Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TheGamingFloppy on November 12, 2025, 01:43:58 pm

Title: Need help with my PCL84 amplifier/radio project
Post by: TheGamingFloppy on November 12, 2025, 01:43:58 pm

Hi everyone,
I’m 15 years old and recently got interested in vacuum tubes. I’ve already built a stereo amplifier using two PCL84 tubes and an EM84 magic eye as a VU meter — it works great and sounds good.

After that, I found an EBF80 and an EBF89, and since I know they were used in AM/FM radio receivers, I thought it would be cool to build a tube AM/FM tuner to go with my amp. I’d like to make it so that I can switch between AM and FM with a selector switch, just like in vintage radios. The problem is, I can’t find any suitable circuit diagrams for the EBF89, so I don’t know how to start designing the tuner section.

For the amplifier part, I also found an EL84 and an ECC40, and I’d like to use them for a subwoofer amplifier. My idea is this:

- Use the ECC40 to mix the left and right channels into mono.
- One triode of the ECC40 would preamplify the left channel, and the other triode would preamplify the right.
- Then I’d combine the two signals, send them through a low-pass filter, and feed that into the EL84, which drives the output transformer and the subwoofer.

My problem is that I can’t find any schematic for an ECC40 + EL84 combination like this, or for how to properly combine both stereo channels after the ECC40 stage. I’d like to avoid mixing them before the preamp stage because I’m afraid that would cause crosstalk between channels. Also, I’d like to use both triodes of the ECC40.

If anyone has experience or ideas about:

- The correct way to mix the stereo channels into mono after the ECC40, and
- How to design the entire subwoofer amplifier stage using the ECC40 and EL84,
- or if you have example schematics for EBF89-based AM/FM tuner circuits (with AM/FM switching)

I’d really appreciate any advice or example circuits!

The linked video shows the current state of my project — it’s still in the testing phase. I’m currently using the only schematic I could find for the PCL84 amplifier section, while the EM84 VU meter circuit was designed by me.
The light bulb you’ll see in the video is there because I accidentally overwound the 6.3V filament winding on the power transformer — it gives about 6.8V under load, only for the testing i connected a bulb to help bring it down to around 6.4V under load. (Update: I managed to fix the transformer winding)

Here’s a video showing the current state of my amplifier:

Thanks in advance!