Need advice on how to troubleshoot a hum

[Reeek]

A short while ago, I stated that I commited the ultimate sin in tupe amp world and ran one of my cherished Sano 160R's for over a minute without the speaker connected before my stupid brain realized that all the cords were plugged in except the speaker leads.

I finally got the amp out to start troubleshooting the damage. When it happened that day, I just hid the amp in the back room and pouted for the past few weeks about it, not even looking it over.

Well the results are that I'm pretty sure the OT is toast.

There is no audible hum anywhere without the power tubes in which is probably to be expected. And when the power tubes are inserted, the hum gets louder as the volume is turned up. If the volume is turned all the way down, it's quiet. I tried different power tubes too. Same result.

So now I'm looking at OT's I guess.

I want to "upgrade" the OT this time around and this is my first time doing any trannie. So my questions are:

1. What does "upgrading" an OT mean?

2. Does it simply mean a better constructed OT such as Mercury Magnetics versus a Hammond 125 series?

3. Do audio quality OT's make for better guitar OT's? The big 4 tab iron things versus the 2 tab paper wrapped?

4. This amp is a 1967 ish Sano 160R running two EL84 in class A, diode rectified, cathode biased with tube trem and tube reverb. Probably 12-15 watts. 1x12

5. The current OT had a secondary impedance of 8 Ohms

6. I was thinking about something around 8000 primary impedance. Does that sound in the ball park?

7. What exactly is it that I need to beef up if I were looking for maximum headroom? In this case, thats what I want and I want to minimize or eliminate any OT saturation perceived or real.

8. What are the effects if I were to over do it on the primary impedance or under do it on the primary impedance?

9. Anything else you care to add?

Thanks!

Rick

 

[Swarty]

I think you have more troubleshooting to do. That fact that you get hum with the tubes in and it gets louder when you turn up the volume control is a good sign that the OT may be OK.

 

[Reeek]

Originally posted by Swarty
I think you have more troubleshooting to do. That fact that you get hum with the tubes in and it gets louder when you turn up the volume control is a good sign that the OT may be OK.

Thanks for the reply, Swarty.

You may be very right. Trannies are the one remaining mystery to me although I am merely a home enthusiast with respect to tube amp repairs and troubleshooting.

Help me out here then, AFAIK, the primary danger with running tube amps without a speaker load is the output transformer and I had always been lead to believe that this was the best chance of damage when no load was present for a period of time.

What other things should I look for? Not the power transformer I hope. I always thought the OT and the tubes were the first to go but I'm not an expert.

Thanks!

 

[Swarty]

What happens when you plug a guitar or other signal into it?

 

[Reeek]

Here's the story:

I had the chassis out of this amp ( I have two identical amps like this but one is a year or two newer) to retube it just for general purpose. The tubes hang horizontally out of easy site when the back panel is off so that the circuitry is exposed for easy repair. The speaker leads have to come off when the chassis is out.

So I closed the damned thing up forgetting to reinstall the speaker leads. I fired it up and had no signal of course and I messed with everything except the speaker leads. After I'd spent 45 seconds to a minute plus dinking with cords and the volume knob, I suddenly realized what I DIDN'T do so I all but lunged for the power cord and yanked it.

Before all this, it sounded great with only a faint 60 cycle hum that I was living with but afterwards, it's basically unplayable. I reinstalled the old tubes and a third known good pair and same thing.

I played a guitar through it that day but will do it again tonight when I get home.

Thanks!

 

[Wakarusa]

Take a look at the screen resistors on the output tubes.

If the OT survived (and I agree with Swarty, there's a good chance it has), and the trouble ain't the tubes (as substitution would suggest), then it's time to start looking for secondary effects.

Either the flyback from running into open, or the overload from running into a dead short (is your speaker jack a shorting jack?) can stress the screen resistor to the point of failure (it's basically there as a fuse anyhow ). A pentode with a disconnected screen will hum like crazy. A shorted screen'll do it too depending on how the power supply is set up.

While you're in there, check on grid stopper resistors gone awry, and, finally, check for arcing (carbon traces) on the tube sockets themselves -- both inside the chassis and out.

 

[Reeek]

Thanks, Todd. I'll check it out during the game now. I'll report back. The speaker does not have a jack. it's direct out of the OT . . .

 

[Reeek]

Just to be sure, the screen resistors are coming off pin 2 on an EL84, right?

 

[Wakarusa]

Originally posted by Reeek
Just to be sure, the screen resistors are coming off pin 2 on an EL84, right?

Nope.

1 - not used
2 - control grid
3 - cathode (and suppressor grid)
4 - heater
5 - heater
6 - not used
7 - plate
8 - not used
9 - screen grid

So if your amp has 'em, they'd be off pin 9

 

[Reeek]

I found that in my books since I left.

No screen resistors then. Pin 9 on each EL84 are tied to each other with a red wire.

The only resistors coming off the power tubes are off pin 2 and there are three resistors coming off each pin 2.

On each tube:

1 goes to ground
1 looks like to goes to the trem depth pot
1 goes through the .047 caps to the preamp

 

[Reeek]

I just did something that may or may not eliminate the OT. I had read where the resistance between the common tap and the impedance tap is generally predicatible. Supposedly the reading should be "about" .75 Ohms for an 8 Ohm tap. The reading on the humming Sano 160R is .6. So I took a reading on the non-humming Sano 160R and the readin was exactly .6 on that one too. Both without the speaker attached.

This is supposed to chack for continuity on the OT.

Is this valid do you think?

In the end, I'll just box this thing up and send it to a very good tech I guess because they sound so extraordinary.

 

[Wakarusa]

Originally posted by Reeek
I found that in my books since I left.

No screen resistors then. Pin 9 on each EL84 are tied to each other with a red wire.

The only resistors coming off the power tubes are off pin 2 and there are three resistors coming off each pin 2.

On each tube:

1 goes to ground
1 looks like to goes to the trem depth pot
1 goes through the .047 caps to the preamp

Okie. One assumes that the red wire then wanders of to somewhere in the B+ chain. The resistors, in order, are ground reference (probably somewhere in the 100K-220K range), trem feed (no idea on value), and grid stopper (usually 1K to 4K7).

And yeah, it sounds like time to go to a tech. There are more thorough ways to test an OT (though I'd still bet yours is fine). It's time to put 'er on the scope and see where the hum is coming from. There is also a strong possibility that while you had the chassis out you disturbed a ground or moved a wire to an unfortunate place and introduced the hum -- i.e. the "no load" may be coincidental and not the true cause of the problem.

 

[Swarty]

One more thing to try is to check reistance between the center tap (B+ connection) and the plate connections on each side of the OT. A substantial mismatch would cause hum and also indicate one side being damaged.

 

[Reeek]

Thanks for the time and effort, Todd.

BTW, you estimates on the resistors is pretty spot on

I've heard that older amps were not designed to be overdriven and that's why some didn't have scren resistors. Not sure if this is true though.

I have learned how to do what I do by ruining a couple amps over the years. Basically I solder well and can replace components including multi-can caps. It pretty much stops there.

I'll just concede and send it in for repair to someone who is qualified since I do not want this amp to die forever. It is really a special amp and only those who have heard them understand.

For anyone with a suggestion; Todd, are you willing to take a look at it if I send it out? If not, who should I send it to? Reno is not well known for tube amp repairs. I use Uncle Spot who is building me a SR clone now but he is always backed up beyond comprehension. I'm willing to box it up properly and pay two-way shipping to make it right. It's worth that much to me. And I do have the original schematic I can copy and include.

At least I have the other one to keep me sane.

Thanks!

 

[Wakarusa]

Reeek, check your e-mail

 

[CarlJ]

Originally posted by Reeek
the hum gets louder as the volume is turned up. If the volume is turned all the way down, it's quiet.
Rick

Reek, that says it's probably in the preamp section.

 

[Reeek]

CarlJ _ I think you were right. When I swapped tubes before, I only swapped the power tubes. I just swapped ALL tubes this morning and the amp is back to normal! Sounding as good as ever too

Thanks everyone!

Todd, I won't be sending it in on Monday afterall! I appreciate it though

Rick

 

[Reeek]

Update - I had a cherished vintage Tesla ECC803S and a Conn branded RCA clear glass 12AU7 in the preamp section as called for in the schematics. I started eliminating the tubes to find the cuplrit last night.

The Conn 12AU7 was fine
The Mullard EL84's were fine
Unfortunatley, the Tesla ECC803s (original) was bad. Fortunately, I have 7 more NIB stashed

Last night, I put the Mullard EL84's back in. I had some vintage GE oval plates in there and they sounded really nice but the putput was lower than the Mullards. I installed a Mullard 12AX7 in the 12AX7 socket and a USA made black plate Hewlitt Packard branded 5751 in the 12AU7 socket and whoa! Does this thing just bloom with tones!

Thanks everyone . . .

Rick

 

[CarlJ]

Congrats,
As I recall from some clips you posted at carvin a while back that is a beautiful sounding amp. It'd be a shame for it to die.

 

[Reeek]

I remember you, CarlJ The same clips are still posted on the soundclick page in my sig

Thanks again!