Help wanted with testing and finetuning. - 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 1st April 2013, 12:45 PM   #1
jazz is offline jazz  Netherlands
diyAudio Member
 
Join Date: Dec 2001
Location: leiden, physically that is...
Default Help wanted with testing and finetuning.

Hi,

I've just finished a small el84 pp power amp inspired by the vacuum state pp1c. The little amp worked perfectly from the first power on and has a nice, fast detailed and very clean sound, way better then what i expected. The only comment i have is that is a little bass shy and less detailed in the bass then the pp6v6 i have and which uses the same trannies (from an old Lafayette la224B).

Now i want to do some testing and finetuning and would like your critique and advise. I've got a scope (philips pm3230), function generator (GW gfg-8015G) and a testload (8R/50W in parallel with 1,8uF times two).

Now here's my question:
-What to measure/test and how?
-What to try? (maybe some feedback like 3-6db?)

attached are the schematics (with measured voltages) and some pics.

thanks,
Joris
Attached Images
File Type: png el84 psu.png (17.3 KB, 163 views)
File Type: png el84.png (17.7 KB, 163 views)
File Type: jpg 002.jpg (922.6 KB, 162 views)
File Type: jpg 003.jpg (960.8 KB, 150 views)
__________________
oh, what a beautifull day
I wanna go out and play!
  Reply With Quote
Old 1st April 2013, 02:26 PM   #2
Loren42 is offline Loren42  United States
diyAudio Member
 
Join Date: Sep 2008
Location: "Space Coast" Florida, USA
I can't see how it works if you built it as per the schematic. The lower input tube has no signal to it and the grid is grounded via a 100Ω resistor.

Is this supposed to be a balanced line input?
  Reply With Quote
Old 1st April 2013, 03:25 PM   #3
jazz is offline jazz  Netherlands
diyAudio Member
 
Join Date: Dec 2001
Location: leiden, physically that is...
Yes it is meant for balanced input but basically the input stage is just an ltp sittin atop a constant current source. The lower input tube gets it's signal through the cathode so it works fine.

Joris
__________________
oh, what a beautifull day
I wanna go out and play!
  Reply With Quote
Old 1st April 2013, 03:38 PM   #4
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
A pentode output and no feedback means you will have high output impedance/low damping factor. Unless your speakers are specifically designed for this unusual scenario you will get a bass peak at the speaker bass resonance and rapidly falling bass below that.

You may also have somewhat high distortion.

Is the CCS circuit correct?
  Reply With Quote
Old 1st April 2013, 04:00 PM   #5
jazz is offline jazz  Netherlands
diyAudio Member
 
Join Date: Dec 2001
Location: leiden, physically that is...
Hi DF96,

Agreed with the pentode no nfb comment. that's why i was thinking about adding that. Just need the figure out how to calculate the appropriate values for feedback from the 16R tap to the negative input on the ltp.

ccs was originally meant for -24vdc supply but otherwise it's straight out of mr Wright's preamp cookbook, sk147 subbed for sk170.

Joris

speakers not designed for this situation btw (sonus faber concertino's)
__________________
oh, what a beautifull day
I wanna go out and play!
  Reply With Quote
Old 1st April 2013, 07:17 PM   #6
diyAudio Member
 
artosalo's Avatar
 
Join Date: Feb 2010
Unfortunately oscilloscope and function generator are not very efficient tools for fine-tuning the amplifier. Instead you should have distortion meter and audio generator having low THD.

You can add the GNFB without any calculations. Just adjust the output level to, say 1...2 W. Then add some 10k potentiometer from the tap of output transformer to the grid of the LTP. Now adjust the potentiometer and monitor the level of output signal at the same time. When the signal is dropped to half, you have reached 6 dB GNFB etc.

Last edited by artosalo; 1st April 2013 at 07:19 PM.
  Reply With Quote
Old 10th April 2013, 11:36 AM   #7
jazz is offline jazz  Netherlands
diyAudio Member
 
Join Date: Dec 2001
Location: leiden, physically that is...
Hi,

Ok so i tried to apply feedback but as soon as i connect it i can actually hear oscillation from the output trannies. I tried 22k, 10k and 3k3.

Could it be i have the output leads reversed? I measured them and they give0.7 and 1 ohms. Right now i use the 0.7 ohm winding for the 8 ohm ls connection and assume the 1 ohm winding to go from the 8 ohm tap to the 16 ohm tap.

any suggestions?

Joris
__________________
oh, what a beautifull day
I wanna go out and play!
  Reply With Quote
Old 10th April 2013, 11:44 AM   #8
diyAudio Member
 
artosalo's Avatar
 
Join Date: Feb 2010
Change primary leads.
  Reply With Quote
Old 10th April 2013, 12:01 PM   #9
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Quote:
Originally Posted by jazz
Could it be i have the output leads reversed? I measured them and they give0.7 and 1 ohms. Right now i use the 0.7 ohm winding for the 8 ohm ls connection and assume the 1 ohm winding to go from the 8 ohm tap to the 16 ohm tap.
You may have 8 and 16 mixed up. The 16 ohm extension to the 8 ohm secondary will have 40% more turns but may be thinner wire so more than 40% extra resistance. If your secondary has just three wires then I would assume the 1 ohm section is for 8 ohms and the total winding (1.7 ohms) is for 16 ohms. Or maybe I have misunderstood what you are saying.

How, exactly, are you trying to apply feedback?
  Reply With Quote
Old 10th April 2013, 12:31 PM   #10
45 is offline 45  Italy
diyAudio Member
 
45's Avatar
 
Join Date: Dec 2008
Location: UK
I tend to agree with DF96, you have mixed up the secondary connections. This could explain why you get oscillations (you are basically applying positive feedback!).
Moreover, as you are using the 8 ohm and 16 ohm taps (i.e. much less secondary turns) your primary load is HUGE! I would guess that if your nominal primary load is 8K with that wrong connection it will be around 50K plate-to-plate. This also explains why you don't have bass (the primary inductance is insufficient). Have you measured the Pout? If my guess is correct your Pout shouldn't be that great....
  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
Wanted: Wanted to buy california Audio Lab Tempest II Special Edition Power Supply Un ebxb Swap Meet 0 22nd February 2012 02:51 PM
Bad tubes wanted for tester testing crj11 Tubes / Valves 4 17th December 2009 02:27 AM
Need help finetuning my Stasis PA-5 dj.barry.v Pass Labs 3 16th January 2009 08:19 PM
testing, testing 123 neil_kaye Full Range 0 25th December 2005 08:21 PM


New To Site? Need Help?

All times are GMT. The time now is 06:10 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