Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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

Can't get full power
Can't get full power
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 14th August 2019, 12:16 AM   #1
liquidair is offline liquidair  United States
diyAudio Member
 
Join Date: Aug 2005
Default Can't get full power

I designed and built a 50W tube power amp based around a pair of 6L6GC, but I cannot for the life of me seem to get 50W out of it cleanly. 28-30W is the max I've seen.

The circuit is your everyday push-pull output with a long tail pair phase inverter and a PPI-MV. I've got a Va of 420V (with up to 600mA avail from the PT) into a 4.2k OT and I'm driving an 8 ohm dummy load. Calculations suggest 50W should be easy here.

I'm measuring with a scope and a Fluke RMS meter, where I'll drive the amp with a sine wave and adjust the level to just before I'm clipping the PI input. I'll then bring the MV up until I just before I see clipping on the power tube grids...this always happens when the signal level exceeds the bias voltage (biased cool at 30 to 38mA, -45 and -42Vdc respectively). The Fluke is monitoring the voltage across the 8-ohm dummy load and it's always ~15-15.5 Vac before distortion sets in, which calculates to only 28-30W.

Is there something I'm missing? Everything seems in order here aside from the power output, which makes me think I'm just calculating power wrong. Does power factor in both the + and - signal when we calculate it or just a single phase?

Thanks all!
  Reply With Quote
Old 14th August 2019, 12:38 AM   #2
tikiroo is online now tikiroo  New Zealand
diyAudio Member
 
Join Date: Nov 2007
Location: New Plymouth
What is the screen voltage, and is the 8 ohm load connected to the 8 ohm tapping? What frequency are you testing at?
  Reply With Quote
Old 14th August 2019, 12:55 AM   #3
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
Please post a schematic.

I have run a Tubelab universal PP driving 6P3S-3 (6L6oid) tubes with UL taps at 400V B+ and a 5K opt and gotten 40W out in AB2 at 8%thd.

I only got 20W before I hit grid current.

With slightly higher B+ and slight lower impedance OPT, I think your output is quite good, depending on what mode you are running (Pentode?, UL, Triode strapped? Other?
__________________
If there are no dogs in Heaven, I don't want to go there.
  Reply With Quote
Old 14th August 2019, 01:28 AM   #4
liquidair is offline liquidair  United States
diyAudio Member
 
Join Date: Aug 2005
Quote:
Originally Posted by tikiroo View Post
What is the screen voltage, and is the 8 ohm load connected to the 8 ohm tapping? What frequency are you testing at?
I just checked and was getting 424V on the screens, 433V on the plate with no signal. Yup, 8 ohm to 8 ohm tap. I've tested at both 350 Hz and 1 kHz and gotten the same result.
  Reply With Quote
Old 14th August 2019, 01:33 AM   #5
liquidair is offline liquidair  United States
diyAudio Member
 
Join Date: Aug 2005
Quote:
Originally Posted by TheGimp View Post
With slightly higher B+ and slight lower impedance OPT, I think your output is quite good, depending on what mode you are running (Pentode?, UL, Triode strapped? Other?
I'm running pentode, fixed bias with switchable negative feedback. Nothing fancy. I'll post a schematic here in a bit!
  Reply With Quote
Old 14th August 2019, 02:54 AM   #6
liquidair is offline liquidair  United States
diyAudio Member
 
Join Date: Aug 2005
Quote:
Originally Posted by TheGimp View Post
Please post a schematic.

I have run a Tubelab universal PP driving 6P3S-3 (6L6oid) tubes with UL taps at 400V B+ and a 5K opt and gotten 40W out in AB2 at 8%thd.

I only got 20W before I hit grid current.

With slightly higher B+ and slight lower impedance OPT, I think your output is quite good, depending on what mode you are running (Pentode?, UL, Triode strapped? Other?
See attached for the schematic.

Thank you for the vote of confidence, but doesn't your Tubelab specs suggest something is wrong with mine? Ignoring your UL connection for a sec, we can assume an effective 350V B+ into 1.25k should get around 98W peak, or 49W. Factor in the UL being somewhere in between triode and pentode operation and 40W sounds right.

Whereas mine would be an effective 370V or so into 1.05k, which would be 130W peak, or 65W. So 30W seems pretty bad, no?
Attached Images
File Type: jpg 50WPowerAmp.jpg (376.9 KB, 320 views)
  Reply With Quote
Old 14th August 2019, 03:27 AM   #7
cerrem is offline cerrem  United States
diyAudio Member
 
cerrem's Avatar
 
Join Date: Jun 2005
Location: San Diego, CA
Measure the power supply B+ at full power...
Also measure you Screen voltage at full power....at pin 4 , since the 1K will also droop the screen supply..
I am assuming your B+ is taking more of a dip than your expecting at full power....
  Reply With Quote
Old 14th August 2019, 03:47 AM   #8
liquidair is offline liquidair  United States
diyAudio Member
 
Join Date: Aug 2005
Ok, just measured at full output and B+ was 408V and Vs directly on pin 4 was 390V.
  Reply With Quote
Old 14th August 2019, 03:55 AM   #9
wg_ski is offline wg_ski  United States
diyAudio Member
 
Join Date: Oct 2007
Your schematic is showing the volume pot inside the feedback loop? That usually doesn’t work well....... but it won’t necessarily cause it not to hit full power. It does need to be corrected - unless you really do intend for the pot to adjust open loop gain and not as a master volume (as labeled).

I’m wit the others on the probable causes - either the B+ is dropping like a stone or you don’t have enough Vg2 to support 350mA of peak current.
  Reply With Quote
Old 14th August 2019, 04:19 AM   #10
liquidair is offline liquidair  United States
diyAudio Member
 
Join Date: Aug 2005
Ya, that's more an experiment where I'm trying to test post PI master volumes, just like the 47pF caps from top to wiper. For the power tests, it ends up that this control is full up, and the 'drive' control is what ultimately sets the max clean volume, if that makes sense.

Don't my measurements above rule out the B+ and screen being issues? Seems like those hold up well.
  Reply With Quote

Reply


Can't get full powerHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
FS Krell FPB 200 (Full Power Balanced) dady Swap Meet 1 16th March 2018 11:30 AM
Power Amplifier and Power Supply Full PWM hien.t.nguyen Class D 35 1st May 2016 12:08 PM
How much power can I get with IRFP460 PWM Full Brdige experimentador Power Supplies 18 24th January 2014 10:31 AM
Running an A7.3 at full power Bob Brines Full Range 107 10th October 2012 07:58 PM
max power for full volume? bob_v5 Multi-Way 7 21st February 2008 05:22 PM


New To Site? Need Help?

All times are GMT. The time now is 09:18 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki