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Old 7th February 2006, 04:57 PM   #11
anatech is offline anatech  Canada
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Hi Owen,
Quote:
it is meant to be a 1K but i am useing a 150Ohm
Sets the current of the cascode stage. Watch your H-K voltage on the top tube.

Your BC3X7-25 transistors. Are they 25V? If so, try something like A970 and C2240, put another transistor stage in between that can deliver a few amps to drive the outputs. This will help raise the input impedance. To generate your bias, use a current source and current sink to isolate from the supply and make the circuit relatively independant on supply voltage. Use a relay to isolate the output stage from the large on-off voltage swings.

Just a few hints, get designing.

-Chris
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Old 7th February 2006, 05:08 PM   #12
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I will change the resistor in the tube stage i think, probably easier.
the BC3X7-25 are 50V as far as i'm aware.. they come in -16 and -25 flavours amongst others, the larger the number the larger the Hfe acording to the datasheet. the problem with adding another transistor stage is that it is currently all neatly mounted on a PCB that mounts to the heatsink, which basically means i can have everythign else wired p2p. Would adding an extra ss stage increase the impedence and help with the problems i'm haveign with the amp? would adding a second tube stage help the situation?
i get what you mean about the current source and sink though, you mean inplace of the 12k /15k resistors? i can do that fairly easily. have plenty of BD139 and BD140 that would be up the the task.. i don't see how this will help with the impedance issues though? I want to get the amp running really before i start modifying it too much so that i have some incentive to carry on the project, i get bored easily with things when they wont work properly..lol..
OWen
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Old 7th February 2006, 05:10 PM   #13
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or did you mean add an extra transistor stage between the tube stage and ss stage to increase the current from the tube stage?
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Old 7th February 2006, 05:18 PM   #14
anatech is offline anatech  Canada
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Hi Owen,
Your supply voltages are +- 50V. You need 120V devices minimum allowing a tiny bit of safety. Another transistor stage will help to increase the input impedance of the output stage, the CCS's work to that end as well. It's all important and inter-related.
Quote:
did you mean add an extra transistor stage between the tube stage and ss stage to increase the current from the tube stage?
No, I do want another stage added (see above). This will reduce the current required from the tube stage and decrease distortion. Leave the tubes alone for now.

-Chris
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Old 7th February 2006, 05:23 PM   #15
ilimzn is offline ilimzn  Croatia
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Sorry to be blunt, but that schematic has so many issues it's not even funny, and your haphazard changes to the component values are making it signifficantly worse.
BC337 and 327 are 50V BJTs in a place where up to 100V can happen across them. The bias setting is connected in such a way that if the pot ever even develops a smidgen of scratchyness, the output stage will blow up as the bias voltage will become about 100V or so. The input impedance is about 7.5k which is WAy too low for a 6922 SRPP. Then you also use a 6 times lower resistor in part of that SRPP and say it's better to change that back to the way it should be, than put proper driver BJTs, the BD139/140 being far closer to that than the BC337/327 - and youi have BD140/139 at hand. I am not even going to attempt a comment on the 'i think the power supply is the wrong way around' statement, my vocabulary fails me.
Basic laws of electricity are not subject to subjectivism (pun intended). So, what worth is it that it sounds good when only the grace of some unknown god is keeping it from blowing up? Or, are you trying to make an even less reliable (I am barely restraining myself from describing the sound!) amp than a Counterpoint SA-100?
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Old 7th February 2006, 05:35 PM   #16
anatech is offline anatech  Canada
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Hi ilimzn,
Yes, correct on all counts. I was trying to get Owen started in the right direction (away from Counterpoint for one). I figured the overall design should be corrected first, then the minor details like bias after. BTW, Counterpoints version is far more scary than this even (SA-100)

Go ahead and describe the Counterpoint sound. I'm sure I'll agree with you at least 100%.

Anyhow, ilimzn, if you read my suggestions above, what do you think of the direction? Keep in mind we are stuck with tube front end and BJT current stages.

-Chris
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Old 7th February 2006, 05:36 PM   #17
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no.. my supplies are +/-25V on the output stage.. same voltage the output stage was running on int he original amp. according to the spec's i found on the net the transistors i replaced were originaly 50V devices aswell so that isnt an issue? or am i missing something?

The way i originally had the power rails is the way that they are in the schematic as i traced it out fromt he output PCB's and they kept burnign out so i swapped the supply rails and now it works. My guess there is that i made a mistake traceing out the schematic and got the pin numbers on the PCB the wrong way around.

I will change the resistor in the tube stage tonight probably as it won't take long, the reason the resistor is 150Ohm and not 1K is that the tube stage is actually a prototype for an SRPP linestage i built and that circuit had a 150ohm resistor in it, that was the only diffrence between the linestage i have built and the tube stage in this amplifier that was why i decided to try this "experiment" out and see if it would work.. because i had a prototype of the tube stage already built from my previous project and i had these cool output boards i wasnt currently doing anythign with.

So I have to add another stage between the tube and the SS stuff so that it dosnt need to draw so much current from the tube stage?
This is the first time i have palyed around with a hybrid amp, just thought it would be quite fun to experiment with **Was bored**

Many thanks,
Owen
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Old 7th February 2006, 05:43 PM   #18
anatech is offline anatech  Canada
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Hi Owen,
Ignore the tube stage for now, it works. Just make sure you are not stressing the heater cathode ratings.

I feel you need more headroom with your transistors even at +-25V. So let's concentrate on the current amplifier stage. Add driver transistors and compensate the circuit design for these. Those small transistors will never supply enough current for the output transistors. ie you are going to blow them.

This is an area where ilimzn can provide a great deal of expert advice.

-Chris
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Old 7th February 2006, 05:51 PM   #19
ilimzn is offline ilimzn  Croatia
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OK, sorry for the reaction, it's one of those days
For starters, PLEASE give us an ACCURATE schemaitic diagram - so far most currections sugegsted ended up being to things that were not really there.
Regarding Anatech's sugegstions, I do agree with them, and we had SA-100 discussions before
There is another thread on a hybrid just below this one, it may prove worthwhile to have a look there too!
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Old 7th February 2006, 05:52 PM   #20
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i get you now. basically scrap the driver transistors and put some thing beefier in their place and then ad an extra stage before the driver stage to drive the new beefier driver stage so that it can supply enough current for the output transistors.

I'll leave the tube stage for now, i'm useing one of my old "experiment only" 6922's in there anyway so i'm not really that bothered if it dies.

I'll go have a play around and see what i can come up with.
Many thanks,
Owen
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