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 Post subject: Re: PP KT88 Amps
PostPosted: October 10th, 2021, 11:23 am 
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Joined: July 24th, 2015, 4:17 pm
Posts: 1489
Location: Parkville, Maryland
I have experience with Williamson and Ultra-Linear amplifiers not to mention my current incumbent -- my Cary 845 based SETs.

My findings were that yes the output transformers will ring or resonant with other reactances in the circuit. What I have done successfully is damp the primary with a clamping (damping) resistor.

For the most part a high-voltage rated 5-Mohm resistor strapped to the primary (plate-to-plate) gets the job done and the result is better transparency like cleaning fog off of a window.

https://www.ebay.com/itm/161128273433?h ... xysstSXSOf

The damper provides straight DC resistance rather than what some manufacturers use such as an R/C network (Zobel?) and prevents other issues that would occur due to a capacitor interacting with the transformer inductance.

And no -- the damper does not influence plate load and the relationship between the primary and secondary. That said -- I also have found that allowing the the un-used secondary taps to "float" has its problems. I've used 4-Kohm non-unductive power resistors on the un-used taps and 800-ohms on the tap feeding the speakers.

This scheme really tightens things up -- enables excellent transient behavior and suppresses reactive artifacts that do not serve the music. I've added the feature to five amplifiers to this date with the same positive result.

For the higher-voltage (1,200-volts) SET output I used 20-Mohm.


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Walt
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 Post subject: Re: PP KT88 Amps
PostPosted: October 11th, 2021, 8:09 pm 
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Joined: February 28th, 2013, 1:19 pm
Posts: 745
Walt,

Interesting about the multi-megaohm resistors.

Well back to the amps. Both are done and in the system and working great. I can say that a Williamson front-end, for all the initial trouble I had with it, seems to MUCH better sounding than the same amp using a Mullard topology. Really no contest. It gets tone and space. Gain structure is right. The sound is complete divorced from the speakers. The extra power (50W) compared with the stereo EL34 triode Williamson amp (18W or so) is apparent by better dynamics and lack of strain. Though the little Williamson does have a certain sweetness to the sound. Everything needs to break in, new resistors, coupling caps, solder joints, etc.

A few things to note:

I used Speer 2W and 1W carbon films resistors available at Mouser for most of the signal path. These are the same resistors as Kiwame, I can say that even though they are 5% rated parts, the resistors off the same spool are tightly matched to each other, better than 1%. Matched parts are critical for the concertina phase splitter and driver plate resistors.
The previous version (Mullard) used a 9 pin mini (12AX7/5751/12AY7) for the first tube. I used a 6CG7 instead of an octal 6SN7 to avoid punching a larger hole for an octal socket.

David


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 Post subject: Re: PP KT88 Amps
PostPosted: October 11th, 2021, 10:11 pm 
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Joined: June 4th, 2013, 2:39 pm
Posts: 396
David McGown wrote:
Walt,

Interesting about the multi-megaohm resistors.

Well back to the amps. Both are done and in the system and working great. I can say that a Williamson front-end, for all the initial trouble I had with it, seems to MUCH better sounding than the same amp using a Mullard topology. Really no contest. It gets tone and space. Gain structure is right. The sound is complete divorced from the speakers. The extra power (50W) compared with the stereo EL34 triode Williamson amp (18W or so) is apparent by better dynamics and lack of strain. Though the little Williamson does have a certain sweetness to the sound. Everything needs to break in, new resistors, coupling caps, solder joints, etc.

A few things to note:

I used Speer 2W and 1W carbon films resistors available at Mouser for most of the signal path. These are the same resistors as Kiwame, I can say that even though they are 5% rated parts, the resistors off the same spool are tightly matched to each other, better than 1%. Matched parts are critical for the concertina phase splitter and driver plate resistors.
The previous version (Mullard) used a 9 pin mini (12AX7/5751/12AY7) for the first tube. I used a 6CG7 instead of an octal 6SN7 to avoid punching a larger hole for an octal socket.

David


That's good to know about the Speer. I like WKiwame but they're a little scarce right now.


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 Post subject: Re: PP KT88 Amps
PostPosted: October 11th, 2021, 10:30 pm 
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Joined: February 28th, 2013, 1:19 pm
Posts: 745
I should say KOA Speer. SPR Series.

David


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