Help: Interstage Transformer for 7788 (E810F) Tube in Triode ?

[HP8903B]

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 ?

 

[cotdt]

Hashimoto A-105 would be a great choice. It can handle 30mA.

 

[HP8903B]

So a 5k 1:1 interstage ?

What makes you think that this would be a great choice ?

 

[cotdt]

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.

 

[HP8903B]

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.

 

[cotdt]

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.

 

[euro21]

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.

 

[cotdt]

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?

 

[HP8903B]

Thank you
I will look into the D3a.

What interstage impedance and ratio would you recommend for the D3a ?

 

[cotdt]

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.

 

[HP8903B]

Thank you
I will try both

 

[euro21]

Which do you prefer, D3a or 7788?

I use D3a, E280F (triode mode, mostly 10mA CCS load) as amplifier's first tube. I have about prefer of them Siemens E280F. Best holographic space, microdynamics and clarity -IMHO-.

I tried 7788 (I have four Philips SQ, and two Valvo pairs, best of them Valvo -green print Philips Herleen made-), but not of my cup of tea. It seems a bit boring.

 

[cotdt]

Ah I see. Thank you. I also have the Siemens and love the sound.

 

[HP8903B]

What do you think would be good bias voltage and current for the D3a to drive a 45 tube ?

 

[cotdt]

What do you think would be good bias voltage and current for the D3a to drive a 45 tube ?

Try the 10mA to 25mA range. It sounds a little different, so you have to decide for yourself what you like. It tests lower distortion at higher bias currents, but the sound can get a little edgy (excellent realism, but more fatiguing after a period).

I have capacitor coupling, with interstage coupling it will sound different. Interstage sounds sweeter, has more harmonics.

 

[euro21]

What do you think would be good bias voltage and current for the D3a to drive a 45 tube ?

Depends on the interstage.

For example if you use Lundahl LL1660 (4:4.5) 10mA, D3a bias 2V (in my practice simple green LED in the cathode), anode voltage about 175V. B+ about 185V.
Amplification about 50.

 

[Seta45]

7788

Hi,

I ran a 7788 triode connected through a Magnequest RIT-5 into a 45; worked well.

 

[TerribleT]

Hello Seta45,
Can the Rit-5 jr be used? Please post a schematic if you can.
Regards,
David

 

[Seta45]

Hi,

Attached is a copy of the schematic; the 7788 was later replaced with an EC8020 which "sounded better". The RIT-5 Jr will handle 85v peak so you should be fine.

b

 

[tcqanh]

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.

I used both D3a and 7788. They are great for power tube which need below 40-50V gain. IT 5k:5K is so good.

In your case, Pout 5W is quite impressive . how much is Pa of tube 45 ?