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Post subject: Plate Diodes
Posted: Wed Apr 25, 2012 4:03 pm
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What is purpose of the diodes that Fender hangs off the plates of the 6L6GCs in the Blues Deluxe/Deville and Hot Rod amps? I also see them on the Super Sonics. I seem to remember reading that they were for "protection", but protection from what? A search was not successful in pulling up anything.

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Post subject: Re: Plate Diodes
Posted: Wed Apr 25, 2012 4:11 pm
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"Flyback"

When you remove the B+ by switching to standby or turning the amp off, the collapsing field from the OT discharges through the diodes instead of through the output tubes.

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Post subject: Re: Plate Diodes
Posted: Wed Apr 25, 2012 4:21 pm
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Ok. That's what I vaguely remembered. They do not appear on Tweed, Blackface, or Silverface amps that I can see. A quick check on Ampwares looks like they started appearing in the 90's.

What drove Fender to install them in modern amps?

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Post subject: Re: Plate Diodes
Posted: Wed Apr 25, 2012 4:33 pm
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Tubes and iron were a bit sturdier "back in the day".

Arjay

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Post subject: Re: Plate Diodes
Posted: Wed Apr 25, 2012 4:40 pm
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So was I! :lol:

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Post subject: Re: Plate Diodes
Posted: Wed Apr 25, 2012 5:05 pm
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All right, I can buy that. I assume they have no impact on amp tone so removing them has no benefit. But again, I vaguely remember some discussion about removing them for some reason or am I hallucinating (again)?

The reason I am asking is because I am contemplating a few mods to my BDRI this weekend. The PI and power amp stages are within a resistor value or two of being identical to the 59 Bassman Reissue/vintage 5F6A (the feedback resistor being the most obvious) so I am thinking about changing them to see what the effect would be. Also, the tone stack is the same as the Bassman Reissue (which is different from my 5F6A clone) so I am thinking of changing the $@!&# resistor and bass cap to the vintage values which will roll off the low bass a little bit quicker. I find the bass of the stock BDRI to be a bit boomy with the Eminence Cannibis Rex and a humbucker equipped guitar. I much prefer the low end response of the vintage Bassman.

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Post subject: Re: Plate Diodes
Posted: Wed Apr 25, 2012 5:05 pm
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shimmilou wrote:
So was I! :lol:


"Sturdier" or "studlier"? :lol: :lol: :lol:

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Post subject: Re: Plate Diodes
Posted: Wed Apr 25, 2012 5:24 pm
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Both! :o

I wondered the same thing, so I pulled one side of both diodes in my HRDlx a while back (a couple of months ago or so) and there was no difference in tone that I could detect. I have several hours playing on it since then, and forgot about the diodes being disconnected until your post. I just loaned my amp to a friend last week, and it is still working fine, but I'll reconnect the diodes when I get my amp back as there was no tone advantage having them disconnected, and the diodes are there to protect the tubes.

P.S.
I would be interested to hear if you notice any sound difference with the diodes disconnected, if you decide to try it. Mine were close to the edge of the tubes' circuit board, and fairly easy to unsolder and pull one side without removing the board, they should be the same for the BDRI.

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Post subject: Re: Plate Diodes
Posted: Wed Apr 25, 2012 6:59 pm
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shimmilou wrote:
P.S.
I would be interested to hear if you notice any sound difference with the diodes disconnected, if you decide to try it. Mine were close to the edge of the tubes' circuit board, and fairly easy to unsolder and pull one side without removing the board, they should be the same for the BDRI.


I doubt I'll bother. I do have to question the need for them though. The JJ 6L6GCs seem to be pretty tough tubes. I'm using the same tubes in my 5F6A clone with no diodes and I doubt the OT in that amp is vintage spec. It seems to have no problem when shutting the B+ off.

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Post subject: Re: Plate Diodes
Posted: Wed Apr 25, 2012 7:07 pm
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If it ain't broke, don't "fix" it.

:mrgreen:

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Post subject: Re: Plate Diodes
Posted: Wed Apr 25, 2012 7:30 pm
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Retroverbial wrote:
If it ain't broke, don't "fix" it.

:mrgreen:

Arjay


Its just the engineer in me asking "Huh?" :lol:

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Post subject: Re: Plate Diodes
Posted: Wed Apr 25, 2012 8:16 pm
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Problem with "flyback" is it can cause a catastrophic failure in a big amp (like an Ampeg SVT). Or small damage to the output tube's grids in smaller units that run over-sized OPT's. Something you may not hear or notice immediately, but may contribute to shorten tube life, with time.

Leave them in. You may not have the luxury of using super-tough tubes, the next change-out.


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Post subject: Re: Plate Diodes
Posted: Wed Apr 25, 2012 8:22 pm
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Now that's irony bluesky636, because it was, in all likelihood, an engineer that decided to add the diodes in the first place. :lol:

Great point BMW2002Ti, I'm sure that's what they are for, and they don't seem to affect tone.

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Post subject: Re: Plate Diodes
Posted: Wed Apr 25, 2012 10:17 pm
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Hey guys, I'm not disputing that they are probably a good thing to leave in and that they don't affect the amp tone. I'm just curious as to what drove Fender to install them in the first place. Did some sort of major failure occur? Or is it really more of a solution to a theoretical problem? Have any of you ever known an amp to fail because of "flyback"?

Looking at the amp schematics on Ampwares (http://ampwares.com/), these diodes seem to have appeared almost universally on high powered Fender amps in the 1990s. They don't seem to appear on modern reissues of vintage amps like the Tweeds or Blackfaces. Am I to beleive that the tubes and OTs in the reissues are really better than what is used in "modern" amps and that the diodes are unnecessary? I guess I'll have to read Richard Kuehnel's "Vacuum Tube Circuit Design: Guitar Amplifier Power Amps" book to see if he talks about these diodes. I'm still working my way through his book on preamp design at the moment.

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Post subject: Re: Plate Diodes
Posted: Wed Apr 25, 2012 11:30 pm
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Here is my short analyse of the circuit with the same conclusion as Shimmilou.

The diodes avoid to have a negative voltage more than -0.6V on the plates regarding the ground.

In normal operation they are in reverse mode, then only a very small "varicap" is added to the circuit, it may be not detectable by "standard" human hear.

When these diodes can have current ? --> when the forward voltage is rougly 0.6V.
When it is possible ? --> the transformer has a leakage magnetic coil (L) and idle current (I), then when the idle current is switched off, the current decrease abruptly from idle to 0. This variation of current in the coil produces a voltage that is negative. According to the model V = L *d(I)/dt, shorter the time is, higher negative voltage is. In order to protect the power valve, the diodes pass this transient current instead of the valve as Shimmilou explained. These diodes can be called freewheeling diodes.

BTW, with an ideal transformer connected on a resistive load, there is no overvoltage possible. May be during '80, the transformer changed, an hypothesis could be more isolation and consequently less coupling between primary and secondaries given more leakage coil.

Probably, this introduction is the consequence of deep study of transient during design or redesign phase, and "easily" studied with spice simulators populed in the '80.

8)

From my point of view, a design center should no put any spare components on a product that is cost driven.

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