Guitar Gear Forum

The sticker on the output transformer states 22K ohm - 0 - 22K ohm

To me this looks like a typo and it might more likely be 2.2K (?)

Has anyone here measured the turns ratio and can share their findings?

The reason I would like to know is that mine shipped with one faulty 6L6 and that mere fact has started me thinking of a whole bunch of things. The logical cheap approach would be a pair of Sovtek 5881s and be done with it. But I am also contemplating alternatives that may or may not also need a PT and / or a OT change. EL34, KT66, KT77 all seem like fairly easy changes and the JJ KT77 in particular here in UK is not that much more ££s than an EL34, while the KT66 is possibly too expensive.

I am also looking at DC heaters for the preamp tubes (I will write that up in another thread) as mine has unacceptable levels of hum, which try as I might I cannot make any quieter with the existing arrangement off a single heater supply winding. I have proved that running the 12AX7 heaters on DC makes it completely free of any mains related noises. If I do this it will free up about 1.75A capacity on the existing heater supply and then a bigger heater draw output tube becomes a possiblity.

Cheers, Nick

Yeah 2k2 seems a reasonable primary. 22k is ballpark for a sub 5W PP amp using dual triode. What is the plate voltage? I guess its in the region of 450-500VDC...

As for the hum, are you certain its caused by the heaters? If so then either add a humdinger to the heater or check the heater balance resistors if it uses them as they might not be that well matched. Also think about elevating the heaters. 70-80VDC elevation will help a lot and its easy to do.

The B+ voltage on mine is about 487v.

Re the noise, I am 100% sure that is from the heaters in the preamp, some in the LTP and a lot in V1 and V2.

I have experimentally powered the complete preamp from a DC lab power supply and it is totally quiet (bar some acceptable hiss).
I tried trimming the 0v reference with a pot (a humdinger) and TBH it was as good as was possible already out of the box (I could only make it worse). I haven't successfully got AC heater elevation with DC to work, my attempt made no difference at all and so I've sort of given up the idea of using AC, given that I now know DC is a working option.

I know DC heater supply is much harder to implement, I'm only going on what I have proven to be a way that works for me.

I have just done some testing of the OT and it looks to be 23:1 turns ratio.

I found an AC wallwart and connected it across the secondary 8 ohm tap to ground.

The input was measured at 10.78v with a Fluke 73. The voltage across the primaries with the output tubes removed was 250.8v

250.8/10.78
Comes out to about 23:1
23*23=529
529*8=4232 or about 4.2-4.3K ohms.

So there you have it...unless anyone can see an error in my math or method I answered my own question.

Nick

Yes that is about right for a 50W amp with that sort of plate and screen voltage. I was thinking of a 100W amp earlier. My bad!

It would be interesting to hear from a JCA100H owner to see what the sticker on the transformer says. Maybe the sticker on mine is incorrect and shaould have been on the 100w version and then the typo 22k to 2.2k would make more sense.

Hiya - welcome to the forum!

We've got a JCA 'club' set up here. See my signature for the link. On the first page there are several reference links to JCA documentation and mod threads.

This is the one for the 100H, which includes pics of the transformers: http://www.guitarampboard.com/forum/viewtopic.php?f=53&t=3280

Also, here's one for my 50H: viewtopic.php?f=53&t=3239

I've got a 50H, too. I've done a few mods for tone, and am now very interested in getting the amp quieter at idle - way too much hiss when the amp is cranked. Please share whatever you do with your amp and let me know so that I can add the links to the main club thread.

You could always think of adding a coring filter to it to reduce hiss. Peavey use this method on in some of the sillier designs like the xxx under the guise of it being a noise gate. There is a fair bit of information on the sloclone forum on this if you happen to be registered over there. It does add a small amount of crossover distortion though which isn't a problem for high gain tones but it will make cleans sound a bit meh from what I understand on the circuit.

Using 2W MF resistors for the preamp plates will help a little too.

Hello guys, today I feel I've been an idiot, but now an enlightened one!

As soon as I saw the sticker on the linked photo of the 100H OT I realised that my own measurements on my OT are correct but also that they DO match my original guess at what the sticker should really say.

I measured 4.2-4.3K across the ends of the primary but was forgetting that the sticker reads from the centre tap.

If you correct the typo from 22 to 2.2 you get 2.2K ohm - 0 - 2.2K ohm = 4.4K across the ends.

My measured 4.2-4.3k is about 4.4K which then makes perfect sense.

For the 100H, 1.4K ohm - 0 - 1.4K ohm would be 2.8K ohm across the ends.

FWIW then the JCA50H has a similar OT primary impedance for it's pair of 6L6 as Fender would have used. Marshall tended to use about 3.4K ohm with a pair of EL34.

I'm now going to read up about a coring filter as I have never heard of such.

I will write up my DC heater findings in a another thread.

quickclicknick wrote:I am also looking at DC heaters for the preamp tubes (I will write that up in another thread) as mine has unacceptable levels of hum, which try as I might I cannot make any quieter with the existing arrangement off a single heater supply winding. I have proved that running the 12AX7 heaters on DC makes it completely free of any mains related noises. If I do this it will free up about 1.75A capacity on the existing heater supply and then a bigger heater draw output tube becomes a possiblity.

Cheers, Nick

I have modded my JCA50 with a Hammond filament transformer to power the preamp tubes. The pre tubes have a DC elevated center tap. The elevated center tap will get you 90% of the hum reduction you could get with DC heaters and is far easier to do. My original intent was to allow the amp to run EL34 outputs and not tax the PT heater windings. The noise reduction in the preamp was a bonus.
Many high gain amps use DC filaments on the V1 and V2 positions and AC for the rest of the preamp and PI. There are some posts about this over on the Jet City thread.

It was actually your post on your Hammond transformer that inspired me to go down this road.
It would be interesting to know exactly how you implemented the DC elevation to the centre tap. I tried hacking a quick experiment to do this on the existing PT but it made no difference.

http://www.valvewizard.co.uk/heater.html

Some details can be found in the link above. In high gain designs people often recommend taking the high voltage from the screen filter.

When you tried this before did you unground the heater ct, assuming it has one? The circuit references ground though the ht supply when applying DC elevation. You might need to play with the amount of elevation to get the circuit quiet. I find 70-90vdc does the job usually.

I read the very same from Valve Wizard.

The existing standard arrangement has the centre tap connected to ground. I lifted this wire and created a potential divider off the HT to give about 70V and connected the tap to this. I also tried a negative voltage from the Bias supply. I'm not 100% confident that I implememnted this correctly. Whatever I did it seemed to make no difference at all. I did not think about or try the screens.

Try ~90V. If that doesn't do it then something else needs looking over in the amp. It isn't critical to take it from the screens but the additional filtering will help the nose floor a little.