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Old 20th June 2013, 11:31 AM   #1
Renioo is offline Renioo  Poland
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Join Date: Dec 2012
Default Marshall Plexi Clone

Hi,

I recently exchanged my old amp for a clone of what should be a 20Watt Marshall Plexi clone. Everything was excellent, it had this bright ear-hurting lead sound and a muddy rhytmic one. However, I had to move it from one room to another and when I plugged it in it had a distorted sound, of rather low volume, with a lot of harmonic harsh in the background. It seemed rather odd it had done this in a moment, because it was plugged in for an hour and acted excellent before the trade and in my home. I don't think it was any defected amp. I found the pot, which I think can be a bias or something - I had to move it nearly to the limit and it worked fine - but a bit muddy, it lost its brightness and volume. I know this pot works very quick, 0,5cm of turn and it goes from around 15V (when it sounds kinda allright) to 30V (it distorts badly or even stop to make a sound). The pot does not affect the voltages, so I think it's a bias pot.

I know, and I measured it has a mod - the 2x EL34 works on 560V plate voltage. I can't find a spot in which I can measure a voltage drop - it has always 560V or 2x 280 in every place where high voltage goes.

There seems also to be strange thing, in my opinion making the sound that muddy and and not loud - there is a 64V, 56V and 50V on 12AX7 which makes a preamp (from third in a stage to first I think) - it looks terribly low.

Do you have any suggestions what to check, change or anything? I think it's a variation of layout i post here. The ceramic resistors are 10K, the pot in it's position is something about 5k, big caps on board are 100uf/385V. Some values are other than on the layout I post, but it's really close.

All connections seem pretty allright. The "bias pot" is without it's case behind big right caps, left to the two small caps. I'm sorry for quality of photo, looking for a camera.
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Old 20th June 2013, 11:47 AM   #2
Renioo is offline Renioo  Poland
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Ok two better quality photos
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Old 20th June 2013, 05:01 PM   #3
Struth is offline Struth  Canada
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Location: books at londonpower.com
Hi Guys

The bias pot is nothing to be twiddled lightly.

The 1R resistor from pin-8 to ground of each power tube socket is there to provide safe ground-referenced measurement of the idle current of the tube. Bis is set with the input volumes or MV set to zero, no signal.

Use an alligator test lead to clip the negative lead of your meter to ground. This means you have only one hand in the live chassis.

Use a DC voltmeter to first set the voltage at pin-5 of either tube to the maximum negative value.

Next, measure the plate supply voltage. Best to probe on the filter cap or the output of the main rectifier. Note this voltage.

Each power tube has a plate power rating. The layout is for a 50W amp and the B+ looks to be about 450V. if you have EL-34s in place, these have a plate rating of 25W, which just means the plate of the tube can throw off 2W of heat and means nothing about the output power that be attained in audio watts.

25W at 450V corresponds to a current of 55mA. This is the maximum current allowable at idle for this tube at this voltage. If the supply voltage was half as high, current could be doubles; if the voltage was doubled then current would be halved. This is just standard power equations and Ohm's Law.

Generally, the idle current is set so that the dissipation is not at maximum but rather a half or a bit higher. This means your target current reading would be more like 27-35mA.

Because those 1Rs are in place, you will not be measuring current, rather voltage. Each 1mVdc = 1mAdc through the tube. So, you will be looking for a reading of 27-35mVdc when you probe either of the octal socket pin-8s.

You cannot set bias by looking at the grid control voltage as that does not tell you how hot the tube is running. Only measuring the tube current and knowing the plate voltage will provide TRUE information about if the tube will be safe.

Usually, when a move is made as you describe and the amp then does not work or sound okay, it is just a bad tube, or a tube is loose in its socket. Combo amps are the worst environment for tubes.

Have fun
Kevin O'Connor
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Old 20th June 2013, 09:33 PM   #4
Renioo is offline Renioo  Poland
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The plate voltage is 560V. I said this is somehow modified version of the layout I posted. Setting the pin 5 to the lower voltage distorts the signal. It can be done by turning the pot I was talkin about and it sound the best when it's fully turned.

I really don't think it's a tube because the problem just magically appeared.

There I think it's also the preamp section low voltage issue.

Are there any other ideas?

Edit: wow, wow the preamp is made using 6n2p tubes

Last edited by Renioo; 20th June 2013 at 09:45 PM.
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Old 21st June 2013, 02:35 PM   #5
Renioo is offline Renioo  Poland
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The DC meter does not show a stable voltage, it goes from 27-32mV across the 1R resistor (but it goes from pin 5 control grid to unused pin 6?) and seems to be OK then with voltage of 560V. I think that stage is working OK . The 100K pot about which I was talking sets the grid 1 (control grid pin5) minus voltage (now it's about -12V, then it was about -30 to -50V and worked like a crap).


What should the voltages been then?

Last edited by Renioo; 21st June 2013 at 02:59 PM.
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Old 21st June 2013, 05:42 PM   #6
Struth is offline Struth  Canada
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Hi Guys

Renloo, there is missing information in your statements about this amp, and you seem to have a total disregard for the safety of the tubes as outlined above.

According to the layout drawing you posted, B+ should be around 450V. Look at the lower right corner of the PT and see 325-0-325. The 325 lines go to each end of the half-bridge rectifier; 0 goes to chassis ground. 325Vac becomes about 450Vdc once rectified and filtered - NOT 560V.

Measure pin-4 of the output tubes. if your claims about the grid voltage (pin-5) is correct then pin-4 should be around 280V.

If the above screen voltage is half the plate voltage (280Vs, 560Va) then the supply is quite different, more like the high-voltage Marshalls from the late '70s. If this is the case, and Vs is only 280V or so, then you would expect to see about -30-35V at pin-5 to keep the tubes from melting down.

-12V on the control grid makes no sense whatsoever - the tubes would be red-plating!

It is more likely that you are not using a proper reference point for the meter readings, or that the circuit ground is liftable from the chassis ground via an undisclosed ground-lift switch (usually on the rear panel), or that your meter is not working, or that you don't know how to use the meter.

You MUST turn the volume controls on the amp to zero to make proper voltage measruements, otherwise you are measuring noise. This is especially important when checking the pin-8 voltages on the outputs.

Have fun
Kevin O'Connor
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Old 22nd June 2013, 11:30 AM   #7
Renioo is offline Renioo  Poland
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According to the layout - I said it is very similiar but have mods - like a higher voltage. So it's different. I looked for bias and for 560V on plate it should be around -25V (class AB) on the grid - then the amp is silent. It is set -25V now.

I don't run it at -12V because I know it's bad and it don't behave good way - so I don't run it like this, I just said then it starts to work, which maybe can be a clue.

There is no ground-lift switch, I know how to use the meter, the ground is perfectly ok.

I have turned the volume controls to zero, according to first reply.

I had checked the preamp and it is adjusted to work with those voltages.

It is my first time working with tube output stage, however I managed to find some good article about biasing now, because the problem lies propably there and everything should be brought back to normal...
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Old 22nd June 2013, 02:54 PM   #8
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plug in a known good cable and guitar only to the amp (this will check for pedal ground loop issues). also check and see if your guitar's pickup needs new batteries (possible issue here too).
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Old 22nd June 2013, 02:58 PM   #9
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I would worry about those low voltages. the phase splitter stage is gain stage near 1 and Marshall like to use a lower voltages in the splitter.
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Old 22nd June 2013, 03:05 PM   #10
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Quote:
Originally Posted by Renioo View Post
The plate voltage is 560V. I said this is somehow modified version of the layout I posted. Setting the pin 5 to the lower voltage distorts the signal. It can be done by turning the pot I was talkin about and it sound the best when it's fully turned.

I really don't think it's a tube because the problem just magically appeared.

There I think it's also the preamp section low voltage issue.

Are there any other ideas?

Edit: wow, wow the preamp is made using 6n2p tubes
Tubes can go bad at any time for any reason. Don't rule out a simple tube swap to fix the problem.
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