801a amplifier

What people are working on at the moment
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8872
Joined: Mon May 21, 2007 9:42 pm

#106 Re: 801a amplifier

Post by Paul Barker »

i wrote my own tool for capacitor calculating decades ago, 6hz -3db with 53.9 uF Im satisfied with that. My caps measure 59.

Image
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8872
Joined: Mon May 21, 2007 9:42 pm

#107 Re: 801a amplifier

Post by Paul Barker »

One of the secret weapons in this amp is the output transformer. It’s bifilar primaries with solid core wound together with litz. The secondary is the usual solid core all joined in parallel. This litz is probably giving its great sound, the lams are cooking waste less EI you’d see in any power transformer. Where the quality of a transformer lies isn’t always obvious. But these sound great. 50 Henry so not worried about their bass response, very few commercial opts sport as much as that, more common is 30h.

They have no bling but speak volumes for themselves in their reproduction.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8872
Joined: Mon May 21, 2007 9:42 pm

#108 Re: 801a amplifier

Post by Paul Barker »

Ive spent a few days studying tube charts with failed results at every chart, until I came to ECL82, which is pretty bang on by just a few volts lacking that I can line up exact with a bias resistor on the cathode of 14 ohm, which will most definately not get a bypass cap. It would work though without a bias resistor, but it would alter operating point of 801a, which Id rather not mess with. 4 triode connected pentodes on the cathode with zero bias to maybe -0.5v to -.75v auto bias to satisfy ocd maybe.

Ive always liked the sound of Russion EL84 equivs, and their ecl82’s are flippin cheap, brand new. I only need 4 (one per cathode bias of 801a). FREE postage 16 us dollars for 8 tubes.

Even though the charts fit perfectly in the requirements of the 801a setup I have, Ill mock up a test bed on a variac for one at a time. So after that experiement is run, theyll probably go into the amp, and I’ll learn what it compares in sound to cathode bias bypassed with pio caps.

Im not new to this process I did it from an old wireless mag in which a triode was used to modulate bias of input valve as a volume control. As far as I recall though it was last century, I spoke about it on tube diy assylum and was shot down in flames. But I liked it. Just a lot of self promoting willy wavers on web in those days criticising. As if internet is any better today!
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8872
Joined: Mon May 21, 2007 9:42 pm

#109 Re: 801a amplifier

Post by Paul Barker »

Balls!

I found a dls10 in a box of valves at Barker Towers. I dont recall buying it but I cant remember most things.

So hexperimented with heater volatge / time delay. Found anything below 6 volt is barely controllable, unexpectedly 6v delivers sitching at 30 seconds. I only need between 15 and 30 seconds to allow d3a’s (the only indirect valves).

So on the test bench it delivered the voltage at 30 seconds bang on, with a pop on the speakers. That was a bit of a surprise, perhaps a gradual start is better than a relay in a vacuum, seems brutal turn on.

Anyway its academic, because this morning at only the second turn on of the ht through the dls10 it self destructed. FEK!

I’ll have to scratch around in more boxes until I find a line output diode to put in series with the b+. slower transition from no HT to full HT is called for!

I know solid state delay circuits are cheap and plentiful, but I have no solid state in my valve amp designs and like it that way.

Sorry guys but no hybrid bridge rectifiers in my amps. centre tapped power supplies every time.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8872
Joined: Mon May 21, 2007 9:42 pm

#110 Re: 801a amplifier

Post by Paul Barker »

Update!

I needed to get round to reducing the poser consumption down on the vr tubes, so did that this morning, they were each shunting 30mA plus each d3a using 15mA meant just the VA section was consuming 90mA, and I could tell from the sound there was something amis. Added two more 50watt 4k7 droppers to the VR’s. So now VR’s each pulling down 18mA with each d3a pulling 15mA thats a perfect shunt reg balance. I cant believe how much difference its made. I was actually exceeding the spec of the main power transformer previously, this has put it within spec, so the b+ has bobbed up to 630v the current through the 801a’s now 37mA. The max power output now 14 watts at 1% 2nd plus 1% 3rd 4th of no significance. Back off to 12 watts distortion below 0.5% Yes dissipation is past 20watts per tube. But the sound is awesome, so Ill run it at this, you only live once. This is an awesome amp. Bear in mind the graphs and data are calced on one tube, you have to double for two and half load. So power is twice what it states load if half what it states current is double. No bright spots on any of the plates! I already changed diodes to 5R4 potatoe mashers prallel connected, so no accidental high voltages, all be it ceramic top caps should be ok. But we have a dog. Safety first these days. The delay tube escapade scared me actually, Ive put a Thermistor inline, and just as an added precaution I plug in signal section first to give d3a,s more time to heat up.
Imagecapital one locations near me

Image

Image


Image

Image

Image
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#111 Re: 801a amplifier

Post by Cressy Snr »

All good stuff Paul. :)
Sgt. Baker started talkin’ with a Bullhorn in his hand.
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8872
Joined: Mon May 21, 2007 9:42 pm

#112 Re: 801a amplifier

Post by Paul Barker »

I know Im always banging on about power supplies, but I discovered the sound of potato mashers first with Darren. So we did a joint purchase. But all I could find was these two. Anyhow two is enough. I think the greatest improvement is that Im no longer over taxing the power transformer, but kant help wandering how involved the potato mashers in the better sound. Or is it that, like so many transmitting valves, the 801a’s are flourishing in the extra voltage and current. All be it Im over the max dissipation curve.

If you do a deep search on the 10y you can find opinions that suggested in aircraft service it was ran at 15 watts dissipation.

I do think the max dissipation spec is over cautious.

Back on the subject of power supply. Im quite liking PS Audio Paul with all this time on my hands. He says a lot I cant fault. OTOH I have a psaudio preamp with phono stage which sounds shyte. Im not sure if I still have it, might have actually skipped it. Now I hope not, maybe I should have investigated why?

BUT (You knew that old Goat would come to it, didnt you?), last thing he was speaking of I watched cant remember what it was, probably a preamp as it looked small, but he said the torroid was ten times the capacity required, and sounded better. I can believe that. I think the biggest mistake the Chinese made when they first flooded the market with 211 and 845 amps 300b amps etc. Which forced people like WAD to give up and contributed to Darren’s demise. Ive bought, found wanting and stripped for components two, a parallel 300b I bought from WAD Richard, and an 845 amp I boght from whoever? cant recall.

Both amps biggest shortfall was inadequate power transformers, and neither arranged for valve fullwave rectification, worse still both used voltage doublers. That right there is enough reason to run from chinese amps. Unless they’ve chaged, I doubt it though. It was all money down the drain for me, I barely salvaged anything at all. Maybe valve bases.

So here I am sat with. 725-0-725 @250 mA Boat Anchor used close to its rating. So it has greater future potential by some multiples, in a future event you may see me wheel in a gynormous HT transformer. Not as if you havent seen that from me before! The biggest one, not quite the size of a pole pig you saw with the Nick and Paul GM70 attempt to drive Colin’s e-statics. It couldnt make the grade but it was fun. I made a mental note never to attempt to make e-stats sound like what they dcan’t again! One minute in Bruce Edgars show and tell, and you’d never think such stupidity. It just makes youre amp look stupid.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8872
Joined: Mon May 21, 2007 9:42 pm

#113 Re: 801a amplifier

Post by Paul Barker »

All I needed one filament death already. Probably I’ll convert it to single ended Gk71 single valve fits the opt’s and with a b+ 615 v. Going forward I can’t maintain an 801a amp with 4 valves needy of £150 a pop to replace in my latter years.

Image

Image
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
User avatar
pre65
Amstrad Tower of Power
Posts: 21373
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#114 Re: 801a amplifier

Post by pre65 »

Have you got a pair of GK71s Paul ?
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8872
Joined: Mon May 21, 2007 9:42 pm

#115 Re: 801a amplifier

Post by Paul Barker »

This concept dawned on me before I built the 801a. GK71 single valve just wants its own filament supply and bias resistor and amp is converted.

Image

Meanwhile, building the Raymond Bates.

Mental health improving incrementally!
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8872
Joined: Mon May 21, 2007 9:42 pm

#116 Re: 801a amplifier

Post by Paul Barker »

pre65 wrote: Mon Jan 31, 2022 9:43 am Have you got a pair of GK71s Paul ?

Thank you ever so much for asking. Im fine Phil I have six.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8872
Joined: Mon May 21, 2007 9:42 pm

#117 Re: 801a amplifier

Post by Paul Barker »

It’ll be a while before I alter this for gk71 so I’ll try to explain where it finally arrived. Ive only been able to hear it on the speakers Anthony built. Ive lifted the top end with some Celestian Alnico mid tweeters, of a bygone era, and those I cant fault they are excellent at that task. The drivers in the speakers are limited in range, but they too are very good at the range they do, to my ear they have no strange peaks or troughs, but they dont do the deepest bass tracks that I like listening at same level. They can give you the notion its there but probably 1/3 rd of the sound level by ear.

So the 801@ amp bossed them, the amp is full range and controls the drivers extremely well, the roll off is from the speakers. The imaging soundstage and depth are all flawless. The voicing is to perfection. These are things we aim for and we mostly get these things in our builds, all of us these days.

I would be very disapointed if the gk71 doesnt exactly mirror this situation. If anything it might do it better. But running an amp that sounds great isnt necessarily a case of using 4 of £150 plus plus plus and climbing all youre life but dying off one by one if its on all the time. By the way filaments are a. and bang on 7.4v so its not misuse. Even though the valves test good and sound great, theyre old and there are no modern production equivalents, and when Im struggling to put money into my van so I can work to earn money, there is no point to this type of build any more. And to be honest a kel84 is in the same ball park as any of our sedht’s. Though I have high hopes for Raymond Bates.

But this amp was a resounding success. I just cant bat in that league forever. When I bought the 801@ s they were £60 to £70 each on average; its a new situation now, I was blind to the fact thre was nothing made today like it, and didnt foresee this situation. Valves are consumables.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8872
Joined: Mon May 21, 2007 9:42 pm

#118 Re: 801a amplifier

Post by Paul Barker »

So inbetween bouts of debilitating pain, Ive had some lucid time on my hands.

GM70’s are cheap enough. Buying the graphite plate ones today you can get them nos delivered for £32 each. So actually no point running the gk71. But its the same base and filament supply, so if you get design right you could use either.

copper plates are more, but against the price of 845’s still very competative. Anyway I have two nos copper gm70’s which are almost indistinguishable from 212’s. In a room full of us types where we werent shown which valve was in, it would be a hung parliament. But the copper plates are not that much better than graphite. Its nothing at all like the difference between a st45 against a globe 45.

So where am I going with this. Now Im juggling ideas again.

So the power supply is at its limit at 615v@max permitted current 250mA arriving at the signal section. That actually sucks. Even now Im using 214 mA. OK the 250mA is delivered through choke input with the biggest mother of a choke Ive ever owned, and series black gates giving 100uF capable of handling 1,000v. But bear in mind the actual peak voltage as it hits the black gates will be more than 615 the 615 is after it is first processed by the black gates. A lot of peoplle miss this affect of raw ripple before its smoothed. In thier amps the first cap will be the first dead cap in their power supply. Caps today are made to the voltage they are specified at. No leway. Youre first cap can see 100v more than you thought at every ac peak.

So in some ways the power supply is still ok for gm70 at 550 v plate, using rest for autobias. current would probably be 80mA so 224mA demand on the 250mA capability.

Which leads to the obvious alternative looking at what I have already. I have a pair of 2920 VCT @ 250mA each power transformers. I have a pair of eminently suitable Tribute output transformers. I have plenty of 1000v poly caps. I just dont have a pair of 250mA suitable for choke input chokes.

So long term game is a new big pair of monoblocks. only held up by chokes in waiting.

With that in mind and looking at all parameters I’ll adapt the 801a amp to single gm70’s. Ive got about 10 graphites from the days they were even cheaper than £32 a pop. They’ll do for the medium term. The bases and heater supply and cathode biasing for parallel 801a’s can remain in situ for the switch back should the 801a sound better of the two. I just fit gm 70 bases and cathode resistors. make a filamnt supply.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8872
Joined: Mon May 21, 2007 9:42 pm

#119 Re: 801a amplifier

Post by Paul Barker »

Regarding GM70 / gk71 costs. Quick servey of ebay, prices per single valve include postage to UK.

GM70 copper : £140
GM70 graphite: £34
GK71 graphite: £24

I bought a pair of used tested “good” copper gm70 for £50 each recently. Might have been worth a punt, Ill only know after the event.

I should have a nos pair at the lockup.

At the moment with graphite gm70’s replacement value at £34, there isn't much incentive for me to fart about with gk71. I think anechdotally people have said they prefer its sound. But I need simplicity right now. The thing that bothers me is unless you go pentode and feedback, youre either in silly low power if you observe the 400v g2 limit as triode, or you're pushing into risk territory. But even if you conservatively push from 400 to 600, power is pants. What’s the point? when gm70’s which you can get an easy 20 + watts class A only at 1,100 volts. You can get a lot more if you build power drive amp. Im not about that these days. But it appears you do need to get above 1kv for best sound from places ive read.

Now with Nick we had it cooking quite hot and though the els panels still didnt sound any good, the amp seemed better at the higher voltage. Whatever that was!

You cant find gm100’s today. I bought two back then I think they were £100 each. Im very unlikely to even heat them. I dont any longer get the race for more power. Its not worh it, and experientially, you cant have the beautiful sound of low powered se valve amps on efficient speakers when playing high powered valve amps on less efficient speakers, and certainly not on the esl’s we did that experiment on. Though my 212 sounded lucsious on Vry’s Martin Logan panels, my God the cellist was sat their I swear down. One of those moments of magic you never manage to achieve when youre chasing it!
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8872
Joined: Mon May 21, 2007 9:42 pm

#120 Re: 801a amplifier

Post by Paul Barker »

Arrived today. Filament continuous, visuals good, and definitely copper.
Image
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
Post Reply