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kevin gilmore

unbalanced/balanced to balanced tube input

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So I've been looking at a bunch of tube/solid state hybrid dynamic amp schematics, and every one of them is some form of bad, or really bad. (one person/ex company in particular)

Low voltage plate, capacitve coupling of a high impedance source into a low impedance, poor balance etc. 

There had to be a better way. true balanced input. dc coupling. low impedance output. no feedback. proper use of a tube (pure transconducance) etc

so I came up with this, work in progress.  4 x 3 inches

ubaltobaltubeschem.PDF

 

ubaltobaltube.jpg

Edited by kevin gilmore
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Odd that a proper Dc coupled input is possible... I wonder if Cavalli would complain about the lack of mosfets...  ;)

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Looks a lot like the front end of an electrostatic headphone amp (T8000-ish), but with relatively low voltage transistors (except for the KSA1156).  You could always substitute a MOSFET current source for the input tube tail CCS. :D

Edited by JimL
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there are now 2 versions of this, one with the tube on the component side, and the other on the back side for mounting megatron style. ask first.

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If I am going to drive high independence headphones with this, what should I do?

1) add triple darlington stage like tubehybrid/CFA did

2) add paralleled darlington stage like dynahi

Edited by joehpj

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2 hours ago, mwl168 said:

Why does the PSU have two sets of F+/F- output?

There's one tube per channel. So one set of heaters per tube

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The filaments don't come from a common source, each has their own regulator. This makes sense because filaments require a lot of current. B+ is pretty low current so just one for both channels should be fine.

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Since this is probably the first thing I will try from the group buy I wondered about the transformer and the VA. Looking at the input ac like this would couple 2 cheap transformers work? The +-15 is from a GRLV or something I guess or would a LM317+LM337  do the job?

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Gonna build this from the group buy soon,

On the board the silkscreen says "tube mounts on the back side of the board" but tracing the boards shows it's the same version as the one in first post. So tubes should be mounted from the top?

I also have never built tube amps so I might be wrong.

some questions I have:

1. How hot is the mosfet on PSU will be? Will heatsinking with chassis floor (3mm aluminum) suffice or does it need heatsink?

2. LT1085 require heatsinks or not?

3. Will it make a good preamp? Do I need to watch/match it's output impedance to the amp's input?

4. How to bias it?

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here are both of them, not sure which one was in the group buy

 

ubaltobaltubeschem2.jpg

ubaltobaltubeschem2flip.jpg

 

3)  Its a current mirror. So output voltage depends on impedance, currently 1100 ohms. Needs an output buffer if you are going to use it for a preamp.

Edited by kevin gilmore
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The group buy is the first one, cool that means it's the correct version. I was counting the tube pinouts counter clockwise.

I'll try looking for a suitable buffer stage.

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I've compiled a BOM from Digikey for the supplied GRHV based on my silkscreen from the recent group buy board. I hope others can help check if I missed anything.

https://docs.google.com/spreadsheets/d/1XMu81oWk6E3mXPW6ahhEuvmhg4U497F5I5W6JPQgnNo/edit?usp=sharing

You might notice there's RES 90.9K and RES 1.10K, these are because I can't find 92K resistors sold without MOQ.

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On 7/29/2018 at 9:29 AM, kevin gilmore said:

here are both of them, not sure which one was in the group buy

 

ubaltobaltubeschem2.jpg

ubaltobaltubeschem2flip.jpg

 

3)  Its a current mirror. So output voltage depends on impedance, currently 1100 ohms. Needs an output buffer if you are going to use it for a preamp.

Kevin, can I use the Diamond output buffer ver. 2.0 ?

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Dear Kevin,

I find the heater power supply portion to be rather handy. 

Would you please  cut the HV supply portion away so  that I can use the heater power supply portion for mini T2 project?

Thanks

Sam

 

 

On 11/13/2017 at 7:45 AM, kevin gilmore said:

board files for you :D

updated including 100v massive overkill power supply

ubaltobaltube.zip

grhv100v.zip

grhv100v.jpg

 

Edited by samsie

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