1979 D103 wierdness

[Wallbanger]

So I've had this Dr103 or some years, used it daily in a band and since it worked fine, clean and loud, it was never on my bench except for rebiases, retube ( different issue than this thread) I have more free time now so I hooked the thing up to my test rig (sig gen, dummyload, two oscopes, power meter. I find that the thing puts out about 17 watts clean, then goes to a crazy looking square wave up to about 130 watts! It SOUNDS clean, just looks hosed up on the scope. Wide open, it sounds like it always did, not sure if I have a problem..? No hum issues at all...its quiet. But is the tone right?

No, my tektronix oscopes and probes are fine., not a measurement issue,

I've found some differences ( factory) in the values of one of the dropping resistors in the string from the main filter supply. Still, it shouldn't cause what I'm seein on on the scope.

Any Dr 103 gurus on here?
What do you guys normally see for undistorted sine wave in the preamp of a DR 103? Sine wave on the grids of the power tubes, max undistorted voltages on the preamp stages, etc. PI grid and plate voltage AC.

I probably have a bad coupling cap or something but I'm trying to narrow it down without molesting this thing too much. I've also never seen a bad mustard cap, which makes me reluctant to do anything to this amp, signed by Harry himself

[martin manning]

I'd be measuring voltages all around the preamp and looking for reasonable DC operating points on all the triodes. Pay close attention to the bias circuit for the PI. Are the bias regulator and PI tubes known to be good? Those three triodes have relatively high cathode-heater voltage, and a heater insulation failure could definitely upset the PI's operating point. If you could find a voltage chart that would be helpful. With a quick look I have only seen one number, 460V at the reservoir.

[jelle]

Paging Mark Huss....

[xtian]

What do your ears tell you? (I'm not being snarky, just curious.)

[mhuss]

What are you using for a test load when you're scoping it? Is it hooked up to the same speakers you normally use?

--mark

[martin manning]

What schematic does this DR103 follow? The hand-drawn one?

[Wallbanger]

I use a 400 watt resistive dummy load when testing, thanks for the suggestions. The thing sounds pretty clean and loud, thats what I don't get. I took some voltage readings on all the tubes, unfortunately I didn't bring my notebook, on the road.

I'll post the info when I get home, I'll grab some scope captures as well so you guys can see what I'm talking about.

I tested all the tubes when I first noticed the odd waveforms, they are are all NOS tubes and are fine, replaced one really weak 12ax7, didn't change the symptoms.

Thanks again for the help!

[martin manning]

Voltages for a "late 70's 2-input" DR103 (from the three hand drawn schematics of the power supply, preamp, and power stage) should look something like this:

B+: 460, 457, 449, 394, 362

12AX7's (a, g, k)
V1a, V1b: 213, 0, 2.03
V2a, V2b: 163, 0, 1.16
V3a: 237, 0, 1.87
V3b: 362, 65.2, 68.5 (PI DC bias)
V4a, V4b: 322, 68.5, 70.3 (PI)

EL34's (a, g2, g1, k)
V5-V8: 453, 450, -43.5, 0 (biased at 37mA Ik)

[Wallbanger]

Dig it!! Thats what I've been hoping to find somewhere, thanks Martin! Mark has some voltages for a 200 watt clone on his site but this is the first voltage info for a 50 or 100 watt that I've seen:) Maybe its similar but I was curious about how much drive is necessary for max output on a quad of el34's. Clean output. I remember that my PI plate voltages were near 400v, so the 322 vdc seems way more"normal" to me, indicating that I have low current in my PI. I honestly don't understand how that PI works, I can see that the extra triode is not processing the signal, basically a voltage regulator providing a DC reference for the PI, but what does that do? I asked the Audio Bros years ago, but the answer was nebulous and I decided that they weren't really sure either:) is it for a bigger swing out of a long tail pi? I have experimented a lot soundwise in my homemade amps with the tail resistors in long tailed PI's with unity gain up to the Vox PI with a gain of 30, there's definitely a lot of mojo happening in the PI in a tube amp. I don't want to experiment with this thing, I'm aspiring to be reverent with this amp, for once, instead of being a serial amp molester:) I have a real (Sprague) cap tester, I just don't want to lift a lead or disturb a single solder joint until I'm sure why I suspect that particular component.

Old Dave certainly knew his stuff, he was one of not many innovators out there, then or now.

[chief mushroom cloud]

I honestly don't understand how that PI works, I can see that the extra triode is not processing the signal, basically a voltage regulator providing a DC reference for the PI, but what does that do? I asked the Audio Bros years ago, but the answer was nebulous and I decided that they weren't really sure either:) is it for a bigger swing out of a long tail pi

http://www.valvewizard.co.uk/dcltp.html

[Wallbanger]

Rrright. i can read and I have his books. What does the voltage reference triode do under heavy load? I understand how a long tailed pair differential amplifier works, why is it advantageous to use a fixed voltage reference?

Chief mushroom cloud, I lurk, I read your posts. I get it, your sense of humor, you are semi funny, mostly excluded, very smart. Not super smart. Thanks a pantload. If you have any insight besides links to Merlins info, cool, awesome, enlighten me. I don't mind being spanked but give me something...

[martin manning]

You kind of have us at a disadvantage, Pete. You've been reading here, have some idea of the level of expertise, and have formed your own opinion of the contributors. To the members you are just another new guy asking for help, and nothing is known about your background or experience. Blencowe is better than most at explaining circuits, so a link to an appropriate page on his site is usually better and more efficient than trying to paraphrase. If I were interested in getting help and advice, I'd have stopped after the first paragraph you wrote above.

This Hiwatt PI is not DC coupled, but the differential amp part works the same way, and there is virtually no difference in headroom or balance (given the same combination of plate loads and tail resistance) between biasing it this way vs. the usual cathode bias method. It does have lower input impedance, making it less susceptible to blocking distortion. Any difference probably comes down to the increased loading on the preceding stage.

If this 103 is a nice original example, how about posting some high-resolution gut shots? You've already got it opened up, and that would be a great addition to the site.

[chief mushroom cloud]

I recant this post....too harsh.

[chief mushroom cloud]

This Hiwatt PI is not DC coupled, but the differential amp part works the same way, and there is virtually no difference in headroom or balance (given the same combination of plate loads and tail resistance) between biasing it this way vs. the usual cathode bias method. It does have lower input impedance, making it less susceptible to blocking distortion. Any difference probably comes down to the increased loading on the preceding stage..

what martin sed

[Reeltarded]

I recant this post....too harsh.

Awwww! Garage it!!