Wonderland/JM100 build

[norburybrook]

I found a 1/2 watt 1.8k so that's in there at the moment. I'll keep rogb's 2w 2.1k on hand in case.

Marcus.

[norburybrook]

OK.

great day in the studio today with Rogb.

he brought me another 100uF/100 and I put it in my new rectifier board.

Fired up and I've got a lovely 39mv bias.

So onwards.

My other B+ voltages going down the string unloaded (no pre amp valves)are:

499, 424 ,424 vDC that's with a 150k bleeder at the end

Is it safe to put the other valves in or do I need to lower these voltages somehow?

marcus

[martin manning]

Woo Hoo! The other node voltages will drop when you install preamp tubes because they will draw more current through the dropping string. I suggest putting them in and (provided that you have good DC operating points on all tubes) then adjust the resistor feeding the PI node to get the V1 plate voltage you want.

[norburybrook]

I think it's got a 6.8k in at the moment, which way do I go to lower the voltage if needed?

Marcus

[martin manning]

Up. Ohms law prevails there... more resistance begets more voltage drop. Hope you guys made some good music, BTW!

What did your bias voltage on pin 5's turn out to be?

[norburybrook]

Martin.


Ok, put valves in and fired up. everything works perfectly

my only thought is it's not that loud for a 100w amp, going to check voltages now.

All controls work, reverb sounds absolutely gorgeous, probably the best spring verb I've ever heard.

plates are around 175 for V1, up to about 185 for V4 and the PI is 290


they seem OK, maybe a bit low perhaps?

any reason the amp sounds quieter than expected?



marcus

[norburybrook]

Up. Ohms law prevails there... more resistance begets more voltage drop. Hope you guys made some good music, BTW!

What did your bias voltage on pin 5's turn out to be?

pin 5, which pin 5's ?

My bias reading were from the resistor on pins 8

M

[martin manning]

I think 175 is fine for V1. Re the volume get some voltages (Va and Vk on all the preamp and reverb tubes) and see if everything looks reasonable.

Pin 5's on the 6L6's... actual bias voltage.

[norburybrook]

ok V1 plates 173
cathodes 1.43

v2 187 and 1.3

V3

V4 182 and 1.38

V5 307,38,61

pin 5's o the 6L6 is -62


Amp sounds lovely just not very loud my Z wreck rocket is a lot louder and that's 30w


marcus

[norburybrook]

sorry here's V3...that looks a bit wrong to me 416 ....

V3
pin 1: 416
pin 2: 0.00
pin 3: 3.7


edit, actually just checked the layout and they've got 373 there. Perhaps a larger resistor to bring down the voltage a bit there. I've got a 10k I there.

edit; put in a 22k and it's down to 390vDc might try a bit higher if I can find one, looks like a 33k would be perfect.

I doubt that's the issue of the low output though.

Any thoughts, could the OT be duff?




marcus

[martin manning]

That doesn't sound right, like that tube is not drawing current. What is the voltage at the supply node feeding it? Missing or poor ground?

[norburybrook]

Martin if you check the layout it says 373vDC so I'm only 20v north of that.

I though exact the same as you originally so i checked the ground from the 'cathode cap' node and re flowed the connection at the valve for that pin.

Marcus

[martin manning]

Ok, the V3 voltage seems good. I wasn't thinking reverb driver or 12AT7, but that's what you have in there, right? I doubt the OT is bad if the overall sound quality is good.

[norburybrook]

yep 12 AT7


the tone is superb it's just quiet for a 100w amp. It's quieter than my 50w 102 I've just built. the donor Peavey was LOUD..even running at 60w with two valves when I bought it.

Is it possible the OT primaries are the wrong way round and it's phase cancelling rather than squeeling? just thinking outside the box here.
Something seems to be sucking power.

As I said everything in the pre amp/tone section, switches ,contour etc works as expected sonically.


I'm home for the night now so will try any suggestions in the morning.

I'm going to take the output form the send on the Dumble into the return on the wonderland and see how that is. and vice versa maybe.


Marcus

[martin manning]

Good plan. You could also try disconnecting the NFB and see what that does.