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help on ul transformer please

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arex

Member
2005-02-21 5:37 pm
france
Hello,

I am on the point to finish my first all-tube amplifier: 6L6 SE ultralinear, bout 5-6 watt, 2 x 12ax7 as voltage amplifier.

Now I am looking for output transformers. (they must be cheap). I found this one: http://store.triodestore.com/tf65wscta48o.html

But it is a GUITAR amplifier transformer (I think, made for replacing a fender one) with 50-20KHz at 20W, 80 mA.

The alternative is Hammond 125ESE, but I must change my final stage, it has no the 43% winding..etc. It has a 150-15000Hz bandwith, 80mA too. but I found somewhere on the internet some test where it makes better than this. (20-20000).

Could somebody more experimented give me an idea, please? Or maybe you know where to buy some other cheap ultralinear otp?

many thanks.
 
I think you're in the wrong direction. That Fender replacement OT is a push pull one. Yes, you can use push pull OTs in a SE amp, but you must use an extremely big one to avoid DC saturation. Better to try some SE OT... try the Hammonds, they're cheap... maybe you can get one, try... if it sounds good, but the other one: if it doesn't, well... keep it for old tube replacement!

Btw where did you found tests on the Hammond Universal Series trannyes? I was looking for a PP one for a guitar amp, but that "150Hz" scared me... that OT has no bass I thought...
 

arex

Member
2005-02-21 5:37 pm
france
sorry; schematic:
 

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You can't modify that circuit to use the 125ESE since the 125ESE does not have a tap for the screen (aka "ultralinear" tap). The possible alternatives would be to operate the valve in fixed screen mode (screen at fixed voltage, as the name implies), or triode mode (screen tied to anode).

The 125ESE has a primary inductance of only around 9.5H or so, therefore it may be useful to drive it with a low-ish impedance source, either a triode, or pentode with local feedback applied - possibly from the anode to grid.

Is this a hi-fi amp or guitar amp?
 
arex said:
I understand that it will be not a ultralinear stage. But I will loose in sonic quakity because of that.? I mean can I manage this new final stage to keep the same quality?

Well, output impedance and distortion will rise, and bandwith will likely suffer, should the output valve be operated in pentode mode without any form of feedback. You can fix this to various extents by applying external feedback around the output stage, either in a loop from the OPT secondary to the cathode of the 12AX7 input valve (like done in the original schematic). Since the OPT doesn't really have massive primary inductance, it might be a good idea to have local feedback around the output stage first, then have a global loop. Anode to grid feedback, or 'partial feedback' can be read about here.

I can't really say whether you will get the same sonic result from operating the valve in pentode mode as opposed to UL even after feedback. They will simply be different... 'better' depends on a whole host of other things, not least on your expectations.
 
Arex,
My advice would be to use the correct transformer for the job, rather than try to make some "cheap" unit work in the circuit.

Look up 6L6 on the Duncan Amps site:
It tells you that a Class A Single ended 6L6 wants a load of 4200 Ohms.

Normally you don't adjust the load for Ultralinear even though the output impedance (rp) of the tube is lower.

Now you want "low cost" - so a Hammond 1628SE is what you want. This unit is 5000 Ohms primary with 40% Ultralinear Tap, and a FULL POWER Bandwidth from 20Hz to 20kHz.

You could probably also use the 1627SE but I think the 1628SE is a better match to your circuit.
Why?
Higher primary inductance => better bass
Higher primary load impedance => lower output impedance => better damping factor before feedback is applied so you can get away with less feedback which will make the amp "livelier".

You will enjoy your first amp MUCH more with with these transformers.

Cheers,
Ian
 

arex

Member
2005-02-21 5:37 pm
france
Hello again !

I asked a question about output transformers. Now I found on ebay 2 French transformers Tu-101 AUDAX, "universal" output transformers. They are push pull, primary impedance is 2 x 2000, 6000, 8000 ohms, max. poxer 27W at 25Hz, bandwith 15-40000 Hz+/- 1dB. Primary inductance 200Hy.
The selles says they can be uses for single ended too.

So, please, could I use them for SE ? It is true ?
I mean with the same good bandwith.

The tubes I have:
3 x 6L6GC
3 x 807
8 x EL84

I could make a push pull with EL84, but They are not matched.
What to choose, please?

many thanks
AR
 

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