50w Build started, have a couple of questions

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

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Explorerman
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Re: 50w Build started, have a couple of questions

Post by Explorerman » Fri Oct 07, 2022 9:05 am

Thanks for taking a look, I’ll check back in a bit later once everything is done - I’m sure I’ll have some more questions when I get up to the testing part.

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Re: 50w Build started, have a couple of questions

Post by Explorerman » Sat Oct 08, 2022 11:06 pm


danman
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Re: 50w Build started, have a couple of questions

Post by danman » Mon Oct 10, 2022 6:26 pm

Lookin good!!

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Re: 50w Build started, have a couple of questions

Post by Explorerman » Thu Oct 13, 2022 8:04 am

Success! Went through the power up procedure (light bulb limiter, slowly increase using a variac, etc) and all looks and sounds good, I have some slight noise from the speaker when I turn up each volume pot.

Just have a question about the bias adjustment as the previous amp build I’ve done were cathode bias. I have a B+ at the HT fuse of 438.4VDC plugged into the wall socket with the bias pot turned all the was clockwise. According to the instructions I need to adjust the bias to below the maximum current of 40mA (17.5w/438.4) as measured by mV across both1ohm resistors. With the bias pot all the way clockwise, it’s only measuring 2.5mV, but as I turn it anticlockwise towards 37-38mV, the B+ voltage reduces. Is this where I now leave it - at about 37 - 38mV?

Heater voltage is 6.075VAC

Also, I’m guessing the OT primary red and white wire connections are correct and in phase as I don’t hear any abnormalities, is there an obvious noise/sound/tone when these are switched?
Cheers

danman
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Re: 50w Build started, have a couple of questions

Post by danman » Thu Oct 13, 2022 8:38 pm

Congratulations on completing the build! When setting the bias, you have to take multiple B+ readings and recalculate the dissipation every time you adjust the pot. So with a B+ of 438vdc, 40ma would be about 70% dissipation. I used the word "about" because when you measure across the 1 ohm resistors, you are reading all of the current that is traveling through the tube. The screens draw about 4-5ma at idle so that leaves the plate drawing the other 35ma. After taking your initial B+ reading and setting the dissipation at 40ma, take a second B+ reading, do the math again for 70% and readjust your bias pot. You will need to do this 2 or 3 times because the B+ will change every time you adjust the bias. The reason it changes is that a hotter bias draws more current through the tube which causes the B+ to fall. A colder bias setting draws less current so the B+ will go up slightly. It takes a few cycles of measuring, adjusting the pot and then remeasuring again to get the bias set exactly where you want it. Do be afraid to try lower settings between 50-70% dissipation. If the amp sounds good to you at a lower dissipation, the tubes will run cooler and live a longer life. To make the bias calculations quicker, I like to use this bias calculator...

https://www.tedweber.com/bias-calc/

If the amp did not squeal, that is a good sign that the OT primaries are correct. I still like to play through the amp a couple times to make sure there are no weird sounds. I did have one amp that would only squeal once the volume was a bit. It didn't actually squeal but the sound was very weird and phasey sounding every time I tried to play a bit louder. Swapping the OT primaries cured the problem.

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Re: 50w Build started, have a couple of questions

Post by danman » Thu Oct 13, 2022 8:44 pm

I forgot to mention that every new set of tubes you install will bias differently and require resetting of the bias pot. Some tubes will bias up easily with the bias pot and others may not adjust hot enough or cold enough with the range of the bias pot. If you can't get the bias to the ma reading that you want, let me know and I can explain to you which resistor in the bias circuit you will need to change. This happens occasionally because tubes can vary quite a bit from one set to another and the bias pot will only have so much adjustment range.

Explorerman
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Re: 50w Build started, have a couple of questions

Post by Explorerman » Fri Oct 14, 2022 12:24 am

Makes sense, I’ll take a look tonight and give it a go.

I’ll report back with the voltage chart as well.

Can’t wait to crank this thing up!

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Re: 50w Build started, have a couple of questions

Post by Explorerman » Fri Oct 14, 2022 1:35 am

I was also going to try this Fluke DC mA current meter that I have on the wire form the 1ohm resistors to ground:

https://www.dropbox.com/s/ydt1qya03bnon ... .HEIC?dl=0

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Re: 50w Build started, have a couple of questions

Post by Explorerman » Fri Oct 14, 2022 4:28 am

Ok, so I’ve done some bias adjustments, see below voltage chart, do these all look ok?

I also have 285v at the V1 node and 317v at the PI node.

After I took the below readings, I tweaked the bias slightly with 37.3mA on V4 and 36.8mV on V5. This Brough my B+ to 421VDC. Max is 41.5mA.

Does this sound ok?

https://www.dropbox.com/s/muw2aqmnj9l64 ... .heic?dl=0

danman
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Re: 50w Build started, have a couple of questions

Post by danman » Fri Oct 14, 2022 8:04 pm

The v1 pin 1 reading is high for some reason. It could just be a funky tube that isn't pulling enough current or the cathode resistor is not properly connected and preventing the tube from drawing current. Does the amp sound good through both channels? If so, then the cathode resistor (820r) is most likely fine. You could try a different tube in v1 to see if the numbers change a bit. If the amp sounds good the way it is then I wouldn't worry too much about it.

The entire preamp section is slightly high but that is no cause for alarm. If it really bothers you and you would like to bring the voltages down, you will need to adjust the first dropping resistor. It is usually one or two resistors located between the nfb resistor and the bias circuit. Marshall has used a single 10k, 10k/10k's and a 10k/8.2k combo for this first dropping resistor depending on which PT they used. What value resistor did you install in this spot on the board?

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Re: 50w Build started, have a couple of questions

Post by Explorerman » Fri Oct 14, 2022 9:39 pm

Thanks mate, I’ve used an 8k2 and 10K in series as per the SDMs 2204 and 1987 layout. However I have a Metro zero loss FX loop to put in once I’m confident everyone is up and working ok, so a 4K7 will be replacing the 8K2 when the loop goes in.

I’ll try a different tube in V1. The amp sounds incredible! In 2204 mode high channel it’s a beast. Nice clean sounds on the low channel, 1987 mode has ‘ that’ old school sound, beautiful. I’m running it through a Marshall TV 4 x 12 Celestian greenbacks. It’s quiet too with master vol and gain knobs at 10.

This Neil special layout is a gem!

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Re: 50w Build started, have a couple of questions

Post by danman » Sat Oct 15, 2022 8:21 pm

I didn't realize that it was a Neil Special so it's entirely possible that the voltage on v1 is correct depending on how Neil set up the circuit. Either way, if the amp sounds good I wouldn't be too concerned about it.

Explorerman
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Re: 50w Build started, have a couple of questions

Post by Explorerman » Sat Oct 15, 2022 8:52 pm

Well his layout on the SDMS site is for a 100w so I made my own modified version for a 50w as well as some extra options
- NFB switchable between 47k/8 (for 1987 mode) and 141K/8 (100k/4 equiv for 2204 mode)
- switchable bright cap
- switchable V2A cathode 0.68uF cap
- Larry grounding

Sound give exact circuits for a 1987 bright and 2204.

This is the layout I used:

https://www.dropbox.com/s/0u5unb7eexbu0 ... 4.pdf?dl=0

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Re: 50w Build started, have a couple of questions

Post by danman » Mon Oct 17, 2022 6:18 pm

I have been eyeing the Neil Special for several years on SDM's website. I may have to build that circuit myself this coming winter.

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