Open secondary on reverb transformer

Tron Pesto

Silver Supporting Member
Messages
920
Sorry for the multiple posts about the same amp, but this really is a different topic, so here goes.

In an Elk Prosonic I'm overhauling, one problem is a shot reverb transformer - the secondary is open. No big deal to replace it, but I have a general question about open secondaries affecting reverb driver tubes.

I understand why one should not start up an amp without the speakers connected. Wouldn't this also apply to a reverb circuit? I'm thinking that is the secondary has an open winding, this will be seen as an "infinite" load on the primary causing the reverb driver tube to try to drive an immense amount of current and fry up. Am I off base?

I ask because I'm thinking I need to address this before I start this amp up or I could cause some damage. Right? Wrong? Stupid? Crazy?
 

pdf64

Member
Messages
8,929
An unloaded secondary puts (voltage) stress on to its primary.
I can’t see how an unloaded secondary would cause the secondary to fail open circuit.
I guess that best practice would be that the reverb transformers secondary has a load whilst on the bench, during testing etc.
But neglecting to do that almost never causes a problem.
 

HotBluePlates

Member
Messages
14,082
... In an Elk Prosonic I'm overhauling, one problem is a shot reverb transformer - the secondary is open. ... I understand why one should not start up an amp without the speakers connected. ... I'm thinking that is the secondary has an open winding, this will be seen as an "infinite" load on the primary causing the reverb driver tube to try to drive an immense amount of current and fry up. Am I off base? ...

Short Answer: Yes, you're off base. The reverb driver tube is probably fine. Just replace the reverb transformer & be on your way.

Longer Answer: No load on a transformer secondary tends to cause flyback voltage spikes on the primary, damaging the transformer primary insulation. This condition generally doesn't harm the tube, and also typically doesn't cause excessive tube (plate) current. With regular output tubes, the no-load on the secondary could cause the tube to operate in a region where screen current can be excessive, but a reverb driver tube doesn't usually have a screen grid. And with output tubes (and pentodes/beam power tubes used as reverb drivers) we usually have screen grid resistors that might help limit the impact of excessive screen current.
 

Tron Pesto

Silver Supporting Member
Messages
920
Short Answer: Yes, you're off base. The reverb driver tube is probably fine. Just replace the reverb transformer & be on your way.

Longer Answer: No load on a transformer secondary tends to cause flyback voltage spikes on the primary, damaging the transformer primary insulation. This condition generally doesn't harm the tube, and also typically doesn't cause excessive tube (plate) current. With regular output tubes, the no-load on the secondary could cause the tube to operate in a region where screen current can be excessive, but a reverb driver tube doesn't usually have a screen grid. And with output tubes (and pentodes/beam power tubes used as reverb drivers) we usually have screen grid resistors that might help limit the impact of excessive screen current.

Thanks for the info Hot Blue Plates! It will be at least a week before I have the replacement reverb tranny - I wanted to test the amp before then (which I will now), that's why I was concerned.
 

Tron Pesto

Silver Supporting Member
Messages
920
An unloaded secondary puts (voltage) stress on to its primary.
I can’t see how an unloaded secondary would cause the secondary to fail open circuit.
I guess that best practice would be that the reverb transformers secondary has a load whilst on the bench, during testing etc.
But neglecting to do that almost never causes a problem.

Thanks for the info pdf64. I don't know what caused the reverb tranny secondary to fail - it was shot when I got it.

And now thinking about it, I've seen plenty of amps run with the reverb tank disconnected/removed but tranny still connected - which I guess is the same as running with an "open" secondary.
 

TimmyP

Member
Messages
2,488
In my 14 years here, I've seen many amps with an open tank input. I've replaced one reverb transformer. I cannot recall whether the tank was also bad.
 

JJman

Member
Messages
994
I would be concerned with WHY the secondary opened. The standard suspect would be that it was driving a shorted load.
 

Tron Pesto

Silver Supporting Member
Messages
920
I would be concerned with WHY the secondary opened. The standard suspect would be that it was driving a shorted load.

Good point - and I "was" concerned, but I've eliminated (ahem *mostly* confident) any driver circuit issues in the rest of the overhaul of the amp - which was a near replacing EVERYTHING in the amp. I have never come across an amp with such ridiculously drifted or otherwise shot components. Coupling and tone caps were anywhere from 5 to 10 times the value - and leaky as hell. Many were oil in paper from the early seventies. I think they all were "Atlas" brand. And the load resistors were crazy noisy - these were those torroidal resistors from Japan.

Oddly enough, the components that seemed to hold up best were the discrete larger electrolytics - like the final filter stage, fixed bias circuit filter cap, and the bypass caps. But I changed those out too.

And TERRIBLE solder joints - components and wires just laid on top of each other - not necessarily terrible per se, but a lot of cold joints with little to no evidence of flux/wetting.

The circuit is definitely "Fender-esque" and my guess is that these Elk/Echo amps were popular in Japan for their reasonable price and decent tone - but there were NOT built to last.
 

HotBluePlates

Member
Messages
14,082
I would be concerned with WHY the secondary opened. The standard suspect would be that it was driving a shorted load.
... shot reverb transformer - the secondary is open. No big deal to replace it ...

Depending on the construction of the reverb transformer (and your motivation to try something silly), you might try repairing the transformer yourself.

I had an RCA VTVM with an open high-voltage winding on its power transformer. Since I'd have to buy a replacement, I had nothing to lose by opening up the transformer to see if I could find the break. After cutting away the outer layers of kraft paper insulation, I found the break in the hair-like high voltage secondary after only taking off maybe 2-3 turns. I carefully scraped away the enamel insulation on that wire, wrapped it on the end of the much-thicker lead-out wire, and soldered. Transformer fixed! (The couple part-volts I lost by removing the few turns were insignificant).

You might try seeing if you can fix your reverb transformer similarly, unless you've already bought the replacement. You could try to figure out why the original burned open, but in my case I never figured out why and the repair never failed (been nearly 10 years since, so I'm guessing it's gonna continue working).
 

MKB

Silver Supporting Member
Messages
9,323
That's unusual to see a open secondary on a Fender type reverb transformer, as IIRC the output Z is low (8 ohms maybe?) and the circuit just can't put that much power into the transformer. However my Yamaha T100C had a open primary on its reverb TX. I replaced it with a Fender type, and it's worked great ever since.
 

Tron Pesto

Silver Supporting Member
Messages
920
Depending on the construction of the reverb transformer (and your motivation to try something silly), you might try repairing the transformer yourself.

I haven't bought a replacement yet and it's supposed to snow this weekend - perfect for a silly new project! Can't break it any more than it is already...
 




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