Building EL-84 push/pull amp out of Zenith Radio Parts

[DRH1958]

Whenever I get back in there I plan to take more extensive measurements for analysis. Maybe not by me.?. But I'll post them and hope that some of the more educated persons here will be able to see what's happening. This is my own personal play/practice amp so I'll never leave it apart for long and I don't tear it open more than necessary. On that note...

I was using a 150R "sag" resistor in the circuit to both add that rectifier tube dynamic and reduce voltage. Plate voltage with this circuit was 360. The tone was very bouncy and dynamic with ample bottom end, but...

I always wondered what the amp would sound like with a more reasonable resistance and a zener drop. So I currently have a string of three 13V zeners in there and a 68k series resistor... Plate volts now are 337 and the sag resistor value has been reduced. The results confirm all my experiences with EL84's...

The tone now has that classic chime. There is a characteristic "ring and hang" to the treble that is so cool. Like the amp really "gets under the notes" and hangs them in the top end. This is a classic characteristic of el84 tubes at this voltage, but... The bottom end is gone! So far it seems that if you want to get bottom end performance from these tubes it only comes at the expense of their characteristic goodness.

I'm still on the fence. I may try a different speaker to revoice the overall tone for the new response character. I may also try removing a diode to bring the voltage to about 350 and hope for the best of both worlds (really long shot I think). I'm really screwed at this point because I want the bouncy, dynamic tone with a bit of bass thump, but I also want that "get's under the note" effect and the classic el84 intermodulation distortions. I don't know if both are possible in the same circuit but I'm sure as hell going to try and tune this thing to find out

The story so far is that el84's perform one way at high plate volts and another at lower plate volts. Both valid. Both great. But can I get a bit of both and will it be the best of both worlds or too little of either trait??? I'll report when I find out.

When I have the amp opened I'll tack solder some things and try to get some meaningful measurements of the zener/resistor interaction too.

I found that if I increased the coupling caps in the preamp by making them switchable and fully bypassed the cathodes in preamp stages, I got a lot of the bass back. I used .022,.047 and .1uF Then lower coupling cap value and also reduced the vol knob on the guitar, got a lot of the chime back. This with 330v plate, 320v screen.

[Chuck H]

I have enough adjustment with the bass control to get my bottom end for clean tones. It's when the power tubes are clipping that I'd like to get my bass back. Not just more low end either, but some punch. IMHE increasing bass in the preamp and power tube clipping aren't such great pals. It's a voltage thing fo shizzle. I'll play with reducing feedback some since this worked for me once a long time ago. The Dudes post reminded me. It was a little Mesa Subway amp. 410Vp and 380Vg2 on a pair of el84's. I reduced voltage and then to get some bottom end back I reduced feedback in the bottom end. Sort of like a fixed resonance control. So, possibly some tweaking and a different speaker. But there's still the more dynamic feel the amp had and I'd like to get a little of it back. I don't know if there's a magic spot with this amp and these tubes but I'm already rigged for an easy increase of 13V B+. So no harm in checking.

[DRH1958]

I have enough adjustment with the bass control to get my bottom end for clean tones. It's when the power tubes are clipping that I'd like to get my bass back. Not just more low end either, but some punch. IMHE increasing bass in the preamp and power tube clipping aren't such great pals. It's a voltage thing fo shizzle. I'll play with reducing feedback some since this worked for me once a long time ago. The Dudes post reminded me. It was a little Mesa Subway amp. 410Vp and 380Vg2 on a pair of el84's. I reduced voltage and then to get some bottom end back I reduced feedback in the bottom end. Sort of like a fixed resonance control. So, possibly some tweaking and a different speaker. But there's still the more dynamic feel the amp had and I'd like to get a little of it back. I don't know if there's a magic spot with this amp and these tubes but I'm already rigged for an easy increase of 13V B+. So no harm in checking.

I suppose it also comes down to the overall design of the amp. I re-build Hammond organ amps for guitar among other things, and increasing the coupling caps in the preamp gives it punch as well as low end. Plenty of low end but still cuts through a mix well. No flabby,muddy bass even when dimed for both pre and power amp vol. Just well defined response. It has a unique front end design that is different than any other amp I've seen so this could be the difference between your experience and mine. I very rarely touch the controls on the amp. It's almost all done with the guitar tone and volume. I just have a simple tone control on the amp for less insertion loss. And they are very generous on the size of the OT. It is the same size as some newer trannies designed for 25 watt output.

[Dave H]

could you measure the ripple both with and without the zener? When the zener begins to conduct, it effectively lowers the resistance of the parallel resistor/diode combination moving the filter on that "node" closer to the first "node". I'm wondering if maybe the resultant increase in B+ is simply due to slightly better filtering of the supply overall.

I'd measure the real thing but my amp has a bridge rectifer and resistor. Here's a simulation of a two diode full wave rectifier. Green is the output of the rectifier, blue is at the capacitor. The first one is with a 150R between the rectifier and capacitor. The second with the 150R and a parallel 12V zener. You can see that the second green trace would measure as a higher voltage on a DC voltmeter.

[Chuck H]

I suppose it also comes down to the overall design of the amp. I re-build Hammond organ amps for guitar among other things, and increasing the coupling caps in the preamp gives it punch as well as low end. Plenty of low end but still cuts through a mix well. No flabby,muddy bass even when dimed for both pre and power amp vol. Just well defined response. It has a unique front end design that is different than any other amp I've seen so this could be the difference between your experience and mine. I very rarely touch the controls on the amp. It's almost all done with the guitar tone and volume. I just have a simple tone control on the amp for less insertion loss. And they are very generous on the size of the OT. It is the same size as some newer trannies designed for 25 watt output.

I do the same with this amp. Set it one way and use the guitar volume control. I do also change the amp setting to idealize for certain tones when I want them "just so". It has about as much gain as something like a TW Express/Liverpool or the modded Fender Mesa mark designs, so the bottom end voicing when cranked is pretty much all in the power amp since there's as much bass going through the preamp as the amp can handle without getting mushy. Since I'm using the same speaker and OT, and I bumped the preamp voltages to be the same as they were at the higher Vp, I have to attribute the missing bass to the power tube operating conditions. I am using .022uf coupling caps now and for my circuit I'm pretty sure increasing preamp bass will mush up the bass response. But I appreciate anyone's experience.

[The Dude]

I'd measure the real thing but my amp has a bridge rectifer and resistor. Here's a simulation of a two diode full wave rectifier. Green is the output of the rectifier, blue is at the capacitor. The first one is with a 150R between the rectifier and capacitor. The second with the 150R and a parallel 12V zener. You can see that the second green trace would measure as a higher voltage on a DC voltmeter.

[ATTACH=CONFIG]34536[/ATTACH]

[ATTACH=CONFIG]34537[/ATTACH]

Thanks for the simulation, Dave. That's what I was getting at, although maybe not explained well. I think that's what Chuck was seeing in his real world tests- a change in filtering because of the zener being turned on and there being less effective resistance before the filter cap.

[Chuck H]

Not trying to hijack the thread, but...

This is a little embarrassing. I have particulars with my current living conditions that dictate when I can play loud and when I can't (somehow I thought I'd have that resolved at this point in my life ). When I tested my amp after lowering the voltage, etc. I had it in a different room than I usually play in. My amp is a 1x12 at 15ish watts, so... Complicating the issue further is that the amp I've had on the bench right before this project was a 2x12 60 watter. Well...

I played the amp today with fresher ears in it's more usual space and it sounded really good. The bottom end isn't quite as tight as it was, but it's all there. The lower voltage on the plates seems to have squeezed things into the mids a little. The glorious treble ring and hang remains. I think all this amp needs is a new speaker to suit the mod. It has a V30 now (which was a bitch to break in) but I'm thinking a G12H30 is the way to go as it is now. I expect it to sound so good that I'll cry a little.

[DrGonz78]

I have put together a preliminary schematic to the EL84 amp design. I am not really sure what this circuit will sound like and if it will even work. Got to start somewhere on this so here it goes... All constructive critique is welcome.

Note that at the last minute I added the Gain pot and extra tube stage. Originally, I had the first preamp stage V1A followed by tone stack then another V1B preamp stage to master volume and then to PI. I added gain pot but not sure if it was a good idea or not. The sag voltages are what I created within PSU2 designer to give an idea what they might be at estimated max current draw.

[The Dude]

Looks good at a glance Dr! (I, of course, may have missed something.) Diggin' the moniker. There's inevitably some tweaking involved along the way, but it looks like a good start. I can't wait to hear and see your creation. Of course, you'll be posting pics and sound samples?

[DRH1958]

There are two things that I see as "maybe to be addressed" items. 1.The slope resistor of the tone stack might be a little low at 33k. Most plate driven stacks have around 100k for this value so the stack's input impedance works better with output impedance of the previous stage which for a 12AX7 is around 38-40k I believe. I think the 33k is usually the value when driven by a cathode follower. 2. The.0047 coupling cap before the tone stack might be a little shy on bass. Maybe a .047 is what you meant? If you ran the sim and these were OK, then disregard. I only go by what I have seen on most amps. One last thing only if you want a little stronger signal on the EL84s. With cathode bias, you can make your 220k grid leaks larger. This will load the output of the LTP less. Otherwise, it looks good!

[DrGonz78]

There are two things that I see as "maybe to be addressed" items. 1.The slope resistor of the tone stack might be a little low at 33k. Most plate driven stacks have around 100k for this value so the stack's input impedance works better with output impedance of the previous stage which for a 12AX7 is around 38-40k I believe. I think the 33k is usually the value when driven by a cathode follower. 2. The.0047 coupling cap before the tone stack might be a little shy on bass. Maybe a .047 is what you meant? If you ran the sim and these were OK, then disregard. I only go by what I have seen on most amps. One last thing only if you want a little stronger signal on the EL84s. With cathode bias, you can make your 220k grid leaks larger. This will load the output of the LTP less. Otherwise, it looks good!

Okay I took your advice there DRH. I went back and reworked the tone stack completely, better now. Yeah I fixed the error and put that coupling cap to .047uf. For the grid leaks I upped them to 330k and I have even seen a heathkit AA-161 with 470k grid leaks. That heathkit is hifi type amp though, but it used 6BQ5's with very high plate voltage. I have to admit that I am not sure about the cathode resistor and cap on the EL84's. It will depend on the tubes I guess.

I have tried to simulate tube amps with LTspice but still a learning curve with that program. When I use LTspice for basic solid state circuits it makes more sense to me. I have downloaded a bunch of files to use like hammond transformers and just can't get it to work correctly. I can get the Fender Bassman circuit from spice yahoo groups to run and I have studied it a bit. All of what I have done on this schematic is just hours and hours of research applied. I am still not completely happy with my understanding of what I have drawn, but it takes time to figure it all out. Thanks for the feedback!

Looks good at a glance Dr! (I, of course, may have missed something.) Diggin' the moniker. There's inevitably some tweaking involved along the way, but it looks like a good start. I can't wait to hear and see your creation. Of course, you'll be posting pics and sound samples?

Thanks Dude! Oh yeah I will be going slow on this thing and documenting the entire process. My amp will be very home brew style and I will even build a head cabinet for it too. I will do some sound samples too. Hopefully you guy's don't mind a long integrated thread? The way I see it is that other people, who are learning like I am, will no doubt find some good use in this type of thread.

[Chuck H]

Just from my own preferences and experience. Use salt liberally...

It looks like this is intended to be capable of some heavy grinding. I think one of two things could be changed about the tone stack. Being after the second stage it will be obliterated by the clipping on the third stage. In this light I think the bass is too deep and the mid scoop is much too deep. IMHE this won't make for good clipping. The TS should be voiced to set up the signal for clipping OR moved to after the third stage to act more as an EQ for the already clipped signal. As it is I don't think you'll get much satisfaction on either goal.

On a similar note I notice that you're only using five stages. That leaves one unused just hangin' around. If you're going to grind, you might as well use it. Perhaps add a cathode follower (to feed the tone stack) or a parallel input stage. Both have benefits and you already have the triode at your disposal.

The preamp has a lot of LF. More than the circuit will be able to reproduce even clean. Probably nothing you need to change now, but you may consider tweaking the preamp cathode bypass cap values and get the parts to do that for after it's built. .68uf, 1uf and 2.2uf.

The way the HV rail is designed you have in phase stages decoupling on the same capacitor. This is a bad idea as evidenced by the Fender HR and Blues series amps that often have problems with oscillations unless a premium cap is in service and never allowed to age much. Save yourself the hassle and separate the nodes by adding a filter and keeping only anti phase stages sharing decoupling.

Two less important notes. The .047uf cap feeding the TS isn't necessary. I know it feels better to decouple prior to a circuit, but it doesn't do anything for you in this case.

You might consider adding a 100p bright cap to the gain and volume controls or they really will dull out the tone at lower settings.

The third stage is pretty much center biased and has a fully bypassed cathode resistor. You have a volume control after, but no fixed voltage division. The LTP PI being fed by this stage will have very generous output. Way more than you could ever need for driving the piss out of the output tube grids. At higher settings this will definitely cause grid loading and, as with many EL84 amps, buzzy crossover distortion. Maybe use the third stage unbypassed. Maybe even make it a cold biased stage. Trainwrecks and some Marshalls use a 10k unbypassed cathode here. Experimenting with the actual value is encouraged in the tweaking process.

It might help to know what sort of tone goal you have and what sort of cabinet you'll be using.

[DRH1958]

I would keep the input cathode bypass cap the same. It seems you want to fully bypass this stage so as to not filter out low frequencies. Otherwise, they are lost right off the bat. Then experiment with partially bypassing the second and third stage using different values.
I like Chuck's idea of decoupling the first preamp stage. I'm assuming that's where he is referring to with modifying the B+ rail. It and the 3rd stage are in phase but on the same cap, although I haven't heard a lot about this being a problem.
Also agree with moving the tone stack after the 3rd stage. With it in the current position, the 3rd stage will be closer to square wave so will make the tone stack less effective at higher gain conditions.
I would use the extra triode for a second input as there is no need for more gain or a cathode follower. You could have a hi/low or norm/bright input scheme.
I like the idea of decoupling the tone stack with that .047 cap so as to keep the high voltage DC off of it right away. This is the value of it, although it will work both ways. And this allows for lower voltage and cheaper(cost-wise) caps for the stack since you have blocked the HV DC.
I like to use a decade resistor/cap box for tweaking values after initial build. Use higher values of resistors that makes sense and then you just parallel different values with the box to lower them for the right sound. Or use lower value caps and then parallel other ones to raise the value to get your sound. Just temporarily tack them in place at first. Then solder in the value you arrived at for the final build. A perfect place for doing this is the grid leaks for the EL84s. If Chuck's assessment of "driving the piss" out of them is correct, parallel them for lower value to load the LTP output some more maybe back to 220k. Remember, it was only an option,not a recommendation. Heck, the data sheet says these can be all the way up to 1Meg(!) with cathode bias. You could also use a 250k audio taper pot for your master vol to attenuate the signal to the LTP if there is too much drive in this stage.
You could also use the decade box to experiment with the NFB resistor value. 100k might be too high a value. More NFB will tame an unruly amp too.
Decisions, decisions!

[DrGonz78]

Okay I am going to rework my schematic to include all the different suggestions given in the last two posts. Both posts are great btw!

I hope I understand this correctly... The bypass capacitor will change the low frequency response of each preamp tube? 2.2uf will have more bass response and I will reduce the uf down to even .68uf to reduce low end response on that gain stage. Is that even close?

Also, I did put that 0.047uf coupling cap before the tone stack, as DRH had stated, as a way to reduce cost on the other capacitors. Looking at it closer the cost of caps is not that much and one less component to add might save space. So I will probably just ditch it altogether.

I understand better about that having, especially the first, triode stage on it's own coupled node. I remember reading about the Fender Hot Rod amps where that last filter cap always caused problems. Good call Chuck!

My tone like I said is hard to pinpoint. If you read another thread I just posted you'll realize I am having a constant tinnitus condition and can't listen or play much music these days. Oh well though this amp project might get me playing some more regardless of the 9khz tone generator in my head, lol. I have a cab here that has a Red Coat Governor speaker. It might sound okay but it won't be the greatest sounding speaker. Describing tone is the hardest thing I can imagine having to type. Something that can play blues rock or even jazz at lower volumes. Clean at lower volume and higher volume break up that does not turn into mud. Good tight punchy low end response without being too bright. I have PRS, 1986 Ibanez Roadstar and Telecaster as my types of guitars. I would like a friend of mine to be able plug in a Gibson ES335 and get a nice sound out of that too. So versatile is the tone I am looking for at the end of the day.

Mainly I want an amp with choices to modify the tone in several stages. I will keep working on the tone stack and I will be placing it after a cathode follower stage at the end of the preamp. From what I know is that a cathode follower stage will have a lower impedance and a less attenuated signal across the tone stack as a result. Sounds great! I am still stuck on the idea that I want two tone stacks. A simpler one between V1a and V1b with the gain pot control. Perhaps that's the bright switch you guys have recommended too. I agree also that if I have that extra triode stage then why not just use it, right?

I will update the schematic with some more changes soon.

[Chuck H]

I don't think you need to change the preamp cathode bypass caps for the design stage. I just think you should have the smaller values on hand for the final voicing of the amp.

Since you like a punchier, thicker sound (but not muddy) you look to be on the right track with the design already posted. Just maybe a little too far that direction. There's a point of diminishing returns WRT to pushing lower mids through a 2xel84 guitar amp I've been there. Since you'll be playing jazz through it sometimes I think the bright switch is going to be a necessity. On my own 2xel84 amp I need to trim the treble way down and use the bright switch to get something resembling a jazz tone. The frequencies enhanced by most reasonable values of tone stack treble cap combined with the standard el84 character seem to keep things in the upper mids. Trimming the treble down and using a bright switch gives me that more balanced low/mid character with a little kiss of top end and just sounds "sweeter". If that makes any sense. Considering your tinnitus, you may want to be able to trim the volume. Since, IMHO, building with el84's is pointless if you're not going to be able to clip them you could include power scaling or an attenuator in the design. A power scaling circuit would also eliminate the need to drop voltage with zeners and could be adjusted for a max voltage that would give you the same result. As well as any voltage below that and notoriously compatible with cathode biased el84 amps. I use a simple attenuator circuit that involves a couple of aluminum housed resistors and a fat rheostat. I've incorporated them into overall designs and retrofitted them to chassis mount on finished amps. Either volume reduction circuit would allow you to clip the power tubes and get you down to a volume level that won't cause as much fatigue.