EH 150 Build Completed

[Colossal]

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!

I am sure I used 1M with a 100k trimmer. Personally, I would stick to the original schematics (Types I, II, III, or IV). If you are after the Charlie Christian amp specifically, follow the attached schematic. Tino's schematic is his take on the EH-150, not the actual deal.

CC EH150.JPG

[GypsyDavey]

Thanks for the help, Colossal! I'm not necessarily going for the Charlie Christian sound (although I do like it). To be honest I was inspired by Johan Segeborn's youtube video of what I believe he said was an EH-185. It seemed to have a pretty unique distortion, and I thought it would be cool to do a build with octals.

[didit]

Hello —

EH-185 has paraphase whereas 150 has interstage transformer to provide PI into power section. Paraphase has quite unique overdrive character.

Best .. Ian

[pompeiisneaks]

Hello —

EH-185 has paraphase whereas 150 has interstage transformer to provide PI into power section.

Best .. Ian

There were actually two variants of the EH 150, the older version had the interstage transformer, the latter was almost identical to the initial EH-185 if I recall (I built a 185 myself) the earlier version is the one I built that used the 6SQ7 Octals for the preamp and 6N7 for the PI, but then the later EH-185 changed it up and used 6J7's for the preamp and still the 6N7 for the PI. I did this a few years back so my memory may not be 100% but that's the gist of it... they were basically identical at one point, but changed.

~Phil

[didit]

Hello —

EH-185 has paraphase whereas 150 has interstage transformer to provide PI into power section.

Best .. Ian

There were actually two variants of the EH 150, the older version had the interstage transformer, the latter was almost identical to the initial EH-185 if I recall (I built a 185 myself) the earlier version is the one I built that used the 6SQ7 Octals for the preamp and 6N7 for the PI, but then the later EH-185 changed it up and used 6J7's for the preamp and still the 6N7 for the PI. I did this a few years back so my memory may not be 100% but that's the gist of it... they were basically identical at one point, but changed.

~Phil

Thanks Phil. Yes, I should have been clearer and said "the EH-150 in the posted schematic".

Best .. Ian

[pompeiisneaks]

Hello —

EH-185 has paraphase whereas 150 has interstage transformer to provide PI into power section.

Best .. Ian

There were actually two variants of the EH 150, the older version had the interstage transformer, the latter was almost identical to the initial EH-185 if I recall (I built a 185 myself) the earlier version is the one I built that used the 6SQ7 Octals for the preamp and 6N7 for the PI, but then the later EH-185 changed it up and used 6J7's for the preamp and still the 6N7 for the PI. I did this a few years back so my memory may not be 100% but that's the gist of it... they were basically identical at one point, but changed.

~Phil

Thanks Phil. Yes, I should have been clearer and said "the EH-150 in the posted schematic".

Best .. Ian

Oh yeah I probably should have noted that myself and put 2 and 2 together sorry.

~Phil

[GypsyDavey]

I finished my EH150 build and am very happy with the result. I built it exactly to Tino's specs, but had some questions about changes that might help reliability. I wanted to know if I should add a 22K resistor to elevate standby above ground as suggested by one of the members here and also should I add screen resistors? I'm a bit of newbie when it comes to understanding exactly what the benefits are, so if someone could let me know I would greatly appreciate it. Also, are there any downsides to these modifications, especially tonewise? The link is to a video of the amp at full volume on the overdrive channel. https://www.youtube.com/watch?v=YpsqbgcdNw8

[M Fowler]

I built my EH150 based on Tino's specs as well. I would add screen resistors and not familiar with the elevation you mentioned.

Mark

Gibson EH150 final 008.JPG

Gibson EH150 final 009.JPG

[Colossal]

I wanted to know if I should add a 22K resistor to elevate standby above ground as suggested by one of the members here

I don't know what you mean by this but there is a 20k 25W resistor to ground off of the power supply rail in the original amp. This is an important feature if you are building a clone as the original (Thordarason) PT had a very high secondary voltage. The original operated at just 275VDC on the 6L6GA plates, true Class A push pull.

[TUBEDUDE]

It is some pretty build David.

I can' help but wonder whether you can get rid of the coax. Forgive me if my tone here sounds pushy -- not meant that way, just thinking out loud that it might be worth trying.

So, just a few ideas for you to consider, FWIW. I'm not saying this would necessarily work or be an improvement. I am one of those people who isn't fond of coax. That's all...

Phil

Phil, that looks like an amp in a suitcase being checked by the TSA X-ray machine.

[TUBEDUDE]

I agree with Phil on the coax.
Screen resistors protect the screens from excessive current. Effects, well, high values can cause a compression. 500 - 2k2 are general values, i usually use 1k5 on power tubes.

[GypsyDavey]

Thanks for the help. I have some 1K 5-watt resistors that I can use. One more question. Because pin 4 on each of the 6l6 tubes is run together, would it be acceptable to use just one 1K 5-watt resistor between the node and the two power tubes?

[Murrayatuptown]

I have been reflecting on the 20K resistor a number of times because of how often people call it 'bleeder', and it's other functions. But I didn't really follow through with the thoughts until today.

The 6L6 plates & screens are fed from the field coil. With the shared cathode current including the plate and screen currents, the 20-23 V/200 ohms = 100-115 mA, and the 20k to ground at that point draws, say, 290/20k = 14.5 mA. I was thinking about currents in comparison to PT current rating, so erring ever so slightly high at 130 mA through the field coil, that's 97.5 V drop across the nominal 750 ohm field resistance, and that is what people cite as typical. All the other variables like what the actual field coil resistance is and how much current is drawn will shift that voltage drop around a bit.

I had taken a 6L6 cathode voltage of 20 V earlier so I had 115 mA theoretically through the field coil.

I've been reading on antique radio sites about a rule of thumb for 1 W of field coil power per inch of speaker diameter. However, going through web archived electronics catalogs from 1936 and 1949, I looked at field coil power specs and they ranged from slightly above that to much less. Power rating and field coil size were pretty small in table & home audio consoles.

But 0.115^2 * 750 is 9.9 W. The original EH-150 speaker was 10".

I have purchased 5-7 (not sure.They haven't all arrived yet) field coil speakers to study. One sure looked like a Jensen but it came from a Coronado radio that had three 12" speakers. Two with 1000 ohm field coils that were in series as filter chokes, and the third had a 6400 ohm shunt field coil (per an old service manual). Mine measured 6440. I took it to the local re-coner (seller had ripped it, canceled the eBay sale & refunded me). I said put it back up on eBay! It's almost 90 years old. I expected it to need help!

I recently got a Style III EH-150 that has some changes but works & sounds good. Instead of putting a field coil speaker back in the amp, I want to put this one into an outboard cabinet with a 'field coil exciter' as they called them in the 30's (DC power supply), and plug that into the EH-150 Echo Speaker jack. 275 VDC/6400 ohms = 43 mA = 11.9 W in the field coil. If I want 10 W, 256 V gives me 40 mA and 10.25 W in the field coil. This is the kind of stuff I was hoping to figure out from a field coil shopping spree.

The speaker people said to put a tube rectifier in the cabinet. I can rationalize it for an amplifier, but it sure seems like a lot of work to power an electromagnet. The current speaker in the amp is a PM type and it sounds fine. Actually, with no amp in the 2nd cabinet either a 5 V or 6.3 V filament winding would be usable but at 40 mA B+, maybe a small 6.3 V rectifier simplifies the PS. I was thinking a 5V and 6.3 V winding and a choke was crazy for just a magnet.

[Murrayatuptown]

I wonder if 40 mA continuous is too much for a 6AL5. Probably OK for a 6X4. Datasheet spec number vs. datasheet graphs say different things...or I read different things.