Re-voicing ODS type

[gktamps]

Hello,

New to this side of TAG - I've spent the past several years building TW and Marshall type amps. Finally decided it was time to expand my horizons, so when an opportunity to trade an unplayed guitar for a commercially-made ODS type amp came up, I took it. This is a combo with EVM-12S, built for a local working guitar player.

Now I find the tone is not to my liking as-is. Clean channel lacks clarity and definition, and treble boost cap helps, but overall the channel seems to lack treble frequencies (is this just how these sound? Am I too used to TW and Marshall tones?). The OD channel has no touch sensitivity - just way lots of overdrive even with drive turned down low, and fuzz with it turned up. Lots of controlled feedback and harmonics, but not a lot of note definition. It's got a bit more treble than the clean channel, but not a lot. To my ear, sounds muddy.

My first instinct is to bring the preamp cathode values back to stock - what do you think? Looking at the preamp cathode resistor/capacitor values, they are not standard; 1k5 Rk and 10uF Ck + 1k5 Rk and 10uF Ck on V1, 1k5/4.7uF and 1k5/4.7uF on V2.

Also, what are your thoughts on upping the main filter caps from 100uF-100uF stacked to 220uF-220uF stacked for a bit stiffer rail? I know this amp design is supposed to have some voltage sag, but the 100 watt versions are running 330uF-330uF stacked...

So far I haven't messed with anything, except for replacing the two artificial center tap heater resistors on one of the output tube sockets which were fried when I got the amp (no other damage noted). The topology of this amp is not a straight clone, and there is a non-standard part of the circuit I haven't figured out yet (third photo), but otherwise most values seem correct.

Thanks for any input you can provide.

Cheers,

Greg

[groovtubin]

Hello,

New to this side of TAG - I've spent the past several years building TW and Marshall type amps. Finally decided it was time to expand my horizons, so when an opportunity to trade an unplayed guitar for a commercially-made ODS type amp came up, I took it. This is a combo with EVM-12S, built for a local working guitar player.

Now I find the tone is not to my liking as-is. Clean channel lacks clarity and definition, and treble boost cap helps, but overall the channel seems to lack treble frequencies (is this just how these sound? Am I too used to TW and Marshall tones?). The OD channel has no touch sensitivity - just way lots of overdrive even with drive turned down low, and fuzz with it turned up. Lots of controlled feedback and harmonics, but not a lot of note definition. It's got a bit more treble than the clean channel, but not a lot. To my ear, sounds muddy.

My first instinct is to bring the preamp cathode values back to stock - what do you think? Looking at the preamp cathode resistor/capacitor values, they are not standard; 1k5 Rk and 10uF Ck + 1k5 Rk and 10uF Ck on V1, 1k5/4.7uF and 1k5/4.7uF on V2.

Also, what are your thoughts on upping the main filter caps from 100uF-100uF stacked to 220uF-220uF stacked for a bit stiffer rail? I know this amp design is supposed to have some voltage sag, but the 100 watt versions are running 330uF-330uF stacked...

So far I haven't messed with anything, except for replacing the two artificial center tap heater resistors on one of the output tube sockets which were fried when I got the amp (no other damage noted). The topology of this amp is not a straight clone, and there is a non-standard part of the circuit I haven't figured out yet (third photo), but otherwise most values seem correct.

Thanks for any input you can provide.

Cheers,

Greg

Try tweakin that white trimmer at front panel edge, ( OFF will prob be your best suit, and that will change OD AND clean sound as well) well as OD entrance, the PCB board is gonna make it tad difficult to play with!

jim@Omegaamps

[Charlie Wilson]

Hello, I am not the most expert on the forum but here is my .02. It looks like you have a low plate skyliner circuit and those bypass cap and cathode resistor values are fairly stock(if there is such a thing with a Dumble Circuit). Look at the layout for #124. I like a Low Plate Classic circuit which I find is very clear in the high end and not so mid heavy. See the layout for the Low Plate Classic. You also have a high frequency taper for the overdrive. So you can adjust that to your taste. That third picture may be the power section for the relays.
CW

[butwhatif]

12s is a middy speaker to my ears--lacking definition,
unlike a 12M. That could be the whole issue, highly recommended is an original G12-65

[martin manning]

Greg, who manufactured this commercially-made ODS-type amp? Anyone we know?

[67plexi]

I would voltage chart the amp out first.
The 33k on pin 7 V-1 is not a stock value 22k is
the false center tap 100 ohm resistors fried may be a problem
set your 100k OD trimmer to 24k and the 10k PI trimmer to 5-7 volts
differential like 298vdc and 305vdc as an example.


Steve.

[gktamps]

Thank you for your comments to date. I was pm'd by the builder, and after giving him more info by phone, he noted some changes made since he built it. He also gave me some things to measure and adjust.
One thing that made a bit of a difference was the OD trigger trimmer was set a bit high at 28k, so I brought it down to 25.6k. The OD contour trimmer gave me some more upper end that improved the tone as well.
Someone changed the V1 bypass caps to 10uF each side, which I might drop back to stock at 4.7k.
I have tried several other speaker cabs, including a 2-12 with an M75 and an H55 LD, as well as a cab with four tens - two Jensen Alnicos and two Mesa Black Shadows, a 2-12 with a Gold and an H55, and a Thiele with an EVM-L. As you would expect, lots of tonal variation, but the EVM-S didn't strike me as the root of the issue, but I will keep an open mind about that.
Thanks!

[vibratoking]

Deleted

[gktamps]

Sorry, Martin - posted a reply before seeing your question.

The amp is made by Bill Dunham at Sebago Sound. I want to state that the build quality is extremely high, with very careful attention to layout, lead dress, tacking leads to the chassis, component quality and soldering, and the PCB traces look beefy. It appears someone was inside the amp subsequent to Bill delivering it to the original owner. This amp is a great candidate for working on and getting dialed in, and with just a few adjustments, the sound has already improved.

I have already spent a few hours reading through the Files section to start to get up to speed on these amps so I don't ask too many dumb questions. In reading the archived posts, I see there have been low plate amps with 10uF bypass caps on V1, as well as 5uF.

'67Plexi: thanks for the voltage chart and the set points. Voltage chart is my next step. I had initially only measured output tube plate voltages when I was checking bias after repairing the CT resistors - 451V. The output tubes look like RCA 6L6GC, not all that well matched at 17.2w and 19.8w dissipation final bias. I'll run through the rest of the amp and get those data.

Thank you,

Greg

[marcos]

Another idea:
If the amp has the LNFB network on V1b, try removing it.
This will add a lot of chime to your clean channel,
it will also make the OD more raw, more rock´n roll,
which may or may not be to your liking.
Good luck Marcos

[10thTx]

Clean channel lacks clarity and definition

In both my Tweed BluezMeister (sort of a Dumblish that is cathode biased in a tweed combo cab) & with my D'Mars (sort of a Marshall preamp into Dumblish OD and LTPI) ...............

I use a 12AY7 in V1 and a 5751 in V2. That works for me and I have as much clean as I want and as much OD as I want. I've also tried 5751/V1 with 5751/V2. Those tubes help eliminate what I hear as muddiness and they add the clarity and touch sensitivity that I like. YMMV.

IF you have a 12AY7 and a 5751, it would only take a minute to swap out those tubes and see if that moves the tone closer to what you want?

With respect, 10thtx

[gktamps]

Thanks Marcos and 10thTX for the suggestions. I'm making changes in small increments, and have now changed the bright cap to 300pF, adjusted the ODS contour pot for more treble and the OD trigger to 24k. I've also temporarily tacked a 66k resistor across the 33k on V1 P2 to bring the grid stopper value to about 22k (didn't want to R&R the 33k at this time). I've also adjusted the PI balance trimmer, but can only get a 3V differential. Resistors on the board are 110k and 120k.

Here is the voltage chart:

Thanks again,

Greg

[gktamps]

Much better now. Good enough to button up and play it for awhile. The 300pF treble cap is a bit too much - a 200 or 250 would have been fine, but dialing the controls does the trick. With the adjustments and a little preamp tube rolling, this amp sounds much, much better. Auditioning more speakers and tube choices can now happen at my leisure.

Thanks for all of your help and comments!

[Structo]

Are you running 6L6 with this amp?

The bias looks kind of hot.

Most Dumbles sound better biased a little cooler, like 55-65%.
Or around 30-35ma per tube.

For the bright cap on the volume pot, I lowered mine to 120pf.

Also I believe that white trimmer is a treble bleed circuit and it can help remove the fizzies.

[gktamps]

Yes, 6L6s. Thanks for the comment on biasing - I'll drop that down. It was even hotter when I got the amp, and I've got them biased quite a bit hotter than 30-35ma. Yikes!

The bright cap was 120pF but I found it too mellow, so bumped it up a bit too much and could stand to drop it down a little. I also adjusted the white trimmer to an improved tone last time around.

Thanks again!