Wonderland/JM100 build

[martin manning]

The other choke lead goes to the screen supply filter's positive end. On the resistor value, you may have to experiment but it will need to be reduced. Maybe 22k? You could also parallel a couple of 6k8's to make 3k4 for the 3k3.

[norburybrook]

thanks Martin, as I thought.

I recon that I will have all the power section done tomorrow. Is there a safe way of powering up to check the power section? obviously I'm not sure what the FWBR voltages will be and same with the bias voltage. Do I connect the OT if I'm going to do this, and if so do I need to connect a speaker/load?



Marcus

[martin manning]

I recon that I will have all the power section done tomorrow. Is there a safe way of powering up to check the power section? obviously I'm not sure what the FWBR voltages will be and same with the bias voltage. Do I connect the OT if I'm going to do this, and if so do I need to connect a speaker/load?

You could test the power supply only, no tubes installed and the OT connected or not, which would allow checking the power supply nodes, and bias range. If you wire the OT and temporarily connect bias voltage to the power tubes you could set the bias and measure loaded B+. Any time you power it up with tubes installed you need to have a load connected to the speaker jack.

[norburybrook]

OK, as I'm using donor transformers and using FWBR for the first time I think I'll power up to check the B+ nodes and bias range as you say.

I think I should be getting around 500-510VDC unloaded from the rectifier. What should my -ve BIAS voltage range be to get the 6L6's biased properly from that sort of voltage range?


Marcus

[martin manning]

I think I'd want to have -60 volts available.

[norburybrook]

Ok, I'll see if I can get that tomorrow

If I need more -ve voltage to get into range which way do I go with the pot resistor....more or less resistance?


M

[martin manning]

It'd have to increase. I forgot you were using the Peavy transformers when discussing the bias resistors above... How much AC voltage do you have available for the bias? The Peavey schematic shows -56V bias, but it has no resistor before the diode. You may have to reduce the 3k3 quite a bit.

[norburybrook]

It's 23-0-23 and i'm not using the center tap.

At the moment I've built it like this with the 1R and 10k

AN_Wonderland_FWB.png

I was looking at someone a schematic someone posted and that had 3.3k and 27k, hence my questions.


M

[martin manning]

46 VAC is only 65V peak, but I think you should be ok with the 1R, 100uF, 10k pot, and 27k to ground. That's a sketch I made, Isn't it?

[norburybrook]

Ok. finished up the power section today.

I've just fired her up and here's what I've got.

Bias supply I'm using a 1R and then a 10K on the 10k pot. I've got -58v max

B+; Main screens unloaded are 490v DC it only drops to about 450vDc by B+ 5.

My dropping string is 5.6k ,22K,2.2k

DSC_1005.JPG

DSC_1006.JPG

DSC_1007.JPG

as it's all unloaded I wondered if this is OK


Marcus

[martin manning]

Seems ok to me, but I think I'd increase the 10k to ground in the bias circuit to something larger like 33k. You will get a finer adjustment on the pot that way.

[norburybrook]

thanks Martin, I'll drop in a 27K as I've got one of those I'm sure.

Got to pack it all away now unfortunately as I'm busy in the studio all week so it has to return to it's proper purpose rather than an amp building workshop!!!!


I'm pleased the whole power section is functioning well, and I've learned about how to get different voltages out of a transformer using different rectification and various combinations of the HT-center tap-HT so all good.........................so far.......

I'm actually excited to have a proper spring reverb on an amp build for the first time


TBC



Marcus

[norburybrook]

[quote=norburybrook post_id=371275 time=1491600575 user_id= Look at the size of those 220uF 500w caps!!!! a bit OTT and ridiculously expensive but they were an ordering mistake.

Wow! Where did you find those?
[/quote]


sorry missed your post. they're from here.

https://www.hificollective.co.uk/catalo ... citor.html

[erwin_ve]

Hi Marcus, Nice build so far, it looks to me as fairly challeging build!
One thing to mention on the dropping string: 5k6, 22k, 2k2 looks very familiar to me with the peavey trannies. When the amp is done you can try 3k3, 24k, 2k2 : it feels less stiff on low volumes.

[norburybrook]

Hi Marcus, Nice build so far, it looks to me as fairly challeging build!
One thing to mention on the dropping string: 5k6, 22k, 2k2 looks very familiar to me with the peavey trannies. When the amp is done you can try 3k3, 24k, 2k2 : it feels less stiff on low volumes.

Thanks Erwin,

It's not too challenging now I've worked out how to do the rectification from my Peavey transformers.

I will bear in mind your dropping string recommendation. I tend not to do 'low volume' though

Marcus