White Oak PL14 upgrade on 700B Quasi Comp

[66stanger]

Hi,

I just finished restoring my 700B Quasi Comp.
The driver card (White Oak) and the output transistors have been replaced (21196).
The bias was set to 350mV.


I am disappointed with the sound quality, certainly the components are lapped but after 1 hour of listening, the sound is worse than my good old PL14A:


- Medium frequencies are not sweet with THD
- Transients on the pianos are not beautiful also with the THD


I think the bias to 350mV is not suited to my listening.
The sound was much better before the "upgrade" !!


What do you think ?


What is the bias on PL14A (300mV) ?


If I measure the value of VR on my PL14A, anybody could tell me what Bias voltage it is?


Thanks for your opinion...


If I can not solve the probème, I'll fix 14A and re-fit on my 700B ...
It was not a problem for me to disconnect the speaker after-turn off ...

 

[laatsch55]

Bias should be 380 mv. With no pics whatsoever, it's hard to say or evaluate your work. Yours will be the first in history that sounds worse after a conversion.

 

[laatsch55]

What did you use for drivers??

 

[laatsch55]

"If I measure the value of VR on my PL14A, anybody could tell me what Bias voltage it is?"


Nope.....too many other variables...

 

[Northwinds]

Bias should be 380 mv. With no pics whatsoever, it's hard to say or evaluate your work. Yours will be the first in history that sounds worse after a conversion.

That was what I was thinking Lee. I suspect his work somewhere because when Blackie was quasi, she could peel the paint of the walls and sound glorious doing it. He mentioned changing the output drivers to 96's but where are the 95's that are used with them? Does that mean he left the 909s or 410s in????

 

[laatsch55]

The bottom row on a quasi has to be different than the rest..preferably RCA-410's...

 

[laatsch55]

No 95's on a quasi.

 

[Northwinds]

No 95's on a quasi.

Could you explain why Lee? I thought the 95's and 96's were just better transistors compared to the 909's and 410s which were pretty much for automotive application? Is it a difference in PNP/NPN or something like that?

 

[MarkWComer]

Could you explain why Lee? I thought the 95's and 96's were just better transistors compared to the 909's and 410s which were pretty much for automotive application? Is it a difference in PNP/NPN or something like that?

A simplified explanation of full-comp amps is here: https://en.wikipedia.org/wiki/Push–pull_output

As I understand it (and smak me wit a brick if I'm wrong...) FC amps invert half of the audio waveform for the negative half and essentially "pull" from the load (speakers) and "push" the positive side of the waveform to the load.

Quasi comp are not "push/pull" in design and merely provide a variable frequency AC current to the load. The difference between the 909s and the 410s is that the odd ones were "pre drivers" and the other were "final output" (and I don't rightly know which was which). The 909s can be replaced with NPN MJW21195, I don't think there's a replacement available for the 410s.

 

[WOPL Sniffer]

I had a Kraco stereo that was Quasi-junk and could be upgraded to Fully-junk with the edition of a matchbook under the 8 track tape.... It was a sight to behold. It was a powerfull 10 watts per and could lay out a smokescreen when I squealed the belts.... Just freakin cool...

 

[Justin Case]

I had a Kraco stereo that was Quasi-junk and could be upgraded to Fully-junk with the edition of a matchbook under the 8 track tape.... It was a sight to behold. It was a powerfull 10 watts per and could lay out a smokescreen when I squealed the belts.... Just freakin cool...

Sounds like my 1967 Fairlane.

 

[WOPL Sniffer]

my 63 comet was so rusty (Minnesota) that you couldn't get a good ground for anything to work.

 

[66stanger]

Bias should be 380 mv. With no pics whatsoever, it's hard to say or evaluate your work. Yours will be the first in history that sounds worse after a conversion.

So, I've a 700B Quasi Comp with PL14A and XPL909/RCA410 (all were stock).
The sound was amazing sweet like tube and extremely powerful like a SolidState. (too easy to drive JBL 250Ti Classic Edition in full range).
You can find some listenings on my YouTube Channel :
https://www.youtube.com/playlist?list=PL30-r4vXCiRSehK0G73LbmrxiK2juRjDv

Few months ago, one channel was mute and the VU in MAX (driver board or XPL faulty ?).
It was the good moment to upgrade the PL14A to Joe PCB and XPL to 21196 (it's a quasi, no 21195, only NPN).
I had the components and board in my workshop since 1 year.

So I've replaced the driver board by REV D and 21196 (all matched) from Joe.
The bias is 350mV @ 5mn warm up :


After one hour of testing, the sound is very different, VHF, MF and LF are good but the HF are a little bit aggresive and the THD are bad, the piano is not real with distorsion and terrible on transient.
The voices are not sweet like before with the old setup (stock electronic).
I hope that it's the bias...



Regarding the sound, what is the difference between 350mV & 380mV ???





Please let me know your opinion !

Thank you,

 

[laatsch55]

If you have 21196's for outputs, what do you have for drivers?

I can not differentiate between 350 mv and 380 mv by listening g to it.

 

[66stanger]

If you have 21196's for outputs, what do you have for drivers?

I can not differentiate between 350 mv and 380 mv by listening g to it.

It's RCA 410.

 

[laatsch55]

Is the ground wire from the negative speaker posts to the ground plate on the RCA jacks?

 

[66stanger]

Is the ground wire from the negative speaker posts to the ground plate on the RCA jacks?

Yes, of course.
Nothing change regarding wiring only driver PCB and 21196s...

 

[Northwinds]

Like the WOPLs, don't you need to leave it on for a few days for it to settle in. How long were your caps discharged? Do you have to reform them? Just asking questions about stuff discussed in previous threads

 

[Gepetto]

So, I've a 700B Quasi Comp with PL14A and XPL909/RCA410 (all were stock).
The sound was amazing sweet like tube and extremely powerful like a SolidState. (too easy to drive JBL 250Ti Classic Edition in full range).
You can find some listenings on my YouTube Channel :
https://www.youtube.com/playlist?list=PL30-r4vXCiRSehK0G73LbmrxiK2juRjDv

Few months ago, one channel was mute and the VU in MAX (driver board or XPL faulty ?).
It was the good moment to upgrade the PL14A to Joe PCB and XPL to 21196 (it's a quasi, no 21195, only NPN).
I had the components and board in my workshop since 1 year.

So I've replaced the driver board by REV D and 21196 (all matched) from Joe.
The bias is 350mV @ 5mn warm up :
View attachment 22614

After one hour of testing, the sound is very different, VHF, MF and LF are good but the HF are a little bit aggresive and the THD are bad, the piano is not real with distorsion and terrible on transient.
The voices are not sweet like before with the old setup (stock electronic).
I hope that it's the bias...

View attachment 22615

Regarding the sound, what is the difference between 350mV & 380mV ???

View attachment 22616

View attachment 22617

Please let me know your opinion !

Thank you,

Hi Cyril
I recommend that you consider replacing Q1-Q4 which were put in backwards during your assembly process. Give that a try.

Thanks
Joe

 

[laatsch55]

When you say the THD is bad , have you measured it? When you ask for help, having an accurate description of prior problems really helps.