Nothing In Particular

Subjects that don't have their own home
Michael L
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#13426 Re: Nothing In Particular

Post by Michael L »

I thought a 300b amp with transformer coupling to the o/p stage would sound better than one with capacitor coupling all else being equal. I'm thinking about a single ended amplifier like my Border Patrol SE300B. I realise component quality and circuit design are key.
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Nick
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#13427 Re: Nothing In Particular

Post by Nick »

Michael L wrote: Tue Nov 02, 2021 3:55 pm I thought a 300b amp with transformer coupling to the o/p stage would sound better than one with capacitor coupling all else being equal.
1. All else is never equal
2. Still not seen a definition of "better"*
3. See my repeated posts about dogma.

*you may be able to have a subjective definition, but yours is almost certainly not the same as mine so will be of no use for such a comparison.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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IslandPink
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#13428 Re: Nothing In Particular

Post by IslandPink »

Re. Nick's point No.1 above :
I came to the conclusion, over the years, that the benefits of transformer-coupling to 300B's were much greater in PP than in SE, and there was a serious downside for SE.
SE amps are typically compromised by having lower inductance in the Output TX's due to the gapping for the standing current. The bass (phase) response is likely to be worse for SE.
If you build everything else as good as possible , not to add any additional effects ( generous cap sizes on cathodes, and the coupling cap/resistor combo for low Hz, but ideally also main power supply regulated ) then you can still get pace and satisfying bass.

As soon as you go to a transformer-coupled driver, you add in another set of the same bass compromises that you've already got with the output TX.
It has to be gapped, and the bigger the driver current, the more gap you need, so going to a higher-current , lower Zout driver doesn't always get you anywhere.
Also, interstages are not always transparent at low signal levels, and the core materials that are the very best for low-level signals ( eg. nickel ) won't give you as much inductance as steel. I have found that it's hard to match the low-level transparency of a good capacitor, with an interstage.
"Once you find out ... the Circumstances ; then you can go out"
steve s
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#13429 Re: Nothing In Particular

Post by steve s »

Personally I think a choke loaded driver stage is the best compromise.. but thats me..
The tube manual is quite like a telephone book. The number of it perfect. It is useful to make it possible to speak with a girl. But we can't see her beautiful face from the telephone number
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Nick
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#13430 Re: Nothing In Particular

Post by Nick »

All good points. But remember this is only a 10k amp so there will be options.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
Ant
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#13431 Re: Nothing In Particular

Post by Ant »

Got a pair of lencos and a 1988 roksan xerxes in from a customer this morning, the xerxes has the classic sagged top plate making it unusable. said id take a look at it to see if it can be properly fixed rather than bodged with a plate and a bit of thick rubber. Hopefully i can remanutacture a new top plate for it out of something that wont sag. Maybe some 30mm bbp would do the job. Be expensive to get some though.
I havent heard a xerxes in years, i remember quite liking the one i heard. That had an artemiz on it, this has an rb300 with a cracked bias outrigger on it. that bit is a complete pig to replace as the whole arm has to come apart so that is probably toast, but im quite looking forward to digging into this deck
It depends on wether its economically viable to fix it, but i hope it is.
The lencos will be built up into a type 3b like the small black one, with 2 arms on, a modded rb250 and an audiomods series 6. 2 decks to pick the best bits off.
Also starring Rex Hamilton as Abraham Lincoln

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Ray P
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#13432 Re: Nothing In Particular

Post by Ray P »

Not familiar with the Roksan Xerxes Ant, can you post some pictures of the top plate and the sagging? How thick does the top plate need to be?
Sorry, I couldn't resist!
vinylnvalves
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#13433 Re: Nothing In Particular

Post by vinylnvalves »

The xerxes - had a MDF top plate- which was supported on rubber blobs as far as remember, to decouple it.
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Ray P
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#13434 Re: Nothing In Particular

Post by Ray P »

I wonder how some aerolam or cored (end-grain balsa or foam) carbon laminate would work as a top plate?

https://www.easycomposites.co.uk/foam-c ... ibre-panel
Last edited by Ray P on Sun Nov 07, 2021 7:06 am, edited 1 time in total.
Sorry, I couldn't resist!
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Ray P
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#13435 Re: Nothing In Particular

Post by Ray P »

Maybe one for Mark?

Sorry, I couldn't resist!
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IslandPink
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#13436 Re: Nothing In Particular

Post by IslandPink »

Somebody just linked me to that via email - good call !
He said "look at the shot at 2:27"
I went up through the same ( Tan y Grisiau ) caverns in 1993 or 1994 with him and his two young sons. I took a photo ( on 6 x 4.5 cm B&W ) from the back of the cavern with him ( Hywel ) , Robin and Rhys silhouetted in the light, in exactly the same way. It was subsequently used as the centre spread of a gatefiold album cover by Robin ( R Seiliog ) about 10 years ago.

That is a great bit of work that those guys have done though - at no small risk to themselves. Some of the tunnels that Hywel used to explore about 20 or 30 years ago have since collapsed.
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Ant
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#13437 Re: Nothing In Particular

Post by Ant »

This xerxes is an interesting design, the top plate is just straight 25mm mdf so it can be re made from bbp. Sits on three towers consisting of a bolt with a set of rubber bushes on them.
The cutout in the top plate is why it sags, the arm and bearing weight just bends it over time.
Ive had it to bits (obviously) and it looks to be pretty well made for what is basically 2 slabs of particle board and some rubber bushes

Depending on what he wants to do with it, ill start a thread on fixing it.

The most important thing is going to be accurately plotting all the cutouts, holes and rebates so i can remanufacture a top plate for it.

Wont be particularly cheap though, bbp prices seem to be astronomical compared to the last time i bought some.....
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Nick
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#13438 Re: Nothing In Particular

Post by Nick »

steve s wrote: Tue Nov 02, 2021 7:26 pm Personally I think a choke loaded driver stage is the best compromise.. but thats me..
Of course its always personally, but I have been thinking about this, in a way a choke loaded driver has the worst features of both transformer and cap coupled stages. Most of the problems with cap coupling can be avoided by cap coupling to a cathode follower and then to the output valve. But again IMHO and it adds a lot of extra requirements in a extra power supply for the CF tail. You could choke load the CF tail and avoid that though :-)
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Ant
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#13439 Re: Nothing In Particular

Post by Ant »

Ah well, the roksan had more problems than first appeared. unfortunately the psu appears to be kaput. It would spin the motor under no load, but put any load at all on it and it just stalled completely. As if there wasnt any power behind it to pull the platter round. There are a pair of what look like current amps in the psu, presumably one per phase which look to have gotten very hot at some point. Several caps and resistors around them also show signs of heat. Something in there didnt like being hot. Having had a discussion with him about it it looks to be more than the deck is worth to repair the psu and remanufacture the top plate, so it looks like it will be sold for parts rather than chucking money at it.
A bit of a shame
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Ant
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#13440 Re: Nothing In Particular

Post by Ant »

Balls. Dropped my blummin dial gauge and now its not reading properly.
Need a new one, the busted one is a finger type gauge, im wondering if a plunger type one is a better bet.
I usually use it for chucking up bar in the lathe because im using an independent 4 jaw one rather than the 3 jaw scroll chuck as I found i can make better parts with it. i have also used it to check the column on the mill was trammed in properly
Havent used a plunger type so i could do with some advice before i stick my hand in my pocket
Any pros and cons i should know?

At least it wasnt a mitutoyo or similar that i broke...
Also starring Rex Hamilton as Abraham Lincoln

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