Looking for the EH-150 Schematic out of Tino Zottola's book

[M Fowler]

I ended up running Hammond iron 375-0-375 with 220R bias resistor and 220uf bypass cap.

I was running the 5u4 but I am quite sure I have the 5AR4 in there now because I needed that 5u4 for another amp

Mark

[David Root]

That's about 490V plate, less whatever the cathode voltage is! Are you running 5881s in it?

I also had another question. Did anyone consider DC heater supply on the 6SQ7s?

These are hi-gain tubes, even though operated in a low-plate manner. I know from experience with 6SL7s that if you push the gain on them you have to use DC heaters on them or they hum badly, because they're not humbucking filaments.

[M Fowler]

I'm running 6L6GC but I am going to go back to the 5U4 once they come in. To many amps.

I did not want to try DC heaters in this build but if I was to make another there are a few things I would do differently, including maybe not use that 6N7 in the PI for one. Perhaps even fixed bias. Take out the OD channel and just have two inputs high/low. Re-evaluate the voltage requirement for this amp as spec'd was 750v CT but I really think that could be lowered.

Heck I think Dave's had even higher voltage then mine.

Mark

[David Root]

Why not the 6N7?

I picked a PT at 325-0-325, so with a 5R4GYB I should get right about 400 Vp and maybe 18-20 Vk. That will be OK for my WW2 6L6s. I know they can handle 415 Vp when I ran them in my harp amp.

[M Fowler]

Because I think without the NOS 6N7 I can improve on noise maybe?

Not sure just guessing.

Mark

[David Root]

I'll use the 6N7 and see!

[Colossal]

I'm running 6L6GC but I am going to go back to the 5U4 once they come in. To many amps.

I did not want to try DC heaters in this build but if I was to make another there are a few things I would do differently, including maybe not use that 6N7 in the PI for one. Perhaps even fixed bias. Take out the OD channel and just have two inputs high/low. Re-evaluate the voltage requirement for this amp as spec'd was 750v CT but I really think that could be lowered.

Heck I think Dave's had even higher voltage then mine.

Mark

Hi David,

I did not go with DC heaters either. The amp was dead quiet on the bench. I did elevate the heaters to DC however by tapping off the output tubes cathode bypass cap (I used 100uF/100V) for about +41VDC. The amp was also extremely quiet for the client but developed a buzz later on and I'm pretty sure we have ID'd that as his house wiring being suspect. His house is old school and has some grounded outlets and others not and it would hum a bit depending on which room he played in (and what appliances were on the same circuit). I am however inclined to believe that the common anode mixer in the amp can be prone to crosstalk, picking up idle noise; hence Mark's suggestion to ditch the overdrive channel. I built the amp to the classic spec. The client wanted that as well as the capability for two players to be able to plug in; one on clean, the other overdrive, or two on clean etc. But he really only uses the clean channel.

Yes, my voltages were higher than Mark's. We both used 5U4Gs and some NOS. I liked the sound of the 6SQ7s over the GTs in the Mic (overdrive) channel and it did take a bit of tube rolling to find "the one". I think we used a GT in the Instrument (clean) channel, sounded great. Mark and I both used Tino's 1M/100k voltage divider on the Mic channel and it seemed to be sensitive to lead dress and oscillation. I did used shielded wire for that and got it quiet. I also used a trim pot for the 100k resistor in series with a fixed low value just so I could find the sweet spot (and keep it from sounding too overdriven and garbled). The plate load could probably be split to eliminate the 1M/100k pair (less noise that way) or just lowered significantly. Or just do away with the Mic channel altogether. The Instrument channel will get some overdrive at about 1 o'clock to dimed, just enough to add some hair around the notes. Very good sounding.

Dave

[David Root]

Thanx for those tips. Misunderstood you on the crosstalk, was thinking about the common cathode on the 6N7, not the tweed anode on V1 & V3. If you split out the cathodes, 2K and 10uF each, would that not help?

Splitting V1/V3 plate load resistor out, and lowering them sounds like a good idea.

I'm going to think about that a bit more. I wonder what the "rule of 66" is for this tube. It's half a 12AX7 (sort of, I think the plate resistance is higher though, will have to check that). Gm is slightly lower.

[David Root]

Mark/Colossal, did you use power tube G2 or G1 grid resistors?

The schematics of course show none, I have a pair of '43/'45 NOS JAN 6L6GA that I am reluctant to use without some protection. They are basically irreplaceable. Vp/Vs is going to be close to 400V.

[Colossal]

David,

I definitely used screen grid resistors (1k 5W IIRC) but I put a fat power resistor in front of them, lowering the grid voltages down into the 380s. My plate voltages were around 499-502V. I biased to exactly 70% of max dissipation.

I have been thinking about the notion that the EH150s were touted as Class A. This may have been the case with the Type I and IIs (using those oddball 6N6s) but I keep wondering about the Type IIIs (the iteration we've been discussing). On the original schematic the (6L6) plates and screens were both run off B+1 (500V) and there was no screen resistor. This looks like triode operation to me. There was also no cathode bypass resistor. So, I need to run a load line for a 6L6 in triode mode. It very well could be Class A; I don't know...I need to do the math. In studying amps like this, I often think that some of the designs of yester-year may have involved some guesswork and that the designers "got away" with some things we wouldn't do today in that the tubes they used were so much more robust and tolerant of less-than-perfect operating conditions. Just speculating on that though.

[David Root]

What I see is not triode connection. That is a DIRECT connection of plate and screen together, PRIOR to delivery of the HV, that is different.

There are some older tweed Fenders connected plates and screens SEPARATELY but to the SAME HV supply, this is what we see here.

I will use screen resistors, probably 470R 3W, which seem to work well and are smaller than the 5W sandboxes. Remember I'm going in with about 400-420V HV, not 490V. Grid resistors, I don't know. The standard Fender value is 1K5.

[Colossal]

You are indeed right sir. I just looked at the original schematic and it's not triode connected. I haven't looked at it in a long time and I know Tino's wasn't but somehow got to thinking the original was. I used 1k5 grid resistors as well.

[andrew]

Hi, I found a metal 12sq7 from an old RCA 78 RPM phonograph. Does it differ much in tone from a 12ax7?

[David Root]

OK, I'll use 470R screen and 1K5 grid. Thanx Colossal.

[David Root]

Just found a '61 Jensen P12N, original cone, probably out of an old organ. I think it should do the trick!