vox berkeley II 1081

[Mars Amp Repair]

hello folks,
I've looked through the posts for this model & Can't find any mention of what the power transistors are in this amp. They are T03 style, but the ones that were in this amp were strange plastic versions that had extra aluminum brackets to hold them tight to the heatsink.
Oh yeah I should mention, the pins on these weird replacement plastic transistors are not a pin, per-se, but a 1/2 curved metal. the original problem was when you tapped the heatsink, one of the transistors would act as though it was disconnecting.
We ended up soldering the transistors into the sockets & tightening the collector bracket...all to no avail. It appeared that on of the transistors was internally intermittent after using heat & freeze spray. At that point, we decided to stop mickey-mousing around with it & replace the transistors with the original T03 case type.
Unfortunately, the numbers have been worn off these strange transistors. I had thought they were the germanium variety, but I can't get the NTE121's (pnp germanium) to do much of anything but hum.
For grins I tried a couple of silicone transistors to no avail. These are transformer driven, so there isn't much else that can go wrong in this stage other that the biasing resistors.
Does anyone have any insight as to what the original transistors are supposed to be? BTW the 1081 schematic seems to be the closest to what we have, but the arrangement of the bias & other resistors in the power stage don't seem to match exactly. thanx, glen

[JHow]

I'm sure RG Keen will weigh in on this, but I got NTE 179 GE PNP to work on the output of my Thomas vox pathfinder. Later I swapped both outputs to MJ15016G PNP silicon, but you have to change the bias resistor to do this. R.G.'s site has a good write up on these amps called "Bulletproofing Thomas Vox" at:

New Page 1

I think most of the NPN silicon are 3055 equivalent.

Here's some additional (probably useless) info:

The orginal (dead) GE PNP in my pathfinder is labelled 86-5043-2, the V1101 schematic called for a 86-54090-2. These are vox numbers, so not very useful at finding a replacement. There is some Thomas Organ parts info here, but I don't see these numbers:

http://www.organservice.com/thomas/thomas.htm

I told you it was useless info.

[Mars Amp Repair]

Well, it's nice to know that you can get silicons to work in here. I tried that on a Rhodes power amp once & could not get the idle current to be low enough as well as since there was not bias tracking, the thing would go into thermal run-away. Running a business that has 4 techs & backlogged 2wks doesnt' allow much time for messing & modifyiing one amp, so sometimes having all the ground work done helps alot. I'm just a very busy guy these days.

The links you have on your reply take you to either an effects type page with no reference to 'bulletproofing...' and the other takes you to a Thomas Organ page, but I can't find it there either. could you be more specific or possibly have your link go directly to the page?

thanx, glen

[JHow]

For some reason I can't link to the sub page, but if you go on geofex, then follow link to the "tube tech page", then go "Bulletproofing Your Thomas Organ Vox" you will see RG Keen's article. He describes possible alternate transistors PNP and NPN and how to go from ge to si. I had to try several resistors to get it right but ended on 18 ohms.

[tboy]

Here's the direct link to RG's article:

Thomas Vox Solid State Amps

[R.G.]

hello folks,
I've looked through the posts for this model & Can't find any mention of what the power transistors are in this amp. They are T03 style, but the ones that were in this amp were strange plastic versions that had extra aluminum brackets to hold them tight to the heatsink.

Someone's worked on it before you.

Unfortunately, the numbers have been worn off these strange transistors. I had thought they were the germanium variety, but I can't get the NTE121's (pnp germanium) to do much of anything but hum.

Probably because the first "repair" mucked with it to get the replacements to work.

Does anyone have any insight as to what the original transistors are supposed to be? BTW the 1081 schematic seems to be the closest to what we have, but the arrangement of the bias & other resistors in the power stage don't seem to match exactly.

Yep. That about nails it. Somebody mucked with it before.

Put in silicons of the correct polarity and change the biasing. These are biased by a low impedance bias from the resistor divider chain. All that you really have to do is change the smallest resistor on the divider chain for each output to a somewhat higher value, usually under 22 ohms. If you just do the math to calculate the base bias voltage from the stock divider, it's going to come out about 0.1 to 0.2V, for the germaniums. Silicon needs about 0.4V in this kind of setup to get to just-barely-on.

Well, it's nice to know that you can get silicons to work in here. I tried that on a Rhodes power amp once & could not get the idle current to be low enough as well as since there was not bias tracking, the thing would go into thermal run-away.

It works because of the very low resistance of the bias string. Quite different from the usual way modern amps are biased. Thermal runaway is still possible - the Thomas amps do it sometimes - but just swapping bias resistors can get you there.

[lowell]

Got one of these in for repair. Cool amp! Do any of you have a schematic avail?

[JHow]

Got one of these in for repair. Cool amp! Do any of you have a schematic avail?

I do, but I'll have to get it at home. I think it's available online as well. If nobody posts it I'll get it to you later tonight.

[DrGonz78]

Here it is...

[lowell]

Awesome thanks! This thing sounds amazing. One of the best ss amp overdrive sounds I've heard. I like my overdrive. It's also an extremely simple circuit, which I'm fond of as well.