My 50 watt #183 with footswitchable FET build.

[Matt J]

Did a little more work today and found something out, I think the voltages are a bit too high for what the amp should be running at.

First, I did add in the LNFB on V1B with the proper values this time. I found it did cut some of the noise and raised the headroom of the clean channel up so it sounded better with a bit more "spank" to it. I'm pleased with that and will leave it in now.

I also upped the snubber cap values. I ended up with 500pf caps. I found they rounded off any treble spikes in OD mode the best, though I may play with them more in the future after I take care of other things.

So I double checked the voltages and found things in B+3 and beyond to be about ~30 to 50 volts DC at least higher than what they should be around. The PI is a little high, but V1 and V2 are certainly higher than what they should be!! I'm cautious this is causing some of the preamp hum.

Here's the data, everything is in DC:

Power Rail
B+1 = 472
B+2 = 471
B+3 = 454
B+4 = 385
B+5 = 379

V1
Pin 1 = 245
Pin 3 = 2.01
Pin 6 = 239
Pin 8 = 2.10

V2
Pin 1 = 241
Pin 3 = 2.1
Pin 6 = 236
Pin 8 = 2.23W

V3
Pin 1 = 310
Pin 3 = 63.3
Pin 6 = 314.6
Pin 8 = 63.3

V4
Pin 3 = 465.3
Pin 5 = -46
Pin 6 = 468

V5
Pin 3 = 466
Pin 5 = -46
Pin 6 = 466

I used the normal dropping resistors seen in most Dumble amps = 2.7k/5W to 22K to 2.2K.

The plate values are as they are shown in the schematic and layout = 110/100k on the PI and 150/220k on V1 and V2.

I've gone over everything with my DMM to see if a resistor could have drifted and caused the voltage issue. Everyone of them checked out ok.

All of the preamp tubes are JJ Gold Pin 12AX7s.

That's the latest so far.

- Matt J.

[norburybrook]

220vdc is the highest I've ever used.

I'd adjust your dropping string so they're nearer 200vDC


M

Sent from my Nexus 6 using Tapatalk

[Matt J]

220vdc is the highest I've ever used.

I'd adjust your dropping string so they're nearer 200vDC


M

Sent from my Nexus 6 using Tapatalk

Yep! That's my plan for now.

I'm calculating out the resistor values I will need to get to bring things down. I'll place an order for them tomorrow morning.

- Matt J.

[Matt J]

I'm trying to measure out the current across the dropping string resistors so I will know what new values I will need to order.

I've never tried this before, so It's all experimental to me. I'm hoping someone here can double check my math and methods.

Starting after the B+2 point, I get a total resistance of 26990 ohms to the B+5 point. Using the B+2 voltage of 471 DC, I calculate a total current out to 17mA. Is my method correct, and should I start using that current to calculate the new resistor values.

This part is all new to me. So even if it is trial and error, I'm at least learning more about electronics to use in the future!!

- Matt J.

[norburybrook]

Is your FET in the build? I can't remember now if it's in or out. If it's out you need a 150k on your dropping string to ground to bring your voltages down.

M

Sent from my Nexus 6 using Tapatalk

[martin manning]

I agree, put the 150k to ground from B+5 in. Then I recommend keeping the 22k and 2k2, and adjust only the first dropping resistor to get V1 plates to 180-190V.

[Matt J]

I agree, put the 150k to ground from B+5 in. Then I recommend keeping the 22k and 2k2, and adjust only the first dropping resistor to get V1 plates to 180-190V.

Thanks for the advice! What value do you suggest I first try?

- Matt J.

[martin manning]

Do you have the FET boost connected, or a 150k to ground to simulate it? If so, what are the plate voltages on V1?

[Matt J]

Do you have the FET boost connected, or a 150k to ground to simulate it? If so, what are the plate voltages on V1?

No FET board connected or 150k to ground. V1 plate voltages 245 and 239 VDC. Would the lack of the FET circuit cause those higher voltages? I never considered that! (The more you know!)

- Matt J.

[martin manning]

It’s certainly part of the reason. I’d add a 150k 2W to ground and see where you are.

[amplifiednation]

The 150k to ground will drop that voltage right down.

Can you kill the hum completely with the preamp gain?

Did you try swapping V1 tube?

Check the solder joints on the heater string. Make sure you have a good solid connection at each of the preamp tubes on pins 4/5.

[Matt J]

Hey everyone!

I’ve been out of the loop for a bit due to my grandfather being hospitalized because of a broken hip. I have found time the past few days to work some more on the amp and have some good news to report!

The 150K resistor to ground off of B+5 did the trick brought the voltages down to their correct levels. The clean and OD channels sound MUCH better after this.

New voltages:

Power Rail
B+1 = 467
B+2 = 458
B+3 = 439
B+4 = 338
B+5 = 331

V1
Pin 1 = 208
Pin 3 = 1.73
Pin 6 = 203
Pin 8 = 1.86

V2
Pin 1 = 210
Pin 3 = 1.84
Pin 6 =203
Pin 8 = 1.97

V3
Pin 1 = 306
Pin 6 = 307
Pin 8 = 61.8

V4
Pin 3 = 468
Pin 5 =-45
Pin 6 = 463

V5
Pin 3 = 467
Pin 5 = -45
Pin 6 = 463

Concerning the hum, I believe what it comes down to is the poor wiring in my amp/work room at my house. The house is old, built in the 1940s, and while some of the wiring has been converted over the years, I believe much of it is still rather old and not completely adequate for modern amplifiers that use require a three-prong outlet. My younger brother is an electrician and did some rewiring on the house after our recent issue with a lightning strike. He installed grounding poles outside to the Earth and inspected some of the internal wiring. He said things look ok if you’re using stuff like TVs or computers, but something that easily picks up electrical interference like a tube amplifier there’s probably going to be some hum issues. I tested this idea out by trying the amp at my neighbor's house and it sounded just fine. No large background hum. All you got was that delightful "WOOSH" sound when a tube amp is fired up and ready to go.

I use surge protectors on most major appliances in the house for safety reasons. This is an issue that's dogged me for a while, so I'm probably going to have to invest in a power conditioner or de-hum device to use stuff in my amp room. I've emailed Furman's tech support to see if they can recommend a device. I've also looked at the Ebtech Hum-X attachment a few places sell.

Concerning tone, oh man is this thing wild! I wouldn't describe it as a "singing" type of lead you would see with #102 or 124, but much more aggressive, almost like a Marshall but with considerably better note definition. You can still get some breakup on the clean channel if driven too hard, but it is not as harsh as it was before I brought the voltages down and added the LNFB section. I'm still dialing in the OD tone I want by tweaking the OD trimmer and Drive control. There's a LOT of burn in to do with this amp, so it's a perfect excuse for me to play it a lot! When I'm not doing that, I have it hooked up to an 8 ohm dummy load I built with a signal running through it. I'm using a ledger sheet to track the amount of time it's been on. I want to check the voltages at 25, 50, 75, and 100 hr marks to see how the bias and other things might have drifted. Hopefully next month I'll pick up a pair of Winged-C EL34s to put in it.

Obviously I want to thank EVERYONE who gave me advise and pointers on this! (You all know who you are)

I'll keep posting more pictures and notes as I tweak and burn it in, so stay tuned....

Now to build a Dumblelator for it and start playing with modulation effects....

- Matt J.

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[Matt J]

I hope anyone else who is interesting in building a #183 can use this thread to help learning about and trouble shoot different portions of the amp. Considering the amount of information people have provided to me on here about amp stuff, I figure posting as much info as I can in this thread is the least I can do!

- Matt J.

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[Matt J]

I love the subtle glow of tubes in action in a dimly lit room!

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[martin manning]

What’s up with the FET boost? Did you ever get back to that?