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Old 5th November 2013, 01:55 AM   #1
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Default 60 Watt Mono Build

Just finishing my prototype mono amp, it has some very great features. One feature is an on-board meter to measure bias that can be switched out curcuit.
Some really nice sound coming out of this thing. The meter will arrive toward the end of the week.
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Old 5th November 2013, 01:57 AM   #2
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Old 5th November 2013, 10:32 AM   #3
MikeR is offline MikeR  United States
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Nice looking enclosure.

What tubes are you using and what circuit boards are those?
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Old 5th November 2013, 04:54 PM   #4
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Originally Posted by MikeR View Post
Nice looking enclosure.

What tubes are you using and what circuit boards are those?
Thanks, I'm using Sovtek 6550's, one 6AN8 driver tube, and one 5AR4 rectifier. The boards I designed myself, using ExpressPCB. I really love their service, and quality.
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Old 5th November 2013, 07:01 PM   #5
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Do you dare to show the schematic ?
It would be interested to see and compare to mine.
I am just prototyping with a EL34 PP with combined cathode feedback (at output tubes) and GNFB.

Click the image to open in full size.

Last edited by artosalo; 5th November 2013 at 07:05 PM.
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Old 5th November 2013, 07:32 PM   #6
MikeR is offline MikeR  United States
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Originally Posted by denny9167 View Post
Thanks, I'm using Sovtek 6550's, one 6AN8 driver tube, and one 5AR4 rectifier. The boards I designed myself, using ExpressPCB. I really love their service, and quality.
A Dynaco III clone?
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Old 5th November 2013, 11:13 PM   #7
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A Dynaco III clone?
Yes it's based on the Mark III, with a few modifications to allow for the meter,as well as some individual component mods.

I thought about using a different front end, but I've read more positive things about the simplicity and performance of the single gain stage and cathodyne phase inverter. David Hafler did extensive testing on the drive of the triode stage of the tube. From what I understand the 6AN8 has more than enough headroom to drive the grids of the 6550's.

Last edited by denny9167; 5th November 2013 at 11:26 PM.
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Old 5th November 2013, 11:20 PM   #8
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Originally Posted by artosalo View Post
Do you dare to show the schematic ?
It would be interested to see and compare to mine.
I am just prototyping with a EL34 PP with combined cathode feedback (at output tubes) and GNFB.

Click the image to open in full size.
Keep us informed. I've read a little on the use of cathode feedback on the output tubes.would be interested to see how it performs.
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Old 5th November 2013, 11:38 PM   #9
Junm is offline Junm  Philippines
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Quote:
Originally Posted by artosalo View Post
Do you dare to show the schematic ?
It would be interested to see and compare to mine.
I am just prototyping with a EL34 PP with combined cathode feedback (at output tubes) and GNFB.

Click the image to open in full size.
Nice one ARTO...another 6F12P front end magic.
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Old 6th November 2013, 07:28 AM   #10
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I built three dissimilar amplifiers:
- UL-connectd
- pentode connected
- pentode connected with cathode NFB.
-
The output transformer was Indel TGL40 having 4k to 8 impedance ratio.
This transformer has four parallel connected secondary windings and is therefore very suitable for cathode NFB tests.

In all three cases I used quite same amount of total NFB. When compared to pentode circuit, the UL-circuit has 2,4 dB of local NFB and pentode circuit with cathode feedback has 6 dB of local NFB.

The supply voltage in all three cases were 460 V. The quiescent current was set to minimum distortion.

Below are the results in brief:


Click the image to open in full size.




My conclusion is that the pentode connected output stage with cathode NFB is the ”winner” of this test.

Pros:

- quiescent power dissipation is lowest of all
- maximum output power is higher than UL has
- required GNFB is lowest and therefore the loop stability problems are least likely
- lowest THD (at low and mid power range)
- performs best at 4 ohms load

Cons:

- high driving level of output tubes – up to 46 Vrms – is required
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