Wonderland/JM100 build

[dhudson]

didn't see your last post, going to far like a 220k will introduce some distortion , which may not be what you want.
100k was just a quick guess I made looking at schematics.

dave

[norburybrook]

yes a resistor to ground , closer you go to zero ohms the more signal is taken from the path, that is a simplification of what's happening there of course.

also 1 megohm trim pot at that location would allow you to adjust it to your taste.

would that be in place of the 1M that's there now with the 68k?

I'm not taking the board out now to put two more eyelets in but I can certainly experiment with different resistance values there.

what will happen if I go too far, will it just overdrive the next stage?

edit ;just seen your reply

M

[ToneMerc]

yes a resistor to ground , closer you go to zero ohms the more signal is taken from the path, that is a simplification of what's happening there of course.

also 1 megohm trim pot at that location would allow you to adjust it to your taste.

On my board I have a 250K trimmer wired as a variable resistor in series with the 68K.

TM

[norburybrook]

is that instead of the 220k on the plate of V4b?


M

[M Fowler]

Off the wiper of the 1MA volume pot (to reverb and the junction of the 1m and 68k is the reverb circuit loop. Look at schematic.

Must be something wrong because the JM 100 is not a low volume amp, it isn't crazy loud like some amps we all have played but certainly 100 watts of very rich tones.

Mark

[dhudson]

is that instead of the 220k on the plate of V4b?


M

no, I would just change the 68k for a 100k and start there.

dave

[norburybrook]

Off the wiper of the 1MA volume pot (to reverb and the junction of the 1m and 68k is the reverb circuit loop. Look at schematic.

Must be something wrong because the JM 100 is not a low volume amp, it isn't crazy loud like some amps we all have played but certainly 100 watts of very rich tones.

Mark

Sorry Mark not sure what you're referring to

I agree there's something not quite right but I'm having a difficult job working out what it is!!


Marcus

[erwin_ve]

Marcus, I think Mark is referring to this junction:

[norburybrook]

Thanks Erwin, yes I knew what junction he was referring to just didn't understand what it was in relation to.

Today is another day so I'll do some more experimenting and checking to see if I can find anything. I might have to take it over to my friends house who has a scope to see what's happening.

It does seem strange that that 68k/1M voltage divider cuts such a massive amount of gain at that part in the circuit, as someone said it's like having a master volume on 2.

I've looked at the pics of the JM amp and it seems like that's exactly what is there if the colour bands on the resistor are correct, they are hard to make out though. So it would seem that that is indeed the correct values, ,but as Mark Fowler said, it's still a loud amp. ??????????????


Marcus

[martin manning]

That voltage divided is obviously cutting signal there, but be aware that if you alter it you are altering the balance of wet/dry signal fed into the mix stages. Is it true that you have the master volume (Level) at 12:00?

[norburybrook]

Yes, I always make a first impression of my amps , in fact every amp with all controls at 12 O clock.

I Know John Mayer has his set about 2 O clock and he's playing big stages.

If you need to have a master volume full up there's no point on having it in the first place IMHO.

I was judging it level wise against my 100W blues master and with the master on 12 it's VERY loud on the clean channel.

I was expecting fender twin type volume levels with perhaps the edge taken off by the twin stage reverb which would be perfect.


Marcus

[norburybrook]

Ok, I changed the 68k to a 100k and that's made a big difference.

I looked at the Dumbleland/SSS layout and that has a 250k trim pot instead of the 220k resister so there's 30k nearly difference if it's on full, so my 100K going to the 220k resistor is about the same.


So just when it was all going well and I was happy, I thought i'd re check my voltages etc after the amp had been on for an hour.

I slipped when checking the power valve plate voltage and obviously shorted something there was a flash and then a loud hum.

I checked the 100r heater CT resistor and that had gone so I replaced it but I've still got a hum that wasn't there before.....ARGH!!!!!

everything is working as before on the amp but I've got a hum now.



Any ideas?

I feel stupid even having to post this


Marcus

[erwin_ve]

Hi Marcus, bad luck after a good troubleshoot session, I think we all had similar experiences at one point in time. I know I have.
The power valve might be gone, creating a inbalance in the push pull power section, creating hum. You can use a other matches quartet of power valve.
But before doing that:
Check the other 100r resistor and the screen resistor. Hopefully it's only that part.

Worst case might be one side on the primary of the ot transformer has shorted(enamel wire melted).
You can check voltages on the plate to make sure.
Hope for the best,

Erwin

[martin manning]

I agree you probably now have one or more bad valves, but my guess is you shorted anode to filament (pin 3 to pin 2), which fried your 100R, and also damaged the heater cathode insulation in the valve(s).

[norburybrook]

Martin,

I thought about the valve but I removed it and it's twin and it made no difference to the hum.

replacing the 100r reduced the hum as it was quite loud but it' snot where it was pre slipup!!


Marcus

p.s. could it have shorted out any of the other valves? the other 100r was fine