The ShadowMusic Web Community

Did JMI ever produce an AC30 with a solid state rectifier?

I have an AC30/6 Top Boost to repair (output valve with spectacular internal shorting). It has a copper colour control panel with 'A JMI Product' lower right and a rotary voltage selector. The metalwork is clearly original and has no hole at the location where the valve rectifier is normally located. There is no rating plate as it has been re-covered since the current owner purchased it second-hand in, he thinks, 1966 or 1967 and the rating plate was not re-fitted. There is a paper label on the chassis with the code 'AC30 7410D1299'.

Any suggestions as to the vintage of this amplifier?

Eric

According to Stephen Grosvenor's AC30 Service Guide, 1968-9 is the first time solid state rectifiers appear. He doesn't give any info on later build dates. The 1990 circuits here have SS rectifiers too -- http://www.voxamps.com/support/circuits/

Vox, I know but I couldn't locate any JMI info

Any other clues? Type of speakers, handles, vents etc

Roger

Hi Roger,

I too have consulted Stephen Grosvenor’s excellent publication for some guidance. In addition to the features already given the loudpeakers are Vox Blue, the handles are plastic as are the ventilators of which there are three.

Here's a picture of the chassis.

Eric

Hi Eric
The guys on the Vox board usually date amps down to the hour
http://vintageamps.com/plexiboard/viewforum.php?f=2&sid=530b94437495f4f17213cb270aebd391

When you find out please come back here and let us know.

Erik

“The guys on the Vox board usually date amps down to the hour”.

Well, not quite but the paper label on the chassis identifies it as manufactured by Dallas Arbiter in October 1974. So it looks like the chassis has been changed at some time but the original JMI control panel retained.

So it looks like I have a hybrid with a 60s JMI cabinet (albeit re-covered) and Vox Blue speakers and a 70s Dallas Arbiter chassis.

Eric

“The guys on the Vox board usually date amps down to the hour”.
Well, not quite....

Hi Eric
Would proably have helped if you had been able to post pics

Your conclusion could well be right - but then again, a bit odd. No offence, but how well do you know/trust the owner, memory can play a lot of tricks you know.

Erik

Hi,

The 1970 / 71 AC30 did include solid state rectification and PCB's for the
Preamp and poweramp stages, the power amp boards caused problems due to the heat from the
EL84's being conducted through the valvebase pins to the soldered joints which eventually caused
dry joints and eventually board damage due to Arcing and caused burn holes in them.
These amps were made by Stolec..

The chassis you have is indeed a CBS - Arbiter chassis, due to unreliability the PCB's of the 70 / 71
era were dropped and the construction went back to Tag boards as in the JMI amps. The solid state
rectifier was retained because it was cheaper than GZ34's and also pushed the output up to 45 watts.
These chassis started production in 1972..

This came at at the price of shorter EL84 life due to increased dissipation and valve matching was very
important in these amps to minimise this..

One other bit of info, if it had been a complete CBS / Arbiter amp then it would have Ceramic speakers
as opposed to Alnico..

Regards

Amanda