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Anyone ever tried building the Feral Eye 2A3 PP?
Anyone ever tried building the Feral Eye 2A3 PP?
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Old 19th May 2016, 01:56 AM   #1
Sodacose is offline Sodacose  United States
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Default Anyone ever tried building the Feral Eye 2A3 PP?

Here's a link to the description.

And here's the schematic.

So I looked up the specs of the discontinued Tango iron and found this:
  • TC-160 choke: 160H @ 15mA or 40H @30 mA
  • NC-14 interstage: 1+1:1+1, 50H primary, 160Vrma max output, 30mA DC current max
  • XE-45-5 output: 5k primary, 45W, multiple secondary taps, some kind of FB winding?

Looks to me like the following Lundahl would be a close match:
  • choke load, LL 1668, $100
  • interstage, LL 1660, $115
  • output LL 1620CFB has the feedback taps, 6k primary, $260
So...any takers?
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Old 19th May 2016, 04:56 AM   #2
45 is offline 45  Italy
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In general, be careful about the amount of cathode feedback as the driver might run short of swing. The Lundahl has rather high amount at 25%.
I don't know the Tango XE-45-5.

However I can't see the point of using the 2A3 with cathode feedback. Very low distortion can be obtained without. Damping factor would not change much because the open loop gain is low and you still have the full Rdc of the transformer in series. The added complexity is not worth for me.
Besides if you use for example the Lundahl LL1623 @ 0.2 dB power loss instead of the LL1620 @0.5 dB power loss the Rdc of the LL1623 is much lower!

Apart from the cathode feedback issue the transformers you want to use are ok.
I personally don't like LC coupling.


The Hashimoto A105 is identical to the Tango NC-14 in everything except the DC current it can work at. This is precisely 1/2. This also results in a lower max rms voltage output (at low frequency) because at any equivalent configuration the inductance is the same but max DC current is 1/2.
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Old 19th May 2016, 05:18 AM   #3
45 is offline 45  Italy
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P.S.
The 4P1L is a better driver.
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Old 19th May 2016, 12:57 PM   #4
Salectric is offline Salectric  United States
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45, what is it about LC coupling that you don't like?
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Old 19th May 2016, 01:31 PM   #5
45 is offline 45  Italy
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Quote:
Originally Posted by Salectric View Post
45, what is it about LC coupling that you don't like?
Because you add a resonance.

CCS and R load, C coupling and DC coupling in any mix or proper interstage transformer all work better for me in that position.

In this case 160H + 0.1 uF means 40Hz resonance. Despite the fact the peak might be damped by the large grid resistor the phase won't change.
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Old 19th May 2016, 01:40 PM   #6
Salectric is offline Salectric  United States
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That makes sense. Thanks for the explanation.
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Old 19th May 2016, 07:10 PM   #7
Sodacose is offline Sodacose  United States
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Quote:
Originally Posted by 45 View Post
However I can't see the point of using the 2A3 with cathode feedback. V
In that case, the LL 1663 @ $145 might be another potential OPT?

I wonder if a resonance at 40Hz was part of what the author liked about the original design. If using full range speakers in modest cabinets, that might make sense.

This is coming from the choke load and coupling cap, correct? I was looking at the interstage at first and trying to figure out what you meant.
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Old 19th May 2016, 10:12 PM   #8
45 is offline 45  Italy
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LL1663 is good and actually more in line with the power rating of your amp. It is rated 40W @30Hz, so it's still 18W @20Hz. A bit less if some small unbalance is present. Power loss 0.3 dB still better than 1620 used @0.5 dB power loss.

I cannot say about the author's choice. I have seen this other times and those times where it wasn't properly calculated I asked and realized people were not aware it is a resonant network.
The transformer has no resonance at low frequency and if properly made will have his first resonance well above 20KHz.

I used LC coupling once for a 300B driver with the 46 triode in the prototype stage while waiting for the IT transformers. It was something like 40H + 4.7uF high quality film cap and 15K next stage grid resistor and it didn't sound bad. However once replaced with the Tango NC-20 interstage the difference was almost like night and day.

Last edited by 45; 19th May 2016 at 10:18 PM.
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Old 18th February 2018, 12:13 AM   #9
prinzinio is offline prinzinio  Poland
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I have built Feral Eye amplifier as per original schematic, however I faced some issue with the 6V6G tube which I have bought from this auction

6V6G CV509 FIVRE NOS BOXED VALVE/TUBE | eBay

When I first turned on the amplifier I didnt heard any sound. After measurements I have found that there is ~250VDC on pin5 of 6v6g. I thought that maybe coupling cap has leakage but its not the case. I have also lowered grid resistor from 820k to 300k, however it also did not helped.
To make long story short - I cannot figure out why there is no minus voltage on 6v6 grid - any ideas?

In the meantime I made EL84 to 6v6 adapter and with EL84 it works good, no issues at all, same circuit.
Can it be the case that there is some internal short in 6v6g? I have two pcs, both bought as NOS with the same issue.
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Old 18th February 2018, 01:26 AM   #10
Technical is offline Technical  United States
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What are the voltages on each of the pins of the 6V6?
What’s the voltage from the cathode to ground?

Also, I breadboarded this amp several years ago. It sounded great. I tried with and without the cathode feedback. It measured better with the feedback but it really sounded the same with or without it.
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