Ampexs 620 suitcase amp low output

[Vyvyan]

I've been working on modifying my ampexs 620 to the provided schematic I got from another forum here: Ampex 620 speaker/amps

Was wondering what the power resistors in parallel are for. Everything is working but way less volume than before. I usually play guitar thru another tube pre into the ampex, but don't have any other pres on me at the moment so maybe that's all I need

But I Was wondering what the power resistors in parallel are for? And in general how would you recommend I squeeze some more volume out of the circuit?

 

[Khron]

But I Was wondering what the power resistors in parallel are for? And in general how would you recommend I squeeze some more volume out of the circuit?

If you mean the ones in the power supply section (bottom of schematic), they're there to form RC low-pass filters in conjunction with the capacitors to ground, to filter out noise, and as a more-or-less intended consequence, also dropping some voltage.

Regarding volume, you could try adding an electrolytic capacitor in parallel with R303, maybe 22uF to start with, and go up in value if you don't have enough bass. Perhaps even a treble bleed capacitor between the input & wiper of the "volume" pot?

 

[Vyvyan]

If you mean the ones in the power supply section (bottom of schematic), they're there to form RC low-pass filters in conjunction with the capacitors to ground, to filter out noise, and as a more-or-less intended consequence, also dropping some voltage.

Regarding volume, you could try adding an electrolytic capacitor in parallel with R303, maybe 22uF to start with, and go up in value if you don't have enough bass. Perhaps even a treble bleed capacitor between the input & wiper of the "volume" pot?

Thanks, I was just confused as to why they were in parallel rather than just using a single resistor of their combined resistance.

 

[Khron]

Also economies of scale, and spreading out the power dissipation.

 

[radardoug]

Cap on R303 will short out the nfb and increase gain. Maybe OK for a guitar amp.

 

[Vyvyan]

Cap on R303 will short out the nfb and increase gain. Maybe OK for a guitar amp.

thanks im just about to start working on this. thinking of putting a pot in place of the 15k nfb just to find where i like it best and measuring the resistance and possibly using a switch for the r303 bypass cap

 

[MagnetoSound]

“I usually play guitar thru another tube pre into the ampex, but don't have any other pres on me at the moment so maybe that's all I need”

Yep, what you have there is a speaker amplifier with a line input.

 

[Bo Deadly]

What you mean by it has "less volume than before". Did you modify the circuit at all? How exactly?

That circuit looks like it has at least 20dB of gain to and can probably make about 10W. That's should be plenty loud. And you should be able to plug the guitar in directly.

 

[Vyvyan]

What you mean by it has "less volume than before". Did you modify the circuit at all? How exactly?

That circuit looks like it has at least 20dB of gain to and can probably make about 10W. That's should be plenty loud. And you should be able to plug the guitar in directly.

Here's the original schematic but there was a Hammond transformer on the input that I'm guessing someone added at some point it's a Hammond 835 here's the only info I've found https://www.mouser.com/ProductDetail/Hammond-Manufacturing/835?qs=1ADZffG2MqvTeKKesWPBlQ==

Maybe I should try adding it back into the circuit?

 

[Hubbub]

If you are looking to drive this from a high impedance source do not add the Hammond 835 back in. If you have a good low impedance (1000 Ohms or less) source use the 835 wired 10K in, 40K out, which will give you 6dB gain there. Also note that putting 22uF across R303 cancels all negative feedback, taking the amp virtually open loop. Probably not pleasant. A better approach is to use a 2.5K Ohm pot in series with the 22uF cap, then across R303, giving you a range of gain adjust.