Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: misterc57 on December 24, 2025, 01:06:06 pm

Title: Vox AC30/6 Tremolo Woes c.1960
Post by: misterc57 on December 24, 2025, 01:06:06 pm
Vox AC30/6 Tremolo Woes c.1960

Hey Folks.  Beed trying to get the tremolo to function on an old AC30/6. 

There are two tubes in play. 

Replaced many caps and resistors, swapped tubes.  Installed a red LED in place of the cap/resistor at pin 3 cathode of the oscillator.

The circuit needs to be grounded via the footswitch which is missing so I hardwired a ground to the circuit at the proper 1M resistor.

Worked on this about two months ago and had it working briefly with the LED blinking in unison but eventually it slowed down and stopped.

ECC82 modulator with the following readings

pin 1  75   VDC
pin 2
pin 3  3.4  VDC
pin 6  75   VDC
pin 7
pin 8  3.4  VDC

ECC83 oscillator with the following readings

pin 1  220   VDC
pin 2
pin 3  1.5    VDC
pin 6  198   VDC
pin 7  12.5  VDC
pin 8  21.6  VDC

I am baffled by the ECC83 pin 7 and 8 readings.  They seem to feed from the ECC82 pin 2 which has no voltage so I am not sure where the voltage is coming from
but I see the VDC at R71, and I do not think there should ever be DC on a grid.

R71 to pin 7 (green wire) is 3 components to the right of the trim pot lower center of photo.

Schematic I am using courtesy of Jennings is attached.

Any thoughts and help would be greatly appreciated. Thank you.  Mark
Title: Re: Vox AC30/6 Tremolo Woes c.1960
Post by: pdf64 on December 24, 2025, 04:08:57 pm
V9 pins6,7,8 triode is a cathodyne circuit, it takes the oscillator output and creates identical but opposing polarity signals at its anode and cathode.

Scroll down to 'wurlitzer vibrato' for the circuit's theory of operation https://www.effectrode.com/knowledge-base/history-of-vibrato/
Title: Re: Vox AC30/6 Tremolo Woes c.1960
Post by: misterc57 on December 27, 2025, 08:53:14 am
Interesting and I read through that link.  Thank you

So that part of the circuit seems okay?

Can anyone advise on why the tremolo pulse is not engaging?  Thank you

Title: Re: Vox AC30/6 Tremolo Woes c.1960
Post by: pdf64 on December 28, 2025, 12:21:18 pm
Maybe one or more components have gone bad. eg R54-59, C22-27?
Title: Re: Vox AC30/6 Tremolo Woes c.1960
Post by: misterc57 on January 02, 2026, 08:10:51 am
Quote from: pdf64 on December 28, 2025, 12:21:18 pm
Maybe one or more components have gone bad. eg R54-59, C22-27?
  Thank you.  I have stepped through the circuit numerous times, many components replaced.