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Has anyone tried 8417's in their Stereo 70? I happen to have a bunch and am thinking about trying them.

The 8417 is an extremely high Gm tube, and would require changes to at least the bias system, and also the NFB loop to return its level to normal. Other than that, it should be a fairly straight forward exchange. Check out the MK VI that used a quad of them with a rather standard Dynaco driver board. In my experience, they will not produce quite as much power output as EL34s will. Let us know the results!

Dave

Hi,

I'd up the grid snubber too - maybe 22K. High Gm tubes love being VHF radio transmitters.

Looks too like UL may be out.... http://tdsl.duncanamps.com/show.php?des=8417 shows a big difference between Va and Vg2, while the EL34 doesn't give a hoot.

Cheers!

I have owned a number of Quicksilver mono block amp and currently still have a pair of 8417's that I converted to use KT88's. Other than bias and feedback consideration I believe they will work fine. I will try at my convenience and post the results.

By the way the stock Quicksilver 8417 is a basic copy of something.

http://www.bonavolta.ch/hobby/en/audio/8417.htm#8417 Push-Pull

How did you like the change to KT88s in your Quick Silvers? I would think that was a good move sonically besides resolving basic issues associated with the 8417. But in going from 17s to 88s as you did, besides the bias and FB considerations, I would also want to make sure that the phase inverter stage could adequately drive the KT88s as well.

To me, the 8417 is just a weird tube. It is a frame grid tube, that (IMO) is not nearly as beefy as more traditional big bottle output tubes are. It is also a cavity anode design, which is what I believe gives it some of it's rather peculiar characteristics. Finally, while it's capabilities appear to be on par with that of the 6550, it must be remembered that the maximum ratings for the 8417 were developed under the Design Maximum guidelines, where as those for the 6550 were developed under the older more conservative Design Center standards (at least as published by RCA). On an equal playing field, I believe that the 6550 is a much more hearty tube.

A decade or so before the Internet, I could find little to no design data on the 8417, so at the time, I manually ran and developed my own set of plate curves. It's been a long time since I reviewed the data, but I remember the tube as being a much lower impedance device than traditional ouput tubes are. However, in my testbed amplifier incorporating a Dynaco A431 OPT, the tubes consistently delivered about 10 watts less available power than 6550 class or EL34 tubes could develop when using tapped screen operation. After that, I really did very little more with the tube.

In the MK VI, the OPT provides a rather typical load environment for the tubes, with the OPT representing a 2200 ohm primary, and screen taps placed at 40% of the winding. I've never had a MK VI on my bench to know of its actual performance capabilities, but this is not significantly unlike what a pair would be loaded with when using an A431 (or an A470 for that matter). The Quick Silver shows a nearly identical 4200 ohm load being supplied to the tubes. While I don't know what position the screen tap occupies on the QS's primary winding, based on my experiments anyway, I would think that the move to KT88s was a positive one.

In the end, I think I became so gun shy of the tube, that I never performed any significant listen sessions with it. I'll be interested to know what yours are.

Dave

To be honest I really like the sound of the 8417's better than the KT88's or 6550's in the Quicksilver. I placed a link to the triode site I believe that discribes their changes to the Quicksilver circuit in order to drive different tubes. I have done this change in one pair and left another pair stock except for bias and feedback changes. The stock basic Quicksilver circuit will drive KT88's or 6550's or EL34's and even 6BG6GA's as is other than what I mentioned about the bias and feedback changes. What I have found out about 8417's is they do not like the high 150K resistors and do much better with 100K. There are some bad 8417's out there and they are prone to oscillation. I have some Phillips /ECG and some RCA that work flawlessly. To be honest I don't think that I got as much power out of the Quicksilvers with KT88's or 6550's. That has been a few years back and I don't remember.

I have a mint pair of Dynaco Mark 6 120 watt mono blocks that use 8417 tubes that I don't have and would like to convert the amps to use KT88's. If anyone has an idea how to convert it before I experiment, please let me know.

Thanks.

Sal

Hi Sal -- The easiest thing to do is to simply copy (or copy as much as practical) the PPP KT88 circuit Hafler provided in the later version of the Dynaco transformer catalog, which is widely available on the net. That circuit basically uses a very lightly modified Dynaco MK III driver circuit to drive PPP 88s, which is similar topology to the basic MK VI design.

That amplifier was shown in the catalog to provide an example circuit for use with the A450 OPT. And while I don't know the exact specs of that transformer (other than those shown in the catalog), I do know the specs for the MK VI OPT, which will work very well indeed with PPP KT88s.

Of course there are other options as well, but this might be the simplist approach if you are still trying to keep your MK VIs a Dynaco centered product.

I hope this helps!

Dave

Dave,
Thanks, I didn't think about that. I am sure I have a copy of the Dynaco transformer catalog. I want to minimize the mods on the board as much as possible.

These Mark 6 amps are really nice and weigh 55 pounds each... :-) Actually I think they were built a few years ago that is why they are new looking.

Regards,
Sal

Sal, here's a link to the Dyna transformer page. The circuit basically uses a 6AN8 board from a MK-3 and PPP outputs which is basically what a MK-6 is. For the most part the bias circuit will need some changes to achieve more neg voltage.

http://www.the-planet.org/pdf/transformer.pdf

Those are really nice, I hope you post more pics, and some inside pics too!

Terry,
Thanks for the link, I'll download the catalog now.

I'll take some pictures of the business end when I do the mod on my amps.

Thanks,
Sal

Here are triode's tips for converting amps using 8417s to KT88s. It applies to more Quick Silver amps, but it should help.

http://www.triodeel.com/8417.htm