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Old 18th April 2010, 01:11 AM   #1
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Default 8417 Output Transformer

I have six NOS 8417's and I am just chomping at the bit to build an amp.
This would be a P-P amp with a pair of 8417's in the output stage.
Phase inverter would be a long tail pair using a 12AX7.

I have available to me an output transformer with a primary resistance of 1900 ohms.
This is the same output transformer that an amp kit supplier uses for both 6550's and EL-34's.
So...I'm guessing that it can be used with 8417's assuming that what I read about swapping between the mentioned tube types is correct.

The power transformer is also available and it is also the same one used with the aforementioned tubes.
Does anyone see any major stumbling blocks here?
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Old 18th April 2010, 02:28 AM   #2
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IMO 1900 ohms is too low an impedance for a PP pair of either 6550 or EL34. It's also too low for PP 8417s. All want to see at least 4000 ohms plate to plate. And maybe a little more for the EL34s. 1900 ohms would be good for PP parallel pairs assuming it has the primary current capacity. To use it you would have to fudge the impedance up by using 8 ohm speakers on the 4 ohm secondary. Something I don't really like to do.
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Old 18th April 2010, 02:31 AM   #3
llwhtt is offline llwhtt  United States
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The impedance of that OPT is too low for just a pair of output tubes, 2 pair would work OK. What brand of 8417s do you have, if they're Sylvanias that's good, if they are GEs that's bad. Those are very touchy tubes and the good ones are nearly unobtainable. So if you do build this amp plan ahead in the bias circuit so you use KT88s or 6550s when the 8417s crap out.

Craig
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Old 18th April 2010, 03:01 AM   #4
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First off, we need to know if you want a HiFi amp or a MI (guitar or bass) amp. I am guessing that given your user name this is a bass amp.

Second, we need to know what kind of output power you are expecting, and how many power tubes you plan to use to get there. The 1900 ohm transformer is about the right impedance for 4 or 6 tubes. This should get 200 to 400 watts if the transformer is rated for this much power and the circuit design supports it.

If you are planning to build an amp of this caliber, and you plan to keep it for several years, I must offer this suggestion. 8417's are rare and expensive. Their cathode to grid spacing is very small, which led to more failures in the 8417 than other tubes of similar power. If this is a bass amp head that will sit on top of a speaker cabinet the excessive vibration will make matters worse.

As with any any MI amp, tube replacement WILL be inevitable. Are you prepared to pay whatever it takes to keep this thing in tubes for the rest of its life? Will you ever be gigging on the road where possible tube replacement in a strange town is needed. I am in favor of using whatever tube you want for your personal HiFi amp, and maybe even your own MI amp, but if the amp will ever be sold, or used professionally, it has to use tubes that you can get at any guitar shop.

My advice would be to sell the 8417's to someone who needs them to fix their Quicksilver (or whatever) and buy some 6550's. You will have plenty of money left over to spend on the other parts that you will need.

Last, this amp will operate at lethal voltages. It will be a complex and somewhat expensive design. Do you, or someone who is closely supervising the build up, have the skill level and experience to pull it off? An amp of this caliber is not a 100% success even for an experienced amp builder. The probability of success is virtually zero for an inexperienced builder.
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Old 18th April 2010, 11:11 AM   #5
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electbass4

Tubelab,llwtt, and Hollowstate have all given you good advice. The 8417 are a good sounding tube that is why I have kept the dozen that I have in a coffee can unused. They can be very nasty to work with. Use some other tube that is easier to find and certainly cheaper.
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Old 18th April 2010, 12:12 PM   #6
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OK, OK, OK.....It seems that it will be a much smarter move to use 6550's. The 8417's go on eBay forthwith. The fragility of this tube is probably why there was only one MI amp (that I know of) that used them. The other manufacturers were obviously smarter folks.
I'll post other transformer questions that I have on another thread.
As for skill level, I have been fixing, not building, MI amps since I got out of tech school in 1981. I have one build under my belt and I am looking for another. Apparently the kit builder that I am using has made decisions that I may not agree with...hence the next thread on my transformer questions...look for it.!
Thanks. I got great advice on transistors form the solid state guys on this forum and now great advice from the tube guys...what is not to love???
Thanks
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