PT15 - finally done it !

[Pedro]

Hi folks,

she lives, she breathes.....she sings.......

I finished the PT15/C3g this morning on the breadboard !!

Sounding great - even though my PS is very basic and the HT drops too much under load due to the old guitar power TX i'm using to get it up and running.

But wow.... what a sound , and with the borrowed 10K TX's from Nick its sounding really "bold" and detailed at the same time . PLENTY of BASS !

I just listened to Nimrod and the hairs lterally stood up on the back of my neck. Then "little girl" from maddonna's Ray of Light......and now Goodnight Song by Tears for Fears.

Lovely .....Goodbye 300B Fi-Primer...

lots of tweaking ahead.

Thanks for a wonderful circuit James...

...now how about a nice power supply fellas ?

will post links to pics etc when I have more time ( I'm supposed to be painting the kitchen ceiling )

Rgds
Pete C

[Pedro]

Oh....... and thanks of course to SteveS for the PT15s and the motivation !

Rgds
Pete

[The Shadow]

Fantastic valve the PT15 isn't it Pete.

Is your PSU DC?
Reason I'm asking-DOES IT HUM?

If it doesn't, it'll give me a bit more confidence in the results when I
start on my winter build.

The bass from this valve is phenomenal both in tightness and quality as well as sheer oomph.

It seems to be one of those valves that sounds big, bold, subtle and delicate at the same time. Not in least bit hi-fi (in the pejorative sense) but everything to do with music.

Well done

Steve.

[Pedro]

Hi Steve ,

PS is pretty basic at present - and i havent measured the residual ac ripple at the speakers yet Steve, but I have to say the hum is no more noticeable than on my 300B SE which seems to hover around 2mv-4mv.

I'll measure sometime this weekend when the kids are out of the way a bit.

PS is basically this : TX 350-0-350 > 5AR4 > 8uF Dubilier Oil Cap > BIG 10H Parmeko choke > 47uf 900v ( bank of 4x47uF maplin 450v jobbies ) > HT line.

Ive just remeasured HT and found that HT on the anodes is as low as 360v so much too low for my chosen Op point, but still sounding OK, just goes to show what poor regulation the scrap mains TX has - it measures 502v without load.

Also I'm using different light bulbs in the cathodes as I only had 1x100W and 1x60W spare !!

The amp is simply great - I want to get the HT right and play with Op point, and then get a regulated PS done for the finished amp over the winter

My aim is to have this as my main amp by next Spring set up in the living room.

I like the breadboard "dissected look" though ( blame Nick ! )

Must have listening session involving SteveS as well sometime on these PT15s. - you'll have to put yours back together now

will post more listening impressions once I get the higher HT sorted out.
cheers
Pete

[Pedro]

forgot to add ....

the heaters for the PT15s are indeed DC, but the C3gs are just lashed up AC from a spare guitar amp mains TX.

Also the PT15s are sharing the same 6vDC supply for now which I guess will be breeding crosstalk etc so needs sorting. I must get hold of SteveS 6.3v TXs in return for some Xmas booze ( steveS ...hint ! )

not much hum at all........

cheers
Pete

[The Shadow]

Originally Posted by Pedro

forgot to add ....

the heaters for the PT15s are indeed DC, but the C3gs are just lashed up AC from a spare guitar amp mains TX.
not much hum at all........
cheers
Pete

Yessss! That's what I wanted to hear.

Must have listening session involving SteveS as well sometime on these PT15s. - you'll have to put yours back together now

Oh it's definitely going back together over the winter with all the stuff I learned about eg pentode drivers, regulated supplies DC heaters, so should be ready in the spring all polished up in a nice case.

The power supply will be brand new with a proper HV transformer putting about 600V onto the PT15 plates and another much lower voltage transformer for the driver stages, all with proper regulation and smoothing.
So I'll be doing quite a bit of pestering James Nick, Paul and Dave Dove.

The Rocky pre/power system will remain functional on it's outboard supply.
That's not coming to bits, It's far too nice. It's got that classic creamy smooth tube sound I remember from my very young days in the early '60s.

Steve.

Edit - Now there's a thought. We might end up with a PT15 extravaganza at the next Eggborough meet: yours, Philip's and mine

[Pedro]

interesting Steve,

Its my first experience using a pentode input stage as well, ( except EF86s in guitar amps) and I'm very satisfied with the result.

I'm convinced that what will turn a great amp into a superb one will be the PS. I'd really like to know how to rig up a shunt reg supply for this amp but I'm not fully up to speed with that yet. I too will need guidance from the "elders" ! I'll help you pester Nick, James, Paul and Dave...

I intend to try DC heaters for the pentode input stage as well as the PT15s - but dont have all the right stuff at moment. I may have a spare 25A bridge rectifier around somewhere so I might try that in meantime just for fun.

I've just done one more listening session before I had to pack the breadboard away because of the kids , and I definitely need to find a better mains TX quickly - the HT is a lot lower than I want. But the sound is still really nice.

The thing I like best so far about the circuit ( and I dont know whether its James' design skill or the PT15's doing this ) is the ability of the amp to produce very dynamic low mids - so that sudden transients pop out like a prizefighters punch - without overpowering anything else in the mix.

I also like the less "barky/honky" midrange vs my 300B Fi Primer, and the more defined sparkly top end ( Glittery is the best description I can think of )

One downside is that I've discovered that my TWQT cabinets need bracing or sealing much better than they are at the moment - they are "flapping" a bit now with the PT15's finding the points of weakness. I built them with removable sides for stuffing/access. I need to sort that out before I listen to any Massive Attack for sure.

So next steps are to check rest of circuit voltages vs James' schematic and check my calculations on voltage measurements. I know there will be some tweaking to get the C3g's sitting at the 200v op point I want. Then its get the mains TX and permanent Output TXs ordered. In meantime its researching a suitable PS.

All PS ideas welcome gents ( and ladies ? )

cheers
Pete

[Pedro]

http://pedroc.myphotoalbum.com/albums.php

hope the link to pics of the breadboard works...

Rgds all
Pete

[Dave the bass]

That looks nice Sir, but someones stolen one of your light bulbs

DTB

[James D]

Nice one Pete!

There is no doubt the PT15 is a nice little valve and much more neutral than a 300B (spit! spit! ). Like the best DHTs (211, PT15, PX25, etc...) the final sound is only as good as the driver and the C3g is exceptional but what gives it the ability to stay open and even and transparent at all levels is the VR tube on the screen grid. It is so important to the sound of this amp...

more on the psu later....

We may well have my PT15 SE at the next EggFest too

James