Bias problem?

[soje]

I have a clone of the #124 on my test bench. Even at low volume there's an unmusical distortion. Sort of uncontrolled.

I've tried to adjust bias, and it does remove the undesired distortion, but it results in redplating

Here's my plate voltages (with tubes):

V5 (6l6gc)
pin3: 435v
pin4: 430v

V4 (6l6gc)
pin3: 434v
pin4: 430v

bias set at -45V, I read 37mA across the 1R resistor I but on pin8 of V5

V3 (12ax7)
pin1: 277v
pin6: 281v

V2 (12ax7)
pin1: 182
pin6: 183

V1 (12ax7)
pin1: 182
pin6: 183

I'm using a Dynaclone 470 from Triode Electronics

Any ideas?

Thanks!
soje

[Bob-I]

37ma should be fine. If you're getting crossover distortion I'd start by checking the current and voltage on all power tubes.

[glasman]

You voltages look fine and the bias is fine.

I suspect it is time to drag out a scope and trace a signal through the amp. If you have distortion at low volumes there could be a multitude of things that could create the problem. I am guessing that this is happening in clean mode, correct.

I would probably start by paking sure that I had a clean power supply and go from there.

Gary

[soje]

Hmm, I traced the signal with a scope, and it seems fine all the way to the power tubes. The signal into both tubes looks fine.

However there's a difference in the output voltage swing. When one side put out about 22VAC, the other is at about 8VAC. I tried to swap tubes, but it didn't change.

The OT is the Dynaclone 470s from Triode Electronics. It has 5 (rather thin) wires on the primary side: red, blue, green, blue-white and green-white. Triode told me the last two was for ultra linare, so I taped those off. The red goes to B+1, and the other two to pin 3 on the powertubes.

Have I connected it wrong, is it defect or is this something completely different?

[heisthl]

I vote defect but to be sure disconnect the negative feedback and either swap the output plate wires or output grid wires and see if it stays with that side of the OT.

[glasman]

Hmm, I traced the signal with a scope, and it seems fine all the way to the power tubes. The signal into both tubes looks fine.

However there's a difference in the output voltage swing. When one side put out about 22VAC, the other is at about 8VAC. I tried to swap tubes, but it didn't change.

The OT is the Dynaclone 470s from Triode Electronics. It has 5 (rather thin) wires on the primary side: red, blue, green, blue-white and green-white. Triode told me the last two was for ultra linare, so I taped those off. The red goes to B+1, and the other two to pin 3 on the powertubes.

Have I connected it wrong, is it defect or is this something completely different?

Check the Triode site. The wiring diagram show the ultralinear taps as being green and green/white.

The Blue and Blue-White wires go to the plates and the RED to B+. I would guess the Blue wire would be on the first PA tube (Top on most schematics )and the Blue-White to the second (bottom).

Gary

[odourboy]

I would say that Gary has nailed it! Using on UL tap and one regular tap would NOT do wonders for your output section.

[collinsamps]

I've seen a UL twin wired the same way with the same problem.

[heisthl]

I wish transformer companies would standardize their color codes - especially on output transformer primary and secondary taps. I've caught myself wiring the different impedances wrong, in one case a player used his amp for 6 months (heavy usage of at least 20 hours a week) and sent it back for a mod, when I opened the amp I discoverd a wiring error that meant he had been running a 2 ohm load from the 8 ohm tap the whole time. Amazingly the transformer was undamaged. In my defense it was the first time I had used a tranformer from that particular vender.

[soje]

Now THAT would explain it. In MY defence, I wrote Triode before connecting it, because the diagram they shipped with the trafo didn't show colors.

Here's their reply:
This transformer has screen taps for ultra-linear mode, and sometimes the extra wires confuse people. I can give you a basic description of where they all go, but I could be more specific if I knew more info about the amp it's going in.

BLUE/WHITE & GREEN/WHITE: These are the screen taps, and are not commonly used in most guitar amp circuits, so if you don't wish to use them, tape them off.
RED: This is the center tap of your transformer and is connected to the last filter capacitor stage(B+).
GREEN: Goes to the Plate of one power tube.(pin 3 on a 6L6)
BLUE: Goes to the Plate of the other power tube.



.....guess they haven't checked diagram of the original either

...I'm glad I didn't cut any wires yet......

[glasman]

I would say that Gary has nailed it! Using on UL tap and one regular tap would NOT do wonders for your output section.

Actually it was Henry when he made the comment about a defective tranny. I just had to look at the diagram.

Gary