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Old 21st December 2011, 01:43 PM   #1
jooch is offline jooch  Netherlands
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Default Upgrading/repairing vintage amps

Hello,

I have been repairing and upgrading vintage amps for many years with great results, a good example is this one i did a while ago link

It sounds amazing after all the mods i've done.

I have a question about the Ft values of transistors though. I experimented with faster transistors in the past, but did not get the results i was hoping for.

Someone told me you can't replace transistors with types that are much faster than the original ones. Is this true and why is that?
Furthermore, what would you have to modify in order to do so?
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Old 21st December 2011, 02:19 PM   #2
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that is way too long to talk through in a single post .

In my company we repair./upgrade an average of 400 amplifiers per year which most of them ( 75% ) is vintage .

Often a non wise choice of any semi in the wrong place might take you to oscillations that can also not be seen at first look ...

Also playing with semis here and there will alter the sonic signature of any amplifier ....Given the opportunity this is simple a question of choice either the owner will like such a thing to be done or not ...

i f you choose to walk on the wild or even extreme side you need to let the costumer decide about that and most important you need to quadruple verify the changes with scopes loads and generator

statistic will say that most of our costumers will look towards a small margin of changes ( even if this alters the sonic signature ) that will include opening of bandwidth removal of VI limiters beefing up of idle and so on ....

Obviously changes like that will have a negative effect on the safety aspects of the amplifier ...costumer should also know that and decide about that on his own .

kind regards sakis
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Old 21st December 2011, 03:38 PM   #3
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If you replace a transistor with a newer type with more bandwidth, the frequency compensation that was designed in might not be correct anymore and you now have an oscillator. That's the short version. You can also check things out with SPICE.
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Old 21st December 2011, 06:34 PM   #4
jooch is offline jooch  Netherlands
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@sakis
I consider myself owner of all amplifiers as long as they sit in my workshop
As far as the change in sound, i consider any reduction in distortion an improvement, this usually means the need for newer more linear components.
Thus far my usual modifications have been:
- improving power supplies
- improving capacitors with better sounding types and tighter tolerances
- replacing small signal transistors with more linear types
- beefing up the idle current

In that sense only the 'wild' option has been left on the todo list

@ingenious
It's as i thought then, an amp will need to be re-engineered with such modifications otherwise the transistors will oscillate, right?

Is there some beginners information on how to use spice?

Cheers, Jochum
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Old 21st December 2011, 07:33 PM   #5
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spicing things like that will tell you only partial information there is a lot of other issues that cannot be seen in simulations ( such as PCB ,signal, power , ground rooting and so on )

your consideration as owner of the amplifier for a while will make a hell of nice technician .... vintage audio likes people like you ( and me )

Since i am mostly work with Japanese equipment allow me to be bit skeptic regarding replacing small signal transistors with more linear types since in many of those the hart and soul is those small signal often magic transistors .....

though your approach is very good ...

kind regards sakis
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Old 22nd December 2011, 12:23 AM   #6
jooch is offline jooch  Netherlands
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Thanks for the info Sakis. I realize the limits of spicing, but it should at least reveal major problems, right?

About replacing small signal transistors, the reason why is because of serviceability. Many times the original components aren't sold anymore and in many cases not even the crossed referenced ones!

However after 30 years or so these transistors will have considerable wear or corrosion, so they must be replaced to get the amp to its full potential again.
This is also the reason why i never buy a used Class A amplifier unless i know how many hours it has been in service.

When i replace a small signal transistor i always check the following:
- The gain of the new transistor is close or equal to the original one
- The ft of the new transistor is close to the original one
- The voltage, current, wattage are equal or better.
- I manually check each new transistor for gain and match the ones i use for the input stage.

You won't believe what a difference a few fresh transistors can make to an old amplifier.

Last edited by jooch; 22nd December 2011 at 12:25 AM.
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Old 22nd December 2011, 12:25 AM   #7
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Quote:
Originally Posted by jooch View Post
Hello,

Someone told me you can't replace transistors with types that are much faster than the original ones. Is this true and why is that?
Furthermore, what would you have to modify in order to do so?
An amplifier is usually slowed down by capacitors at the VAS and sometimes in the feedback loop to stop oscillation.
If you put in faster transistors then these capacitors might no longer be large enough to damp oscillation.
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Old 22nd December 2011, 12:57 AM   #8
jooch is offline jooch  Netherlands
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Hi Nigel,

Thanks for your help. If we assume we were to modify the output transistors of the amplifier shown below with types that are twice as fast.
Would we then be looking at R3, R7 & C5?

Click the image to open in full size.
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Old 22nd December 2011, 01:03 AM   #9
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Originally Posted by jooch View Post
Hi Nigel,

Thanks for your help. If we assume we were to modify the output transistors of the amplifier shown below with types that are twice as fast.
Would we then be looking at R3, R7 & C5?

Click the image to open in full size.
C8 and C9 are the capacitors that affect the speed.

I would first try the amplifier without any changes under various loads and volumes and look for oscillation on the output. If it is ok then I would leave it. If it oscillates then look at making c8 and c9 larger. If it oscillates switch it off quick just in case it fries.
Sometimes the oscillation can be quite small just sat on top of your test sine wave.
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Old 22nd December 2011, 01:09 AM   #10
jooch is offline jooch  Netherlands
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Originally Posted by nigelwright7557 View Post
C8 and C9 are the capacitors that affect the speed.

I would first try the amplifier without any changes under various loads and volumes and look for oscillation on the output. If it is ok then I would leave it. If it oscillates then look at making c8 and c9 larger. If it oscillates switch it off quick just in case it fries.
Sometimes the oscillation can be quite small just sat on top of your test sine wave.
Ahah, so the drivers are the most important link then?
BTW: Are c8 and c9 matched to the output transistors, the driver transistors or the entire darlington circuit?
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