300b SE

[Paul Barker]

Below is the 10Y distortion



I at first looked at 6em7 va and driver but distortion way out weighed the 300b distortion. So I chose a driver which has barely any distortion. Then a va more naturally linear with sufficient gain. Barely any distortion there either. So all the 2nd harmonic is coming from the 300b.

Input sensitivity 1v peak.

I had a 30 watt gm70 penned out but … not yet. Lets do something with a little more power than the el38 se and considerably less distortion, along the way. Oh and the bench power supply can easily give 480v at up to 550 mA, as a true choke input with Black Gates.

I also had a look at the full music mesh plate PX25. But power output not enough for the project barely any more than the el38 se. I could use a true px25 aswell but power output almost same. With my 87 db speakers no.

[RhythMick]

Look forward to watching this develop.

My only question would be the input sensitivity seems a bit low? What sources are you using?

[Paul Barker]

Solid state phono stage upstairs mm cart, if I put mc cart on Id use 10 X wear and Wright reslo microphone transformers, 3rd floor and apple music ground floor. Im not ready to trust my left hand to not batter the mc cart during set up if it starts the violent shake which I cant completely control yet.

Upstairs the 4.5 watt el38 is plenty of power for 87 db speakers.

[simon]

Reminds me of my Madness amp - 26 with gyrator load cap coupled to 10Y IT to 300B. That was a BIG amp

[Paul Barker]

Reminds me of my Madness amp - 26 with gyrator load cap coupled to 10Y IT to 300B. That was a BIG amp

Yes I’m using resistors for the lack of space on the small chassis. The larger r cores are like the fat man on the aeroplane seat next to you. No room for AE interstages.

Before back tracking to this sensible next step, I had sat the small chassis on top of the tributes which filled all the space underneath. Had two px4’s and two gm70’s on top and two AE interstages but it was crowded. Two d3a’s completed the line up. When its time comes, it will be done roaming. But probably on a bigger chassis. 30 watts. All in good time.

Im gradually clearing my Lockup and saw a smallish valve among the dusty concrete floor. It was a Brimar 2a3. Had a friend with me but not as a helper.He is the guy who helps me over my wife’s memory loss. He looked after his mother with same. We meet for coffee often. He wanted to see what I faced at lockup. I looked up the 2a3 on ebay we found one same Euro 800. I could have stood on it. I still have more valves at the lockup. Im too busy to build amps but I need to get this amp built. Its about time.

[Paul Barker]

Reminds me of my Madness amp - 26 with gyrator load cap coupled to 10Y IT to 300B. That was a BIG amp

Why I tried the 10y on paper was because its hard to find an iht with sufficiently low 2nd h distortion. The 10y is bringing timbre but sufficiently low distortion so as not to dominate the 300b with greater distortion by quite a factor. I looked at nesh plate 27 globes but no, 6J5 is an excellent valve to match the 10Y. One day I may get a time to give the 112, 26 or 27 an outing maybe into a 46. I still have the plasterboard open baffles at the lockup.

[Paul Barker]

The crowded gm70 which would be stood on Trubute opt’s. It needs a bigger chassis.

[Paul Barker]

I mis understood, in my head I thought the 10y had more mu than the px25. But I was wrong. In fact the way I have decided to operate the px25 I can leave the bias resistor unbypassed and still get a gain of 6.8. Ive ordered a pair of br4 bases and a pair of U4A bases, both of which I shall parallel mount so that I can either plug in the pair of full music px25’s or the British PX25’s. I dont understand the Chinese px25 filament situation, it appears you can run it at 5v or 4v. But at 5v the filament will run hotter . Anyway I’ll use 4v anyway as the Brit PX25’s are definately 4v.

There is more distortion than the 10y but less distortion than the 300b. So the way I see this is the grid drive of the 300b at 30mA by the way, with get out of phase distortion of less than the distortion it generates, so mitigation is at play. I think about 1% thd at the loudspeater.

Now that I only have one stage with a bypass cap I can make that -3db at 3hz. After all the opt is probably going to roll off at 30hz in any case and non of my speakers show any noise below 30hz except the sub and that can amplify for itself.

The 0.1 uf coupling cap stages are each into 250k (px25 does not state grid resistance auto bias but the 300b states 250k so for safety Id treated the px25 same) so if I use a 1uF coupling cap from the 6J5 to the px25 -3db is 0.5 hz. Now for the second coupling I want a small cap to drive the 300b so a 100 nF cap will roll off 3db at 5.5hz. I think I can pay that price for better drive of the 300b grid up to zero volts.

So Ive ordered the parts. First turn on will be with the Chinese mesh plate px25’s.


The dc 5v power supplies I can use a trimmer to adjust it to 4v. I havent shown the first coupling cap change to 1 uF.

[Paul Barker]

So the layout on top a 2x100uF 350v black gate for the 300b bypass caps. Two 8mA inductors for the 6C5 input valves. One 75mA 10h inductor for smoothing of both channels from the driver up to the other inductors. Caps will go inside for the inductors pio.

Only the chinese px25’s or the British PX25’s will be fitted. Both sets of bases parallel wired up to filament dc supplies 4v. Anode pin and grid pins also parallel. But only one valve each side occupying space.

This time its a project that will fit the chassis.

[Paul Barker]

There is a spanner in the works.

https://web.archive.org/web/20071228203 ... 00bdrd.php

Curiosity killed the cat. I had to try this for once. Substituting Jacks choke for two series hammond I get the inductive reactance sufficient to get gain from the e88cc parallel, to achieve full power from 1.1v peak signal for 9.8 watts.



I have to do this first.

[Paul Barker]

I think I’ll drop the b+ to 600v, drop the cathode current to 70mA and achieve 7 watts.

The one eighty volt remains via a change of 2k3 resistor to 2k5.

[Paul Barker]

[Paul Barker]

So with the ultrapath circuit there are just four valves. Mounting the 9 pin bases wasn’t without my spasticity syndrome I got one screw hole nothere near and had to drill another. And somewhere in the process I forsed a broken drill bit abot half into my thimb through my thumb nail. But its good that I made a start. The two polly caps are 100uF for the ultrapath caps. I’ll just glue them in place.

[Cressy Snr]

Ouch!

[Paul Barker]

Well left hand is accident prone