SE output transformer

[tonie]

Hi everyone,

My brother in law has started playing guitar, so we want to start building amps again. (I've been on the previous ampage board, but then the interest faded due to a lack of time.) As a first amp we want to make something like the ax84 Hi-Octane/Marshall 800 preamp with SE power stage suggested by KOC.

I have read KOC chapter on the champ in TUT3 and the forum on ax84 but still can not decide what OT to use. At ax84 everyone uses a 125DSE (or similar) because of the gap. Unfortunately this transformer is less versatile and more expensive than the 125E, which they say sound harsh. According to KOC you have to use a feedback loop, ultra linear operation or triode operation to get rid of the floppy bass response of a single ended power amp.

On paper, the 125E in ultra linear operation looks the best option to me, but I'd like to hear your opinion/comments/advice on this. Any help is appreciated!

Tonie

[mooreamps]

Ultra linear mode is a straight line, and thus the tone a little flat sounding since it nulls out the even harmonics. NFB will tighten up the bass responce. Just don't over do it. Kevin will write about "all" the different ways one could design and build an amplifier. But he will leave it up the the builder to figure out how he/she wants to do it.

The 125E is a favorite amonst small builders, hobbists, and the self appointed internet forum guru's. Not sure why. All the models in that series sound equally good to my ears....

-g

[Chuck H]

Why do you need versitility if you plan to use the OT in a particular design where one model has already been ear tested by many? I'd use what's known to sound good for the project at hand. In other words, a tractor is more versitile than a moped but I would pick the moped if it's dedicated task was to get around town.

I don't have any personal experience with that amp project. These are just my observations based on the considerations presented.

Chuck

[tonie]

Hi guys,

Thanks for the reply! I see what you mean, chuck, but the idea is that we will make this amp as a learning project, so when it is finished, we have some practical experience. Maybe it sound great and he will keep it, or maybe it is not what he is looking for (probably because we are too greedy to buy a decent OT ) and then we can take it apart and make another amp with mostly the same components, for instance with a PP power stage.

So basically the question is: is ultra linear mode a good alternative for the air gap in a SE OT, or is the air gap just superior to anything?

Thanks anyway!

[jimboyogi]

Tonie, I don't have any experience with the Hammond OT's that you mention. However I have built a couple of SE amps, one running a 6V6 and one running a 6CA7. On the 6CA7 build I tried a number of OT's:

First a 10W rated 70v line matching transformer (no air gap)- got the correct impedance, but volume was low, bass response was poor and on a CRO the bass waveforms (100Hz) showed a lot of distortion on sine wave testing

Second a 20W rated 100v line matcher (no air gap)- as above, but more volume and better bass response/less distortion

Finally a proper air-gapped SE OT from Allen amps - Much better bass response and therefore much more apparent volume, much less bass freq. distortion and therefore the effect of greater headroom. I wouldn't hesitate to buy another of these, the difference in sound quality is definitely worth the extra ~$20!
Scroll down to the TO11C - Allen Amplification - Parts Order Page

I had NFB switched on/off, bass response was a bit tighter with NFB, but not too bad without for me. You can easily try a NFB switch on your build and decide for yourself.

[tonie]

Hi Jim,

Thanks for your reply. I heard some good stuff about the Heyboer transformers from others, but their site is down, so I cannot see if they have a supplier in Europe (I'm from Holland, in case you did not noticed from my bad English) and because transformers are relatively heavy, it does not make much sense to import them myself. There is also not much to build with the small power transformer needed for this amp, only something like an AC-15, so I have to spend more money for the PT then I would save on the OT, so it has to be a keeper.

What is a line matcher? Just another term for an Ultra linear tap? And NFB? No Feed Back?

Thanks!

[jimboyogi]

Tonie,

Sorry I didn't realise that you were in Europe, your English is excellent! One difficulty that you will have if you want to order the OT from Allen Amps, is that they do not export outside of the USA. I had to get a friend in the states to order them and send them across to me in Australia, I bought 2 at once to save on shipping.

One thing that I have noticed is that there seems to be a lot of transformers for sale in Europe on ebay, maybe you could pick up something cheap and local there?

I don't quite understand what you mean re. the PT. You can use a PT with more current capacity than you need for this build, as long as the voltages are correct. Too much current capacity is a good thing. Then if later you want to use it for a higher power build, it will have enough juice.
I have been using these Antek PT's - Antek - AN-1T250 , 6A of heater current and 0.4A of B+ makes them suitable for a wide range of builds, and these folks do export outside of the USA. Once again if you get a number sent at once then the shipping is greatly reduced per unit.

A line matcher is a transformer designed for using multiple speakers in Public Address situations. Some great information here - Cheap Output Transformers
They can be set up with U/L taps, but are really only very useful for push-pull output stages, as they have no air gap in the core and therefore saturate with the DC standing current in a SE amp. On this topic, I don't understand how using an U/L screen tap would help to minimise DC standing current in a non air gapped OT?

Also, sorry about the jargon, NFB is an acronym for Negative FeedBack.

[tonie]

No problem, Jim. I'm still learning and your help is really appreciated. Maybe it isn't that clear form the start post, but I wanted to know if it make sense to build a single ended amp with an output transformer without air gap, so I can use this transformer in another amp another time and the hammond 125E is almost half the price of a hammond 125DSE. Therefore I asked some opinions about a suggestion of KOC: use a universal push-pull output transformer and use the center tap as ultra linear tap. So it's not about minimizing the standing current, but keeping the learning costs as low as possible. The comment about the power transformer was also with the idea in mind to use it in another amp, but the 20 euro's I save on the output transformer I have to spend on the power transformer, otherwise the whole idea about using the transformers again doesn't make any sense.

I was planning to get the transformers from a Hammond distributor in Holland, but those Antek transformers look like a good value for money too. I don't know about Australia, but in Holland we have the customs and they search for parcels from outside the European Union and if they think that it's worth more then 22 euro's you have to pay taxes and some money to the postman, because they paid your taxes in advance. So I can only get one at a time if I don't want to pay the taxes and I have t check if it is still saving me some money. Anyway, thanks for the link. Can I easily combine the secondary windings to use it as if it has a center tap?

Thanks for all your input. We will start searching for parts now and hopefully be able to show you a nice amp in one or two months.

[pdf64]

My understanding is that an air gap is essential for the OT in SE amps.
Push-pull OTs used in SE amps have their magnetic circuit saturated by the SE dc, resulting in them losing inductance and so bass response - just what you don't want.
See
http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#gaps
and
http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#PPtoSE
and
http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#PPtoULSE

[jimboyogi]

No problem, Jim. I'm still learning and your help is really appreciated. Maybe it isn't that clear form the start post, but I wanted to know if it make sense to build a single ended amp with an output transformer without air gap, so I can use this transformer in another amp another time and the hammond 125E is almost half the price of a hammond 125DSE. Therefore I asked some opinions about a suggestion of KOC: use a universal push-pull output transformer and use the center tap as ultra linear tap. So it's not about minimizing the standing current, but keeping the learning costs as low as possible. The comment about the power transformer was also with the idea in mind to use it in another amp, but the 20 euro's I save on the output transformer I have to spend on the power transformer, otherwise the whole idea about using the transformers again doesn't make any sense.

I was planning to get the transformers from a Hammond distributor in Holland, but those Antek transformers look like a good value for money too. I don't know about Australia, but in Holland we have the customs and they search for parcels from outside the European Union and if they think that it's worth more then 22 euro's you have to pay taxes and some money to the postman, because they paid your taxes in advance. So I can only get one at a time if I don't want to pay the taxes and I have t check if it is still saving me some money. Anyway, thanks for the link. Can I easily combine the secondary windings to use it as if it has a center tap?

Thanks for all your input. We will start searching for parts now and hopefully be able to show you a nice amp in one or two months.

Yes you can, that's exactly how I run mine into a full wave rectifier.