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Messages - Loudthud

#1
Honey Amp / Re: Transformer Question
July 09, 2024, 02:21:03 AM
After rectification and a filter cap, the output will be close to 24VDC. To get 16VDC, you will need some kind of Voltage regulator.
#2
Matched components are not really needed and are a waste of money unless you get them from a trusted source.

If you are using ESP's circuit boards, make sure you don't mix up the two R24s and Q12s. The amp won't work and you will likely damage several components.
#3
Generally, to prevent hard saturation of transistors (the kind that takes significant time to undo) you install a Schottky or Germanium diode between Base and Collector. I question whether it makes an audible difference in an audio circuit. The way this guy has installed the diodes, it makes sure any big negative input is able to degrade the Base-Emitter junction. 
#4
Quote from: Randy Lahey on May 19, 2024, 06:35:53 PMTransistor pin resistance measurements:
Original Gate - Source: 550 ohms
Original Gate - Drain: 306 ohms
Original Drain - Source: 300 ohms

New Gate - Source: 11M ohms
New Gate - Drain: 320 ohms
New Drain - Source: 10M ohms

The Gate should measure like a diode to Drain or Source. If you measure from Drain to Source you should see the ON Resistance of the JFET (a couple of hundred Ohms). If you short the Gate to one of your meter leads while you are measuring resistance from Drain to Source, the resistance will go down slightly on one lead and rise slightly on the other lead. Those readings of 10M or 11M are probably just the resistance of you fingers across the leads.

From your schematics with the Volume pot at min and max it looks like the JFET is installed with Gate and Source reversed. It may pass a little signal but will have a gain less than one.

See my comments in this thread: https://music-electronics-forum.com/forum/amplification/guitar-amps/build-your-amp/debugging-your-build/20115-please-help-me-out-debugging-my-ruby-bassman-amp
about how to Ohm-check a JFET out of circuit. Note that with the vast majority of JFETs, the Drain and Source are interchangeable.
#5
Were the Voltages in post #2 before or after you changed Q2 ?

You didn't say that you Ohm-checked the new or old Q2. Do you know how ? There is no reason for the pin-out to be different if the new part in an actual 2N5484. Is it a Chinese fake ?
#6
Q2 is not conducting any current. Were Voltages measured on the resistors or directly on Q2's pins ? Verify that the Source of Q2 is connected to R9 and that the Drain is connected to R8. Did you install Q2 facing the correct direction ? Did you Ohm-check Q2 before installing ? Did you Ohm-check the bad Q2 ?
#7
Amplifier Discussion / Re: Switching Jacks Question
April 10, 2024, 03:06:02 PM
Not sure what you want to do exactly.

Is the combo amp like a Fender where the internal speaker(s) just plug into the back of the amp chassis ?

Are either of these amps tube amps ?

Franken amps are notorious for poorly done grounding. Do you get any hum when you just connect a wire between the chassis' ?

You might risk damage to one or both amps unless they are both OFF when you plug the cable between them. Can you do that or do you need to make this idiot proof ?
#8
Amplifier Discussion / Re: Acoustic 134 reverb
March 22, 2024, 12:51:24 AM
The Acoustic tube amps probably use the same tank as the tube Fender amps with an 8 or 10 Ohm input impedance.

Check the 230 or 330 amp schematics to see if the drive circuit is similar to the 134. That would indicate that the 1BB1D1B is the correct tank.
#9
Amplifier Discussion / Re: Watts vs Volume (db)
March 20, 2024, 07:27:43 PM
What is VAR and what does it have to do with anything ?
#10
Problem 2 would seem to be Q9, part of IC2. I think you can still get these parts at Jameco in the USA. Check the -15V supply for ripple, it comes from A1A.
#11
Probably a bad connection on the preamp circuit board.

Do you have the schematic correct for you amp? Post it or a link.
#12
With a DVM, first check a cap in circuit for a short or very low resistance. Then, remove from circuit and check for a short again. Normally when you check a cap for resistance, at first it will seem like it's a low resistance but as the cap charges up, the resistance reading will increase until the DVM indicates over-range. If the DVM can measure capacitance, try that to see if you can get a reading close to the value and tolerance of the cap.

Verify that any rectifier(s) are good with the DVM's diode check feature.

Check for shorts around the power transistors.

A light bulb limiter will usually prevent fuses from blowing. A Variac is a better option, but should have a way to measure current to avoid blowing fuses. Sometimes you can use the Variac to apply a very low Voltage to the amp and find which circuit or part of a circuit is drawing excessive current. This is a procedure for a qualified and experienced technician. 

You should not connect a speaker to the amp until it can operate without blowing fuses and it is verified that there is no DC on the speaker output.
#13
Usually, the problem is electrolytic caps. They are generally available but usually smaller than they were forty some years ago. This means you may have to improvise how the caps are mounted.

Spare circuit boards will only be available from some amp that has been cannibalized, they will likely have the same problem your amp has.

See if you can find a tech who knows what a "Light Bulb Limiter" is and knows how to use it.
#14
The Newcomer's Forum / Re: LA4282 in BTL bridge mode?
February 26, 2024, 03:42:52 AM
Quote from: Maddus on February 26, 2024, 03:24:54 AMIs it bad for the chip when only one channel is mostly used? So my idea would be to hook up one channel to an internal speaker and the other to an extension speaker jack.
That's not a bad idea, it shouldn't harm the chip. The heatsink should be adequate for two channel operation.
#15
The input is simply a dual differential pair, one NPN the other PNP. Each pair has it's own active current source. The outputs of the diff-pairs feed current mirror driver transistors where most of the Voltage amplification takes place. Bias for the output stage comes from a Vgs multiplier which matches the temp-co of the output MOSFETs much better than a Vbe multiplier. The source follower type output clips softly because the output Voltage can only swing to about 5V less than the rail Voltage. In addition, the zener diodes across the feedback resistor can cause soft clipping depending on the diode's characteristics.

Hard to tell what the open-loop gain is without modeling or building one, I don't speak German or trust any translation on the net.

Designs like this can be found on the diyaudio.com forum.