs'.
 
Two lesser points, and certainly not intended to disagree with good posts thus far, but for folks less acquainted with tube design, to avoid going down a wrong route:

The amplifier (in this case) does not actually provide a certain output impedance as in 8 ohms (thus two in parallel being 4 ohms). What actually happens is that when an 8 ohm load is connected to the secondary, the tube(s) see a certain load into which it/they can work optimally. Other loads connected to the secondary will reflect other impedances to the tubes into which they will do work, but not optimally so. (If one wants to be pedantic about secondary load 'presented', in the case of a pentode that would be 100s of ohms because the pentode rp is quite high.)

Thus two '8 ohm' windings in parallel do not actually 'give' a 4 ohm output impedance; it again depends on the optimal loading, which in this case will be around 4 ohms. Semantics perhaps - but there is a difference in the viewpoint.

Then I also see less danger in 'driving' the second OPT from the (loaded) secondary of the working one, as e.g. in producing damaging transients, as might be the case if an active device was driving the primary, with open (secondary) load. Again a small matter and not encouraging dangerous practices, but again a basic difference.

(The rest of the circuit is not relevant to this thread, but I hope there is not global NFB involved in each amplifier. That will change matters significantly.)
 
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