Medium power SET with parallel OP tubes - 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 22nd May 2010, 10:50 PM   #1
diyAudio Member
 
Joshua_G's Avatar
 
Join Date: Nov 2008
Location: Small village, Israel
Default Medium power SET with parallel OP tubes

Background:
Recently I purchased an amazing low cost SET integrated amplifier: MP-401 EL156 Integrated Amplifier [MP-401] - $599.99 : Musical Paradise - Audio Art . It's the first SET I owned or listened to and I'm highly impressed by it's sound quality, which, as I understand, is to be expected from good SET amps.

Alas, this amp with it's declared 15Watt doesn't have enough power to drive my speakers to their full ability. Also, in high listening volume it distorts.

I guess I need about 30-50 Watt. Commercial SET amps of such power are way beyond my finances. Being retired, I just cannot buy a commercial one. So, I'll have to build one myself.

The Musical Paradise MP-401 comes with Chinese EL156 and I like their sound very much. I tried Chinese KT88 and Russian (SED) 6550 on this amp and a Push-Pull one and I didn't like their sound. So, the amp I'm going to start with will be based on the Chinese EL156. In order to get more power I'll have to use parallel output tubes.

Question:
Since I have zero experience with SET amps, I'd like to hear from you, experienced DIY'ers: when all else is equal, should I expect degradation of performance when parallel OP tubes are used in a SET amp? (Yes, I'm aware of the fact that the amp's sound quality will be up to the OP trafo used).

Basic decisions:
1. HT of about 420-450 Volt (for maximum OP power).
2. Class A1 operation (for purest sound).
3. Very little local negative feedback (for somewhat reducing harmonic distortion without big undesirable impact on the sound).

The options:
Right now I see 3 basic options:
Option 1: Fixed bias from regulated negative supply, adjusted individually for each one of the 3 OP tubes.
Benefits:
A. Avoiding the dreadful capacitor necessary for cathode bias.
B. Maintaining equal quiescent current for each one of the 3 OP tubes.
Drawbacks:
A. Should the bias voltage change or fail, the OP tubes will burn out.
B. Higher parts count than on the other options, especially input capacitors.

Option 2: Fixed bias from regulated negative supply, adjusted to bias shared by all 3 OP tubes.
Benefits:
C. Avoiding the dreadful capacitor necessary for cathode bias.
D. Lowest parts count.
Drawback: Should the bias voltage change or fail, the OP tubes will burn out.

Option 3: Cathode bias, shared by all 3 OP tubes.
Benefits:
E. The conventional method usually used in Class A1 SET OP amps, safe for the OP tubes.
F. Low parts count.
Drawback: the dreadful capacitor necessary for cathode bias.

All comments and suggestions are welcomed.
Attached Images
File Type: jpg SET 1.jpg (64.4 KB, 339 views)
File Type: jpg SET 2.jpg (53.3 KB, 326 views)
File Type: jpg SET 3.jpg (48.7 KB, 326 views)
  Reply With Quote
Old 22nd May 2010, 11:48 PM   #2
DrRick is offline DrRick  United States
diyAudio Member
 
Join Date: Nov 2004
Location: New Jersey, US, Third Stone From The Sun
Josh,
Given your financial restraints for this project, you may first want to explore the cost of 15-30 watt SE output transformers.
__________________
Peace,
DrRick
  Reply With Quote
Old 23rd May 2010, 01:12 AM   #3
Grin is offline Grin  United States
diyAudio Member
 
Join Date: Apr 2010
You already have a nice tube amp. So you just need to change you speakers, believe me, 10-12W is more then enough for 30-40 m2 room if you will get 92dB - 95dB speakers.
  Reply With Quote
Old 23rd May 2010, 01:46 AM   #4
diyAudio Member
 
Joshua_G's Avatar
 
Join Date: Nov 2008
Location: Small village, Israel
Quote:
Originally Posted by DrRick View Post
Josh,
Given your financial restraints for this project, you may first want to explore the cost of 15-30 watt SE output transformers.
That much I believe I can afford.
  Reply With Quote
Old 23rd May 2010, 01:55 AM   #5
diyAudio Member
 
Joshua_G's Avatar
 
Join Date: Nov 2008
Location: Small village, Israel
Quote:
Originally Posted by Grin View Post
You already have a nice tube amp. So you just need to change you speakers, believe me, 10-12W is more then enough for 30-40 m2 room if you will get 92dB - 95dB speakers.
Oh no, I'm not going to replace my speakers, which are the best part of my stereo setup, Brodmann VC2: Bösendorfer Audio - Tubeprofi . 15 Watt isn't enough. That is, I get enough loudness, but it's not there yet.
  Reply With Quote
Old 23rd May 2010, 01:57 AM   #6
diyAudio Member
 
Joshua_G's Avatar
 
Join Date: Nov 2008
Location: Small village, Israel
I'm going to build this amp anyhow, so, any comments on the design options will be highly appreciated.
  Reply With Quote
Old 23rd May 2010, 02:10 AM   #7
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
Quote:
Originally Posted by Joshua_G View Post
I'm going to build this amp anyhow, so, any comments on the design options will be highly appreciated.

If you regulate the bias supply, you MUST regulate B+ too.

"Fixed" or combination bias is a good idea, when employing paralleled "finals". The "idle" currents must match or "smearing" occurs.
__________________
Eli D.
  Reply With Quote
Old 23rd May 2010, 10:34 AM   #8
diyAudio Member
 
Joshua_G's Avatar
 
Join Date: Nov 2008
Location: Small village, Israel
Thanks Eli.
I intended to regulate the +B initially. I thought of minding the power supply after having the amp's circuit.
By smearing, do you mean smeared sound? When idle currents are matched, should there be no degradation in sound, compared to conventional SET with only 1 OP tube?
  Reply With Quote
Old 23rd May 2010, 11:01 AM   #9
diyAudio Member
 
Globulator's Avatar
 
Join Date: Aug 2004
Location: Cambridge, England.
I would use individual biases but a single cap driving all the tubes (actually I'd allow space to 2 caps in parallel) - allowing for interesting combos.

I'd also go back to your original amp and experiment at clipping (maybe into a big resistive load!) with an oscilloscope, as the operating point of the driver and/or output tubes may be clipping asymmetrically reducing the potential output.

Speakers look really good BTW, and fairly sensitive. With my 13wpc Sweet Peach GU50 powered SE amp I get adequate levels from my 87dB/W Usher X-718 speakers (also with wood and piano black finish!).
  Reply With Quote
Old 23rd May 2010, 12:14 PM   #10
diyAudio Member
 
Joshua_G's Avatar
 
Join Date: Nov 2008
Location: Small village, Israel
Quote:
Originally Posted by Globulator View Post
I would use individual biases but a single cap driving all the tubes (actually I'd allow space to 2 caps in parallel) - allowing for interesting combos.

Do you mean individual fixed grid bias as in my first option with the 3 grids fed from 1 input cap? In such a case it will be impossible to match the idle current between the 3 tubes, since all 3 grids will be DC connected and will see the same bias.


Quote:
Originally Posted by Globulator View Post
I'd also go back to your original amp and experiment at clipping (maybe into a big resistive load!) with an oscilloscope, as the operating point of the driver and/or output tubes may be clipping asymmetrically reducing the potential output.

Clipping asymmetrically between the L and R channels? Both the driver and output tubes for each channel are single ended.


Quote:
Originally Posted by Globulator View Post
Speakers look really good BTW, and fairly sensitive. With my 13wpc Sweet Peach GU50 powered SE amp I get adequate levels from my 87dB/W Usher X-718 speakers (also with wood and piano black finish!).

I also get adequate level, but distorted on high listening levels. I assume the distortion comes noticeable even before clipping.
  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
Make a power amp out of op-amp ICs in parallel? Kolloi Solid State 10 11th March 2008 07:40 AM
Need medium-power transistor recommendations abzug Solid State 4 20th December 2007 01:46 PM
power tubes and learning SET design cbutterworth Tubes / Valves 34 16th December 2007 01:25 PM
advice on a set of medium sized computer speakers Dr. Photon Multi-Way 13 11th May 2005 05:14 PM


New To Site? Need Help?

All times are GMT. The time now is 01:18 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