NEED HELP! 50w kit

Get support and show off your MetroAmp 50 Watt kit builds.

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Brad H
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NEED HELP! 50w kit

Post by Brad H » Tue Jun 20, 2017 8:06 pm

Okay guys, just finished my first 50w build, used the 70's and up specs for the board. At final testing, I'm getting readings on everything just like the instructions say. BUT...when I try and bias the amp, I get no reading across pin 1/8. I swapped the tubes with known good el34's from my jubilee, and same result. Literally every other readout in the amp is as expected. The only thing I noticed I did differently, I routed the 1ohm resistor through pin 1, and bridged pin 8, rather than the other way around. But that shouldn't matter...right?

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Re: NEED HELP! 50w kit

Post by ValveStorm » Wed Jun 21, 2017 3:12 pm

So you jumpered 1 and 8? And from there the 1 ohm resistor goes to gnd?
Your looking for a small voltage there... .036mv.
Does the amp make sound?
Post pics.
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Brad H
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Re: NEED HELP! 50w kit

Post by Brad H » Wed Jun 21, 2017 4:26 pm

Okay, to answer questions, I have a super low reading in mV, jumps from 0.0 to 0.1, and back to 0.0. Yes pins 1/8 are jumped by the 1ohm resistor lead. Even with my bias rite, it was weird. Plate voltage reads 454 on both tubes ( :thumbsup: ) but bias on both reads .5 all the way clockwise, and 26.2 all the way counter clockwise (backward?). So right now it's in the middle, 3.8 on the bias rite. SMH??

Pin 5 reads -37.37 to -53.37. Every pin on every tube, all switches and fuse holders, bridge rectifier, everything reads as expected. Same thing with a known good set of tubes.

I'm attaching a pics (hopefully they show), but with all of the readings being dead on except bias, I plugged in to check it.

In both number 1 inputs (1 at a time of course) I get very low volume through the cab with the volume at zero. And the volume is very bright, and very touchy. Jumps very loud at 1.

In number 2 inputs, it acts normal, still super loud, but that's a 1987.
Last edited by Brad H on Wed Jun 21, 2017 6:50 pm, edited 3 times in total.

Brad H
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Re: NEED HELP! 50w kit

Post by Brad H » Wed Jun 21, 2017 4:27 pm

Not seeing the pic! Something about a board attachment quota.

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Re: NEED HELP! 50w kit

Post by danman » Wed Jun 21, 2017 7:32 pm

The very loud jump in volume is normal with that channel because of the large bright cap over the volume pot. 5000pf is very large and will cause that quick jump in volume. Lowering it down between 100-500pf will help with the abrupt volume jump or you can remove it altogether if you prefer.

Concerning your biasing problem...with your meter set to ohms, check to see that each 1 ohm resistor still measures properly. You may see a reading of 1.2 ohms or so because your meter's test leads will add a small amount of resistance to the reading. If you can't get a proper reading across both resistors then they are either bad or your meter is not functioning properly. If you do see the expected ohms reading, then set your meter for the lowest possible DC voltage scale and try to get a millivolt reading across them. If your meter is auto-ranging, it should find the correct scale on it's own as long as you are set to DC voltage. Two things that could also cause a problem are weak batteries in your meter and the possibility that you used a 10 ohm resistor on each socket and not a 1 ohm. The 10 ohm would still allow you to check your bias but your actual reading would be off by a factor of ten.

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Re: NEED HELP! 50w kit

Post by Brad H » Wed Jun 21, 2017 7:43 pm

Cool, thanks! I'll check that Later tonight when I get home. I think the bright cap is a 500, but should it allow volume to the cab even on zero? It's not much volume at all, but it is there nonetheless. That's what alarmed me most about it. Wish I could attach a pic for you to look at, I contacted Admin about it.

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Re: NEED HELP! 50w kit

Post by Brad H » Wed Jun 21, 2017 8:05 pm

Well, just checked my zoomed in pic, the resistor is definitely 1 ohm (brown-black-gold) metal film.

Also checked the bright cap, its 500 +/- 5%.

At this point, I'll read out the resistors when I get home and check them. Before the build I read every resistor in the kit, even the ones that weren't used, and they all checked good.

I'm confident this thing works fine, but I don't like not getting a bias reading. Makes me wonder if something came loose under the turret board when I heated up the turrets to solder components. Don't even want to think about pulling the board and checking.

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Re: NEED HELP! 50w kit

Post by Brad H » Wed Jun 21, 2017 10:21 pm

Resistor reading were pretty jumpy, but settled at 1.1 ohms. Put new batteries in my Fluke and did it again, still jumpy, but still settled at 1.1. Think tomorrow I'm switching out the bright cap to 250, or even a 100, I have both laying right here ready to go.

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Re: NEED HELP! 50w kit

Post by Brad H » Thu Jun 22, 2017 8:14 am

Okay, here's what I got with the bias pot pegged all the way left? Must be backward:

Pin 3 to pin 3 (amp off, caps drained): 77.2 ohms
CT to V4 pin 3: 39.2 ohms
CT to V5 pin 3: 38.0 ohms

Voltage drop of OT primary legs (expected these to be around 1.5 Vdc).
CT to V4 pin 3: .90 Vdc
CT to V5 pin 3: .90 Vdc

Voltage drop of screen resistors
V4: 3.18 Vdc
V5: 3.97 Vdc

Smh?? Thinking my resistor to the bias pot is limiting my bias range. It's a 47k.

What do you think?

Brad H
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Re: NEED HELP! 50w kit

Post by Brad H » Thu Jun 22, 2017 8:56 am

If I swap out that resistor, won't that also affect the negative voltage at pin 5? With a negative voltage range of 37.37 - 53.37, how much of a change in the resistor do I need? Was hoping my bias range would have been like 28-48mA...NOT 0-23.

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Re: NEED HELP! 50w kit

Post by danman » Thu Jun 22, 2017 9:17 pm

Changing the 47k to a different value will affect the negative DC bias voltage at pin 5 of each socket. If you have a particular set of power tubes that will not bias up properly with the 47k, you will need to change it's value or try a different set of tubes. If your current pair will not bias up warm enough (too much negative bias voltage), you can lower the value to 33k. This is usually enough to warm up the bias and get your current reading across the 1 ohm resistor up into the proper range. If your tubes are running too hot, you will need to raise the 47k up to a 56k. This will increase the negative bias voltage and lower the tube's current draw. This is completely normal as any particular pair of tubes could have as much as a 25ma variance so sometimes it is necessary to adjust the bias voltage range. I had to run a 33k in my 50 watter to get the Mullard El34's I installed to bias up properly.

Brad H
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Re: NEED HELP! 50w kit

Post by Brad H » Thu Jun 22, 2017 9:23 pm

Awesome!! Thanks for the help on that one. I can't get the bias high enough, and I have a 33k sitting right here! I've tried two sets of el34's, a brand new matched set of JJ's, and half of the tube set from my 2555x...both had the same result. Going to handle this one tomorrow and run the bias again!

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Re: NEED HELP! 50w kit

Post by danman » Thu Jun 22, 2017 10:08 pm

I forgot to mention that my bias pot also seems to work backwards like yours but it's just the nature of the circuit design. When I put together a 2204, the bias pot worked exactly opposite of the 1987. As long as you are getting the proper bias voltage adjustment at each socket, yours is working normally. That resistor swap should get you up and running in the correct range.

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Re: NEED HELP! 50w kit

Post by Brad H » Fri Jun 23, 2017 2:27 pm

Okay, first off, thanks for all of the help!! I was incorrect about having a 33k resistor...all I had was a 27k. Since I buried in zero bias, I went with it, and it's perfect!! The sweet spot for bias is dead in the middle of the bias pot range. The swap brought my negative voltage to just a hair over -33 Vdc, and my plate voltage down to 436. I set bias just under 70%, it's at 38.2 on each tube, and the amp sounds great!!!!

Thanks guys!! I'll try again to post pics.

:hairband:

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Re: NEED HELP! 50w kit

Post by danman » Fri Jun 23, 2017 9:32 pm

Cool!! Glad you got it worked out :rock:

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