UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Other Discussions > Homebrew Equipment

Notices

Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members.

Closed Thread
 
Thread Tools
Old 16th Sep 2006, 6:23 am   #1
adibrook
Heptode
 
adibrook's Avatar
 
Join Date: Dec 2004
Location: weymouth
Posts: 377
Default 807 power amp #1

Its 6.08am now and i'v been sitting in my shed for like 6 hrs, building this.

Usually, i spend this part of the day watching Cops/Cheaters on channle 151 with our drummer. However, today channle 151 have become very annoying and did a whole day of stuff that wev seen before. So a new activity was needed.

I thought, even tho i'v got a number of 807's, and i'v lost interest in them, its worth at least building a prototype amp to see if 1) i'm capable of building a power amp of considerable power and 2) to actually see what the 807's are like out of curiocity.

So i built this power amp, and to be honest it aint too bad. Sounds nice, and 807's dont look TOO bad in the dark. In fact, these 807's seem to have a slight blue glow in the glass, which makes it look even nicer.

I'm using a 5Z4 rect at the moment, which is abit small i think but works. For the smoothign choke i used the primery of a mains transformer form a transistor amp, which seems to be quiet effective.

This also happens to be the 1st time i'v ever built a long tailed pair phase splitter. I used a 6SN7 and it worked 1st time no pronblem. I think the trick was to draw the circuit in a way that seemed logical to me. In schematics its usually not how my brain would consider logical. But...as soon as i redrew it i was like OOOOOOH i see and now i can build them with no problem, and feels like i'v known it all my life .

I gave it a blast with my minidisk, and these 807's really sound ok. I doubt anyone appreciates listening to black metal at 6am, but they should be used to it by now.

So, the 807 isnt such a bad valve after all. And looks like i AM capable of building one of these. Even when slightly tired.

The search for an even cooler power amp valve still continues tho.

I will put a pic and a schematic up in the ''morning'' (3pm).
adibrook is offline  
Old 16th Sep 2006, 7:42 pm   #2
adibrook
Heptode
 
adibrook's Avatar
 
Join Date: Dec 2004
Location: weymouth
Posts: 377
Default Re: 807 power amp #1

Ok

Here it is.
Attached Thumbnails
Click image for larger version

Name:	8071.GIF
Views:	153
Size:	17.2 KB
ID:	5482   Click image for larger version

Name:	P1010056-1.jpg
Views:	148
Size:	116.8 KB
ID:	5483  
adibrook is offline  
Old 16th Sep 2006, 8:48 pm   #3
jim_beacon
Heptode
 
jim_beacon's Avatar
 
Join Date: Mar 2003
Location: Bishop's Waltham, Hants, UK.
Posts: 939
Default Re: 807 power amp #1

Hi Adi,

your phase splitter doesn't look right to me - as far as I can tell, yyou're driving both grids in phase (at the splitter and the ouput). I think on of the phase splitter grids should be connected to a fixed bias potential, with no signal.

Jim.
__________________
Jim Beacon G1JBG
www.g1jbg.co.uk
jim_beacon is offline  
Old 16th Sep 2006, 10:45 pm   #4
Skywave
Dekatron
 
Skywave's Avatar
 
Join Date: Jun 2006
Location: Chard, south Somerset, UK.
Posts: 6,367
Question Re: 807 power amp #1

Quote:
Originally Posted by jim_beacon View Post
Hi Adi,

your phase splitter doesn't look right to me - as far as I can tell, yyou're driving both grids in phase (at the splitter and the ouput). I think one of the phase splitter grids should be connected to a fixed bias potential, with no signal.

Jim.
Yes - it does look a bit strange at first sight.

The two halves on the 6SN7 form a long-tailed pair with the grid of the second triode at ac ground. That's fair enough, but I would have expected to see the 1.2M R going from triode 1 grid to deck, not to the 0.47uF cap., and another 1.2M R from the grid of the second triode to ground and this R in parallel with the 0.47 uF.
That way, the second triode acts as a grounded-grid phase follower and the first triode as a phase inverter.
I can see that as it stands it sorta works - but I'm convinced that it ain't quite right!

Al.
__________________
"Your idea is crazy! But the question is this: is it crazy enough . . . . to be true?" (Niels Bohr)
Skywave is offline  
Old 17th Sep 2006, 12:01 am   #5
ronbryan
Octode
 
Join Date: Jan 2003
Location: Guildford, Surrey, UK.
Posts: 1,585
Default Re: 807 power amp #1

The phase splitter as drawn looks peculiar because it does not have any DC return path for the 6SN7 grids, they are just floating.

If the coupling capacitor between the EBC33 anode and the 6SN7 grid is removed and replaced with a direct connection, the problem is solved.

This minor change gives a phase splitter circuit that is fundamentally similar to the famous Mullard 20W amplifier or indeed the Mullard 5/10.

Ron
ronbryan is offline  
Old 17th Sep 2006, 12:20 am   #6
arjoll
Nonode
 
arjoll's Avatar
 
Join Date: May 2006
Location: Invercargill, New Zealand
Posts: 2,472
Default Re: 807 power amp #1

Quote:
Originally Posted by adibrook View Post
Its 6.08am now and i'v been sitting in my shed for like 6 hrs, building this.
Quote:
Originally Posted by adibrook View Post
I will put a pic and a schematic up in the ''morning'' (3pm).
Looks like you're stuck in NZ time!
arjoll is offline  
Old 17th Sep 2006, 6:33 am   #7
adibrook
Heptode
 
adibrook's Avatar
 
Join Date: Dec 2004
Location: weymouth
Posts: 377
Default Re: 807 power amp #1

Yeah, the phase splitter is copied from some kind of hifi amp design i think. Although the original design had no dc blocker cap, as you said, i stuck one in. I thought if it doesnt work right i can allways remove it later.
adibrook is offline  
Old 17th Sep 2006, 9:41 am   #8
Skywave
Dekatron
 
Skywave's Avatar
 
Join Date: Jun 2006
Location: Chard, south Somerset, UK.
Posts: 6,367
Talking Re: 807 power amp #1

Quote:
Originally Posted by adibrook View Post
Although the original design had no dc blocker cap - as you said - I stuck one in.
I thought: "if it doesn't work right, I can always remove it later" . . .
Tut, tut! You wouldn't be the phantom core-twiddler would you?

Best wishes with the amp.

Al
__________________
"Your idea is crazy! But the question is this: is it crazy enough . . . . to be true?" (Niels Bohr)
Skywave is offline  
Old 17th Sep 2006, 9:42 am   #9
jim_beacon
Heptode
 
jim_beacon's Avatar
 
Join Date: Mar 2003
Location: Bishop's Waltham, Hants, UK.
Posts: 939
Default Re: 807 power amp #1

At low frequencies (<500Hz), there will be an in phase signal passed to both ouput grids, and that is not good! (if you don't believe me, do the maths - the component on one grid is very small, but it will cause some distortion). The circuit you have is a frequency dependant potential divider, so long as the capacitor is large, the effect won't be noticeable, but if it was less than the current value, it would. If you want to stick with this circuit, increase the cap to say 1uF, or even more

You should seperate the AC ground of the two grids really.

Jim.
__________________
Jim Beacon G1JBG
www.g1jbg.co.uk
jim_beacon is offline  
Old 17th Sep 2006, 10:52 am   #10
ronbryan
Octode
 
Join Date: Jan 2003
Location: Guildford, Surrey, UK.
Posts: 1,585
Default Re: 807 power amp #1

Jim

I make the reactance of a 0.47uF capacitor equal to 677 ohms at 500Hz. This is not really comparable with the phase splitter 1M2 inter-grid resistor, so phase shifts due to the CR network at this frequency are small (<<1 degree).

Even at 50Hz, its reactance is 6k7 ohms, which is several orders of magnitude smaller than 1M2 and the phase shift of the network is less than a degree.

Ron
ronbryan is offline  
Old 17th Sep 2006, 12:34 pm   #11
Merlin
Banned
 
Merlin's Avatar
 
Join Date: Apr 2006
Location: Shrewsbury, Shropshire, UK.
Posts: 67
Default Re: 807 power amp #1

Adi, that LTP is meant to be DC coupled, you can't just jam a cap on the input and hope for the best. As it stands, your LTP grids are at 0V while the cathodes are at 100V or something- it's biased WAY cold!

Can't you just copy the LTP from a Fender/Marshall or something? Basically you need two extra resistors, a bias resistor and another grid leak, so that each 6SN7 triode has its own grid leak.
Merlin is offline  
Old 17th Sep 2006, 5:48 pm   #12
jim_beacon
Heptode
 
jim_beacon's Avatar
 
Join Date: Mar 2003
Location: Bishop's Waltham, Hants, UK.
Posts: 939
Default Re: 807 power amp #1

Quote:
Originally Posted by ronbryan View Post
Jim

I make the reactance of a 0.47uF capacitor equal to 677 ohms at 500Hz. This is not really comparable with the phase splitter 1M2 inter-grid resistor, so phase shifts due to the CR network at this frequency are small (<<1 degree).

Even at 50Hz, its reactance is 6k7 ohms, which is several orders of magnitude smaller than 1M2 and the phase shift of the network is less than a degree.

Ron
Yes, but it is still there, and for the sake of a couple of components, it can be removed. As I said, with 0.47uF it works, but with a smalller capacitor, you could run into problems.

Having said that, with the music Adi plays, he may prefer to put in a smaller cap

Jim.
__________________
Jim Beacon G1JBG
www.g1jbg.co.uk
jim_beacon is offline  
Old 18th Sep 2006, 3:26 pm   #13
adibrook
Heptode
 
adibrook's Avatar
 
Join Date: Dec 2004
Location: weymouth
Posts: 377
Default Re: 807 power amp #1

Thanks for the advice. I will take the cap out and just connect the grid directly.

I think overall the prototype went quiet well tho. It worked 1st time, which i cool.

I didnt have any 807 sockets so i just cable tied some wires to the pins and drilled holes in the wood for the pins to go into.

I thinki will try out some more power valves and then finally start buildign the real thing.
adibrook is offline  
Old 18th Sep 2006, 8:47 pm   #14
Skywave
Dekatron
 
Skywave's Avatar
 
Join Date: Jun 2006
Location: Chard, south Somerset, UK.
Posts: 6,367
Wink Re: 807 power amp #1

Quote:
Originally Posted by adibrook View Post
I didn't have any 807 sockets so i just cable-tied some wires to the pins and drilled holes in the wood for the pins to go into.
Excellent! Now that's what I call bread-boarding!

Al.
__________________
"Your idea is crazy! But the question is this: is it crazy enough . . . . to be true?" (Niels Bohr)
Skywave is offline  
Old 18th Sep 2006, 8:58 pm   #15
adibrook
Heptode
 
adibrook's Avatar
 
Join Date: Dec 2004
Location: weymouth
Posts: 377
Default Re: 807 power amp #1

I'm pretty sure thats a sort of acceptable method of doing it. I made friends with the 83 like that, and i tried out directly heated triodes with AC/pens and stuff liek that too. It seems to work for prototyping. Plus the fact that i prototype stuff on a piece of plywood with nails as terminals makes it easier. The wires are soldered to nails round the ''socket''. That keeps everythgin stable.
adibrook is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 8:14 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Copyright ©2002 - 2019, Paul Stenning.