You might want to increase the value of the 68k resistor on the OD entrance 100k trimmer. I suggest 150-470K maybe 220K. Also the 500K level pot at the end of the OD might be too high. Most schems show a 100k. You could add a resistor across it to lower the impedance. I haven't been able to listen to the clips so maybe it sounds great already.
Very heroic conversion BTW, I love it!
Jelle
Thank you for the comments. That 68k was just a ohm reading from a bunch of resistors I had stacked on top of each other to try and get more gain onto the OD section. After replacing 10k Mid pot with 100k I had a lot more gain in the clean section. I put the amp back in the cabinet yesterday but forgot to write down what I wound up with there. The OD is a little brighter than the Clean, will a higher value here tame some of the highs?
While waiting for my 100k pot to come in the mail I tried all my spare pots in that 'Level' positon. Had a 220k across the 500k pot, then took the resistor off to see what would happen. The 100k came yesterday, so I put that in before putting it all back together.
Now that it is all back together, Ill post some pictures.
I don't like going back and updating the schematic in a prior post, because, for those doing research, it violates the principles of archeology that is the oldest stuff is always should be deepest!
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The higher impedance at the OD entrance will allow more lows to enter the first OD stage. This means more/different clipping so you might want to reduce the signal entering that stage. Since the ratio of the voltage divider is changed, you can probably leave the setting of the trimmer or put it at 40% from ground. The increased low end will make it sound more smooth and less bright.
So after using the amp for a week or so I had the following problems to solve:
1) Still having loud popping with JAZZ/ROCK switch (others were cured with high meg resistors)
2) OD is still brighter than the non-OD
After re-reading those anonymous Dumble tweaking notes that are floating around I decided to try increasing the series resistance after OD2.
I increased the 100k resistor to 220k. While I was there I also took the jumper resistor off of my 220k on the OD entrance, bringing that back up to 220k.
Then I added ONE snubber (270p) on OD1 first stage only.
To cure the JAZZ/ROCK popping I put a 22M resistor in parallel with the .0047uf capacitor of the circuit.
Sound clip to follow.
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You may want to try adding a 270pf on OD2 and reducing the OD2 series resistor to 180k. On non-HRM amps, I have never liked the sound of no snubbers. It gets kind of sizzley on the edges.
I'd also strongly suggest tacking an additional 4.7uf on both V1 cathodes. And listening to the results. All the "grail" Dumbles I have seen with 100K plates, had either 10uf or 25uf there. Try it and see.
Lastly, measure your OD pots. I'd open them and scrape them up to 100K if they are low. I have seen 87K! Or, you can raise the OD1 series resistor to 110K or 120K to compensate. It will fatten and smooth things a hair. It all adds up.....
ic-racer wrote:So after using the amp for a week or so I had the following problems to solve:
1) Still having loud popping with JAZZ/ROCK switch (others were cured with high meg resistors)
2) OD is still brighter than the non-OD
After re-reading those anonymous Dumble tweaking notes that are floating around I decided to try increasing the series resistance after OD2.
I increased the 100k resistor to 220k. While I was there I also took the jumper resistor off of my 220k on the OD entrance, bringing that back up to 220k.
Then I added ONE snubber (270p) on OD1 first stage only.
To cure the JAZZ/ROCK popping I put a 22M resistor in parallel with the .0047uf capacitor of the circuit.
I totally agree with those tweaks! It will help a lot.
Scottie, did you see that 500uf cap on v1a? I'd use the values Scott suggested. Everything looks cool but I'd add 270pf across both OD stages or maybe even a 330pf across the second one is you think it is too bright compared to the cleans.
One thing you'd def should change..sorry I did not see that earlier....is the 100K master volume pot. The secons OD stage is loading into something like 70K? That's gonna leave you without low end out of the OD chnl. Put in a 1M log If you like the OD to be clean....I know that sounds wrong.... .....then I'd try a .022uf v2a out and a .01 v2b out instead of .01 and .005 that you are using now. You play strats right?
Please let me know how she sounds after these mods....clips?
Have fun,
jelle
dogears wrote:You may want to try adding a 270pf on OD2 and reducing the OD2 series resistor to 180k. On non-HRM amps, I have never liked the sound of no snubbers. It gets kind of sizzley on the edges.
I'd also strongly suggest tacking an additional 4.7uf on both V1 cathodes. And listening to the results. All the "grail" Dumbles I have seen with 100K plates, had either 10uf or 25uf there. Try it and see.
Lastly, measure your OD pots. I'd open them and scrape them up to 100K if they are low. I have seen 87K! Or, you can raise the OD1 series resistor to 110K or 120K to compensate. It will fatten and smooth things a hair. It all adds up.....
ic-racer wrote:So after using the amp for a week or so I had the following problems to solve:
1) Still having loud popping with JAZZ/ROCK switch (others were cured with high meg resistors)
2) OD is still brighter than the non-OD
After re-reading those anonymous Dumble tweaking notes that are floating around I decided to try increasing the series resistance after OD2.
I increased the 100k resistor to 220k. While I was there I also took the jumper resistor off of my 220k on the OD entrance, bringing that back up to 220k.
Then I added ONE snubber (270p) on OD1 first stage only.
To cure the JAZZ/ROCK popping I put a 22M resistor in parallel with the .0047uf capacitor of the circuit.
dogears wrote:
I'd also strongly suggest tacking an additional 4.7uf on both V1 cathodes.
Thanks for these additional suggestions. In terms of the V1 caps, that 500uf is not a misprint in my schematic. The Original Fender schematic shows a 750uf cap on V1 (it is pins 6,7,8, but indicated as V1a on the Fender schematic), however, on examining my amp, it came from the factory with a 500uf there.
I currently have it hooked up to toggle between the 500uf and a 25uf to try and figure out what they were thinking with that...
The following Strat clips are made with just the one snubber and the bright is ON. Vol=5, Treb=5, Mid=5, Bass=5, Pres=5 Drive=5 mid=off deep=off reverb=bypass R/J=Rock.
jelle wrote:In addition to the 100K master I see another load lowering impedance: the reverb ckt. Can you hear a difference when you bypass it?
Nice playing! I do hear low end loss but it actually sounds very cool!
jelle
The reverb sucks tone and db. I read somewhere that a 2:1 passive mixer will HALF the output of the original signal. So, with the reverb not bypassed it is like a 90w amp. (there is less gain in the reverb now because I converted it to a single-tube circuit.). The bypass switch has a real effect on this passive mixing circuit, sounds much better and louder with the reverb out of the circuit.
I actually am not much of a Strat player. I have over 25 guitars, but only one Strat! I always considered the Strat a cheap guitar and could not get good sounds out of it. Now that I am getting this Fender amp tweaked I am finding the Strat fun to play and good sounding (its a '93 American Standard, rosewood neck, with Lindy Fralin pickups and .009 strings)
Checked over the 'parts bin' and this is what I can try before another order to Mouser.
I have a 500k CTS I can try for the Master (instead of a 1M).
I have some extra 25uf caps from the old distortion circuit and I can put those on V1 cathode.
I can put another snubber on V2b and decrease the series resistance to 180k from 220k.
Other things on the "to do" list for this amp are to re-do the preamp-board in the same manner as I did the relay board. That is, to use the existing backing board to make a new eyelet board that can put the 'Dumble' components for V2 in a better position. Too bad I can't just use a standard Dumble layout, but the tubes are right behind the pots so all the wires need to come off the same side of the board. If someone knows of a preamp layout for this style of setup, please point me to it.
ic-racer wrote:Checked over the 'parts bin' and this is what I can try before another order to Mouser.
I have a 500k CTS I can try for the Master (instead of a 1M).
I have some extra 25uf caps from the old distortion circuit and I can put those on V1 cathode.
I can put another snubber on V2b and decrease the series resistance to 180k from 220k.
With that 180W clean poweramp, you won't need that master to be dimed. I'd use the 500K CTS with a 470K to make it ca 1M.
that is the oldest stuff is always should be deepest!
I'd use the values Scott suggested. Everything looks cool but I'd add 270pf across both OD stages or maybe even a 330pf across the second one is you think it is too bright compared to the cleans. 
.....then I'd try a .022uf v2a out and a .01 v2b out instead of .01 and .005 that you are using now. You play strats right? 
(its a '93 American Standard, rosewood neck, with Lindy Fralin pickups and .009 strings)