For Nick - Yet another Pentode input amp

[James D]

Too late!

We have all read about how good LCR RIAA stages are and how good the E810F is... Expectations are high and getting higher

James

[Stuart Dean]

James
Any ideas on an SE PT15 design.
Pentode driver and min component count.
My PT15's are getting bored sitting in a box after their exiting life in a Lancaster bomber. Thats what I like to believe anyway.
Pentode drive Ok by me. My Mullard 3/3 built in 1992 is regularly dusted off and given a workout.
Uses the same Mullard EF86/EL84 valves I plugged in in 92.
I can find transformers up to 500/600v. Not really bothered if it's valve or SS rectified. Various chokes also available.
Thanks in advance for anything you come up with.
STU

[James D]

As it happens I have but I don't have it drawn up electronically yet. I used C3g with 200V on the anode and 150V on the screen grid with a VR tube screen grid to cathode. C3G set for a gain of 60 so Ra about 4-5K. Make sure the VR tube has 15mA current through it minimum for minimum noise. Cap coupled to PT15 in triode mode. Used a 5.2K output trannie. $00V Va on the PT15 50mA Ia and 40Volts on the cathode so 450V B+.

PSU has two options - the ultra quiet one I posted on a seperate thread or a conventional LC one.

Any meduim to high Gm pentode can be used. I have used from C3m at 6mA/V to PL802 at 40mA/V. EL84 are fine but most of the others sound better as a driver! Secret goodies that I have tried are C3g, 12GN7 and PL802. Also good are C3m and the EF180, 280 and 810.

I'll complete the drawing overnight and post something tomorrow.

I think you will be surprised just how good the PT15 sounds

James

[Stuart Dean]

Thanks James
I'll see what In can find in the way of pentodes.
I had already thought of using an EL84.
STU

[NickG]

James: actually I am rather happy with it now, my worry about the harsh top end being the result of oscillation has been carmed now, as I said the grid stoppers sid help a bit as have the ferrites. But without doubt the worst part was from the 10uf solen coupling cap. He is a crafty chap that Thorston, I had seen the original circuit used a big cap bipassed with a small, with a 56R sesistor in series with the big, and suspected it was more than just a rumble filter, but looking at it a bit more I realise it means the 10u cap is the main cap below about 8khz, crossing over to smaller but better cap for the HF. Almost the best of both worlds. I think I must try that idea in a few more places...

Stuart: if you need a couple of EF180's to try let me know, I have a few more than I need (too good a deal to pass).

The only question with the phono now, is why the d3a's were so dull. Maybe they are just old and tired. I can't expect too much I guess at 10 for £30.

[steve s]

I'm driving my se pt15 with a high gain triode,

if i where looking to pentode drive..

i'd use a tetrode....something on the lines of a kt61,,
you could nearly use the kt61 as a 1 valve amp

i'm sure it will give a much better bottom end than many...


Don't leave them pt15's in the boxes too long..
I 'm running mine trioded at only at 300volts -22grid volts.. I think thats around 40-50 ma and getting about 3 watts output

sounds really good to me...

steve

[Paul Barker]

Yes I loved the sound of your pt15 amp at aggfest, possibly prefer it to the px4 and px25.

[steve s]

thanks paul.. i will have to have another go with the px's in the amp.. but not before the det's

i'm under pressure not to have something that looks like its from a old horror film in the lounge... so may have make a choice and build a new amp soon

maybe in time for eggfest 4...

cheers steve

[Stuart Dean]

Hi Steve
Might try KT61 or maybe an el37
I had thought of experimenting with the KT61 as single valve amp.
I did try this with an el84 and with a high output source it was quite impressive 'bout as simple as you can get and absolute minimum component count.
Quite like the look of James's Quiet PS but just can't come to terms with BG prices.
STU

[James D]

I made a 12GN7 spud amp. Three times the Gm of an EL84 so it did the job easily. Its now the output stage of my sons latest guitar amp but it made a terrific little hifi amp when given an easy load and had loads of gain - only needed 1V for full output.

James