Stereo amp

[jfutral]

Just in time for 5.1+ type sound to be all the rage, my recent foray into a guitar amp for a friend has reignited my desire for a tube amp for my "stereo" (how old fashion does that sound?). I've loved every McIntosh amp I've heard.

But I also figure, schematics alone don't make the McIntosh amp so appealing. Or am I wrong? Is a McIntosh just ("just", ha!) a more carefully laid out and circuited guitar amp? Or am I about to find out why those amps cost in the thousands of dollars, such as the cost of higher grade transformers, more chokes, etc?

What would I be getting myself into that is different or more intense from a guitar amp?

Thanks,
Joe

[Gibsonman63]

To bridge the gap between guitar amps and a McIntosh tube stereo amp, you can pick a couple of Bogen mono-blocks fairly cheaply on fleabay. ... or build your own.

[jfutral]

If I could afford the speakers this would be the way I would go! This is an old article my brother sent me from way back when I first got interested in building a tube amp.

Back to the future

The original article posting had a few more images of the speakers and amps, but the article is still quite interesting.

Joe

[imaradiostar]

Why not just build a circuit that looks like a pair of 50 watt Fender or Marshall output sections (without the guitar-oriented frequency compensation) with an extra tube in front for a little more gain? Transformers from Edcor are top notch and perfect for what you're building. Hi-fi guys will scoff but it'll sound better than you'd expect. Don't get sucked into hi-fi land of capacitor cork sniffing and all that jazz. My Scott 222c is built with the cheapest stuff that would do the job and to this day it sounds amazing.

jamie

[jfutral]

Cork capacitors, huh? Do those have a more vintage sound than paper capacitors? ;-) From what I've read about that Scott 222c, it is pretty darn nice.

Joe

[Chuck H]

You could do OK with a pair of mono blocks built similar to guitar amp PA sections, but... Bridging the gap between guitar amps and home audio is a whole butload of refinements in amplifier function that simply never applied to plain 'ol geetar amps. NFB loops are typically phase error corrected and finely tuned r/c networks. Often operating on a global system instead of just the PA. Bias and tube matching becomes more critical and just a bunch of other stuff I don't know a lot about. Basically your average guitar amp PA will typically have a 3%, 4% or even higher distortion figure compared to as low as .01% for a top end tube home audio amplifier. Anything better than .1% distortion is generally plenty good enough and there are many old designs that meet this criteria that were available as kits for home builders even back then. Often with transformer information included. Old Heathkits and the like.

If it were my goal I would probably build a kit with a good rep or restore a classic favorite. To build from scratch would mean I'd have to learn a whole different end of this hobby and I have enough hobbies as it is. There are geek forums like this one for home audio as well and they can be a helpful source of info.

JM2C

Chuck

[imaradiostar]

How about some of the simpler designs from the Triode Electronics website? Perhaps an Eico HF-86 or 87. Edcor transformers are excellent and modern El34's and EL84's should sound excellent in a circuit like that.

I seriously doubt that you'll hear the difference between .1 and 1 percent distortion. You will, however, enjoy that you built the amp!

jamie

[MWJB]

Right now I am listening to a CD through a fixed bias champ/Princeton hybrid (5F1 essentially, no preamp bypass cap, hi-cut tone pot, 2K input cathode resistor, 175v on the 5751 plate)...I thought, this sounds rediculously good so I bought a well regarded entry level hi-fi amp (Fatman I-Tube, cheaper than the champ, but if the champ was made in China too, they could build a stereo head for a comparable price - Fatman is on lots of magazine's must have lists) ...the Champ just kills the Fatman. I have had the urge to build a tube hifi for a while, but I always have guitar amps on hand, so I do the obvious thing.

I'm with Iamaradiostar, have a listen through some guitar amps to start with, you have to sum the L&R by using a 1/4" Y connector & 2x RCA to 1/4 adaptors, plug into the low gain input, sometimes it's easier to dial in volume by using the headphone out jack, rather than the line level out from a CD/DVD player. An amp with 2 channels in phase like a tweed bassman allows you to plug Left in one channel, Right in the other, using the on board speakers you lose stereo imaging & many guitar speakers are crap for this application (especially 12"), Fane AX10/Medusa 75 are pretty cool though & frequency response extends far further than the spec sheet suggests. If you have a parallel speaker out option you can obviously wire up some hi fi speakers with a suitable load. You may want to curb gain some, pull back preamp voltages?

A few years ago, I built a middle of the road 2x6V6 guitar amp...it did a nice "clean w/drive tone", "Hey Joe" style, I wasn't using it much so I was going to rebuild it, unfortunately I plugged my CD player into it (it almost made me weep!) & ended up just adding some switches to turn off cathode bypass caps/bright switches. When you are listening to something and you think, "I don't know how this could sound better", what more do you want. I'm not the kind of guy who is easily impressed by amps either, I don't often hear amps that leave me catatonic for the length of a whole CD.

There are reasonable dedicated tube amps & hifi kits & schems around, I'm not suggesting the guitar amp approach will sound better, as you note, you really have to build stereo amp. My next project is a stereo version of my champ, because I know it works, plus I like the idea of multiple applications for one amp (tightwad). To be honest, there are Chines tube hifis that are cheaper assembled than a decent kit costs (Jolida, Separo), you can afford to "kiss a few frogs" in this respect. There are kits based on proven designs (Dynaco) but as soon as I start reading some of the low scale hifi designs the blurb gets my alarm bells ringing, designers talk about circuits & techniques but never about what the damn thing sounds like.

Randy @ Lone Wolf Blues Company has built a stereo hifi amp, build is documented on the website.

Google "tube hifi amp/schematic".

[imaradiostar]

I once had an amp built with a twin reverb power transformer and a pair of New Sensor 50 watt universal output transformers. I don't care what all the hifi tone snobs say- it sounded fabulous in spite of being a standard Fender output section with 6L6's. It had plenty of power to drive modern speakers and I had a lot of fun with it. I still have the transformers and I'm thinking I might try something similar with EL34 outputs and 6u8 drivers like a Scott hifi amp.

I still would love to build a huge power amp with 4 kt88's, edcor outputs and 100 or more watts per channel.

jamie

[Chuck H]

+++
I was just pointing out that to do it right, as in like a true hi fi, there are finite considerations that go way past what we do with guitar amps. If you are a finicky or compulsive sort (guilty) these things will matter. And I do think there is a substantial difference in the sound quality too. Have you ever put a square wave signal into your guitar amp input and checked it with a scope? I do think the Champ would be better for home audio than many others because class A single ended just has the right attributes built in.

OK, I'm getting a little off track... Sure a guitar amp will work alright. But a design specifically for home audio will be better and there are designs, kits and old restorable units available.

[Steve Conner]

McIntosh amps were built with the best quality components. In particular the extremely high quality OTs with multiple feedback windings.

Triode Electronics sell a nice kit for a reproduction of the Dynaco ST70.

Hi-fi amps have more feedback than guitar ones. About one extra tube stage worth inside the NFB loop. The extra feedback reduces distortion and damps the speaker for a flatter response. Modern stereo speakers can sound pretty weird without the damping.

My stereo amp is a restored Crown SXA. scopeblog » Crown Super SXA It's no McIntosh, but it was a lot cheaper than one. I'm currently running KT88s in it.

[Chuck H]

That's a real pretty amp Steve. How do you measure the distortion content? I've often wondered about that. I can certainly look at a scope pic and see if an amp is clipping, but how do you measure such tiny amounts of distortion and even the associated harmonic?

Some of the best valve amps quote strangly low distotion figures. I wonder if the method used to obtain these figures is at all like these amps are in actual use.

[Steve Conner]

I used a distortion analyzer, a Tektronix DA4084. It was a US government surplus one that I bought off a friend.

It basically shows you the difference between the actual output of the amp and an ideal sinewave. Either as % THD on a LED readout, or you can look at the leftover mush (the "residual") on a scope. The second picture shows the amp output (which looks perfect) and the magnified residual beneath it. Those traces were from one of my solid-state builds, running 50W into 4 ohms at 10kHz.

No tube amp designer would ever admit to owning such a thing: the "mojo" is supposed to be maximized by ear. So I guess I shot myself in the foot right there.

[Chuck H]

That's great. No "mojo" monkeys here, as you know. We can leave that to the other forums. I've looked at distortion analyzers but was put off by the cost since I don't really need one for what I do.

It makes good sense that you would shell out the clams because of your bredth of interest and investment in this field.

[J M Fahey]

Hi chuck.
If you are interested, you can build a reasonably simple THD meter (it would a bit much to call it "analyzer") if you stick to a single frequency, usually 400Hz or 1KHz.
You can build either:
http://www.kennethkuhn.com/electroni...n_analyzer.pdf
or:
http://greygum.net/sbench/sbench102/TubeMisc/notch.gif