• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Series connecting output Transformers.

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#21
Sorry Mississippi
I missed your post about school - I didn't do electrical engineering in school so I no doubt lack the basics but am willing to learn

Please bear with me if my questions are stupid, I'm a "noob to tube"

John
 
#23
>> I have two PS toroids 230V 25-0-25V 160VA - based on my calcs I would get 1.2k Pri impedance running 15ohm speakers from one secondary of this trafo. How does dual sec come into the picture? Do I need to unwind about half the secondary to get it down to about 8V to get 9K impedance?

First - is this a center-tapped (or dual primary) transformer? If not, it will be tough to use, as you need to balance out DC flux in a toroid core. For a single-ended amp, you could use parafeed - a choke for DC, AC coupling to the transformer.

If the center isn't potted, you could wind your own secondary on the outside - 10% is a good ratio for cathode feedback - use the 25V winding there. For good coupling, you want full coverage with the secondary - multi-filar winding would be good.
 
#24
Tom,
Yes of course - I forgot about it not being centre tapped - got carried away on a sea of calculation & missed the big picture.

I may well use another toroid I have which is single tapped 230V 35V Sec - strip it down & rewind. I have a question about multifilar winding - I believe that it means combining all wires together into a multi strand & winding with this. How do I wind fewer secondary windings (to get smaller sec voltage) if all wires (pri & sec) are wound together?

Again, probably a stupid question but I'm ignorant about these matters.

By the way, I hope I haven't hijacked this thread if I have then I will start a new one instead to answer these matters.

Thanks all

John
 
#29
Shooq,

.....you are difficult 🙂

I also find no reason why this would not work, except for perhaps the P-S leakage reactance in case of NFB stability. I do not know toroidal transformers; is that leakage necessarily low? But you can presumably measure that as well as primary inductance, so?

There is also not the usual "full coupling" between windings, since you now have two separate magnetic circuits. It could be argued that the split primaries and parallelled secondaries will look after this, but that is by back induction. An interesting experiment; one would like to know what the above parameters turn out to be.

Regards
 
#30
I may well use another toroid I have which is single tapped 230V 35V Sec - strip it down & rewind. I have a question about multifilar winding - I believe that it means combining all wires together into a multi strand & winding with this. How do I wind fewer secondary windings (to get smaller sec voltage) if all wires (pri & sec) are wound together?
Click to expand...

Suitable toroidals aren't that expensive from Farnell. Your asking for a hell of a lot of grief if you go rewinding a toroidal core from scratch. Think in terms of hundreds of turn, chances of doing that without making a huge mess are slim.

You wrote:
'Microwave oven transformers make food parafeed plate chokes.'

Could you expand on this please?
Click to expand...

Someone on this forum suggested this to me. I am using it in a parafeed version of the RH807. I am using the primary winding of a transformer from a salvaged microwave oven (standard domestic). The secondary can be wired in series with the primary, but it doesn't really matter if it is left hanging. It works surprisingly well, though not quite as well as a Tube CCS.

Shoog
 
#31
In order to use the CCS (LM317) I need to measure the voltage drop accross the 130R bias res currently in the Rogers Cadet and using this calculate the current. Then set this current with the LM317 res (bypassed by good cap) to ground.

Is this the correct way to proceed?

John
 
#34
One other question - does each tube need a CCS to it or can a pair of tubes share one CCS?
Click to expand...

You can do it either way, but if you use a single one and there is imbalance between the output tubes, this will be present in the transformer. It takes very little of this imbalance to compromise the bass response of a toroidal.
I have adopted the approach of using one for each tube to overcome this. It introduces its own imbalance (ie the tubes produce slightly different waves and so the merged wave is slightly smudged) but I think this is by far the lesser of two evils.

Shoog
 
#36
That is one way to do it. Unfortunately the down side is that you will have to do it every time you change a valve, and then at regular intervals throughout the life of each tube. Tubes operating points drift over time, and not in a uniform manner.
Save yourself the hassle and give each tube its own CCS.

You can salvage LM7905's and LM7805's from old PC power supplies (along with good caps), which will do nearly as good a job as the LM317's.

Shoog
 
#37
So giving each tube its own CCS delivers the same current to each tube without adjustment - but what is the imbalance you talk about above?
It introduces its own imbalance (ie the tubes produce slightly different waves and so the merged wave is slightly smudged) but I think this is by far the lesser of two evils.
Click to expand...
 
#38
Each tube is working at a slightly different operating point. This effectively makes them slightly different valves, with slightly different operating curves. Therefore the two output waves that are presented to the transformer will be slightly different. In the ideal situation they would be identical and so would combine perfectly. In our theoretical situation, when they combine they produce a new waveform which is a product of the two combined waves. This a form of distortion. This type of distortion would be most noticeable in the higher frequencies, but should be reduced if gNF is been used.
I could not measure its overall effect, but I would estimate that this is a very slight factor - and as I say a much better result than having current imbalance. It might be speculated that this is a form of distortion which is inherent in any PP circuit. Don't worry about it.

By the way, if you go down the path - bare in mind that the valves can only work in class "A". This may be a limitation, but it almost forces you to go class "A" triode, which is a very good thing in my book.

Shoog
 
#39
jkeny said:
Do you use a pot to balance the current to each tube in pair like in this schema? http://www.diyparadise.com/buildel84c.html. This seems like the way to do it.

John
Click to expand...

That link didn't work for me, but here's an article showing the use of a balance pot and CCS on the driver (but same principle applies). I did something similar on a 6CK4 amp, including adding an inexpensive meter (described very briefly in the article linked on the site) so that I could easily trim the balance to within a mA or better IIRC. It works nicely and the drift is minimal. I'm not using toroids though.

http://www.wimdehaan.nl/tubeamps/el84dc/index.html


Sheldon
 
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