Help smoothing out an ODS

[Joe6v6]

 
I need some help tweaking this amp to make it sound better than it does considering its leniage. The amp I need help with is an OverDrive Special following the #102 schematic. For the most part all the values on this schematic have been followed, with these exceptions - change the filter caps from the 2x220uf to a single 100uf & then 4x33uf  as opposed to the 20-20-40-40 shown on the schematic.  .  It only has 2 6l6 power tubes,  5k presence pot with 1uf to ground.  .    FET input has been eliminated. The power trans is a P-T290DX  its 325-325 @ 200ma , 5v tap is used for the relays,, The output is a Classic Tone 40-18001 w/4.2K secondaries.
        The amp works as it should, all the controls work, very little noise no hiss or humm & the relays work fine with no noise. The main problem is with the overdrive channel. The overdrive is not as smooth as it should be, it is gritty & seems to lack the sustain it should have. Just the overdrive does not seem very musical or plesant sounding – I have built a ODS #124 that sounded fantastic so much so that the first person I lent it to wanted to buy it so that’s why I wanted another for myself.
 The main differences that I see between the 102 & the 124 are in the Preamp plate Resistors & the cathode resistors & bypass cap values but Im not sure if changing those will help matters or possibly make things worse.  Is component selection possibly a contributing factor in this sub par performance ? I used carbon film resistors thruout but on the other one I built I used metal film on the preamp plates.
          Tweak & Mod suggestions are welcome , I have posted the schematic I used & a picture of the innards.    Thanks : Joe

[tubenit]

How well do the voltages match the schematic, especially on the overdrive?

Speakers can sure make a difference in tone on these amps.   

You can always try a 5751 on V2?

You can try a 120-220p "enhance cap" across the 110k phase invertor entrance plate resistor.

You can add a 220p across the plate resistor of V2 in the first gain stage in the OD section.

You can add a tone stack to the OD section.

You can try a 250ka ratio pot and then lower the settings on the trim and especially the level pot.

Just ideas to consider .........................

With respect, Tubenit

[92Volts]

Have you messed with the OD trim pot? While one value is reported, it probably exists to be tweaked to taste (or variation in other components).

Type of resistor is unlikely to make a big difference.

I hate to say this since it's not too insightful... but I'd look for a wiring error... somewhere. You know how the amp should sound, or very near it. If it's wrong, rather than just "not your cup of tea", something isn't wired right.

I consider myself a decent builder, but I've screwed up plenty of times myself  the tonestack is a good candidate for an error to affect the sound, but not make it fully non-functional.

It looks mostly good, but the blue cap towards the lower-middle of the main turret board looks like it could use some more solder.

[Joe6v6]

How well do the voltages match the schematic, especially on the overdrive?

V1 & V2 voltages are higher than the schematic - On my amp V1B is 208 & V1A is 223V , V2B is216 & V2A is 224V , I dont understand how the schematic has A & B on both tubes at the same voltage (200v on V1 and 205v on V2)  when the plate resistors are diffrent values and if the schematic is correct why are my voltages so different from one side to the other?

You can always try a 5751 on V2?
You can try a 120-220p "enhance cap" across the 110k phase invertor entrance plate resistor.
You can add a 220p across the plate resistor of V2 in the first gain stage in the OD section.

All simple enough to try - but im wondering if the V1 & V2 voltages should be addressed first.

Have you messed with the OD trim pot?

Yes- its mounted on the rear of the chassis, so yes ive messed with it but I did wind up measuring 25v to ground just so I know wher that is.

It looks mostly good, but the blue cap towards the lower-middle of the main turret board looks like it could use some more solder

Probably just the picture, I could not get a higher res. to upload. That is however an electrolitic when a non electrolitic is called for?


Now im really wondering about those Preamp voltages & if bringing them into line should be the first thing I do. The rest of the listed voltages are with in a few volts of where they are on my amp.   .     


                                                                       Tx fo the input : Joe

[SILVERGUN]

Besides the above mentioned possibilities, one issue to consider is how you have handled negative feedback.
Dumble was very specific about the importance of the use of NFB in his designs and this grabbed my attention because you mentioned how you changed the value of the pot and cap in your version.
You should also take into account that his original design was based upon the open-loop forward gain of a 4- 6L6 output stage and you have eliminated 2 of those tubes.
I'm not smart enough to do the math but I figure I'd mention it and someone else might be.
There is a lack of consistency in the transcribed schematics, mostly due to the fact that it's very easy to just cut and paste pieces together without considering the actual circuit requirements.

[Joe6v6]

Very good points Silvergun - Hopefully some others will let me know the effects my changes have made and if they need correcting. I used a 5k pot there becaues thats all I had.  Thanks : Joe

[92Volts]

V1 & V2 voltages are higher than the schematic - On my amp V1B is 208 & V1A is 223V , V2B is216 & V2A is 224V , I dont understand how the schematic has A & B on both tubes at the same voltage (200v on V1 and 205v on V2)  when the plate resistors are diffrent values and if the schematic is correct why are my voltages so different from one side to the other?

The differences between yours, and yours vs. the schematic, are within normal variation of tubes/components. The tube with the smaller plate resistor COULD have the same or lower voltage because the smaller cathode resistor lets it draw more current. But I assume 200v is a ballpark, not exact measurement. Even with the same circuit it's unlikely they'd be within 1v of each other due to tube/component variation. And you are in the right ballpark.

Have you messed with the OD trim pot?

Yes- its mounted on the rear of the chassis, so yes ive messed with it but I did wind up measuring 25v to ground just so I know wher that is.

Is that a typo, or did you actually measure 25 volts at that location? There should be zero voltage on that pot. It suggests the 0.047u capacitor following V1B is shorted or missing. 

Probably just the picture, I could not get a higher res. to upload. That is however an electrolitic when a non electrolitic is called for?

Like resistor type, capacitor types would have a small impact on sound (not noticeable to many people). Of course, an electrolytic is polarized so if it were installed backwards, that would be a problem.

[Joe6v6]

Is that a typo, or did you actually measure 25 volts at that location?

Yes , sorry 25k to ground

[SILVERGUN]

Very good points Silvergun - Hopefully some others will let me know the effects my changes have made and if they need correcting. I used a 5k pot there becaues thats all I had.  Thanks : Joe

Just swapping the 2K for the 5K pot won't make a big difference as long as you kept the 390 ohm resistor in there....did you?

[Joe6v6]

Just swapping the 2K for the 5K pot won't make a big difference as long as you kept the 390 ohm resistor in there....did you?

Yes the 390R is in place, the 2.2uf cap on the schematic is 1uf.   .   Joe

[Joe6v6]

I added a Treble Bleed that Ive seen in another schematic at the ratio pot and changed the (2) 250pf caps on V2 between the plate & cathode to 750pf. I did both at the same time so im not sure if one had more effect than the other but definetly a step in the right direction.  .    .     Joe

[brewdude]

You might try adjusting the lead dress of the wiring around V2b.  I found that there was a wide range of tonal tuning possible.

[92Volts]

Yes the 390R is in place, the 2.2uf cap on the schematic is 1uf.   .   Joe

Try upping that cap to 2.2uf. I'm not sure if 1uf is what you had available, or if you scaled the cap to "match" the higher impedance of the pot. That's not the only change that needed to fully adjust for the 5k pot... For now, up it to 2.2u (if available) and just keep the pot at lower settings. When the resistance is between 0 and 2k ohms, it will behave like a 2k pot would.

You might try adjusting the lead dress of the wiring around V2b.  I found that there was a wide range of tonal tuning possible.

Lead dress shouldn't have a big effect on tone unless parasitic oscillation is starting/stopping with those changes... which is actually worth considering here. Joe6v6, do you have an oscilloscope?

[brewdude]

I’m my limited experience, the position of the wires around the tube sockets on Dumble style circuits had a great influence on the signals tonal character—position the wires bunched together and the tone is more midranged and horn like, while spreading them further away from each other increases the bandwidth (more highs and more lows).  My kids (who don’t give a crap about tone) could hear the difference in the next room as I fiddled with the wires. 


However, this may not be what’s wrong.