My Tabor Clone is finished. - Page 2 - 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 8th April 2008, 01:04 AM   #11
diyAudio Member
 
Gary P's Avatar
 
Join Date: Apr 2002
Location: Portland OR
I think what you are missing is that pentodes do not have lowest distortion with a fixed screen voltage. Look at the average transfer characteristics of the 6AU6-A comparing plate current to grid voltage or screen current to grid voltage for example (page 5 of the 1960 GE 6AU6-A data sheet ET-T916A, 4-60) . The average transfer characteristic is not a straight line but a curve. The curve gets steeper as the plate current increases. You will also note that the screen current tracks the plate current closely. By adding resistance to the screen you allow the screen voltage to drop as the plate current rises. This helps straighten the characteristic curve for lower distortion. The trade off is it also lowers the effective transconductance. This is another way of achieving lower distortion using local feedback on pentodes.
  Reply With Quote
Old 8th April 2008, 08:28 AM   #12
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
Thanks for the explanation. I will try that today.

Shoog
  Reply With Quote
Old 8th April 2008, 08:21 PM   #13
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
Implemented that mod.
Got a very slight reduction in gain, but also a rounding out of the sound - a worthwhile improvement.

This amp has amazing control over the bass - very tight.

Shoog
  Reply With Quote
Old 8th April 2008, 09:52 PM   #14
JoshK is offline JoshK  Canada
diyAudio Member
 
Join Date: May 2004
Location: Toronto, ON
Roughly how many watts do you get?

Looks like a very cool project.
  Reply With Quote
Old 8th April 2008, 11:10 PM   #15
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
Quote:
Roughly how many watts do you get?
Haven't really tested power output. I would estimate something like 20watts of class A. I'm going to put her back on the bench at the weekend to check for residual ringing. I'll see if I can push her to clipping. My speakers only need a few watts so its a bit of overkill really.

Shoog
  Reply With Quote
Old 9th April 2008, 12:03 AM   #16
diyAudio Member
 
Join Date: Mar 2003
Location: Zürich
Hi Shoog!

Congratulations on the job!

Quote:
My speakers only need a few watts so its a bit of overkill really.
That is a real demonstration of rationality!
__________________
my surname is indeed 'de Best': neither misspelling nor snobbism! Ask SY!
  Reply With Quote
Old 9th April 2008, 12:41 AM   #17
diyAudio Member
 
Gary P's Avatar
 
Join Date: Apr 2002
Location: Portland OR
Hi Shoog,

Glad to hear the screen resistors helped.

Experiment 1: Increase the DC bias on the cathode bypass caps if the current voltage is not ~80% of voltage rating.

Looking at the cathode bypass caps- by their size, I'm guessing that the cathode bypass caps on the output stage are electrolytic. What is their voltage rating? At this time you only have 18V bias on the capacitors. If the voltage rating is 50 volts or higher you may want to increase the DC bias on the caps. The current 18V may be enough for 35V parts.

To increase the bias on the cathode bypass caps you could split R4 into a 2 resistor voltage divider then connect the end of R17 that is currently connected to ground to the middle of the new voltage divider. Adjust the resistor values to set the voltage on the bypass caps to ~80% of the rated voltage.

Experiment 2: Remove the need for cathode bypass caps.

Now that you have tried the higher screen resistance put a multi turn pot in the screen circuit like in the Tabor Rev. 3. This allows you to set the DC balance of the input stage. Balance the input stage while measuring the DC voltage between the cathodes of the output tubes. If you can set the amp up so that there is 0V DC between the cathodes and it seems stable you can put a jumper across the cathodes removing the cathode bypass caps from the circuit. Now you can hear what getting rid of the last capacitor in the signal path does for you.

Have fun!
  Reply With Quote
Old 10th April 2008, 05:26 PM   #18
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
I had her back on the bench. I had real difficulty getting a clear scope reading - there was all sort of wierd stuff going on with the signal. However I eventually established that there was still a resaponse peak centered around 50Khz. This is surprisingly noticable to the ear. I also did some listening tests and found that the bass was tight and punchy, but there was a definate suck out where the bass guitar should have been. I think there was a low frequency resonance which was picking up the lower bass but leaving a dip in the mid bass. Wierd sounding.
So today I changed out the LL1517 for the 110V:24V+24V toroidal transformer. I put a load resistor on the secondary of 120K. I didn't do any frequency sweeps but put her straight back into the main system. She sounds very similar to my DC7 amp (6080 PP with interstage toroidal transformer), but the top end is quite a bit more extended. She now has a very balanced sound with cymbals having real naturalness. Also the extra step down has put the gain into the right ball park for my system.
I am going to leave her at that for a while to see if anything subtle presents itself. Still I have a hunch I can put this one to bed now.

So I was thinking of trying my hand at a MOSFET based partial feedback amp with input and output transformers, as a way of utilizing the LL1517. Projects projects !!!

Shoog
  Reply With Quote
Old 8th January 2011, 07:36 PM   #19
Tyimo is offline Tyimo  Hungary
diyAudio Member
 
Join Date: Nov 2002
Location: Hungary
Hi Shoog!
What happened with your input toroid transformer? Is it works good??
I am also looking for an input phase splitter transformer in 1:1+1 configuration.
I don't know what would be the optimal inductance for a line level transformer. May be 5H? It would be like the classical 600R imp. transformers.
Greets:
Tyimo
  Reply With Quote
Old 8th January 2011, 08:33 PM   #20
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
I used a step down ratio of about 5:1+1 using a small toroidal. I recently dropped the secondary load from about 14K to 1K as it was sounding a bit harsh. This smoothed things out a lot. Ringing is the real serious issue I have come up against when using toroidals as phase splitters. I think interwinding capacitance maybe an issue when trying 1:1+1, which will lead to roll off and peaking in response somewhere between 30-60K. If you can manage 2:1+1 then I would say you would have a better chance. Really its a suck it and see situation. Toroidals definitely do work though.

In a previous EL82 headphone amp I used microphone transformers from OEP for phase splitting. I had massive amounts of front end gain and these were configured as 9:1+1 - they sound absolutely fantastic.

Since then I have built another amp of almost the same design as the Tabor but used OEP transformers which are relatively cheap, sound sweet and can cope with 1:1+1 without issues. They are designed for 600R professional loads. I think I paid about €40.00 for a pair with screening cans.

Shoog
  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


Similar Threads
Thread Thread Starter Forum Replies Last Post
I Have Finished My Standell 25L15 Clone guitar88 Instruments and Amps 115 31st May 2013 03:24 PM
The Clarro alias Totem Arro Clone. Finished!! FrankDIY Multi-Way 26 9th February 2010 03:47 AM
Solid State Tabor Amp jkeny Solid State 11 25th June 2008 10:24 PM
my finally finished gain clone (the whole story) loudlemmy Chip Amps 0 1st April 2005 05:00 PM
Question regarding ProAc 2.5 Clone and Tempo 2 Clone zoooh Multi-Way 5 11th March 2003 08:19 PM


New To Site? Need Help?

All times are GMT. The time now is 05:28 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