DR-103 hums after new trans install

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DR-103 hums after new trans install

Postby GeorgeDelves on Wed Sep 16, 2015 5:14 pm

Hello all:

The original thread regarding the replacement of power transformer is here: http://tinyurl.com/oe2ckzn
I suppose that after a while the old threads are not seen as I had posted a few questions that went unanswered.

Anyway, I have installed the PT I ordered from Mojotone together with JJ filter caps from grangeramp.com and also replaced both switches as the power switch was intermittent, (the on position for these is down, right?), but now I am hearing a hum when powered up with tubes and the current limiter bulb I have in series with the amp is lighting up pretty brightly. I have looked-over the wiring but I don't see any caps wired backwards or any star-ground screws loose,

Any ideas?

By the way, the ac bias leads do run alongside the rear turret strip on their way to the new voltage doubler circuit but there is no short in this circuit. Also there is no short -- meaning the limiter lamp doesn't light -- where there are no tubes installed.

Thanks!!
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Re: DR-103 hums after new trans install

Postby pdf64 on Wed Sep 16, 2015 6:52 pm

Assume that with the power tubes out, it's fine?
If so, is the bias voltage on pin 5 sufficient?
Sorry, not much knowledge of these amps so can't advise of expected voltages.
But the above is fundemental, in that if pin 5 is insufficiently negative then the tube will draw too much current / run hot / worst case red plate.
So if the old and new PTs have bias winding voltages that are a bit different then this will happen.
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Re: DR-103 hums after new trans install

Postby GeorgeDelves on Wed Sep 16, 2015 7:43 pm

Thank you for responding.

I have also installed the Mark Huss voltage doubler bias circuit.
My reading are as follows:

from the transformer: 51.7 vac

leaving the doubler (before the pot circuits): -70 vdc

At pin five on V-1 & V-2: variable between approximately -6.07 to -12.39
At pin five on V-3 & V-4: variable between approximately -4.45 to -6.8

I may have just spotted at least part of the problem -- on the v3/v4 side: I left the 47K resister in that was a part of the original bias circuit... :oops:
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Re: DR-103 hums after new trans install

Postby GeorgeDelves on Wed Sep 16, 2015 8:20 pm

Okay, I removed the 47k from turrets 5 & 8.

Now at pin 5 of V1 & V2 (left side from the rear) I get, variable from: -6.11vdc to -12.45vdc each.

Pin 5 of V3 & V4 is, variable from: -6.26vdc to -12.07vdc each.

Here is the layout and bias circuit I am working from. NOTE: I have severed the uninsulated lead beneath the turret board between turrets 10 and 11 to facilitate separate biasing for each pair of valves. The one 100k and two 22k resistors per side are still in place between where the leads from the new bias pots enter at turret 10 & 11 and pin 5 of each valve.

http://s28.postimg.org/kvwacpjz1/working_Layout.jpg
---------------------------------------------------------------------
So, I guess it's a matter of changing the resistor values in either the Mark Huss bias mod or in the rest of the circuit somewhere to bring it into range for the EL34.
Which brings me to another question I had from the other thread ( viewtopic.php?f=3&t=109177 ) that given the apparent abundance of bias voltage available with the new transformer together with the voltage doubler mod -- once the resistor values are sorted -- could it be tweaked to provide enough bias voltage to properly bias a 6L6 or even 6550?

Thanks again, pdf64, OldSchoolDave, Ian_Frankish and MHuss, et al. I greatly appreciate your help! :excited:
Last edited by GeorgeDelves on Thu Sep 17, 2015 8:53 pm, edited 2 times in total.
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Re: DR-103 hums after new trans install

Postby GeorgeDelves on Thu Sep 17, 2015 4:20 pm

Here's where I am so far:

Image

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Last edited by GeorgeDelves on Fri Sep 18, 2015 1:37 pm, edited 1 time in total.
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Re: DR-103 hums after new trans install

Postby pdf64 on Thu Sep 17, 2015 10:02 pm

Now at pin 5 of V1 & V2 (left side from the rear) I get, variable from: -6.11vdc to -12.45vdc each.

Pin 5 of V3 & V4 is, variable from: -6.26vdc to -12.07vdc each.


These voltages are way insufficient, there would be immediate red plating with full VB+ applied.
Something must be screwy with the circuit between the bias supply output (voltage doubler) and the bias trimmers, ie dropping way too much voltage.

Have you got a schematic link to save me trawling?
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Re: DR-103 hums after new trans install

Postby GeorgeDelves on Thu Sep 17, 2015 11:26 pm

The circuit is pasted onto the power stage layout here:
http://s28.postimg.org/kvwacpjz1/working_Layout.jpg


Here is the full page the circuit came from, Improved dr103 dual pot circuit:
or click this to see full size: http://s1.postimg.org/9mra1dgu7/Bias_Ckts2rightway.gif
Image
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Re: DR-103 hums after new trans install

Postby GeorgeDelves on Fri Sep 18, 2015 3:06 am

U P D A T E :

After replacing the 220k resistor that were in series at the pot with 22k resistors, I got the following readings at pin 5 (with no tubes installed:

V1 & V2 variable from -34.2 to -47.3
V3 & V4 variable from -34.5 to -47

After setting both sides to -36vdc, I switched off and installed power tubes only and restarted. Same symptoms: as the tubes warm up the begin to glow the amp begins to hum fairly loudly and the current-limiter light begins shining about half brightness.

I had tried putting the power tubes in in pairs in various combinations hoping to isolate a shorted tube if one were present but I observed no change.

Any ideas??? :dumass:
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Re: DR-103 hums after new trans install

Postby pdf64 on Fri Sep 18, 2015 7:47 am

I don't think that you can have implemented the circuit correctly, as the control range should extend to a max of almost the full output of the voltage doubler, ie -70V.
Where are you losing that voltage?
The voltage dropped across the 220k grid leaks (if those are the 220k resistors you are referring to?) should be insignificant unless something is badly wrong.

Also note that when powering up a new build / new PT / different tubes / change to something significant, set the bias voltage to its max (ie most negative) value.
As -36V may be insufficient with the tubes / operating conditions that you've got.
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Re: DR-103 hums after new trans install

Postby GeorgeDelves on Fri Sep 18, 2015 12:25 pm

I tried maxing-out the bias voltage to -47 and no improvement.

The 220k resistors I am referring to are the ones connected to the trim-pot wiper in the Mark Huss adj. bias circuit mod above. When I replaced those with 22k instead, I got the -47 to -34 volt (instead of -12 to -6 or whatever it was).

How many -vdc should be at pin 5?

Have a look at the photos I've uploaded, the 50vac from the PT secondary runs right along beside the rear tagboard . . . but that wouldn't account for the current draw. As I've said, once the tubes warm-up there is a loud hum (with no preamp tubes installed) and the current limiting 250x incandescent bulb I have in series with the amp lights-up to about half brightness (and the pilot light on the amp dims). Is it possible that all of the tubes shorted when the last PT failed? (it was a crazy military surplus transformer -- you can see it on the other thread: http://tinyurl.com/oe2ckzn )

I have a set of 6L6 valves that I know are good -- should I try those even though I know they will require even more -vdc?

Update: 6L6s do the same thing. It's almost as if the filter caps are bad but I just replaced them. I did notice, however, that the three 220uF, 350v caps each had a small bubble between the terminals which I thought was odd. Could I have been sent bad caps? Seems unlikely. Nevertheless, when I return from my morning errands I will remove and test them just to be sure.

Thanks for your help, pdf64 and everyone!
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Re: DR-103 hums after new trans install

Postby GeorgeDelves on Fri Sep 18, 2015 7:09 pm

Okay all fixed. I knew it would be something I overlooked. I wired the doubler circuit wrong. I'll upload a photo later.

Meanwhile, this thing sure is loud and sweet sounding -- I may need to hang on to it for further testing!! ;-)

Photos to follow!
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Re: DR-103 hums after new trans install

Postby pdf64 on Sat Sep 19, 2015 8:17 am

Good news, well done!
Those 220k resistors may be better described as being in parallel with the bias trimmers (rather than 'series 220k'). Their function is just to maintain a path for the bias in case the trimmer fails and becomes an open circuit.
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Re: DR-103 hums after new trans install

Postby GeorgeDelves on Sat Sep 19, 2015 8:25 pm

Yes, thank you! It was a stupid mistake; that's what happens when you're not paying attention.

I'll be doing the 1 ohm resister trick on all four sockets to make biasing without a jig easier and then I'll want to do a bit of cosmetic work before I take it back to my friend.

Image
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Re: DR-103 hums after new trans install

Postby GeorgeDelves on Tue Sep 29, 2015 11:50 pm

Spoke too soon...

I am having problems. The hum is back. Should these 220uF caps in the voltage doubler be rated higher than 100v?
What should be the voltage coming from this circuit and what should plate voltage on this amp be?

I'm getting around 315 at the plate -- with tubes in -- but this figure is slowly decreasing. I don't want to leave it on for long as I fear it may damage the new tubes.

I also get, variable, between 73mv and 168mv at the top of the 1 ohm, 1%, 1w resistor that has replaced the shunt to ground from the cathode.

At the voltage doubler circuit, at the top of the two 68k resistors which then split-off to the pots with their two resistors, I get -142vdc. But where they join the turret strip and on to the 100k which then splits to the two 22k per side, I get, variable, between -12.86 to -24.69 on the one side and from -12.86 to -25.91 on the other side. Is this enough bias voltage?
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Re: DR-103 hums after new trans install

Postby pdf64 on Wed Sep 30, 2015 11:34 am

Looks like there's a problem between the voltage doubler and the bias trimmers.
There's nowhere near enough output from the trimmers, I'd be looking for a max of ~-50V.
Remove the power tubes until this is sorted.
Unfortunately once they've red plated heavily they become damaged, not able to run stably.
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