Gibson eh-150, version 3, field coil question

[centervolume]

Greetings one and all

Restoring this interesting piece after a previous 'tech' got under the hood and left it in some disarray. I've gone through to get it as per the only schematic I could find (with 6F5 v1 for mic pre see schematic here http://prewaramps.org/gibsoneh-15.htm). Then I took voltage measurements (ordered a new set of 6C5 and 6F5 tubes, but using old (originals?) for now. The amp is passing a very faint signal at the moment and all the transformers are showing no shorts or open windings.

Other than very low plate voltages on v2, 3 (60vdc), I was struck by the fact that the voltage dropped over 100 volts across the field coil (435 after rectifier/before field coil and 320 after). I haven't yet measured Ohms of the speaker coil (they are supposed to be approx. 800 ohms) but I was wondering if 115 volts lost across that field coil is problematic.

Other than the electrolytics, most of the other caps are original. About the 3 cathode bypass caps on v1-4 - if they are either leaky or inert/holding no capacitance, what sort of effect would that have on those tubes' operation?

For now, plate voltages are as follows:

V1: 122
V2: 60
V3: 60
V4: 155
V5, 6: 320

Nodes:
1. 435
2. 325 (after field coil)
3. 319 (after choke)
4. 221

Lastly, someone installed a .01@600 cap between the two 6L6 plates. Not in schematic but I think some versions of these early gibson EH series amps may have used this set up. Any guidance appreciated as always!

[sluckey]

Other than very low plate voltages on v2, 3 (60vdc), I was struck by the fact that the voltage dropped over 100 volts across the field coil (435 after rectifier/before field coil and 320 after). I haven't yet measured Ohms of the speaker coil (they are supposed to be approx. 800 ohms) but I was wondering if 115 volts lost across that field coil is problematic.

Probably all those plate resistors have increased in value and that would cause the plate voltage to be low. 115v across the field coil is probably fine. It has the current for the entire amp plus current through that 20K bleeder resistor flowing through it.

I would replace every CC resistor and every cap in that thing and then re-evaluate. I'd remove the cap that's across the 6L6 plates.

[centervolume]

Thanks Sluckey, great advice as usual. Hard to believe how old it is until you try and move a wire and half of it turns to dust.

[sluckey]

I had the same problem with a 1941 radio a couple years ago. The only things salvageable were the cabinet, chassis, and PT. Even the PT leads crumbled but I was able to take the bells off and replace the leads. I built an Ampeg J12B in it. 

Take your time. I bet you can make that amp sing again. You may have to get another speaker though.

[PRR]

> I was struck by the fact that the voltage dropped over 100 volts across the field coil

That's probably normal. The field coil must eat DC power to make magnetic field. It usually eats a large fraction of whatever the amplifier output power is. 100V and 110V field coils were common values. Let it be.

> someone installed a .01@600 cap between the two 6L6 plates.

Standard item for no-NFB pentodes driving cone speakers. Speaker impedance rise above 1KHz causes rising treble. "Screech". A cap limits the rise. Taking 5KCT load and 0.01uFd we get response falling past 3.5KHz which is ample for guitar, and trims the harsh high overtones of steel strings.

The low plate voltages may be normal for designs of that day. Today I would do it different (that plate mixer sucks) but it surely worked when new and I don't see reason to fret yet.

I would like to see all grid and cathode voltages. These voltages hint at leaky caps. The ratio of field coil drop and 6L6 cathode resistor drop confirms current is not leaking away (allowing for the few mA to the small bottles).

Good DC voltages but poor signal can be near-open coupling caps or dirty jacks.

[centervolume]

Ok gents thanks for the good words. Out of town on holiday til this weekend will get more
Measurements then and check out tubes

[centervolume]

Got back under the hood today for a few minutes to check the field coil speaker. There is a 4-connector plug that connects the chassis to the speaker in addition to 3 thumbscrew-style lugs on the side of the speaker. I was looking for measurements of about 700-800 ohms for the field coil and then 4-8 ohms for the speaker. I thought I'd be able to get these all on the male ends of the 4-connector plug but only found the 736 ohms there.

Below, the 4 wire plug colors and info are to the left and the info relative to the 3 lugs on the speaker frame are to the right. Note that the 4 wires from the connector plug go directly into the dust bell area of the speaker and the 3 lugs are just connectors on the frame of the speaker (i.e. the plug wires that are showing continuity with the lugs are connected internally).

The 4 plug wires are not all showing direct connection to the 3 lugs, but are showing the following connections, resistances

WHITE -            connected to (0.2 ohms) bottom of 3 lugs on speaker (6.5 Ohms resistance to middle spkr lug)

YELLOW  \
               736 Ohms                                                                                 
BLACK    /

GREEN -             connected to (0.2 ohms) top of 3 lugs on speaker (no connection, infinite resistance to middle lug on speaker)