Jörgen Schou transformer?...

[Johan]

has anyone used/know about these..
..I got myself two "JS Type 0,32M NO1290" transformers the other day..

the primary is 50/250/600 ohm and the secondary is 100kOhm. with the primary at 250 ohm I get 1:20, so sending this to a 100k pot then straight into one side of a 12au7, with a little luck, I could get something like 45-50db gain..it would be the absolute minimum, only the transformer and one gainstage(300volt, 15-22k plate resistor, bypassed cathode), but would that be too simplistic to be any good? ..I live in a 10k enviroment, so I dont need a 600 Ohm output..
I realize with the pot between transformer and grid, the highs might be affected by the millereffect at low gain setings, but lets ignore that for now..
what about the transformers themself? ..are they any good, what about when straped for high turn ratio?..would there be any significant difference in sound between 50,250 and 600 Ohm?

any thoughts/input?..
..I almost wish I didnt get these right now..I'm not finished with my two NYD 1B's yet.. ;D

thank's
johan

 

[Jonte Knif]

the primary is 50/250/600 ohm and the secondary is 100kOhm. with the primary at 250 ohm I get 1:20, so sending this to a 100k pot then straight into one side of a 12au7, with a little luck, I could get something like 45-50db gain..it would be the absolute minimum, only the transformer and one gainstage(300volt, 15-22k plate resistor, bypassed cathode), but would that be too simplistic to be any good?

Would you _really_ like to have an input impedance of 250 Ohms? Kind of hard on any microphone! Might work sometimes just fine though.

I realize with the pot between transformer and grid, the highs might be affected by the miller effect at low gain setings, but lets ignore that for now..
what about the transformers themself? ..are they any good, what about when straped for high turn ratio?..would there be any significant difference in sound between 50,250 and 600 Ohm?

The Miller effect will be your enemy, but not in the way you think. High step up (high leakage inductance) will make the transformer secondary extremely sensitive to capacitive loads, and therefore you probably get pretty bad treble resp. If you have high fidelity in your mind I think a combination of 1:20 trannie with simple triode stage is not the way to go. 

Combining acceptable input impedance+pot before 1st stage+good noise performance at -6dB of max gain is not possible.

furthermore combining high step up with almost anything else than pentode or triode with feedback is going to ruin your treble, more or less.
You got to give up something. What do you prefer?

would there be any significant difference in sound between 50,250 and 600 Ohm?

Even without knowing the transformers, yes, probably. Depending on the quality and overall design the higher step up is going to be a problem either in the bass or in the treble. 1:20 mic transformers can work quite nicely with low source Z and high load though. 100kOhm load will kill your treble. Miller will probably do the same with a nasty resonance.

12AU7 output impedance is anyhow fairly high. 10 k input in the next equipment will be lowering your gain, increasing the distortion and you can not have very long cables without treble roll off

-Jonte

 

[Johan]

thank's Jonte
so, the only really good way af adjusting the gain when using high ratio transformer, is throught feedback, like the one-bottle or all the other vintage pre's, then  ;D
..have to rethink things..
thank's
johan

 

[Sorr]

You could try feeding a square wave into the input and adjust the secondary load resistor for best response.
Sometimes a small value capacitor is used in series or in parallel with that resistor
From the Sowter data sheet for the 9045 for example,
"A damping network of 45k0 ohms in series with 25 pF across the secondary may be used depending on the input capacitance of the amplifier."

Using it as a mic I/P Tx I would use the 600 input.

 

[Johan]

thank's all
I've been home with the sick for a few days, but being restless I had to do something, so I drew up and built this on my breadboard

obviously more than a little inspired by the REDD47 but using parts I had around..so 6ak5 instead of EF86 and 12au7 instead of ecc88 and runing on lower voltage..I scaled the resistors to those in the datasheet for runing these tubes at this voltage and re-arranged the gainpot/selector...have a little hum at the moment, but it sounds really good...
..if anyone can give any suggestions to improve it ( before I get a chassis and start drilling) without actually building a REDD( we allready know that one sounds great, so where is the fun in that?.. ) I'd be thankfull

johan

 

[Jonte Knif]

Hi,

I tried to look for REDD schema. Could you give a link.

Any way, one key improvement in this topology could be made. Look at for example V72 or V76 schematics. (perhaps the REDD too? I'm sorry that I don't know it) Look at the cathode circuit of the first stage. A couple of resistors+cap lift the transformer secondary somewhat above ground. This is done to have a bigger cathode resistor value. Why? To make applying feedback easier. At low gains the output stage does not get loaded too much and you can go lower in gain if needed. In your design you have a lot of gain even at lowest setting. Combining this with high step up doesn't make the most versatile amp perhaps.

What is the local feedback through that 1M8 resistor there for? Just curious. Just wondering if it is wise to have that 330k resistor to raise impedance and therefore make the 2nd stage miller a serious concern.

The way to really optimize things gets past the scope of this (or any) post, so I don't know if this is just confusing more.

-Jonte

 

[Johan]

thank's Jonte.
I found the schematic for the REDD in a thread at diystompboxes.. http://www.diystompboxes.com/smfforum/index.php?topic=73025.msg591807#msg591807
as you can see, I basicly just simplified the feedback then took the datasheet numbers for the 6ak5 running at the voltage I get from my breadbaording PSU and put them in place of the resistors in the REDD circuit...obviously, a schematic on the net shouldnt be trusted, but it sounds good and the gain does work ( but I do hear what you say..)
j

 

[emrr]

what about the transformers themself? ..are they any good

Not sure which ones, but there' a JS type I've seen sell on ebay at $600/pair, if that means anything.  If possible, that's enough to buy almost anything you want.