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Post subject: Re: What determines bias reading?
Posted: Wed Jan 29, 2020 7:10 pm
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No,

I set the bias on one tube at .038, plate voltage was 365, grid screen was 364

Edited to show my error...plate voltage was 465, not 365, grid screen was 465, not 365.


Last edited by royb3988 on Sun Feb 02, 2020 2:37 pm, edited 2 times in total.

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Post subject: Re: What determines bias reading?
Posted: Wed Jan 29, 2020 7:37 pm
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Sorry, I was back on page 2, didn't see the new reply for a bit. I assume you mean 465vdc and 464vdc @ 38ma (a reasonable bias, imo @17.7watts). Check both tubes or each pair. Thats getting darn close to a hot screen grid, when the plate voltage drops below the screen grid voltage you could have problems.
You should check the screen grid resistance w/ tubes in power off, black to ground red to pin 4 of each 6l6. W/ a matched set they should all be nearly the same. This measures the resistance in the circuit as well from the PSU resistor to the 470 ohm screen resistor and the tubes resistance. If one is off considerably from the others they are not as matched as they should be. Have you checked all the resistors on the tube sockets, the more closely matched the resistors of each pair are the better.


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Post subject: Re: What determines bias reading?
Posted: Wed Jan 29, 2020 8:13 pm
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sfceric64,.

I have not...I am admittedly somewhat new to this level of trouble shooting and repair. I will look up the value of all the resistors that are attached to each socket. I assume I can get that info from a schematic.

One of the tubes in the inserted pair reads 26 mA while the other reads 38 mA...could that, in fact be the culprit?

Thanks,

RB3988


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Post subject: Re: What determines bias reading?
Posted: Thu Jan 30, 2020 2:01 am
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Plate voltage and screene voltages are very close,


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Post subject: Re: What determines bias reading?
Posted: Thu Jan 30, 2020 2:11 am
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royb3988 wrote:
sfceric64,.

I have not...I am admittedly somewhat new to this level of trouble shooting and repair. I will look up the value of all the resistors that are attached to each socket. I assume I can get that info from a schematic.

Yes

One of the tubes in the inserted pair reads 26 mA while the other reads 38 mA...could that, in fact be the culprit?

Thanks,

RB3988


Culprit for what ? redplating ? No.

You did not write before you buy a matched pair ? Tubes are not matched.
Matched less than 8 ma is very good.

Tubes redplating yesterday and now they work fine :shock: ; Something you don't see or write about the amp .
Damaged/ dirty/ loose socket contacts ?

To help you with no amp in front, is not easy.
Post good pictures.


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Post subject: Re: What determines bias reading?
Posted: Thu Jan 30, 2020 2:14 am
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royb3988 wrote:
Stratele,

Tubes are installed...inside pair, tubes have subtle orange glow at top and bottom.

RB3988


Is the heater or low redplating ? Post pictures.


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Post subject: Re: What determines bias reading?
Posted: Thu Jan 30, 2020 4:35 am
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If the plate voltage falls below the screen voltage, you can burn up the screen grid(redplate-so to speak) a screen grid that has gaping holes in its structure due to high current will not function correctly. A red plated tube sometimes exhibits the signs of a bad screen grid for example, varying size spots on the plates from electron bombardment. You really need 5x-10x magnification to see these on a tubes plate. Whereas, a real red plated tube will generally scar-discolor the plate enough to be visible w/ the nekid eye when cold.

From the closest schematic i have for your amp, the spec says @ -52vdc on the bias supply to each tube the plate vdc is 450 while the screen grid vdc is 443. The goal electrically speaking.
Each tubes current draw is dependent on its internal components and its socket specific resistors.
Quote:
One of the tubes in the inserted pair reads 26 mA while the other reads 38 mA...could that, in fact be the culprit?

In your case, 26ma and 38ma in the center pair of sockets imo is not closely matched(how does it sound?) You cant make a bad/worn-out tube better. It is what it is.
Swap the tubes w/ each other, does the ma reading follow the tube? if yes the socket resistors are fine and one tube is weaker than the other or not matched. Try other available tubes in each socket until you find the best match you have available. If no, then you likely have other circuit issues that need troubleshooting.
To clarify matched pair tube definition i gave earlier in the thread: you buy a matched quad of 6l6's.
Each of those tubes should yield the same or nearly the same ma's when used in the same socket.
It does not mean they will yield the same ma's in any other socket.
Each socket shares from the power supply, resistors and capacitors network to provide plate and screen grid voltages.
Each socket shares the bias supply voltage, adjustable by either bias or balance potentiometer or a fixed resistor.
Each socket has its own resistors for screen grid and grid.
Each tube has its own internal resistance for each section.

The only issue I see if the amp sounds good, is
As you said earlier, your plate voltage dropped to w/in one volt of the screen grid voltage when biased to 38ma.
I would check the remaining tube sockets to see if they respond the same using the same tube in each socket. For me, if the plate to screen voltages responded the same across all 4 sockets, I would adjust the PSU screen dropping resistor(schematic says 1k , I would increase to 1.2k-test 1.4k-test, etc.until i reached the schematics 7 volt difference at bias voltage of -52). I would test all the socket resistors and closely match tube socket pairs. I would consider replacing the bias potentiometer if its not smooth functioning or cleaning at a minimum.

To little difference between plate and screen voltages can lead to melt-down of the screen(lower resistance) shorter tube life, greater chance of red plating and lower output(screen grid receiving more electrons than the plate).
To much difference between plate and screen voltage will impact the tone of the amp. Headroom, touch dynamics/feel, compression effects and/or SAG are all effected by the relationship between these voltages. Bias type does play a role in the operation of the tube and the relationship between plate and screen voltages.


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Post subject: Re: What determines bias reading?
Posted: Thu Jan 30, 2020 10:30 am
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sfceric64,

Is there a way to read the bias mA on each tube from inside the chassis...so I do not have to switch back and forth from using the bias tool adapters to my Fluke VOM probes? I will be getting a second VOM soon but until then it would save time if I can just read the bias settings using the probes.

Thanks,

RB3988


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Post subject: Re: What determines bias reading?
Posted: Thu Jan 30, 2020 11:27 am
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You can read bias on each tube across a 1 ohms resistor between cathodes and ground.

Voltage and ampere are same in 1 ohms resistor ; for 35 ma bias ,voltmeter across resistor will read 35 mv
Resistor could stay there forever in the amp.

I sometimes use it.

Image[url=https://flic.kr/p/2in4LLT]


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Post subject: Re: What determines bias reading?
Posted: Thu Jan 30, 2020 1:47 pm
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Based on this picture from your previous "tremolo" thread, it appears you already have 1 ohm resistors on the cathode pin of the power tube sockets.
Image

Unless you removed them.

Which is why posting some fresh pics would be helpful.

royb3988 wrote:
No,

I set the bias on one tube at .038, plate voltage was 365, grid screen was 364


royb3988 wrote:
sfceric64,

Tubes in, tubes out? I measured all 4 tube sockets...inside pair has tubes in it, all 4 are 465 DC volts.

RB3988


So, which is it? 365 or 465 or 470?

I would be very leery of any tube that has red-plated, even once.

Based on what you have said so far, you have:
a. A power supply problem
b. An output transformer problem

I hesitate to even post this, because you're getting good advice from Stratele52, sfceric, and BMW2002ti, but you don't seem to know what you're doing, and anything I say will only serve to confuse you more I fear.

I realize you're trying to learn something, but I'm not sure a vintage 52 year old AB763 Dual Showman is the best amp to learn on. A simple mistake can mean damaging another part of your amp, like an output transformer, or a power transformer, severely devaluating your amp.
I had a 1967 Telecaster that I "experimented" on relentlessly in the '70s, getting different sounds. I regret that now. I ended up selling certain parts of that guitar, but I could of gotten a lot more money if it were intact today. My point is if there are components that have already been replaced, like capacitors or resistors, the damage is already done, replacing them again will make no or little difference.

In that thread about the tremolo of your amp, it was recommended that you replace all of the electrolytic capacitors, was that ever done? Based on the pictures you posted, they had been replaced before, but it was not possible to determine when.


Post some fresh pictures of the power tube sockets. Not just one, but all four.

I'm going to stop posting in this thread, because I don't want to add to your "confusion".

If you ask why other people are posting the advice they give you I'm pretty sure they would be happy to explain.

vinyl


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Post subject: Re: What determines bias reading?
Posted: Thu Jan 30, 2020 2:02 pm
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Vinyl...

Yes, I located them and ran the test. The previous owner probably had the resistors installed...I just didn't know where to look but found them on pin 8.

Thanks,

RB3988


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Post subject: Re: What determines bias reading?
Posted: Thu Jan 30, 2020 2:08 pm
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vinyl wrote:


Based on what you have said so far, you have:

a. A power supply problem
b. An output transformer problem
l


And more.....

It is too much work to help you, need too much time. You may learn something with us but you need to learn electronic first like a amp tech.

I will not follow anymore.


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Post subject: Re: What determines bias reading?
Posted: Thu Jan 30, 2020 2:49 pm
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Quote:
Is there a way to read the bias mA on each tube from inside the chassis...so I do not have to switch back and forth from using the bias tool adapters to my Fluke VOM probes?

To do both simultaneously you need two meters. Otherwise, as stratele52 has suggested and vinyl has confirmed w/ other thread photos you can measure across the 1ohm resistor to get the current measured in vdc.
red on cathode pin 8 blk on ground side of 1ohm resistor


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Post subject: Re: What determines bias reading?
Posted: Thu Jan 30, 2020 6:01 pm
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Vinyl...I understand your frustration trying to help out, I figured this forum, while not being able to solve all my amp problems, would be a good place to learn a thing or two...and I have.

To everyone else:

After extensive swapping, measuring and examining all 4 tubes, one tube, no matter which socket it is in...either by itself, in a pair (outside or inside) or as part of the quad...always runs the amp into a slow thumping that rapidly increases in speed resulting in red plating...as I call it.
One other tube will not bias higher than 26 up to 28 no matter at what level I set the bias for any other tube.

So...I had an old pair of Ruby 6l6GCMSTR tubes...and I tried them one at a time and then as the outer pair, then the inner pair and the amp runs fine. Those two bias at .028 and .033, plate voltage at 455.

The two Svetlanas I kept will each bias within 3 mA at 32 and 35 each. Those have a plate voltage of 455...I can set them higher but don't want to put too much juice into the Ruby's...they glow somewhat brighter then the Svets.Image

The amp plays great. I'm going to figure it's time for a new, "matched" quad of good power tubes.

Edited to add: the pair of Ruby's that are the inside pair glow the same way if put outside with the Svets inside.


Thanks,

RB3988


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Post subject: Re: What determines bias reading?
Posted: Thu Jan 30, 2020 7:15 pm
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RB,

I'm not really frustrated, I just "suffer" from a good memory. I wish it was photographic, but it's not.

For advice about which tubes, I don't think there is anyone here that knows more than BMW2002ti. If I were seeking advice about a matched quartet of power tubes, his advice would be the one I would seek.

Arjay (Retroverbial) would be the next. Then Stratele52. Others too.

I'm glad you have seemed to resolve your problem.

Cheers,
vinyl

P.S. Solving problems here is possible, but the devil is in the details. Pictures can be a huge part, but beyond that, you will find people here that are at least willing to try and help you.


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