JCM 800 Plexi 2204 build Low Volume

[Varginator]

This is my second build ever and at no point do I claim that I know what I'm doing. That being said I was able to wire everything up and the amp fired up. When I turned the volume up I noticed that it is not loud at all. I found a few forums stating the capacitor at V3 should be 470 ohms. I put in a 47pf capacitor like the diagram states. Any help would be great.

My first build I had to bring to a guy that works on amps to get it to work. If I cant figure this out I will do the same thing.

[cbass]

470 ohms would be resistance. I'm not a pro but as far as I know that 47pf cap is there to bleed of high frequency. So a larger value cap would bleed off more lower frequencys

Someone should be along soon that knows a lot more than I.

[Varginator]

I Did try to switch it out but it actually lowered the volume so that makes sense. Any help would be great! I know I'm close just defiantly a novice, but I enjoy the challenge!

[cbass]

470 ohms is not a measurement for a capacitor.
Ohms is the measurement used for resistors. So im confused when you say a 470 ohm capacitor.

[Varginator]

Like I said it was a suggestion I saw on another forum which was bad information. If I removed the capacitor completely would that increase the volume?

[cdemike]

These might help narrow down the cause of the issue:
-How low is the volume? Still loud enough to play over a drummer, or little 1W amp loud?
-Have you checked that all of your joints are good? I like a trick I learned here that involves working on the amp with all the caps drained and removing all the tubes. Then you use a spare strip of wire connected to alligator clip leads to the multimeter and plug that into your tube sockets. Check resistances from there to the various leads to and from the sockets. You can do the same with plain multimeter leads to and from the tone stack, to check continuity to ground (both where there should and shouldn't be continuity to ground), and to check if your heaters are all connected well.
-What are the voltages like in the amp with the volume and gain at zero? What about 10 with nothing plugged in? And what about with the guitar plugged in?
-Does the amp get louder with higher settings then get quieter at very high settings?
-Did you change the circuit at all?
-Does the volume issue improve if you disconnect the OT from the NFB network? Quickest way to check would be to just un-solder R22.

The 47pF cap, as cbass pointed out, is just to cancel out very high frequencies at the phase inverter. Some call it the "anti-fizz" cap. So that shouldn't cause any issues unless you have a very defective cap there (measuring much, much higher than 47pF). And that would cause low volume at all settings. Some of the loudest amps in common use--100W Marshalls--use these, so there shouldn't be any volume drop associated with it.

The 470 ohm resistor is a different story and helps bias the phase inverter, so if that's gone that could be an issue. Looking over your pictures I'm seeing a somewhat strange mixture of parts on the plates of the phase inverter -- just to confirm, the plates are wired to resistors of the correct values?

[pdf64]

The cathode bias resistor on V3 LTP should be 470R, whereas the value that seems to have been fitted looks like 470k.
The effect that would have aligns with the reported symptoms.

[cdemike]

The cathode bias resistor on V3 LTP should be 470R, whereas the value that seems to have been fitted looks like 470k.
The effect that would have aligns with the reported symptoms.

Good catch. Just noticed that mistake is there also in the layout portion of the layout provided at the bottom of the original post as well.

[Varginator]

These might help narrow down the cause of the issue:
-How low is the volume? Still loud enough to play over a drummer, or little 1W amp loud?
-Have you checked that all of your joints are good? I like a trick I learned here that involves working on the amp with all the caps drained and removing all the tubes. Then you use a spare strip of wire connected to alligator clip leads to the multimeter and plug that into your tube sockets. Check resistances from there to the various leads to and from the sockets. You can do the same with plain multimeter leads to and from the tone stack, to check continuity to ground (both where there should and shouldn't be continuity to ground), and to check if your heaters are all connected well.
-What are the voltages like in the amp with the volume and gain at zero? What about 10 with nothing plugged in? And what about with the guitar plugged in?
-Does the amp get louder with higher settings then get quieter at very high settings?
-Did you change the circuit at all?
-Does the volume issue improve if you disconnect the OT from the NFB network? Quickest way to check would be to just un-solder R22.

The 47pF cap, as cbass pointed out, is just to cancel out very high frequencies at the phase inverter. Some call it the "anti-fizz" cap. So that shouldn't cause any issues unless you have a very defective cap there (measuring much, much higher than 47pF). And that would cause low volume at all settings. Some of the loudest amps in common use--100W Marshalls--use these, so there shouldn't be any volume drop associated with it.

The 470 ohm resistor is a different story and helps bias the phase inverter, so if that's gone that could be an issue. Looking over your pictures I'm seeing a somewhat strange mixture of parts on the plates of the phase inverter -- just to confirm, the plates are wired to resistors of the correct values?

-The volume is sounds like its set at "2". Loud enough to play with but not loud enough to play over a drummer.
-All the joints seem to be fine
-Yes I believe all the values are correct.

The cathode bias resistor on V3 LTP should be 470R, whereas the value that seems to have been fitted looks like 470k.
The effect that would have aligns with the reported symptoms.

Good catch. Just noticed that mistake is there also in the layout portion of the layout provided at the bottom of the original post as well.

Should I try using a 470R at the V3?

[cdemike]

There is a key value that's incorrect. You currently have a 470K resistor where there should be a 470 ohm resistor. The layout for the phase inverter and schematic in the schematic document is correct, but the second layout is not. So yeah, the 470R at V3 should fix your issue.

[Varginator]

Changed the resistor today to a 470 ohms. Fired it up and its STILL not as loud as it should be. it did make a little difference. Any ideas?

[Varginator]

I got it fired up and Working!!!!! Thanks for the help, I checked the readings on the resistors and had those few wired up wrong. Its wicked loud now!!!

[cdemike]

Congrats!