EH 150 Build Completed

[David Root]

I couldn't find the other thread on this that had two others builds in it. Colossal was one. Just finished wiring it up. Haven't put the juice to it yet.

It is the Type III with 6SQ7s and a 6N7 PI, released by Gibson in 1941 IIRC.

It's in a Princeton Reverb chassis I got, blank, from RJ of rjguitars. But 3/32" aluminum instead of steel. The circuit doesn't lend itself to a Fender layout, which is not surprising since the original was laid out point to point, of course. That's why there are five coax runs in it.

Modern (relatively) alnico speaker, a '61 P12N, not the field coil original type. Nice line-up of old glass tubes including a nice pair of WWII era 6L6GAs, all NOS.

[Colossal]

Nicely done David! Bet it sounds really good with the old glass. I've got iron to build another one myself. What do you think of the alnico speaker with it?

[M Fowler]

David,

Nice looking build. I like your layout.

I really enjoy my EH150 totally different sounding amp great for blues.

Mark

[David Root]

Haven't cranked it up yet either. It's an original cone, 220147, no bell, probably from an organ. I have a nice aged tweed cab for it.

I put a 50K pot in the voltage divider 100K, plus a fixed 51K. One of you recommended a trimpot there to control gain. Put that on the faceplate. I also used coax where you suggested, and a couple other spots too!

Now I have to get a faceplate & backplate for it.

[Colossal]

You need to crank it! It is not a particularly loud amp (at least the way I biased the one I built) but comfortably so with a 1x12 speaker. I love the simplicity of the amp and the single volume and tone is all you need really. I found that the tone is quite transparent (you can hear the tone of whatever guitar you use) but warm, woody, and very musical.

[David Root]

Believe me, I will! I guess it ends up in high AB cranked, but with no cathode bypass cap I am looking super for blooz tone!

If I add the 25 uF cap and change the cathode resistor to 250 ohms, Fender style, may get some extra watts, but the tone will change towards fixed bias tone.

I figure the way it is with 200 ohm cathode and no bypass cap I am at about 90% of 19W plate dissipation at about 400Vp plate, assuming 20V at at the cathode.

[M Fowler]

I actually like the clean side better so if I build another I may just eliminate that OD section.

Sure hope she sounds great on fire up

Mark

[rp]

Inspired build, looks fantastic, nice to see it laid out clean on a board. I so want to build one of these (not that I really know why). I got a few NOS matched coke bottles that have been itching for some voltage for near 70 years. I can hear them fidgeting in their boxes. And, isn't it simply amazing how you can take a tube that's been idle for 70 years hit it with 400V and it lights up sings like a day hasn't passed since 1940 and the Andrew Sisters are still topping the charts. No need for rust removal, 3&1 oil, elbow grease, or raw fingers to get them working. I'm awed every time I take these relics out of the box and plug 'em in.

Pity with all the shielded cable, it looses a bit of the 1948 vibe. It's low gain enough that maybe you can swap out a run at a time and see what happens, might not need it all. Is it a dark or bright amp?

I figure the way it is with 200 ohm cathode and no bypass cap I am at about 90% of 19W plate dissipation at about 400Vp plate, assuming 20V at at the cathode.

Even w/o the bypass cap it's gotta be pretty loud, no? Maybe cap on a switch, pull boost on the volume?

BTW What iron did you use?

Please after you play it a while report back on what you find.

[David Root]

Back to work today, but I'm off Friday and will crank 'er up then. I love those little "time capsules" too, as you can see!

Not sure how much vibe it will lose with the coax, that is MIL-C-17 RGU-142, with 18 ga. conductor. Best I could find.

PT is Allen Amps' TP25, this is a Fat Stack Princeton Reverb centers 325-0-325, 150mA, with enough heater current for a pair of KT66s. Only 2A @ 5V so the rectifier is a 5R4GYB, not a 5U4GB. Made by Magnetic Components.

OT is Allen Amps' TO35MT, another Fat Stack designed specifically for a pair of 6L6. Made by Heyboer.

Choke is a Hammond 158Q 5H, 150mA.

Tube line up, L to R is '52 Sylvania 6SQ7, MO Valve CV1991, '62 RCA 6SQ7, two JAN CHS Sylvania 6L6GA, '43 and '45, and a '61 RCA 5R4GYB w/tan base. No idea how old the CV1991 is. All NOS.

I am wondering how the Fender blue molded .1 uFs will work out, they are from a '67 AB165. Also the .01 and .001 caps are both polystyrenes, thought I'd try those there.

[Colossal]

Believe me, I will! I guess it ends up in high AB cranked, but with no cathode bypass cap I am looking super for blooz tone!

If I add the 25 uF cap and change the cathode resistor to 250 ohms, Fender style, may get some extra watts, but the tone will change towards fixed bias tone.

I figure the way it is with 200 ohm cathode and no bypass cap I am at about 90% of 19W plate dissipation at about 400Vp plate, assuming 20V at at the cathode.

David and RP,

The original type III EH150 had 275-300VDC on the plates, a 200R cathode resistor (no bypass cap), and no screen resistors. I biased mine to about 800R (IIRC) [higher plate voltages] via a pot in series with the 200R (41V at the cathode) and dropped the screen voltages to around 380V so as not to beat too heavily on today's production 6L6s. I used some GT 6L6GEs and think they sound quite good. I also added 100uF for bypass FWIW.

I would be interested to hear if you get any crosstalk/bleedover from your overdrive channel on the common anode mixing stage adding noise to your clean channel. I built mine to historic spec (with a few liberties taken in the interest of modern sensibilities) and the schematic Mark provided.

[edited to correct an incorrectly cited plate voltage]

[rp]

I am wondering how the Fender blue molded .1 uFs will work out, they are from a '67 AB165. Also the .01 and .001 caps are both polystyrenes, thought I'd try those there.

If the amp sound wonderful then what ever the old caps are up too doesn't matter. If it sounds less than wonderful you will always be suspicious. Did you test them for spec and leakage first? Leakage is impossible without a fancy tester, but you can lift them in the circuit and check that way, but you know all that. Best way I found that eliminates doubts is to start out with cheap new caps of a known quality like 6PSs or 150s get the amp running 100% and then sub in the oldies 1 or 2 at a time and listen for good mojo or bad mojo. Easy with a board, hard ptp.

[M Fowler]

I used a Hammond 400-0-400 secondary and 4k OT.

Cathode bias with bypass cap. Can't remember off hand the values.

The new production 6SQ7 tubes I got from China made a big difference over the NOS tubes.

[David Root]

What about that field coil in the original, at 750 ohms? That must have taken the voltage down a bit.

I can't see how a 1941 6L6 metal or 6L6G could have handled 500V with a 200 ohm cathode. Not for very long, anyway. No matter how you cut it that has to be way over 19W plate dissipation, not to mention the screen at the same voltage with no screen resistor. Are you sure it was 500V?

I thought Tino Zottola was a bit high with the 365-0-365 PT he spec'd, so I cut mine back to 325-0-325 which I am guessing should come in right around 400V plate/screen. This should be pretty close to class A1. 470 ohm 5W Mills wirewound screen resistors, 1K5 1/2W carbon comp grid resistors.

On the anode mixer I split the 1K/25 uF common cathode to two the same 2K/10uF. We will see if it bleeds across or not.

Mark, how did the Chinese 6SQ7s sound compared to NOS?

[David Root]

rp, I measured the blue molded caps for uF, all good and the PI couplers almost perfect match. If they show the symptoms of DC leakage I will check them out further.

The Jupiter paper/beeswax are probably the best for this circuit but too big and a bit pricey, I think you likely get a better bang for your buck with NOS tubes and original cone old speaker(s).

[Colossal]

What about that field coil in the original, at 750 ohms? That must have taken the voltage down a bit.

I can't see how a 1941 6L6 metal or 6L6G could have handled 500V with a 200 ohm cathode. Not for very long, anyway. No matter how you cut it that has to be way over 19W plate dissipation, not to mention the screen at the same voltage with no screen resistor. Are you sure it was 500V?

I thought Tino Zottola was a bit high with the 365-0-365 PT he spec'd, so I cut mine back to 325-0-325 which I am guessing should come in right around 400V plate/screen. This should be pretty close to class A1. 470 ohm 5W Mills wirewound screen resistors, 1K5 1/2W carbon comp grid resistors.

On the anode mixer I split the 1K/25 uF common cathode to two the same 2K/10uF. We will see if it bleeds across or not.

Mark, how did the Chinese 6SQ7s sound compared to NOS?

The original EH150 used a 408-0-408 PT. With a 5U4G rectifier this would have been 500-ish volts. But at 110V mains probably a bit less and losses in the field coil might have shaved it down a bit more too. I had a conversation with some colleagues about this as I was trying to make sense of the amp's history and understand the intention behind the design and their feeling was that many designs of yesteryear smack of trial and error with brute force solutions. It seems illogical that the PT had a 408V secondary and the power tubes at 200R cathode bias but that is what it was. The degenerative feedback at the cathode may have lowered the output some.