Diode/GZ34 wiring... I'm confused!

[bluesbear]

I've seen a number of was to run diodes across a GZ34 as protection but I don't understand this one. As you can see, the diodes are run from pins 7 to 6 and 5 to 4. I've seen pins 7 and 5 to pin 8 or between the PT and pins 5 and 7. What is the reason for this particular wiring scheme and how does it work?
Thanks!
Dave

[sluckey]

The ss diodes are simply in series with the tube diodes. Pins 5 and 7 are not connected inside the tube and are just being used as a convenient terminal to mount the ss diodes.

[drgonzonm]

I see the nice red wires, but where do they tie into in the circuit?

[bluesbear]

If pins 5 and 7 have no internal connection, what are the diodes in series with? It makes zero sense to me. There must be more to it. Those diodes weren't added for beauty... unless, of course, this drawing is incorrect. It's supposed to be from a Bluesbreaker Ceriatone.
Thanks,
Dave

[blackcorvo]

Pins 5 and 7 are connected to the PT, and the silicon diodes are connected to pins 4 and 6 (the plates for the GZ34's diodes).



That means the silicon diodes are connected between the PT and the GZ34.

In other words:

PT----|Silicon>|----|GZ34(---|
                                          |_____B+
PT----|Silicon>|----|GZ34(---|

I hope this can clarify thigs for you now 

I remember seeing people using these NC pins to add an optional SS rectification, so they could switch between the two if desired.

[stratele52]

+1 blackcorvo, that is what bleusbear need

[Willabe]

I see the nice red wires, but where do they tie into in the circuit?

The red wires are the high voltage leads from the PT secondary.

[Willabe]

Hi bluesbear, maybe this will help?

I left out the ceramic disc cap, it's for killing the ss diode hash/noise.

Edit; Doh! I had 1 leg of the 5v heater supply going to pin 1 instead of pin 2. Drawing fixed. Sorry all.    

[PRR]

.

[mresistor]

Is this a safety feature or for some other purpose?

[Shrapnel]

Is this a safety feature or for some other purpose?

Safety feature. If the tube recto were to short out, it would overload the secondaries (possibly) of the PT, but it will also dump AC into your amp, and this can destroy your filter caps, sometimes violently if they are very intolerant of reversed voltages. The SS diodes in series with the recto plates will prevent this.

[stratele52]

Is this a safety feature or for some other purpose?

Safety feature. If the tube recto were to short out, it would overload the secondaries (possibly) of the PT, but it will also dump AC into your amp, and this can destroy your filter caps, sometimes violently if they are very intolerant of reversed voltages. The SS diodes in series with the recto plates will prevent this.

If this append the fuse will blow ....normally. But these diodes ere a good protection for a very cheap price.

[plexi50]

Another diagram/

[bluesbear]

Sorry, I see it now that it's not midnight. Of course, the red wires from the PT go to 6 & 4, not 5 & 7. I was going to use a terminal strip to add the diodes. This is better. This WAS a case of a dumb question... or a dumb response to the correct answer.
Thanks!
Dave

[stratele52]

Another diagram/

Your diagram Plexi50 is very interesting for bypassing tube rectifier with silicon Diodes. This will remove tube's sag and boost voltage. With the switch , you can choose.
It is not a safety diodes use  like the post by bluesbear

[HotBluePlates]

Sorry, I see it now that it's not midnight. Of course, the red wires from the PT go to 6 & 4, not 5 & 7.

No, you had it right the first time.

The ONLY reason for these diodes is if one or both tube rectifier sections shorts from cathode to plate. If you have the solid state diodes added as per your original drawing, then when a side of the rectifier is shorted, the solid state continues to rectify the B+.

Yes, the red wires normally run to pins 4 and 6, but you will be using those pins as one mounting point of your series diodes. Since the convenient point on the socket for mounting the other side of the diodes are pins 5 and 7, then your red wires will run to those pins.

When the tube rectifier is functioning properly, the solid state rectifies the B+, and the tube drops the voltage as normal. If a tube rectifier section shorts, the output voltage will rise somewhat, but the amp will continue to function properly. A more severe short (probably external to the tube, from a socket pin to chassis) will be required to cause a fault that will pop the fuse.

[bluesbear]

I'm also fusing the circuit. A 5AR4 shorted in my son's amp and took out the filter cap can and the PT. That was way too expensive! It's not going to happen again if I can help it... and I think I can.
Thanks,
Dave

[xm52]

Adding a silicon diode in series with the plate of the tube rectifier increases the peak inverse voltage capacity over just just that of the 5AR4. Doing this allows you to put a larger capacitor after the tube rectifier. Without it, you can get flash over in the tube when you first power the amp up due to the current draw of the charging capacitor. It serves to protect the tube rectifier.

As an example, Fender does this on their 57 Deluxe reissue amp http://support.fender.com/schematics/guitar_amplifiers/57_Deluxe_Fender_57_schematic.pdf.

[Fresh_Start]

R. G. Keen wrote about this use of diodes in Immortal Amp Mods.  There are 3 additional installments, all having to do with protecting the power supply and transformers.

Cheers,

Chip

[stratele52]

I'm also fusing the circuit. A 5AR4 shorted in my son's amp and took out the filter cap can and the PT. That was way too expensive! It's not going to happen again if I can help it... and I think I can.
Thanks,
Dave

I'm afraid that even with these diodes, if the the tube rectifier short this can destroy you PT . A short at the secondary of the diodes will draw too much current from the Power Transformer.  And if fuse at the primary don't blow your PT will be destroy.

Is this make sense for you ?

[stratele52]

IMO in the Fender 57 Deluxe schematic , it is F2  the best ( and only ? ) protection for saving PT from rectifier tube shortening or any B+ line short . Semiconductor Silicon diodes will only protect filter capacitors from AC voltage in case of rectifier tube short .

[bluesbear]

Well, I've added every protection shown in the 57 Deluxe schematic plus a couple more, thanks to the "Immortal Amp" mods. It should be as safe as is possible to make it. As I've been playing since 1965 and this is the first time I've had to deal with it, I feel pretty safe. There's no such thing as perfect but I've done all I can. The only alternative is to quit playing. Ain't gonna happen!
Thanks,
Dave

[John]

I did the diodes on the little SE I built as rect. protection, but didn't install the .001/3KV cap across them. Could that be the cause of what sounds like faint speaker buzz, but isn't? I tried another speaker to eliminate that. I'm the only one that hears it, and only then if I'm listening for it, but still. Taint right!

[stratele52]

Well, I've added every protection shown in the 57 Deluxe schematic plus a couple more, thanks to the "Immortal Amp" mods. It should be as safe as is possible to make it. As I've been playing since 1965 and this is the first time I've had to deal with it, I feel pretty safe. There's no such thing as perfect but I've done all I can. The only alternative is to quit playing. Ain't gonna happen!
Thanks,
Dave

First time this append from '65 ! So you don't need to re design an amp for one bad luck in 45 year. Can we say today's tubes are cheaper so this can append more often ?

[Willabe]

Could that be the cause of what sounds like faint speaker buzz, but isn't? I tried another speaker to eliminate that. I'm the only one that hears it, and only then if I'm listening for it, but still. Taint right!

Take the ss diodes out and see if you still hear it.

Then you'll know.


            Brad     

[John]

But I'm too busy playing it to do that. 

[bluesbear]

I don't mind the rectifier tube burning out. In fact, I expect to replace them occasionally. I'm only a little upset that the filter cap can was destroyed. It's a pain but not too expensive. What really bugs me is losing the power transformer. That is way more trouble and expense to replace than adding the safeties.
Dave

[bluesbear]

What is the purpose of the .01uf 3KV cap between pins 4 and 6? I hadn't noticed that until I started soldering.
Thanks,
Dave

[John]

Dave, that is the cap I was talking about too. It is supposed to filter/smooth out the sound of the diodes operating, if I understand correctly. When I get a chance I'm going to tack one across the diodes on that little SE and see if it doesn't get rid of the faint "speaker buzz" that isn't speaker buzz.

[xm52]

Yes, as John stated, that capacitor filters high frequency hash. I've never found the need for one with fast switching diodes but it doesn't cost a lot and would come in handy of noise is present.

In some amps you will see a large value capacitor to filter 60 or 120 Hz and there is a small value capacitor, often ceramic, in parallel as shown below. In the circuit, the capacitors are part of an LRC circuit. The larger capacitor usually has a low resonant frequency and is effective at filtering hum but not high frequencies. The ceramic capacitor, with a high resonant frequency, doesn't do anything at the lower frequencies but does filter at very high frequencies. So each capacitor covers different bases.

[MaJiG]

This thread is pretty old, but I just to have to add a caveat. The idea to use the unused pins of the GZ34 socket to add the series resistors is attractive one. When I first started adding the "safety"/"helper" diodes I did use them, however now I don't.

The problem is after the mod is made and the amp is returned to the user. If anytime in the future some guy gets the the bright idea of dropping in a different rectifier tube that might use one or more of those "unused" pins, you run the risk of an arc welding demonstration. These days I place the diodes between the plate pins and the primary winding to avoid any problems downstream.

Of course I keep leads short and use shrink wrap.

I started by using 1N4007's, and then swapped to 1N5399's (thanks to Gerald Weber) which added 50% more current handling. Lately I found the UF5408, which has twice the current handling of the 1N5399 and three times the 1N4007's max current.

The UF5408 is a FRED (Fast Recovery Epitaxial Diode) and seems to have reduced switching transients (when the slow tube is shorted or bypassed). I am going to get a scope on these when I get the time.

Just my tuppence...

[sluckey]

The problem is after the mod is made and the amp is returned to the user. If anytime in the future some guy gets the the bright idea of dropping in a different rectifier tube that might use one or more of those "unused" pins, you run the risk of an arc welding demonstration.

Which rectifier tube would that be???

[MaJiG]

Well, my post was generic and was more a corollary to Murphy's law,  but the 5X4 has its plates on 3 and 5, for instance.

I stand by my modest caveat on general principle and everyone is free to accept or reject it.