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Old 29th February 2008, 08:16 PM   #1
lpd is offline lpd  Canada
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Default Quicksilver conversion

How does this conversion form 8417's to 6550/KT88's look?

conversion
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Old 1st March 2008, 05:06 PM   #2
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It looks good I've converted 1/2 dozen of them so far.

I would also modify the bias to allow for individual tube adjustment.
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Old 3rd March 2008, 03:10 AM   #3
lpd is offline lpd  Canada
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I thinking of converting a pair of Bogen MO-100A's to KT-88's. Four per channel, rebuild the front end, individual bias pots for each tube, triode/ultra linear switch.

Looks like a great candidate for medium power tube amp? Here's the schematic. Any thoughts?

Schematic
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Old 3rd March 2008, 10:32 AM   #4
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Commercial 70 volt power amplifiers aren't really known for their low end so I wouldn't expect too much there. Actually it will run 6550's or KT88's the way it is. I have heard Bogans that people have just popped in some different output tubes and they sounded ok. I would loose the input transformer and the music/speech switch if it were me. If you wanted you could copy the Quicksilver which would allow you to use the easier found front end tubes.

The 8417 Quicksilvers will also work fine without changing the front end. A bias upgrade however I would highly recommend.
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Old 3rd March 2008, 03:11 PM   #5
lpd is offline lpd  Canada
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What about the MO-100's? Look identical to the Quicksilver 8714 mono blocks to me or very close. I have access to a pair of these as well. I'm thinking of using the utility 9 pin socket for another stage of gain. These use 6550's of coarse, but look very similar to the Quickies.

shematic
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Old 4th March 2008, 10:38 AM   #6
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Why do you need another stage of gain on the MO-100?

I like the MO-100A chassis the best and it is fine with small modifications. Change the bias circuit and loose the speach/music switch and the input transformer.
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Old 8th May 2015, 01:38 PM   #7
87gnx is offline 87gnx  United States
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Default I tried the converstion to KT88

I get way less power and the wavform at any audio frequency produces non symmetrical waveform, what gives?
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Old 8th May 2015, 03:03 PM   #8
87gnx is offline 87gnx  United States
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Hi Burntfingers I hear you are the GOD of Quicksilver 8417 amps..Mine never sounded right once i did the KT 88 upgrade. Sign wave not symmetrical under load...Both amps have the exact same issues with theKT 88 's..Got the amps 7 years ago from my brotherinlaw , who was original owner...Reason for the change to KT88's, Two 8417 got damaged in shipping.Please help, Thanks Joe
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Old 10th May 2015, 01:11 PM   #9
87gnx is offline 87gnx  United States
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I did not change the front end and the result was 45 volts peak to peak at 8 ohms...Are you getting full clean 60 volts peak to peak? I looked at a schematic from a Dynaco MKIII, they only use one tube 6AN8 to drive the KT88's..I'm wondering this is what I have to do get get the drive for the KT88 for full output,. Your thoughts ?

One of the links on that blog page you sent me Schematic, no longer exists
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Old 10th May 2015, 05:25 PM   #10
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Quote:
Originally Posted by 87gnx View Post
Quicksilver 8417 amps..
I just did a load of work on the Bogen which is almost identical.
KT88 are a totally different animal from 8417, but already the driver circuit is too weak in the original 8417 circuit to give good performance.

8417 are a very high gain high voltage modern valve (ie. low impedance) quite unlike anything else and a similar family to the modern 7591, which again neither resembles a 6V6 nor 6L6.

Click the image to open in full size.

See it's quite similar

Click the image to open in full size.

Substituting for the 7591

Drive requirements as a result are about 15-18V on the 8417, like the 7591 Versus about double or more on the KT or EL34.

If your circuit can't deliver linear drive and plenty of current (dictated by the line requirements of that type of phase splitter), then you will get lots of distortion in the driver, and mismatch in the OPT.

In this case I would say you will get more luck trying some high mu sweep tubes & reducing the HT rather than the mundane normal stuff.
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