MULLARD 5-20 build with mods - diyAudio
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Old 13th September 2013, 09:03 PM   #1
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Default MULLARD 5-20 build with mods

I'm building two amps using the Mullard 5-20 circuit with mods. One is using negative bias on the op valves. Another is replacing V1 the EF86 with an ECC83.
Any thoughts?diyAudio

Note the use of the negative bias rail and driving both grids of V2. What gain and distortion can I expect compared to the original EF86 circuit.
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Old 13th September 2013, 09:07 PM   #2
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It's not really a 5-20 since you have the input stage set up as a diff amp. Your issue is likely to be in the second stage, since the first stage plates are unlikely to have the same DC voltage on them.
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Old 13th September 2013, 09:18 PM   #3
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Your issue is likely to be in the second stage, since the first stage plates are unlikely to have the same DC voltage on them.
Why is that, SY?
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Old 13th September 2013, 09:24 PM   #4
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Sections are not tightly matched, and if they are, they won't be over time. It doesn't take much to upset the applecart- if there's a 5 volt difference, that will saturate the second stage.
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Old 13th September 2013, 09:34 PM   #5
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Good point. A rethink is needed.
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Old 13th September 2013, 09:44 PM   #6
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I'd stick to the original topology, a single-ended input stage direct coupled to a long-tail pair. An ECC83 with a very high plate load and minimal cathode degeneration should work great for that. You might think about biasing it with an LED in series with a 47R (47R connected to ground), then return the feedback to the junction of the 47R and the LED. A CCS plate load would be icing on the cake and further improve linearity.

Because the ECC83 current is low, you'll want to run a resistor from the B+ rail to the top of the LED to increase the current through it, if you don't use a CCS at the plate. If you size it so that 6mA of current run through the LED, the 47R will contribute another 0.3V or so, so you might want to use an IR LED for that position to get the right bias voltage on the ECC83 cathode.
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Old 13th September 2013, 10:00 PM   #7
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I still have lots to do. It'll give me time to think this over.
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Old 13th September 2013, 11:55 PM   #8
multi is offline multi  Australia
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I am thinking of using the Mullard circuit too;
On another thread I found the following.
Best to keep the EF86 and get rid of the 12AX7 and use a 6CG7/6SN7 from what i have read.
Phil
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Old 14th September 2013, 06:44 AM   #9
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Originally Posted by payitforwardeddie View Post
What gain and distortion can I expect compared to the original EF86 circuit.
It is essential to know that the distortion is mainly generated by the output tubes, not by voltage amplifying stages. And the THD is mainly reduced by GNFB.
Therefore there is no big difference on THD level if anything is modified at the front end of the amplifier assuming the amount of GNFB is not changed.
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Old 14th September 2013, 06:49 AM   #10
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Originally Posted by payitforwardeddie View Post
I'm building two amps using the Mullard 5-20 circuit with mods. One is using negative bias on the op valves. Another is replacing V1 the EF86 with an ECC83.
Any thoughts?diyAudio

Note the use of the negative bias rail and driving both grids of V2. What gain and distortion can I expect compared to the original EF86 circuit.
what output transformer will you use?
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