My very first breadboard

Dave the bass · Post by **Dave the bass** » Thu Apr 03, 2008 7:49 pm

andrew Ivimey wrote:DTB, back to 6B4Gs for a moment.

I preferred kenrads and Sylvanias to the ruskies but these are the only ones I tried.

Fankyew Mr I.

Partial Hijack over.

As you were

DTB

Cressy Snr · Post by **Cressy Snr** » Mon Apr 07, 2008 10:46 am

I got the PX25s this morning. My enthusiasm was slightly tempered by the 13.50 Parcelforce charge plus the VAT.

They look to be very nice valves. All I need now are the caps for the PSUs and I can start on the breadboard.

Steve

Cressy Snr · Post by **Cressy Snr** » Sun Apr 13, 2008 5:32 pm

Here's the design tweaked up a bit more.
The PX25s are being run at around 75% of maximum, pulling 45mA per triode with 30V of bias.

The valve requires 60V pk-pk from the driver to develop its max power within the operating range imposed on it. The parallel connected choke loaded 6A6 input/ driver valve will easily provide 70V pk-pk with little distortion for a 2V rms input.

The driver valve however will worked quite hard, so we'll have to see how it goes with 2 stages. There is always the option of direct coupling a 76 to the 6A6 grid, to give an extra boost, as with 418V, I have plenty of HT to raise the 6A6 to the required levels for DC coupling, simply by changing the dropper resistors that feed it.

Steve

Cressy Snr · Post by **Cressy Snr** » Mon Apr 21, 2008 8:16 pm

I must have been inspired by this weekend's events as I have been quite busy preparing the breadboard for wiring.

Here are a couple of pictures of the progress so far.

The Sowter mains TX was intended for the ill-fated OTL, which was SS rectified so it has no 5V winding. Therefore to the rear of the rectifier we have a pair of 2.5V 4A heater transformers that will be series connected to supply the rectifier heater.

Four big LCR motor run caps at the front, form part of the power supply.

I have three 6.3V windings on the mains TX. Two of them will power the PX25s and the other will feed the 6A6s.

The PX25s themselves are mounted on wire fan guards above a large hole to provide ventilation from below.

The two chokes in front of the pair of 6A6s provide the anode loads for the input/drivers. I've left a space to the left of the valve board so that, if necessary, a pair of 76s or ML4s as voltage amps can be plumbed in.

I'll try the 6A6 first then I may make it a three stage amp, with 76s or ML4s, at the front followed by 26s to drive the PX25s. One thing at a time and steady as we go.

Version 1 of the breadboard should be ready by this weekend.

Thanks go to Simon for the loan of the output transformers

Steve

Greg · Post by **Greg** » Mon Apr 21, 2008 8:45 pm

Hmmm, nice one Steve. Brings a whole new concept to breadboarding

Go on, have a play with some clip leads

andrew Ivimey · Post by **andrew Ivimey** » Mon Apr 21, 2008 9:21 pm

yes isn't it! It is far too tidy by half - onwards!

Cressy Snr · Post by **Cressy Snr** » Wed Apr 23, 2008 4:45 pm

Greg wrote: Go on, have a play with some clip leads

Will this do Greg?

The heaters and filaments are done and the PSU, as we can see, is producing a healthy HT supply.

Busbars in and audio circuit next.

Steve

simon · Post by **simon** » Wed Apr 23, 2008 5:20 pm

Come on Steve, that's not a breadboard, that's one of your normal builds without a top on

.

Out of interest, what is the PSU? I was looking at the bits and couldn't quite work it out.

Cressy Snr · Post by **Cressy Snr** » Wed Apr 23, 2008 6:03 pm

Hi Simon.

I really don't get breadboarding it seems.

OTOH the build has been extremely quick and easy
using the bit of wood so I get at least part of it.

Here's the PSU. It's a dead simple brute force unregulated one
to get the amp going. The resistances are a start as I'm probably going to need twice what I've got to get the right voltage coming out.
These will be adjusted on test.

The 40 and 50uF caps are made up from 2 x 20s and a 20 and a 30.
Those are the blue LCR motor run caps you see. The 33uF is a Sprague
Atom and can just be made out nestling between the big polyprops.

Steve

simon · Post by **simon** » Wed Apr 23, 2008 6:10 pm

Ah, I hadn't noticed the Sprague. What's the voltage rating of the LCRs - didn't think it was that high?

Cressy Snr · Post by **Cressy Snr** » Wed Apr 23, 2008 6:19 pm

The PSU is not under load in the picture so the HT is a lot higher than it will be in use. There are also no series resistors in the rectifier High voltage AC cct. It was just a lash up test to see if it worked.

The choke is rated at a max of 800VDC

There'll be about 420V going through the PSU when it is set up properly.

The big LCR caps are rated at around 700VDC (440VAC) and the Sprague is a 500V one that Philip supplied with the Loftin White I built. This had about 430V at the anodes of the 2A3s IIRC.

Steve

simon · Post by **simon** » Wed Apr 23, 2008 9:19 pm

I have the very same Sprague

. Didn't realise the LCRs were as highly rated as that - very useful...

Cressy Snr · Post by **Cressy Snr** » Thu Apr 24, 2008 10:07 pm

It's up and running!

The voltages check out OK and it sounds very very nice indeed. Plenty of liquid top end and the usual DHT midrange magic. I didn't realise how much I had missed that sound after jettisoning the 2A3s a couple of years ago - beautiful. The TJ PX 25s are quite a beefy but delicate sounding valve if you know what I mean.

Those paralleled 6A6s make nice driver valves.

Of course with AC on 4V filaments it hums sufficiently to be heard at the normal listening distance so the next job will be a regulated DC 4V supply. I'll try a bit more filtering on the PSU first but I'll probably have to bite the bullet eventually and go DC on the PX25s at least.

All in all though I'm very pleased with PX25 version 1.
I'll be picking Nick and Andrew's brains for help with the DC supply.

Here's a pic of it playing.

Post by **Nick** » Thu Apr 24, 2008 10:11 pm

|Having a resistor in one leg of the heater supply and then taking the center tap to the carhode circuit will make hum much worst. You might be better disconnecting the center tap and using a hum pot, or at least a pair of resistors to create a center.

Or at least split the resistor over both legs so the center is back to being the center.

cressy · Post by **cressy** » Thu Apr 24, 2008 10:23 pm

yes i thought that too!............ looks good dad, those px 25s do look awesome glowing away like an electric fire at least you wont be cold but please put it away next time the kids are up!!!!!!!!!!!!!!! (smaller fingers bigger risks though with the little sods behaviour at the mo it might not be a bad idea..........please dont call social services!!!!)

audio-talk

My very first breadboard

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