#26 pre amp - Page 225 - 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 3rd December 2012, 04:19 PM   #2241
Magz is offline Magz  United States
diyAudio Member
 
Join Date: Oct 2009
Location: New Jersey, USA
I'm happy enough with the bias battery under the input transformer that I've permanently changed the preamp, removing the input capacitors and grid bias resistors, and relocating the batteries to where the input caps were previously.

It sounds much better this way, and the signal path is a little bit neater as well. Pic below.
Attached Images
File Type: jpg 26 Preamp Battery under L1676.jpg (153.0 KB, 570 views)
  Reply With Quote
Old 3rd December 2012, 04:20 PM   #2242
diyAudio Member
 
Join Date: Jan 2010
Location: Harrow, UK
Beautiful............... :-)
  Reply With Quote
Old 3rd December 2012, 10:04 PM   #2243
diyAudio Member
 
Join Date: May 2010
Hello, just a question to Magz.

I have a 26 preamp with input capacitors and battery bias and just so happen to have two LL1676 input transformers laying around. I probably need to have them wired in 1:1 but how do you calculate the RC network and what is the purpose of the battery bypass?

Very nice work on your preamp by the way...
  Reply With Quote
Old 3rd December 2012, 11:08 PM   #2244
Magz is offline Magz  United States
diyAudio Member
 
Join Date: Oct 2009
Location: New Jersey, USA
Quote:
Originally Posted by Vbenonisen View Post
Hello, just a question to Magz.

I have a 26 preamp with input capacitors and battery bias and just so happen to have two LL1676 input transformers laying around. I probably need to have them wired in 1:1 but how do you calculate the RC network and what is the purpose of the battery bypass?

Very nice work on your preamp by the way...
Mine are wired 2:1 parallel/parallel and that is perfect for use with the autoformer outputs/volume controls. How you wire them determines the RC network you need, so parallel/parallel, parallel/serial, serial/parallel and serial/serial all have somewhat different resonances and require somewhat different networks. I used a function generator and oscilloscope to determine what gave the best square wave performance; the easiest method is to wire the cap to a pot and vary the pot to get the best setting, then replace the pot with a resistor. I split the resistor value in two and used an RCR setup to isolate the transformer somewhat from the cap.

I put in a small bypass on the battery (.01uf polystyrene) in case there is any HF noise that needs to be bypassed to ground and may be stopped by the battery.
  Reply With Quote
Old 4th December 2012, 01:22 AM   #2245
diyAudio Member
 
Join Date: May 2010
I thought that it was not recommended to use the secondaries in parallel on the LL1676 ("frequency abberations" according to KevinC) but I'll guess it depends on the expertise of the user.

I have no scope but perhaps you can recommend some starter values on the RC network just to try? Or maybe it's best to use nothing just to hear if the transformer has any potential of bettering my Duelund Cast?
  Reply With Quote
Old 4th December 2012, 02:12 AM   #2246
Magz is offline Magz  United States
diyAudio Member
 
Join Date: Oct 2009
Location: New Jersey, USA
Quote:
Originally Posted by Vbenonisen View Post
I thought that it was not recommended to use the secondaries in parallel on the LL1676 ("frequency abberations" according to KevinC) but I'll guess it depends on the expertise of the user.

I have no scope but perhaps you can recommend some starter values on the RC network just to try? Or maybe it's best to use nothing just to hear if the transformer has any potential of bettering my Duelund Cast?
I know of Kevin's thoughts, but my scope showed the 2:1 (parallel) actually had less HF ringing than 4:2 (serial) for a 2:1 connection. I was using a CAST 0.22, but in addition to the LL1676; the transformer alone is definitely better.

I'll tell you what..I have a spare LL1676. Tell me how you want to hook it up, and I'll scope it and recommend an RC.
  Reply With Quote
Old 4th December 2012, 02:22 PM   #2247
Magz is offline Magz  United States
diyAudio Member
 
Join Date: Oct 2009
Location: New Jersey, USA
Vbenonisen,

I had some time this morning, so I wired up the LL1676 in 1:1 as 1+1:2||2. The RC network that worked well was 2nF and 2Kohm.

2K square wave scope shots below: first is the LL1676 with no RC, second is with 2nF+2Kohm.
Attached Images
File Type: jpg LL1676 1to1 with no snubber, 2kSQWV.jpg (238.3 KB, 472 views)
File Type: jpg LL1676 1to1 with 2nF, 2k snubber. 2kSQWV.jpg (224.4 KB, 422 views)
  Reply With Quote
Old 4th December 2012, 04:27 PM   #2248
diyAudio Member
 
Join Date: May 2010
Thank you Magz! Much appreciated. It sure is some helpful people on this thread.
  Reply With Quote
Old 5th December 2012, 07:17 AM   #2249
Richard is offline Richard  Australia
diyAudio Member
 
Join Date: Apr 2005
Location: Melbourne
Quote:
In any case, the transformer wouldn't be damaged by the DC, the core would saturate, which can be remedied by passing an AC signal at the saturation level...demagnetizing, in other words.
Thank you for the feedback Magz. I'll give it a shot.

Cheers,

Rich
__________________
"A little learning is a dangerous thing"
  Reply With Quote
Old 6th December 2012, 11:50 PM   #2250
diyAudio Member
 
Join Date: May 2010
Default RC network on VR tubes?

Is there somebody out there who knows what's the appropriate RC network to shunt a 0D3 VR-tube with to get a flat power supply impedance curve?

V
  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 01:04 AM.


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