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Re: Best 6550 tupe amplifier

Posted by shermanr@prw.net on April 5, 2006 at 10:35:49

In Reply to: Re: Best 6550 tupe amplifier posted by afilado on April 5, 2006 at 08:51:48:

I created a spreadsheet that has the actual costs that I incurred to build these. It took me several years to accumulate parts and decide on the Mk3 Board from Triode Electronics. This is on my web page. The Grey Hammond paint on the chassis was more of a decision based on the fact that the paint did not get ruined as I covered the chassis with masking tape throughout the metal work and final drilling. When the tape was pulled-off 3 years later, I found that the Grey paint looked great as it was.

Now that these amps are built and working, I have not used any of my other tube amps on a regular basis. These amps deliver the goods better than any prior amp either home-brew or vintage. And I have 60 watts to play with per channel, vs. 35 watts from my next most powerful amplifier, the Harman Kardon A700.

With careful ordering of parts to reduce the shipping costs, these amps can be built with all Hammond Transformers for under $500 USD the pair, maybe even closer to $400 dollars. Of course you must add the costs of the tubes you choose.

I may just go ahead and make a 'replica' of this one with Hammond transformers so as to compare the performance vs. the Bartolucci outputs and sell it if it works as good as the one with the special iron.

The Magnequest Power transformer wound by Mr. LaFevre is now unfortunately not available any longer. I uderstand that Mr. La fevre prefers a lower volume, higher margin, specialty output and inter-stage transformer business than winding replacement vintage mains transformers.

Yet, I am almost positive that the sonic benefits are there with the magnequest transformer vs. OEM wound tranny such as Hammond.

With the cages installed, the bias potentiometers are not accesible. It would be a disaster for someone to confuse these shafts for volume controls..... (yikes!)

I have not had the time to shoot some pictures of the bottom of the amp, but this will be done over the weekend or sooner.

Anyone wanting to build these monoblocks, you can rest assured that they will perform well. The Dynaco MK3 replacement board I used works great. I can imagine how much of an improvement this board would make in a real Mk3 amplifier. I modded the board by removing the 10K AC Balance Control and replacing with a 10K-1watt resistor, making the Long tailed phase splitter non-adjustable. Anyone wishing to have the splitter AC balance control, I highly recommend a 10K-2watt potentiometer installed on the chassis with jumpers soldered to the PC board.

The neat thing about these amps is that the bias is adjustable without any VOM or calculator to divide the Cathose Voltage by the Cathode resistor value. In this amp, the cathodes each have it's own Shurite meter in series. The small toggle switches place a short across the meters 1 ohm internal resistance, effectively tying the cathodes direct to ground. Shurite verified that this is perfectly acceptable with these meters.

The red light is the 120 VAC AC Pilot from Radio Shack and the green light is also from Radio Shack, but it's a diode that is forward biased through a 150K/2Watt resistor. This diode indicates the presence of High DC Voltage, you can watch the caps charge-up. And when the amps are powered down it acts like a bleeder for the capacitors in the Power Supply, the diode will dim after about 20 seconds.

As a final description I must add that the Metal can caps are the ones' sold by Steve at Angela.com. The values are actual Dyna MK3 Power Supply: (30-20-20-20) uF. As there are no electrolytics in the power supply system (apart from the two in the bias supply from the transformer bias tap), the amps perform very well with these Oil filled caps. They sound and perform as good or even better than the SCR/Solen Mettalized 600V caps I have used in my other amps.

I heartily recommend that anyone try out building your next project with oil filled type metal can caps, they sound great and my ears cannot hear any hissing from the tweeters with my Dyna PAS2 at 12 O'clock. At full throttle I can hear the faintest hiss from the tweeters with my ear right on-top.

This post is made possible by the generous support of people like you and our sponsors:

Atma-Sphere Music Systems, Inc.

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