Last post by phatt - December 04, 2022, 10:42:46 PM
Hi Joe, Yep can be confusing. My understanding, it's about the Z (impedance) of the input. A low Z input on guitar amps is for big signals or even line input. While a Hi-Z input is very sensitive so as to pickup very small signals. In the early days guitar PU's were low output and needed a very sensitive input to amplify well. (Hence the 1Meg grid resistor on the old Valve gear) These days even the cheapest guitars have much bigger outputs than the 40's 50's era so the preamps will redline much earlier on the dial using the pu's around nowadays.
The old Fender circuits used 68k resistors on the Grids of V1 for LOW input. while the HIGH input switched the input Z to 1Meg. far more sensitive.
With super hot pickups you are likely better served by limiting the input Z to about 220k. I've noticed this with some of the hotrod boutique Amps. A Carvin Legacy is an Example of this where in put grid R is 220k. With multi stage preamps this keeps thing from going stupid and also helps keep circuit noise lower. Phil.
Are you using two different guitars when you try the high and the low input ? Does one of the guitars have active pickups and the other normal pickups ? Some guitars have active pickups that have a battery and electronics installed in the guitars, they put out a larger signal then standard guitars with normal pickups. That's the reason why some amps have a High and Low input (as far as I know). One input is for the smaller signal of a normal guitar and the other input is for a stronger signal from an amplified active pickup guitar. This is not to be confused with amps that have more then one channel, usually one clean channel and one or more channels with distortion.
I think you will find that the amplifier is working like it should. Does one input sound louder then the other with the controls set the same ?
Last post by phatt - December 02, 2022, 10:02:52 PM
And after that I'd be looking into the limiter wrapped around the Power amp section, as it maybe faulty. The master volume switch might be connected to that as well ,, hard to tell. Also there is a feedback setup in the power amp but not connected to the output?? maybe a drawing omission.
I bought a used Gear4Music house brand 35W solid state amp. 2 inputs (High & Low), TMB EQ stack, Master Volume. The PCB has "BELCAT CORP. SED-03" printed on it.
I have no schematic for the circuit.
On the Low input, it works fine, but on the high input it distorts when I play hard. Particularly low notes.
What could be the fault? I am not an electronics expert, but I can certainly manage to replace a capacitor or resistor. I assume the High input only has a few extra components so that it will be easy to replace just those.
Of course, if anyone has the schematic on hand, that would be useful.
First check the voltage on C102 in both AC and DC. Should be approx 98V DC. Check the voltage on the speaker out with no signal both AC and DC. If you have a large DC on speaker out then don't leave a speaker plugged in.
I recently acquired an Acoustic 230 guitar head, sold "as is" for quite cheap. It did not include foot switch to initiate the EQ, reverb, or master volume circuits, so I rigged one up myself after finding a Jones 6-pin connector online. Upon testing the amp with my 2x12" cab (4 ohms/2x 8 ohm V30s), the preamp section appears to be find and sounds good, but the output level is noticeably quieter than it should be with an amp like this and the cab (when I run the cab with my Silvertone 1464, I can't play it above ~1 without it being too loud for my apartment). When the Master Volume channel is turned on, the entire output is dramatically reduced; like not much louder than iPhone speakers even when everything's maxed out. Patching the pre-amp out and power amp in around back on the unit with a 1/4" cable seems to make a barely noticeable improvement but nothing significant. I have tested the preamp with another power amp and it seems to be fine, so it's looking like the issue is in the power amp section somewhere. Not sure if it's worth mentioning, but the only other issue with this head is the on/off switch, which really needs a good punch sometimes to turn over, and sometimes doesn't for a few tries. Anyways, if anyone might have some insight to what is causing this issue of weak output in the power amp section and what components I should focus on testing right away as potential culprits, please let me know. Myself and a friend who is an amateur tech are in the process of doing some troubleshooting. Also to clarify, the output level does not increase or decrease over time or fluctuate; it remains steadily weak. Thanks in advance.