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Old 10th November 2008, 09:54 PM   #11
chrish is offline chrish  Australia
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Thanks for taking the time to respond Eli, and sorry for any thread hijack. If things look like moving away from the original question on the thread I will start another thread.

The output iron is Tamura F684. It has UL taps and "a third winding exclusive for NFB use" 16R.

I thought that I read in Morgan that the original Mullard 5-20 had high sensitivity and poor noise performance. I am guessing the replacement of the EF86 with the ECC81 etc and the sand source followers address these issues.

Very interesting. Have two more assignments due in the next week, then finished till Feb. so will be able to start designing something...

Thanks again,

Chris
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Old 10th November 2008, 10:40 PM   #12
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Quote:
Originally posted by rockgardenlove
I'm aiming for around 400 bucks. I don't care about audiophile components or anything. I have 1/4 watt resistors and caps.

Thanks!
Take a look at http://www.diytube.com. The construction manual is here: http://www.diytube.com/phpBB2/viewtopic.php?t=837

The ST-35 PC board makes it very easy to build an EL84 Dyna ST 35 clone that should set you back less than $400.
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Old 11th November 2008, 01:18 AM   #13
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The DIYtube ST35 makes a very nice first project. I cobbled together my first one with less than $100 of iron (salvaged power transformer, $20; Hammond 1620 output transformers, $65 for the pair) plus $100 for the board and parts. I stole the tubes out of another amp and the ST35 sounds just beautiful for not a whole lot of dollars.

Having everything on one circuit board (power supply, tube sockets, everything!) makes construction a total no-brainer. Results are virtually guaranteed. Highly recommended.

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Old 11th November 2008, 03:28 PM   #14
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Quote:
Originally posted by chrish

I thought that I read in Morgan that the original Mullard 5-20 had high sensitivity and poor noise performance. I am guessing the replacement of the EF86 with the ECC81 etc and the sand source followers address these issues.



Chris
Strange, the completed Mullard 5-20 spec quotes a s/n ratio around -90dB down. That's quiet enough. Maybe one was looking at the EF86 spec alone and not the completed amp.
Somewhere is a design of the Mullard 5-20 using the EF86 as a triode and 15dB sens drop but with far reduced noise.. The characteristics then become a ECC82 look-a-like, but with a far better HF response. Obviously the drop in loop gain will req. nfb component changes.

richj
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Old 11th November 2008, 09:08 PM   #15
chrish is offline chrish  Australia
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I am not an expert, or even an informed commentator, just remembering what I saw in Morgan... I think what Eli is suggesting is going to be different enough from the original design for those comments in Morgan to be irrelevant.

I am reasonably good with a soldering iron and a drill press and following a schematic, but I am no electronics engineer. I would be grateful for some gentle guidance

So, for a 6L6 based design, I am guessing around the 300 volt mark B+? So a transformer something around 370-0-370 200mA (for monoblock) mark? Plus I will need a -ive bias. Excuse the ignorance again, but what kind of current draw is there on the bias? I would imagine quite low...

Thanks for the help so far. If this turns in to another design, I will start a thread

Chris
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Old 11th November 2008, 10:08 PM   #16
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Quote:
Originally posted by chrish
I am not an expert, or even an informed commentator, just remembering what I saw in Morgan... I think what Eli is suggesting is going to be different enough from the original design for those comments in Morgan to be irrelevant.

I am reasonably good with a soldering iron and a drill press and following a schematic, but I am no electronics engineer. I would be grateful for some gentle guidance

So, for a 6L6 based design, I am guessing around the 300 volt mark B+? So a transformer something around 370-0-370 200mA (for monoblock) mark? Plus I will need a -ive bias. Excuse the ignorance again, but what kind of current draw is there on the bias? I would imagine quite low...

Thanks for the help so far. If this turns in to another design, I will start a thread

Chris

Starting a new thread that links this thread is an excellent idea. Title it AB2 6L6GC or something similar.

B+ for this project will be about 450 V. Look at the 6L6GC data sheet. While you are at it, pay close attention to the plate curves of positive g1 voltages. Notice that when g1 goes substantially positive, quite a bit of current is present. That current is associated with (among other things) the negative rail. B- current requirements here are substantial.

The "idle" current of the 6L6GCs gets controlled by what goes on at the MOSFET gates. Remember, the "beamie" control grids are DC COUPLED to the FET source electrodes. An initial WAG on my part for the 6L6GC idle current is 10 mA.
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Old 12th November 2008, 02:32 PM   #17
chrish is offline chrish  Australia
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Thanks Eli, new thread started here: 6L6 AB2 Amp
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