Gibson EH-185 6J7 version

[centervolume]

Hello all

I was thinking this question may have already been asked but I can't seem to find it here so I'll go ahead and pose it.

The mic input appears to have an extra gain stage (v1 perhaps, i.e. it;'s own 6J7 tube) before signal passes to the remaining 2 6J7 stages for tone control and perhaps recovery gain. The Instrument channel ONLY passes through those 2nd two 6J7 pre's and has a significant drop in overall gain to that channel (presumably because mic levels are often much lower themselves than instrument).

So the question is as follows: are there any simple ways to boost the gain on the instrument channel (which is basically operating like a clean channel) without messing up the performance of the mic channel?

[sluckey]

So the question is as follows: are there any simple ways to boost the gain on the instrument channel (which is basically operating like a clean channel) without messing up the performance of the mic channel?

no

There is a not so simple way though. You could add another tube between the instrument jacks and that second 6J7.

[PRR]

The old amps were for two guitars playing *behind* a singer.

If you want your guitar out front, plug your axe in the "MIC" input and let the singer find some other place to plug in.

[DummyLoad]

strap V2 as a pentode  - as built V1-V3 are strapped as triodes.

resistor, cap, and a single pole double throw switch. see attached: SW2, R8, C6

switch will POP - LOUD POP when powered up. switch modes with power off, or try adding a 10M-22M across the poles of the switch.

--pete

[centervolume]

Thanks for input. The mic input is already being used so that option is out. Will look at other options.

Are there threats to using a 5AR4 rather than 5u4 in this amp to get a few extra watts?

[bmccowan]

I built an EH-185 from a PA a few months back. There is a short thread on here. That original circuit, but 6SJ7s instead of 6J7s. As you found, ch2 is very weak. No singers in my house so my solution has been PRR's, use the mic channel. I'm curious, did you follow the "correction note" for V2. I did not and followed the original, but Gibson schematics are an adventure. Is yours an original amp? I have a spare octal socket that was used as a connector. I thought I might wire that for a pentode to add a tube as Sluckey suggested.
BTW - I used an inductor for the bass control but will likely rewire to something more modern (like 1940s )
I don't think switching rectifiers will get you there.

[shooter]

have you put a  gain/boost/tube screamer in front to see what extra gain sounds like?
no soldering required 

[pompeiisneaks]

I've also built an EH185 but it was the 6SN7 version, but also it had a decent 'instrument' channel, great for cleans, but the mic channel was so driven I had to setup a voltage divider on the first stage for the output to attenuate it a bit, and I still can't turn it up to max volume without it going mad :).  It sounds growly and punchy up til that point, though.  I still have thought I should go back and add something else to tame it just a touch more, so i can push it a bit harder, but honestly, with the volume at the right level, it sounds perfect, so I guess I just need to live with it as is and never turn it up too hot :D

~Phil

[bmccowan]

I still have thought I should go back and add something else to tame it just a touch more, so i can push it a bit harder

Since Gibson schematics cannot be trusted, I do not hesitate to try various circuit mods, especially if not monkeying with a valuable vintage amp.
I have read a lot of accolades regarding the EH-185, so wanted to try it, and when I picked up a PA with four 6SJ7s, the opportunity was there. I now know what it sounds like built to the schematic, and will try to get a bolder 2nd channel. Shooter suggested a no-solder required approach to which I say, "what forum is this?"
Anyways, since I have a spare socket I will try either a pentode 6SJ7 (not triode strapped) > volume pot > phase inverter. That should (I think) produce a channel that sounds similar to the BR-6, and the early GA-25/30s, which is a good thing in my opinion.

[shooter]

has a significant drop in overall gain

Shooter suggested a no-solder required

fastest way I know to see what more gain would be like 
but hell, solder away, unintended consequences are always fun 

[bmccowan]

Yup Shooter - I knew what you were getting at, and agree. But I had a needle in my hand and needed to use it. You seemed a good candidate.

[jjasilli]

Looks like a lot of voltage voltage dividing @ each of the Instrument input jacks.  Maybe get a passive "boost" by bypassing or reducing the value of one of those 100M input resistors.

[shooter]

I had a needle in my hand

I've been to Trinidad, voodoo dolls are over rated 
now smoking with the Rastafarians in Jamaica.........

 

[bmccowan]

reducing the value of one of those 100M input resistors

Schematic is from the 1930s when M was often used for thousand - so today those would be labeled 100K

[PRR]

> from the 1930s when M was often used for thousand

I have an audiometer schematic from 1947 where ω means naked Ohms and Ω means MEGohms. I had never seen that before and it took a while to derive the convention from schematic clues. I still can't "read" that plan at a glance because so many values have to be mentally(ha) translated. (Also because it gets ultra cramped and cross-wired in the middle.)

[bmccowan]

I have an audiometer schematic from 1947 where ω means naked Ohms

Go figure - I thought ω meant naked Dolly Parton.

[jjasilli]

reducing the value of one of those 100M input resistors

Schematic is from the 1930s when M was often used for thousand - so today those would be labeled 100K

Doesn't matter M or K, it's still a 1:5 ratio.  The input voltage is being multiplied by 5/6.  So, 200mVAC from the guitar is reduced to 167mVAC. 1VAC is reduced to .833VAC. 

As the series resistance R1 approaches -0-, the denominator in that formula {R2 / (R1 + R2)} approaches 5.  5/5 = 1; whereby there is almost no input voltage reduction, if the series resistor is bypassed.  This results in almost a 20% "gain", for free. Soldering is required. 

[bmccowan]

Doesn't matter M or K, it's still a 1:5 ratio.  The input voltage is being multiplied by 5/6.  So, 200mVAC from the guitar is reduced to 167mVAC. 1VAC is reduced to .833VAC.

As the series resistance R1 approaches -0-, the denominator in that formula {R2 / (R1 + R2)} approaches 5.  5/5 = 1; whereby there is almost no input voltage reduction, if the series resistor is bypassed.  This results in almost a 20% "gain", for free. Soldering is required. 

I want to be careful to not hijack CenterV's original post, and I'm not sure if his is an original Gibson or a build. My build does not have that input configuration on channel 2. I used a single jack > 33K grid stopper > 500K volume pot > 100K grid stopper > V2. I'm used to the instrument channel being the weak sister in early amps, but it is unusually low in this one. So unless another idea pops up - I will try adding another tube in my spare socket.
Centervolume; you out there? I'm interested to hear more about your amp.

[shooter]

you out there

they all left, but the lights are still on 

[jjasilli]

Doesn't matter M or K, it's still a 1:5 ratio.  The input voltage is being multiplied by 5/6.  So, 200mVAC from the guitar is reduced to 167mVAC. 1VAC is reduced to .833VAC.

As the series resistance R1 approaches -0-, the denominator in that formula {R2 / (R1 + R2)} approaches 5.  5/5 = 1; whereby there is almost no input voltage reduction, if the series resistor is bypassed.  This results in almost a 20% "gain", for free. Soldering is required. 

I want to be careful to not hijack CenterV's original post, and I'm not sure if his is an original Gibson or a build. My build does not have that input configuration on channel 2. I used a single jack > 33K grid stopper > 500K volume pot > 100K grid stopper > V2. I'm used to the instrument channel being the weak sister in early amps, but it is unusually low in this one. So unless another idea pops up - I will try adding another tube in my spare socket.
Centervolume; you out there? I'm interested to hear more about your amp.

Your 33K R is actually not a grid stopper, because it's on the input side of the grid leak R (here, a 500K pot).  (OTOH, the 100K is a grid stopper).  The 33K is R1 in a voltage divider circuit with R2, the 500K pot.  If you bypass R1, the input gain stage will bet more signal voltage from the guitar.  Don't know if this will make enough of a difference.  Easy enough to jumper it and see.

[bmccowan]

jjasilli - thanks; I am often confused on the nomenclature. When I get back on that amp, before adding an additional tube, I will try bypassing that resistor.

[PRR]

> The 33K is R1 in a voltage divider circuit with R2, the 500K pot.  If you bypass R1, the input gain stage will bet more signal voltage from the guitar.  Don't know if this will make enough of a difference.

Just like that, the drop is 500/533 or 0.93 or half a dB. Hardly audible. Much less than one number on the VOL knob.

I have seen 3-input plans where using just one input gave a larger drop, like 1/3rd. But even this does not usually justify an additional stage, because stage-gains are 15-50 so you go from slightly-low to way-loud and have to re-think the gain again.

[jjasilli]

To illustrate PRR's point, thought I'd try my first modeling job in LTspice.  One file shows the mV output of the 33K/500K voltage divider with 1 VAC input from 1Hz to 10KHz.  The other shows the same in mdB (1/1000 dB).  The output is down about 550mdB = about 0.5dB, which is insignificant.  3dB is barely significant.

[PRR]

> my first modeling job in LTspice

Good work. (Agrees with my TI-30Xa.)

Having done such things before, it is better for quick-grasping to scale to some human size. 5db, 10dB, 20dB... we know(*) these are significant changes. Also plot the no-loss 0dB line to show what we are comparing to.

My 20dB range is probably too large to squint half-dB changes; yet too small to display "an added stage" (25dB-35dB).

(*) https://www.audiocheck.net/blindtests_level.php
Yes, this may show sub-dB detection on instant A/B checks. The dB as minimum-audible difference is subject to many quibbles. But if two amplifiers are 0.555dB different, you won't be sure after plugging your axe one to the other, or if the VOL is anywhere below Max.

[bmccowan]

I have seen 3-input plans where using just one input gave a larger drop, like 1/3rd. But even this does not usually justify an additional stage, because stage-gains are 15-50 so you go from slightly-low to way-loud and have to re-think the gain again.

Not wanting to mess with the Ch1 gain is why I was thinking of rather than adding a gain stage to both channels, having channel 2 input go to a pentode preamp tube (6SJ7) to a Vol pot and then to the phase inverter. Keeping ch1 as is, but adding a different flavor with ch2. Sensible/dumb?
The LTspice modeling is interesting - I must try that someday.

[jjasilli]

Adding any type of pre-Gain stage will produce way more voltage than you need (even with a low mu tube), as PRR said.  Whatever tube you choose will need its output attenuated, along with frequency (tonal) adjustment.  The only reason to choose a pentode is for your tonal preference, not voltage gain purposes.  If you choose a pre-Gain stage, I suggest you plagiarize from proven designs.

[shooter]

+1
once you get this far into re-design a scope is almost essential for getting the "correct" AC outta the pre and onto the PA grids

[bmccowan]

Adding any type of pre-Gain stage will produce way more voltage than you need (even with a low mu tube), as PRR said.  Whatever tube you choose will need its output attenuated, along with frequency (tonal) adjustment.  The only reason to choose a pentode is for your tonal preference, not voltage gain purposes.  If you choose a pre-Gain stage, I suggest you plagiarize from proven designs.

I'm not intending to add a pre-gain stage. I understand the consequences. My intent is to remove the existing channel 2, which now sends the signal to V2, and replace with a channel that includes only a single pentode w/volume pot and introduces its signal at the grid of V4 (6N7 phase inverter). Ch1 would remain intact as stock. The reason is as you state; offer a different tone than ch1. I am not looking to boost the gain of ch1, just to have ch2 (instrument) be usable. Am I making sense?

[jjasilli]

Yes, it does -- Vox thinks so!


OTOH, KOC recommends a pentode later in the chain so it can be overdriven.  Been meaning to try this. 

[bmccowan]

OTOH, KOC recommends a pentode later in the chain so it can be overdriven.  Been meaning to try this.

As does Merlin, referencing the Matchless Clubman. I loosely followed the Clubman preamp using a 6Sj7 as V2 in a conversion of a SE 6L6 Bogen Challenger; V1 is a 6AT6. Its good tone and I've been meaning to build a P/P Clubman type using a 5879 as V2. I have a few Hammond organ chassis that should be good donors for that.

[RWood]

I have read that from Merlin, too, but there aren't many examples besides the Clubman where the pentode is preceded by a triode. What did you use after the Triode > Pentode in your Bogen build? And were you pleased with the result?

I am interested in the 6SJ7 because of the built in shield!

[sluckey]

All of the old Sunn amps have a pentode after the tone stack. I doubt that "overdrive" distortion was a design consideration though.

[brewdude]

I have recently built a SE amp with a 12ax7 1st triode -> volume pot -> 2nd triode -> Voigt TB tone stack -> 6SJ7 -> master volume pot -> 6L6 SE PA.

[bmccowan]

I have read that from Merlin, too, but there aren't many examples besides the Clubman where the pentode is preceded by a triode. What did you use after the Triode > Pentode in your Bogen build? And were you pleased with the result?

I am interested in the 6SJ7 because of the built in shield!

The Bogen CH-8 - is 6AT6 (can sub in 6AV6 for more gain but I prefer the T)>6SJ7>6L6 5U4 rectifier. And yes I am happy with it. It seems the early Clubman had a 6SH7. and then later went to an EF86.
6SJ7 should get more love IMO. A very common tube in old PAs, old Gibson, Magnatone, Valco. Very sturdy. 5693 is an "improved" version, but I have not found them to be necessarily better in a guitar amp. But they are red and look cool. Interesting info on the Sunn amps Sluckey - I'm really not familiar with those amps.

[sluckey]

The Sunn power amp is just a clone of the old Dynaco Mark III power amps. And I really think of that pentode as part of the PI since the amp uses a cathodyne PI that has no gain. But the pentode is still located after the tone stack.

[PRR]

...the early Clubman had a 6SH7....

I think I know why. Look at the 39-cent special, lower-right, in 1956.

[bmccowan]

Dynaco Mark III

Funny; I bought a pair of Mark IVs at a pawn shop years ago. Friday I decided that I should dig them out and recap them. I found one right away. I looked for the other one for 2 hours - nothing! WTF?
And PRR - I saw that ad, but my mom would not give me the 39 cents. I was only three, but I never really got over it.

[centervolume]

Does anyone have an actual example of this amp to measure hole location distances for power transformer?

[bmccowan]

Does anyone have an actual example of this amp to measure hole location distances for power transformer?

I built mine from an old PA, so do not. But, it'd be helpful to know what you are doing. Rebuilding an original? building from scratch? repurposing an old chassis? etc? Not sure what you are asking - placement of PT? dimensions/stud spacing?
My experience is that if you start with a description of what you are doing - you will get great help from others on this Forum.

[centervolume]

Hi Mac

Thanks for your input, Mac. Apologies I got sidetracked and no longer have the amp on my desk. The heater winding for the rectifier appeared to have quit as that tube was not warming up unlike the others. The new owner was asking me what to do about it and didn't have a meter to verify the winding was busted. I had him put in different known-good rectifiers to ensure it wasn't just a tube problem. Due to their lack of being technically inclined and out of concern for their not getting ripped off, I was trying to locate a drop-in PT for that amp so I at least knew what the options were (that's why I needed the mounting locations). Because of the relative complexity of all of this and the overarching fact that none of it moved this thread forward I guess the issue petered out. I hope the fact of this topic's fading from relevance hasn't been too distressing.

[bmccowan]

too distressing

Not at all. As someone who built an EH-185 circuit and is still tweaking it, I was interested in how you were getting along. I hope the new owner finds a reasonable solution.