Low voltages pream tubes on Metroamp 1959 100w plexi superlead

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stevespring
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Low voltages pream tubes on Metroamp 1959 100w plexi superlead

Post by stevespring » Mon May 28, 2018 1:57 pm

Hi,
I've build a Marshall 1959 folowing the instructions of the metroamp site.
I live in europe (sorry for my english) so I've modify a little tad kit bought in germany following the instructions of metroamp.
Europe socket is 230v.
Problem is that I have a very lower voltage respect the reference chart on pin 1 (85v vs 230v of the manual chart) and pin 6 (80v vs 229v) from v3 and no voltages on v2 and v1 with preamp tubes installed. I've already try to change with new set but no success.
Testing with no tubes voltages drop up to 165v on pin 1 and 160v on pin 6 but still lower than 230v.
So i stopped myself. It's my first amp build and i'm a little scared to do any damage.

The good new is that the left section of the turret and power tubes voltages are ok, in line with the chart reported on final pages of the manual. No blowing fuses.
Problems start after 10k/1watt resistors because almost the right part of the board is dead in voltage.
Tad kit give just carbon comp resistors, don't know if this could be the problem. On original instruction the first upper resistor on left is of 8.2k insted of 10k tad give me.
Following a link with a picture of my build with respectives voltages on pins of the board and capacitors. I can't understand where the problem could be.
Please, any help is appreciated.
Thanks!

https://drive.google.com/drive/folders/ ... hWrlhGh_Ew

Tazin
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Re: Low voltages pream tubes on Metroamp 1959 100w plexi superlead

Post by Tazin » Tue May 29, 2018 2:07 pm

Looks like a 100K (brown, Black, Yellow) resistor instead of a 10K (brown, black, orange) on the long-tail of the phase inverter, so you might what to double check that.
Use you multimeter set to the appropriate ohms scale and take a resistive reading across that first 10K/1w resistor where the voltage drops from 114Vdc to 0Vdc....It should read 10K.

EDIT: Forgot to mention the safety precautions....Make sure the amp is unpluged and drain the filter caps if needed before you measure the resistance across that 10K/1w resistor.

stevespring
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Re: Low voltages pream tubes on Metroamp 1959 100w plexi superlead

Post by stevespring » Wed May 30, 2018 8:01 am

Thanks for your answer!
I've checked all 10k/1watt resistors and they are fine.
Respect the picture they are 10.8k upper left, 10.2k lower left and 9.5k the right ones.
Any other suggention?

Tazin
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Re: Low voltages pream tubes on Metroamp 1959 100w plexi superlead

Post by Tazin » Wed May 30, 2018 10:14 am

Make sure that the 1K/5w resistor on the output tube socket V4 isn't accidentally touching the solder lug where the Black heater wires connect...It could just be the picture angle that makes it look closer than it really is.
Double check the jumper (sleeved in Yellow) that runs from the 10K/1w resistor to the junction of the plate resistors (100K & 82K) for the Phase Inverter V3 and make sure it isn't accidentally making contact with the solder turret for the 47K negative feed back resistor.

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Re: Low voltages pream tubes on Metroamp 1959 100w plexi superlead

Post by danman » Wed May 30, 2018 7:27 pm

At the turret between the 10k and 8.2k, do you possibly have something soldered under the board we cannot see? It is also possible that something is touching that point under the board. If you follow the yellow jumper wire from v3 to v2 and then v1, it appears that you missed a link that would carry the B+ voltage to v2 and v1. You could go down the B+ line from turret to turret with your meter set for a resistance reading or "beep"and check each point along the yellow wire. Somewhere between v3 and v2 you have missed a connection or have a bad solder joint.

stevespring
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Re: Low voltages pream tubes on Metroamp 1959 100w plexi superlead

Post by stevespring » Thu May 31, 2018 3:25 am

Ok I will go to check what you suggest. Maybe the problem would be after 0v reading on right (v2 and v1) and not before as i was thinking. From picture seems that the resistence on v2 in touching pin 4&5 but now i'm not at home so this evening i will check that pins and let you know. Thaks again

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Re: Low voltages pream tubes on Metroamp 1959 100w plexi superlead

Post by danman » Thu May 31, 2018 7:58 pm

Your voltage drops after the 10k/8,2k resistors an extreme amount. This is what makes me think that something is wired incorrectly or that something is touching those turrets under the board but that should result in a blown HT fuse. Be sure those two resistors measure close to spec. After the 10k/8.2k dropping resistor setup, the B+ voltage travels along the yellow jumper to the 82k/100k PI plate resistor node. The voltage there is also lower than it should be. Check all solder joints along that B+ path and be sure that none of the yellow wire runs have an internal break inside the insulation. The "beep" function on your meter is handy for checking this.

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Re: Low voltages pream tubes on Metroamp 1959 100w plexi superlead

Post by Tazin » Thu May 31, 2018 9:27 pm

danman wrote:
Thu May 31, 2018 7:58 pm
Your voltage drops after the 10k/8,2k resistors an extreme amount. This is what makes me think that something is wired incorrectly or that something is touching those turrets under the board but that should result in a blown HT fuse. Be sure those two resistors measure close to spec. After the 10k/8.2k dropping resistor setup, the B+ voltage travels along the yellow jumper to the 82k/100k PI plate resistor node. The voltage there is also lower than it should be. Check all solder joints along that B+ path and be sure that none of the yellow wire runs have an internal break inside the insulation. The "beep" function on your meter is handy for checking this.
To have like a 250 volt drop across that first 10K/1w resistor is way out of whack considering there is no load (current draw) on the H.T. rail after that first resistor when the tubes aren't installed. Equally interesting is the fact that you go from 494Vdc to 114Vdc over the span of two 10K/1w resistors in series with no other connections at their junction.

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Re: Low voltages pream tubes on Metroamp 1959 100w plexi superlead

Post by Tazin » Fri Jun 01, 2018 1:47 pm

Is the filter cap for the Phase Inverter (the filter cap located near the Fuses) grounded?...I can't quite make it out in the photo so I figured I'd ask.

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Re: Low voltages pream tubes on Metroamp 1959 100w plexi superlead

Post by neikeel » Fri Jun 01, 2018 4:35 pm

Tazin wrote:
Thu May 31, 2018 9:27 pm
danman wrote:
Thu May 31, 2018 7:58 pm
Your voltage drops after the 10k/8,2k resistors an extreme amount. This is what makes me think that something is wired incorrectly or that something is touching those turrets under the board but that should result in a blown HT fuse. Be sure those two resistors measure close to spec. After the 10k/8.2k dropping resistor setup, the B+ voltage travels along the yellow jumper to the 82k/100k PI plate resistor node. The voltage there is also lower than it should be. Check all solder joints along that B+ path and be sure that none of the yellow wire runs have an internal break inside the insulation. The "beep" function on your meter is handy for checking this.
To have like a 250 volt drop across that first 10K/1w resistor is way out of whack considering there is no load (current draw) on the H.T. rail after that first resistor when the tubes aren't installed. Equally interesting is the fact that you go from 494Vdc to 114Vdc over the span of two 10K/1w resistors in series with no other connections at their junction.
Exactly. The choke appears to be correct, presume OT centre tap and HT and first choke connection go to corner turret and choke out goes to the turret with screens wire attached?

If the screens were grounded (i.e. shorted) I would expect fuse to go. Presume you checked all those big ccs are 10k with a meter?
Neil

stevespring
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Re: Low voltages pream tubes on Metroamp 1959 100w plexi superlead

Post by stevespring » Fri Jun 01, 2018 5:07 pm

I think to solve the problem...
I was searching at the beginning but it was after the first 10k/1 watt of the phase inverter.
There was the blue cable jumpered with the red one (to the f1 cap) that give back a beep with the chassis. Moving it with a plier to the solder joint in the v2 socket it solve that problem. To be sure i have clean and resolder all the pins with the respective cables.

https://drive.google.com/file/d/1vDlZPK ... p=drivesdk

I have posted another picture with the voltages i tested in v1-v2-v3 pins just a little lower than chart.
Are these values depending by tubes brand or what i measure is acceptable?

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Re: Low voltages pream tubes on Metroamp 1959 100w plexi superlead

Post by Tazin » Fri Jun 01, 2018 6:21 pm

The voltages look pretty good now.

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Re: Low voltages pream tubes on Metroamp 1959 100w plexi superlead

Post by danman » Fri Jun 01, 2018 7:41 pm

A little lower is ok. This could be from differences in component values, the tubes used and even how hot or cold you bias the power tubes. A hot bias will lower B+ throughout the whole amp and a cooler bias will raise it. Congratulations on finding your issue...how about some clips of the amp in action... :hairband:

stevespring
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Re: Low voltages pream tubes on Metroamp 1959 100w plexi superlead

Post by stevespring » Fri Jun 15, 2018 4:35 pm

Hi,
I wanna thank you all for the support you gave me.
It was appreciated and helped me to solve the problem.
So, I'd like to share with you some clips of my build.
I've tested it with some different bright cap values and I have my opinion about my favourite take but I'd like to know what is your opinion.

My chain is

recording take test:
Cort guitar Z-22 > Radial JDV MK3 > Focusrite 8 Pre > Apogee Symphony I/O MK1

repro & recording take test:
Apogee Symphony I/O MK1 > Radial Reamp JCR > Marshall Plexi (see the settings in the picture) > Marshall Power Brake > Marshall 1960 AV 1998 (4x 70W 16 ohm Celestion G12 Vintage) > Royer R-121 > Focusrite 8 Pre > Apogee Symphony I/O MK1.

For the amp setting see the picture in the folder.
For the sound clips see bounces shared folder.

https://drive.google.com/drive/folders/ ... hWrlhGh_Ew

danman
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Re: Low voltages pream tubes on Metroamp 1959 100w plexi superlead

Post by danman » Fri Jun 15, 2018 7:08 pm

They all sound good to me! Congratulations on getting it running properly. :rocker:

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