David Root wrote:There does seem to be an interaction between the two channels, but it only seems to affect the hum/noise level as far as I could see. I found no tonal difference in the clean channel by cranking up the overdrive volume pot. Not the other way either.
Yep, that's what I found too. Just a bit of unwanted noise on the Clean side if the volume to the OD channel is left up. A consequence of the design and shared anode; grid noise from OD bleeding in. Mark suggested a switching jack to alleviate it and that's a good idea.
I'm thinking about an EH150 build. I think I've read every thread here concerning these amps, and im confused about the PT. I see Mark used a 375-0-375, David used a 325-0-325 with ~350 plates, and Colossal has on good authority the originals were ~408-0-408 with ~300 plates. Im also assuming we're talking about the style4, with the 6sq7s and 6n7 PI. I've seen 2 schematics : https://tubeamparchive.com/download/file.php?id=24587 https://tubeamparchive.com/download/file.php?id=16384
The only major difference I see is the 20K 25W to ground and of course the choke and FC.
Colossal, what is causing the massive voltage drop as opposed to say David's, where Vp is higher than PT secondaries(and like every amp I've built functions)?
Thanks,
Jeremy
Smokebreak wrote:I'm thinking about an EH150 build. I think I've read every thread here concerning these amps, and im confused about the PT. I see Mark used a 375-0-375, David used a 325-0-325 with ~350 plates, and Colossal has on good authority the originals were ~408-0-408 with ~300 plates.
Hey Smokebreak,
I can confirm absolutely that the original Style III and IVs had Thordarson T-40069 PTs (made in Chicago) and they were indeed 408-0-408. I was just in a real Style III recently and measured it from stem to stern.
The first schematic is Tino's interpretation and uses a 350V secondary and no shunt resistor to ground. The second is the later 1937ish Gibson Style IV. If you want to be dead accurate, build the Gibson and include the 20k 25W resistor. Charlie Christian's was the Style III using a 6F5 and 6C5s and an interstage transformer for the PI, not a 6N7 dual triode as in the Style IV. The power amp in both the III and IV is the same, dual 6L6s.
Colossal, what is causing the massive voltage drop as opposed to say David's, where Vp is higher than PT secondaries(and like every amp I've built functions)?
Thanks,
Jeremy
The field coil does play a role in dropping voltage as does the voltage divider created by that 20k to ground. The plate voltage in these amps are running 275-290VDC in true class A with 20VDC on the cathodes. You can use a choke in place of the field coil but you will likely need to add some series resistance to get the plate voltage right, depending on the DCR of your choke. The amp I was in was very quiet with only a soft hiss and very little ripple voltage at idle. They are not loud amps, but man, what a tone with an ES style hollow body and a neck pickup or gypsy jazz guitar in capable hands. It'll take you back to Paris ca. 1940s!
I followed Tino's instructions; therefore, I used Hammond iron because they had what I needed.
I still have oscillation from gain channel past noon on the volume dial. Changed the voltage divider several times. I think I just need to sit down and redo my wiring. I should have built it PTP rather then turret board in large chassis wiring is too long.
Has very nice tone and potential I just really need to clean my amp up.
Ok thanks, I think I'm beginning to understand. I mistook the 20K25W for a typical cap bleeder resistor and not a PS voltage divider. It must have massive effect on V drop, as I see the field coil can be substituted with a choke with a 750r resistance, which would only drop say 20V or so? Do I have this right so far?
If that's the case, smoothing effect of choke aside, and for the sake of discussion, why not sub a 1K25W for the FC/choke(ala Express), and use a 250-0-250 PT, ommiting the 20K25W, to get close to 300V plates?
I guess I'm still having a hard time understanding how a modern interpretation of this amp with 350V PT sec and 400Vp is even close to the same animal with 400V PT sec and ~300Vp(original). I mean, regardless of how we get there, the most important thing is getting that 270-290V on the plates right? I may be grossly overlooking the effect the cathode resistor has...
Finally, assuming I'd only be using 1 input at a time, and historical accuracy aside, It seems the instrument channel's 6SQ7 could be viewed as redundant. Why not split the cathode, run instrument channel input to the top of mic channel pot, and save a tube and a pot?
Yikes that's a lot of questions! Colossal, I'm very interested in this paper you've put together, but can't seem to find it. I've studied Django and Western Swing(mainly the Barnard stuff) for a long time now, so I'd love to see what you've put together.
Edit: Thanks Mark, I missed your reply because it took me forever to post. I think I'll try the new prod 6SQ7 as you suggested in another thread. I've spent a lot of time troubleshooting noise when it turned out to be bad octals.
J
Let me rephrase my main question, as I can get a bit longwinded. If the goal is getting ~ 300V on the plates, what is the difference between using a 400v secondary PT, and dropping it 200v w/ the 20K25W, vs. just using a ~250V secondaries PT, and getting to 300Vp that way?
Or if you keep the pot, you can limit the max b+ by putting a fixed resistor in the wire to lug 1.
Very interesting. I can only imagine that much sag contributes greatly to the vibe. Perhaps I'll ballpark the PT based on what I find and VVR it. That will be new for me.
I am about to start an EH-150 build using the Tino Zottola book. I would like to know what resistor and pot combo I should use in the voltage divider circuit on the overdrive channel instead of the 100K resistor. I was hoping you guys with completed builds could tell me what works best and what sort of range I should have available. Thanks!
