Hi, A couple of quick questions on OPTs for a GM70 output stage. My design has a max power output of 25W from the GM70. Ua = 770V, -65V bias, 80mA.
1. Should the OPT be spec'd for 25W + % headroom? If so, what % is reasonable to use?
2. I am targeting a 7K primary and 8ohm secondary. There are some 3.5K primary OPTs available and therefore, can I use the 3.5K primary with 4ohm secondary tap and get the same results as a 7K : 8 ohm? Is this as simple as primary : secondary ratios?
Thanks.
1. Should the OPT be spec'd for 25W + % headroom? If so, what % is reasonable to use?
2. I am targeting a 7K primary and 8ohm secondary. There are some 3.5K primary OPTs available and therefore, can I use the 3.5K primary with 4ohm secondary tap and get the same results as a 7K : 8 ohm? Is this as simple as primary : secondary ratios?
Thanks.
You can use a transformer at twice it's design impedance but you'll lose an octave at the low end - i.e. if it's good for 30 Hz at 25 W at 3.5K, it'll do 60 Hz at 25W as a 7K.
It depends on what you want. If you are looking for good bass performance your point 2) is not good idea. Using a 3.5K OPT will generally result in low inductance (and so high distortion) when you use it like a 7K OPT even if the power rating is good. It's very unlikely that you will find a 3.5K transformer with 40-50H inductance.
There is no reason to specify the transformer with more headroom. The correct way is to specify the power at the lowest frequency you want to reach. Normally 30Hz is the best compromise between performance and size, maybe 25Hz.
This is an important point. The power rating is derived from an AC voltage limitation, and is a strong function of frequency.
High-power, high-impedance SET outputs are often compromised in power bandwidth even if the small-signal bandwidth goes deep. Ideally you need to know the frequency and DC current at which the power rating is quoted. Using less than the rated DC current provides some extra margin for more power. It's complicated ...
I was just about to post a similar question about overhead.
So, a 7W rated OPT (like Hashimoto’s H203S) is adequate for a 300B SET amp?
So, a 7W rated OPT (like Hashimoto’s H203S) is adequate for a 300B SET amp?
In my opinion the H203S is borderline for 300B. It's 7W at 50Hz and that means that below 50Hz undistorted output will be less. inductance is low. It's a "budget" transformer. Unfortunately these products are way more affordable in Japan and much less outside.....
The H20-3.5U is a much better choice. Yes, it's almost twice the price but it ticks all boxes. The H203S is ok for 3.5W 2A3 amp.
The H20-3.5U is a much better choice. Yes, it's almost twice the price but it ticks all boxes. The H203S is ok for 3.5W 2A3 amp.
Looking for comparisons of capacitors, vacuum tubes, resistors, LOTS of information.
Trying to find comparisons of OPTs, I’m coming up short.
(It’s my google-fu, master, it’s very weak)
Trying to find comparisons of OPTs, I’m coming up short.
(It’s my google-fu, master, it’s very weak)
all valid comments, many thanks! And indeed, comparison of OPTs are rather rare. Member George (Tubelab) did some, about 20 years ago there was a shootout of SE OPTs at the European Triode Festival. Probably because OPTs are rather expensive, or, as I often read: get the best OPT you can afford, as this is key for the overall performance. And the best is very often also the most expensive. Caps and resistors are cheaper and easier to replace in an amp.