Clamping diodes on OT primary?

[JazzGuitarGimp]

Hello All,

I've seen schematics and worked on a few amps that have what appear to be clamping, or flyback diodes, either two or three 1N4007's in series from each of the OT primary windings to GND. It appears that the end result of the diodes is to keep the winding from going negative when the output tubes go into cutoff. But what is the _purpose_ of these diodes - do they lengthen tube life, protect the OT, or something else?

Many thanks,
JGG

[Jana]

They are there to give a false sense of security in that they will protect the amp from all sorts of irregularities, including, but not limited to, solar flares, nuclear emp pulses, rabid skunks, and women with PMS and a loaded 45.

[ampdoc1]

Check out this from Ken! Toward the last pages, it shows the diodes are for tube and OT protection.

There is a lot of other good info here.

[martin manning]

I've seen several reports recently saying that removing these diode strings eliminated some strange sounds. Personally I don't mess with those, skunks, or anybody with a loaded gun.

[dobbhill]

..not much help when they short......
My direct experience after removing them from a handful of amps....
D

[JazzGuitarGimp]

Thanks to all for your replies. The Trainwreck pages is a great resource - thanks ampdoc1 !

[soma_hero]

If you're doing a build with real high voltages on the primaries then they can save your tubes from arching. I do a build with 700V on the primaries of 6550s or KT88s. Had a ton of arcing problems before I started adding flyback diodes.

Not necessary in lower voltage amps but with high voltages they are essential. Something up to twice the B+ can be generated on the plate of the "Off" side during cutoff through inductive flyback. That'll cook tubes or the OT.

[gingertube]

Chased this same issue through another forum.
A couple of amp designer/builders on that forum recommend 330K from each OT primary anode connection to 0V instead of the diodes to "tether" the disconncted end when one tube cuts off as part of normal Class AB operation.
Probably need to made up from series connected resistors as you need 2 x B+ voltage rating.
Cheers,
Ian

[rooster]

gingertube, along these lines look at the schem for a Fender Bassman AB165. Note the 220K resistors that seem out of place, surely the origin of the 330K resistors. It seems to work, or at least let me say that I don't seem to get the artifacts I get with the diodes. I won't use the diodes anymore.

[Structo]

I haven't seen that before.

Is the AB165 the only Fender amp to use those resistors?

[rooster]

As far as I know, yes, the only Fender amp to use this. Odd, isn't it? I assume since it's a bass amp, this was intended to stop some type of issue relating to bass frequencies at the output section, subharmonic or not I couldn't say.

Does it perform as well or better than the diode string? I couldn't say that either.

It doesn't really change the resistance of the primary tap, 220K in parallel with 64 ohms so it isn't hurting anything that I can see. ?? To see another poster mention this type of circuit being used by another builder or two is interesting. For that matter there's prolly a Western Electric white paper on it somewhere because it's certain Fender didn't dream it up. ....And of course Fender didn't keep it in their amps for that matter, but the company was sold at this time so who know what might have happened. It does seem to be one of the most popular Bassman heads ever sold by Fender. They say Duane Allman used one at Muscle Scholls, and I'm sure there are other famous users because these things were everywhere.

[desu boi]

gingertube, along these lines look at the schem for a Fender Bassman AB165. Note the 220K resistors that seem out of place, surely the origin of the 330K resistors. It seems to work, or at least let me say that I don't seem to get the artifacts I get with the diodes. I won't use the diodes anymore.

Those resistors are not along these lines (flyback damping). They are negative feedback loops. They restrict the output tubes.

[husky]

Hello All,

I've seen schematics and worked on a few amps that have what appear to be clamping, or flyback diodes, either two or three 1N4007's in series from each of the OT primary windings to GND. It appears that the end result of the diodes is to keep the winding from going negative when the output tubes go into cutoff. But what is the _purpose_ of these diodes - do they lengthen tube life, protect the OT, or something else?

Many thanks,
JGG

I used to do that many years ago but when they do fail and I have seen them short they need to be replaced. I had a very pissed of Pat Thrall call me from Europe once cause replacing the power tubes still had a failed amp. The diodes had shorted. Now yes you could argue it could have been the OT that shorted and these saved the OT. However.... in at least the amps I have built I have not once seen a blown OT for any reason. I don't bother with them anymore.

[Diablo1]

The diode strings appear on Ampegs from the early 1970s, like the V4 and V2. 580 volts on the plates is not unusual for the V2. Ken Fischer worked for Ampeg during that era, so it's likely he got the idea of using the diode strings on Trainwrecks from the Ampeg engineers.