Magnatone Starlet bZZZZ

[samgraysound]

Client brought me a magnatone starlet that had developed a buzz.  I told them I needed to install an iso transformer and fuse before I did anything else on it. They agreed, I did that then started working on the buzz. The amp was heavily modified before I touched it and I have attached a traced schematic showing the current circuit. The buzz is present with the volume control all the way down, indicating to me it is not in the first amplification stage.

Here's what I've tried to eliminate the buzz.

Checked the filter caps by bypassing them with a good cap
Tried replacing all the tubes
Retouched solder on all ground connections
replaced all coupling caps
replaced all resistors  in the power stage
replaced all resistors in the 2nd stage
replaced the resistor in the power filter

I''m not sure what else to do at this point. Suggestions?

 

[walter]

Pull the pre-amp tube, does it still buzz? Connect a capacitor to ground and touch it to various points in the circuit to identify where the buzz is originating(shorts the signal to ground). Ohm out all the grounds. Clean the tube sockets. I would do all this before, but it may help at this point.

 

[dmp]

Checked the filter caps by bypassing them with a good cap

Replacing the 50 uf filter caps would be the first thing I would do. Do you mean you put a good cap in parallel with the existing caps? Because the current ones need to be out of the circuit. 

If it is heavily modified who knows if it ever worked right too. Heater wiring, etc You could post a picture of the electronics

 

[scott2000]

Did it work correctly after any mods???

What purpose is that 270k resistor that was removed???? 

 

[samgraysound]

Pull the pre-amp tube, does it still buzz? Connect a capacitor to ground and touch it to various points in the circuit to identify where the buzz is originating(shorts the signal to ground). Ohm out all the grounds. Clean the tube sockets. I would do all this before, but it may help at this point.

• pulling the preamp tube makes the buzz worse
• using a 47uf cap, coupling the plate or grid of the power tube to ground eliminates the overtones of the buzz but leaves a 60hz hum
• I previously redid all the grounding to a star ground. Just did an ohm check for each ground point and they all read 0.2 or 0.3 ohms
• cleaned tube sockets, no effect

 

[samgraysound]

Replacing the 50 uf filter caps would be the first thing I would do. Do you mean you put a good cap in parallel with the existing caps? Because the current ones need to be out of the circuit. 

If it is heavily modified who knows if it ever worked right too. Heater wiring, etc You could post a picture of the electronics

It seems to me that if the caps had gone open then bypassing them would reduce the ripple voltage. If they were shorted then the fuse would blow. If they were leaking, then the B+ voltage would be low. Also they were clearly replaced with a modern can cap when the mods were done.

Picture attached.

 

[samgraysound]

Did it work correctly after any mods???

What purpose is that 270k resistor that was removed????

The mods were done before the current owner purchased it. He was unaware of them. He says that it works fine for a long time and then one day started buzzing.

The 270k resistors and .05uf cap created a virtual ground separate from chassis. This was how they made it "safe" to not have an isolation transformer.

 

[scott2000]

Have you tried moving stuff around with a chopstick???? Hard to tell from pic but some things are pretty close to each other......

Not sure that grounding set up is the best???? I wouldn't know though.....

Why is the original 107 drawing online different from the "un-modded" version here???

 

[PRR]

> Why is the original 107 drawing online different

"107" was a model number on a contract. Montgomery Ward might have ordered 10,000 of product "107". As long as what was delivered looked like the sales-sample, and played "OK", who cared if the guts were different? If one tube got cheaper than another, or the factory had over-stock of 470K, changes happened and nobody needed to know.

Anyway: sudden buzz means a connection went bad or a cap went bad. I do not know what the client can afford. If it is truly beloved, it looks over-due for a complete re-soldering, even to socket replacement, and fresh new filter caps. All this crap was low-low-bid when new, a long time ago, and the "mods" or repairs seem to be a similar make-do philosophy. SAVE all the parts; the "magic tone" might be a loose-wound cap which might have more life in it.

 

[walter]

I'm not a fan of rearranging ground schemes. I had a similar problem, where I did a cap job and replaced can caps with axial. There was one can near the input that was originally grounded to the chassis and I moved that cap and ground to near the rest on the other end of the chassis. There was this mystery hum. I moved that one ground back to the original location and that fixed it. That may not be what's going on here since it hummed before you moved grounds. Maybe remove the cathode caps from the pre-amp to reduce gain that is amplifying hum? or not, if the hum is worse with pre-amp tube removed. Is there any sign of shielding in the cab? Try putting the amp back in the cab? Add thick aluminum foil to the cab so it touches the chassis when assembled? shielding issue? does the hum change if you rotate the amp?

 

[dmp]

This thread has good info on putting in an isolation transformer. You can compare yours to the pics.
https://music-electronics-forum.com/showthread.php?t=38004

I'm guessing you have compounded problems now. The original cause of the hum (bad filter cap or bad connection) plus the star ground / wire routing to the iso transformer. 

What is the big white power resistor in the upper right, which seems to be in between the iso transformer and the rectifier?

 

[samgraysound]

This thread has good info on putting in an isolation transformer. You can compare yours to the pics.
https://music-electronics-forum.com/showthread.php?t=38004

I'm guessing you have compounded problems now. The original cause of the hum (bad filter cap or bad connection) plus the star ground / wire routing to the iso transformer. 

What is the big white power resistor in the upper right, which seems to be in between the iso transformer and the rectifier?

Yeah I implemented the iso the same as that thread. ie eliminating the floating ground.

The power resistors. My iso was putting out 140vAC on the secondary for some reason with a 120vAC input, so I added those to drop it down to 120.

Why would the star ground cause hum? How can I be sure the filter caps are good?

 

[samgraysound]

> Why is the original 107 drawing online different

"107" was a model number on a contract. Montgomery Ward might have ordered 10,000 of product "107". As long as what was delivered looked like the sales-sample, and played "OK", who cared if the guts were different? If one tube got cheaper than another, or the factory had over-stock of 470K, changes happened and nobody needed to know.

Anyway: sudden buzz means a connection went bad or a cap went bad. I do not know what the client can afford. If it is truly beloved, it looks over-due for a complete re-soldering, even to socket replacement, and fresh new filter caps. All this crap was low-low-bid when new, a long time ago, and the "mods" or repairs seem to be a similar make-do philosophy. SAVE all the parts; the "magic tone" might be a loose-wound cap which might have more life in it.

I'm pretty much at budget already. Or rather I was before I replaced all those resistors .

 

[dmp]

Yeah I implemented the iso the same as that thread. ie eliminating the floating ground.

Not really the same since I think you drilled a new hole in the chassis and have wire routing come up differently.  When you are trying to make a tube amp that has low noise / hum, the wire routing matters  - particularly with AC wires.  The way it was done in the other thread was cleaner and had less AC wiring running around. Too late to change that though.

The power resistors. My iso was putting out 140vAC on the secondary for some reason with a 120vAC input, so I added those to drop it down to 120.

Strange. Sounds like iso transformer was wrong.  It is not typical to put a resistor like that before the rectifier. I'm not sure if that is a bad idea or not.  I don't like components floating like that as amps get moved around and it will bounce around. Fatigue could eventually cause it to break a connection. You notice in the picture of the other amp all the components are fixed at both ends so they won't move around.

Why would the star ground cause hum?

Read about ground loops and the ground scheme of vintage tube amps. This is a good summary:
https://el34world.com/charts/grounds.htm

A correct star ground connects all the grounds together at a single point at a chassis attachment. You however have several grounds at the star and a few others at the input jack and possibly even more locations (like the solder tabs on the tube sockets?). So you have multiple paths connected to the chassis and you do not actually have a star ground (the original had multiple connection points too).  You changed the grounding so it might have more/less hum than the original design.

Why does poor grounding cause a hum? The chassis isn't necessarily a solid 0V ground, it will have voltage fluctuations / currents moving through it. The point that needs the most stable ground is the 1st preamp tube since it sees the most amplification. Any fluctuations in the ground of that 1st stage will show up as noise / hum on the output.

How can I be sure the filter caps are good?

You can measure the capacitance & leakage current but it is easier to just detach them and put in a new cap to see if it eliminates the hum.
I would probably get a replacement cap can like this before doing anything else and replace the one in there (depending on the diameter needed).
https://www.tubedepot.com/products/jj-can-capacitor-50uf-x-50uf-500v

I'm pretty much at budget already. Or rather I was before I replaced all those resistors .

Replacing all the resistors and coupling caps was unnecessary and a bad decision imo. If that had been a vintage fender or other valuable amp it would have been tragic and would have hurt the value considerably.
Time to take a step back and develop a good plan for how to get the amp healthy and back to as good as it can be now.   

The recommended service (IMO) for a vintage amp:
- New filter caps (replace all electrolytics with correct physical types: i.e. cap can, axial, etc...) In some cases increasing the voltage rating if it was close originally.
- Remove any death cap, upgrade to three prong cord
- Swap to known good tubes and clean sockets with deoxit. You can clean the pots with deoxit at the same time.
- Check for bad connections: clean tighten input & speaker jacks, check for cold solder joints.

Just doing this standard stuff on a vintage amp will fix a lot of problems.  There might still be problems after doing this with out-of-spec resistors or caps, but doing the above eliminates 90+% of issues older amps have. Adding the iso transformer and removing the floating ground (& changing the V1 cathode resistor) was necessary on this Starlet also.

 

[PRR]

> iso transformer

"Unity ratio" transformers are unity at Full Load.

All transformers sag. To hit unity they must be turns-ratio a little more than 1:1.

Small transformers sag 20%. 120:140 suggests 17% over unity. This would be in the right ballpark.

It does suggest either that "140V" was seen NO-load, or that the iso is grossly over-size for this load.

And yeah, you do see series resistors on some old off-line stuff, to prevent huge rectifier surge. Moreso when solid-state rects appears, but also with bottle rects.