18W Hifi amps

Post by **Graydon** » Fri 07/16/04 9:37 am

Hi Guys,

Here is what I have been building over the last two weeks. I got them finished a couple nights ago and today I reversed the polarity of one and suddenly all of my music seems to be more defined!

They aren't really loud but they sound very nice. With a pair of horn-based speakers I think they would be great. I plan to add some hardwood ends to close out the chassis. Here's a photo...

Dai · Post by **Dai** » Fri 07/16/04 10:31 am

nice G!

MichaelJ1965 · Post by **MichaelJ1965** » Fri 07/16/04 10:36 am

What OT is that? I hooked up the preamp ouputs from a car stereo and it sounded great. The speakers were the g1280's. It probably rolled off the high frequencies because of the OT and the speaker.

Post by **Graydon** » Fri 07/16/04 11:31 am

MichaelJ1965 wrote:What OT is that? I hooked up the preamp ouputs from a car stereo and it sounded great. The speakers were the g1280's. It probably rolled off the high frequencies because of the OT and the speaker.

That is a GDS/Heyboer 18W OT but it is like my new Vintage 18W OT with ultralinear taps and multiple secondary windings just like the original Radiospares OT. The only other difference is purely mechanical in that it has end bell covers and wires instead of a tagboard.

There is no lack of treble response. It sounds great actually. The circuit is a simple phase invertor that is very similar to our 18W guitar amp PI with a few other details from a Dynaco 2-EL84 amp and a Heath 2-EL84 amp. It has negative feedback and a 1M input volume which seems to work very well. I used the EZ81 rectifier tube and a 32/32 capcan. I think I sold a pair of 50/50 capcans that I was saving for this so I might buy another pair of those to try. It has a very even response although I haven't instrumented it yet to see what it really produces. I built two completely separate mono amps with separate power supplies and everything for a stereo pair.

danelectro · Post by **danelectro** » Fri 07/16/04 11:50 am

thats awesome. i tweaked an older Marshall stereo power amp once to play a little cleaner and it sounded really nice for audio. a tube hifi project is on my 'someday to do' list

loverocker · Post by **loverocker** » Sat 07/17/04 4:52 am

Great job, Graydon.

I think that horn speakers would be perfect.

Little story: I saved a '50s Lowther hi-fi amp that was destined for the junk heap. 2xEL34, huge trannies, huge choke, scary safety issues - didn't know what to do with it. Thought about restoring it, imagined it'd sound 'wooly' and lo-fi by today's standards. Thought about using it as a parts donor for guitar stuff, decided the values were all wrong. Put it aside for months. By chance I found out about a hi-fi guy who was building a Lowther museum, so I went over there. He had about 20 Lowther horn speakers (it's what they were more famous for).

He hooked one up to demo it... again, I though this was going to be lo-fi, slow and dull. Was I wrong

- it sounded incredible. Huge amounts of detail and real presence - especially for vocals. It's probably not the best set-up for loud reggae or numetal, but for anything recorded simply and not processed-to-hell, it was awesome. Best hi-fi I ever heard.

If I had the space - these horn speakers were about 6ft tall, and two foot deep and the same wide - I'd have one in a second. Not sure how they get along in stereo, and I don't know anything about the smaller horn enclosures I've seen, but it could be a real ear-opener. Please post follow ups

EBE · Post by **EBE** » Sat 07/17/04 10:25 am

I have a Heath Kit AA-151 that has been waiting for a recap. It is in really good shape but I just haven't gotten around to fixing it up. It is a stereo 4 x EL84 amp. Supposed to sound pretty good.

Norm

Post by **Graydon** » Sat 07/17/04 11:40 am

Barry,

You're right about horns. I have a buddy that has a pair of spekers that he built with exponential horns with a 15" woofer on each side. The rest of the speakers are standard circular speakers like we are accustomed to, but those horn bottoms produce fantastic bottom end. I've also listened to Klipsch horns a few times. A pair of those would go great with my 18W amps.

I am building an addition on my house, which will include a new amp/guitar room in the basement. So, while I am pouring concrete anyway, I am building two huge concrete speaker boxes into the foundation of the addition. They will house two 18" woofers on each side which should go down well below 20Hz!

I may try to build an exponential horn to fit within these boxes though. We'll see...

Post by **zaphod_phil** » Sat 07/17/04 11:48 am

Graydon wrote:That is a GDS/Heyboer 18W OT but it is like my new Vintage 18W OT with ultralinear taps and multiple secondary windings just like the original Radiospares OT.
.... It has negative feedback and a 1M input volume which seems to work very well...

Hmmm, the hi-fi guys usually prefer not to have NFB and use UltraLinear operation instead. Have you thought of trying that approach..?

Post by **Graydon** » Sat 07/17/04 1:08 pm

zaphod_phil wrote:Hmmm, the hi-fi guys usually prefer not to have NFB and use UltraLinear operation instead. Have you thought of trying that approach..?

I have both. I am using the UL taps on the OT and I have a 10K NFB resistor bypassed by a .001uF cap. All of the '60s circuits I looked at had NFB. I need to read up on modern 2-EL84 technology to see what is being used on the more modern amps.

loverocker · Post by **loverocker** » Sat 07/17/04 2:40 pm

Graydon - wow! Sounds like one helluva cool music room

By the way, I just quickly checked my copy of the '50s Mullard's "Circuits for Audio Amplifiers" reference book and in all cases amps do have NFB. For example, in the 2xEL84 "10W amplifier" it's done via a 15K and 220pF pair for a 4 ohm load (diff values for the 8 and 16) and runs back to the cathode of the stage before the PI.

Post by **zaphod_phil** » Sat 07/17/04 5:33 pm

Yes, the vintage books do really favour NFB, almost as a kind of ultimate solution. But they were forgetting about how that affects dynmic behavior and transients. In modern hi-fi NFB is therefore pretty much out of favour. But with all these things, your ears should decide....

Post by **jaysg** » Mon 07/19/04 3:34 pm

I have a Dynaco SCA-35 under my desk at work. UL taps and no nfb. I've never found anything about doing guitar amps with the iron, so there it sits.

GlennG · Post by **GlennG** » Mon 07/19/04 9:32 pm

Graydon

As always , nice job. A few years ago I "needed" a tube amp for music in my shop. I made something very similar to your pair and had great results using this Eico schematic

http://users.rcn.com/fiddler.interport/HF14.HTM

I have found that for audio tube amps, unlike guitar tube amps, a SS rectifier is a big help. I had to fiddle with resistors to get the voltages right but the SS power supply was the ticket.

For speakers I use a pair of "somewhat efficient" JBL 3-way 8" monitors with good results.

Glenn Gertis

Post by **Graydon** » Tue 07/20/04 12:47 pm

Glenn,

I've looked at that schematic as well. I thought about trying it because there are some good reviews of it on the net.

I will try a SS rectifier then. Thanks for that tip.

BTW, I am running these through my MTM speakers right now. Look here. these are the ones I have... http://mywebpages.comcast.net/kidder/Au ... _alpha.htm

Keith Kidder is quite a speaker builder. His workmanship is great and his speakers sound great.

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18W Hifi amps

18W Hifi amps