VOX AC30 1971 Birch Stolec Reverb Tank Problem

[Input-Output]

Hi everyone,

I have a VOX AC30 1971 Birch Stolec with reverb but there is no reverb tank in the amp.

I'm having trouble looking for the correct reverb tank for this amp and it isn't the usual 10 Ohm input 2275 Ohm output as the driver for the reverb is off the anode via a capacitor so this makes it high impedance.
The recovery is the standard circuit.

I'm also thinking that perhaps the capacitor goes into a transformer and then drives the reverb in the usual way but I was unable to find any transformer of this sort on the chassis or on the cab.

Has anyone got any ideas what the model number is for this reverb?

Link to the circuit bellow of driver and recovery attached for the Birch Stolec 1971 amp, not the 1978 later amp.

https://www.dropbox.com/s/ofaxktwrwn...54.23.png?dl=0

Thanks,
I-O

[AttachMan]

Local copy:



Full schematic PDF:
http://music-electronics-forum.com/a...8&d=1499706324

[Jon Snell]

A photo and schematic of the unusual AC30 would be useful.

Ah ha! The new look site is the other way up. Eg; Newest posts at the bottom of the screen.
Now I see the schematics etc.

[Input-Output]

Hi,

The schematic with the circuit in question has been kindly added above, the rest of the circuit is standard VOX AC30 stuff.
I cannot attach photos at the moment but here are links to the inside of the amp.

https://www.dropbox.com/s/qbmdivw1hu...819D.jpeg?dl=0
https://www.dropbox.com/s/uati4m29gz...5035.jpeg?dl=0
https://www.dropbox.com/s/47itd1id8o...0819.jpeg?dl=0

Thanks
I-O

[Jon Snell]

Accutronics makes a replacement; 9EB2C1B Reverb Tank, it is 17" Long and has an 800 Ohm input impedance.
I include a youtube video on the tank.
It's a bit cheesy!
https://www.youtube.com/watch?v=O_OMz2Tq0uY

[AttachMan]

Local copies:

[Input-Output]

John,

Thank you for the reply, the guy in the video has a C2 VOX which looking at the circuit diagram has the standard setup with a step down transformer in the anode of the valve, my amp is a 'AC30 TB Rev'. The problem here is that the driver is off the anode via a capacitor so all high impedance. May be the 800 Ohms would help but I've just thought of a trick, I'll try and find a step down transformer, attach it to the cap and the secondary to the reverb unit and see if that makes any difference.

[Jon Snell]

The Accutronics 9EB2C1B has more options. Here is an explanation.


Hope that is useful.

[Pedro Vecino]

It´s possible that with this system (input from the anode through a capacitor) the tank has the highest input impedance. I have remembered that the Carvin Legacy use this system (I do not remember the tank model) and after searching a little I have found these two references:
https://www.tubeampdoctor.com/en/9gb...an-3x2-springs
https://www.banzaimusic.com/accutronics-8gb2c1b.html

In both the letter "G" appears, corresponding to the highest input impedance. Here's more information:

https://music-electronics-forum.com/...nk-code?t=6597

[Helmholtz]

I would definitely use the "G" type. The reverb drive transducer is a motor and needs a little power. Lower input impedance means less driving power, especially as the single ECC83 driver has quite high output impedance. Also the low value coupling capacitor suggests high impedance.
The circuit shown has no separate rev recovery stage, the rev output directly feeds the LTPI. So you probably want as much reverb drive as possible.

[vintagekiki]

Vox AC-30/TB "Stolec" in his reverb pan uses piezo cartridges as drive and receive transducers.

https://music-electronics-forum.com/forum/amplification/guitar-amps/vintage-amps/45198-vox-ac-30-tb-stolec-needs-mystery-reverb-tank?t=44067

http://www.voxshowroom.com/uk/misc/reverb_hood.html

http://i.imgur.com/PT6Ksk1.jpg

[Helmholtz]

Vox AC-30/TB "Stolec" in his reverb pan uses piezo cartridges as drive and receive transducers.

That explains a lot. What a poor solution!

[vintagekiki]

Some old Ampeg also uses a piezo reverb unit.

https://usermanual.wiki/Collections/magnet/files/9365%20Audio%20Schematics%20and%20Service%20Manuals/Ampeg_gv22_gemini22.pdf

https://usermanual.wiki/Collections/magnet/files/9365%20Audio%20Schematics%20and%20Service%20Manuals/Ampeg_V-2_(6-71)_Schematic.pdf

https://usermanual.wiki/Collections/magnet/files/9365%20Audio%20Schematics%20and%20Service%20Manuals/Ampeg_V-4,_VT-22_Schematic.pdf

[Input-Output]

Hi everyone who replied!

I will go through each one of your posts and reply back.

Thank you!
I-O

[Input-Output]

Vintagekiki,

Thank you very much for the links and write up, I looked on the voxshowroom website but clearly didn't dig deep enough. Interesting history behind the development of the unit and yes, the 'recovery' is the LTPI.
I'll play around with a high to low impedance transformer connected to the capacitor and then to the input of the reverb unit and see if this works.
I guess even a 8K input on a reverb tank might not be enough for this setup as piezo cartridges are higher.

Once again, thank you to everyone.

I-O