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Re: PP KT88 Amps

September 8th, 2020, 9:34 am

It is interesting to see all the machinations that need to occur to try to deal with an inherently flawed device like an output transformer. Then you look at the ZOTL circuit that David Berning uses to eliminate the transformer and all the associated warts they have. Much better way to do it and make sense of using tubes in a situation where impedances have to be matched. Transformers have their places but those places are getting fewer and further between every day.

Re: PP KT88 Amps

September 8th, 2020, 9:35 am

Great! That sounds about right. It's amazing that he gets such a stable 10kHz square wave with just a stock Williamson shelf filter and phase lead cap. I have to use a much more aggressive filter with my Peerless to avoid HF ringing, Probably also a testament to the very tight build he's done underneath. That power tranny is perfect, too! I'm really looking forward to seeing how this turns out. When you get started,you should begin a new thread.

I have to say that it's also amazing to me that anyone lavishes such care and interest on original Williamsons these days. You don't find many DIYers interested in that anymore. That was a very instructive video.

Re: PP KT88 Amps

September 8th, 2020, 11:11 am

tomp wrote:It is interesting to see all the machinations that need to occur to try to deal with an inherently flawed device like an output transformer. Then you look at the ZOTL circuit that David Berning uses to eliminate the transformer and all the associated warts they have. Much better way to do it and make sense of using tubes in a situation where impedances have to be matched. Transformers have their places but those places are getting fewer and further between every day.


Tom,

Using a ZOTL circuit is beyond most DIYers, and not even sure that Dave has the information for designing them in the public domain. Yes, transformers have their flaws, but nothing is perfect and designing around those flaws is part of the challenge. There is a long history with transformers, and information is abundantly available for building tube amplifiers with them.

Also, when it comes down to it, linear power supplies are also obsolete technology today, and switching power supplies in principle can be developed to perform better than linear power supplies. But they are complicated for the hobbyist to design and build, require greater attention to filtering, and require test equipment beyond what most hobbyists have. A linear power supply is pretty easy to design for filtering and low noise from 60Hz/120Hz input.

David

Re: PP KT88 Amps

September 8th, 2020, 12:21 pm

Grover Gardner wrote:Great! That sounds about right. It's amazing that he gets such a stable 10kHz square wave with just a stock Williamson shelf filter and phase lead cap. I have to use a much more aggressive filter with my Peerless to avoid HF ringing, Probably also a testament to the very tight build he's done underneath. That power tranny is perfect, too! I'm really looking forward to seeing how this turns out. When you get started,you should begin a new thread.

I have to say that it's also amazing to me that anyone lavishes such care and interest on original Williamsons these days. You don't find many DIYers interested in that anymore. That was a very instructive video.


That was a great video, and thanks for passing that along to me. I will let you know when I start (posting a new project thread). Don't know if I will do tube or solid state rectification, it is always problematic using a single tube rectifier (like a GZ34 in a Dynaco ST70) in a stereo amp since they get hammered. Could use a hybrid (i.e., a damper diode or GZ34 after a solid state rectifier for delayed turnon). Will use electrolytic caps instead of oils to save space. Have a heavy gauge aluminum cake pan the right size I picked up for a chassis some time ago. A nice priming and paint job would do it!

David

Re: PP KT88 Amps

September 8th, 2020, 12:25 pm

David McGown wrote:
Grover Gardner wrote:. . . it is always problematic using a single tube rectifier (like a GZ34 in a Dynaco ST70) in a stereo amp since they get hammered.


2x6AX4 works great in my stereo 845 amp ;)

Roscoe

Re: PP KT88 Amps

September 8th, 2020, 12:44 pm

David McGown wrote:
tomp wrote:It is interesting to see all the machinations that need to occur to try to deal with an inherently flawed device like an output transformer. Then you look at the ZOTL circuit that David Berning uses to eliminate the transformer and all the associated warts they have. Much better way to do it and make sense of using tubes in a situation where impedances have to be matched. Transformers have their places but those places are getting fewer and further between every day.


Tom,

Using a ZOTL circuit is beyond most DIYers, and not even sure that Dave has the information for designing them in the public domain. Yes, transformers have their flaws, but nothing is perfect and designing around those flaws is part of the challenge. There is a long history with transformers, and information is abundantly available for building tube amplifiers with them.

Also, when it comes down to it, linear power supplies are also obsolete technology today, and switching power supplies in principle can be developed to perform better than linear power supplies. But they are complicated for the hobbyist to design and build, require greater attention to filtering, and require test equipment beyond what most hobbyists have. A linear power supply is pretty easy to design for filtering and low noise from 60Hz/120Hz input.

David:

This was a statement on output transformers in general, not specifically aimed at DIYers. I also still use linear supplies in a lot of my applications because design of a switching supply is beyond me. However, where it makes sense I buy commercial switchers with very high performance and low costs. Dealing with output transformers or the complexity of a DIY ZOTL are some of the reasons why I switched to solid state. To each his (her) own.

David

Re: PP KT88 Amps

September 8th, 2020, 12:52 pm

David McGown wrote:
Grover Gardner wrote:Great! That sounds about right. It's amazing that he gets such a stable 10kHz square wave with just a stock Williamson shelf filter and phase lead cap. I have to use a much more aggressive filter with my Peerless to avoid HF ringing, Probably also a testament to the very tight build he's done underneath. That power tranny is perfect, too! I'm really looking forward to seeing how this turns out. When you get started,you should begin a new thread.

I have to say that it's also amazing to me that anyone lavishes such care and interest on original Williamsons these days. You don't find many DIYers interested in that anymore. That was a very instructive video.


That was a great video, and thanks for passing that along to me. I will let you know when I start (posting a new project thread). Don't know if I will do tube or solid state rectification, it is always problematic using a single tube rectifier (like a GZ34 in a Dynaco ST70) in a stereo amp since they get hammered. Could use a hybrid (i.e., a damper diode or GZ34 after a solid state rectifier for delayed turnon). Will use electrolytic caps instead of oils to save space. Have a heavy gauge aluminum cake pan the right size I picked up for a chassis some time ago. A nice priming and paint job would do it!

David


It looks like he's using a SS voltage doubler in his. Dual 5AR4s would probably be best if you can fit them in. You don't want much more than 460VDC B+ and those would about land you there. A big pair of damper diodes would work, too, though you might end up a bit on the high side.

Also, these 600VDC snap-in electrolytics are handy for this sort of circuit. I've used a Mundorf 10uF Tube Cap at the input and then these for the rest of the power supply, they seem to hold up well:

https://www.mouser.com/Passive-Componen ... 9sQAvD_BwE

Re: PP KT88 Amps

September 8th, 2020, 1:11 pm

Roscoe Primrose wrote:
David McGown wrote:
Grover Gardner wrote:. . . it is always problematic using a single tube rectifier (like a GZ34 in a Dynaco ST70) in a stereo amp since they get hammered.


2x6AX4 works great in my stereo 845 amp ;)

Roscoe


Roscoe

I use a pair in my big GM70 SET amps as tube rectifiers. Also have used them singly in a hybrid application as a slow turnon tube. They work great. They are cheap, and I have a good stash of them, along with 6AU4. Although I have not tried them that way, I have heard they do work with a 5V heater.

David

Re: PP KT88 Amps

September 8th, 2020, 1:19 pm

Grover Gardner wrote:
David McGown wrote:
Grover Gardner wrote:Great! That sounds about right. It's amazing that he gets such a stable 10kHz square wave with just a stock Williamson shelf filter and phase lead cap. I have to use a much more aggressive filter with my Peerless to avoid HF ringing, Probably also a testament to the very tight build he's done underneath. That power tranny is perfect, too! I'm really looking forward to seeing how this turns out. When you get started,you should begin a new thread.

I have to say that it's also amazing to me that anyone lavishes such care and interest on original Williamsons these days. You don't find many DIYers interested in that anymore. That was a very instructive video.


That was a great video, and thanks for passing that along to me. I will let you know when I start (posting a new project thread). Don't know if I will do tube or solid state rectification, it is always problematic using a single tube rectifier (like a GZ34 in a Dynaco ST70) in a stereo amp since they get hammered. Could use a hybrid (i.e., a damper diode or GZ34 after a solid state rectifier for delayed turnon). Will use electrolytic caps instead of oils to save space. Have a heavy gauge aluminum cake pan the right size I picked up for a chassis some time ago. A nice priming and paint job would do it!

David


It looks like he's using a SS voltage doubler in his. Dual 5AR4s would probably be best if you can fit them in. You don't want much more than 460VDC B+ and those would about land you there. A big pair of damper diodes would work, too, though you might end up a bit on the high side.

Also, these 600VDC snap-in electrolytics are handy for this sort of circuit. I've used a Mundorf 10uF Tube Cap at the input and then these for the rest of the power supply, they seem to hold up well:

https://www.mouser.com/Passive-Componen ... 9sQAvD_BwE


Nice selection of 600VDC capacitors, alot easier than using series 300 or 350V caps with balancing resistors. Back a few years ago it was tough to find anything over 450VDC in a El-cap.

David
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