50 watt high voltages, no cathode voltage v1 v2

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whopperplate
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50 watt high voltages, no cathode voltage v1 v2

Post by whopperplate » Sat Dec 08, 2018 2:53 am

Hello everyone,

I have a new 50 watt Marshall 1987 I built that is throwing high voltages throughout the B+ line. There is no voltage drop across any dropping resistor and I have the same +-420vdc on every plate with no tubes installed. What might cause such strangeness? Don’t understand how the dropping resistors aren’t doing there job...thanks for any help , much appreciated.
Last edited by whopperplate on Sat Dec 08, 2018 8:20 pm, edited 1 time in total.
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FourT6and2
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Re: 50 watt high voltages

Post by FourT6and2 » Sat Dec 08, 2018 6:37 pm

You need the tubes in.

whopperplate
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Re: 50 watt high voltages

Post by whopperplate » Sat Dec 08, 2018 7:49 pm

Sorry , meant to mention with or without preamp tubes installed. I was getting 420vdc on p1 v1, but since I swapped a suspect filter cap I am getting 365vdc. I am using two 8.2k dropping resistors instead of 10k, but that seems substantially higher than what the voltage charts from the metro instructions indicate
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whopperplate
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Re: 50 watt high voltages

Post by whopperplate » Sat Dec 08, 2018 7:56 pm

Well I thought it was bad tubes; wasnt Getting cathode voltages on v1 and v2 . Replaced but no luck. Tubes get heater voltage fine, but they are cold to the touch . Cathode on v3 is reading normal. Thanks for the help!
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whopperplate
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Re: 50 watt high voltages, no cathode voltage v1 v2

Post by whopperplate » Sun Dec 09, 2018 12:41 am

So I assume v3 is pulling current and functioning , but v1 and v2 are some how not. I have measured for continuity and possible shorts between all b+ turrets, having full continuity evidenced also by being able to measure voltage on every plate resistor down the line. Pretty confusing . Running at 110vac V3 reads p1 220 p2 17 p3 28 p4p5 2.96 p6 370 p7 25 p8 28 p9 2.96. Not suppose to have voltage on grids correct?
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whopperplate
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Re: 50 watt high voltages, no cathode voltage v1 v2

Post by whopperplate » Sun Dec 09, 2018 1:08 am

Could a faulty preamp capacitor somehow cause v1 and v2 to not pull current ? Only thing I can think of
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whopperplate
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Re: 50 watt high voltages, no cathode voltage v1 v2

Post by whopperplate » Sun Dec 09, 2018 4:26 pm

Well, replacing the 32/32 cap in the pre didn’t change anything. Time to to keep hunting for shorts I suppose
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RockinRocket
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Re: 50 watt high voltages, no cathode voltage v1 v2

Post by RockinRocket » Sun Dec 09, 2018 6:31 pm

Up load some photos would probably help

danman
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Re: 50 watt high voltages, no cathode voltage v1 v2

Post by danman » Sun Dec 09, 2018 7:49 pm

If you have voltage at the plates of v1 and v2 but the tubes are not conducting, I would double check cathode resistor values and be absolutely sure they are grounding properly. Other than that, I would check that the sockets are making proper contact with the tube's pins.

whopperplate
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Re: 50 watt high voltages, no cathode voltage v1 v2

Post by whopperplate » Sun Dec 09, 2018 10:49 pm

All resistors have been measured before installing (I have to, color blindness is too risky, plus part tolerances are all over the place) , but I double checked anyways and they all check out; 820/360uf and 2.7k/.68uf and 820/.68 v2. I measure continuity To ground from all the necessary turrets, and continuity from the tube pin sockets themselves to all necessary connections. So weird that both v1 and v2 do not conduct; I can’t think of any other thing to try. Just changed out the other 50/50uf cap to make sure it was not faulty; no difference either. Would be so simple and easy if it were a faulty ground, but I have zero resistance from any point measured.

FWIW , Is the dc voltage on p2 and p7 of v3 negligible or possibly a negative symptom as well ?
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danman
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Re: 50 watt high voltages, no cathode voltage v1 v2

Post by danman » Mon Dec 10, 2018 7:36 pm

You mentioned that v1 and v2 were cold. Could you try taking ac voltage readings on the tube side of the sockets to be sure that the tube pins are actually getting 6.3v. I had a build recently where the filaments were measuring correctly inside the amp, but a bad solder joint on the socket was preventing the voltage from reaching the tube's pins. Reflowing the socket fixed the problem.

whopperplate
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Re: 50 watt high voltages, no cathode voltage v1 v2

Post by whopperplate » Mon Dec 10, 2018 7:48 pm

I could reflow connections, but I have checked continuity to each tube pin , even backing the tube out just enough to probe with a multi meter and still solid continuity.

Feel like I have attempted everything short of replacing cathode caps and resistors themselves , but if they measure in spec I don’t know how they can be at fault.

Further analyzing the voltages, there is only a very low voltage drop on the 100k plate resistor on V3 p6, drops from 371vdc to 367, but there is a large voltage drop on the 82k. Down to 222vdc :? This, coupled with the DC voltage on p2 and p6 of V3 , together i hope they can perhaps be enough clues to isolate the issue...what could seemingly limit current so much?
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RockinRocket
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Re: 50 watt high voltages, no cathode voltage v1 v2

Post by RockinRocket » Mon Dec 10, 2018 9:20 pm

Can you post photos? May give us an idea.
Have you gone over the build schemaic/layout by each connection?

I triple chech all my work and after 20 builds or so Ive only had one cold solder joint on the first NOS parts build I did ( got to remove all the oxidation!!) and the first build with beer I forgot a solder joint lol.

whopperplate
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Re: 50 watt high voltages, no cathode voltage v1 v2

Post by whopperplate » Mon Dec 10, 2018 10:31 pm

I will get pics ASAP

This build is all new stock parts, except for an AB 100k resistor on v2 , and I clean the leads anyways before I install each component.

I have gone over this build connection by connection , over and under, both visually and probed with a multi meter , and I can’t find any thing shorting or obviously amiss . My grounds are clean with zero resistance, cathodes firmly connected with zero resistance to tube pins, my plates have voltage , but no tube conductivity.

Fwiw I disconnected the NFb resistor to eliminate any variables, but nothing changes. Also fwiw each tube behaves the same regardless of how many preamp tubes are installed. Only v3 draws current .
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whopperplate
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Re: 50 watt high voltages, no cathode voltage v1 v2

Post by whopperplate » Tue Dec 11, 2018 12:10 am

So here is a link to the amp photos

https://postimg.cc/gallery/xis5em7u/

Please, before anyone jumps to any conclusions , the unsoldered turrets are verifiably conductive and have full continuity between the components and the turrets . I tension each component into place so there is a solid mechanical connection before I solder. The points left not soldered are where I will be swapping components for final tone tweaking and adjustments. I did this with my previous NOS build without so much as a hiccup.

To play it safe I have gone through and soldered more connections as I have troubleshooted to eliminate variables, but alas nothing has changed.

Since I swapped the filter caps the leads to top cans are currently tensioned into place and again have full continuity to the relative points, in case anyone was wondering. All symptoms remain the same regardless.
Musicians are like a bowl of cereal
If they aren't Fruits or Nuts they're Flakes

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