Help with a new Build ODS

[Wardy]

Hi All,

I'm getting near my wits end with a new build ODS. It a 50w kit from Tubetown in Germany and I believe its based on 102 but with an added single tube reverb.
I have the amp working OK but I can’t get much volume out of it. Compared to a blackface 40w I made it seems very quiet. The overdrive channel gets loud but not the clean channel. I have been through and checked everything thoroughly. Voltages look OK. I tried the PI balance on my Siglent scope as indicated in the build instructions, but I can’t get anywhere near the same output level.

I’ve attached a voltage chart and some readings I took with the all controls set at 5.
The clean channel breaks up if I go higher than 7 on the volume. The fet channel of course depends on how you set the trimmer.
If I max out the amp on the clean channel I can only get about 8w output not 50. I was wondering if this is just the way the amp is as it only has a single tube for the clean channel.

If I hook up a dumbleator I made then the extra gain stage can bring the level to what I would expect. without it the clean channel is very weak.

sine readings.pdf

Any thoughts?

ODS Voltage chart.xlsx

gut shot.pdf

kit-dmb50-ODS.pdf

[xtian]

No, this is not normal. And you've already spotted the location of the trouble--you list 32v signal entering the Master Volume, but only 2.4v out.

[Wardy]

Ok thanks for confirming that.

i already tried swapping out the MV pot. the reading was taken with all controls at 12 o' clock so there would be a fair bit of attenuation. i'll redo the reading with mv on full and see what it says. i don't think thats it though.

best

Steve

[alnight]

I once had a similar issue. The clipped end of a resistor lead had just ever so slightly penetrated the heat shrink on the end of one of the shielded cables and contacted the shielding, grounding out the signal. It was a devil to find. One teensy little unintentional connection can really ruin your whole day.

[Wardy]

Yes this amp is driving me nuts!

I got a whole set of different reading this eve. If I use a Dumbleator I can get proper volume out of it because of the extra gain stage but without it the clean channel would be useless on a gig. I've been over it many times but your probably right its something like that, just need to find it.

[alnight]

So, I've studied your gut shot and gone back a few years in my mind to my very first D'Lite build, which had a very similar issue at first. This and that are two different things, but you might take an alligator clip test lead and just run from the ground buss to the unused lug on the brite switch. Just to see...

[Wardy]

Many Thanks,

I'll try that later. I spent hours last night with the scope and got a different set of readings throughout the amp. I had gone over it and touched in any suspect solder joints although I think they are pretty good connections. Been researching more this morning and looking at utube videos. I noticed on one site that in a marshall amp with a 1k 200mv sine going in, the reading after V1a was 12.5v pk-pk. A gain of maybe 62.5.
At the same location I'm only getting double the input so I think this area could be the culprit. I can't yet see anything wrong here though. Could a defective cathode cap cause this? I already checked all the resistor values.
sorry for my lack of knowledge, this is only my third amp and I started from scratch/zero knowledge in Jan this year.

best
Steve

[alnight]

You're fine, we all start somewhere. The wonderful thing about problems like this is that once you solve it you will have learned something, and it will be knowledge that you have earned.

I think you are on the right track looking around V1 and the front end of the preamp. Further, I would suspect that the issue is not on the board but somewhere around the tube socket or on the control panel, because that is where I have most often made frustrating and memorable mistakes. An overlooked ground connection, and a tiny drip of solder or whisker of wire making unintended connection between two preamp tube pins are two that come to mind.

[Wardy]

So I tried the alligator clip from the brite switch but no change. i've now spent probably 2 days going through the amp with a magnifying glass, checked the values of every resistor, changed v1 tube several times, lifted and tested cap on V1. Tonight I even lifted the board to see if there was anything fouling underneath. Nothing makes sense. All the voltages look great. the amp is fairly noise free. there is plenty of volume when the overdrive channel is on.
I scoped a tube preamp pedal I made to compare V1a and sure enough on the preamp I was getting a pk-pk increase from 200mv to about 8 to 10 volts depending on settings.
On the Dumble the output on V1a would be something like 500mv for a similar input. The only thing I haven't tried is replacing the valve socket. I've checked and rechecked. I would like to know what a high plate 102 ODS expected pk-pk should be on V1a from 200mv signal.

I'm starting to come to the conclusion its just the way the amp is given there's only one tube really for the clean channel. My last build was a heavily modded blackface at 40w with a London Power preamp mod and it was miles better. Not sure what else to try with this amp.

[dbharris]

Did you happen to take pictures of the underside of your board? You may just need to reflow your solder joints. Have you tried to short out the reverb section to bypass it?

More pics around v1 would be helpful also.

It's frustrating, but I'm sure we can help you get to the bottom of it.

-Dan

[Wardy]

kit-dmb50-layout marked up.pdf

Thank for the encouragement Dan, I'm losing the will wilt this!

I didn't take any pics. It all looked very good. I did trim some excess length of a few connections that were sticking down through the board that I had soldered in after it was in place. I'm sure they were not shorting but just to be sure. I did reflow some points earlier from above the board. I know all the eyelets were filled from below but I also in desperation filled them from above.

I have attached what I think is the problem marked on the layout on pins 2 and 1 of V1. I think the gain from V1a is way too low showing an increase of roughly 200mv to 500mv. I think this second reading should be showing maybe 8 to 12v but I wonder if anyone could verify what this should be here and give any possible causes. I have already done the obvious stuff like swap out the valve (tried 3) and checked the values of resistors.

many thanks

Steve

[dbharris]

Can you measure the mv present immediately after the the bass cap, the 0.1uf on your board? Please measure at the leg of the cap closer to the tone stack side of the board.

It seems like you have done a great job narrowing down what part of the circuit is the issue. So now we just need to go over all of the tone stack wiring again. Confirm the parts are the correct value and the wiring matches the layout.

If you can upload more pictures and angles of the v1 wiring, that part of the board, and the tone stack, we may be able to spot something.

If it were me, I would also take some compressed air and liberally spray those areas of the chassis, including while holding it upside down.

-Dan

[Wardy]

Thanks Dan,

thats given me some more things to try. I already tried air from a spray can but not upside down yet. I'll take some pics later. I did check to see if there was any DC voltage on the wrong side of any of those caps and there wasn't. I'll recheck the values.

best

Steve

[dbharris]

Good luck! And I should have specified, I was suggesting you measure AC on the other side of that cap. Also on the other side of the mid cap. Versus the grid of v1b. Just trying to narrow down where you are losing signal.

What changes with the r/j switch with those measurements when in rock, jazz, or PAB mode?

-Dan

[Wardy]

Ok took a few pics but I'm up against an upload limit.

I did the upside down thing and blasted everything with an air duster. looking at the pics there is still crud everywhere so probably would benefit from a soft brush. I checked all the cap ratings using a boroscope so I could read the codes. I didn't measure them as would have to lift an end and thats a pain with eyelet boards. I did lift one end of V1a cathode bypass cap and verfify that was ok though as I was suspicious. Readings are also attached. I checked for any dc on all those caps with a DVM. I finished before I saw your last message so didnt take readings of the Rock/jazz switch but generally speaking the signal would drop by maybe 25 to 30 % if setting on jazz. I didn't get all the ac readings only the Pk-pk signal on the bass cap. I can check the others later.

Pics & readings (optimized).pdf