I found this schematic in the web and I want to build one of this amplifier, but the problem is what type of output transformer is using in this schematic? I'm living in Malaysia and here is very difficult to find output transformer, even if you can find one, it will be very expensive. I think custom made is the only choice here. Anyone out there can give me some useful guide to make this output transformer?
Making output transformers is something of an art. Nevertheless, the opriginal Williamson article gave details on how to make the output transformer and that transformer would be entirely suitable for your circuit (which bears more than a passing resemblance to a Williamson). I believe Hi-Fi World is available in Malaysia, and their subsidiary World Designs sell a booklet comprised of all the original Williamson articles.
More like a Mullard, I think? This appears to be a circuit out of Audio from 1959 (March). The transformer is an S152A, 4000 ohms plate-to-plate. The amp is good for 50 watts as shown.
If you can find iron out of an old Dynaco Mark III, that will work just fine. There are quite a few commercially available transformers with 50-60W ratings and 4000-5000 ohm primaries that shoudl work just fine.
If you can find iron out of an old Dynaco Mark III, that will work just fine. There are quite a few commercially available transformers with 50-60W ratings and 4000-5000 ohm primaries that shoudl work just fine.
Hey Matthewong,
Nickc, DIY member here, would be able to help you out.
Promitheus Audio
regards,
zul
Nickc, DIY member here, would be able to help you out.
Promitheus Audio
regards,
zul
Williamson was cathode bias, this is fixed. Same idle current balancing topology, though.
Interestingly, the original article put all the focus on the use of silicon rectifiers, a novelty at the time. The doubler was used because of the difficulty of getting a rectifier with more than a 400PIV rating.
The voltage divider on the screen of the input tube, referenced to the cathode, is an interesting trick.
Interestingly, the original article put all the focus on the use of silicon rectifiers, a novelty at the time. The doubler was used because of the difficulty of getting a rectifier with more than a 400PIV rating.
The voltage divider on the screen of the input tube, referenced to the cathode, is an interesting trick.
I'm going by what Claus Byrith stated in his article "Power Amplifiers with Valves," in which he partially redesigned the Mullard 5-20. One of the changes he made was to replace cathode bias with fixed bias. He stated that the ideal load p-p for the EL34s in UL would be 6.6k but he was obliged to use a Lundahl OPT and the closest they had available at that time was 5k p-p.
ray_moth said:
For me it’s quite confusing why Byrith is referring to those curves since the curves are for cathode bias.
An externally hosted image should be here but it was not working when we last tested it.
IIRC David Hafler very deliberately chose the 4.3K plate to plate load and the specific operating conditions (UL/Fixed Bias/idle at 100mA/pair) in the ST-70/MKIV to minimize 3rd harmonic which is not cancelled in the output transformer, reasoning that most of the 2nd which would then be somewhat higher would mostly cancelled anyway.
Output power should be >40Wrms and depending on the supply and output transformer could easily reach 50Wrms.
I have built similar designs in the past and with a good output transformer this should sound pretty good. Note that you will need to tweak the compensation as it intended for the recommended transformer.
Some of the output transformers used in clones (Chinese and American) of the old Dyna ST-70 would be a good match. There are several Chinese vendors that make transformers that ought to be suitable like ASL in Hong Kong for example.
Output power should be >40Wrms and depending on the supply and output transformer could easily reach 50Wrms.
I have built similar designs in the past and with a good output transformer this should sound pretty good. Note that you will need to tweak the compensation as it intended for the recommended transformer.
Some of the output transformers used in clones (Chinese and American) of the old Dyna ST-70 would be a good match. There are several Chinese vendors that make transformers that ought to be suitable like ASL in Hong Kong for example.
Hi,
There is a chap in Malaysia I know who winds excellent OPT.
Check this chap out, ew@audiohobbyist.com
Excellent quality, in the same class of Audo Note & other big names, at extremely affortable price... all hand wound and measured. He custom wind to your requirements... by hand...
even have c-core... I bought a few pairs from him, both SE & PP.
A strong recommandation from a very satisfied customer.
Cheers
A-I
There is a chap in Malaysia I know who winds excellent OPT.
Check this chap out, ew@audiohobbyist.com
Excellent quality, in the same class of Audo Note & other big names, at extremely affortable price... all hand wound and measured. He custom wind to your requirements... by hand...
even have c-core... I bought a few pairs from him, both SE & PP.
A strong recommandation from a very satisfied customer.
Cheers
A-I
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