Adding an isolation transformer to an older amp

[Steve A.]

tboy hasn't posted here in 156 days

I just checked and his last visit here was yesterday around noon. Just because he doesn't post a lot doesn't mean that he's not around.

As for Steve Conner he hasn't been around since November 27th.

[tedmich]

Ok so we all hope Steve M. is aok and that Steve C. doesn't electrocute himself!

[J M Fahey]

Sorry for writing this but TBoy might be dead or in ICU or have become a Monk (not kidding , just reasoning with an open mind) or had a stroke or be battling some illness (my ex-partner is fighting last stages Lung Cancer and obviously dissappeared everywhere so such behaviour is an ugly daily reality to me).
The ghost appeareances here might be wife/son/daughter/friend checking now and then from his computer but doing nothing.
Our own Enzo disappeared for a few months during a bad health problem, and Mrs Enzo checked now and then, so that is a distinct possibility.

But anyway what Alex R suggests in Post#9 sounds like a realistic solution to me.

[JOHNS]

Good evening Gentlemen,
I am sorry to have restarted this heated debate. The Amp I am repairing is for a great friend it was handed down from his now deceased Dad. From the info gleamed from Alex and what I have read I have accomplished the following:
Installed a ISO transformer (Triad 50va)
Installed a 1 amp fuse inline with the primary side of the transformer (also added a new 3 prong cord)
I hooked the output of the iso transformer to the exact points the old line voltage was but did not remove the cap or resistor yet
Ordered new tubes from Tube Depot which should be here by Monday next week.
Any suggestions as to next steps? The amp works with no hum but there is a erratic static from it
I did clean the volume pot with oxidation gold cleaner

Thank you in advance for any suggestions please make them easy to understand as I am still learning

John

[JOHNS]

Good evening Gentlemen,
I am sorry to have restarted this heated debate. The Amp I am repairing is for a great friend it was handed down from his now deceased Dad. From the info gleamed from Alex and what I have read I have accomplished the following:
Installed a ISO transformer (Triad 50va)
Installed a 1 amp fuse inline with the primary side of the transformer (also added a new 3 prong cord)
I hooked the output of the iso transformer to the exact points the old line voltage was but did not remove the cap or resistor yet
Ordered new tubes from Tube Depot which should be here by Monday next week.
Any suggestions as to next steps? The amp works with no hum but there is a erratic static from it
I did clean the volume pot with oxidation gold cleaner

Thank you in advance for any suggestions please make them easy to understand as I am still learning


John

Also I would love to replace the OEM speaker for him as well I found Jensen 8" on Amazon however I am not sure if I need 4 or 8 ohm speaker

[bluto]

Also I would love to replace the OEM speaker for him as well I found Jensen 8" on Amazon however I am not sure if I need 4 or 8 ohm speaker

If you disconnect at least one wire from the existing speaker and read the resistance of the voice coil it will typically be about 2/3 the impedance...i.e an 8 ohm speaker will read about 6 ohms, etc.

[JOHNS]

Great Thank you for the advice on the speaker I try measuring across the voice coil good info Thanks again

John

[JHow]

My first question is about the .05/170K combo that separates the ground from earth. What does it's function? This particular model I have does not have the 170K resistor.

I thought the cap's purpose was to reduce hum by grounding any stray ac on the chassis and the resistor is to discharge the cap when turned off?

[JOHNS]

I did not remove the capaictior or resistor

I thought the cap's purpose was to reduce hum by grounding any stray ac on the chassis and the resistor is to discharge the cap when turned off?

The little amp sounds pretty good but there is random static I think maybe from an old dry speaker
I think I got it the best it can be

[Chuck H]

The little amp sounds pretty good but there is random static I think maybe from an old dry speaker
I think I got it the best it can be

More likely a cold solder joint or dirty connection. Isolate the existing speaker. If the problem is the speaker the noise will continue. If the problem is vibration causing an intermittent connection it may stop. If it IS an intermittent contact just reflow the joints, clean the tube sockets and jacks and it'll probably go away.

[JOHNS]

Wow Thats great advice! I actually did that yesterday and its quiet as a mouse Thanks

More likely a cold solder joint or dirty connection. Isolate the existing speaker. If the problem is the speaker the noise will continue. If the problem is vibration causing an intermittent connection it may stop. If it IS an intermittent contact just reflow the joints, clean the tube sockets and jacks and it'll probably go away.

This is a great place to learn I cannot thank the members who have helped me. Last night I returned the Starlet 107 to its owner who was thrilled.
He has a bunch of Fender amps handed down from his Dad I hope to help him keep them all working good.

Any input on Amps which are not used every day? How often should tube amps be turned on warmed up or left on if they are not being used every day to keep the things viable.
I want to give good info to my friend. I know in my Ham Radio world I turn my old Drake gear on as much as possible to warm up those tubes.
I am assuming its the same with music amps?

Have a great afternoon all
Thanks to all of you

John

[bob p]

I think he and Steve Conner ran off together .

I guess I'm lucky in that I can take off for a few months and nobody notices that I'm gone.

[Leo_Gnardo]

I guess I'm lucky in that I can take off for a few months and nobody notices that I'm gone.

Don't worry, I noticed, and that goes for your little dog too. Figured you were out making big piles of money somewhere kaCHING ! ! ! Good to see you back.

[AZJoe]

Need Clairfication on the Grounding

Post #1 by Gaz 05-27-2013, 05:40 PM

I've read about transformerless 'widowmaker' amps before, but this is my first experience with an amp which has no power transformer. My friend who owns the amp complained of a buzz, and I said I'd take a look at it.

The amp in question is a Magnatone "Starlet" 107 (schematic below).



My first question is about the .05/170K combo that separates the ground from earth. What does it's function? This particular model I have does not have the 170K resistor.

Before I had this amp, my friend told me that a shop added a 3-prong cord. From my research it seems like it was the wrong thing to do, and it turns out the added earth connection to the chassis via the 3rd prong was the cause of the hum.

I think I just need to add this isolation transformer, then I can add the earth connection again. Is that correct?
Is there anything else I need to do? What about the mysterious .05/170K?

I also plan on adding a fuse to the primary of the iso transformer.

Thanks for any clarification on this process.

Gaz


Post #8 by Gaz 07-10-2013, 08:20 PM from original thread

Hi, Just wanted to post an update. I installed the isolation transformer, and simply removed the .05 cap, and connected the V- directly to the chassis. I did not make any other changes, and the amp is very quiet now. There is also a proper safety ground now with its own dedicated ground lug.oldgoodfornothinghusbandmaker.jpg

Here's a picture of my handy-work. Not exactly how I would have ideally done it, but the transformer was way too big and heavy to mount on the inside or outside of the tiny flimsy steel chassis.



I also added a primary fuse you can see in the photo, but am not sure how to calculate the proper value. Any ideas?

[ATTACH=CONFIG]24197[/ATTACH]

Submitted by Steve A. 5/25/14

Guys,

I have one of these Magnatones I'm working on for a customer and I wasn't sure on how you tied in your ground so you don't get ground loops. The -V side of the output is connected to the amp chassis, correct? Where do you connect the mains ground wire from the plug? Can it be connected to the shield on the output side of the transformer? I first tried connecting the main ground to the chassis and isolate the -V side internally NOT connected to ground or chassis but, I get some hum and it gets worse when you turn up the volume. I tried the filter caps, no help. I connected my O-scope to the input after the coupling cap on the Vb 12ax7 and I see about 28mV sine wave and a little noise on the power tube as well. Do these amps inherently have a little noise on them or am I just over reacting on the noise. I think the 28mV or maybe it’s more like 280mV is about what you have on the main neutral line to ground. Thanks for any help with this.

Joe C