Valve Junior Combo Octal Conversion - progress and questions...

Huddy

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So there are always these valve juniors in my area for sale and I got this one for pretty cheap because it was always crackling. Didn't sound like a pot crackle. I opened it up once I got it home. The board is all kinds of brown and it looks like the EL84 Cathode resistor is cooked. The PT and OT seem fine. After i figured out what I was really working with I decided to just gut it and start from scratch. But I wasn't sure about mounting the tubes because the current mounting holes are too big for my noval sockets and I didn't want any janky mounting. So I can enlarge them just a bit and mount some octal sockets. I have a few 6SC7s but I wanted a split cathode. No 6SL7s on hand. 6SL7s are expensive. Remembered that the PT has a 12v winding. Found a 12SL7 and 12V6 on ebay for $11 delivered. They arrived today and they look great. So I got to break the chassis down to enlarge the mounting holes for the sockets. The PT doesn't have a 0-12 winding. It has a 12-0-12 winding.

Can I use half of that 12-0-12 winding for heaters? Maybe do a little artificial center tapping?
 
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Huddy

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So now I'm thinking... I have no idea what the intended use of that 12-0-12 winding is or it's current rating. I think I need to go through my tube stash and find something with 6 volts unless I can find out for certain.

EDIT: So the 0-6.3v winding has 4A slow blow fuse labeled on the holder? That seems excessive. The 12-0-12v winding has 2x 1.6a slow blows on either side. Sounds like I should be good. I'll look at the actual fuses in the morning with good light and fresh eyes.
 
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2L man

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If half of 12-0-12 VAC current rating is too low you could use DC filament and balance current for both. Octal tubes filaments are not bifilar wound and filtered DC make them hum less! :)

Bridge rectifying and filtering would produce about 16-0-16V filtered DC and it is enough for positive 7812 and negative 7912 regulators . They can be made 12,6V installing two diodes between them and zero. Regulators are usually cheap, less than $1. Screwing regulators to chassis should be also enough for their heat reduction.
 

Huddy

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If half of 12-0-12 VAC current rating is too low you could use DC filament and balance current for both. Octal tubes filaments are not bifilar wound and filtered DC make them hum less! :)

Bridge rectifying and filtering would produce about 16-0-16V filtered DC and it is enough for positive 7812 and negative 7912 regulators . They can be made 12,6V installing two diodes between them and zero. Regulators are usually cheap, less than $1. Screwing regulators to chassis should be also enough for their heat reduction.
Very good info! I've done DC heaters before on a high gain Marshall - Fun Fact: the Valve Junior versions 2 and 3 have DC heaters on the 6v winding. have DC heaters on the 6v winding.

Would it look something like this?

If you happen to have an example schematic I would be very appreciative! ;-)

Googling now but I'm not the best googler. Gonna look on Rob's site and continue reading valve wizard as well.

DC Heater.jpeg


Thanks!
 

andrewRneumann

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All the VJ schematics I googled have only a single 6.3V heater winding. Has yours been modified? You are saying it has one 6.3V winding and then another 24VCT winding?
 

Huddy

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All the VJ schematics I googled have only a single 6.3V heater winding. Has yours been modified? You are saying it has one 6.3V winding and then another 24VCT winding?
That's correct. I assume the PT is used in some other off shore model from another brand that has the low voltage winding for channel switching or something like that.

Now it might just be the early combos… I don’t recall later heads having the 24vct winding but I don’t know if I was paying attention when I had one.

5B28E12A-F3C5-4503-9CE7-9307D36DCFEC.jpeg
 

Huddy

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Another alternative would be to wire the 12v tube filaments in series... though at this point I feel like I'm just doing something different for the sake of being different whereas my intent was a quick/easy path to overcoming an obstacle.
 

andrewRneumann

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Another alternative would be to wire the 12v tube filaments in series... though at this point I feel like I'm just doing something different for the sake of being different whereas my intent was a quick/easy path to overcoming an obstacle.

That is exactly what I was going to suggest. Are you confident in the amperage rating of the 24VCT winding?
 

printer2

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Stick one tube on one side of the windings, another tube on the other 12V winding. No need to try and balance the current through the tubes in series. Also a 6SC7 can run two different stages in series if the cathode bypass capacitor is made really big. A 6V 1,000 uF would let you play around as long as you do not mind the same bias point.
 

Huddy

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That is exactly what I was going to suggest. Are you confident in the amperage rating of the 24VCT winding?
I'm not. All I know is that it's got 1.6a slow blow fuses on either side of the center tap. 12v6 heater is .225A and the 12SL7 is .15A. So that's 0.375A. Should be good, right?
 

andrewRneumann

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I'm not. All I know is that it's got 1.6a slow blow fuses on either side of the center tap. 12v6 heater is .225A and the 12SL7 is .15A. So that's 0.375A. Should be good, right?

Doh! That's not going to work. If they are in series, Kirchhoff says they HAVE to pull the same amount of current at all times. Listen to @printer2 :)

The problem I see with using each side of the the 24VCT for it's own tube is that the voltages won't be balanced. One side will be pegged at 0V and the other side will be 12VAC. Would this be noisy? The other potential issue is that since they pull uneven amounts of current, I *think* there will be net DC in that winding, which is usually a bad thing for PTs.
 
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Huddy

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Doh! That's not going to work. If they are in series, Kirchhoff says they HAVE to pull the same amount of current at all times. Listen to @printer2 :)

The problem I see with using each side of the the 24VCT for it's own tube is that the voltages won't be balanced. One side will be pegged at 0V and the other side will be 12VAC. Would this be noisy? The other potential issue is that since they pull uneven amounts of current, I *think* there will be net DC in that winding, which is usually a bad thing for PTs.
Well what I could do is use the 12SL7 in with one heater and the 6V6 with the regular heater. Surely half of the 24v winding can handle 150 ma. Yes? No?🤷‍♂️

Other option is use a 6v6 and 6sc7 and just not have a cathode bypass cap on the first stage. I'd like to stick with either very cheap - or parts that I have on hand. I have plenty of 6v6s just only 6sc7s - no 6sl7s.
 




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