#26 pre amp - Page 197 - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 27th August 2012, 02:31 PM   #1961
euro21 is offline euro21  Hungary
diyAudio Member
 
Join Date: Sep 2004
Location: Budapest
Quote:
Originally Posted by airegin View Post
anyone tried BATTERY filament bias ?
10V bias + 1.5V heater + 4V Rod Coleman regulator overhead : 15.5V
15.5-16V battery pack, 1A current, 16VA power.
5 hour operation 80VAh ..... need 120-150VAh battery pack.
Recharing time aprox. 10h.

Wasting power, wasting money.
  Reply With Quote
Old 28th August 2012, 11:38 AM   #1962
diyAudio Member
 
Join Date: Apr 2006
Location: copenhagen
Hi all,

First of all, thank's to everyone who has contributed to this long thread. I think I have read through it all, and it has been very, very helpful for me to get along with my #26 preamp.

I have the amp breadboarded now, and I feel that besides from the PSU and the AVC, there are just small modifications to be made. I run the tube at 120V@8mA. Probably a little bit hot, but I will adjust it down to 5-6mA later on as Kevin suggests. I have tried out both battery grid bias, bypassed cathode, and filament bias. Filament bias is staying, I like it a lot. The sound now is amazing and highly musical with no audible hum in my 98dB speakers.


But what I really need some help with is the PSU. As I don't have acess to a scope, I can't make any measurements. I have have simmed it in PSUD II, but I really don't know what to look for to make i optimal besides ripple.

I attach the PSUD file, a screenshot, schematic and a photo of the nest. If you want to use the PSUD file, remove .doc and rename it.psu
I would like a passive PSU for this amp, preferrably with choke input.
I have one more pair of 20H chokes if that should be needed.


Theo
Attached Images
File Type: jpg schem.jpg (84.0 KB, 611 views)
File Type: jpg lcl.jpg (150.5 KB, 568 views)
File Type: jpg 26pre.JPG (119.9 KB, 523 views)
Attached Files
File Type: doc 26lcl.doc (222 Bytes, 39 views)

Last edited by Nightpuma; 28th August 2012 at 11:43 AM.
  Reply With Quote
Old 28th August 2012, 12:37 PM   #1963
diyAudio Member
 
Vinylsavor's Avatar
 
Join Date: Feb 2002
Location: Lindau
Hi Theo,

congrats to your 26 preamp! Why do you feel you need to change the PSU? Do you have any hum left?

PSUD2 is a great tool, but it will tell you little about how a PSU impacts the sound. It's a great tool to get your voltages and ripple numbers.

What would be worth trying in your PSU is swap the 8uF and 100uF caps. Put the 100uF after the first choke and the 8uF after the second choke. The last cap is in the signal path and you want the highest quality cap there. It might be worthwhile to increase the 8uF to 20uF, but try the 8uF alone first.

I would not terminate the secondary of your line transformer with 600 Ohms. Try it unterminated or with hoigher resistance values like 10k. 600 Ohm loads the preamp unnecessarily.

Best regards

Thomas
__________________
http://vinylsavor.blogspot.com/

Last edited by Vinylsavor; 28th August 2012 at 12:39 PM.
  Reply With Quote
Old 28th August 2012, 01:34 PM   #1964
diyAudio Member
 
Join Date: Apr 2006
Location: copenhagen
Quote:
Originally Posted by Vinylsavor View Post
Hi Theo,

congrats to your 26 preamp! Why do you feel you need to change the PSU? Do you have any hum left?

PSUD2 is a great tool, but it will tell you little about how a PSU impacts the sound. It's a great tool to get your voltages and ripple numbers.

What would be worth trying in your PSU is swap the 8uF and 100uF caps. Put the 100uF after the first choke and the 8uF after the second choke. The last cap is in the signal path and you want the highest quality cap there. It might be worthwhile to increase the 8uF to 20uF, but try the 8uF alone first.

I would not terminate the secondary of your line transformer with 600 Ohms. Try it unterminated or with hoigher resistance values like 10k. 600 Ohm loads the preamp unnecessarily.

Best regards

Thomas
Hi Thomas,

Thank's for the advice. There is no hum left, and it sounds very good. Last cap is Black Gate WKZ. I also tried solen polyprop not so good. So I suppose I just need some experienced builders to tell me it looks good or bad. What do you think about adding another LC stage and removing the last cap, leaving the 30H before the ultrapath and OT?

Regarding the termination of the OT. I was under the imperession it is like loading a MC step up. So when I load it with 600R the tube sees 20K load. Have I misunderstood?


Theo

Last edited by Nightpuma; 28th August 2012 at 01:52 PM.
  Reply With Quote
Old 28th August 2012, 01:53 PM   #1965
diyAudio Member
 
Vinylsavor's Avatar
 
Join Date: Feb 2002
Location: Lindau
Hi Theo,

I just saw that you have that other 14uF cap in ultrapath connection there. In fact with filament bias, there is not much difference between that and the 8uF since the cathode resistor is low in value. I would leave just a single cap there. You can experiment what sounds better to have it connected to filament or ground. If you have hum, add another LC stage. If you don't have hum, I see no need for that.

Best regards

Thomas
__________________
http://vinylsavor.blogspot.com/
  Reply With Quote
Old 28th August 2012, 06:42 PM   #1966
diyAudio Member
 
Susan_Parker's Avatar
 
Join Date: Oct 2004
Location: London, UK.
Default A scope of some sort...

Congraats on the progress

Quote:
Originally Posted by Nightpuma View Post
Hi all,
...

But what I really need some help with is the PSU. As I don't have acess to a scope, I can't make any measurements. I have have simmed it in PSUD II, but I really don't know what to look for to make i optimal besides ripple.

Theo
Anyone working on this stuff really does need a basic scope or something equivalent.

The simplest is to use a buffered op-amp with gain switching to go with a standard 10x cheepie probe, into the line input on the computer. 2 off is even better of course.

You may only get 22kHz (more with modern computers) but that is enough for 99% of analogue audio and power supply work.

There are programs out these that will do a basic display to look at signals or do FFTs even. Free or nominal fee.

e.g. Zelscope: Sound card oscilloscope and spectrum analyzer

Or look out for an Android or iPhone app for a smart phone if there is a mic or line in option.

Best wishes,
Susan.
  Reply With Quote
Old 28th August 2012, 07:31 PM   #1967
diyAudio Member
 
Susan_Parker's Avatar
 
Join Date: Oct 2004
Location: London, UK.
Default Simple tube oscilloscope

....

And if one really want to keep with the tube idiom then something like this could be a nice side project.

Parcgwyn DIY Electronics Web Site - Simple Oscilloscope

A search of a certain on-line auction site shows good availability of the DG7-6 CRT tube.

I built something like this when I was a teenager!

Best wishes,
Susan.
  Reply With Quote
Old 28th August 2012, 07:40 PM   #1968
diyAudio Member
 
Join Date: Apr 2006
Location: copenhagen
Quote:
Originally Posted by Susan-Parker View Post
Congraats on the progress



Anyone working on this stuff really does need a basic scope or something equivalent.

The simplest is to use a buffered op-amp with gain switching to go with a standard 10x cheepie probe, into the line input on the computer. 2 off is even better of course.

You may only get 22kHz (more with modern computers) but that is enough for 99% of analogue audio and power supply work.

There are programs out these that will do a basic display to look at signals or do FFTs even. Free or nominal fee.

e.g. Zelscope: Sound card oscilloscope and spectrum analyzer

Or look out for an Android or iPhone app for a smart phone if there is a mic or line in option.

Best wishes,
Susan.
Hi Susan,

Thank's a lot for taking the time and effort to answar my post. However, I do not understand much of what you are talking about
Either I rely on my ears or I simply have to get in touch with someone who has a scope.

Theo
  Reply With Quote
Old 28th August 2012, 07:46 PM   #1969
diyAudio Member
 
Join Date: Apr 2006
Location: copenhagen
Hi,

I have redrawn the schematic so it is easier to read.

I will experiment a little bi with the PSU.

Theo
Attached Images
File Type: jpg schem2.jpg (116.8 KB, 560 views)
  Reply With Quote
Old 29th August 2012, 02:31 AM   #1970
diyAudio Member
 
albertli's Avatar
 
Join Date: Nov 2004
Location: Vancouver
I lately was gifted a pair of NOS CX326 as my birthday present. Can I say it sounds bad. Eh........it sounds mellow and sweet.
Attached Images
File Type: jpg small 326.JPG (182.2 KB, 518 views)
File Type: jpg small 326.1.JPG (169.8 KB, 233 views)
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



New To Site? Need Help?

All times are GMT. The time now is 05:14 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2