Remember These?

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Cressy Snr
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#601 Re: Remember These?

Post by Cressy Snr »

So....in addition to the 15K, g1 grid-stoppers on the output stage we now also have 2.7R, 3W anode-stoppers, soldered right up against the anode tag on the socket. John Broskie advises 1R anode-stoppers on the 6C33C single ended OTL, as well as 10K grid-stoppers; these anode resistors, I had completely overlooked until I read the article again the other day. Don’t know why I missed them, but there it is.

Anode stoppers are usually between 10 and 100R and are (according to Merlin Blencowe’s preamp book) usually worked out at 2/gm for hi-gm preamp valves. Trying to be scientific about it for power valves, I worked out my own resistor value for the 13E1, based on 1/(gm +1). This was done to avoid too much of a voltage drop, given the high 500mA idle current involved, and I figured that the value that came out should be sufficient. This gave a value of 2.777R and I had a pair of 2.7R, so in they went.

Broskie’s 1R value is probably used for the same reason (avoiding too much voltage drop whilst still stopping parasitic oscillation) and will probably be the minimum value he could get away with on the 6C33C amp.

You know when you listen to something and you just know it sounds right? Your ears seem to relax. Well that’s what happened after the installation of the anode-stoppers. You know that something that was interfering with the music is no longer there. It’s true that you don’t notice these very low levels of background rubbish until they are absent.

The incremental gains I’ve had since I finished the boxed up amp and started the detailed tweaking, have added up to produce a beautiful sounding piece of equipment. In the context of a small room system with high efficiency speakers, couldn’t really ask for a lot more.
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Mike H
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#602 Re: Remember These?

Post by Mike H »

I'm guessing the anode stoppers are wirewound?
 
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Cressy Snr
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#603 Re: Remember These?

Post by Cressy Snr »

Mike H wrote: Sat May 25, 2019 4:41 pm I'm guessing the anode stoppers are wirewound?
Yes, so there’ll be some inductance involved.
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Ray P
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#604 Re: Remember These?

Post by Ray P »

That's interesting about the anode resistors Steve; I thought I had mentioned the use of them previously here but checking through the thread it looks like another one of those moments... I've included a couple in my build - can't recall the specific value off the top of my head but probably a little lower in value than yours.

Looking forward to hearing your amp next Saturday.
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Cressy Snr
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#605 Re: Remember These?

Post by Cressy Snr »

Ray P wrote: Sun May 26, 2019 8:45 am That's interesting about the anode resistors Steve; I thought I had mentioned the use of them previously here but checking through the thread it looks like another one of those moments... I've included a couple in my build - can't recall the specific value off the top of my head but probably a little lower in value than yours.

Looking forward to hearing your amp next Saturday.
Yep, looking forward to giving it an outing. The anode stoppers were staring me in right the face when I returned to the Broskie article. As you say, they weren’t on the diagram right at the start of this thread, maybe because the top of the pic had been cut off? I don’t know. My own fault anyway for not seeing them much earlier when Wolfgang referred me to the original Broskie article later on in thread. You can’t miss them on Broskie’s schematic. All that matters is that they are installed now. :)
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Nick
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#606 Re: Remember These?

Post by Nick »

The anode stoppers were staring me in right the face when I returned to the Broskie article
Do you have a link to the original article, I am wondering if they are "anode stoppers" or there to provide a bit of anode degeneration?
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Cressy Snr
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#607 Re: Remember These?

Post by Cressy Snr »

Nick wrote: Sun May 26, 2019 7:37 pm
The anode stoppers were staring me in right the face when I returned to the Broskie article
Do you have a link to the original article, I am wondering if they are "anode stoppers" or there to provide a bit of anode degeneration?
http://www.tubecad.com/2018/04/blog0419.htm

The 6C33C OTL bit starts about half way down the page.

He mentions “plate stoppers” in the last sentence before the section on impedance multiplier circuits begins.
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Nick
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#608 Re: Remember These?

Post by Nick »

Fair enough.
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IslandPink
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#609 Re: Remember These?

Post by IslandPink »

Broskie is so readable, once you get on there you can't stop !
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Cressy Snr
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#610 Re: Remember These?

Post by Cressy Snr »

I’ve given up with the tube rolling on the input stage of this amplifier.

I put back the Electro Harmonix 6H30pi gold pin valves this morning and the fact is that the music just sounds best with these on input and driver duty. The 6N1P and 6CG7 are more impressively “hi-fi” but I’ve concluded that they don’t gel or flow as musically, in comparison to the 6H30pi.

It shouldn’t be a surprise really, since I designed the amp around the 6H30pi. The amount of feedback in particular, was worked out and the square waves scoped to give the “squarest” waves with the 6H30pi in there, but you have to have a play around or it wouldn’t be DIY :lol:

The only future tube rolling ought to be between the EH, the Sovtek and the prized Reflektor (read bloody expensive) DR versions of the 6H30 but TBH, I can’t be arsed.
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Mike H
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#611 Re: Remember These?

Post by Mike H »

You're fiddling with it! Stop fiddling! LOL .. :lol:

Image
 
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Cressy Snr
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#612 Re: Remember These?

Post by Cressy Snr »

Mike H wrote: Wed May 29, 2019 6:04 pm You're fiddling with it! Stop fiddling! LOL .. :lol:

Image
I know, I know! :D

It’s not really fiddling anymore. All I’ve done is put the original driver valves back in their sockets.

Long term, what I’d like to do, is maybe to DC heat the 13E1s. I have a couple of otherwise OK ones that vibrate internally when AC is applied. Whether grounding their heater centre taps would balance the mechanical noise out is something I haven’t tried as of yet, but DC would definitely shut them up.

The two 1950s ones I have, with only the two cooling fins, are mechanically the quietest and they are easily the best sounding out of the quartet I have. It’s a pity that one of them doesn’t like being run at 75W anode dissipation, glowing dull red in parts, unless of course that was being caused by the oscillation I’ve now eliminated. I may try the offending valve later; you never know.
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Mike H
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#613 Re: Remember These?

Post by Mike H »

Alright then, I'll let you off. Image
 
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IslandPink
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#614 Re: Remember These?

Post by IslandPink »

It might class as 'unfiddling' .
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Mike H
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#615 Re: Remember These?

Post by Mike H »

While Rome un-burned
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
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