Question about newly finished D-Lite. FIXED... or?

[Svempan]

Hi everybody,

There seems to be a lot of helpful people here with great experience of self made ODS style amps. Therefore I hope someone can help me with this question.

I have just finished building a D-Lite (from a Brown Note kit) for a friend of mine. It is a standard kit but with 6L6s as output tubes.

I have many years of experience of servicing and repairing tube amps, but my experience of building amps like this is limited. I have only made two amps from scratch like this before and those have been pure Fender clones. I have very big experience of Fender amps so those amps have worked tremendously from the start.

The D-Lite works quite well. There’s almost no hum or that kind of noise. The overall sound and the overdrive sound is good. But there is one problem that I don’t understand.

The first gain stage distorts a little when playing hard. As it is the first stage that distorts, the Volume (or the other controls) has no effect on the amount of distortion (until other stages turns into distortion, off course).

My guitars have heavy strings, I play rather heavy and even if the pickups are vintage style, they give comparably high output. Despite that I don’t think that the fist gain stage ever shall distort. With my Stratocasters there’s not that much of this distortion but with the neck pickup on my SG it sounds awful.

I have now removed the bypass capacitor from the first tube’s cathode resistor and the problem is gone, but I don’t think that’s the correct solution. There got to be a clean sound in this amp with its’ original design!

Have anybody of you had the same problem? Any ideas of why it distorts like this?

Here are some gut shots.

Thanks for your help,

Svempan

[Svempan]

Some more pics

[Svempan]

Here are schematics of the amp in question:

[Normster]

These amps respond to a light touch as designed, but I think you're on the right track with the cathode bypass cap. Try dropping it to a 5uF or 1uF. I use 10uF to give me more of a Fender clean, but I have a VERY light touch.

Another thing that may help is to increase the size of the V1a grid resistor. Try bumping it from 22k to 33k (or even 47k).

[Bob-I]

What is the voltage on the plate and cathode of V1a?

[Svempan]

What is the voltage on the plate and cathode of V1a?

B+ to V1: 334 V
Plate 1: 210 V
Cathode 1: 1.8 V
Plate 2: 218 V
Cathode 2: 1.6 V

Increasing the B+ to this tube doesn't make much difference on the distortion in question. Changing the V1A's plate and cathode resistors to 100k and 1.5k doesn't make much difference on the distortion in question either.

[odourboy]

I agree with you - you should not have been getting distortion on that first stage like that. Could the bypass cap you removed been bad? I presume you did the usual tube swap to eliminate a bad tube from the equation?

(As an aside - looke like a silver mica on the V1B grid resistor bypass.... nasty!!)

[Bob-I]

All the voltages are correct, everything is wired correctly (form what I can see) you MUST have a faulty component. I'd suspect Ck or the tube. This stage should NOT distort even with your high input.

[Structo]

Nice clean build!
I have a D'Lite 44.
I am no expert on these amps but I guess I wasn't expecting chimey Fender cleans on this amp.
I can keep it pretty clean until I get it up there loud.

Especially if you have the volume dimed and turn the gain up on the master.
It seems to go into saturation pretty easily on my amp.
Isn't this typical of this amp?

BTW, where did you get your relay PCB's?

[dogears]

Try these all together.

Raise V1a grid to 33K.

Change the bypass on V1a and V1b to 4.7uf.

Change the silver mica 390pf on the V1b grid resistor to a 500pf ceramic.

Change the treble cap to a 330pf ceramic.

Replace the .0012uf on bass pot with a .001uf. Type doesn't matter.

Try lower CL2 cap. Maybe .02 PS series.

Reduce the negative feedback resistor to a 4.7k

Replace PI plates with 110K/120K

Replace output grids with 5.1K

Replace screen resistors with 500ohm

Lower the choke replacement resistor to a 330ohm

Raise the main cap equalizing resistors to 270k

Raise the 1K dropping resistor to a 2.2K

Lower the FET simulator to 150K

Possibly go back to the 22K/2.2K dropping as well.

[dogears]

Is the input gain pot linear? Linear pot there will give the impression of no headroom.....

[phsyconoodler]

Just a silly question; what are your output tubes biased at?I can get very high clean volume levels on mine.Much better than a Super Reverb.

[Svempan]

All the voltages are correct, everything is wired correctly (form what I can see) you MUST have a faulty component. I'd suspect Ck or the tube. This stage should NOT distort even with your high input.

I've already checked with other tube and cathode capacitor, but of course I will try som others. I haven't changed the plate resistor though. I will check them.

[Svempan]

...Raise V1a grid to 33K.
Change the bypass on V1a and V1b to 4.7uf.
...

Is the input gain pot linear? Linear pot there will give the impression of no headroom.....

Thans for all the tips!

Raising V1a grid resistor is a good suggestion. After checking for faulty components I will probably try that.

I’ve already tried changing the bypass capacitor on V1a and V1b to 4.7uf

I guess you mean the Volume pot with "the input gain pot". It's not linear. The distortion doesn't change at all in different positions off this pot. With the Master Volume at full, the distortion is the same at any position on the Volume pot, from just audible to when the rest of the amp starts to distorts.

By the way: The distortion sounds almost as it does in some amps when the Volume control is all the way down, but there’s still a little sound . If you’ve tested many, especially vintage amps, you probably have heard this.
One thought now hits me: I haven’t checked the 1M grounding resistor on the control grid of V1a. Could it be that the grid is loaded in som bad way?... I will check.

...Change the silver mica 390pf on the V1b grid resistor to a 500pf ceramic.
Change the treble cap to a 330pf ceramic.
Replace the .0012uf on bass pot with a .001uf. Type doesn't matter.
Try lower CL2 cap. Maybe .02 PS series.
Reduce the negative feedback resistor to a 4.7k
Replace PI plates with 110K/120K
Replace output grids with 5.1K
Replace screen resistors with 500ohm
Lower the choke replacement resistor to a 330ohm
Raise the main cap equalizing resistors to 270k
Raise the 1K dropping resistor to a 2.2K
Lower the FET simulator to 150K
Possibly go back to the 22K/2.2K dropping as well...

I see these suggestions as modifications. Mainly not as fixing for the problem. What do you think?

Regarding the feedback resistor, I will probably lower that. I like the sound with more negative feedback. This is a good place for tweaking, especially when using the amp at higher sound levels.

Regarding the values of the dropping resistors: As I see it, the exact values is not as important as what B+ voltages it will be with the respective resistor. The power transformer in this amp gives rather low voltage so I have tweaked the resistors to values that give the B+ voltages I want to the pre amp.

The FET simulator resistor surprises me in any case. My intuition says that it should be removed totally. I don’t see the reason for it (other than discharge the filter caps when the Stand By is off). Please correct me if I’m wrong. The distortion problem should definitely not be there without the resistor and probably not with values from there down to 150 k.

Your "Raise the main cap equalizing resistors to 270k" makes me think that there's something i don't know or yet understand. For me the ONLY reason for theese resistors (the two 220 k in parallell with the two main filter caps) are to get the same voltage drop over the two caps. But I may, of course be wrong. I wonder what you mean with equalizing resistors?

I’m grateful for your suggestions, so please excuse that I’m questioning some of them .

[Svempan]

Just a silly question; what are your output tubes biased at?I can get very high clean volume levels on mine.Much better than a Super Reverb.

I don't think this is what makes the distortion problem.

They are biased for about 27 mA cathode current per tube when idling.

The distortion is there at ANY POSITION of the volume pot! With the Master Volume full up, the distortion is there, and is the same at volume positions from just audible to when the rest of the amp starts to distort.