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How to match a replacement OPT to an existing one.

Ken Boyd

Ken Boyd

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Thanks in advance for your assistance on this subject.

I was curios as to if there is an easy way to pick out an OPT from a new supplier if you have an old one in hand. I am not so much inquiring as to if it would be as good based on something special about an old one, but more in line as figuring the best matched based on specs. Guessing impedance, or resistance, not really sure, that is why I am asking.

The power supplies seem pretty straight forward, primary voltage and secondary taps voltages and amps and if you need center taps, but I am unsure as to the OPT's.
 
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Not sure if it helps, but I have a Scott LK48B amp, I've been working on that came with one original OPT and one jap OPT that is about 2/3 the size of teh Scott,,, it specs to pretty close to teh Scott but still needs a bit more "balance" knob towards teh replacement side to sound equal on both channels,, I see the output balance with teh scope set on both channels on top of each other in Mono..
Consensus on AK is the feed back has to be altered to match them,,, Other than balancing side to side they sound pretty much identical..
However I'm still looking for original for this amp!
 
Ken Boyd said:
Thanks in advance for your assistance on this subject.

I was curios as to if there is an easy way to pick out an OPT from a new supplier if you have an old one in hand. I am not so much inquiring as to if it would be as good based on something special about an old one, but more in line as figuring the best matched based on specs. Guessing impedance, or resistance, not really sure, that is why I am asking.

The power supplies seem pretty straight forward, primary voltage and secondary taps voltages and amps and if you need center taps, but I am unsure as to the OPT's.
Click to expand...
What is the model and year of the amp?

Hammond and other transformer manufacturers have cross-references to new part numbers.
 
Selmerdave said:
Pretty sure it's only really possible if the new one is expressly designed as a copy of the vintage one. Having the same specs doesn't ensure the same behavior in an amp.
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Even if he had a NOS transformer made by the same company, on the same date as his original, it wouldn't sound the same as the old one in his unit.

Just like NOS tubes of the same manufacturer and year of manufacture, replacing the old ones. Things wear out with use.

If he wants to maintain semi-balance between channels, he would prolly need two new transformers. Whether off-the-shelf or custom-wound.

:thumbsup:
 
I really don't care about it sounding just like the one it will replace, what I am looking for is one that will replace one and not be a mismatch that will match well enough to not become a problem.

I have tree exisiting part numbers that RCA used;

RCA # 100536
Stancore A-3800
Triad S-35A

No match with Hammonds search tool but using there tool and picking a type of amp, the desired wattage, and the tube type I think I found what will be compatible.
Mr Boochie said:
What is the model and year of the amp?

Hammond and other transformer manufacturers have cross-references to new part num
Click to expand...
 
Below is what A.I. puked out on those. Based upon that, you definitely want the same winding ratio, to give 5K plate to plate for given rated output load. (which tap are you using?) I am assuming these are not Ultra Linear.

As long as the power rating is in that 20-30 watt range, and the winding/impedance ratios are the same, to a first order you are in the ball park. Where it falls apart is the resonant tendencies of the transformer, which if materially different will require adjustments to the feedback and/or compensation, and the frequency limiting and stability tuning. Also, the performance under power (mostly power bandwidth) may be different given differences in core construction and winding technique. Since the feedback compensation and stability tuning is the biggest headache, finding a vintage unit or a decent clone is usually the easiest path.

Stancor A-3800
    • Type: Audio Output Transformer
    • Power Rating: 30 Watts
    • Primary Impedance: 5,000 Ohms Center-Tapped (CT) @ 80mA
    • Secondary Impedance Taps: 4, 8, 15, 250, and 500 Ohms [1]

Triad S-35A
    • Type: High-Fidelity Push-Pull Output Transformer
    • Power Rating: 20 Watts Audio
    • Frequency Response: 20 Hz to 20,000 Hz
    • Primary Impedance: 5,000 Ohms Center-Tapped (CT)
    • Secondary Impedance Taps: 4, 8, and 16 Ohms
(RCA was an in-house number so no data.)
 
Need to figure out what the impedance is. If you have one working one, that makes this easier.

Voltage from plate to plate if push-pull, or plate to B+ lead if single ended. Record input voltage and voltage across the full secondary with as many decimal places as your meter will give you. I typically do this at 60hz using a variac and an isolation transformer since basically any meter is accurate at that frequency, and if you get the secondary up to about 1 volt it has decent enough resolution for this to come out fairly accurately.

Divide input voltage by output voltage. This is the turns ratio
Turns ratio squared is the impedance ratio
Impedance ratio times load impedance will be the plate to plate impedance.

Find a transformer that matches on the primary side within as close as you can.

Power output can be guesstimated by output tubes. A pair of 6V6 in push-pull for example will typically do 12-15, so you need at least a 15 watt transformer, larger is fine.

If the old transformer is just shot, best guess the impedance by tube type and what the datasheets have to say about it. Also possible its in the Sams, they often give impedance values however make sure what they say seems sane. Once in a while they have values that simply do not make any sense. Can also look at other amps with the same tubes and verify sanity. 6V6 typically run 8-10K primary. Parallel will be half that.
 
I have a couple of used transformers but I would like to use try out a new transformer on my test bed. Copying the RCA SP20 drawings I am looking at a hammond 1620 OPT, and using a Hammond 273bx power transformer.
 
Ken Boyd said:
Just how close of a match is needed before it becomes an issue?
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Are you keeping the original 5K-ohm one on one channel?

Most tube amp manufacturers suggest a 10% nominal tolerance between channels. You'll prolly hear a difference between 6600-ohm OPT on one channel and 5000-ohm on the other.

If you get two new 6600-ohm OPTs on both channels, you may not hear a tonal difference. However, the ultimate output power may be slightly lower than that of two 5000-ohm OPTs.
 
I will be buying two OPT's and two Power transformers, new chokes, everything will be new, just copying the schematics for design. I have a RCA RS147 that is almost the same as the SP20, so I have that as a building guide
 
Ken Boyd said:
I will be buying two OPT's and two Power transformers, new chokes, everything will be new, just copying the schematics for design. I have a RCA RS147 that is almost the same as the SP20, so I have that as a building guide
Click to expand...
Then, two 6600-ohm OPTs will work. Be aware that you will prolly not get the same output power from whatever output tubes you use — if they want to "see" 5000-ohm primary windings.

This is a basic impedance mismatch.

:thumbsup:
 
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