Madamp M15MK1 build help....

Tube or solid-state, this section goes to eleven!
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ryanuk
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Post by ryanuk »

Please could someone offer some advice? I'm building the m15mk1 amp (18w marshall lite typology) from a kit and had come across some problems;

I completed all the wiring and triple checked everything. Voltages are all fine as per the schematic that was provided +/- 5%.
However, when I plug in, I get a horrible distorted gated sound (with a low level hum in the background).

Ive don't some tests and discovered that if I remove one of the power valves, it sounds amazing and as Id expect (save for the low level hum) - both channels function etc. This indicates to me that both powers tube should be ok, as the amps works with either power tube ands sounds good. But with both together, its gated and horrible.

I thought it could be the phase invertor - but checked and double checked this.

Any suggestions anyone? All the wiring seems ok, so I'm thinking maybe its the output transformer?

Can anyone help????

regards

RyanUK
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Groovenut
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Post by Groovenut »

I sounds as though you may have your output transformer leads reversed and the amp is oscillating. Try swapping the OT leads that connect to the output tube plates and see if it corrects the issue.

The low level hum may be a grounding issue, but you'll have to deal with that after you get the amp functioning IMO

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Post by alexradium »

reversing OT leads is only effective with feedback,which i don't think its this case.
I suspect some kind of mistake(wire in the wrong pin) or a bad soldering,or a capacitor leaking or shorted (that happens with low cost kits)

post the layout you followed and pics of the assembled amp.

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Post by ryanuk »

Thank for the help guys.

First and foremost, I'm going to go back to basics and check (and recheck) all the wiring, and test point voltages before I do any further tinkering.

Yes - I agree that the hum and "output" problems are 2 separate issues. Ill resolve the output first . My thinking is that it will be one of three issues;

1. Output transformer wiring - I found the 16ohm a struggle to wire as I had to tie 4 taps together and wires kept breaking. I may disconnect the 16ohm output to isolate this as a problem
2. Phase invertor wiring - this area of the build is very busy with 3 resistors wired above the tube socket.
3. Other wiring issue.
4. Faulty OT?

Re the hum. I think the heater voltage is too high @ >7VAC. Possibly because its a european kit and our UK voltage is slightly higher than anticipated. If its still high after debugging, Ill use a pair of 0.22ohm 3w resistors to drop the voltage.

One question, the design uses an artificial centre tap (via 1 x 100 ohm) with a ground reference to the cathodes of the EL84s. Most designs Ive seen just ground these resistors to the star ground. What's the advantage of reference to the cathodes?

I'm not confident with posting pics of the design as I'm aware this could be regarded as an IP infringement.

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Post by alexradium »

my crystal ball doesn't work,ask to those who sold the kit for support,if the schematic is "secret",they have to be magicians and can solve any problem with the magic stick or dust!

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Post by ryanuk »

I was aware that might attract a sarcastic response.

Just being cautious as a guy with the same kit got some grief for posting the plans online.
Thanks for your help though.

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Post by ryanuk »

alexradium wrote:my crystal ball doesn't work,ask to those who sold the kit for support,if the schematic is "secret",they have to be magicians and can solve any problem with the magic stick or dust!
alexradium - Ive emailled the supplier. If they dont respond, I will post the layout etc. Of course, youre right, its no big secret. I do need help getting this resolved and would appreciate any expertise you can provide.

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Post by mlp-mx6 »

Schem and layout are on this page:

http://madamp.de/index/lang/en/artikel/17

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Post by ryanuk »

You're right. Hi Res documentation attached.

I did some more debugging last night. Just rechecking wiring. Plan to do some more work next week.
If anyone has any ideas please let me know.

I'm also now wondering whether the "Euro"/UK voltage difference (220 vs 240) makes a difference - hence the higher heater voltage.

Also the design uses an artificial centre tap (via 1 x 100 ohm) with a ground reference to the cathodes of the EL84s. Most designs Ive seen just ground these resistors to the star ground. What's the advantage of reference to the cathodes?
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M15Mk1.pdf
(155.33 KiB) Downloaded 59 times

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Post by alexradium »

center tap can be referred to ground or a different voltage,usually higher,the amp will be less prone to hum,there should not be problems to connect it there,but in case a power tube will go short maybe it could damage those resistors or the other filaments,who knows.

Its better to decrease the filament voltage,otherwise tubes won't last long.

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Post by alexradium »

it sounds to me like there is some mismatch in the OT transformer wiring to the power tubes,recheck everything there.

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Post by ryanuk »

alexradium wrote:it sounds to me like there is some mismatch in the OT transformer wiring to the power tubes,recheck everything there.
Attach is a schematic of the OT xformer.
Can I clarify something. Does is matter which tube is connected to A1 and A2?

I had difficulty connecting some of the wires to connect the 16ohm output. So I envisage a problem there.
Thanks alexradium
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ATRA04027.bmp

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Post by ryanuk »

Hi
Update to my issue.
I triple checked my wiring against the schematic and layout plan. I did some rework in places where I thought I may have dry joints and the busy phase invertor wiring with all new components. No change to the problem. Sound is still gated and horrible.

I rewired the OT, and tried swapping the plate wires round. No change to the problem. Sound is still gated and horrible.

I then noticed one of the (multiple) secondary OT wires hanging by a thread! It inspected it and it broke in my hand. Ive come to the conclusion that this connection was already broken and only held in place by the potting compound. Im appealing to the website to provide me with a new OT, or the cost of one. Sound is still gated and horrible after testing with the broken OT.

Whilst I source a new OT, I have a few more questions re some of the amp voltages.

All the amp voltages at the test points, are too high and just out of recommended tolerance. I believe this is because the kit is built for a 220v AC wall voltage, and in the UK we have 240v.
Ive used Ohms law and sourced a pair of 2W resistors to drop the heaters down from 7.4VAC to the recommended 6.6VAC.
However, because the B+ is higher, so is the the resulting bias voltage (10.7V - the manual says not to exceed 10.5 as I assume this will take me above the recommended plate dissipation);

1. Is this amp Cathode Biased? Im assuming yes, and the Bias is determined by R28 and R29 in the schematic I provided.
2. Should I bring the overall B+ down by adjusting R33, or shall I just adjust the bias of the tubes by adjusting R28 and R29? The sensible approach seems to be adjust R33 as this will bring all the voltages down proportionally?

Can anyone help?

RyanUK

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Post by alexradium »

What is the plate/cathode voltage on the power tubes?

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Post by ryanuk »

Bumping my own thread!

I managed to fix this amp eventually; after much checking and head scratching I could find no defective components or bad/incorrect connections.
I negotiated a credit for another output transformer from the supplier. The amp now produces a rich and complex sound!

One minor criticism is that when the guitar is directly connected to the amp, and I roll the guitar volume, it produces a scratchy sound. Its not a scratchy guitar pot as I've compared to my guitars plugged into other amps. Also, the problem goes away if I plug into a pedal first. So it sounds like an impedance problem.

I've checked the input wiring several times - its correct according to the provided schematic.
The input is switched to ground when jack is unplugged. When plugged in, the input has a 1M pulldown to ground, goes through a 33K resistor before going into the first preamp stage. Seems typical of many others amp schematics online (eg. Princeton).

Does anyone have any experience with this? Any advice?

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Post by alexradium »

ryanuk wrote:Bumping my own thread!

I managed to fix this amp eventually; after much checking and head scratching I could find no defective components or bad/incorrect connections.
I negotiated a credit for another output transformer from the supplier. The amp now produces a rich and complex sound!

One minor criticism is that when the guitar is directly connected to the amp, and I roll the guitar volume, it produces a scratchy sound. Its not a scratchy guitar pot as I've compared to my guitars plugged into other amps. Also, the problem goes away if I plug into a pedal first. So it sounds like an impedance problem.

I've checked the input wiring several times - its correct according to the provided schematic.
The input is switched to ground when jack is unplugged. When plugged in, the input has a 1M pulldown to ground, goes through a 33K resistor before going into the first preamp stage. Seems typical of many others amp schematics online (eg. Princeton).

Does anyone have any experience with this? Any advice?
scratching is due to dc voltage appearing on the grid of the first tube,yes,it goes away with pedals because you are breaking the dc path with the capacitors through the pedal circuit.
Probably you have a defective tube or the plate voltage is too low,anything less than 150V has the tendency to put that tiny dc voltage on grid,so try replacing tubes or connect a 100n cap in series with a 1M to ground before.

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Post by ryanuk »

alexradium wrote:
ryanuk wrote:Bumping my own thread!

I managed to fix this amp eventually; after much checking and head scratching I could find no defective components or bad/incorrect connections.
I negotiated a credit for another output transformer from the supplier. The amp now produces a rich and complex sound!

One minor criticism is that when the guitar is directly connected to the amp, and I roll the guitar volume, it produces a scratchy sound. Its not a scratchy guitar pot as I've compared to my guitars plugged into other amps. Also, the problem goes away if I plug into a pedal first. So it sounds like an impedance problem.

I've checked the input wiring several times - its correct according to the provided schematic.
The input is switched to ground when jack is unplugged. When plugged in, the input has a 1M pulldown to ground, goes through a 33K resistor before going into the first preamp stage. Seems typical of many others amp schematics online (eg. Princeton).

Does anyone have any experience with this? Any advice?
scratching is due to dc voltage appearing on the grid of the first tube,yes,it goes away with pedals because you are breaking the dc path with the capacitors through the pedal circuit.
Probably you have a defective tube or the plate voltage is too low,anything less than 150V has the tendency to put that tiny dc voltage on grid,so try replacing tubes or connect a 100n cap in series with a 1M to ground before.
Thanks Alex - will try that!

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Post by alexradium »

i checked your schematic,and yes,its 123 to 133V on the first stages,that calls for dc leakage...

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Post by ryanuk »

Thanks Alex
I did swap the tubes - improved things but still noticeable.
Ill try the cap you suggest. To confirm, you suggest putting this in series with the 1M at the input. Like a DC blocker?

Ry

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Post by alexradium »

insert the cap between the input jack tip and the joint of 1M and the 33k resistors

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