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Old 17th August 2003, 10:05 AM   #1
jevta is offline jevta  Serbia
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Unhappy OTL ver.6 Ciuffoly

Hi,
I`m finishing my amp ver.6 Ciuffoly .It`s different version from very popular ver 8.2. on DIY. I decided for this version for any enhance (indifferent 4 bias supply, respected ecc83, and very clear situation with second part e182cc..).
But, I have any obscurity with >
1.On shemme ver6. resistor on output have 56 Ohm vs ver.8.2 1KOhm. Output have not different why is resistance so different and what is better?
2. I blindly followed shemme and installated for high voltage only block conds MKP like on shemme. I `m sceptic for good filtring less big capacity 470uF. What thinking about this?
3.Using excellent perfomance FET include via 2N5458 and inverting voltage I have 4 V on cathode ECC83 but I have to change 2 pieces during testing. I dont be sure for reason but perheps put bigger like bipolar 2N6488?
3. I reserved for OCL on output. Older constructors ver 8.2 what your opinion and experience.
Please, all arguments be beneficial for me .
Jevta
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Old 17th August 2003, 07:34 PM   #2
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Hi,

BTW, the version you posted is a slightly modified version of V8.2.
I added the separate biasing scheme to allow for easier biasing.

1. They're two different versions of this amp.You have to look at the amp as a whole, not just the output stage alone.
The resistor at the output just makes sure the amp see a load with no speaker connected.
This is only necessary when the no feedback position is selected.

2. I don't quite follow here....you mean the B+ for the input stage?

3. You mean the 2N5458 went up in smoke during testing?
The trouble with that first stage is that A. Ciuffoli doesn't indicate how the trimpot must be adjusted nor what to measure.

3. (bis) I see little point of building an OTL and then send the lot through a huge output cap. Why don't you just try it with the cap first, monitor the output for DC for a few months and decide afterwards if you can leave it out.
If the tubes are reasonably matched and biased correctly, no DC will be present at the output.

I know of at least one member who's built version 8.2 as posted by you and is happy with it.

Why don't you do the same? You just need to burn of 40 V for the B+ of the input stage, some new components and an ECC82 instead of the ECC83.
Easy enough to do if you hardwired the amp.

The pic I post is version 6.0.

Cheers,
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Old 20th August 2003, 04:44 AM   #3
jevta is offline jevta  Serbia
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Thank you fdegrove,
My problems are because too late connect on DIY, after finished and start with testing and measurment.
Mu dilleme now bigger after first result>
1.on one channel have voltage on anode +90V and - 260V (I installted per channel 2x3200uF Sprague and 4x1500uF Siemens and I located problem.
2. For input stage I stricly use per schemma power block capacitor
and don`t sure that right solution.
Thank for advice I be to close for make decision .
Cheers
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Old 21st August 2003, 12:09 AM   #4
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Hi,

Quote:
1.on one channel have voltage on anode +90V and - 260V (I installted per channel 2x3200uF Sprague and 4x1500uF Siemens and I located problem.
That would blow up the filter caps...

The problem stems from the way the PSU is presented for the outputstage.

If I find the time next week, I'll correct the drawing and present it for you here.

Cheers,
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Old 21st August 2003, 04:57 AM   #5
jevta is offline jevta  Serbia
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Hi,
Thank you Fdegrove I appreciate your time
I don`t told that before instalationed I measured all caps and pairing in section. Expecally this is strange becouse second channel are very identical. This voltage measured without 6C33B . In case with tubes there is blow up protected resistor 1 Ohm . Second channel work with 150 mA on +_ 170 V but have problems with fet. Now I have to for delivery from Farnell it`s rarely feet.
Thank you,
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Old 21st August 2003, 06:06 PM   #6
jevta is offline jevta  Serbia
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Hi,
Thank you Fdegrove I appreciate your time
I donít told that before installations I measured all caps and pairing in section. Especially this is strange because second channel is identical but difference voltage between two balanced section insignificance. This voltage measured without installations 6C33B. In case with tubes there is blow up protected resistor 1 Ohm . Second channel works with 150 mA on +_ 170 V but have problems with FET. Now I have to wait for delivery from Farnell. Thank you for suggestion for protection speakers I should that added caps.
Thank you,:bawling
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Old 31st August 2003, 05:53 PM   #7
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Hi,

In order to understand the odd voltages you measure you need to know that;

On one side of the PP you measure the anode to ground voltage, on the other the cathode to ground voltage.

Assuming an imaginary OTL with a bipolar supply for the outputstage of +/- 170VDC and -85V bias supply, you will find the following voltages with respect to ground:

+170 VDC -85 VDC = + 85 VDC on the upper banks' anode.

- 170 VDC - 85 VDC = - 255 VDC on the lower banks' cathode.

This is because the bias supply is referenced to the bipolar supply as shown on the diagram.
If you'd disconnect the bias supply you should measure +/- 170VDC on the rails.

I have no idea how the FET that serves on the input tube should be set but I found someone in Sweden building the same version of this OTL as yourself.

His e-mail address is at the bottom of the page.
If you like I can e-mail him and hopefully he can put you back on track.

OTHER BUILDER


Cheers,
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Old 1st September 2003, 05:03 AM   #8
jevta is offline jevta  Serbia
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Thank you Fdegrove,
I give up of any change for inverted FET. It`s best solution and this 2N5458 is good choice( inverted voltage, small resistance and work up to 10 V)
I delete problem with voltage. Import fact for construction and determinate point for virtual ground. I change this point for right channel and now have only 5 V difference . Last problem with 8 V on speaker have to enquire. I suppose that problem are same or of bias current.
Just finished.
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