help me find a transformer for my amp??

benmurray85

so I've apparently blown the transformer in my 5E3 clone. all i know is that the numbers and info on it are 40-18022 166-0933 magnetic components inc.

can anyone recommend what i should be looking for and where to start looking online?

knuckleberryfinn

Argh how did you do that?! I assume it's the OT. Hammond 1750e or 1760e depending on what secondary impedance you want. Bluebell audio do them. So do Mouser UK.

Cirrus

This is the website for Magnetic Components transformers; http://www.classictone.net. Just find your amp on there or drop them a message. Or you can get a uk made one from http://livinginthepast-audioweb.co.uk. I've got a classictone in my ac30, it's good.

benmurray85

knuckleberryfinn said:

Argh how did you do that?! I assume it's the OT. Hammond 1750e or 1760e depending on what secondary impedance you want. Bluebell audio do them. So do Mouser UK.

hmmm no idea. it just went up in smoke one day. changed the valves and they overheat after 15 mins. my mates sorting it for me but might be a long while till funds allow for the repairs. just glad it didn't go at a gig!!!!

cheers above for the help

ICBM

Which transformer has gone? While it's possible, that doesnt sound like a blown output transformer really. Or even a blown power transformer, if the amp basically works and doesn't blow fuses.

Are you sure there isn't some other fault (eg a shorted cathode cap) which could be causing the valves to overheat? Find the problem before buying expensive parts!

Misterg

This looks like your transformer:

http://www.classictone.net/40-18022.html

But there are UK alternatives if you are *sure* that it is the problem.

jpfamps

As ICBM says I would be 100% there is a transformer problem before you order anything.

For a 5E3 clone I tend to favour Mojotone for the OT (can be ordered via Allparts in the UK), and Hammond for the mains transformers.

benmurray85

Sorry to bump necrothread. Haven't touched the amp since. Can anyone advise whether this transformer will work with my clone? http://www.ebay.co.uk/itm/151416770134?_trksid=p2060778.m1438.l2649&ssPageName=STRK:MEBIDX:IT Thanks guys

exocet

Looks about right...whether it will mount on your chassis (size / hole location) may need further investigation.

ecc83

I echo the caution about proving out the transformer (and I STILL don't know which one!) as the culprit but might I add another caveat?

On another forum a guy swapped an OPT for a supposed replacement only to find the amplifier was instable, motor-boating. He found that lifting the NFB network, paralleled 3k3 and 10nF, cured the problem so it looks as if the new transformer was not up to the quality of the old!

I have no idea whether the amp in question here has a feedback loop but be wary!

Dave.

ICBM

Steadily overheating valves are still far more likely to be a biasing problem than the OT.

First lift the cathode cap and see if the voltage across the cathode resistor changes. If it doesn't, what value is the resistor? Many cathode-biased designs use too low a value, especially as many modern ones have higher HT voltages than the original amps.

benmurray85

guys i should point out I'm absolutely useless with stuff like this. i wouldn't have a clue where to start. i use a local amp tech and have done for years. i trust him implicitly but all these comments are making me think twice. I may try and get a second opinion. Anyone know anyone in the east lancs area?

benmurray85

and as before thanks guys. really do appreciate the help

jpfamps

ecc83 said:

I echo the caution about proving out the transformer (and I STILL don't know which one!) as the culprit but might I add another caveat?

On another forum a guy swapped an OPT for a supposed replacement only to find the amplifier was instable, motor-boating. He found that lifting the NFB network, paralleled 3k3 and 10nF, cured the problem so it looks as if the new transformer was not up to the quality of the old!

I have no idea whether the amp in question here has a feedback loop but be wary!

Dave.

No negative feedback in 5E3.

Did he really mean motor boating (defined as LF instability), or increased hum due to HF oscillation?

Regardless, doesn't mean that the OT was "better" or "worse" than the original, just that the circuit hadn't been stabilized for that component.

ecc83

jpfamps said:

ecc83 said:

I echo the caution about proving out the transformer (and I STILL don't know which one!) as the culprit but might I add another caveat?

On another forum a guy swapped an OPT for a supposed replacement only to find the amplifier was instable, motor-boating. He found that lifting the NFB network, paralleled 3k3 and 10nF, cured the problem so it looks as if the new transformer was not up to the quality of the old!

I have no idea whether the amp in question here has a feedback loop but be wary!

Dave.

No negative feedback in 5E3.

Did he really mean motor boating (defined as LF instability), or increased hum due to HF oscillation?

Regardless, doesn't mean that the OT was "better" or "worse" than the original, just that the circuit hadn't been stabilized for that component.

Pretty sure the guy meant motor boating but of course, remote diagnosis is tricky to say the least!

Yes, I know the new traff might be as good as the original but it eventually transpired that the old one was physically bigger! Not an absolute sign of better quality but a pretty good one.

Still, if no NFB, no problem.

Dave.

knuckleberryfinn

5E3s are so simple that you can diagnose the problem yourself by doing what ICBM said.  Look at the picture below; the two components circled are the bias resistor (white one on the left) and the bias cap (on the right).  All you have to do is unsolder the bias cap at one end.  Obviously unplug the amp from the mains first, and make sure there's no (or very little) DC voltage across the left-most filter cap.  If the amp works after lifting the bias cap (though it will probably be noticeably lacking in gain) then that tiny 75p cap was the culprit (and not a forty quid transformer).  I've heard that a bias cap and resistor soldered too close together can cause the cap to fail owing to all the heat coming from the resistor.  So that might be the cause - if it is, solder the replacements as per the photo below. 

jpfamps

Incidentally, if the problem is the cathode bypass cap, when you replace it us a 105degC rated part, rather than a 85 degC rated cap as it will enhance reliability.

jpfamps

ecc83 said:

jpfamps said:

ecc83 said:

I echo the caution about proving out the transformer (and I STILL don't know which one!) as the culprit but might I add another caveat?

On another forum a guy swapped an OPT for a supposed replacement only to find the amplifier was instable, motor-boating. He found that lifting the NFB network, paralleled 3k3 and 10nF, cured the problem so it looks as if the new transformer was not up to the quality of the old!

I have no idea whether the amp in question here has a feedback loop but be wary!

Dave.

No negative feedback in 5E3.

Did he really mean motor boating (defined as LF instability), or increased hum due to HF oscillation?

Regardless, doesn't mean that the OT was "better" or "worse" than the original, just that the circuit hadn't been stabilized for that component.

Pretty sure the guy meant motor boating but of course, remote diagnosis is tricky to say the least!

Yes, I know the new traff might be as good as the original but it eventually transpired that the old one was physically bigger! Not an absolute sign of better quality but a pretty good one.

Still, if no NFB, no problem.

Dave.

A smaller OT is likely to have a higher LF roll off, and could provoke motor boating, however this would not be cured by snubbing caps, but by moving one of the other LF constants within the feedback loop (which incidentally why it is not always a good idea changing the power valve grid coupling caps is an existing stable design).

ecc83

jpfamps said:

ecc83 said:

jpfamps said:

ecc83 said:

I echo the caution about proving out the transformer (and I STILL don't know which one!) as the culprit but might I add another caveat?

On another forum a guy swapped an OPT for a supposed replacement only to find the amplifier was instable, motor-boating. He found that lifting the NFB network, paralleled 3k3 and 10nF, cured the problem so it looks as if the new transformer was not up to the quality of the old!

I have no idea whether the amp in question here has a feedback loop but be wary!

Dave.

No negative feedback in 5E3.

Did he really mean motor boating (defined as LF instability), or increased hum due to HF oscillation?

Regardless, doesn't mean that the OT was "better" or "worse" than the original, just that the circuit hadn't been stabilized for that component.

Pretty sure the guy meant motor boating but of course, remote diagnosis is tricky to say the least!

Yes, I know the new traff might be as good as the original but it eventually transpired that the old one was physically bigger! Not an absolute sign of better quality but a pretty good one.

Still, if no NFB, no problem.

Dave.

A smaller OT is likely to have a higher LF roll off, and could provoke motor boating, however this would not be cured by snubbing caps, but by moving one of the other LF constants within the feedback loop (which incidentally why it is not always a good idea changing the power valve grid coupling caps is an existing stable design).

Not snubbing caps, the guy disconnected the 3k3 feedback resistor. Coupling caps? Yes This is a HI-FI design he is playing with (and I am still not sure if he intends it for hi fi or guitar!) and the couplers are 0.5mfd. I have told him already that for guitar use they should be reduced considerably in value.

Dave.

benmurray85

benmurray85