Equip your electronics bench- Help!!!

[finom1]

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.

The 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....

OK, got it. For some reason my head hurts now!!!

[oldway]

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]

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.

Great advice, thank you!!!

[Enigma-man]

@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 !!!! 

[finom1]

@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 !!!! 

I purchased this last year - Variac Variable AC Power Transformer 0-130 VAC with Meter (TDGC-1KM) Max.10 A

 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!!!

[nanofrog]

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]

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.

OK, it was less then $100. I will sell it.

What brand & model should I buy that has been a test bench standard over the years?

Thank you for sharing!!!

[oldway]

There is no switch mode power supply in guitar tube amplifiers ....

Amplifiers have already an isolation power transformer, no need of a second one....

People are answering this topic without reading and knowing what you intend to do....

I remember that you and your son intend to modificate old tubes amplifiers to make guitar amplifiers. 

[john_p_wi]

I've been building guitar amps and pedals for more than 20 years.  As I get older, I find that I am more into the convenience of things than the equipment.  Most of my test gear (I have two benches set up with nearly identical equipment) is of 80's and 90's vintage, Tek scopes and Wavetek 164 signal generators.

Make sure that your bench is set in a way that DVM's, scope probes, dummy loads, signal generators etc can be easily accessed without the spaghetti nightmare of cable entanglement.  Buy Fluke probe clips that you can hook on to the test circuit and leave while you test under power.  I can't stress enough the importance of test lead management and dedicated leads for each piece of equipment.  The last thing you want to do is have a tangle of wires over an open chassis with 500 volts exposed.  Also have dedicated amp cradles, even if it is wood blocks etc, vises for the pcbs etc.  I can't express the importance of being methodical and deliberate.  Trust me, whatever size bench you think you will need - double it.  Most likely you need some type of lighted magnifier too.

Regarding the soldering iron question a few pages back, this is the one area that I spoiled myself with a good Metcal with desoldering gun.  I have 10 tips or so with the Metcal and can solder anything from 0402 surface mount to the heaviest ground plane connections by changing the tips.  Look at something like the MX-5251 - when using it with the old tag board / eyelet circuit boards it is very convenient to pull the component lead out of the eyelet using the soldering hand piece then immediately "clean" out the eyelet with the vacuum desoldering gun.  It is also great for cleaning out the solder lugs on pots etc along with the normal pcb desoldering work.

Finally, all of the info regarding variacs, iso transformers etc is solid.  I also have several multi channel power supplies for pedals and a large 20 volt x 20 amp power supply for heater strings etc.  Good, solid, working equipment of 20+ years is more than adequate.  I'd focus more on quality test lead sets, scope probes, patch cables etc - there is nothing more frustrating than second guessing / trouble shooting your cables / probes.

Good luck.

[Enigma-man]

@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.

[oldway]

@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.

Did you ever work with tube amplifiers ? I think not !!!!!
There are a lot of components who are at 300 V and more, up to 800V dc (I have seen a public adress amplifier with EL34 working with 800V anode voltage !), in reference with chassis ....
Only very few components are on main voltage: input fuse and on/off switch...
This is a minor concern.

If anyone does not work with safety, he can't repair or modify tube amplifiers.
This is a very dangerous kind of equipments
An isolation transformer does not improve substantialy the safety in the case of tube amplifiers.
It is very fair to have principles, but you have to think if they are applicable in this case.

[Enigma-man]

@ all:

FWIW:
I was just trying to help after seeing finom1 was asking a lot of questions based on his experience.

Recently I acquired an Eldorado Electronics 4.5 digit voltmeter with Nixie readouts.  It's from the 70's and I had no service data.
While working on a problem with the analogue board, my hand brushed against some foil traces from another pc board carrying
120VAC.  I got quite a jolt and without the isolation transformer... I might not be typing this.  I am very careful around electricity
but things happen no matter how good or careful you think you are. 

Good luck with your equipment search and amplifier projects finom1 and son.
If I have anything further to contribute I will PM you.
There is more than enough information here and likely more to come from others to steer you in the right direction or confuse you further... 

Take care.

[oldway]

Speaking about safety:

It is obvious that choosing tube technology as first experience in electronics is not a right choice.
Risks are high because of high voltages used in tube technology.
I hope finom1 knows this and that he assumes the responsability of his choice.

He should learn about safety and pay attention to what he do.
For exemple, never touch a component and the chassis at the same time. One hand in the pocket is a very good safety rule.

Beginning with help of somebody who has experience in tube technology would be very helpful.

... things happen no matter how good or careful you think you are.

I don't agree, you must learn to prevent accidents to happen and that's possible.
Use your brain, work with safety and pay attention to what you are doing.

I worked all my life with high voltage, high power electricity and electronics, I would not be alive anymore if I had not learned to prevent accidents. (High voltage tests up to 150Kv, 120 Kva installed power test equipment, tests of 3000V 1500A rectifiers, test and repair of dc drives up to 8000A 550V dc, No-breaks up to 1 Mva, industrial battery chargers up to 600A 500Vdc, inverters, SMPS, ....)

[john_p_wi]

oldway, Enigma-man is only trying to be helpful.  One thing that is concerning is that the OP has the intent to modify old radios to guitar amps, if I'm not mistaken some of these were transformerless.  In this scenario, was the line not directly connected to the heater string and the heaters voltages set in such a way that the series connection of the tubes equaled the line voltage?  In this case I believe that the "neutral" was directly connected to the chassis.  IF a non-polarized plug or the duplex was wired incorrectly this could potentially put line voltage on the chassis, especially since the "neutral" was either terminated at the chassis or was connected via caps.  In cases like this, an isolation transformer is a must, but is not a substitution for a proper protective earth and knowledge.

Agreed, knowledge and safety are paramount - unfortunately there are very few versed in these old radios / amps today, and all cautionary advice should be welcome.

[oldway]

Finom1 wrote that he and his son want to build a hi-quality less then 150 watt amp
I have a couple of Hammond & Wurlitzer amps that we want to convert into Cigar Box Guitar amps!!!

Nothing to do with "old radios" .....

Those old transformless radios had a class A output stage between 2 to 5W audio power.
This has absolutely no interest to be converted as guitar amplifier.
Moreover, it would be difficult if not impossible to do this because the tube filaments were connected in series to run on the mains voltage.
It should therefore be necessary to leave all the high-frequency and intermediate-frequency tubes which would be absurd as they would become useless.

Besides, finom1 never said he wanted to convert old radios in guitar amplifiers and it would be very unsafe and lethal as the electric guitar is conected to the chassis of the amplifier.

Finom1 was ready to sell his variac because it is not isolated....
One of the most important equipment, he would sell it. 
You call this be helpful ? 

[john_p_wi]

Alright, let's keep this in line for the OP.  IF he plans to build cigar box guitars 2-5 watts is plenty. 5 watts through a modern 4x12 cab with efficient speakers will make your ears ring.  Trust me I know.

Additionally, 5 watts SE can be a glorious thing.  Try it.  You'd be amazed at how many albums have been recorded with such an amp.

I don't have a dog in this hunt, nothing to prove - just trying to help a newbie out who wants to spend time with his kid which is priceless.  Please don't scare him off - let's nurture and educate the father / son team without the measuring stick bs - we all started somewhere and had mentors along the way, I can certainly name mine.

Honestly the fastest way to learn is with a few solid state parts and a LM386 building a noisy cricket amp with a simple jfet buffer in front of it.  A very loud and cool distortion at 1/2 watt that will fill the cigar box vibe while teaching the basics of amplification, grounding, shielding and decoupling at 9 volts dc.  All completely safe.

[finom1]

Thanks everyone for looking out for our safety.

We did pick up a couple of vintage upright radios around 3' tall. Wattage was 100 watts and up. No small radios so far, it just turned out that way! WE also have some grate vintage 12" & 15" speakers from organs. All three radios have 12" speakers in them. They all have room for 15" speakers inside.

We have a bunch of vacuum tube organ chassis, that we accumulated over the last year.

So we have parts, we then needed a safe test bench for the long haul. Everyone here has been making this happen for us. We now have a Pre-Start Up & Start Up Check List. Our safety awareness has increased immensely! Thank all of you for that!!! WE also made great Simpson meter purchase.

I am waiting for two people to get back to us concerning capacitor testers. One person has a Heathkit IT-28 the other person has a Eico 950B both around the same price shipped.

Thanks for keeping us safe and sharing the wisdom you have attained over the years by repairing & building electronic equipment . It's very exciting!

Thanks again!

[finom1]

It all started over a year ago. We were watching people turn small radios into cigar box & lunch box guitar amps. WE loved the idea!!!

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.

I just thought this would be a great way to share some of life's experiences building guitar amp projects with my son while in retirement.

Thanks again to all who have taken the time to share with us!!!

[Shock]

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.

First video I see, transformerless design.

[oldway]

We 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]

We 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

I purchased a 8'x8'x20' shipping container a few months ago so I could finish the basement to make room for a work area. The chassis and radios are in there. I have plastic totes full, stacked up on the radios. I have to turn them around and see if I can find a name plat & how many/what type of power tubes are being used. That way I can provide more accurate information to you. Some of the organs had metal name plates with 150 watts and up on them but nothing on the tube chassis themselves. Now going by your tube breakdown I will know close to what the output is for each chassis.

Two organ name plates on my desk have on them, Lowery Organ 115 volts 60 cy 2.5 amps 275 watts, accessories 4.1 amps 500 watts ( this had multiple tube chassis in it) & The Conn Organ volts 115 herts 60 watts 140 amps 5.

Thanks for the detailed break down!!!

[oldway]

Ok, here we are ....the wattage you said, it is the power consumption and not the output power....
That's very different because tube amplifiers have a very very low efficiency.

[finom1]

My son just said I could have this - Transmation Digital Calibrator/Process Signal Indicator, 1045 - http://www.ebay.com/itm/Used-Transmation-Digital-Calibrator-Process-Signal-Indicator-1045-/191411160754?hash=item2c90fe86b2:g:ffAAAOSwYshUY5Ty

He restores cars, trucks & motorcycles and this was in one of the trucks. He buys vehicles that have problems, fixes them then sells them to afford the next one at a higher starting point.

I do not know what it is or how to use it. Is it something I should keep for our hobby or sell it to buy a scope?

Thank you for any advice!

[rsjsouza]

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!

[finom1]

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!

Great advice, I will do that. I found a pdf manual. I have to find some leads then I will put it on ebay.

Thanks!