Help with ef86 mic pre input

[Vyvyan]

ive built a few of these and love them working on one now and the gain seems much lower the my previous builds.

im using a berlant t-3344 transformer from a concertone 20/20 which has the transformer feeding a 12ax7 with a 1m resistor on the input to audio ground. so my question is should i up the 500k on the bruteforce build to 1m? osis that to high for an ef86?

also can anyone explain the purpose of the 243 series resistor post input transformer.

the schematic is for a single channel but its a 2 channel build so the power suply schematic has provisions for 2 channels

 

[marcuswilson007]

The EF86 should handle the 1M but it will likely make no difference to that transformer which has an effective output impedance of 45K. The 243R resistor is a snubber to prevent the valve oscillating at 10s of MHz. It should be directly on the valve socket.

I have just mounted 2 Gates 5215A preamps in a 2U case. They are similar to your design except for a single ended output triode and some negative feedback. I like the cascode output setup you have, it should give you lots of drive.

 

[Tubetec]

I would think a 1meg resistor would have an effect in that position , top loss .
try 100k and see how it sounds .

Connect the 100k directly across the transformer secondary ,that way when you interupt the input with the DI you still have the 500k or 1M load at the grid .

 

[radardoug]

Why would it introduce top loss?

 

[marcuswilson007]

Exactly!

Hi Doug, I hope you are well.

 

[radardoug]

Yes I'm good Marcus, still alive!

 

[Vyvyan]

The EF86 should handle the 1M but it will likely make no difference to that transformer which has an effective output impedance of 45K. The 243R resistor is a snubber to prevent the valve oscillating at 10s of MHz. It should be directly on the valve socket.

I have just mounted 2 Gates 5215A preamps in a 2U case. They are similar to your design except for a single ended output triode and some negative feedback. I like the cascode output setup you have, it should give you lots of drive.

thanks, not my design but i love them! theres i have a ton of akai/roberts pres that ive been building these out of. theres also an insturment amp version that can fit in a unit alongside the mic channel. very useful tools.

Im thinking of building a pultec out of one of these as well, do you think the power supply would be adequate? with the 250-0-250v xformer, 6H choke and 6x4?

also thinking of the attached eq/line amp schematic from the same designer

 

[Piedwagtail]

That C2 in the Gates 5215 is interesting. I'd like to hear the difference between terminating into ground against into the 2nd stage cathode.
6eu8.....

 

[Tubetec]

Wouldnt the effect of any incidental capaciatnce across the secondary make a difference between 100k and 1meg load values ?.
I always thought around 100k was the most youd load an input transformer with for good high frequency response .

 

[radardoug]

Only if you have a very high impedance secondary. Most transformers are much lower. Jensen use 10K termination on their 10K bridging transformers.

 

[marcuswilson007]

thanks, not my design but i love them! theres i have a ton of akai/roberts pres that ive been building these out of. theres also an insturment amp version that can fit in a unit alongside the mic channel. very useful tools.

Im thinking of building a pultec out of one of these as well, do you think the power supply would be adequate? with the 250-0-250v xformer, 6H choke and 6x4?

also thinking of the attached eq/line amp View attachment 109436schematic from the same designer

I think the Akai power supply would have enough grunt for a Pultec clone. The recorder has an EL84 for speaker output which draws lots of power, when that is removed there is a lot of power available.

 

[marcuswilson007]

Wouldnt the effect of any incidental capaciatnce across the secondary make a difference between 100k and 1meg load values ?.
I always thought around 100k was the most youd load an input transformer with for good high frequency response .

The load resistor doesn't normally determine the impedance at the transformer secondary. The impedance is reflected by the source impedance (the microphone or whatever) and the transformer's impedance ratio.
So if you take a Jensen JT-115, made for FET or valve preamps, it's turns ratio is 1:10, so the impedance ratio is 1:100. If you have a mic with 150 ohms impedance the reflected impedance at the secondary would be 15K ohms. A 1M resistor will have no effect on that. Due to winding resistance and various other losses, Jensen give the effective source impedance of the secondary as 17K.

 

[Vyvyan]

I just found 4 beyerdynamic 1:20 transformers from an old nagra mixer i want to try in this circuit.

Any suggestions for loading? or just stick with 500k?

 

[ruffrecords]

500K will 'look like' 1200 ohms at the primary which is a reasonable load for your typical 150 ohm mic. It might be worth experimenting with the polarity of the secondary. Some of these high ration mic input transformers are known to have better HF response one way than the other. And of course keep the leads from the secondary to the grid of the first tube as short as possible/

Cheers

Ian