High voltage on 2204 V1/V2 Plates

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Elegost
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High voltage on 2204 V1/V2 Plates

Post by Elegost » Fri Aug 31, 2018 8:42 pm

Hi folks,

I built a 50w JCM800. Having issues finishing it, it is CRAZY dynamic.

Without tubes, the voltages and on pins 1 and 6 of both V1 and V2 are very high, around 461-469v. I'm getting 519v out of the rectifier. This is way too high, right?

From mhuss's voltage chart for the 2204 i should be closer to 300v on V1 pin 1 and 245v on V1 pin 6.
https://mhuss.com/MyJCM/JCM800_2204.gif

my best guess was that the big 10k resistor on the B+ line before V1 had failed open but I measured it in circuit and it doesn't seem fully open or anything.

Could it maybe be that the preamp filter cap has failed open and is letting big voltage past that resistor? it's under the board and i'll have to really tear things apart to get to it.

Any help on places to look would be really appreciated!

Thanks

Elegost
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Re: High voltage on 2204 V1/V2 Plates

Post by Elegost » Sat Sep 01, 2018 10:23 pm

More data:

The V1-V3 plate voltages are more reasonable if i have the preamp tubes in and the power tubes NOT in.
With just preamp tubes:
V1 pin1 - 295
V1 pin6 - 353
V2 pin1 - 216
V2 pin6 - 408
V3 pin1 - 286
V3 pin6 - 265

However, if i put in the power tubes, voltage at the V1 plate starts rising and rising as the power amp tubes heat up until it gets close to 500v and i chicken out and turn off the amp.

what does this mean? do i have a blown power amp tube or is it something more serious?

danman
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Re: High voltage on 2204 V1/V2 Plates

Post by danman » Sun Sep 02, 2018 7:04 pm

Without tubes in an amp, there is no load to draw current from the power supply. Because of this, the voltage from the 1st filter cap all the way to the end at the plates of v1 will be the same and be very high. Usually around 500v. With no load on the power supply, the 10k dropping resistors are basically doing nothing and the voltage just passes right through them. Now, when you load tubes and they begin to draw current, the voltages in the amp come down to normal range. Preamp tubes are a small load so the voltage drop will be small. Power tubes draw the most current so the b+ will drop quite a bit when they are installed. If you bias the amp hotter for more current draw, the b+ falls. If you bias the amp colder, the b+ goes up a bit. I would hook your meter to the first filter cap or pin 3 of one power tube and power up the amp with all tubes installed. Voltage will shoot high for a few seconds but as the tubes begin to warm up and draw current, the b+ voltage will begin to settle down to normal. This is why you must use 450-500v filter caps throughout the entire amp because voltage does go very high until tubes warm up and begin conducting. It would be wise to set the bias pot for the highest negative dc voltage you can dial up before doing this though. Clip your meter to pin 5 of a power tube socket and set your meter for the lowest dc voltage scale (or use an auto ranging meter). Now with the amp powered up and no tubes installed, turn the bias pot until you see the highest negative voltage, maybe -50vdc or so. This will bias the power tubes very cold but will prevent redplating. Now you can start the amp with all tubes and do your voltage testing. Remember that voltages will still look slightly high because the amp is biased cold. If everything looks normal, you can now do the math and use a bias probe or 1 ohm cathode resistors to set you bias. The b+ will now drop a bit throughout the entire amp as you warm up the bias.

Elegost
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Re: High voltage on 2204 V1/V2 Plates

Post by Elegost » Mon Sep 03, 2018 2:17 am

Thanks for your help! I took a bunch of readings today with tubes in, I will post those below with links to a bunch of pictures.

Pictures:
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Elegost
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Re: High voltage on 2204 V1/V2 Plates

Post by Elegost » Mon Sep 03, 2018 2:19 am

I'm getting 122v from the wall
I'm getting 360vac from the hv leads off the power transformer
i get 494 vdc after the rectifier diodes
bias voltage is -49
All my heaters seem to be functioning, i'm getting about 3.3vac on all 5 tubes
I'm using a 1x12 cabinet with a 200w EV in it as a load while testing.

preamp tube readings with no power tubes:
V1 pin 1 - 295
V1 pin 2 - signal
V1 pin 3 - 2.6
V1 pin 4 - 3.3
V1 pin 5 - 3.3
V1 pin 6 - 350
V1 pin 7 - signal
V1 pin 8 - 3.6
V1 pin 9 - 3.3

V2 pin 1 - 216
V2 pin 2 - signal
V2 pin 3 - 1.5
V2 pin 4 - 3.3
V2 pin 5 - 3.3
V2 pin 6 - 408
V2 pin 7 - 216
V2 pin 8 - 223
V2 pin 9 - 3.3

V3 pin 1 - 286
V3 pin 2 - signal
V3 pin 3 - 39.6
V3 pin 4 - 3.3
V3 pin 5 - 3.3
V3 pin 6 - 265
V3 pin 7 - signal
V3 pin 8 - 39
V3 pin 9 -3.3

Readings with power tubes, immediately after powerup, all knobs at zero
V1 pin 1 - 281
V1 pin 2 - signal
V1 pin 3 - 2.34
V1 pin 4 - 3.3
V1 pin 5 - 3.3
V1 pin 6 - 331
V1 pin 7 - signal
V1 pin 8 - 3.3
V1 pin 9 -3.3

V2 pin 1 - 204
V2 pin 2 - signal
V2 pin 3 - 1.43
V2 pin 4 - 3.3
V2 pin 5 - 3.3
V2 pin 6 - 382
V2 pin 7 - 204
V2 pin 8 - 205
V2 pin 9 -3.3

V3 pin 1 - 269
V3 pin 2 - signal
V3 pin 3 - 35
V3 pin 4 - 3.3
V3 pin 5 - 3.3
V3 pin 6 - 252
V3 pin 7 - signal
V3 pin 8 - 35
V3 pin 9 -3.3

V4 pin 1 - gnd
V4 pin 2 - 3.3
V4 pin 3 - 465
V4 pin 4 - 458
V4 pin 5 - -49
V4 pin 6 - 467
V4 pin 7 - 3.3
V4 pin 8 - gnd

V5 pin 1 - gnd
V5 pin 2 - 3.3
V5 pin 3 - 465
V5 pin 4 - 458
V5 pin 5 - -49
V5 pin 6 - 467
V5 pin 7 - 3.3
V5 pin 8 - gnd

After a few minutes the power tubes get very hot. the preamp tubes are still cool to the touch. The voltages in the preamp spike to this:

V1 pin 1 - 440
V1 pin 2 - signal
V1 pin 3 - 0
V1 pin 4 - 3.3
V1 pin 5 - 3.3
V1 pin 6 - 440
V1 pin 7 - signal
V1 pin 8 - 0
V1 pin 9 - 3.3

V2 pin 1 - 440
V2 pin 2 - signal
V2 pin 3 - 0
V2 pin 4 - 3.3
V2 pin 5 - 3.3
V2 pin 6 - 445
V2 pin 7 - 440
V2 pin 8 - 0
V2 pin 9 - 3.3

V3 pin 1 - 273
V3 pin 2 - signal
V3 pin 3 - 28
V3 pin 4 - 3.3
V3 pin 5 - 3.3
V3 pin 6 - 440
V3 pin 7 - signal
V3 pin 8 - 24
V3 pin 9 -3.3

Thanks for any help!

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neikeel
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Re: High voltage on 2204 V1/V2 Plates

Post by neikeel » Mon Sep 03, 2018 6:00 pm

With tubes out the amp voltages might be up 10% or so but if the B+ is that high you are always going to have issues.
The stock Marshall has two series B+ droppers between choke and PI (any combination of 8k2 or 10k) also the mid board also has two 10k droppers not one (take a peek at the schems or layouts)

What mains transformer tap are you using for your wall voltage?
Your heater readings are just a little high (should be 3.15vac each half for total 6.3vac
Yours are reading 6.6v which is 5% over and if you apply this to B+ you are about right.
Maybe your wall voltage is too high - will a variac help?
Neil

Elegost
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Re: High voltage on 2204 V1/V2 Plates

Post by Elegost » Sun Sep 09, 2018 2:36 am

It was v2 pin 8. I worked on it for many nights but eventually I just took a chopstick and poked wires and connections until I found the intermittent fault. Remade the connection btw pin 8 and and the board and it seems fixed! Thanks everyone,

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