# The two channel amp 5C2+6SJ7 optional grid leak/cathode biased circuit

#### Jerry garrcia

##### Tele-Meister
A picture helps. Beside the triode and the pentode symbols I put the sample internal impedance of the tubes, 68k for the triode and 200k for the pentode. At the second stage of the triode we have a pair of 200k resistors. At point A the AC impedance of the circuit has the 100k resistor goes to ground through the filter capacitor. It also has the internal resistance of the triode in series with the 1.5k resistor. The 68k and 1.5k equals 69.5k (the internal impedance of the tube is different for different triode types and can change depending on how much current is going through the tube). This 69.5k resistance is in parallel with the 100k plate resistor. the two parallel resistances combined have a equivalent resistance of 41k.

Say the triode at point A is making a signal voltage of 20V. It has a load of the following resistors, 220k and 220k, 440k. Now we calculated the impedance of the first stage as an equivalent resistance of 41k. This is the source resistance, the 440k is the load resistance. Without the load resistance the first stage is making 20V and if we measure the voltage at point A we will read 20V. But if we connect the 440k to it we now have current flowing through the circuit. The total resistance would be 41k and 440k in series for a total of 481k. We use Ohm's law to calculate the current through the loop. 20V / 481k = 0.041mA (or 0.000041A, it is best to use Amps and resistance values in ohms to calculate with.

Now we take either the 41k or the 440k to calculate one of the voltages across a resistor, I will pick the 440k. 440k x 0.000041 = 18.1V, this is the voltage across point A to ground The original voltage the triode could produce without a load was 20V but since the triode circuit has its own internal resistance and a current going through a resistance causes a voltage drop, 20V - 18.1V = 1.9V is dropped internally in the triode. The lower the resistance of the load resistance the more voltage that gets dropped internally in the triode. If you had a short to ground the only resistance in the circuit would be the equivalent resistance of 41k and Ohm's law says all the voltage will be dropped internally and point A would be at 0V. So with no load the open circuit voltage would be 20V, a short would be 0V and and as the load resistance goes up more of the voltage appears at point A and across the load resistor. This is why we use higher resistances in order to retain as much voltage as possible. When you went from 220k resistors to 470k resistors you end up losing less signal voltage.

View attachment 988447
Now the easy part, the output of the first triode goes to the top of the first 220k resistor and the voltage is split between the two resistors. So if we have 18V at the top resistors and since they are the same value we will have 9V at point C.

Now jumping to point E. Just to make it easier we are going to say the pentode is putting out 10V at the top of the pot. And we have the bottom pot at 0%. In this case we have the voltage split equally across the 220k resistors with 5V at point E. If we turn the top pot up to 100% we have that point to ground with our equivalent resistance of 41k. So the pentode circuit will see 220k + 220k + 41k = 481k. At point E we with then have the 10V signal split between the 220k resistor and the 220k + 41k. And using the voltage divider rule we have 10V x (261k / 481k) = 5.4V.

So the mixing resistors act as a voltage divider. If the top pot is set at 50% and I am going to wave my magic wand at the triode resistance equivalent and say it is 0 ohms rather than 41k, just to make the math easier. So the 1M pot at 50% we have two 500k resistances in parallel, which is 250k.

So now with the pentode pot at 100% putting out 10V we have the 10V split across 220k, 220k, 250k. With the triode side of point E we have 10V x ( (220k + 250k) / (220k + 220k + 250) ) = 10V x (470k / 690k) = 6.8V

So in the end we will end up with roughly 5.4 to 6.8V of the 10V signal. Now if we did not have the mixing resistors we could get the maximum voltage from the pentode side when the triode side pot is set to the middle of the resistance range of 500k to ground (remember 50% of rotation of a log pot is not 500k). Now if you turn the triode pot to 0% all the signal from the pentode is shorted to ground and you will have no signal at point E. If you have the triode pot at 100% you will effectively shorted out the signal with the 41k load on the pentode side.

So the mixing resistors is a necessary evil if you just want to use the volume controls and have them operate as you would expect. But if you have the opposite pot turned up or down you would have little signal from the channel you are using and you may think something is wrong. You could turn up the pentode channel to a point and then rotate the triode pot to find the point where you have an equal resistance on either side of the pot and therefor not load down the pentode channel.

You can jumper across the two 220k mixing resistors and see how the signal of the one channel can be loaded down by turning down, or up, the other volume control.

A lot here again, once you see the concept you start seeing how the AC signal is effected by the other parts of the circuit.

The pentode side should be operating as the other amps (if you have the switch on the grid leak bias circuit). Something is causing it not to it seems.
Holy cow! I think I’ll need to bring out my defibrillator, make a bilateral temporal bipolar brain shock to reset it all after this reading. Was flipping through “War and peace” to get in the right mood for a good night sleep. Now it feels like I hung out with Ken Kesey at an illegal rave party. How will I be able to sleep after this?

Will need to read it a couple of times more and I guess my patients tomorrow will meet an absent doctor flipping through his old high school physic’s books.

So for the layman:
- tomorrow I’ll will update the schematic with all the changed stuff I’ve done.
- start up the amp and plug a guitar in to the pentode jack. Turn the volume at a 100% of the volume pot of the pentode. Then playing while I’ll adjust the volume pot of the triode to see if I’ll get a increase in output volume of the pentode and try to find the sweet spot?
- then place a jump wire directly from the volume pot, of the chosen channel, to the turret that connects to 470K grid leak resistor of the 6v6. Then do the same as above and repeat it with the other channel (the un-bypassed mixing resistor channel is still connected to the 470K (previous 220K) mixing resistor from the volume pot)?

One thing that struck my mind. One thing that differs from this build than from the previous, that includes a high gain 6SJ7 circuit) is that in this build 1) both channels has their separate tone control and
2) the previous builds with two different channels the both has an extra “gain stage” (if you could call the paraphrase phase inverter a gain stage).

Or didn’t I choose this version since the triode had to little output volume ?

Don’t remember now and will have to check. Ether way in the last two schematics the output volume stuff was that the triode had a higher volume than the pentode. Now it is the opposite….
Tone circuit?
In this build this schematic is almost correct except for the grid leak resistor for the pentode is 470K, cathode resistor 890R. For the triode there is an added 1M grid leak resistor. For the 6v6 the cathode resistor is 470R and increased the resistors between the filter caps and increased the mixing resistors to 470K ones. Must really update the schematic…

Strange and will now contemplate what you @printer2 really wrote.
And if anybody has any good idea what could be the cause of this and what to make of the tone circuit of the reverb amp I’m all ears. Actually thinking about skipping it since it now mostly make something great a bit less great. Maybe a bias switch instead?

#### Jerry garrcia

##### Tele-Meister
Last updated schematic. Will change the plate resistor of the pentode tomorrow to a 220K to see if there will be a difference. Hard to think that everything till be solved by that.
Will also do a complete measurement of the whole circuit. Might be something that I have missed.
Might also connect another pentode preamp build to this circuit. Thinking from the coupling caps of the high gain pentode amp (same circuit except for the bias switch) to the volume pot of this amp as a way of finding where the problem is.

Will add the voltages on the schematic tomorrow as well. So glad that I don’t have any Russian or Chinese tubes in any amp at home.

Found this amp for a low price. Seems like a nice circuit so I might buy it. Anybody knows how it sounds and functions?

### Dynacord Rex Combo Amp 1965 Two-Tone​

#### Jerry garrcia

##### Tele-Meister
Did an experiment last night. Took another grid leak biased 6Sj7 amp (named amp2) with the almost the same pentode circuit as this one (amp1).
Started amp1 and low output volume from the pentode in both bias modes. Triode runs fine.
Started amp2 and both the pentode and the triode (12ay7) runs fine and good reverb circuit (changed the tone circuit to only run with the pentode).
1) I disconnected the wire from the coupling cap of the pentode to 1M audio pot on amp1.
2) I took a jumper from the coupling caps of amp2 and connected it to the 1M audio pot of amp1. (Disconnected the wire from the coupling cap to its own volume pot on amp2).
3) attached a 8ohm speaker to each amp.
4) started both of them up.
5) on amp1 the triode runs as usual but really low output of the pentode. Lower than before.
Was on my way to continue the experiment but the son needed to run to a gig and wanted a small amp so I had to instal amp2 in the nice cabinet that amp1 was in…

So my conclusion of the experiment:
The problem lays probably between the volume pot and the mixing resistors.
Correct?
Any inputs in how to proceed except for resolder everything from the pot to the mixing resistors, change pot, measure all resistors and recheck the circuit?

#### printer2

##### Poster Extraordinaire
I knew an engineer that worked at a place that re-manufactured turbine engines. They had one engine on the test stand that was making 5% less power than it should have. The went over the engine fuel system, sensors, electrical system, they could not figure it out. After a couple of days on the test stand with no further things to try the engineer took a rubber chicken (no idea why they had one) and said "Ooga-booga mumble this jumble that." or something, doing a voodoo chant while circling and hitting the engine with the chicken.

He said to run up the engine again, everybody looked at each other and thought why not? When it got up to speed it made the correct power. Nobody expected it. Then the big question, what changed? I can't remember what it was but sometimes you run out of theories what it can be and have to either isolate and/or measure everything and even then things may seem ok but in a combination it still does not work. Stereo amp are great when one side is not working right, you can compare the one side to the other to help find the problem. I would have done the same thing at this point, use another circuit and hook it up with the amp to see how it fairs. Mind you I do have a signal generator and oscilliscope to dig a little deeper into a malfunctioning amp.

There are some techs that just throw parts at a problem until the problem goes away. There is a woman on Youtube that did just this with a vintage amp, replacing the plate resistors for a problem. It did not make any sense to me but after doing that she did some other things and found the problem. But by that time she replaced a bunch of parts that should have remained in the amp considering that they make the amp more valuable (good or bad). Sometimes you do have to replace parts to find the bad ones, always best to reinstall the part if it does not seem bad though. In some cases you need to redo a section of circuit if you find it to be a problem. It sucks but what can you do? The good thing is if you get it right afterwards you end up with some knowledge that you did not have before. I have a supply of alligator clip jumpers to quickly put parts in and out of a circuit or to make changes.

I think you may have to pull the pots out of the chassis and look at the operation then. I am sure others here have had a problem with something mounted in a chassis when it works fine outside. I had that with an amp, drove me nuts until I pulled the chassis and found the cabinet misalignment cause a part to move and connect another part of the circuit. You are close to finding the problem, there is no short cut to suggest at this point. Just have to bite the bullet and get across the finish line.

#### Jerry garrcia

##### Tele-Meister
The masochistic person in me really loves them cranked up 5F1/5F2 chassis. It’s a dream to redo all the soldering when everything is mounted into place. Which I had that rubber chicken. ThNk you for the encouragement. Hopefully I’ll get to it tomorrow or the day after.

#### Jerry garrcia

##### Tele-Meister
Finally. I’m just to stupid. It was a bad pot. Everything suggested to that problem but I didn’t listen. Also changed the plate resistor to the pentode to a 200K.
Beautiful sound from both channels and the bias switch works like a charm.
Thank you everybody!
Will post a YouTube video in a couple of days.

Now totally locked at a EH-185, 6SQ7/6N7 one. Will be the next build. Thinking of a turoidal transformer to get the weight down. What to make a light weight.
Any suggestions to any mods on that circuit?

Final schematic

#### printer2

##### Poster Extraordinaire
See, I knew you could do it. With the 185, I would change the 6N7 tube to 6SN7. The 6N7 is more a power tube than a signal tube. The 6SN7 is more common (to put it lightly) and has the same gain as the 6N7. There is an amp discussion on jazzguitar.be where a guy is selling reproductions of the 185 and he used the 6SN7 also. I think this is the guy, http://vintage47amps.com/VA-185G/

On light weight, think you should go with a light pine cabinet rather than a Birch plywood one. I am not sure you will save all that much weight with a toroidal transformer.

#### Jerry garrcia

##### Tele-Meister
See, I knew you could do it. With the 185, I would change the 6N7 tube to 6SN7. The 6N7 is more a power tube than a signal tube. The 6SN7 is more common (to put it lightly) and has the same gain as the 6N7. There is an amp discussion on jazzguitar.be where a guy is selling reproductions of the 185 and he used the 6SN7 also. I think this is the guy, http://vintage47amps.com/VA-185G/

On light weight, think you should go with a light pine cabinet rather than a Birch plywood one. I am not sure you will save all that much weight with a toroidal transformer.
It’s because all the help I got. Especially from you. Grateful.
Regarding the 6SN7 it was actually what I was thinking. Great minds think alike .
Must probably change the circuit a bit then. Was also thinking about using the LNDVERB in the build as well. Thinking that a reverb unit would suit that amp a lot.
Might have to start a thread. Heard that the circuit would benefit from a mod. Is it the 6SN7 tube people are talking about?

#### printer2

##### Poster Extraordinaire
I was going to mention adding reverb (I like a touch) but thought maybe you had an amp with it and had enough. Heck, great minds. Google for "EH-185 schematic" and look at the images, oh hell,

And it gives a lot of links of a lot of the same couple of schematics (Gibson has been known to change things) but a few different threads of people doing builds. Always take things with a grain of salt, but there might be a thing or two you might like to take with you. Also that grid leak bias switch, if you like it, it will work with a triode as it does with a pentode.

#### tobyk

##### Tele-Meister
It’s because all the help I got. Especially from you. Grateful.
Regarding the 6SN7 it was actually what I was thinking. Great minds think alike .
Must probably change the circuit a bit then. Was also thinking about using the LNDVERB in the build as well. Thinking that a reverb unit would suit that amp a lot.
Might have to start a thread. Heard that the circuit would benefit from a mod. Is it the 6SN7 tube people are talking about?
If you did the input grid leak switch, may I ask if you notice any major difference when switching? I also did the switch in a 6SJ7+6SL7 single ended amp, but only notice an ever so slight bump in gain when in grid leak mode. I also think you should up the 470K input grid leak – right now you only have 430K grid leak when in cathode bias mode.
Up it to 1.5M and you’ll have less signal going to ground (4.7M+1.5 in parallell = 1.1M, which is close to the standard 1M.