JM wonderland 100W problem solved <please delete>

[Drew.12345]

Hello all,
It’s been awhile since I posted here but was wondering if anybody could offer a fresh set of eyes or help me out with diagnostics.
I followed Taylor’s layout and all my preamp tubes are working fine. I should add that I’m using a brand new matched 6L6 quad from antique electronic supply.
I’m using a brand new set of mercury magnetic iron for a fender blackface twin variant.
The amp powers on and no fuse blows but the plate and bias voltages are way too low. All the preamp controls and switches work just no reverb. The amp sounds fizzy and I don’t want to run it longer than a few seconds at a time to check voltages.
I’ve checked the tank, my ground connections and re-soldered the rca jacks to no avail. Before I start gutting it or make a mess of it I thought I’d see if anyone could offer some insight.
I’ll post a few pics of the insides as well as voltage chart.
I’m going crazy trying to figure this out so any help would be greatly appreciated.

Drew

[rccolgan]

Hello! Beautiful boards! I would double-check the power filter section. I believe there was an error (or something missing, I can't recall) from Taylor's layout that didn't get updated.

For cross-reference, I would look at the Ceriatone JM100 http://www.ceriatone.com/ceriatone/wp-c ... t-2015.jpg and see if that helps in the power section or anywhere else. Hopefully, it's a simple fix!

[pompeiisneaks]

Two things:

1. I'd highly recommend putting a rubber bushing around the reverb transformer wires in that hole so they don't wear and short out over time.
2. I'm confused about the first power filtering stage. It seems to me that there should be a different wiring there. It looks like that the first 220uF capacitor is just going through a 220k resistor to ground and the second 220uF cap is just going through a 220k dropper and both would basically be at the same voltages and they may be in bad shape. Shouldn't there be a jumper between the anode of two and the cathode of 1 and a load resistor going from anode to cathode of each cap instead? or is there an under board jumper I'm not seeing?

Other that that, voltages being lower that expected could mean that either there's a path to ground somewhere unexpectedly with some decent resistance causing the voltage to drop before it gets to the preamp tubes. I'd check the power rail's B+ nodes with a resistance to ground measurement and see if any node seems to see some path to ground. They should be 0L on a DMM for all points along that path to ground. If not then possibly a tube is semi shorted and dumping a lot of voltage across it?

You could try pulling one preamp tube at a time form V1 backwards to see if the voltage drop suddenly goes away?

~Phil

[Drew.12345]

Thanks for the quick reply guys.
I tried to my hand at some rosewood boards but getting the turrets in there was a huge pain!!
1. Planning on getting some rubber bushings when my local stores open back up (lockdown due to Covid and I can’t deal with the cut side pickup thing).
There is a jumper wire going underneath the power filtering stage from anode to cathode but you just can’t see it (under the board).
Thanks Phil, I’ll check the power rails B+ nodes later this evening.
All preamp tubes are working (pulled from my 50w version of this amp)

Thanks again

Andrew

[Richard1001]

Hello all,
It’s been awhile since I posted here but was wondering if anybody could offer a fresh set of eyes or help me out with diagnostics.
I followed Taylor’s layout and all my preamp tubes are working fine. I should add that I’m using a brand new matched 6L6 quad from antique electronic supply.
I’m using a brand new set of mercury magnetic iron for a fender blackface twin variant.
The amp powers on and no fuse blows but the plate and bias voltages are way too low. All the preamp controls and switches work just no reverb. The amp sounds fizzy and I don’t want to run it longer than a few seconds at a time to check voltages.
I’ve checked the tank, my ground connections and re-soldered the rca jacks to no avail. Before I start gutting it or make a mess of it I thought I’d see if anyone could offer some insight.
I’ll post a few pics of the insides as well as voltage chart.
I’m going crazy trying to figure this out so any help would be greatly appreciated.

Drew

According to your voltage chart there is 6 volts over the 470 ohm screen resistors. This means 12,8 ma screengrid current. This is way too high for your amp in idle. It should much lower like 1 to 1,5 mA in idle.

So either your measurements are not right or the tubes are drawing a lot of current either due to an issue with the bias, wiring or hf oscillation.

BTW on pin's 8 i saw 1.55 mA. How is that measured? Are you sure this is right? You probably measure a voltage over a resistor. What is the voltage and what is the value of the resistor?

[Drew.12345]

Good eye Richard!!
Thanks for that and I had to double check again.
I’m not running the amp for longer then 30 second intervals as the bias is so low. Fluctuating wall voltage can be frustrating here so that chart was made over 10 minutes.
I measured it again to see if I was off and it is actually pretty close (within 0.1-0.2 on my dmm) good catch.
I was measuring the bias using my dmm set to ma and on pin 8 of 6l6 tubes on the amp.

[xtian]

Re bias, if this has not been sorted yet, pull out all your power tubes, and get the bias voltage sorted BEFORE you install power tubes again. For new builds, this is a critical first step before you power up with power tubes installed!

[erwin_ve]

Good eye Richard!!
Thanks for that and I had to double check again.
I’m not running the amp for longer then 30 second intervals as the bias is so low. Fluctuating wall voltage can be frustrating here so that chart was made over 10 minutes.
I measured it again to see if I was off and it is actually pretty close (within 0.1-0.2 on my dmm) good catch.
I was measuring the bias using my dmm set to ma and on pin 8 of 6l6 tubes on the amp.

Based on your pics I see measuring points for pin8 with a 1 ohm resistor? If so you should set your dmm to mV. Because of Ohms law everything measured in mV equals the mA.
I also see you you use wooden circuit boards? This might be a problem with high voltage.

[Drew.12345]

I went ahead and replaced the bias pot and was able to bring bias up to 35Ma on each tube. This dropped the plate voltage to 299v so I’m going to go through the boards once more and do some more tracing over the next few days and try to figure out why I have an audio signal without a reverb tank plugged in. There’s definitely something going on around there or some leaking somewhere.

Thanks again people

Andrew

[Richard1001]

I went ahead and replaced the bias pot and was able to bring bias up to 35Ma on each tube. This dropped the plate voltage to 299v so I’m going to go through the boards once more and do some more tracing over the next few days and try to figure out why I have an audio signal without a reverb tank plugged in. There’s definitely something going on around there or some leaking somewhere.

Thanks again people

Andrew

You have a problem in your highvoltage supply you need to fix first. It is not normal that the highvoltage drops 60 volts when your powertubes draw a little current.
Either there is a resistance somewhere between the transformer, rectifier and caps / centertap to ground or you forgot to connect your filtercaps to ground properly / did not wire the first two caps in series. (Negative side of the first 220uf cap which connects to the rectifier has to be connected to the positive side of the 220uf cap which is connected to ground) Check all connections with a continuity meter. (Beep setting on your multimeter)
Make sure all your connections are wel solderd and strong. The solderjoints on the first filtercaps look less than optimal.

If you find an error and fixed it, make sure to first turn your bias pot back to -50 volts on the grids before powering up the amp. If the highvoltage stays stable, you can set the bias.

Before you have a stable highvoltage, there is no point in troubleshooting anything else!

[sluckey]

Is your amp plugged into a current limiter light bulb?

[talbany]

Sounds like something is drawing too much current?.
Did you check to make sure those boards weren't conductive before you started drilling them.?
(I had some fiberglass boards go conductive on me from sitting in a basement too long )
Isolate the Power supply.
Then go down and check continuity on each node on both sides of each cap GND then to their assigned plate resistors.
I generally start there (ohm-ing out the power supply for any shorts, poorly soldered or missed connections and bad caps.I do this before I ever flip the amp on).

Good Luck?

Tony

[Richard1001]

I went ahead and replaced the bias pot and was able to bring bias up to 35Ma on each tube. This dropped the plate voltage to 299v so I’m going to go through the boards once more and do some more tracing over the next few days and try to figure out why I have an audio signal without a reverb tank plugged in. There’s definitely something going on around there or some leaking somewhere.

Thanks again people

Andrew

You have a problem in your highvoltage supply you need to fix first. It is not normal that the highvoltage drops 60 volts when your powertubes draw a little current.
Either there is a resistance somewhere between the transformer, rectifier and caps / centertap to ground or you forgot to connect your filtercaps to ground properly / did not wire the first two caps in series. (Negative side of the first 220uf cap which connects to the rectifier has to be connected to the positive side of the 220uf cap which is connected to ground) Check all connections with a continuity meter. (Beep setting on your multimeter)
Make sure all your connections are wel solderd and strong. The solderjoints on the first filtercaps look less than optimal.

If you find an error and fixed it, make sure to first turn your bias pot back to -50 volts on the grids before powering up the amp. If the highvoltage stays stable, you can set the bias.

Before you have a stable highvoltage, there is no point in troubleshooting anything else!

If you used this layout, it is probably your 220uf caps that are missing the wire connecting them in series. This connection is missing on the layout.

Wonderland 100w jan2017 (1).jpg

They should be wired like this:

1620921713198-258233204.jpg

[Drew.12345]

Thanks all,
Sorted the problem out this morning. I had it wired correctly but had a leaky F&T 220uf. I measure/check all my resistors before but figured/counted on brand new caps were working.
Swapped it out for a used one I had just to confirm and sure enough voltages up to 480 OT CT and 450 plates and was able to set bias without an issue and it was stable.
Ordered a 220uf replacement and will wait for it in the mail before burning amp in.
Thanks for the help and thanks Richard for the drawing and your time.
Cheers


Andrew

[Richard1001]

Great news Andrew!