Gibson EH185/150 Question

[surfsup]

I am brainstorming a build using the core design components of the EH185 and/or EH150. The actual amp schematic is here for the version fo the 185 I'm looking at:

http://www.gibson.com/Files/schematics/EH-185%20Amp.pdf

The bass control uses an inductor and I found a photo with "GQ-13" stamped on it. That's all I know. Anyone know the value of this item? I know the Recording King amps were built by Gibson, same as EH185, but schemos for these amps are impossible to find. I can't seem to find any info on this item. Any help is greatly appreciated.

The 20k 20W resistor in the PS will drop the B+ rail if I include it. I am planning to omit it right now. Does this resistor somehow affect how the PA or the preamp will behave under load? or can it just be removed and subbed for a 220k 3W bleeder?

[Colossal]

Surfsup,

In my build, I used the EH150's tone control arrangement, not the 185's so can't tell you about the inductor. The 20k 20W resistor is an integral part of the EH150's design. I can confirm positively that the original EH150 ran in pure Class A with plate voltage on the 6L6s about 275-285VDC. The original PT was 408-0-408 (T-40069). Thordarson Chicago is the original manufacturer.

Dave

[surfsup]

I found a doc on a thordanson tone control that looks eerily similar to the tone control in the eh185 design I'm checking out. It looks very cool as the treb adn bass are totally independent of each other. I emailed thordanson as they are still in business, to find out the specs on the choke used in that circuit because there was an item #. Who knows maybe I will get a reply.

[rdjones]

http://www.gibson.com/Files/schematics/EH-185%20Amp.pdf

M=K ?

rd

[Colossal]

http://www.gibson.com/Files/schematics/EH-185%20Amp.pdf

M=K ?

rd

Yep, M is old school for one thousand. So 20M would be 20,000 ohm.

[martin manning]

http://www.gibson.com/Files/schematics/EH-185%20Amp.pdf

M=K ?
rd

Yep, M is old school for one thousand. So 20M would be 20,000 ohm.

Real old school... since Roman times ;^)

[rdjones]

http://www.gibson.com/Files/schematics/EH-185%20Amp.pdf

M=K ?

rd

Yep, M is old school for one thousand. So 20M would be 20,000 ohm.

If so, then the question must be asked ... What is x1,000,000 ?
(inquiring engineer brains want to know)

I've never seen this done before, it opens up a lot of possibilities for problems.

reddog

[martin manning]

On the linked schematic the guy wrote MEG.

[surfsup]

M is 1000
MEG is 1000000
Threw me at first.

Going back to the 20k20w it doesn't drop the b+, the field coil does. Supposedly these are about 750r. If the current through the amp is 100mA, or with 6L6 maybe 120, that's 100v drop. So, if I sub a choke and series resistor, I will need more than 290/290 PT...

Any comments on the bass coil?

[M Fowler]

I tried to drop the voltage by using the 2ok/20w but nothing. I am not sure why they spec over 400 volts but colossal confirmed this with an expert with these old Gibsons and Mercury also confirmed.

I used a 375-0-375 Hammond I believe in my EH150.

[Colossal]

Going back to the 20k20w it doesn't drop the b+, the field coil does. Supposedly these are about 750r

The field coil forms a Pi filter of 10uF - Field Coil - 10uF after the 5U4 rectifier. Sorry, but the field coil does not provide any significant drop in voltage despite what the Gibson schematic might show. Also, the field coil is located on the + side of the Pi filter in all of the amps I have seen. As Mark mentioned, I have verified the operating voltages in an original EH150 at idle. The 185, although it has a different preamp configuration, the power amp is the same as the 150 and the same PT. While analysis and simulation shows that the 20k 20W resistor appears to play no significant role in dropping much voltage, at idle, the voltage taken at the plates shows a drop of some 200V+. The amp is operating in pure Class A with about 280-285V at idle with about 21V or so on the cathodes (I don't have my notes in front of me). The rectified voltage is about 489V after the 5U4G rectifier and only slightly lower after the Pi filter. The field coil is acting as a choke and was close to the 750R shown on the Gibson schematics for the 150 and 185 that are available.

[surfsup]

If you move the field coil to the positive side, you have 381V on one side and 275 on the other, correct? So the 750R coil drops 106V. The current through the amp is therefore 140mA. (106/750=.141)

Ohms Law, the 20k20W resistor sits between 275 and 0 volts. 275/20000=14mA which means the rest of the amp is pulling 126mA.

In fact, regarding the 20k, it seems to do the opposite, because the only thing I could think of is under loads, the amp is drawing more current, the sag across the choke/coil will lower the voltages more due to more current so the voltage at that node will go down from 275 (with 275 being a presumed maximum I guess). With load, and lower voltage, the current through that resistor will be less i.e. 250V/20000=12.5mA as opposed to 14mA at idle. So what am I missing?

[Colossal]

If you move the field coil to the positive side, you have 381V on one side and 275 on the other, correct? So the 750R coil drops 106V. The current through the amp is therefore 140mA. (106/750=.141)?

The rectified voltage is about 489V, not 381. The original amps used a 408-0-408 secondary PT and I have confirmed this positively through multiple sources. All of the 150s and 185s had that same PT. There is a LOT of variance in parts between the various years of production so you cannot take the two schematics that are available from Gibson as verbatim. I have seen variance amongst amps that were produced in the same year. I do not know the OT primary of the Type II-IV EH150s but I am guessing around 5k. I show an idle voltage of 66mA with load line analysis using a 6L6 in Class A with a plate voltage of 275V, but I think the idle current may actually be lower. I cannot verify any of this however until I get my voltage document back.

I am working on a paper on the history of these amps specifically because of the link with famous swing jazz players such as Charlie Christian and Django Reinhardt who were known to use these amps during their careers. I was fortunate enough, after a LOT of effort, to meet a gentlemen with a large number of these amps in his personal collection. He had his tech go through one of them that is in pristine condition and took voltages at every point I specified, including the very points you are trying to make sense of.

I suffered a hard drive crash recently and although I got 100% of my data back, I did not get any information stored in the operating system's library file so this key file was unfortunately stored in my email database and was lost. I am hoping to get it back in the next day or so.

Surfsup, you might be better served to fit your assumptions to a lower PT secondary voltage and build your power amp to suit. Gibson did what appeared to be some odd things by our standards, but I have begin to work out their reasoning behind the choice of PT for instance and it is not haphazardly chosen.

Hope that helps.

[Colossal]

In fact, regarding the 20k, it seems to do the opposite, because the only thing I could think of is under loads, the amp is drawing more current, the sag across the choke/coil will lower the voltages more due to more current so the voltage at that node will go down from 275 (with 275 being a presumed maximum I guess). With load, and lower voltage, the current through that resistor will be less i.e. 250V/20000=12.5mA as opposed to 14mA at idle. So what am I missing?

Ok, I have my notes back. Since the amp is operating in pure Class A, it's maximum output is at idle and no signal. The center point is around 56-66mA with a plate voltage of 275-300V (depending on the PT and mains voltage). The Gibson (Thordardson) PTs varied by +/-20% of nominal. Cathode voltage is about 20-23V with a 200R unbypassed bias resistor. Both the screens and the OT primary are getting B+ from the same point. The plate voltage is about 2V lower than the screens and there were no screen resistors. Yes, there is a drop in voltage at the point just after the field coil (B+1). Per the tech that was in a real amp, "the 20k resistor stabilizes the voltage, it draws it down (and helps reduce hum) so that there isn't so much fluctuation when a signal is presented". The actual measured value of this resistor in the sampled amp was 19.2k, the field coil 730R, and the PT AC secondaries measured 392-0-392 open circuit. The drop across the 20k resistor at idle is around 15.5mA.

[M Fowler]

I need to get back inside my EH150. I had messed with the voltage divider bu the OD circuit can still oscillate but at much higher volume then before.

This would be a great amp without the second channel.