Future plan for 300B XLS amp

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Paul Barker
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#166

Post by Paul Barker »

Mark, thanks, I hadn't noticed a difference in noise when I use VR's but I am not using them on the screen of a pentode, which would explain that one.

Good to know that you have discovered two great benefits.

If I didn't need some 2nd harmonic distortion I would be doing your driver right now. But with a PP output stage I dare not. there is no doubt in my mind that a PP output stage with a little 2nd helps it to sound better.

In my case the VA and driver is near enough the same as my low powered SE amp. The PP output stage just provides power to that formula. Sadly though it does not come near to the wanderment of that element on low level work. Neither does it do it as well as an SE 833a. But it is small light cheap and practical and much better than most other amps I get to hear.
Last edited by Paul Barker on Sun Jun 02, 2013 7:19 pm, edited 1 time in total.
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Nick
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#167

Post by Nick »

200 = 16.4
180 = 17.1

I used

Av = Gm * rp * Rbc / ( rp + Rbc + Rk * ( Gm * rp + 1 ))

Where Rbc os RL || Following grid resistor (470k in my calc)

But anyway, sounds like you have found the cathode resistor was not having that big effect on gain..
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Paul Barker
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#168

Post by Paul Barker »

Nick wrote:200 = 16.4
180 = 17.1

I used

Av = Gm * rp * Rbc / ( rp + Rbc + Rk * ( Gm * rp + 1 ))

Where Rbc os RL || Following grid resistor (470k in my calc)

But anyway, sounds like you have found the cathode resistor was not having that big effect on gain..
Yes I must have made an error last night but not worth looking for it now.
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#169

Post by IslandPink »

The following grid leak is 100k .
If I apply a 7.8k anode load, and use 300R cathode resistor with a cathode by-pass cap, in Spice, I get a mu of 33 .
You were using Rp as 15k - what's this ? - is it the internal resistance ?
Ri must be about 150-200k , based on 250k for a C3m .
If there's no bypass cap and still with 300R on the cathode you get only 14x gain .
I reckon I have about x20 with the extra current from the VR tube, because it's almost exactly the same as I had from a 6072/6N6P aikido last week .
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#170

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IslandPink wrote: You were using Rp as 15k - what's this ? - is it the internal resistance ?
If you are talking to me I was guessing 15k anode resistance. I made my guess from the figures given on these pages. Go down to the pentode ccurves for 80v G2 voltage. Below those is a chart giving s/mu and Rp.

I still had to guess because he doesn't give figures at your settings.

likewise, looking at his page the figure for s of 6 is probably wrong.

So we are only able to guess with what little we know.

Plenty of people have put snipets of information up about these valves but we don't have all the information we would normally get from a western valve data sheet.

As a side note it is almost impossible to google this valve and not come up with 100 links to Bartola Valves. thank goodness I found the above link eventiually but you have to wade through a Bartola Valves bombardment to find it.
Last edited by Paul Barker on Sun Jun 02, 2013 9:10 pm, edited 1 time in total.
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#171

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IslandPink wrote:The following grid leak is 100k .
If I apply a 7.8k anode load, and use 300R cathode resistor with a cathode by-pass cap, in Spice, I get a mu of 33 .
You were using Rp as 15k - what's this ? - is it the internal resistance ?
Ri must be about 150-200k , based on 250k for a C3m .
If there's no bypass cap and still with 300R on the cathode you get only 14x gain .
I reckon I have about x20 with the extra current from the VR tube, because it's almost exactly the same as I had from a 6072/6N6P aikido last week .
I was just basing my numbers on what I believed Paul had used to calculate gain. Its was the connection between small change in cathode resistance and a large change in gain that I did not understand. Sorted now I think.
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#172

Post by Paul Barker »

Nick wrote:
IslandPink wrote:The following grid leak is 100k .
If I apply a 7.8k anode load, and use 300R cathode resistor with a cathode by-pass cap, in Spice, I get a mu of 33 .
You were using Rp as 15k - what's this ? - is it the internal resistance ?
Ri must be about 150-200k , based on 250k for a C3m .
If there's no bypass cap and still with 300R on the cathode you get only 14x gain .
I reckon I have about x20 with the extra current from the VR tube, because it's almost exactly the same as I had from a 6072/6N6P aikido last week .
I was just basing my numbers on what I believed Paul had used to calculate gain. Its was the connection between small change in cathode resistance and a large change in gain that I did not understand. Sorted now I think.
Indeed. I didn't find where I went wrong but it's history. Our two ways of measurement bring minor different result but not any reason to change either of our ways.

Having uncertainty over mu/s and Rp makes it all nothing more than a paper excercise. Mark will find out the facts by empirical measurements eventually.
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#173 Picture

Post by IslandPink »

Finally, here 'tis :
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#174

Post by IslandPink »

Having a bit of e-mail chat today about the strengths/weaknesses of this amp & debating ...
The upper mids and top end are just great . Makes the 300B sound like a 45 or an 801A . So much natural detail, sweetness, and low-level presence.
On the other hand the bass is not great and separation & organisation on complicated stuff is not as good as the C3m driver.

It does seems there's a bit more 2nd-harmonic than before ; this can result in more muddle, but explains the sweetness and the great performance on spacious , acoustic music .

There are some other things to try - proper DHT supplies for the 4P1L's , for starters. Anti-microphonic ideas ( heavy collars ) on the valves perhaps . Maybe go back to the L-C decoupled supply for the drivers instead of direct from the main PS .

Fun place to be, anyway :wink:
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andrew Ivimey
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#175

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but apart from the excuse that (initially anyway) the 4P1Ls valves are new, exciting and cheap as chips why not stick with the C3m as it delivers the goods in a very respectable way!?
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#176

Post by IslandPink »

Well, because the 4P1L is showing potential to be a better overall package than the C3m . The improvement in the top-end and the low level detail is not subtle - it's operating at a new level for this amp - better than when I had the 300B driven by a 45. So far, I haven't done enough work on the set-up and all the variables ( like filament supply ) to say it won't come good in the bass.
Even if 4P1L isn't the way, there will still be 2E24 ( DHP ) to try before I go back to the C3m.
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#177

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Anyway, to continue this semi-monologue, I'm making some slight progress on understanding the issues on the 4P1L driver.
If I get some weighty metal collars from the B&Q tube-damper section , and put them on the drivers ( this evening ) , it gets a touch better in the bass and timing department. Not a lot , but worth having .

Now, later on, I got out the oscilloscope and coaxed ( co-ax-ed ? ) it into life by feeding it some electricity and fondling all its potentiometers and switches for a bit .
What I found was there isn't much evidence of RF in there, or at least not in the low-MHz region. However, there is an annoying whistle on one or both channels that seems to consist of about 0.2v pk-pk on the grid of the 300B at about 2kHz . This seems very stable in level and frequency , although from time to time it stops , mainly it's absent for a little while at switch-on and then comes back .

I guess this will intermodulate with a lot of the upper mids and treble and perhaps emphasise the treble over the mids and bass. So, if anyone has any ideas what to do next let me know.
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Paul Barker
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#178

Post by Paul Barker »

That's anoying. Bet you're glad you didn't use these Cisco power supplies. Bad enough having an oscilator in the amp. Apart from having the wrong transformers I suspect the power supply is the main reason the PP amp cannot come anywhere near the 6em7 SE amp. But that could be fixed. If I get to the end of the road having fixed the power supply quality and transformer quality and still prefer SE nobody can argue I didn't try! I am prepared, when I get myself back into transformer winding to sacrifice my 50% Nickel C cores to this project. Nickel is really only appropriate for PP and parafeed anyway, as the Senior Mr Sowter wrote in a much quoted paper "the extra permiability of Nickel just falls through the gap".

I have no idea, apart from short leeds and grid stoppers. More expect it to originate in the 4p1L. We'd better not have our amps too close together, yours will detect all the RF in my power supply!

What is the B and Q tube damper section? PTFE tape tied in a knot around the valve? O ring department? Or those flexi-mounts you used previously? The latter I am guessing.

At least there is one thing to be said for my old school VA approach of using low transconductance high mu triode; it is no good as a detector.
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#179

Post by IslandPink »

Hardware shown below !
Thanks for advice.
Talking to Nick today, he pointed out that some valves just ring in a self-sustaining way when they are hot or the HT is on them. Given I have one on the right channel that is quieter, I should just get another couple of valves out and see if I can get a quiet one for the left. May not be anything to do with the circuit.

I'll report back on that one .

Yes, sharp-eyed readers might warn that the dampers sat on top of the 4P1L's there could easily slide off onto the Mills resistors which are carrying 465V at one end. It worries me too :shock:
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#180

Post by simon »

IslandPink wrote:Simon : if you can find the 6N6P's and check the date codes ( under the 6N6P logo usually, eg. '8811' on mine ) and the type of packaging, we can add it to the circumstantial evidence. My ordinary 6N6P's were as good as RCA 5687's when used in the Aurora PP amp .
Hi Mark,

Just back from holiday (in time for Owston :-D) and catching up with posts. Mine look to be from Dec 77, Oct 78 and Jan 80. I hadn't appreciated there was such a range so can't definitively say they're all mediocre.
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