Project questions - Gibson EH-185 6J7 preamp version

[bmccowan]

The recent discussion about the EH-185 got me wanting to try that circuit. Since I have a donor amp with 4 6SJ7s, I am trying that (6J7) version, rather than the 6SQ7 version. There is a lot of love for this 1939 circuit.
To start, I am going to marry the 185 preamp to the existing PI and power amp in a Muzak - Strongberg Carlson. An earlier thread describes that amp.
Questions: the schematic does not show grid 3. I plan to tie it to the cathode. Is that likely just assumed?
The preamp tubes are triode strapped pentodes. I added a 100 ohm grid stopper (per Merlin and others) to Grid 2. Smart? Not needed?
I found two schematics for this version - one with "corrections" for V2, that are designed to reduce gain - I'll try the original first, which includes an elimination of the speaker field coil, presumably by a user who switched to a PM speaker.
The tone stack is a strange one and includes an inductor (choke.) I plan to substitute a resistor; 10K as a starter?
I found a cool website on early Gibson history: http://www.gibson-prewar.com/ Take a look at the early amps he collected!

[PRR]

Your V4 has no bias and will clip far too early. I think you need to steal a little more of this stage.

Yes, G3 to K. (Or, as Musak did, G3+G2 to plate.) And on small tubes I see no point in a G2 resistor-- everybody just tied it to plate.

[DummyLoad]

Your V4 has no bias and will clip far too early. I think you need to steal a little more of this stage.

Yes, G3 to K. (Or, as Musak did, G3+G2 to plate.) And on small tubes I see no point in a G2 resistor-- everybody just tied it to plate.

i'm curious about how the bass control is going to work as proposed...  i believe that an inductor is needed hanging off the wiper of the bass pot and not another resistor. a resistor there it seems would be just another level control...


--pete

[bmccowan]

Hi,
Thanks PRR - I deleted too much when I linked the two circuits.  I have now deleted the G2 resistors that I had added to the original 185 schematic. Updated pdf attached.
Pete - looking at the schematic again, I see what you mean. I had read that an inductor used there was a hum issue. But I have several new and old chokes hanging about, so maybe I should stick to that for now and see how the noise level is. Something to be said for getting the original circuit to work before changing things. The next version of the EH-185 returned to the older 150 configuration with a single tone control.

[kagliostro]

One thing Is an inductor on a Power Supply one  other thing Is an inductor on a Tone Control


Franco

[bmccowan]

Understood Franco, but are you saying that its not a likely hum inducer on a tone control?

[st]

Quite the opposite i'm afraid...

[kagliostro]

I'm not a great technichan

what I was meaning is that an inductor is used in a Power Supply as part of a filter that "stops" unwanted nois to go on the supply chain

in a Tone Control an inductor is used to achieve a particular filtering effet to have control on the frequency that can pass and go on for the further stage

Here in the forum there are many people who can explain this much better than what I can do

Franco

[bmccowan]

Franco - yes, I understand the difference. I will likely try one or more inductors that I have and observe the results. The Gibson schematic only has a part # and Web searching has not revealed the spec. But others here have suggest the H value range that should work.
Thanks

[bmccowan]

An update - and some questions
I have the amp up and working. I have left the tone stack and channel 2 out for now, as I wanted to stage the trouble shooting. Good news is that it works and sounds great. I was not expecting such clear sailing, so I'm waiting for the other shoe to drop! It is surprisingly quiet for having 4 6SJ7s.
Questions:
The Muzak had a humdinger - I kept it, but its in rough shape and contact lube did not help much. I could bypass it, or replace with one I have from a Hammond organ chassis - worth-it?
I kept the Muzak PI which is a 6SJ7. EH-185 has a 6N7. I am not good at predicting what something will sound like from looking at the schematic. Opinions are welcome as to whether or not I'd hear much difference by going with the original Gibson PI circuit.
Attached are the progress schematic and the original Gibson schematic.

[shooter]

not good at predicting what something will sound like from looking at the schematic

that's just phase I, follow the schematic, get

it works and sounds great

Phase II you gotta be a musician, or know one, in my 'ol life I called it tuning n matching
Phase III, jam til it breaks or you get bored 

[silverfox]

I do not understand the -106 volts on ground.


silverfox.

[bmccowan]

I do not understand the -106 volts on ground.

Nor do I Silverfox. And if you look at the other EH-185 schematic in my first post (the one with "prewar" stamped on it) the CT goes right to ground and there is no large resistor bleeding voltage off to ground. I suspect that neither schematic is fully original or accurate. I'm wondering if someone was trying to note that they were scrubbing off 106 volts, but I think that 20 watt resistor would need to be more like 5K to do that. The fact that the notation for that resistor is K when the rest of the schematic uses the old M for 1000, is a further clue that its not in the original schematic.
I've worked on a lot of old Gibson amps - the schematics are often just suggestions!

[sluckey]

More than you ever need to know...

https://www.aikenamps.com/index.php/what-is-back-biasing

[bmccowan]

Thanks Sluckey. Do I fully understand it? No. Will I ever? Probably not (but there are many things in that category)
For the EH-185, since no negative bias voltage is needed, this was done (maybe by Gibson, but maybe by another) only to reduce B+ voltage? Do I have that right? 275V B+ is pretty low for a 6L6 amp.
Interestingly, the Muzak amp I converted had a high wattage resistor on the PT-CT that had been bypassed with a jumper. The instruction on the Muzak schematic said to install the jumper for 8watts power. In the past I have used a 20-30 watt resistor, B+ to ground, to bring down voltage, but first I'd seen it on the CT.

[sluckey]

this was done (maybe by Gibson, but maybe by another) only to reduce B+ voltage?

No. It was done in order to operate the field coil, which is just an electro-magnet. The fact that the B+ was reduced is just a secondary side effect.


Think of the power supply as a battery. The battery positive terminal is the cathode of the rectifier tube. The battery negative terminal is the center tap of the HT winding. Now think of the amplifier as just a resistive load placed across the negative and positive terminals. That's how most amp circuits work. Now consider putting another resistor (in this case, a field coil) in series with amp's resistive load. It makes no difference whether you connect this field coil resistor to the positive terminal or the negative terminal. Remember it's just a simple series circuit. The same current flows through the amp's load and the field coil load. Some voltage will be dropped across the field coil load and the rest will be dropped across the amp's load. So the current flowing through the field coil develops a magnetic field required to operate the speaker. The voltage dropped across the field coil will subtract from the entire "battery" voltage, thus reducing the voltage available to the amp circuit load.


So, it's just a simple battery with two resistors connected in series. The confusing part for most people is that the battery negative terminal (HT centertap) is no longer called ground. Now ground is moved to the other side of the field coil. There is nothing magical about ground. Ground is just a reference point that we use to measure voltages. We can call any point in the circuit ground if we want to.

[bmccowan]

Thanks! That definitely helps.
As I add the tone stack to this amp, I'll likely be back with more questions.

[PRR]

> I do not understand the -106 volts on ground.

It's not "on ground". (CT is not automatic "ground".) It is 750 Ohms away from ground. It has the full amplifier current flowing in it. Apparently 141mA, making 106V drop. Since "ground" is relative, and this is in the negative leg of the rectifier, it goes to negative 106V.

(BTW, that drawing is missing a ground at 6L6 cathode return. Green dot added.)

It's not back-bias either. That has little point when the power tubes are the only large load in the amp. (A small benefit because of the 20k power-soak, but that is not-much compared to 6L6 current.)

It *appears* (as much as any old Gibson makes sense) that they were happy with 250V plate-cathode on the 6L6es. But e-caps are 450V. And they needed some DC power for that Field Coil. And it is good to have filtering on the plate power. With this config, they make 381V of DC, a better fit for 450V e-caps. 106 goes to field coil, 270V to 6L6es (of which 20V appears as bias). Even at that point the field coil needed more current than they wanted in the 6L6es, so the 20K passes a little more current (perhaps over-fussy, perhaps it sounded better with the specific field coil they had that week). And the C-L-C filter makes 6L6 plate power so clean they could skip filtering for 6L6 screens and the driver stage.

[ALBATROS1234]

i looked at the schematic at first and was like wow theres a ton of gain then i looked closer and thought it  seems wasteful to me to waste 3 6j7 pentode preamp tubes triode strapped like that which is basically 1 1/2 6sn7 . i really like early gibson stuff with 6sj7 pre tubes and 6j7 is basically its older brother . but i guess the point is you are trying to mimic an amp you like he sound of. i respect that.

[bmccowan]

Albatros - I also like the early Gibsons with the 6SJ7 preamps; BR-6, early GA-25/30, etc. When I read the posts regarding the EH-185, I recalled that I had the Muzak PA on the shelf with 4 6SJ7s, and thought, "hey why not." Building from scratch I would not go that route. And I like working with those 50s era PAs - the chassis are rock solid, the iron is good, and the octal sockets are easy on my old eyes and fingers. And I think they look cool. I recently received an old Bogen from the Grandson of a square dance caller who passed away. I tried to convince him to keep it in the family, but clearly not everyone treasures the same things.

[ALBATROS1234]

 I am just like that as well. I like taking old reel to reel tape recorders etc and mod them for the same reasons .not long ago I converted a Kodak projector which has 2 x 12ax7 and 2 x 6L6 which turned out nicely. One if my favorite amps I built is a hybrid of one channel of ga-20 preamp with a 12SJ7 and it uses a 1/2 12SL7 as a gain stage then the other half as a pi set up like a 5e3 deluxe ,except it's 12AQ5s instead of 6v6s. I was designing it for a guy to use as a harmonica amp but then he didn't have the money to buy it and since it sounds great for guitar I kept it. I would love to hear a sound clip when you are done.

[PRR]

Side-drift about Kodak split and moved:
https://el34world.com/Forum/index.php?topic=24902.0

[bmccowan]

Ok - back to the main drift. I added in the tone stack. I used a 20H choke for the bass pot.  I left the leads long at first so that I could move it around and see the effect on hum. I found that if I kept it away from the OT, it was good. Question - it's pretty effective but there is hum when the bass pot is turned all the way down, or full up. In between - its fine. I'm fine using all but the extremes, but I'm curious about this, and I bet some of you know the answer.
Next - I installed channel 2. I followed the original schematic that has the input direct to a pot and then a 500K grid stopper, bypassed by .001 cap, and then into the grid of V2. It worked, but gain was way too low. So, I tried a 100K grid stopper. Much better. Until - I plugged into channel 1 again, and channel 2 had sucked all the balls out of channel 1. So I guess some of the signal is being scrubbed off through channel 2. Should I remove that .001 bypass cap, it is not on one of the early schematics I posted, and/or go back to a 500K grid stopper? Or is there something else going on?
I'm fine using the amp as a 1 channel amp, but am curious and would like to get it working as original before modding things. It's a damn good sounding amp for that early jazz/blues sound.
Thanks in advance for any advice - this forum is the best!

[bmccowan]

So I guess some of the signal is being scrubbed off through channel 2.

I woke up this morning and it occurred to me that out of habit, I used a grounding jack for that ch2 input. I think I have an unintended path to ground. I'll change that and try again.

[PRR]

> hum. I found that if I kept it away from the OT, it was good. ...hum when the bass pot is turned all the way down, or full up...

The hum is not from the OT but the PT or line wiring.

Pot in the middle it does little/nothing. (Actually it injects to both sides and cancels.) Tune your hum at one extreme.

You may only be confirming what is common wisdom in audio: chokes are trouble especially in the audio path.

[bmccowan]

You may only be confirming what is common wisdom in audio: chokes are trouble especially in the audio path.

Thanks - yes, its been my intention to build this close to the original EH-185 (given that Gibson often changed circuits and/or had inaccurate schematics) to see how it sounded and to hopefully learn some new stuff.  And then likely change the tone stack to a more common B-T stack, eliminating the inductor.
By luck only, I did tune the hum at one extreme of the bass pot. So far its been a fun process. And channel 1 sounds great - channel 2 sounds weak, but I have worked on many 40s & early 50s amps where the instrument channel has little character and the mic amp has good volume and tone. So given the extra gain stage of channel 1, that also may be just the way it's gonna be. Eliminating the unintended path to ground brought back the balls of channel 1, but channel 2 still sounds like a weak practice amp.