New PP Pentode Monoblocks

What people are working on at the moment
steve s
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#211 Re: New PP Pentode Monoblocks

Post by steve s »

Looks good steve, and the system sounded good with the el34's to me those old kt66's are fairy plentiful and relatively low cost so they are good for day to day use.
Nice...
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Cressy Snr
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#212 Re: New PP Pentode Monoblocks

Post by Cressy Snr »

Cheers Steve, it was nice to see you and I'm glad you liked what you heard.
The system now seems to be making all the right noises. Speakers are right, amps are sounding lovely, the vinyl source is a cracker, the digital and streaming bit is spot on and I'm happy as a pig in the proverbial. :D
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steve s
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#213 Re: New PP Pentode Monoblocks

Post by steve s »

Yes im stll in the dark ages when it comes to streeming and computer sources, seeing your set up just reminds me by how much! yours is a nice system and it works very nicely in your room, seeing it reminds me how much i need to do, but my adventures are on hold at the moment.
i started some new speakers before christmas and they are waiting in the wings to be completed. The motorhome conversion is taking all my time up.. Role on retirement ?
It was good to see you...
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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Paul Barker
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#214 Re: New PP Pentode Monoblocks

Post by Paul Barker »

I could retire now by moving into a motorhome or a canal barge. But neither would suit her indoors.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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#215 Re: New PP Pentode Monoblocks

Post by Cressy Snr »

Yes, life in a motorhome, would suit me nicely. :)
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steve s
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#216 Re: New PP Pentode Monoblocks

Post by steve s »

Paul Barker wrote:I could retire now by moving into a motorhome or a canal barge. But neither would suit her indoors.
It would not suit me for too long but a couple of 6 week breaks in the sun every year is the plan
..to be honest i had not realised how much work was involved, its been every night and weekend and a few days hols used but theres still a fair bit to do, lets not talk about the expense.
Worst is i have nightmares about rattling cupboards... Every piece of wood is screwed/ bolted and then glued to make sure there is no rattles ... Fingers crossed
There are alot of poeple who head for spain and the likes every winter, you cant blames them
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
Cressy Snr
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#217 Re: New PP Pentode Monoblocks

Post by Cressy Snr »

Amps have been getting noisy of late, with lots of crackling and fizzing for the first five mintes of operation.
The culprits turned out to be the pots between the screen grids of the input push-pull pair of pentodes.
These cheapo carbon track pots had deteriorated and the wipers were intermittent until things had expanded on warm-up. Now seeing as the screen grid is just about the most important element in a pentode valve, this was far from ideal.

I disconnected the damned things, fitted individual screen resistors to the input pairs and fed both halves separately from the regulated screen supply. This now means that the input pair cannot be balanced up, but measuring revealed they are not far out and the bias on the cathode followers that are directly coupled to the plates of these pentodes, simply readjusted themselves to give the same differences grid to cathode.

Things have cleared up nicely and the sound is clear and clean. I hadn't noticed the fall-off in sound quality, as it had happened so gradually. The input pair is now, in theory, adding a bit of second harmonic love to the proceedings, due to incomplete cancellation but there you go; it's better than the input pair being noisy due to inadequate balance pots. The whole thing is a lot quieter now. Funny how slow deterioration creeps up.
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#218 Re: New PP Pentode Monoblocks

Post by Cressy Snr »

Finally got around to taking out the "temporary" zener shunt stabiliser to the screen grids of the 6BS7 input stage.
I replaced it with a 0B2 VR tube, the socket for which, was fitted in the hole left by the removal of the redundant screen balance pot.
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#219 Re: New PP Pentode Monoblocks

Post by Cressy Snr »

So the 300B OTL experience got me thinking. So refined was it and sooooo chock full of that SET cream with the addition of the startling clarity, that is the stock in trade of these OTL things, that it set some serious think bubbles appearing above the old head.

Well as Dr Gizmo famously said about push-pull pentode amplifiers, there is just no way you are ever going to get that direct heated SET cream out of an amp festooned with five-electrode, indirectly heated glassware. With that unavoidable fact in mind I resolved to see what I could do to close the gap a bit.

So the task now, with the Barbarik monoblocks, was to try to get as close to that 300B magic midrange and cream as possible, while keeping that kick-ass, rock'n roll pentodosity, Gizmo used to go on about.

Thinking back and having a read through the bit in this thread about Mike H's EL84 push-pull, all pentode jobbie, I saw that he was running his EL84s at 10W dissipation; 2W away from the max, (give or take a bit of screen current) IOW he was biasing them well into Class A. My Barbariks OTOH were being run, pretty cool-biased, and were Class B above a whisper.

I had been quite happy with this arrangement....until I heard the 300B OTL running at about 4/5 of a Watt, giving decent levels on my OmniMets, and outperforming the Barbarik monos, in terms of hear-through clarity, richness and tonal colour saturation. I mean the Barbariks were good, but not in that class.

So in the light of hearing the OTL do its thang, I cranked up the bias on the EL34s from the 70mA for the pair, I had been running them at, to 110mA. So now we were at 20.5W dissipation per valve out of a max of 25 (again give or take a bit of screen current)

As far as I was concerned, there was nothing to lose, and EL34 are cheap enough for a quad, plus the HT winding of the Danbury mains TX is good for 250mA.

The result of this is a hot pool of rock'n roll pentode magma, dripping with Kodachrome saturation, EL34 tone, a whacking great soundstage and a great big grin on my face. Plus those imperturbable 200J, defibrillation capable power supplies on each monoblock get to show what they can really do.

Is it as good in the clarity dept as the 300B OTL? Of course it isn't......BUT.... it is good enough to cause me not to miss the 300Bs too much, and the power and the dynamics of the Barbarik monos, knock the low power 300B OTL into a cocked hat on less than 100dB/W speakers; which they would do.

That's what I like about this hobby. Listening to other gear, especially in your own system, can be a good motivator to re-evaluate, and often, that leads to you getting something good out of the experience.
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Cressy Snr
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#220 Re: New PP Pentode Monoblocks

Post by Cressy Snr »

Now that Owston is over and the perils of transporting valves in the car have receded for another year, I have reinstalled my quad of old, used, grey glass KT66s I bought off Steve, biased and balanced them up and given them a listen.

These and the new production 6L6s I bought recently tend to give the best all round sound, in this particular amplifier, even though in theory, the OPT is wrong, being 5K a-a instead of the 8K the old data sheets specify.

After having got these venerable old tubes, I thought, what was the point of buying them, if they weren't going to be used.
OK, they are, on the one hand, precious old valves, but on the other, these things were meant to play music, not to be kept in their boxes for eternity. It's a dilemma.

Currently they are singing loud and clear, with that extra something that proper old valves just seem to have over new production, no matter how good the latter get. New production has improved tremendously, but they are not quite there yet.

To test that theory to destruction, I would, of course, have to acquire some new production New Sensor, Genelex KT66 Gold Lions to compare to the old ones.
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Mike H
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#221 Re: New PP Pentode Monoblocks

Post by Mike H »

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Cressy Snr
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#222 Re: New PP Pentode Monoblocks

Post by Cressy Snr »

Tonight I've also gotten around finally to sorting out the input stage idle current. I've only had the amps 14 months :lol:
It has been too low all that time and the daft thing is that I knew it but was too idle to reduce the cathode resistor values. I'll sort the final nth degree of adjustment tomorrow.
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Cressy Snr
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#223 Re: New PP Pentode Monoblocks

Post by Cressy Snr »

steve s wrote: Thu Jun 15, 2017 8:24 am
Steve that's a point thats often missed
too many times it's easy to blame the speakers when it's something in the chain, its a very skilled person who can identify every issue by listening

It was funny how those horn mouthed speakers woke your amp up.. Was it the single driver ?

Those comparisons we do are invaluable in my view
Thought I'd continue the discussion on my amp thread rather than the Owston thread.
Yes this business of amps has begun insidiously to get on my wick over the years since I built that wonderful 6B4G set of monoblocks that brought a tear to one of our members eyes on hearing "Save Me" by Joan Armatrading. That 6B4G creation was just lovely and I'm seriously beginning to wonder why I abandoned it to mess about with pentodes.

I've spent so much money on pentodes that I could have bought a couple of those PX25s with the 1960s, GEC KT66 type bulb, off you Steve and still had enough change to buy a pair of tribute output transformers, and would have had an amp to treasure, instead of a load of frustration, because after over a decade of building valve amps, I still cant get the blasted gain structure of the bloody things right! I mean FFS everyone else can, so why the hell not me? I don't get it! Clue me in somebody.
TBH I'm glad that the Dropbox links to 99% of my past circuits have gone. :mrgreen:

The only three amps that have come close to being right in terms of delivering the power output they were supposed to, have been the Bluebell Loftin White from 2006, the aforementioned 6B4G monoblocks and a KT120 Alex Kitic based SEP. The rest of them?

Time after time I have demmed at Owston and had to wind the wick up and up and up just to get a reasonable level, then when my gear has been asked to drive anything real world it has simply not been able to cut it. It's just embarrassing and perplexing.
It's like the plucky underdog who keeps on keeping on and on and on but is never going to play with the big boys.

This was finally brought home to me this time, with the commercial 25WPC Japanese amp DTB brought that was kicking the Doc mods B&Ws, Doc mods Celestions and the BBC LS35As along at a cracking pace, with some beautiful John Martyn and some fabulously specialist King Crimson. I left for home half way through the LS3/5a a bit depressed.

OK, so this time I managed to deduce the problem of too cool bias on the input diff pair and sort it, but the nature of the direct coupling to the 5687 cathode follower driver, means that although I have achieved a useful improvement in the sound I have now transferred part of the initial problem onto the cathode follower driver. This is now biased too cool and the signal is bumping along the bottom of the plate curves on the negative swing. The sound is far better, but because of the cathode follower now being too cool, the problem has not entirely been solved.

I've recalculated the cathode resistor for the follower, to put the bias back where it should be and have ordered the requisite Mills 12W resistors from HFC, and of course, that is all fine and dandy, except that now, the heaters are lifted too high so they need to be dialled down by redoing the bloody potential divider from the HT, that connects to the centre tap of the heater supply to the follower.

It's all far too much blooming complication and stuff in the way of purity of the signal.

OK, academically it shows that I am capable of wrestling with such problems and coming out on top eventually after five years of fookin about with the mass of support circuitry that comes with the territory if you choose to be daft enough to build with pentodes. Aren't I clever?......NOT! I mean valve circuits are supposed to be simple aren't they! If going to go to all that trouble you may as well have used transistors in the first place.

I sincerely hope that this last ditch effort will be worth it, because I don't know if I have anything left in me.

To be quite honest with you all, I have lost interest in valve amplifiers, and having discovered that after nearly a forty year gap, I have not forgotten how to design crackingly good, simple crossovered loudspeakers, particularly tapered quarter wave transmission lines. Ive therefore decided that speakers are the way forward and are the things that are giving me the greatest pleasure.

I've got Dave and Meredith, who wouldn't be parted from their five-foot Metronomes, and the highly positive comments I had the other week about the flatties and OmniMets from people I respect are extremely encouraging.

I need an amp that will work both as a main listening amp and be capable of doubling up as an evaluation tool for voicing speakers up properly, and not wilt at the slightest impedance dip. To this end I have a transistor amp coming from a well known cantankerous old bugger who's middle name is forum conflict. :wink:

I'll fix up the Barbariks so that they are giving their full potential at last, then it's over and out in terms of amps.

Speakers are the thing for me now.
I know it makes sense, why? because the missus said so and if there's one thing women do in situations like this it's talk sense.
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Max N
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#224 Re: New PP Pentode Monoblocks

Post by Max N »

Steve
Don't forget that your previous incarnation of the pentode amps were bought by Paul B - that's a bloody good recommendation/sign that you have made amps to a high standard as well.
If speakers are your thing then from now then go for it, but don't be down on your amps
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Paul Barker
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#225 Re: New PP Pentode Monoblocks

Post by Paul Barker »

Please dont put me on a pedestal. But yes Steves amps do the do.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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