Industrial vs Commercial amps and OPT's

[homemade]

Hi , I'm fairly new to working on tube amps and trying to learn as much as I can without getting overloaded. I picked up a pair of DuKane 1A775 mono amps , each with 4-8417's and rated at 100wpc. They required very little to get them working , but I'm hoping to determine their potential. These are industrial type, with 70v , 50v , 35v and 25v output taps. Very massive iron and robust construction. 40lbs per side. I purchased 2 sets of 'matched quad' 8417's , so I think to tubes I have are ok. I'm using them on the 25v tap to my speakers(6ohm 'test' speakers). Sound ok , if not what i might expect from 100 tube watts. All caps test good and no hum. High seem rolled off and bass is fat. I guess my question is if this is how these type amps 'sound' compared to audio OPT type. Will I be able to retrofit different transformers (any suggestions?). I having fun either way. Opinions appreciated.

[cerrem]

Industrial can be used for lab purposes...and sometimes that will can require good bandwidth....
McIntosh had Industrial amps that were very good....

Chris

[Tom Bavis]

for 100W,

25V = 6 Ohms,
35V = 12 Ohms,
and 50V = 24 Ohms

Conviently close to nominal speaker impdeances...

[tgarson]

I just ran across this thread today, so please forgive me for picking it up two years late.

I have four Dukane 1A775 amps and would like to acquire two more. Stock, they are not especially impressive performers. In fact, they can be unstable, being potentially subject to parisitic oscillation at a frequency well above the limits of the audio spectrum.

The common "cure" for this tendency, probably instituted by Dukane, was to add series plate resistors, or RFCs (r.f. coils), which further degrades their audio performance.

The output transformer is so so. The Hammond 100 Watter is better, especially for the low end as the 1A775s transformer does not have adequate primary inductance to work well below 40Hz.

Having said all that, with sufficient modification, including completely reconfiguring the circuitry of all three amplification stages and changing the output tubes to 6550s, they can be made to perform fairly well as a mid range quality hi-fi amplifier, although it's a lot of work.

The reason I want two more of them is to power a 6 channel home theater system. I know it's a nutty idea, and would be better done with any number of other amplifiers but, as previously mentioned, I already have four of them.

So, for two Dukane 1A775s in good physical shape that have all original parts, including good working transformers, but excluding original tubes, I am willing to trade one Altec 1569A in near pristine condition, guaranteed to operate as new.

[yel17]

Hello,
I had a 1A775 back in the late 70's. It was a conservatively rated amp, it put out 120 watts rms at 1khz before clipping (using an 8 ohm load on the 25v output (6.25 ohm). Unfortunately I sold it to a fellow tube-o-phile who wanted to use it for a guitar amp booster. When Dukane's CSD division was sold in 2002, I acquired several transformers, a couple of them being the 1A775 power transformer. The 1A775's show up on Ebay from time to time, Good luck with your search.

tube.guy

[Original Burnedfingers]

These are industrial type, with 70v , 50v , 35v and 25v output taps

These amplifiers are designed to drive speakers with step down transformers. The load figured like 70.7 X 70.7= 4998.49 ohms / 100 watts = a 49.9849 impedance. This is the MAXIMUM load this amplifier can drive. While some of us have driven say a single 8 ohm speaker on the 25v tap it really isn't recommended.

A much more pleasing effect would result from removing the output transformer and replacing it with a 16,8, 4 ohm type.

[tgarson]

What the transformer windings are called is immaterial to their use. The most important parameters are, in no particular order, core material, primary inductance, interwinding capacitance, wire gauge (DC resistance) and turns ratio.

With a nominal 8 Ohm load on the secondary designated as 25 Volt, the reflected load is a good match for the power tubes and their load line. That's what counts.

The only significant negative with respect to the output transformer Dukane used is the insufficient primary inductance, which manifests itself in output power and quality deteriorating significantly below 40 Hz.

[Original Burnedfingers]

What the transformer windings are called is immaterial to their use.

Well, the point I was trying to make is that the step down transformers in most cases are not of high enough quality to pass a signal of say below 100 HZ. The output transformer on the amplifier wasn't designed to pass low frequency either.

The amplifier was designed for commercial use to drive a load of cheap 8 ohm $5 a piece speakers will little consideration to high quality. In other words the signal is processed and its not a high fidelity signal that would satisfy a audiophile.

With a nominal 8 Ohm load on the secondary designated as 25 Volt, the reflected load is a good match for the power tubes and their load line. That's what counts.

As long as the load doesn't dip below 6.25 ohms in the case of this amplifier it will probably tolerate the load. In some cases however it is very possible that the impedance of an 8 ohm load or driver will fall to around 4 ohms or less at low frequencies. It is important to remember that commercial/industrial amplifiers need to be loaded so that the maximum load will not be greater than what their power capacity is capable of running.

There are exceptions when dealing with tube amplifiers since they are a lot more forgiving that their SS counterparts are.

I would highly suggest that the output transformer in this amplifier be replaced with one that is designed to run a 16,8, or 4 ohm load because the transformers make up/design, the way its wound will better compliment the load the owner wishes to run.

[tgarson]

You've missed the point.

First, I'm not looking to change the transformer. Second, the existing output transformer works quite well into an 8 Ohm load. If anything, the diifference between a 100 Watt/25 Volt output and an 8 Ohm output works to the advantage for an 8 Ohm load, as the nominal impedance of that 25 Volt output is 6.25 Ohms.

You are correct that in commercial amplifiers, the output transformer often has insufficient, or inappropriate, core to sustain performance at low frequencies and may also have had poor winding techniques used that result in other problems. However, this is not universally true and the degree varies from amp to amp.

In the case of the Dukane 1A775, I already know what the primary inductance (measured at 40Hz), differential and common mode capacitances, and dc resistances are. I used that information to "build" a spice model using current and voltage sources, not the defective mutual coupled inductor model, to simulate the transformers performance prior to working on the amp. I have installed improved driver and pre-driver circuitry in the amplifier and am speaking from a position of having confirmed, by actual bench testing, that my model is valid. This also means that I have done dynamic and static load tests on the improved design. The transformer works as well as many that are sold for "hi-fi" applications.

As I previously acknowledged, there absolutely is a compromise in performance below 40 Hz. At 100 Watts, The 1% THD point is reached at about 30 Hz. Still, that's not too bad. From 60Hz to 10kHz, THD is below .1%, gradually rising to 1% at about 35kHz.

I have compared the Dukane transformers in circuit performance with that of the 100 Watt Hammond utra-linear transformer (taps not used). The Hammond has better low end, as expected, due to its higher primary inductance. However, the Dukane transformer actually slightly surpasses the Hammond in high frequency performance.

[yel17]

Thomas, it appears that you have done your "homework". That's a big project you have going, nothing beats the sound of a good vacuum tube amplifier. It's a good thing that decent 6550's are available at reasonable prices these days.
Oh, one more thing, If memory serves me correctly, I think there was a thin sheet of asbestos under the heat reflector in the 1A775's. Take care.
tube.guy