SE el84 with tremolo

[j-stylez]

Hello,
I have a question about this schematic I found regarding a small tube amp with 2 small tubes and an el84.





I dont understand the section of the circuit starting from 100k trim pot going to the 2 halves of the ECC-85. I read through some information on valve tremolo circuits (The Valve Wizard- Tremolo Oscillator), but I dont understand how this one works.
Why is there nothing connected to the grid? Also, what is the left half of the ECC-85 doing, and how does it work together with the first half of the ECC-83?
Why is there an "overdrive" control within what I thought was the oscilattor circuit?

Any ideas are highly welcome! Thanks for your help.

[tubeswell]

Hi niggez, where's the schematic?

[j-stylez]

you should see it now.

[tubeswell]

Where did you get that schematic? Some of the stuff about the trem circuit looks wrong (the grid of the LFO stage isn't connected up to the RC network, and there is no grid load on the CF stage for starters)

[j-stylez]

It was from the build report of some guy who built it...I was searching for a simple design with tremolo and an el84. I tried to contact him but there was no way to do that.

If I want to incorporate this type of circuit:


Is it correct that the wave connects to the cathode of the first gain stage, as in the schematic of my first post? I dont understand why...

[d95err]

The oscillator modulates the bias of the first gan stage, which in turn modulates the gan of the stage. It is the same kind of tremolo as in the Fender Vibro Champ or Marshall "18 watt".

[j-stylez]

Oh I think I understand. I looked at the Vox AC4 and the tremolo looks very similar. It also shows the grid connection that I was wondering about. (It was missing as you also pointed out)

The Vox ac4 seems like the thing I was looking for. Am I correct that the ef86 used in the ac4 is wired as a single triode gain stage?


So, if I plan to make something similar to that, but using a dual triode for the first valve, I could add another gain stage to the design.

[j-stylez]

One more question that I cant seem to figure out. I want to use transformers I salvaged from an old tube device. This is a schematic of the transformers:



The power transformer is clear, but I am wondering about the OT shown on the left side of the picture. I dont understand what the extra windings on the primary side are for. In case you dont speak german, it is labled 2840 turns (550 ohm) and 30 turns (8,5 ohm) on the primary side.

The anode of the power tube was previously connected to point 3 on the OT transformer.

[tubeswell]

The Vox ac4 seems like the thing I was looking for. Am I correct that the ef86 used in the ac4 is wired as a single triode gain stage?

It is wired as a pentode (with a 5M6 screen supply resistor and a .1uF screen bypass cap). If it was wired as a triode, these bits would be missing, and the screen would be connected to the plate, possibly with a smallish screen grid stopper.