Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Papa Jim on October 29, 2018, 11:38:00 am

Title: How to test pre-amp stages as they get completed.
Post by: Papa Jim on October 29, 2018, 11:38:00 am

I am building an amp from an existing schematic. As I complete each preamp stage I would like to test it as each successive stage is complete. I have a scope to look at the signal being amplified. Would a resistor to ground at the output of each stage be required of the value of the input impedance of the next stage be the way to do that? I need help because I am a much, much, better builder of electronics than I am at understanding it totally.

Title: Re: How to test pre-amp stages as they get completed.
Post by: sluckey on October 29, 2018, 11:56:45 am

You can do that, but why? I've never seen anyone build an amp that way. By the time you get to the power amp your B+ voltages will have changed considerably, making your previous testing results kinda useless.

Title: Re: How to test pre-amp stages as they get completed.
Post by: terminalgs on October 29, 2018, 12:15:22 pm

There are easier ways to successfully double-check your work.  Once wired (the entire amp or a portion..) ignore wiring diagrams for a moment and compare your wiring to the schematic. measure resistances to ground from cathode, grid, plate.  Do the same from your power supply B+. comparing these measurements to the schematic, you should be able to 'yes, the ohm-meter is correct' for each measurement.  'open' can be the correct measurement, based on the circuit.  (amp is OFF for ohm readings...)

Title: Re: How to test pre-amp stages as they get completed.
Post by: PRR on October 29, 2018, 12:48:41 pm

Yes, be sure signal passes. Put coupling cap and 220K dummy in place of the next stage. Generally signal gets bigger at each stage, though tone or mix networks add loss. Might be good to spot-check 100Hz 400Hz 3KHz, because a crap connection may pass 10KHz "OK" but massive bass fall-off. Each stage should normally be able to output 10V clean. Do not expect +/-0.5dB or 0.01% THD: this is a guitar tone-shaper, not a hi-fi.

We normally feed preamp power from power amp power. If the power amp is un-built the B+ voltages may be WAY high. Be sure your caps can stand it for more than a minute. (I hate it when I put 490V on 450V caps and they blow up.)

Personally I'd work the other way. Build the power amp, be sure it biases smoke-free. Build the driver, most power amp drivers can be driven hard from most signal generators. Work through mid-stages and tone, then input stages. This way the B+ voltages start near-normal and you can even ear-test from another preamp or hot iPod.

Title: Re: How to test pre-amp stages as they get completed.
Post by: Papa Jim on October 29, 2018, 07:30:47 pm

Thanks everyone. I believe from all of the advice I am gonna build it up all the way thru the power amp before testing. This is how I have done it in the past on 2 other amps I have built and it worked out ok. Don't know why I was getting so concerned about it.