Leak TL/25 Gain control?

[6B4G]

LeakTL25plus.gif (59.15 KiB) Viewed 4422 times

LeakTL25plus.gif (59.15 KiB) Viewed 4422 times

Me again with a another question.
A friend came over after hearing that I was working on a LEAK and asked me weather the gain control that was removed from his amp (tl/25 plus)
could be put back.
I said that it can.......................
Here's my question.
There are two resistors in series that come from pin 9 of the EF86 that go to ground (22k and 1Meg) and the input signal enters at the
junction of the two resistors. The 22k is the one connected to pin 9 and the 1Meg to ground.
Do I just remove the 1Meg resistor and add a 1Meg pot or do I do what is shown on the tl/50 schematics, add a 250k pot?

[6B4G]

Sorry for the two 25 schematics they are both the same

[Gingertube]

The 22K is the grid stop to supress any tendency to parasitic oscillation - it should stay.

The value of the pot is not critical. Its value will affect High Frequency Roll Off at the input, Pot effective resistance + Input Tube Miller capacitance forms a low pass filter which is the same as saying a high cut filter.

Since the input tube in this case is a pentode then Miller capacitance is extremely low and the pot value can accordingly be higher than if the input tube was a triode.

Even so you don't want that pot any higher value than required by the preamp that drives it. Ideally a 50K pot is what you want but a tube preamp may not drive that well and so you might want to go to 100K or 250K. Of course a 1M would work but yo may suffer some High Frequency loss.

So choose the pot value depending upon what is driving the input. If using a passive preamp (i.e. just a selector switch) then you want to err on the high side, say 500K max.
If using a Solid State Preamp then 50K is the go.

Why not just use the 250K value on the 2nd schematic.

Cheers,
Ian

[6B4G]

Hi Ian and thank you.
I could honestly say as a person that has been playing around with tube amps for the last
7-8 years and that has asked many a question, due to my lack of in depth knowledge.
I am truly impressed with the answer that you gave me, it was direct and to the point with
lots of info that I could understand and implement.
Ian truly thank you for your help.

Best Regards
Greg.

P.S.
I wish there were books out there that would explain tube amps and pre-amps like that.
Ever thought of writing one?

[6B4G]

Ian I just saw you are also a Adelaide boy, I was born and lived in Findon, now OS.

[Shannon Parks]

I wish there were books out there that would explain tube amps and pre-amps like that.
Ever thought of writing one?

Do you have Morgan Jones' book? Highly recommended.
http://www.amazon.com/Valve-Amplifiers- ... 0080966403

Shannon

[6B4G]

Thanks Shannon it'll be my X Mass present to me.

[Gingertube]

Ian I just saw you are also a Adelaide boy, I was born and lived in Findon, now OS.

Hi Greg,
I live in Propect but work in Findon (Valetta Rd.). Design EE for Fugro LADS Corp. LADS = Laser Airborne Depth Sounder.
Small World!!!
Cheers,
Ian

P.S. I have that Morgan Jones book on my bookshelf, in fact I had the 1st edition and was impressed enough to buy the 3rd edition and pass the old one to another hobbiest, I note it is now in its 4th edition - RECOMMENDED.

[6B4G]

Thanks Ian and all the best.
Merry Christmas and a Happy New Year

Best Regards
Greg

[johnf]

As you mentioned Morgan Jones, you might look at the section on running EF86 as a triode. I did this with a pair of TL50 amps and they ran much quieter. These amps were built for a 150mV input so that level of gain is no longer needed. I also replaced the 250k pot with a voltage divider set for a gain level determined with the pot. I put the mods back to original when I sold them.

[Shannon Parks]

P.S. I have that Morgan Jones book on my bookshelf, in fact I had the 1st edition and was impressed enough to buy the 3rd edition and pass the old one to another hobbiest, I note it is now in its 4th edition - RECOMMENDED.

I have the 3rd edition, but I've seen bits of the new edition online with completely new material. Very tempting....

Shannon

[kheper]

Unless my eyes are shot, the filtering in the PS is only 16 or 32 uF - depending on the schematic. Substituting a 32 uF after the rectifier and an 80 uF after the resistor should be a big improvement.