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 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Tue 24, 2020 11:40 pm 
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Location: Bend OR 97703
After running for a couple hours the OT is hardly warm at all. The PT is pretty warm, but I can still hold my fingers on the laminations comfortably.

Vast improvement - and it still sounds great!

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 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Wed 25, 2020 5:14 pm 
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Location: Powell River BC Canada
The way to adjust for best IM ( 60-8000 Hz) distortion that I ever did upon
replacement of output tubes was a model that had pots to set the individual
screen voltage to a sweet value.

It was a PA amplifier, forgotten the name just now.

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 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Sat 28, 2020 10:29 pm 
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radiotechnician wrote:
The way to adjust for best IM ( 60-8000 Hz) distortion that I ever did upon
replacement of output tubes was a model that had pots to set the individual
screen voltage to a sweet value.

It was a PA amplifier, forgotten the name just now.

Thanks for that! I will look into setting up an IM test in the future to assist with optimum biasing.

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 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Sat 28, 2020 10:36 pm 
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I think I've got it.

So, what I settled on is an optimum primary winding voltage drop (each leg) of 6.2 volts, after about 5 minutes warm-up. Of course I am VERY careful taking these measurements because the meter leads are elevated to nearly 700 VDC, never moving the probes when energized, etc..

I was able to dig out three sets of output tubes to move around. I got two that match within 0.2 VDC, which I think isn't too bad.

A couple of the tubes when swapped into place put the measured primary voltage drop at over 12 VDC, which indicated to me a big difference in output tube voltage drop.

At any rate, I have my tubes reasonably balanced and my bias set, though I will be looking in the future to try the small cathode resistors Tom suggested. Than you Tom Bavis so much for your help!

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 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Sat 28, 2020 10:47 pm 
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Location: Boston, MA USA
The DC resistance of the two legs of the OT primary winding never matches, because the winding pattern requires that the two legs be be of different lengths of wire. So by adjusting for balanced DC voltage drop between the two legs you are ensuring that the cathode currents are NOT balanced. That's one reason that current adjustment is always done with cathode resistors. The other reason is to avoid the high potential of the plate circuit which can exceed the voltage rating of some meters.

-David


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 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Sat 28, 2020 11:57 pm 
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Location: Powell River BC Canada
I don't know if balancing biasing adjustments can affect IM, in the same way
screen voltage does.

IM was mostly for the film crowd to set up the various modes of optical
sound recording.

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 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Mon 30, 2020 5:58 pm 
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dberman51 wrote:
The DC resistance of the two legs of the OT primary winding never matches, because the winding pattern requires that the two legs be be of different lengths of wire. So by adjusting for balanced DC voltage drop between the two legs you are ensuring that the cathode currents are NOT balanced. That's one reason that current adjustment is always done with cathode resistors. The other reason is to avoid the high potential of the plate circuit which can exceed the voltage rating of some meters.

-David

Thank you David. Where were you the previous page?

The DC resistance of the two legs matches within 2 ohms, as measured. That's a difference of 1/2 of 1%, as opposed to the idle current difference of the tubes, which is a little over 3% difference.

And, as I discussed earlier, it is a series circuit. Put the cathode resistors in the circuit, and the transformer is STILL in the circuit, still part of the current flow in each leg.

So, the real purpose of the cathode resistors, it seems to me, is to avoid measuring anything at the high potential of B+, but even then I have done so safely, with care, I swear.

Otherwise, someone needs to convince me that the idle current flow through the transformer has nothing to do with the idle current flow in that circuit. If you can do that, I'd really like to hear it.

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 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Mon 30, 2020 6:02 pm 
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radiotechnician wrote:
I don't know if balancing biasing adjustments can affect IM, in the same way
screen voltage does.

IM was mostly for the film crowd to set up the various modes of optical
sound recording.

What I'm really trying to do here is match idle currents, on the hopes that the rest of the operation of the tubes will be similarly matched. Yeah, I know - big if.

But that said, IM distortion is created by putting a signal through a non-linear device, yes? A push-pull circuit made up of non-matching legs would be even more non-linear, yes?

That was my thinking when I said that.

And that said, I listened to my amp for a couple hours last night, and I was not satisfied. It just sounds... off. Will keep you all posted on my continuing explorations. I'm going to try some more tube substitutions first, see what that does.

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 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Mon 30, 2020 7:33 pm 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
Intermodulation is when the overall system can impress a lower
frequency on a higher frequency. An example is hum in the background
of soft passages of music.

A question with music is, does the bass riff affect how a triangle
is heard. The conductor has an ear that pick can out a sour note in
an orchestra and know who, when and why it was bowed or blown.

The bias for a given tube affecting distortion may be differences in
individual tubes of a given type.

As a technician, you need to listen to what the owner of the amp
was saying. After all, there were far more people in the game than
there was easy money most times.

And the subject of sound has been around.
Attachment:
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helm i.jpg [ 284.12 KiB | Viewed 437 times ]
Attachment:
helm ii.jpg
helm ii.jpg [ 505.03 KiB | Viewed 437 times ]

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 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Mon 30, 2020 8:28 pm 
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radiotechnician wrote:
Intermodulation is...

Thanks again.

https://www.audiocheck.net/testtones_imd.php

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 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Tue 31, 2020 12:32 am 
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The first tube I substituted was the 6CG7 PA driver. Immediate improvement in sound. This amp also uses a 6CG7 as the voltage regulator. I'm thinking I had one in the driver socket that had previously seen extended voltage regulator duty, so wasn't very well balanced.

listened to very critically, it still wasn't quite where I thought it should be. On a whim, I opened my Champaign face DB130, the one WITH the balancing circuit, and measured the voltage drop across each leg of the OT.

Remember I had measured the DC resistance of each leg of the OT and calculated the voltage drop to bias at 15mA, which figured out to 6.2 VDC. I had gone through the biasing procedure for the Champaign face (the manual actually has one for this version of the amp), which measure 1.5 VDC across 100 ohms - but when I measured across each leg of the OT, it measured 8.8VDC. They have the same part number OT, so I don't get it - but I will investigate that tomorrow.

For tonight, going back to my original brass face version, I played around with the biasing. Too low of bias and it sounded terrible. Going by ear on the fly the sound seemed to clear around 5VDC... so I bumped it up to 6.9 VDC and restarted testing. At that level the two legs also equalized - I no longer had the 0.2 VDC difference between the legs.

I've been playing it for a couple hours and it is sounding really good. I'm also using a CD player as the source, instead of my phone. The transformers are running pretty warm but hanging in there. I can still keep my hands on them no problem.

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