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Old 26th May 2021, 1:05 am   #10
Join Date: Feb 2016
Location: Tauranga, New Zealand
Posts: 118
Default Re: Advice on this linestage build

Originally Posted by Robert Gribnau View Post
The EF86 was designed for low level audio applications. It has a bifilar filament (like the EF40 and the CF50 before it) to minimize heater hum. With respect to noise and microphonics I think they are rather good. The EF86's I use in the first stage of two of my guitar amplifiers are not prone to microphonics (but they are stand-alone amplifiers and the filaments are being fed dc voltage).

The EF184 was designed for the IF stages of televisions, so also mostly for low level signals. But they were not designed with audio in mind (as far as I know, they do not have bifilar filaments). I know that Radford used them in their STA-100 amplifier as phase splitter but there they operate with high input signals.

The forerunner of the EF184 was the EF80. Some two years ago there was a discussion on this forum about its usability for audio:
Interesting what you say about the EF184, I have no idea if it has a spiral heater or not.
I guess going to DC heaters could solve this?
I know both the EF80 and EF184 are popular tubes with builders.

Originally Posted by Robert Gribnau View Post
What you perhaps could do is use the schematic of the passive tone control on the site you linked to, preceded by an EF86 in triode mode + a 12AU7 cathode-follower (dc-coupled). The EF86 gives you a gain of about 20, so of about 26 dB, while the tone control takes 20 dB off of that. You than have about 6 dB headroom.
I was actually wondering about that myself.
I had also considered using it as the first gain stage, but I think impedance matching would be an issue?

From what I understand, with a line stage, particularly if you are using it as a buffer with little gain, is you want to keep output impedance as low as possible, but able to match various loads on the input?
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