Vintage Ampeg ID help

[Tabl43]

Ok folks. I know there are some really knowledgeable ppl on here when it comes to Ampegs. Please tell me what I have here. I am aware of the 815 series of "Bass Amps", but what is this "Guitar Amp"? I can't seem to find any similar models especially with a gain switch.

 

[beans-on-toast]

some schematics here.

https://www.talkbass.com/wiki/schematics/#-pre-portaflex-amplifiers

Ampeg offered a lot of custom variations so finding the exact model could be difficult.

 

[Tabl43]

Thanks, I'll poke through those. It's just wild that I can't find another example of one with the gain switch or the labeling. My goal is the rehouse this in an appropriate cab.

 

[Tabl43]

....apologies, I just realized I wrote "815" above. I meant 835.

 

[gtmalone]

....apologies, I just realized I wrote "815" above. I meant 835.

The 835 was a Bassamp but Ampeg might have made a Guitaramp version.

https://reverb.com/item/37599240-ampeg-model-835-bassamp-around-1960-rare-historic

 

[Tabl43]

Ok, so I've rebuilt the amp keeping as much original as possible and built a temporary housing for it. Somebody had been inside it years back and things were not hooked up properly so I'm just working off the 625 schematic. I'm using that as wiring matches up better and it does not use an 0C3 as the 835's do.
The preamp is definitely out of a Dolphin, it has the extra 6SN7 stage and and components match up. One thing I cannot figure out (as it was disconnected), is how the "gain" switch was wired. I know for certain one side was tied to ground. I cannot find any other examples with this switch.
Any ideas?

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[gtmalone]

Ok, so I've rebuilt the amp keeping as much original as possible and built a temporary housing for it. Somebody had been inside it years back and things were not hooked up properly so I'm just working off the 625 schematic. I'm using that as wiring matches up better and it does not use an 0C3 as the 835's do.
The preamp is definitely out of a Dolphin, it has the extra 6SN7 stage and and components match up. One thing I cannot figure out (as it was disconnected), is how the "gain" switch was wired. I know for certain one side was tied to ground. I cannot find any other examples with this switch.
Any ideas?

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The gain switch is probably an input pad. There would be a resistor from the switch to one of the input jacks to load the input and reduce the level. Here's an example from a PF500. Ampeg used a DPDT swith in parallel for some reason. For the resistor value, I have no idea. Take a look at an amp with high and low gain inputs and see what was done there.

 

[Tabl43]

Hmm, that makes sense. There is also an input matching pot (also on the Dolphin schematic as optional), seems crazy to have both doing essentially the same thing.

 

[beans-on-toast]

In the early days, there were no pickup impedance standards. There were crystal mic and magnetic pickups. The input matching was used best bridge the pickup impedance being used to the input of the first tube stage. This isn't necessary with a modern pickup.

In this example the input pad is achieved by switching in a 47.5K resistor to ground that acts as a simple voltage divider.

 

[Tabl43]

Great! Thank you.
Since this thing isn't going in a museum, any suggestions as to what I can do in place of the impedance pot? Maybe a vol/master volume?

 

[beans-on-toast]

I would get it going first, then add whatever you want. A pre-amp out/power amp in is a possibility. See a B-15 schematic to see how that is done. Schematics are in the TB portalex wiki or Vintage Blue