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One Tube Mono Blocks

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Payback time...

I have gained so much from this forum and others that I'd like to chip in my tiny bit. It is a design for a one tube mono block amp that is pretty good and fairly cheap considering the return. Skip to the bottom if you are an experienced builder and have some fun with this very good and easy to build 13EM7 spud.

I must note that I have borrowed ideas from some far better minds though I may have gone off the reservation. Credit to Dan Wildt, Sean Richardson, Larry Moore, Jim Dowdy, Dave Slagle, Jeff Medwin, Thomas Mayer, Al Klappenburger, John Browskie...the list is long. The first two fellows taught me so many things hands on, the others have given both direct and indirect guidance in copious quantities and believe me there are more people out there helping in ways too numerous to enumerate!

The thought here is bang for the buck, cheap yet good. It is in fact, Very good.
I would not offer it up otherwise, there are so many designs with this venerable tube from the glory days of tube TV. I started with scrap box parts but found every small investment paid dividends.

So we start with an isolation transformer I got off that famous auction site, donated capacitors, lots of terminal blocks from China and a visit to the Hammond website for bargain hunting. One key element in this design is the pair of Hammond 156C chokes. Oh, cheers to Andy Evans for this tip :) mark the wires in and out, same on both chokes, then align the mounting holes bottom to bottom. Now chose one to be the first choke, send it's "out" to the "in" of the second and you have a humbucker and a real humdinger of a choke for this amp. The chokes in the PSU are also hammond. I went with the 159ZA but you could go with a lesser model. The point here is that you don't need huge chokes, only .3H and less than 10 Ohms each. The PSU will be fast and silent. Using a six pack of smaller caps for the one large filter cap is smart and good. I think it sounds better than a single. You can also isolate that bank from the next stage with a single diode if you worry about the sound of electrolytics but I don't think it makes a big difference. The final cap 50uF Vishay (actually 2 x 25uF) is the important one.

I use terminal blocks and spade connectors more than I solder because I am always experimenting with components and placements. Feel free to just lock everything down, solder at will. You see from the photos I used a sort of sled (1/2" Baltic Birch plywood) inside of a slip case. The sled is 8.5" by 22". I used both the top side and the underside to fit everything. My schematic does not note that I used Rod Coleman voltage regulators for the (yes indirectly heated) heaters nor the Brumfield industrial timer to warm up said heaters. Nor does it note the voltage meter on the front for AC and the meter on the back for DC. (The heat sink is for the regulator.) It is mostly for showmanship but I will say the Rod Coleman regulators surprised me for the detail they added (note, the DC meters of those heaters need to be switched off during play to get the best use of the regulators). I used James 6123H output transformers I had from a previous project, those are very nice. I would not poo poo Edcore for the job, they are a bargain! The coupling cap is important. I have been collecting Russian PIO caps for a while, a very good bargain too!

The bypassing of the final cathode with so many smaller caps is up to you. I like it, I think it helps and it's cheap to do with the afore mentioned Russian PIOs.

That's about it. The linear arrangement keeps the magnetic fields away from the signal end. I always use a Corcom IEC connector with fuse and sometimes with all the filters. I use CL60s after that because my household AC is 5% strong and there is that surge at startup to alleviate. The color light is from screw-in Christmas LEDs in concert with the timer for start and run conditions. I took the scale card off the AC voltage meter to let the light shine through it. All the aluminum is from the scrap yard. Screws and such from the hardware store. I can recommend an eight inch long 3/16" bit for drilling holes for blocks and chokes so you can reach around them for the pilot holes. The silver screen is Home Depot radiator screening. I use 6/32 cap screws for everything I can. Drill and Tap is easy on T6 aluminum also readily available at the hardware store. This is probably more detail than you wanted but I DID invite the experienced to jump to the schematic and just plow on ;-)

Cheers!
 

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from grid leak to grid choke

Hi, I'm back.

It did have a grid leak resistor I left off my schematic. I apologize and thank you PRR for spotting that. It had a 0.5M resistor. But, in opening it up and investigating I decided to try a grid choke in place of the grid leak resistor, its that tiny thing next to the potted James output transformers...

Nice!

It is NOT day and night different BUT it is in every way, better. Not many changes are all to the good, usually there is give and take. This was all good. So, for those of you thinking of giving a grid choke a try, here is one case where it went well.

I have altered the schematic to include the grid choke, and include a few shots of the cases opened up for people who like "nudes". :rolleyes:
 

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Coming up now on one year since I built these amps. The only further ateration was to switch to a Jupiter Paper/Wax coupling cap. I have been listening to them a great deal they are my "daily driver" since I started a rebuild of my main amps thinking they'd tide me over... seven months ago and counting.

I come to like them even more and though I'd acknowledge familiarity factor I think there is something to that Japanese notion that transformers need break in too. Or manybe it is not the transformers but some other part but they make better music than they did when I built them. My take away is that I may not have been giving my other amps the chance they deserved before moving on to some other design. But hey, we're builders right?

I'd still change a couple of things here like maybe that huge C1 but I'm not going to touch it. It is a great amp. I got very lucky with this build/design. It's a honey.

On a sad side note: I think James transformers has dropped off the map. I would have bought another set of 6123h if I had known...
 
Why the use of chokes (two!) as a triode plate (anode) load? Apart from space and cost they add a frequency dependency (LF and HF roll-off), also distortion from core hysteresis. In a signal channel they can also pick-up hum. A single resistor is far better (use a metal film type.) It's simple to make up the voltage drop with a higher B+ (HT+) from the P/S.
Cheers,
Roger
PS. Also, why battery bias on the first stage? A bypassed cathode resistor is easier, cheaper and zero maintenance.
 
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Joshvito, the amp is a single tube amp, a dissimilar triode: 13em7 (the 6em7 has become pricy) Output is about 2W so you need high (95dB) efficiency speakers, that's always the rub for so many folks. I play these amps through Klipsch La scala speakers but have also run the suple clever Frugle Horn XL. I highly recommend that build!
 
To gem_piano, I'm sorry, you can argue until the cows come home... listening is a skill and a talent. Engineering is a different skill and talent. I'm not going to argue who has more or which is more important. If you cannot hear a difference, lucky you. You don't have to go through all this nonsense that "we who hear things" must.
 
Very cool looking chassis ideas and I like all the junk-box parts usage!

I you have an old solar garden path light (even the bargain-bin Chinese new ones only cost 7 bucks at Home Depot). Those use a single 1.2v NiMH battery. Maybe you could rip one of those apart, extract the small solar panel and charger to keep the bias battery trickle charged?

I think I understand the whole circuit except for... Why all those cathode bypass capacitors and what is the thought process behind selecting their values/types?
 
Why the use of chokes (two!) as a triode plate (anode) load? Apart from space and cost they add a frequency dependency (LF and HF roll-off), also distortion from core hysteresis. In a signal channel they can also pick-up hum. A single resistor is far better (use a metal film type.) It's simple to make up the voltage drop with a higher B+ (HT+) from the P/S.
Cheers,
Roger
PS. Also, why battery bias on the first stage? A bypassed cathode resistor is easier, cheaper and zero maintenance.

I have played around with using two hammond 156Cs in series as a plate load.

Remember that if you are using a proper plate choke, the load line is virtual horizontal (its actually elliptical, but for practical purposes we say its horizontal). This helps reduce non linearities in the tube and lowers distortion.

The humbucking configuration also helps to reduce hum pick up. Im not quite sure by how much, but every time I have used them in this configuration I wasn't able to hear or measure any sort of hum what so ever.

If you use the plate chokes with the right tube, in the right circumstances, they can sound a LOT better than a resistor.
 
To gem_piano, I'm sorry, you can argue until the cows come home... listening is a skill and a talent. Engineering is a different skill and talent. I'm not going to argue who has more or which is more important. If you cannot hear a difference, lucky you. You don't have to go through all this nonsense that "we who hear things" must.
My only comments are:
1. "... skill and a talent."? Yes, ears need educating! Go to more live concerts (acoustic instruments only) and listen very carefully. What you hear is the audio reproduction objective. My systems get quite close...
2. Audio power amplifiers should not have any "sound" at all! They should behave as close as possible to a "short, thick wire with power gain". Then, any residual distortion can be laid on the source (mostly tape, CD, DVD, FM) and the speaker... and the latter is a challenge.
BTW, Star-ship "Enterprise" Chief Engineer, "Scotty" was (or will be!) right when he said (or will say!) "Captain, I canna change the laws of physics!" Nor can we!
Cheers,
Roger
 
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