Build Your Own Clone Message Board

I finished the Brit 45 build and began following instructions for testing and power up. At the point when I installed the GZ34 tube, power on and stand by to on resistor 32, the 10K 1W on board started to smoke. After removing power and inspecting I noticed the 0.1uf cap in the middle of the three had started to bulge out on the end as well. I have checked and re checked my wiring, re flowed solder joints, checked continuity to ground at all points I can't find the problem. Does anyone know a good spot to check or see something I missed in the pictures?

I have built many pedals and the tweed royal, I have a meter for testing. I have not done any voltage readings as I do not want to turn the standby switch to on again and possibly let the smoke out of more components.

Looking at the Brit 45 schematic, that 10K 1W voltage dropping resistor should nominally have about a 70V voltage drop across it (from 380V down to 310V). The power generated for that voltage drop and resistance is calculated as the square of the voltage drop divided by the resistance, i.e. 70^2/10,000 = 0.49W. So a 1W resistor has twice the power rating it should need. What I suspect that you have going on is a short to ground somewhere downstream (i.e. on the preamp side of the circuit) from that resistor. In that case, the calculation would be 380^2/10,000 = 14.4W, which would obviously burn up that 1W resistor very quickly. In fact, it wouldn't even take a complete short to ground--I would guess that a voltage drop much over 150V would fry R32 pretty quickly.

First thing I would check is for continuity/resistance between the tube side of R32 and ground. The 310VDC that should be present on that side of the resistor supplies the plate voltages for the triodes of both V1 and V2, so I would inspect the tube socket pin connections on both of those tubes very closely. On V2, make sure that the lead of R12 connecting to pin 6 isn't making contact with pin 5--it looks pretty close in your photo. See my blow-up of that area attached below.

Clearly, R32 will need to be replaced before the amp is energized again.

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“My favorite programming language is SOLDER” - Bob Pease (RIP)

My Website * My Musical Gear * My DIY Pedals: Pg.1 - Pg.2

Nice math DVM!


Caps bulge when they fail. You need to replace this cap. From your description, it sounds like you are referring to C13, correct?



If so, that means that either that cap was just bad and failed, or there was a severe overvoltage situation in the power amp.

In addition to checking and replacing R32, you need to check the 16µf cap can and the 16µf cap on the board. Check continuity to ground on both the positive and negative side of each cap. Desolder the positive side of each cap and measure the resistance from the positive side to ground (it should be well over 1M). If your meter has a capacitance setting, use it to measure the capacitance of both caps. DMM's aren't typically very accurate on filter caps, but it should show you that each cap has over 10µf of capacitance. If it shows low, the cap is bad.

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MasterDelayer/Reverbrador/Ampaholic/TopJacker

I meant to comment on that. I noticed a couple of those yellow axial film caps, between my Brit 45 and TMB18 kits, that had a noticeable bulge in the plastic at one end while still brand new/unused, so this might just be a manufacturing artifact and not an indication of anything wrong. It's certainly worth checking, but unless it was defective from the factory, I can't see why a 630V rated cap would be bulging in this application.

To the OP: With the mains switch on but the amp still in standby mode, did you check the voltage at the OFF side of the standby switch? If not, could you do that? It would be useful to know what DC voltage the rectifier is putting out. Remember, only one hand reaching into the chassis with the DMM probe at a time!

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“My favorite programming language is SOLDER” - Bob Pease (RIP)

My Website * My Musical Gear * My DIY Pedals: Pg.1 - Pg.2

If it is C13 that you're talking about, I wouldn't worry about it. Like Duhvoodooman said, you've probably got a short somewhere in the preamp section, and C13 is a decoupling cap between the phase inverter and power tubes. If the cap fails it will bulge in the middle, not on the edges. That's probably extra epoxy.

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*patience is a virtue*

Please do not PM me. email is prefered. keith@buildyourownclone.com

Meh....you can get away with continuing to use the resistor as is for testing purposes. Fortunately, whatever the problem is, it's not catastrophic when you turn the power on. In fact, you shouldn't replace the resistor yet till you know you've fixed the problem. Don't bother turning the amp back on again till you find what you think is the problem and fix it, but the worst that is going to happen is that it will just start smoking again. If you turn it on, and it stops smoking, then you know you fixed the problem. Then you should replace the resistor.

_________________
*patience is a virtue*

Please do not PM me. email is prefered. keith@buildyourownclone.com

I guess I just have a deep-seated aversion to smoke coming out of ANYTHING in an amp!

_________________
“My favorite programming language is SOLDER” - Bob Pease (RIP)

My Website * My Musical Gear * My DIY Pedals: Pg.1 - Pg.2

I suppose it's a little unnerving. It just makes more sense to fix the problem first before you replace the resistor, because you'll just burn up the new resistor if it's not fixed. Less desoldering and resoldering. Think of it as a controlled burn.

_________________
*patience is a virtue*

Please do not PM me. email is prefered. keith@buildyourownclone.com

When turning mains on and standby in standby I have about 375 VDC at the off side of the standby switch.

I was talking about C13. I would have also thought it was like that from the factory but what drew my attention to it was the creaking noise as if something were expanding and stretching. The noise could have been something else though. I killed power pretty which once R32 started to smoke.
Here is a closer view.

It's well known around these parts that it's never good to let the smoke out of any component. The smoke is in there for a reason- once it's out you usually have to replace the part.

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Twisting and tinning is for chumps.

I tried putting the smoke back but most of it escaped before I could catch it.

I replaced R32 and cap13. Turned on power and standby switch and I am reading 490vdc at all three test points described in the instructions. Yellow terminal of 32uf cap, red terminal of 16uf cap and positive side of board mount 16uf cap.

I have continuity to ground at the negative side of the board mount 16uf cap not the positive side.

Did you find/correct anything that could have been causing the problem to begin with?

_________________
“My favorite programming language is SOLDER” - Bob Pease (RIP)

My Website * My Musical Gear * My DIY Pedals: Pg.1 - Pg.2

I did reflow some more solder joints and noticed a wire in one of the kt66 tube sockets was loose and fell when touched. I'm guessing it was shorting to the chassis or a pin next to it.

I put the rest of the tubes in and powered her up. It seems to be working now. The volume is lower then I would expect but I think I need to do some more adjustments to the bias trim pot.

Thank you everyone for the advice and help. It's nice to have some help at the point in trouble shooting when you start pulling your hair out in frustration.

the volume probably shouldn't be lower than expected. should be a very loud amp me thinks!

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Doh! Stupid Diode

Yeah I was expecting a loud amp. when plugged into input 2 there is almost no volume until the loudness 2 knob is about at 2. Then it gets a little more volume until around noon and stays at that level through 10. Inputs 1 and loundness 1 on the other hand is massively loud when turned about a mm up. I'm going to take some reading with my fluke and see if anything seems odd.

There shouldn't be a big difference in volume between the two different channels.

_________________
*patience is a virtue*

Please do not PM me. email is prefered. keith@buildyourownclone.com