Help: Interstage Transformer for 7788 (E810F) Tube in Triode ? - diyAudio
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Old 14th January 2013, 07:53 PM   #1
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Default Help: Interstage Transformer for 7788 (E810F) Tube in Triode ?

I want to use the 7788 in triode to drive a 45.

The plate voltage of the 45 is ~250V.
I plan to use this same voltage source for the 7788.
7788 will draws 15mA.

I want to use interstage transformer.
Does anyone know what would be a good impedance and ratio for the interstage to couple the 7788 triode ?
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Old 14th January 2013, 08:08 PM   #2
cotdt is offline cotdt  United States
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Hashimoto A-105 would be a great choice. It can handle 30mA.
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Old 14th January 2013, 08:13 PM   #3
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So a 5k 1:1 interstage ?

What makes you think that this would be a great choice ?
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Old 14th January 2013, 08:18 PM   #4
cotdt is offline cotdt  United States
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Quote:
Originally Posted by manp111 View Post
So a 5k 1:1 interstage ?

What makes you think that this would be a great choice ?
At 30mA, the 7788 would have an output impedance of 1.1k, which makes a 5k load a good choice. It will give very extended bandwidth both directions. You could also configure it as a 1:2 step-up transformer for more gain, or step-down for less gain.
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Old 14th January 2013, 08:25 PM   #5
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Thank you.

What do you think would be a good bias point for the 7788 to drive a 45 tube ?

I prefer ~250V because I can use the same voltage source as the 45.
But I would build a separate power supply if different voltage is better.
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Old 14th January 2013, 08:33 PM   #6
cotdt is offline cotdt  United States
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I'm using 320V B+ on the 45 at 40mA. I get 5% distortion at 5W, and it still sounds relatively clean at this level.

I've used 5842 and D3a to drive the 45, using a variant of George's TSE circuit. The 7788 is a drop-in replacement for the D3a, but I've never tried the 7788 myself as I don't own this tube. Anyway, I have a CCS which drops the voltage down from 320V to 130-200V (adjustable). I run at 180V. High transconductance tubes have a lot of variation so adjustable bias is very useful to install.
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Old 14th January 2013, 08:50 PM   #7
euro21 is offline euro21  Hungary
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Triode mode E810F is a good tube for small signal amplifiers .... but large signal operation distortion a bit high, in many case twice the D3a or E280F distortion.
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Old 14th January 2013, 08:55 PM   #8
cotdt is offline cotdt  United States
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Quote:
Originally Posted by euro21 View Post
Triode mode E810F is a good tube for small signal amplifiers .... but large signal operation distortion a bit high, in many case twice the D3a or E280F distortion.
Which do you prefer, D3a or 7788?
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Old 14th January 2013, 08:59 PM   #9
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Thank you
I will look into the D3a.

What interstage impedance and ratio would you recommend for the D3a ?
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Old 14th January 2013, 09:07 PM   #10
cotdt is offline cotdt  United States
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Quote:
Originally Posted by manp111 View Post
Thank you
I will look into the D3a.

What interstage impedance and ratio would you recommend for the D3a ?
It would be the same as for the 7788. If you build the amp for 7788, it would work for the D3a as well. The D3a has a bit more gain.
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