Rectifier options M-125/kt120

I am NOT a tech.

The thermisters I ordered (from Mouser) for some old Silvertone guitar amps (they have faux standby switches) were

995-SG317
SG317
120OHM 3AMP

One ran a pair of 6L6's and the other two pairs of 6L6's.

Dahlberg wrote:Ok, an idea. Lets say that I would build a solid state rectifier on a socket and ad the
ntc thermistor there (at the ac input). They get pretty hot but it will be at the outside so....?
I'm still trying to figure out what values I'm supposed to look for. Putting it at the high voltage
beetwen the transformer and the rectifier would lower the current demands for the device.
Still fumbling around in the dark here Any input would be appreciated

McIntosh put their thermistors at the AC input. MC275 schematic attached. See lower left corner.

Dahlberg wrote:Went on a shoppingstreak so now I have gz34's, gz33's, wz68's and
a couple of octal tube bottoms to make my own rectifiers.

Where'd you find all of those beauties? Fleabay?

I belive that it would be a good idea not to let the thermistor take the whole load
of the power transformer, no need to hold the filaments down. Just the gz33's
came from Ebay. A seller in England had quite a few, they are supposed to be NIB.
I saw that the value of the thermistor in the mc275 was 25-100ohm cold.
How about the other values? There is a maximum Joule value that I'm a bit conserned over.

Edit: It was gz37's, sellers called "Langrex", looks solid to me.

Found a comment in the description for the "Ametherm MegaSurge Series Inrush Current Limiter"
that would suggest that the ac input side to the power transformer would be the logical place to put it.
Haven't found that information elsewhere and assuming that they are correct the voltage would be to
high at the transformer ac output.

•While traditional thermistors are only certified for input voltages from 85 VAC to 265 VAC,
UL-recognized MegaSurge devices are certified for single- and three-phase input voltages up to 480 VAC.

But looking at the "MS22 75004" it's specified for "Max Capacitance @ 680 VAC: 480 µF"

I don't know what to make of this.

It was gz37's, sellers called "Langrex", looks solid to me.

Hope so, just ordered a pair from same seller. (Mullard branded)

I got the wrong tubes (cv378 and a partial refund) but have made a second order since it seemed to have been an honest mistake. I will need spares anyway so....

Hopefully I receive the correct ones. Following your M125 build in the photos section. PCB covers look great.

?

Did an ebay search, and no such seller. I did find some rebrands using that name that look sort of Mullardish ...



PS ... you should be happy with the GZ37's ... been running one here in an ST120 for a couple years and nary a problem, and I push it hard with a quad of KT120's and a 6SN7 driver.

The Ebay store is called "Langrex". He has 8 gz37 Mullards left, just search for gz37 in the top left corner.

http://stores.ebay.co.uk/LANGREX?_trksid=p2047675.l2563

Dahlberg wrote:The Ebay store is called "Langrex". He has 8 gz37 Mullards left, just search for gz37 in the top left corner.

http://stores.ebay.co.uk/LANGREX?_trksid=p2047675.l2563

Only 8, maybe I should snag a couple more before they are all gone. Dangerous to rely on Cuba as a new source, they are likely to have Cyrillic lettering and I wonder if the sockets will be compatible!

My guy in Turkey has some (he thinks) GZ-33s that are post-war Russian-made. Wonder how they would stack up against the Mullards. Cyrillic is basically just the Greek alphabet, very phonetic, easy to learn -- that is, if you know Greek or were ever a member of a college fraternity.

Russian GZ33's, now there's something you don't see every day. Wait a minute, how often do you ever see GZ33s anymore? If they are legit NIB 33's I for one would be interested.

This one looks interesting ... it's either a GZ33 or a GZ37 ... oh, wait ... it's both!

At least it's a thermionic valve, albeit of questionable parentage . . . and therefore must be acquired.

Here's what my friend in Turkey has; they are Reflektor (Russian) marked as GZ-33s but I'm not convinced, nor is he, that they are up to Mullard snuff and sure don't have the (ahem) shapeliness of a GZ-33 or GZ-37 Mullard.

http://www.nostubestore.com/2016/10/reflektor-54-gz32-gz33-5z4gt-ussr.html

I've asked him for a pin-out diagram so to see if they might sub for a 5AR4, at least. Or I'll spend the $14 he's asking and try it in an M-125 and see what blows up. I am sure the fuse will be protected.

Speaking of WZ68 Copper caps, the cap on mine came off when I was removing it and that gave m,e the opportunity to drill some ventilation holes in it. I put three holes vertically on opposite sides and a larger hole in the top. I also have a 6-inch fan running at 6 volts behind the power transformer which partially blows on the rectifier. The rectifier is quite cool now. I can actually touch the top of the rectifier for two to three seconds. I also polished mine but it quickly dulls. So, drill those holes.

deepee,

This might look familiar: http://www.nostubestore.com/2012/01/russian-nos-gz34.html

I bought three of them a couple of years ago, and unfortunately I haven't put them in use yet.  Mostly because the plates are frighteningly small, and there is precious little space between the plates and cathodes.  I did look up what they actually are, and it seems they are a Russian 5C4M that does spec to a 5V, 2A heater with a reverse voltage of 1550 volts and a current rating of 300mA !  It's obviously the last part that I have a real problem with.

And as it turns out a real Mullard GZ37 is also available from the same source:
http://www.nostubestore.com/2015/01/mullard-gz37-gtbritain.html.
Not cheap by comparison, but they have been discussed here many times and seems that they are worth the money in the long run.

These tubes do sort of resemble the current Chinese 5AR4s I've seen and bought (and use, though my usage is very light at the moment) and may turn out to be better than expected, but I don't know that I would want to risk other much more pricey components like transformers and such with one of these rectifiers.  Certainly perform the diode mod and then maybe give them a try, at least that would be my plan.

Tony

UPDATE:  I just found a spec sheet that seems to indicate the average current capability is 70mA per plate for a total of 140mA.  Not sounding like a good enough sub to me.  See below:

https://frank.pocnet.net/sheets/084/5/5C4M.pdf