Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Jul Sat 11, 2020 5:50 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 31 posts ]  Go to page 1, 2  Next
Author Message
 Post subject: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Wed 18, 2020 9:38 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1335
Location: Bend OR 97703
So I'm finishing up a Bogen DB130 electronic restoration and it comes to adjusting the HV regulation - and there is no information on what to set it at.

Seriously. I have FOUR manuals for this thing, including the SAMS and they all say nothing about it. All versions of the original Bogen manuals say 'Leave it to a qualified technician'. They DO give a voltage in the schematics, but they all also say "All controls set to minimum" and in fact the voltage reading given is the minimum the regulator will adjust to.

SO anybody who is familiar with these amplifiers know what to set it to??? :?

_________________
I don't believe in experts. That's why I take my car to a plumber and my taxes to a chef.


Top
 Profile  
 
 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Thu 19, 2020 12:45 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4765
Location: Rochester NY USA
Since the negative grid bias is fixed, the screen voltage is adjusted to set the idle current in the output tubes. The 6AV5 is rated for 11 Watts plate dissipation though some will take more; 6FW5s are rated for 18W, otherwise very close to 6AV5. So, with 605V on the plate, 30 mA is 18W, way too much, 25 mA is 15W, 20 mA is 12W, starting to look reasonable.

The Sams folder says to set for 1V to ground at each test point (and balance for zero volts between them) with jumper strap removed, which is 10 mA through 100 Ohms. When the jumpers are reconnected, the bias will be 1V less negative and idle current will go up a little (who knows how much - the tube curves don't extend that far!). I'd prefer 10 Ohm 1% cathode resistors, 150 mV=15 mA, and would remove the jumpers entirely.

Though this amp uses a fairly low idle current, it won't suffer from crossover distortion at higher power due to regulator - screen voltage doesn't sag - it tracks as a percentage of B+. Actual voltage is less important than idle current. If you can't set that properly, check that the 100K pot and 68K series resistor are correct value. The 68K resistor could be increased to lower screen voltage if necessary.

_________________
My web page: https://bit.ly/2rxq4qx


Top
 Profile  
 
 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Thu 19, 2020 1:35 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1335
Location: Bend OR 97703
Thank you Tom!

The thing is I have the original version: Single rectifier tube, no balancing option (and no jumper straps). I didn't even know SAMS had any later versions - perhaps it's for the DB-130A?

Also, because my line voltage is 120VAC my minimum regulated voltage is about 618VDC. Will look at that!

There are at least four versions of this amp that I know of: the original version with single rectifier and no balancing option (brass face) - the original version with dual rectifier tubes and no balancing option (brass face) - the dual rectifier version with balancing option (champagne face) - the DB-130A, with single rectifier and balancing option (gray face).

Anyway, I am finished for the night, will look at it tomorrow and see what I can do with your insight. Thanks again!

_________________
I don't believe in experts. That's why I take my car to a plumber and my taxes to a chef.


Top
 Profile  
 
 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Thu 19, 2020 2:19 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4765
Location: Rochester NY USA
Yes, sorry - the Sams was the DB-130A. But the Bogen schematic I have for the DB130 seems identical in this area.

_________________
My web page: https://bit.ly/2rxq4qx


Top
 Profile  
 
 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Thu 19, 2020 3:50 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1335
Location: Bend OR 97703
This is the original version SAMS schematic.

I have often wondered if there wasn't a way to discreetly add a balancing circuit for the output stage. You said you had some thoughts on that?

Image

_________________
I don't believe in experts. That's why I take my car to a plumber and my taxes to a chef.


Top
 Profile  
 
 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Thu 19, 2020 3:53 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1335
Location: Bend OR 97703
After posting that, I see it isn't very clear. I could email you the high res copy if you'd like. Nice web site you've got, by the way!

_________________
I don't believe in experts. That's why I take my car to a plumber and my taxes to a chef.


Top
 Profile  
 
 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Thu 19, 2020 3:57 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1335
Location: Bend OR 97703
After posting that, I decided to just send it to you, and have. Check your email. :)

_________________
I don't believe in experts. That's why I take my car to a plumber and my taxes to a chef.


Top
 Profile  
 
 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Thu 19, 2020 10:18 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4765
Location: Rochester NY USA
Thanks for the schematic - I may have a copy, several boxes deep in the basement...

First step would be to add 10 Ohm resistors in each 6AV5 cathode. You need this to measure cathode current. A balance control can be added at the bias feed point as shown in the later DB130 schematic. I have that schematic if you don't . Since this will result in less negative voltage to the grids, the screen voltage will have to be decreased to compensate... so you may want to see where the idle current is first, adjust that regulator if it doesn't go low enough. There are a number of high-value resistors in that circuit - make sure they're accurate (the best way to do that is replace 'em all with metal film parts...).

_________________
My web page: https://bit.ly/2rxq4qx


Top
 Profile  
 
 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Thu 19, 2020 11:53 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1335
Location: Bend OR 97703
Thanks again Tom.

I've checked all the resistors, and they are in spec.

When I was first running it I noticed the output transformer getting very warm. Then I found out that the "Increase regulation" arrow on the pot pointed to lowering the regulated voltage. Things cooled down after that.

After running it at volume for a couple hours both the output and power transformers do get pretty warm, together - and man does it sound good! I'm running it with an old University 312 triaxial and it sounds amazing!

Also, as far as the cathode resistors... The idea is to measure idle current flow, correct? To flow, it has to go through the tube, cathode to anode, correct? If that's the case, and we are checking for DC idle current - couldn't we just measure voltage drop across the transformer primaries, calculating with primary DC resistance to get the current?

I mean, I'm probably missing something here, and that 1% cathode resistance would be pretty handy - but if you measure the primary DC resistance on each leg of the output transformer and keep them straight, wouldn't that tell you the same thing?

Listening to how this sounds, I don't think I will be modifying this one - but I have three more brass face chassis to resurrect, so I am still very interested in this discussion!

Thanks again!

Image

_________________
I don't believe in experts. That's why I take my car to a plumber and my taxes to a chef.


Top
 Profile  
 
 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Fri 20, 2020 12:00 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4765
Location: Rochester NY USA
Yes, you can use the transformer primary, but two things to keep in mind: First, the two halves of the primary are likely different in resistance, second, copper has a positive thermal coefficient - resistance goes up as it heats up - enough so that it's used to measure transformer winding temperature. I like measuring voltages closer to ground myself...

_________________
My web page: https://bit.ly/2rxq4qx


Top
 Profile  
 
 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Fri 20, 2020 12:12 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1335
Location: Bend OR 97703
What you said is true - however the temperature rise should be quite manageable at idle.

I just hate to reinvent the whole output stage for something that's solved with matched tubes. I can't help but think there should be an easy way to match current in each leg.

Perhaps use a clamp-on style ammeter for each leg? It's not hard to get one with 1 ma resolution these days.

_________________
I don't believe in experts. That's why I take my car to a plumber and my taxes to a chef.


Top
 Profile  
 
 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Fri 20, 2020 1:42 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1335
Location: Bend OR 97703
I think what I'm going to do is set up one of my chassis' as a test bed - see if I can make a few simple mods that actually improve it.

Besides that, I'm still curious just how they set optimum regulation. It just seems odd that it has this big labeled control on the back of the chassis, and NO info on setting it.

_________________
I don't believe in experts. That's why I take my car to a plumber and my taxes to a chef.


Top
 Profile  
 
 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Fri 20, 2020 6:39 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1335
Location: Bend OR 97703
Anyone?

_________________
I don't believe in experts. That's why I take my car to a plumber and my taxes to a chef.


Top
 Profile  
 
 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Sat 21, 2020 5:07 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4765
Location: Rochester NY USA
The optimum operating point in class A is easy - as hot as possible! Class A amplifiers are generally run near rated maximum dissipation. Of course, tube life decreases with temperature, so you might back it off from maximum.

Class AB is not so simple - at low levels, the tubes are in continuous conduction and distortion will likely decrease as idle current is increased. However, unlike class A, dissipation will increase and reach a maximum as power output is increased, so it is more important to stay below max dissipation at idle. And the distortion gets complicated as well - often there will be a minimum at low power and another at a higher power level. Look at the plate curves for a large power tube - notice how the the spacing increases at higher currents? That increase in transconductance at higher current is a good thing for class AB, since only one tube is supplying all the output at a time at higher parts of the AC cycle while the other one is cut off. And the idle current giving lowest distortion will change depending on the power level where it's measured - there may not BE an optimum.

Bogen gave a rather strange procedure for the DB130A - set idle current at 10 mA per tube with 100 Ohms in series with cathode, then short out that resistor (which changes the bias, and results in somewhat higher current). Presumably, they chose an operating point giving lowest distortion at rated power, while remaining within tube ratings (at least under average conditions). Over 20 mA will exceed the 6AV5 ratings at idle, so there's not a lot of adjustment range. I'd set for 15 mA and be done with it.

_________________
My web page: https://bit.ly/2rxq4qx


Top
 Profile  
 
 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Sat 21, 2020 7:33 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1335
Location: Bend OR 97703
Thanks again for your reply Tom.

As I said earlier when I first turned it on I had the HV reg. control turned all the way clockwise, which turned out to be the wrong way, leading to the output trans rapidly heating up. After turning it off and letting it cool back down I tried again, this time starting with the control all the way counter-clockwise. As a VERY rough setting HV estimate I slowly turned the control clockwise while continuously checking the output trans temperature, eventually arriving at about a 15 degree clockwise setting of the control. This seemed to satisfy the transformers, and seemed a rather low setting, so I went with it for now.

I've been running this all morning, plugged into a CD player. The first time I ran it for about an hour and then it inexplicably started motor-boating. You could still hear the music, but something was wrong. I turned it off for about 5 minutes, then turned it back on. It came back on no problem.

The second run went for about two hours, during which time I played it quite loudly. This time I also activated the variable damping factor. It actually does seem to improve the sound. At the end of the two hours it was quite hot - I couldn't quite keep my fingers on the transformers. They were both about the same temperature.

Rather worried, I hurriedly unplugged it, turned it over, and took the bottom cover off, expecting to see smoke and charcoal - but, nothing. It looked exactly like it had, except all the wires that had goo on them now glistened like they were wet. Now I know where the goo comes from. These amps, probably because of modern line voltage, run pretty hot.

And back to the idle current - I am running matched tubes in the output, so balancing is a secondary problem. What if, in order to solve the idle current problem, I was to put my 10 ohm resistor in the B+ wire leading to the output transformer? This should give a sum of the tubes idle current, I could measure it that way. What do you think?

_________________
I don't believe in experts. That's why I take my car to a plumber and my taxes to a chef.


Top
 Profile  
 
 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Sat 21, 2020 7:58 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4765
Location: Rochester NY USA
I'd put two in the cathodes - 10 Ohms is too low to have any noticeable effect and measuring there doesn't put 600V on the meter leads. Matched tubes may not be matched at 600V, and for that matter may not stay matched.

_________________
My web page: https://bit.ly/2rxq4qx


Top
 Profile  
 
 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Sat 21, 2020 8:04 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1335
Location: Bend OR 97703
Tom Bavis wrote:
I'd put two in the cathodes - 10 Ohms is too low to have any noticeable effect and measuring there doesn't put 600V on the meter leads. Matched tubes may not be matched at 600V, and for that matter may not stay matched.

That is true, but you also mentioned above that putting those small resistances in the cathodes would require other circuit adjustments, especially if a balancing circuit was installed, so I guess I'm a little confused.

One thing to keep in mind is that for the later versions of the DB130, the ones with tube balancing, a different power transformer was used. I'm not sure what the difference is. Those later versions also have dual rectifier tubes, but I have another early version with no balancing and dual rectifier tubes, and it uses the same power transformer.

I can see the need for improvement (Just HOW did they set that HV regulator? WHAT did they measure??) but would prefer any mods to be as simple as they are helpful...

_________________
I don't believe in experts. That's why I take my car to a plumber and my taxes to a chef.


Top
 Profile  
 
 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Sun 22, 2020 4:15 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4765
Location: Rochester NY USA
No, perhaps I wasn't clear - I suggested that adding the balancing circuit might require other circuit changes since it would change the control grid bias (and there might not be enough adjustment range on the screen voltage to compensate). This may be the difference in the transformers - more grid bias voltage.

Anyway, adding 10 Ohm resistors will only drop millivolts - tiny effect on the circuit. You can swap 6AV5s if necessary for best match.

_________________
My web page: https://bit.ly/2rxq4qx


Top
 Profile  
 
 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Sun 22, 2020 4:31 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1335
Location: Bend OR 97703
Thanks again Tom. Thank you for clarifying.

The arrangement you propose sounds very much like the arrangement Bogen used in the RB140, which is the last mono receiver they made. They do away with the straps and the balancing control, but have a "Bias" adjustment and a set of test points, one for each push pull leg. They use 15 ohm @ 1 watt resistors, one for each pair of 6973 tubes. When I restored it I did indeed use the test points to swap tube positions to try to balance out the measurements.

I could use 10 ohm cathode resistors but would still prefer to use 1 watt, just to keep any temperature drift to an absolute minimum if nothing else, so I will have to order them.

In the mean time I am still considering experimenting with measuring the DC resistance of each primary leg of the output transformer and using that as a rough basis for setting it, for a couple reasons.

1) Bogen used SOMETHING to set that regulator, and that is the ONLY thing in the circuit I can see, so they must have used it.

2) It's true that the legs more than likely aren't equal and the DC resistance will drift with temperature, but they are always like that anyway, and they are always part of the circuit, so that difference and that drift will be part of the measurement whether I put in a resistance or not. The matched resistance values just make the measurements a little easier and more convenient. So, I aim to take some measurements and see what it looks like - preferably after it has heated up.

_________________
I don't believe in experts. That's why I take my car to a plumber and my taxes to a chef.


Top
 Profile  
 
 Post subject: Re: Bogen DB130 High Voltage Regulation setting
PostPosted: Mar Tue 24, 2020 10:22 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1335
Location: Bend OR 97703
I took cold and hot resistance readings of the output trans primary.

Readings are (each leg - they measure within 2 ohms of each other)

Cold: 385 ohms

Hot: 415 ohms

Looking at the DB130 procedure I have for setting bias and balancing from the original Bogen manual, it says to set the regulation with your voltmeter on 1-2 to 1.5 volts. That's 15mA through the 100 ohm cathode resistors. Calculating out voltage drop @ 15mA for each leg of the OT primary gives us:

Cold: 5.78 volts

Hot: 6.23 volts

I'm gonna call it good enough at 6.2 volts. Interestingly where I had set it the voltage drop was quite high, at about 16 volts, which is over 40mA!

I obviously am not understanding how this circuit works. :(

Anyway, after letting it warm up so the voltage drop stabilized I set it to 6.2 volts. Interestingly, this put the regulation control in the middle-ish range, so I am hopeful I'm on to something. It's been running about 20 minutes and is still running cool - I'll let it run for a couple of hours, monitoring.

_________________
I don't believe in experts. That's why I take my car to a plumber and my taxes to a chef.


Top
 Profile  
 
Post New Topic Post Reply  [ 31 posts ]  Go to page 1, 2  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 10 guests



Search for:
Jump to:  


































-->


Privacy Policy :: Powered by phpBB