Replacing flyback diodes in vintage Ampeg v4b

[xj98jeep]

Flyback diodes are bad, I know I can leave them disconnected but I want to replace them. OTs are expensive! I'm going to replace them with 1n4007s as per the interwebs suggestion, but how do I know how many to use? There are currently 2 parallel sets of three series wired diodes coming from one of the terminals (r45, I think) and one set of two larger diodes wired in series coming from the other terminal (r46? I'm not looking at the amp so I can't remember.)

How do I know how many diodes to use/what configuration? They do nothing but block reverse current flow, so are there multiples for redundancy, and the two sets wired in parallel so that if one blows it will still work? If I'm correct I could in theory use however many I want, right? 8 per string, with 4 strings wired parallel for crazy redundancy (I won't do that, but just to make sure I understand)

 

[Old Garage-Bander]

Flyback diodes are bad, I know I can leave them disconnected but I want to replace them. OTs are expensive! I'm going to replace them with 1n4007s as per the interwebs suggestion, but how do I know how many to use? There are currently 2 parallel sets of three series wired diodes coming from one of the terminals (r45, I think) and one set of two larger diodes wired in series coming from the other terminal (r46? I'm not looking at the amp so I can't remember.)

How do I know how many diodes to use/what configuration? They do nothing but block reverse current flow, so are there multiples for redundancy, and the two sets wired in parallel so that if one blows it will still work? If I'm correct I could in theory use however many I want, right? 8 per string, with 4 strings wired parallel for crazy redundancy (I won't do that, but just to make sure I understand)

Do you have a link to that interwebs suggestion?
This has me curious about how these, and or, the original diodes were used.

Sounds kind of weird to me. I know why someone might use diodes in series, but parallel?
This clearly is something I've not encountered.
I am familiar with flyback transformers, but never heard flyback used with diodes.
Must be some kind of special circuit?

I can't be a know it all if I don't know it all.

 

[beans-on-toast]

What you have installed in your amp is non original. The original had two series diodes on each leg. The V4B reissue does the same, they use 1N4007's.

These diodes have a way of shorting, chaining them does add some redundancy. In addition, the reverse voltage sums with each diode in series. Two will give you (0.7 x 2) 1.4V. The SVT uses a chain of three 1N4007's on each leg. The diodes will limit the size of the spike, so you don't want an arbitrary number in the string.

These diodes can be damaged if a power tube blows. They serve as a safety device to protect the output transformer. If a power tube blows, test these diodes. It isn't unusual to have to change them.

 

[xj98jeep]

What you have installed in your amp is non original. The original had two series diodes on each leg. The V4B reissue does the same, they use 1N4007's.

These diodes have a way of shorting, chaining them does add some redundancy. In addition, the reverse voltage sums with each diode in series. Two will give you (0.7 x 2) 1.4V. The SVT uses a chain of three 1N4007's on each leg. The diodes will limit the size of the spike, so you don't want an arbitrary number in the string.

These diodes can be damaged if a power tube blows. They serve as a safety device to protect the output transformer. If a power tube blows, test these diodes. It isn't unusual to have to change them.

Awesome, very informative response I appreciate it. You'll have to bear with me then, I thought I understood the diodes' function but based on what you're saying I was incorrect.

It sounds like they don't stop the flow of current in the reverse direction, but merely limit it? How do I know how many diodes to string together in series? And where is the .7v spec you mentioned coming from? I see the 1n4007 is rated for 1000 volts on the spec sheet, no mention of .7v.

Do you have a link to that interwebs suggestion?
This has me curious about how these, and or, the original diodes were used.

Sounds kind of weird to me. I know why someone might use diodes in series, but parallel?
This clearly is something I've not encountered.
I am familiar with flyback transformers, but never heard flyback used with diodes.
Must be some kind of special circuit?

I can't be a know it all if I don't know it all.

It's hard to say as I've spent all day Googling various combinations of Ampeg, v4b, flyback diodes, etc and pieced it together from a lot of different places, but the ampegv4.com forums are pretty handy, and a few other spots. Link Removed was one of the places I saw the three 1n4007s mentioned, and this was one of my starting points.

I'm also kind of "dumb" in the sense that I don't really understand much about the circuitry, simply that the diodes are bad and must be replaced so I can't offer much insight as to the design.

 

[beans-on-toast]

The purpose of the diodes is to limit spikes coming through a power tube. This might help clarify (maybe not): Output transformer protection, flyback diodes, MOVs, 2KV capacitors,.

Note how a voltage spike can punch through insulation and arc between windings. The transformer specs dictate what spike voltage can be withstood. Age and operating temperature also play a roll. Wire coatings of old transformers are more susceptible to damage.

There are different circuits that people use to protect output transformers. One way is to use diodes, another is to use a resistor and series capacitors, MOVs work well, etc. The diode string is cost effective so people use them.

Use two 1N4007 diodes per string since that is what the designer intended. Some people string three together. I remove these diodes in V4B's, I find that they are more trouble than they're worth.

A diode has a forward and a reverse voltage. This is explained in the data sheets. The 0.7V is a forward voltage typical of a diode. To be more precise, the forward Vf of the 1N4007 is typically 1V or 1.1V. It also varies with current. There is also a maximum repetative reverse voltage of 1ooov.

 

[agedhorse]

The purpose of a flyback (or backswing or back EMF or kickback or snubber or any number of other names) diode is to short any inverse voltage generated by a variety of mechanisms, thus clamping the voltage to a safe value.

For example, when clipping an amp (especially with no load), you can generate a voltage opposite of the normal voltage due to the collapse of the magnetic field. Same thing applies to coil voltages on relay and contactor coils when driven with solid state drivers.

For example since an OT primary is typically driven at the center-tap by B+. The ends of the transformer are limited under normal operation to swing no more than 0V to B+. When clipping (under some conditions), or with an open secondary, it's quite possible for the ends of the transformer to rise above B+ following the formula V = L (di/dt) where the higher the inductance of the primary, and the faster the rate of rise of the signal, the higher the voltage. rate of rise can be as simple as the opening of the secondary circuit (a switch theoretically has infinite di/dt). If you connected two diodes with the cathodes at the center tap and one anodes at each end, the diodes will only conduct when the end of the primary exceeds B+ (+0.7V) and will clamp the end of the primary to slightly above B+. Since B+ is connected to the plates, and via screen networks to the screens, it serves to protect the tubes from flashover as well.

The reason to use 2 diodes in series is to increase the reverse breakdown voltage under normal operation. The drawback is that it increases the forward drop to 1.4V rather than 0.7V (insignificant). Paralleling diodes is sometimes done to increase current handling capacity, and also power dissipation under fault events. There are other ways to do this however, primarily by using 3 amp rated diodes rather than 1 amp.

Diodes used to protect screens, drop screen voltage, clamp grid voltages, and such are often a different kind of diode that works in a different way that a rectifier diode like the 1N4007. These are called Zener diodes or avalanche diodes, they exhibit reverse blocking up to a set threshold then conduct in a breakover or avalanche action. These are not used for backswing protection.

There are so many ways to implement protection. Each has its advantages and disadvantages. Alternately, the transformer can be designed with reinforced insulation so that they are robust enough not to suffer undue damage under realistic conditions, though nothing is perfect.

 

[beans-on-toast]

An interesting aside, tube rectifiers, such as a 5AR4 and 5Y3GT, found in amps are essentially two diodes. When the amp is turned on and the power supply capacitors are fully discharged, the current inrush can cause flashover in the tube which can damage it. Since these tubes are pretty expensive these days, one solution is to add a series diode between each high voltage transformer winding and each plate in the tube rectifier. These series diodes extend the peak inverse voltage of the rectifier tubes and prevents the flashover. This technique has been adopted by amp manufacturers such as Fender and used in amps with tube rectifiers such as the '57 Deluxe (http://www.fmicassets.com/Damroot/Original/10001/8150500_gamp_schematics.pdf). There are other ways to limit inrush current and prevent this.

This is another example where increasing the peak inverse voltage with series diodes can be useful.