Looking for some Insight in Dumble Steel String Singer Phase Inverter

[makinrose]

After reading the Dumble chapter in TUT6 (if you don't have it get it!)  I got really interested in the different Dumble circuits. Most are simply variations of what is in TUT6  but the Steel String Singer caught my attention since it's a little different. Basically what I'm seeing is the Dumble clean channel but with a reverb configured a bit like standalone reverb unit and switchable filter.  That makes sense to me.  

However the topology of the phase inverter does not. It looks like a schmitt but has a cathode follower for each drive line.  Is there any benefit to this approach over the standard plate driven configuration? Has anyone here played an amp with this topology? I've attached a schematic!

Hmmm....I've having trouble  attaching  the PDF.  Hopefully you guys can still envision what I'm talking about....

[woodswalker]

Oops, sorry makinrose! No wonder you had trouble uploading your PDF - the Forum's attachment size limit was set too low. We've now set it to 10MB for PDFs, so feel free to try again.

Cheers,
TUT Forum I.T. Support

[K O'Connor]

Hi

Based on your description of the splitter it does not sound specifically new or different. Adding cathode followers after a Schmitt splitter is common in amps with parallel output tubes, as Fender did in their 180W amps, and as I did in the SVT project in TUT5, and as was done in the original SVT and Vp amps from Ampeg. Hiwatt did it and many others.

Of course, you can add CFs even if there is only one output tube per side. The benefits are two-fold: The first is that the gain of the splitter is not effected by the loading of the output tubes or their grid-leaks. The second benefit is that the CFs can drive the output tube grids positive if driven that hard with no recovery issues. There will be no blocking distortion; rather, more clean output or "clean" clipping - no sticking or sonic drop-outs.

The CFs can be direct-coupled to the splitter, direct-coupled to the output tubes, or capacitively coupled to both.

[makinrose]

Kevin,
Thanks for the reply. That makes sense for the design since it's supposed to be loud and clean. These day it seem most of the builds people request are relatively low power (20W is still loud!) so I haven't spent as much time researching the high powered amps where this approach seems to be found. It might be interesting to apply to lower powered amp.

[K O'Connor]

Hey

Amp design is the same regardless of the technology inasmuch as the techniques used to make a powerful amp clean can be applied to lower powered amps. In the latter, the main issue is running out of headroom.

The easy way to attain more loudness is to use big cabinets and more than just one AND THEN don't cluster them - spread them out. The sound will be HUGE. You only need one driver per big cabinet, so you can begin with a multidriver one and spread its drivers over more boxes, saving some $$.

[makinrose]

Good points! I find so many people I build amps for want the HUGE sound you are talking about but don't need the big power. I think a little more clean out of low powered amp with that kind of tone will be useful. I'm definitely going to try it. Thanks for the encouragement and explanations.

In my experience it takes some doing to talk players out of the mindset of tiny combos and stacked cabs but you absolutely right about the multiple cabs spread out! I've been doing myself after I read TUT and SPKR.

[Amptroll]

I have built an SSS style amp.  The only thing that was an issue was coming up with the inductor in the switchable filter.  The one HAD used is not around anymore, I think it was a Triad piece.  I used a 500mH Bourns piece with good results.  If you build one you are in for a real treat, if you really like a clean, glassy sounding amp.  Mine was 150 Watt with 4 x 6550's like he used.  I believe it was derived from a Winterland or a Dumbleland 300W.  SRV used a Winterland in Montrose at a mobile studio that Jackson Browne had there.  It was one that Jackson had stashed for his lap steel player.  Once SRV used it, he was spoiled and that is when he ordered one.  HAD later canceled his order though I read because SRV asked when it would be finished.

    After reading the Dumble chapter in TUT6 (if you don't have it get it!)  I got really interested in the different Dumble circuits. Most are simply variations of what is in TUT6  but the Steel String Singer caught my attention since it's a little different. Basically what I'm seeing is the Dumble clean channel but with a reverb configured a bit like standalone reverb unit and switchable filter.  That makes sense to me.  

However the topology of the phase inverter does not. It looks like a schmitt but has a cathode follower for each drive line.  Is there any benefit to this approach over the standard plate driven configuration? Has anyone here played an amp with this topology? I've attached a schematic!

Hmmm....I've having trouble  attaching  the PDF.  Hopefully you guys can still envision what I'm talking about....