Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on December 05, 2014, 04:40:57 pm
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Hi guys, I still got this http://el34world.com/Forum/index.php?topic=17798.0 (http://el34world.com/Forum/index.php?topic=17798.0) sitting under my desk. The new caps have arrived from Doug (THANKS).
From what has been said about this and the MAJOR is that it needs to be pretty well CRANKED to get some life into it.
In the picture it has a spare hole that could accommodate another TUBE...............
The preamp PCB are in great condition, but to accommodate an extra tube I may etch another board.
I have taken the circuit for the OD directly from the Dumble schems as a starting point. The 12ax7 being redly available I will stick with a 12a_7 type tube.
Space on the front panel is limited so the OD DRIVE will mounted next to the volume pots.
The OD TRIM I think this will be a trimpot mounted on the PCB (set and forget????), I was also thinking that the OD LEVEL could be a set and forget trimpot, maybe???????????
The OD will be switched in/out via a relay.
Do you guys feel that this OD is worth adding??? Thanks
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Do you guys feel that this OD is worth adding???
What you are doing has strong similarities to the D'Mars ODS which is my favorite amp ever.
So, yes, I think it is worth doing. Only thing I would strongly suggest reconsidering is using a 250kl OD level pot along with the 5E3 type tone stack. I find the tone stack very very useful & actually adjust it more then the clean channel tone stack.
This is similar to what I currently have except I have a CF on the OD and a recovery FX stage.
With respect, Tubenit
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Another option to consider is the 5879 OD. I did this one on the T-Lite for my son. I think it sounds great. A different type of tone in
the overdrive. Not as smooth, but very touch sensitive and it has some chime to it.
With respect, Tubenit
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Timbo,
See, I told you Tubenit was the man!
Did you ever check your B+ on that beast? Also you may want to swap in some KT88's to check the flavor if the bias has the range. I like them much better than the 6550's.
Jim
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Hi guys, T, those are great ideas and it will be a case of trial and error as I do not know what this thing sounds like.
I was trying to keep I pretty simple with minimal knobs and a 12A_7.
I guess I am just trying to get this amp to breakup at a lower level.
Jim, I will be putting in a pot to make the fixed bias adjustable. Thanks
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it will be a case of trial and error
trying to keep I pretty simple with minimal knobs
A couple of thoughts ................
It is very easy to get too large of coupling caps in the OD and end up with a very muddy/gritty tone. To keep it smooth, the caps usually need to be pretty close to Dumblish standards. So, I am offering value ranges that I've experimented with ALOT for your consideration.
I think your .02 and .01 coupling caps will probably be too large?
I have found on the TOS, TBM & D'Mars ODS that I don't use the OD drive pot very much. It is pretty much a set and forget for me. I use the OD trim pot and the OD Level but not the drive hardly at all. So IF you wanted to eliminate a pot, I would suggest the drive pot. I did that on my Tweed BluezMeister and have not missed the drive pot at all. Having said that, most of the Dumble cloners use an internal trim pot as a "set and forget".
with respect, Tubenit
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I have taken the circuit for the OD directly from the Dumble schems
Ah. Now I see where that little 2-stage figure is coming from. I keep seeing it inserted into circuits verbatim like as though it is magic or something. Well, it ain't. It's just the two clipping stages in a 4-stage pre-amp with a 100K fetish. It's only cool if the preceding circuit is matched to it, which seems to escape most people. I like the way you use it, though.
Got the .0027uf after the first stage along with the the 560pf/470K thing to match that response to the Distorto figure.
That's interesting how you use the 100K trim pot to radically alter the tone stack response when in OD mode. With the 100K load and the treble up, the lower frequencies are cut way back which is more appropriate for overdriving the Distorto stages. That's a pretty good idea.
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The OD will be switched in/out via a relay.
As long as you're using relays and want to minimize knobs, you could use a second relay with its coil wired in parallel with the first relay. I have attached an altered schematic of one idea in this vein.
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Thanks guys, Looks like A GOOD JOB FOR A BREADBOARD.
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Thanks guys, Looks like A GOOD JOB FOR A BREADBOARD.
Hey man,,,what're you insinuating here? :hello:
Did YOU BUILD ONE yet?
:violent1:
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YOU BET, Knocked this one up this morn'n.
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Hi guys, I'll continue this topic here http://el34world.com/Forum/index.php?topic=17798.0 (http://el34world.com/Forum/index.php?topic=17798.0) Thanks
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Hi guys, I'll continue this topic here http://el34world.com/Forum/index.php?topic=17798.0 (http://el34world.com/Forum/index.php?topic=17798.0) Thanks
My experience with a Dumble overdrive in a Marshall was less than stellar. As mentioned by 2deaf, matching the rest of the circuit to it is critical. I decided to go another way with the overdrive but the Dumble overdrive circuitry works very well in my Dumble clones. Tubenit's advice is valuable when it comes to this type of circuitry.
Thanks
Mike