There is absolutely no need to measure the harmonic distortion of a tube guitar amplifier.
As I have already written and as calexanian has also confirmed, it would just adjust the bias of the output tubes so that there are no visible crossover distortion on the oscilloscope.
So all audio generator with sine / square output is good, even the cheapest one.Quote from: calexanianThe recommended biasing procedure in those days was to apply a sinewave. Typically of the lowest frequency of the -1DB point of the output transformer as full power. I believe its about 100hz for a deluxe reverb. Perhaps a tad higher. Adjust for rated power output and no crossover distortion. return to no signal and make sure the tubes are not dissipating too much power at idle, then take the amp into clipping. Adjust bias up to clean up the square wave a little and check for anodes to not be beginning to glow. Return to clean rated power and observe maximum dissipation and again check for no anode glow. Return to idle and check for dissipation. It was not a really exact science. What sounded good at clean rated output may sound terrible under heavy clipping. That's why they had to be run a little hotter and the transformer impedance was played with to get them in a reasonable spot on the transfer curve of the tube at that point where the distortion was not cold or harsh and the tube was not melting.
https://www.eevblog.com/forum/projects/fender-deluxe-reverb-repair/
Nb: Square wave is needed to check the stability of the amplifier....
The problem you have is that you do not have a clue about what you need to do to transform old amplifier in tube guitar amplifiers.
To find out what you need as test instrument, you must "put your hands in the dough" and you will realize for yourself what you need.
So, first try to do the job, look for experienced tube technicians in your neighborhood and equip you gradually according to your needs.
Obviously you will realize quickly that it is not necessary to have high precision instruments and that the problems are simple and basic ....
One of your main problem is the fact that the tubes in general are no longer manufactured.
As wrote calexanian, we used the tubes beyond their maximum ratings and they were often replaced after each concert.
I remember amplifiers that worked with the anodes of pentodes dark red hot ! ....
So one of the first requirements seems to be a tube tester because they have become scarce and expensive.
Another principle: a tube circuit is generally quite tolerant about the value of the components.
The characteristics of the tubes change during aging and circuits are scheduled to continue operating even in this case.
So do not change all resistances only because they are out of tolerance, it is ridiculous and absurd.
Make a functional test and do not change the components if the functional test proves that's unnecessary.
@finom1:
Hello, fellow retiree...
I don't know if anyone mentioned this in a previous reply and apologies if that is the case.
Get an isolation transformer. I see you are in the USA so a 120VAC in and 120VAC out at about 2.5 amperes or higher.
Everyone who dicks around with electronics, either hobbyist or professional should have one as the most important piece
of gear. Owning one could save your life. It did for me. Back in the very early 90's I was asked to repair computer monitors.
The switching supplies run right off the AC line. I got a few shocks and a few injuries from pulling my hand away.
Needless to say, I don't futz around anything until it plugs into the isolation transformer. Sure you may get an electric
shock but it won't be fatal. The isolation transformers are not cheap, but if your life isn't worth spending $100 on something
safety related.... well, good luck to you.
Here's a pic of my homemade unit with a 5 Amp Variac inside. Make sure you have the necessary skills to wire up the AC.
I used a square receptacle, but one could use a standard two outlet wall receptacle.
Remember, PLAY SAFE !!!!
http://www.ebay.com/itm/Variac-Variable-AC-Power-Transformer-0-130-VAC-with-Meter-TDGC-1KM-Max-10-A/121605198781?_trksid=p2047675.c100009.m1982&_trkparms=aid%3D0%26algo%3DDISC.MBE%26ao%3D1%26asc%3D20131227121020%26meid%3D18133bf93c084b9195b6a2b782367e86%26pid%3D100009%26rk%3D1%26rkt%3D1%26sd%3D301960068642
Does this have the isolation transformer in it?
Thank you for helping us!!!
http://www.ebay.com/itm/Variac-Variable-AC-Power-Transformer-0-130-VAC-with-Meter-TDGC-1KM-Max-10-A/121605198781?_trksid=p2047675.c100009.m1982&_trkparms=aid%3D0%26algo%3DDISC.MBE%26ao%3D1%26asc%3D20131227121020%26meid%3D18133bf93c084b9195b6a2b782367e86%26pid%3D100009%26rk%3D1%26rkt%3D1%26sd%3D301960068642
Does this have the isolation transformer in it?
Thank you for helping us!!!Unfortunately, that unit is non-isolating.
@finom1:
You may as well keep the Variac and attach the isolation transformer to the Variac. I never came across something that had an isolation transformer and Variac
both in one box.
@oldway:
I know that finom1 plans to make cigar box amplifiers for his family and about guitar amps with power supplies.
It is still connected to the AC line when connected to the Variac and the AC supply is not isolated. That is where you might
get electrocuted while poking around in the primary should you forget to unplug the unit from the Variac.
My concern is safety and accidents do and will happen. What people do or do not with information is up to them.
Everyone makes mistakes, they don't have to be fatal.
Better safe than dead.
... things happen no matter how good or careful you think you are.
Then we started watching The Guitologist on youtube, that was crazy fun for us to watch. Then watching people taking someones electronics headed for the junk yard and creating something new with the parts.
We did pick up a couple of vintage upright radios around 3' tall. Wattage was 100 watts and up.
QuoteWe did pick up a couple of vintage upright radios around 3' tall. Wattage was 100 watts and up.If you mean that output audio power was 100W and up, there is something terribly wrong.
NB: With tube amplifiers, output power is always specified as rms power. (root mean square).
Here are the values of output power you can hope within ratings from the most used audio tubes:
http://tubedata.tubes.se/
- EL84 = 6BQ5 Class A: 4.5Wrms
Class AB (2 tubes push-pull) : 11Wrms
- EL34 = 6CA7 Class A: 8.7Wrms
Class AB : 35 Wrms
- 6V6 Class A: 4.5Wrms
Class AB: 13.5 Wrms
- 6L6 = 7027 Class A: 6.5 Wrms
Class AB: 34 Wrms
- KT66 Class A: 5.8 Wrms
Class AB: 30 Wrms
- KT88 = 6550 Class A:
Class AB: 70 Wrms
- 807 Class AB: 75Wrms
- 7591 = 7868 Class AB : 37 Wrms
- 5881 Class A: 6.5 Wrms
Class AB: 18 Wrms
finom1, given what you already have or plan to have (a DMM and a power supply), I don't think this calibrator will be of much use. I would sell it and save for the oscilloscope or even increase the budget for a better one!