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Post subject: Re: Plate Diodes
Posted: Thu Apr 26, 2012 12:43 am
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Tissan, I agree with your great analysis and Shimmy. Also, one other point: Output tubes. The quality and quantity were decreasing, even in the 1970's. So as amps got bigger, the question of the availibilty of tubes to produce the watts needed for these amps became an issue.

The "flyback" issue is well-known in the Ham and Transmitter world. And circuitry to counter its effect seems to be a good solution to the question of the quality of the output tubes.


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Post subject: Re: Plate Diodes
Posted: Thu Apr 26, 2012 4:34 pm
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While I appreciate and understand all the explanations given, they still don't answer the question as to why some Fender amps use the diodes and others don't (that question can be extended to other manufacturers too).

Looking at high power amps listed on this website (40 watts and up), we find the following:

65 Twin Custom Reissue - no diodes
57 Twin Amp Reissue - no diodes
59 Bassman Reissue - no diodes
65 Super Reverb Reissue - diodes
65 Twin everb - no diodes
94 Twin Amp - diodes
Bandmaster/Deluxe VM - diodes
Blues Deluxe/Deville Original & Reissue - diodes
Concet everb - diodes
Custom Vibrolux Reverb - diodes
Hot Rod Deluxe/Deville - diodes
Prosonic - no diodes
All Supersonics - diode
Twin Amp -diodes
Vibrosonic - diodes
Vibroking - diodes

5 amps without diodes, 10 amps with.

Most of the ones without diodes are reissues of vintage amps. Modern amps seem to have the diodes. I think we can rule out tubes since Fender uses basiclly the same power tube in their amps, so that leaves the transformers themselves. Am I to conclude from all this that the transformers used in the modern design amps are inferior to the vintage amp reissue transformers in amps that are made today?

I recognize that this is all probably acedemic, but I am just curious about all this, so please humor me. :D

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Post subject: Re: Plate Diodes
Posted: Thu Apr 26, 2012 6:18 pm
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bluesky636 wrote:
Am I to conclude from all this that the transformers used in the modern design amps are inferior to the vintage amp reissue transformers in amps that are made today?


I can attest that the PT and OT in the DRRI are physically somewhat smaller than those found in the vintage DR. The chokes and reverb trannies appear to be the same.

It's noteworthy that the SRRI uses the flyback diodes while the other blackface re-issues do not. Perhaps an effort to extend the service life of marginally-adequate GZ34's......?

:?:

Arjay

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Post subject: Re: Plate Diodes
Posted: Thu Apr 26, 2012 7:22 pm
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Quite possibly, the OTs themselves would determine whether or not the diodes would be necessary/beneficial. Some transformer designs will have a more flyback than others. I can't see how these diodes would protect anything other than output tubes, each diode is parallel to each output tube, diodes being reverse polarity to the operating tube current. The current is very high during the discharge, and with the diodes, the high current is shunted away from the tubes, but all other parts of the amp circuit still see the same current and voltage. :idea:

I would say the opposite about some newer transformers vs old. Many of the newer ones might be smaller, but are likely more efficient, and can possibly handle higher currents and voltages, thus the need for the flyback diodes. With less losses in the newer designed OTs, they could easily deliver greater flyback.

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Post subject: Re: Plate Diodes
Posted: Thu Apr 26, 2012 7:29 pm
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shimmilou wrote:
With less losses in the newer designed OTs, they could easily deliver greater flyback.


Quite possible.

Thanks for the reminder, shimmilou.

Arjay

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Post subject: Re: Plate Diodes
Posted: Thu Apr 26, 2012 7:32 pm
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Arjay,

And potentially marginal 6L6GC's. Fender or any manufacturer cannot know the future of this tube. It's already known that many labeled "6L6GC" do not meet original RCA standards (like JJ 6L6GC).

Best to have those "avalanche" diodes (often in series to dump higher flyback) sitting on the 6L6GC plates to OPT. Then, the chance of sudden, large current backflow through the output tube is much less.

Some amps, like Ampeg SVT or VT22 put some on the grids to further the safety of flyback jumping pins. Even then, I've seen big amps with carbon tracings across the socket pins, where flyback or instabilities have caused an arc across them. I usually replace these sockets, as there is usually is irreversible damage to the tangs.

Interestingly, I have seen only a few these diodes on any Ampeg schematic. You open the amp and there they are! Like an after-thought.

Here is a schemo of an Ampeg V4 amp with cascading "avalanche" diodes (D7 through D10). If I remember correctly this amp also had diodes on grids of the those wonderful 7027A. A great powerful amp.

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Last edited by BMW2002Ti on Thu Apr 26, 2012 9:02 pm, edited 1 time in total.

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Post subject: Re: Plate Diodes
Posted: Thu Apr 26, 2012 7:59 pm
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BMW2002Ti wrote:
A great powerful amp.


Indeed......MAGNIFICENT!

And look at that OT -- you could use such iron as an at-sea anchor to moor the USS Nimitz.

:mrgreen:

Arjay

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Post subject: Re: Plate Diodes
Posted: Thu Apr 26, 2012 10:09 pm
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Retroverbial wrote:
It's noteworthy that the SRRI uses the flyback diodes while the other blackface re-issues do not. Perhaps an effort to extend the service life of marginally-adequate GZ34's......?

:?:

Arjay


Yes, I noted that in the Super Reverb Reissue.

How do the diodes extend the life of the rectifier tube?

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Post subject: Re: Plate Diodes
Posted: Thu Apr 26, 2012 10:19 pm
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shimmilou wrote:
Quite possibly, the OTs themselves would determine whether or not the diodes would be necessary/beneficial. Some transformer designs will have a more flyback than others. I can't see how these diodes would protect anything other than output tubes, each diode is parallel to each output tube, diodes being reverse polarity to the operating tube current. The current is very high during the discharge, and with the diodes, the high current is shunted away from the tubes, but all other parts of the amp circuit still see the same current and voltage. :idea:

I would say the opposite about some newer transformers vs old. Many of the newer ones might be smaller, but are likely more efficient, and can possibly handle higher currents and voltages, thus the need for the flyback diodes. With less losses in the newer designed OTs, they could easily deliver greater flyback.


What would the impact of higher/lower B+ voltages with older/newer OTs be on flyback? I notice that the original 65 Twin Reverb schematic shows the B+ at 460 VDC

http://ampwares.com/schematics/twin_reverb_ab763.pdf

while in the Reissue the B+ is only 442 VDC

http://support.fender.com/schematics/gu ... ematic.pdf

Neither amp has the diodes.

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Post subject: Re: Plate Diodes
Posted: Thu Apr 26, 2012 10:21 pm
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BMW2002Ti wrote:
And potentially marginal 6L6GC's. Fender or any manufacturer cannot know the future of this tube. It's already known that many labeled "6L6GC" do not meet original RCA standards (like JJ 6L6GC).


How does the JJ 6L6GC not meet the original standards? I thought the JJ was a pretty stout tube. Do you have data sheets for both?

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Post subject: Re: Plate Diodes
Posted: Thu Apr 26, 2012 10:23 pm
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This has been a great discussion, the kind that I really enjoy and I appreciate everyone's thoughts. I've already learned quite a bit.

One last question, then I'm off to bed.

Has anyone ever seen any data on amp failures caused by high flyback currents?

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Post subject: Re: Plate Diodes
Posted: Fri Apr 27, 2012 1:51 am
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I've had issues with the QA/QC of many of the latest JJ output and rectifier tubes. Spec sheets and what the tube is really built to, are two completely different things. Look at the original 1950's TungSol or RCA spec sheet for it's 6V6GT.

http://www.mif.pg.gda.pl/homepages/fran ... /6V6GT.pdf

It recommends 315VDC as max plate voltage. Even, in AB1 push-pull mode. How many TS 6V6GT you think actually ran at those levels? Zero. And everyone knew that, & they could push well over 425VDC and 12 watts dissipation and survive, without a sweat.

Now, look at today's TS RI 6V6GT. They post the same spec's as the 1950's TS data sheets. My experience is that you NEED to run today's TS 6V6GT at close to those ratings. Max of 375VDC on the plates. Max of 8-9 watts dissipation. And closer to a 100k-ohm grid-to-ground resistor, than the 220k-ohms of the original 6V6GT's.

As for these diode saving rectifiers. Same issue. When you turn off your amp, the stored E-M field in the OPT collapses. As is converted to electricity. This electricity then flow backwards to find the easiest path to ground. Which could be your output tubes. Or the rectifier (if the standby switch is left in "on" position). The path usually does not go through a charged filter cap, as this is not the easiest path to ground.

So that leaves the tubes, connected to the OPT. Now, if you attach an appropriate diode to the power supply B+ line. That doesn't allow electrons to go to ground UNTIL a certain level (or avalanche point) is reached. Then, the simpliest path is through these diodes. The instantaneous voltage of a big OPT collapsing can reach 1KV+. So, it's much higher than the average voltage of B+, when the amp is turned on.

These diodes also protect the OPT from catastrophic failure if they are not connected to a proper load (or no load) and the primary side builds up a saturated EM field, without load drain from the secondary side.

http://ampgarage.com/forum/viewtopic.ph ... w=previous

As for amp failures. Yes. I've seen one Ampeg SVT and quite a few hi-end Audio Research amd Conrad-Johnson amps. Those high-end hi-fi amps are very complicated and I've seen my shared of fried output sections, of amps which were worked on by "techs" who were way over their heads, in technical aspect of these amps.

Powerful Ampeg amps can go unstable when parts in the output section fail or go too far out of spec. I usually use WW 1% tolerance resistors in all parts of these amp's output. Grid-to-ground. Screen. And grid stoppers. Both for heat stability under all those 6550's and rock steady ratings. How you dress the section and the wiring looms are critical to a well-behaved unit. I've seen one Ampeg with fried & cracked sockets! You can imagine what those beautiful TungSol 6550's looked like. All of the diodes and a good percentage of the resistors were nuked. Resulted from a period of no load. Then, plugging in the speakers with the thing near half-volume. The stored energy in those OPT is a sight to behold.


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Post subject: Re: Plate Diodes
Posted: Fri Apr 27, 2012 11:18 am
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Interesting writeup BMW. Thanks.

My only quibble is that in your last post you mentioned JJ 6L6GCs not meeting original spec, but then you go on to talk about 6V6GTs. In any case, the JJ 6V6S is rated for and is capable of operating at higher plate voltages as seen here:

http://www.jj-electronic.com/pdf/6V6.pdf

I use the JJ 6V6S in my modded Champ 600 with a plate voltage of 322 VDC. The bias is set to 86% max plate dissipation.

I use the JJ 6L6GC in my BDRI and 5F6A clone with plate voltage and bias setting of 410 VDC/53% and 425 VDC/70% and they sound great.

I have read your posts about problems with the JJ EL-XX tubes. I forget if you said they were all from the same run or not. Don't recall anything about bad rectifier tubes (I use the JJ 5AR4 in my 5F6A clone).

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Post subject: Re: Plate Diodes
Posted: Fri Apr 27, 2012 12:05 pm
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Blue, the JJ 6V6S is not a true 6V6GT. But, more like a 5881 in bottle closer to 6V6GT size. That is why it can take more voltage and current than a typical 6V6GT.

Yes, I have had issues with JJ 6L6GC. The last set I use had one go ballistic, out-of-the-box. Reading other boards, the problems with their octal power tubes and rectifiers were noted. Not that you can't get a good set. It's just that prolly a good idea to have them thouroughly tested and screened before purchase. As the company's QA has become more questionable.


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Post subject: Re: Plate Diodes
Posted: Fri Apr 27, 2012 3:29 pm
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BMW2002Ti wrote:
Blue, the JJ 6V6S is not a true 6V6GT. But, more like a 5881 in bottle closer to 6V6GT size. That is why it can take more voltage and current than a typical 6V6GT.


I've read that about the JJ 6V6S. It does sound mighty nice in my little Champion 600, though. :D

BMW2002Ti wrote:
Yes, I have had issues with JJ 6L6GC. The last set I use had one go ballistic, out-of-the-box. Reading other boards, the problems with their octal power tubes and rectifiers were noted. Not that you can't get a good set. It's just that prolly a good idea to have them thouroughly tested and screened before purchase. As the company's QA has become more questionable.


I buy all my JJ tubes from Eurotubes. They claim to test everthing they sell:

http://www.eurotubes.com/eurotubes-test ... rranty.htm

I have no reason to doubt them. I know I don't use my amps as hard as you guys do that gig them, but I have not had any problems with the JJs from Eurotubes.

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