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Hoping to build a pair of 807 based Mono Blocks on a budget, looking for schematics

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Mike52787

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Joined 2018
#1
Hello Forum, This is my first post here, so go easy on me 🙂

I recently came upon 2 pair of NOS 807s, and many other NOS tubes. I've been thinking of building a tube amp for a long time, and I now have no excuse. I am however... a bit of a noob. I know the basics of tube principles, and how amplifiers work, but I have no experience building anything.

Anyhow, I have been looking around for some schematics, but can't find much. I have a pretty loose budget, but anything in the $500 range would be ideal. I already have most of the tubes I expect to need also.

Any suggestions, help, or schematics are greatly appreciated! Thank you in advance 🙂
 
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#4
starting with a specific tube is not the best approach for your first build imo.

Define your build goals first - if it is to learn practical building skills, thats a fine goal in itself. But some power output and other "hard" goals are useful to help identify a suitable project.

On a more positive note, look for circuits using 6L6 tubes - they are electrically near identical, and 6L6 is more commercially appealing so has a much wider range of circuits available.
 
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#5
Yeah, Looking at the RH807 schematic, it definitely looks like my best choice, as I have all the tubes required and it seems very simple. I do have one concern though... (This may be a stupid question, so don't laugh too hard) The schematic seems to use a 50hz power transformer, Can I sub in a 60hz transformer with no ill effects on the rest of the circut? or do I have to make some modifications?

Also, what output transformers would you suggest for this project?


Thank you for all the help so far!
 
#6
aardvarkash10 said:
starting with a specific tube is not the best approach for your first build imo.

Define your build goals first - if it is to learn practical building skills, thats a fine goal in itself. But some power output and other "hard" goals are useful to help identify a suitable project.
Click to expand...

What I really want to do with this project is learn a bit more before I dive into deeper projects. I have several other things I would love to build. The tube choice is simply based on what I currently have on me, in an effort to save money.
 
#7
here's another:
The VK1SV 6J5/807 audio amplifier

I've got some old 807 & 5933 tubes. You gotta be careful with the top cap. Old ones can be fragile.
I've got some top caps with wire says on the cable ,"PARAGON (F) 20KVDC...")
Make sure you use the proper high voltage wire & top cap.

And as aardvarkash10 states check 6L6 schematics, Also 5933, STC 5B/254M, 5b/255M, FU7 tubes
807 (vacuum tube - Wikipedia)
(STC 807 replacements Advert)
5B/255M @ The Valve Museum

Or just buy one off of theee bay
look up FU7/807 6N8P amp
6N8P =@ 6SN7
 
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#12
I'd advocate building an amp from scratch; it's more fun and you'll learn more. By scratch i mean designing it yourself rather than build from an existing schematic. It will mean a bit of a steep learning curve, but it's do-able.

BTW, there are no dumb question's, we all started off knowing nowt.

Also building an amp based on the OP valve's you have whilst strictly not the way to do it, is perfectly valid. A list of some goal's and a rough idea of what you want, EG 10w output, class AB1 PP, one mains tfmr/PSU stereo all on one chassis.

Next I'd start by having a look at this datasheet - http://people.physics.anu.edu.au/~dxt103/audio/807/807STC.pdf which is the best 807 datasheet. If you look at page 5 it says Class AB1 tetrode connection for an Op of 26.5 watts ( you'll have some gain to throw away on NFB) you'll need 360v plate/anode/HT/B+ at a max plate dissipation of 138mA, so call it 150mA times two will give you your mains tfmr specs, IE secondary of 250v (360 divide by 1.414 = 250v ish) @ two times 150mA = 300mA or 75 - 100VA. This is the rough specs of your mains tfmr. The primary if your in the States is 120v AC 60hz.

We can also get our OPT specs from page 5, it says OP load Z (impedance) is 6600 ohm's or 6k6, this is a common value, so you'll need a primary of 6k6. You could go for UL connection, taps at 43% if it doesn't cost much more ( though 360v exceeds the 807 max screen voltage, max screen dissapation is within limits, so you'll be ok to connect UL) The secondary will likely be your usual 0/4/6/16 ohm's with a power capability of 20w or higher. Getting a higher (more expensive) rated OPT is generally a good idea as it will likely be bigger and thus have a better frequency response at lower frequencies. it also gives you some wiggle room.

Once you have your transformers, you can alter your design to to fit. The "front end" and phase splitter are pretty easy to build once you have an idea of the OP stage. See here for lots of great info - How to design valve guitar amplifiers though not primarally a hifi site, it has I think the best info for beginners.

Lastly remember valves/tubes are very tolerant of mistakes unlike three legged fuses AKA transistors and are thus ideal to learn audio amplifier design on.

Good luck with your build, Andy.
 

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#13
If it's your first amp I'd suggest using a cathode biased design for simplicity. Use a resistor and cap on each cathode for better balance. In the case of the above datasheet image, you'd use a 9k (8k is easier to find, and close enough) Ra-a OPT and double the cathode resistor value to 500. Bypass with 470uF/50V and you're good.
 
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#15
How complicated do you want the amp to be?
You could build 2 push pull channels, or you could build 2 parallel single ended channels.
How safe are you with high voltages?

Sarris did a parallel single ended that is similar to the RH circuit:

807 single ended tube amps | Sarris Custom Tube Amps

It looks like there may be a schematic error or two, but that can be fixed, if you like that amp.
 
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#17
I recently made a long series of youtube videos on a SE 807 amp I built based upon the triode dick design but using commonly available parts here in the US. You could easily adapt that design to monoblock, it currently uses a split rail PS so all you would have to do is duplicate the front half of the PS (xfmr, choke, fuse, switch, etc) and then break the schematic at the split rail for mono.
I was very pleased with how it turned out and sounds.

You can find them under Blueglow Electronics on youtube.
Mark
 
#19
Mike52787 said:
I was thinking of doing a class AB PP design, but the single ended designs seem simpler and easier to understand for a first build.
Click to expand...
Then do it. That was my first build, too.

Here's the schematic of the first power amp I ever built. I used UF4007 instead of the tube rectifier, and a 50R/20R resistor instead of the 8H choke. I was expecting to hear hum when it warmed up, but it was so silent I thought it was broken until I played music through it!

http://www.keith-snook.info/amplifier-hifi-schematics/Acrosound%206L6%20Rebuild.pdf

You could use this package for the transformers
EDCOR - PM-100
 
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