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Old 27th March 2006, 11:24 PM   #1
layt is offline layt  Australia
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Default squealing output transformer

I bought an output transformer from E-bay that was supposedly a drop in replacement for Fender champs etc etc. Anyway I put it in and hit some strings and it worked ok at low volume, great! I thought now lets turn it up a bit. When I did I got no sound what so ever and the 5Y3 started to arc inside the tube. I triple checked everything and made sure all was well and tried again, keeping the volume low I tried to play again (for about 30 seconds) and it started doing the same thing at low volume. Hmmm strange so back in went the hammond transformer and everything works fine again. This morning I was damned pissed off with that "fender replacement" so I tried again to get it to work.This time all I got was this awful squealing noise that changed pitch when I played with the volume. Can I assume the transformer is a piece of junk?
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Old 28th March 2006, 12:24 AM   #2
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sounds like positive feedback. swap the leads on the primary (B+ lead to plate, and vice-versa).
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Old 28th March 2006, 12:32 AM   #3
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Default Re: squealing output transformer

Quote:
Originally posted by layt
I bought an output transformer from E-bay that was supposedly a drop in replacement for Fender champs etc etc. Anyway I put it in and hit some strings and it worked ok at low volume, great! I thought now lets turn it up a bit. When I did I got no sound what so ever and the 5Y3 started to arc inside the tube. I triple checked everything and made sure all was well and tried again, keeping the volume low I tried to play again (for about 30 seconds) and it started doing the same thing at low volume. Hmmm strange so back in went the hammond transformer and everything works fine again. This morning I was damned pissed off with that "fender replacement" so I tried again to get it to work.This time all I got was this awful squealing noise that changed pitch when I played with the volume. Can I assume the transformer is a piece of junk?

This sounds like the scariest thing ever! is there anything I should make sure to do to assure this problem doesn't occur as I build my first amp?
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Old 28th March 2006, 12:48 AM   #4
Trout is offline Trout  United States
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Default Re: squealing output transformer

Quote:
Originally posted by layt
I bought an output transformer from E-bay that was supposedly a drop in replacement for Fender champs etc etc. Anyway I put it in and hit some strings and it worked ok at low volume, great! I thought now lets turn it up a bit. When I did I got no sound what so ever and the 5Y3 started to arc inside the tube. I triple checked everything and made sure all was well and tried again, keeping the volume low I tried to play again (for about 30 seconds) and it started doing the same thing at low volume. Hmmm strange so back in went the hammond transformer and everything works fine again. This morning I was damned pissed off with that "fender replacement" so I tried again to get it to work.This time all I got was this awful squealing noise that changed pitch when I played with the volume. Can I assume the transformer is a piece of junk?

Oh Boy,

I have been fighting this exact problem in a Fender Champ with a Supposed "DROP IN" replacement output transformer also !

I bet I could guess who's you bought.
In my case, I built a Champ AA764. It has a separate bass and treble Vs a single tone control.
So I have the luxury? No wait, I have yet another variable.
I can play mine at full volume provided the treble control is reduced to about 65%.
Otherwise, I get a terrible squeal and will even go to cut-off and get no sound. The 6V6GT runs excessively hot and lowering the bias has/will not improved the situation. Depending on the model Fender you own, I have found clipping the feedback lead kills the squeal, But not the cut-off issue. If its the same brand/source transformer as mine, Simply reversing the primary leads will likely not work. I had about a 4 page thread on this amp here and we never solved it.
Another route, I put in a 12AU7 and made it playable without modifications "BUT" output was much lower, Tone was terrible and I ruled it out as a possible option. One of the Master Builders on this site "Tubelab" has mentioned a source for a far better transformer from Edcor, At $18.00 +SH it looks real promising .

Edor Transformer

Trout

PS, What Brand/Vendor Transformer Is this From?

Old Thread
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Old 28th March 2006, 01:14 AM   #5
lndm is offline lndm  Australia
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There could be a few simple explainations. For starters, the xfmr may roll off the highs contributing a phase shift that upsets the feedback. You may need to re compensate the amp.

Another possibility has to do with the Q of the natural top end roll off which has to do with interwinding capacitance etc (which is often why power xfmrs don't make good OPT's. This may be improved by putting a zobel across your speaker/secondary to damp the highs.

OPT's also 'sing' by physically resonating. Transducers can work forwards or backwards (we're talking microphonics). Try tightening the bolts.
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Old 28th March 2006, 03:33 AM   #6
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I am familiar with the transformer that Trout spoke about. I have had it work well in my breadboarded amps, but when everything is all mounted to a chassis, they will often oscillate even in a design that does not use feedback.

I assume that you are referring to the "
Made precisely to the original Fender specs.

Drop-in replacements for original Fender amps"

transformer. I have no experience with this one.

I learned about the Edcor XSE-15-8-5K through this forum. they outperform any trqansformer that sells for less than $50
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Old 28th March 2006, 05:03 AM   #7
Trout is offline Trout  United States
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If Its the Transformer I am thinking it might be,

Quote:
A bolt-on replacement for Fender Champs, but also useful for many do-it-yourself projects and repairs on vintage amplfiers & radios. Typically used with tubes such as 6BQ5/EL84, 6BM8/ECL82, 6AQ5/EL90, 6CM6, 6V6-GT. Now you can use a wide range of 8 ohm speakers in the ubiquitous Fender Champ, without drilling or hacking up your amp. TF103-48 bolts into the same 2 3/8 inch (61mm) mounting hole centers as the original blackface & silverface Champ output transformer , but it has both 4 & 8 ohm output taps
They are selling them on Ebay as
Quote:
Single Ended 5K Transformer 4/8 Ohm Output Champ NEW
I know a couple other guys on another forum that had the same problems trying to use them as replacements in genuine fender champs.

I wrote the Vendor a while back about the issues, The Comment Back was Simply
" We do not know of any issues, But we will contact the manufacture and get back to you."
That was about 4-6 months ago.

I am learning all over again what I used to preached when I was a national model areobatics champion, "Cheap is often self defeating"
Trout

BTW, I still use the amp, I noise test tubes in it, Modded it for a couple different sweep tubes,Tinkered with it until I was purple, If it ever suddenly worked correctly, I would think something broke haha
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Old 28th March 2006, 12:27 PM   #8
SHiFTY is offline SHiFTY  New Zealand
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The only times I have had similar problems have been with either

1) feedback connected wrongly (is there anything internally connected to the speaker (+) terminal that goes back to the first stage? If so try disconnecting it.) Make sure the speaker (-) terminal is connected to the chassis/ground.

2)Location of input wiring- if your input wiring or tubes is anywhere near your output section, it will squeal. You need to move the input section or output transformer, use shielded cables with the shield grounded for all signal connections, and tidy away all B+ wiring so it is far away from the signal wiring.

3)Output transformer location- it might be radiating signal which is picked up by the input stage. Try rotating it 90 degrees.
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Old 28th March 2006, 02:11 PM   #9
Sherman is offline Sherman  United States
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I didn't drop one in to an existing Fender but I did build a guitar amp recently using one of these and it works fine. About 4.9 watts according to my measurements with no squealing, hot tubes or cut-off issues.

The amp I built was a modification of the AX84. I used the input, driver and tone stack from the AX84 Extreme and the output section of the AX84. It has bass, mid and treble controls, drive and master volume along with a clean-dirty switch. The controls all work as they should.

Bottom line- it is only one OPT in one amp so it is hardly a statistically significant sample but at least some of these OPTs can work as advertised, at least in a complete DIY amp.
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Old 28th March 2006, 04:02 PM   #10
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I have used the TF103-48 in several successful small guitar amp designs. I even tried them in a budget HiFi amp. They all worked. (none used negative feedback)

There is another "drop in replacement" transformer being sold on Ebay in Australia. It is not clear which one the thread originator was using.

As stated before, the first thing to do when an amp oscillates is to disconnect any feedback circuitry and get the amp stable. Then you can optimize the feedback for the given OPT.

When the amp oscillates without any INTENDED feedback source, then there is some feedback loop somewhere. In a compact guitar amp design there is a lot of gain in a small package and there is always the possiblity of oscillation. Just because the amp oscillates with one transformer and not another, does not imply that one transformer is bad, just different.

Until I found out about the Edcor, I had decided that the TF103-48 was good for $14, the next step was a Hammond 125CSE for $29 and then there are plenty of larger transformers for larger money. The Edcor is an anomally, it looks like $29 worth of transformer for $18.
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