Reply
 
Thread Tools Display Modes
  #1   Report Post  
Posted to rec.audio.tubes
Luigi Vogliobene Luigi Vogliobene is offline
external usenet poster
 
Posts: 2
Default BIAS regulation Electro Harmonix KT90 vs Jinvina KT90

Hi to all, I am new in this forum, I own a MingDa MC368BSE audio amplifier and it is time to replace the final tubes, I was suggested to replace the original 4 Jinvina KT90 with a selected quartet of Electro Harmonix KT90EH; I wonder if the BIAS will be OK if not exactely the same (this amp has a fixed BIAS).
Thank you in advance for your kindly replies.
Luigi
  #2   Report Post  
Posted to rec.audio.tubes
Peter Wieck Peter Wieck is offline
external usenet poster
 
Posts: 2,418
Default BIAS regulation Electro Harmonix KT90 vs Jinvina KT90

On Sunday, February 12, 2017 at 12:54:08 PM UTC-5, Luigi Vogliobene wrote:
Hi to all, I am new in this forum, I own a MingDa MC368BSE audio amplifier and it is time to replace the final tubes, I was suggested to replace the original 4 Jinvina KT90 with a selected quartet of Electro Harmonix KT90EH; I wonder if the BIAS will be OK if not exactely the same (this amp has a fixed BIAS).
Thank you in advance for your kindly replies.
Luigi


The circuit uses fixed bias but as this is only adjustable using unlabeled internal controls the task is best left to a dealer; the two meters just show the output voltage at the 8ohm loudspeaker sockets.
Read more at http://programming4.us/multimedia/16...MGZU4VOilTD.99

This is what I got from the net. I suggest you find someone specifically familiar with this amp and have them walk you through the bias process.

Peter Wieck
Melrose Park, PA
  #3   Report Post  
Posted to rec.audio.tubes
Luigi Vogliobene Luigi Vogliobene is offline
external usenet poster
 
Posts: 2
Default BIAS regulation Electro Harmonix KT90 vs Jinvina KT90

Il giorno lunedì 13 febbraio 2017 13:20:07 UTC+1, Peter Wieck ha scritto:
On Sunday, February 12, 2017 at 12:54:08 PM UTC-5, Luigi Vogliobene wrote:
Hi to all, I am new in this forum, I own a MingDa MC368BSE audio amplifier and it is time to replace the final tubes, I was suggested to replace the original 4 Jinvina KT90 with a selected quartet of Electro Harmonix KT90EH; I wonder if the BIAS will be OK if not exactely the same (this amp has a fixed BIAS).
Thank you in advance for your kindly replies.
Luigi


The circuit uses fixed bias but as this is only adjustable using unlabeled internal controls the task is best left to a dealer; the two meters just show the output voltage at the 8ohm loudspeaker sockets.
Read more at http://programming4.us/multimedia/16...MGZU4VOilTD.99

This is what I got from the net. I suggest you find someone specifically familiar with this amp and have them walk you through the bias process.

Peter Wieck
Melrose Park, PA


Thank you, Peter.
  #4   Report Post  
Posted to rec.audio.tubes
Big Bad Bob Big Bad Bob is offline
external usenet poster
 
Posts: 366
Default BIAS regulation Electro Harmonix KT90 vs Jinvina KT90

On 02/13/17 04:20, Peter Wieck so wittily quipped:
I suggest you find someone specifically familiar with this amp and have them walk you through the bias process.


sometimes I forget that not everyone who would be into tube amps is a
qualified electronics engineer...


  #5   Report Post  
Posted to rec.audio.tubes
Peter Wieck Peter Wieck is offline
external usenet poster
 
Posts: 2,418
Default BIAS regulation Electro Harmonix KT90 vs Jinvina KT90

On Tuesday, February 14, 2017 at 9:42:35 AM UTC-5, Big Bad Bob wrote:
On 02/13/17 04:20, Peter Wieck so wittily quipped:
I suggest you find someone specifically familiar with this amp and have them walk you through the bias process.


sometimes I forget that not everyone who would be into tube amps is a
qualified electronics engineer...


Normally, I would point out the process and try to walk someone through it. However, this is a Chinese amp with all that implies. So, find a tech, have that tech walk the OP through the process so that he will be able to do it himself in the future. Fixed Bias in tube amps is an ongoing process. Those who would not learn to do it for themselves ought not to own a Fixed Bias device.

Peter Wieck
Melrose Park, PA


  #6   Report Post  
Posted to rec.audio.tubes
Big Bad Bob Big Bad Bob is offline
external usenet poster
 
Posts: 366
Default BIAS regulation Electro Harmonix KT90 vs Jinvina KT90

On 02/14/17 06:53, Peter Wieck so wittily quipped:
On Tuesday, February 14, 2017 at 9:42:35 AM UTC-5, Big Bad Bob wrote:
On 02/13/17 04:20, Peter Wieck so wittily quipped:
I suggest you find someone specifically familiar with this amp and have them walk you through the bias process.


sometimes I forget that not everyone who would be into tube amps is a
qualified electronics engineer...


Normally, I would point out the process and try to walk someone through it. However, this is a Chinese amp with all that implies. So, find a tech, have that tech walk the OP through the process so that he will be able to do it himself in the future. Fixed Bias in tube amps is an ongoing process. Those who would not learn to do it for themselves ought not to own a Fixed Bias device.


I have to wonder whether or not a 'hybrid' design (part fixed, part
cathode) might be better...
  #7   Report Post  
Posted to rec.audio.tubes
Peter Wieck Peter Wieck is offline
external usenet poster
 
Posts: 2,418
Default BIAS regulation Electro Harmonix KT90 vs Jinvina KT90

On Tuesday, February 14, 2017 at 10:07:17 AM UTC-5, Big Bad Bob wrote:

I have to wonder whether or not a 'hybrid' design (part fixed, part
cathode) might be better...


Forgive an analogy:

Back in the day, VW had drum brakes on all four wheels, typically made by ATE. They used a fairly basic star adjuster system accessed through a hole in the the drum with a flat-bladed screwdriver. This was when pretty much every other maker (Except Mercedes) had switched to self-adjusting (drum) brakes. Once upon a time, VW was asked why they maintained the user-adjust system instead of the then-cheaper self-adjust option.

They answered: Self adjusting brakes are never excellent. Most if the time, they are barely good, as they adjust only when on the edge of failure (true at the time). Our brakes can be excellent.

At about 5 minutes per month, I can make sure that my bias is dead-on, adjusted to the type and age of tubes in place, and changed as needed. Not so easy with other systems. *Although I also have an amp built that way that is just fine.*

Now, very nearly everybody uses disc brakes and the issue is moot.

Peter Wieck
Melrose Park, PA
  #8   Report Post  
Posted to rec.audio.tubes
Patrick Turner Patrick Turner is offline
external usenet poster
 
Posts: 3,964
Default BIAS regulation Electro Harmonix KT90 vs Jinvina KT90

On Monday, 13 February 2017 04:54:08 UTC+11, Luigi Vogliobene wrote:
Hi to all, I am new in this forum, I own a MingDa MC368BSE audio amplifier and it is time to replace the final tubes, I was suggested to replace the original 4 Jinvina KT90 with a selected quartet of Electro Harmonix KT90EH; I wonder if the BIAS will be OK if not exactely the same (this amp has a fixed BIAS).
Thank you in advance for your kindly replies.
Luigi


The Jinvina KT90 looks like a fancy kind of KT90. Any other brand of KT90 should be able to be biased like Jinvina, if the advice in Ming-Da user manual is followed.

HOWEVER, nobody should assume Jinvina is exactly the same as KT90 EH.

THEREFORE, be sensible, and fixed bias should be adjusted so that each KT90 has Idle Pda 25W, so that if the Vadc between anode abd cathode = 450Vdc, then each KT90 has cathode dc current no exceeding 66mAdc. This allows for Pdg2 = 5W at idle, Pda = 25W. It seems to me this will allow for good class AB sound. I do not know if there is just ONE bias adjustment for all 4 output tubes, which IMHO would be BAD engineering, because tubes would have different Ia and thus different temperature. THERE SHOULD be one adjust pot for bias on each KT90.
If so, then each bias pot may need to be adjusted 4 times over a 30 minute warm up period to make sure bias Iadc is the same when all tubes have warmed up properly.

In 2011 I worked on a pair of 85W a Ming Da mono amps made in 1996, each with a pair pf 845 in PP at output. I needed to completely gut them and re-wire them properly.
I hope your Ming Da have much better quality than was normally awful in 1996.
I also hope your amps have active protection to prevent an overheating output tube from destroying an OPT which may be fragile, and now perhaps difficult to obtain as a spare part. Don't say I didn't warn you.
Patrick Turner.

  #9   Report Post  
Posted to rec.audio.tubes
Xtrchessreal Xtrchessreal is offline
external usenet poster
 
Posts: 11
Default BIAS regulation Electro Harmonix KT90 vs Jinvina KT90

On Sunday, February 12, 2017 at 10:54:08 AM UTC-7, Luigi Vogliobene wrote:
Hi to all, I am new in this forum, I own a MingDa MC368BSE audio amplifier and it is time to replace the final tubes, I was suggested to replace the original 4 Jinvina KT90 with a selected quartet of Electro Harmonix KT90EH; I wonder if the BIAS will be OK if not exactely the same (this amp has a fixed BIAS).
Thank you in advance for your kindly replies.
Luigi


The idea of throwing in 4 KT90EH tubes without any knowledge of quad output tube stage or biasing would be reckless. They need to be a matched quad and as stated Jinvina v EH likely have different operating Max watt output. If it were me I'd put in three trim pots one to bias each pair and then one for balance between the pair (not if its a stereo output, 1 pair per channel) and then I would run them very cool for long term use.

I know it is a very different amp but I run tubes in my Marshalls as cool as possible, to be more electrically robust since they are subjected to extreme variances in input voltage 100-130 VAC RMS that range from no ground to well made GFI and Independent grounds. I can also get more head room from input signal to output signal, to some degree, depending on clean or distorted tones, the tone stack seems to provide more range as well. That may a subjective thing I'm hearing, I have not put a freq sweep across it and measured output.

X
  #10   Report Post  
Posted to rec.audio.tubes
[email protected] johnnhelen4@gmail.com is offline
external usenet poster
 
Posts: 13
Default BIAS regulation Electro Harmonix KT90 vs Jinvina KT90

On Friday, June 30, 2017 at 7:34:24 PM UTC-4, Xtrchessreal wrote:
On Sunday, February 12, 2017 at 10:54:08 AM UTC-7, Luigi Vogliobene wrote:
Hi to all, I am new in this forum, I own a MingDa MC368BSE audio amplifier and it is time to replace the final tubes, I was suggested to replace the original 4 Jinvina KT90 with a selected quartet of Electro Harmonix KT90EH; I wonder if the BIAS will be OK if not exactely the same (this amp has a fixed BIAS).
Thank you in advance for your kindly replies.
Luigi


The idea of throwing in 4 KT90EH tubes without any knowledge of quad output tube stage or biasing would be reckless. They need to be a matched quad and as stated Jinvina v EH likely have different operating Max watt output.. If it were me I'd put in three trim pots one to bias each pair and then one for balance between the pair (not if its a stereo output, 1 pair per channel) and then I would run them very cool for long term use.

I know it is a very different amp but I run tubes in my Marshalls as cool as possible, to be more electrically robust since they are subjected to extreme variances in input voltage 100-130 VAC RMS that range from no ground to well made GFI and Independent grounds. I can also get more head room from input signal to output signal, to some degree, depending on clean or distorted tones, the tone stack seems to provide more range as well. That may a subjective thing I'm hearing, I have not put a freq sweep across it and measured output.

X


Variation of bias due to aging or adjustment will not effect the frequency response much at all. But the THD & IM will be. All easily measurable & heard by those who enjoy rock guitar or the classics. And by how much heat is evolved!
Reply
Thread Tools
Display Modes

Posting Rules

Smilies are On
[IMG] code is On
HTML code is Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Recommendatons bias settings for KT88 and KT90 Wessel Dirksen Vacuum Tubes 7 September 15th 07 01:33 AM
My 8585 amp with KT90. Patrick Turner Vacuum Tubes 4 November 10th 06 07:51 AM
KT90 Iain M Churches Vacuum Tubes 7 September 18th 05 02:53 AM
WTB quad KT90 gmgenett Vacuum Tubes 4 December 26th 04 05:49 PM
EH KT90 Patrick Turner Vacuum Tubes 9 November 21st 03 06:19 AM


All times are GMT +1. The time now is 09:37 PM.

Powered by: vBulletin
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 AudioBanter.com.
The comments are property of their posters.
 

About Us

"It's about Audio and hi-fi"