6x5gt rectifryer?

[TurboGuitarMelton]

Hey,


I recently acquired a old reel to reel player. The motor and all werent working so I salvaged the transformers/tubes for a build.


The circuit originally ran a 6x5gt rectifier. I have read some questionable things about these tubes but it looks like it has functioned fine in this old 1950s reel player for a loooong time. You can see the black deposit on the top of the tube around the cathode of the 6x5gt so it must have run for a long time no issues. All the transformers are fine too!


My main question: Should I use the 6x5? I have 3 NOS new in the box 6x5gts on hand too. The circuit is just going to be a champ because the OT is SE 5k to 4ohms.

[tubenit]

I've used them with no issues.  Just watch the 1st capacitor rating in the B+ line up.

With respect, Tubenit

[AmberB]

I've used them with no issues.  Just watch the 1st capacitor rating in the B+ line up.

With respect, Tubenit

Watch the first cap?  What's it suppose to do?

[Joel in Texas]

Watch the first cap?  What's it suppose to do?

“Watch” as in “be careful” as in “do not exceed limits for the tube”.

Different tube rectifiers have different limits on how much capacitance the first filter cap they feed can have. 

Info at this link indicates a limit of 16uf on the first filter cap, for a 6x5gt:
http://www.r-type.org/exhib/aam0006.htm

On the other hand, the data sheet linked on that page gives a 32uf limit.

You can sometimes do stuff like add a lower resistance, higher wattage resistor after an initial lower value cap like 16uf, and follow it with a higher value cap like 47uf; that second node instead serves as your “point A” in the power supply rail. If you can work with the resulting voltage drop, it can be ok.

Also too, one time when I watched a cap after a 6x5gt with a magnifying glass for 16 hours straight, I saw a butterfly with laser antennae pop out of the negative side, and chase miniature centaurs around the top until they all disappeared in a blue flash, back into the positive side. So that’s another reason to watch the cap after a 6x5gt.

[scstill]

I also am building a guitar amp project with reel to reel PT and 6x5.
Mine are from a Webster Chicago 2010 which used a 35uf as the first reservoir cap.
I have also heard that you can fuse the secondary side of the PT for protection.

[HotBluePlates]

... Should I use the 6x5? ... The circuit is just going to be a champ because the OT is SE ...

You're fine with a Champ, though a 6X5 is less capable (70mA) than a 5Y3 (125mA).

... Different tube rectifiers have different limits on how much capacitance the first filter cap they feed can have. 

Info at this link indicates a limit of 16uf on the first filter cap, for a 6x5gt:
...
On the other hand, the data sheet linked on that page gives a 32uf limit. ...

... reel to reel PT and 6x5.
Mine are from a Webster Chicago 2010 which used a 35uf as the first reservoir cap.
I have also heard that you can fuse the secondary side of the PT for protection.

It's mostly unhelpful to think of the situation as a "capacitance limit" because you're stuck with vague "rules" from a variety of sources & anecdotes of "I got away with a larger cap."  The problem is "the right way" to think about it is a difficult design-problem.

6X5GT data sheet
5Y3GT data sheet

Notice both sheets describe a definite d.c. milliamp limit, but start getting fuzzy on a.c. plate current with reference to rating charts.

Caps suck current to charge to the applied voltage.  If there is zero resistance in-series, the cap charges near instantly, but pulls near-infinite current.  Your rectifier tube just popped.  The higher the capacitance and/or higher PT voltage and/or lower-resistance, the higher the charging current.  Rectifier = Popped.

So in accordance with Ohm's Law, we can reduce the current pulled through the rectifier (which is one of the "peak a.c." limits) by either lowering capacitance and/or increasing the series-resistance.  Or we can lower the a.c. voltage delivered from the PT, though usually we picked that because we needed a certain d.c. voltage output.

The PT's winding resistance is part of this "series-resistance."

So the resistance of the PT (as well as the internal resistance of the rectifier) tend to bring the cap charging-current down.  And we don't always have specs on the winding resistance of the PT we use.  Where larger filter caps were used in vintage gear (and the PT's resistance didn't limit the current enough), designers added a resistor per-rectifier-plate to reduce the charging current & protect the rectifier.

So any filter cap limit is a fuzzy rule-of-thumb.  The problem is rectification is non-linear, so mathematically predicting peak charging current is a pain.  The Radiotron Designer's Handbook 4th Ed has a chapter devoted to rectification, and copious charts/graphs to work through to predict the needed external resistance (if any) for the PT, a.c. voltage, desired d.c. voltage, load current, and capacitor µFs.

[DummyLoad]

it's SE. it'll work and sound better with less buzz/hum with silicon.


--pete