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Messages - Loudthud

Amplifier Discussion / Re: Old SS Project Rebuild
March 18, 2021, 07:24:09 AM
I think you have it right. The only problem I see is the diodes from the output to the rails should be 1N400x.
Positive Gate Voltage turns Q315 off. This comes in through R330. Q316 is normally off, but it has a slightly positive base Voltage, so it is ready to conduct. When signal comes in via R335 or R337, Q316 starts to conduct lowering the Gate Voltage on Q315 which turns it ON lowering the Source to Drain resistance. C315 gives the whole thing a fast attack - slow release action.

R331, C314 unknown function, probably for fast attacks.

Note: When replacing semiconductors in an amp this old, double check the Base Emitter & Collector connections. Sometimes the original parts had non-standard pin-outs. Same with the JFET.
Normally the diodes CR301-304 don't conduct. If they do, the input signal is too big. When the signal at the output of the power amp gets high enough, the JFET turns on shunting the input which in turn reduces the output.

A too big signal on the input makes me think the gain of the power amp is too low somehow.

The gain of the power amp is set by R314, R313, R315 and C306. R312 helps set the DC gain (operating point) of the output. The pot R304 is used to adjust the DC (45V) on the output. The low frequency corner is about 30Hz, set by R313, R315 and C306.
Which side of R301 is the waveform from ? One side is essentially the same as the Line outputs, the other might be limited by the JFET compressor circuit.
I think it was mentioned in this thread that many of the connections on the PCB were re-heated and/or re-soldered. Was any flux from re-soldering removed ? I would use a chemical intended for flux removal or isopropyl alcohol, a tooth brush and paper towels. Pay attention to the areas around channel switch IC's.
Can the same be said about some of the more recent Marshalls ? A couple of threads about JCM2000's over at MEF right now. Seems like the quality of transformers used by Marshall has gone down. Has just the reliability changed, or has the sound changed also.
Preamps and Effects / Re: The JFET Bender Preamp
February 16, 2021, 05:26:53 PM
Currently, no-can-do... But I might be able to upgrade my backup phone. Fighting a winter storm right now,maybe when things thaw out next week.
Preamps and Effects / Re: The JFET Bender Preamp
February 16, 2021, 11:10:20 AM
I wish I did, but I'm just not set  up to do it. I could buy some equipment, but I'm not sure what the best way to go is. Got a couple of mics and a few PA mixers. Don't know anything about the digital end of things.
Google is telling me the schematic file you linked does not exist.

Found this thread:
Some amps use a scheme like this to detect when something is plugged in to mute the channel when nothing is plugged in. On this amp, it looks like they are providing power to some kind of active guitar or perhaps an accordion.  A normal phone plug will short it to ground, but it will send a jolt to your guitar if you plug in while the amp is on. Probably a good idea to disconnect it.
I'm not sure how much the Frontman 65 has in common with the Frontman 212R, but the 212R has problems with solder connections fracturing on the PCB. A close inspection under bright light and magnification might reveal the problem.
Tubes and Hybrids / Re: Help with Hybrid Design
December 31, 2020, 08:15:44 PM
It should be noted that when you hijack one of the primary windings to make high Voltage, you should de-rate the VA of the transformer by half.
Preamps and Effects / Re: Old newb questions
December 21, 2020, 12:31:09 AM
You can probably use an effects pedal as a preamp, but not all will be suitable. You don't need a DI box as long as the pedal has enough output to drive the power amp. A wall wart is a good choice for a power supply as long as it has enough current capacity for the power amp. It should be enough for both preamp and power amp. Both circuits can be built into the same box.

A WORD OF WARNING: If the power supply does not have a safety ground, you will probably have hum on the output of your amp. In addition, you may get a mild to moderate shock if you touch someone playing another amp when you both are touching your guitar's strings. BE SAFE, GET A POWER SUPPLY WITH A SAFETY GROUND !
Quote from: shasam on November 29, 2020, 09:50:50 AM
So I should use a circuit like the one in attachment, with no other base resistor?
In a lot of schematics in Douglas Self's book, he use the other circuit (base of the reference transistor connected on the VAS current source emitter). Do you know why, please?

I can't speak for Mr. Self but perhaps he doesn't listen to his amps when they are clipping.  A base resistor on Q90 or Q91 might tweek the temperature coefficient slightly or suppress any tendency of the circuit to oscillate.

Quote from: shasam on November 29, 2020, 09:50:50 AM
Do you have a suggestion for a faster turn on please?

Try one of these:
1) Replace C32 with a zener diode.
2) Replace R156, R157 and C32 with a high impedance current source.
3) Connect the bottom of R157 to the minus rail and change R156 to 22K. Connect the anode of a 1N4007 to the junction of R156, R157 and C32 and the cathode of that diode to ground. This will force C32 to charge fast to the rail Voltage, then slowly charge to about 1.5 times the rail Voltage.