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

[The Dude]

Vodka, Kahlua, Cream.
Leave it to me to dumb down the thread.

[DrGonz78]

Now I want to name this amp "The Dude", but I bet I might get sued by someone. Hell I might just name it "The Dude" anyway.

Is that extra dot left of the third stage grid load just a blob (?) or place to add a resistor to make a divider should you choose to? Because that would be exactly what I was referring to above.

The little blob is a dot from the capacitor component and I thought I was putting in the 470k(just a starting value) as the grid load resistor. I guess I am a little confused at the moment as to what the grid load would be on that stage. Not the value but the placement of this resistor. Should it be in parallel to ground as I have it or should it be in series? Perhaps it should be one resistor in series with another one in parallel to ground? I need a better understanding there... Edit: I am pretty sure the grid load voltage divider is a grid load and a grid stopper resistor together. I was confused that I was just adding a grid load resistor and not getting the voltage divider part.

Now I am not against removing one gain stage if that will lead to better results. Besides I am only really adding the extra triode stage since it is there and it can be used. However, I need to understand better how to tame the beast and researching voltage divider circuits in relation to the grid load resister. Looking at my schematic there is V1-A and then there is the 250k gain pot. Does that 250k gain pot work as a grid load resistor in that sense? Also, note that on the Pignose schematic it has a 500k resistor in series with the 500k volume pot in that very first stage.

Don't sweat having a design similar to others. Especially when you "discovered" the design post facto.

Yeah it was strange when I was trying to see how some amps used the cathode follower stage and then stumbled onto the Pignose G40V. Not only did it shed some light on how to place that stage in my schematic, but it was very similar in other ways too.

Last little tidbit is about not needing to install a standby switch. From what I have learned about tube rectifier tube amps is that there is no need for the standby switch. What are peoples thoughts on this subject. Thanks.

Later Double Edit: I added updated version of schematic to see if what I added was on track. Regarding grid load voltage divider between V1b and V2a.

[Chuck H]

The grid of any cathode biased preamp tube should have a 0VDC reference of not more than 1M. That typically means a resistive path to ground. That path can go through a grid stop resistor or not to a straight up load or a voltage divider being derived from fixed resistors or a pot. As long as the grid can "see" ground (0VDC). That means there can't be a capacitor between this 0VDC reference and the grid because just like a capacitor blocks the high voltage DC it also blocks 0VDC because in electronics "0" can represent one end of a voltage differential.

And... A divider would be a resistor and a load in that order in the direction of the current flow. Because of the high impedance of a preamp tube grid, a grid "stopper" doesn't draw any significant current and there won't be any voltage dropped across that resistor as placed IN FRONT of the load. So, in case you do need to attenuate...

This is what I was talking about:

[DRH1958]

Many times when lower gain from a stage is called for, I just use lower value grid leaks to the next stage. Yes, a lower value value pot will lower the signal at the next stage.
I sometimes analyze things by looking at extremes. If you had a direct short to ground in the signal path, you would lose all of your signal. If you put no connection at all to ground, infinite resistance, you lose none. So if you have 1Meg, you lose less than if you use 250k.
Instead of running endless sims, I just buy a decade box. I then use what I would think of as the largest resistor to use in a certain place and just tack it on with little solder. Then all you have to do is hook up the box parallel to the resistor and decrease it until you get what value you like, then solder in that value. Same with caps, use the lowest value and parallel higher values on until you have increased to the value that sounds best and then solder in that value. It is very gratifying to hear the immediate result of the changes. This process eliminates endless soldering and re-soldering in the design process and I like the hands-on approach. I guess that would make me an old schooler, no?

[The Dude]

Now I want to name this amp "The Dude", but I bet I might get sued by someone. Hell I might just name it "The Dude" anyway.[/COLOR]

Don't get me wrong. I'd be honored to be associated with your creation. But, it is your creation and I think "The Dr Gonz" has a nice ring to it (or "Gonzo" if your into that whole brevity thing).

[The Dude]

How's your project going, Dr.?

[DrGonz78]

Slowed down on it while it has been so hot in the shop during the summer here. I was on the fence about moving to Los Angeles and went out there for a week at the start of August. The idea of moving pushed the project away from my focus the last couple of weeks. Then I got working on the amp two nights ago, and at the end got stung on the foot by a scorpion. So that does not sound as if it is really coming along nicely, but actually got some good stuff done.

Drilled some holes and installed the output transformer. Started to populate the board with the main power filter caps and dropping resistors. I am currently filling up the board with all the caps and resistors. I did a DIY Layout of the wiring diagram which is pretty cool. Took a picture of the amp and then used the DIY Layout program to draw in all the components.

If you look at my wiring diagram attached you will see only 5 zener diodes to drop some B+ voltage. I actually have 6 zeners connected up on the board and I can fit a 7th if needed. Would 7 zeners be too many or does it not matter? I hope I got the correct legs wired up for the pots.



Edit: My previous layout that I attached had an error with the bright cap on the Gain pot. Had it going to the wrong leg.

[DrGonz78]

Well I am getting this guy going along pretty nicely so far. Of course the chassis layout that I attached(last post) had some really dumb errors that I saw clearly as I was wiring stuff up. Learning a lot about the pros and cons to making everything fit together nicely.

I have a question regarding the filament supply and need some advice. I have a 6.3v 2.5A CT heater supply hooked up to everything now, but I am just not sure it will be enough. Three 12ax7's and two EL84 tubes is a total current draw of 2.42A on the filament supply. So the way I have it now will work but it is cutting it very close. I have a second non-center tapped 2A 6.3 winding that I can use too. I am starting to think that I should just go ahead and wire that one up for the preamp tubes. Do you think I should utilize this other heater supply? Or do you think the current draw will be fine the way I have it all wired up now? Any opinions are appreciated thanks.

[Enzo]

It is designed to provide 2.5A and you want to draw not quite 2.5A. I don't see the problem.

[Chuck H]

^^^^^^^^^^^^^^^^^What he said^^^^^^^^^^^^^^^^

The thing about that rating is that while it MAY be a max spec. for current limit, it probably isn't. It's almost certainly a spec for "the amount of current at which the winding provides 6.3V" and it's very likely that winding can provide a bit more current, though the voltage would drop below the 6.3V mark. I use to take that figure as a current limit and would over spec the winding. Now I have a bunch of amps around with high filament volts. Which, as it happens, is really bad for tube life above the stated 10% tolerance. What you have is an ideal scenario. No need for the auxiliary transformer.

[DrGonz78]

Good news! I had done some searches and read some things about recommendations on this matter. One poster was advising to double the current rating as per safety reasons. Lots of guess work I guess...? Anyway thanks for the quick responses

[eschertron]

Good news! I had done some searches and read some things about recommendations on this matter. One poster was advising to double the current rating as per safety reasons. Lots of guess work I guess...? Anyway thanks for the quick responses

There's no reason to double the ratings on the heaters. It's straightforward addition for the loads. As mentioned above

If you were to convert to DC heaters (for example), then there's additional work being done to dissipate amps through the rectifier, and some more math for the load adjustment would be required. I don't know if you're going for 6 stages of preamp gain (!!) but if you were, maybe the OTHER heater winding could be used to make DC for the first couple stages. Just speaking out of turn, I don't know what your design spec is for this amp.

[DrGonz78]

So I am almost finished wiring up everything on this guy! This is my first amp where it was not a kit and the only other one I built was an 5F1 Champ clone. When I did the Champ I pieced it out and purchased all the parts at different times so it was not truly a kit. This one is definitely a home brew amp. I am happy so far with what I have made and of course there is always room for improvements. You learn more by doing than anything else and the next amp I make will benefit greatly from this experience. Overall it has turned into a decent layout and I will begin testing it very soon.

I do have one quick question though. When I hook up the negative feedback to the secondary of the output transformer which tap do I connect it to, 4-8-16ohm tap?

I will post more about the amp as I start to test and eventually the audio samples. Here are some pictures to see it all. I have to still wire up the input jacks a couple more items, but it is close to finished.

[trobbins]

I appreciate the schematic shows PT secondary voltages, but it seems to me there is still doubt on what the actual loaded B+ will end up being, and the sanity check is only going to happen at the end of the build. If you were lucky enough to have a low mains voltage, then you could possibly end up not needing voltage dropping devices, given the cathode bias. Sometimes a quick pre-design check of actual PT output voltages, especially heater voltage, can reduce any uncertainty on what voltages the final amp will operate with.

It can be easier to tune B+ using another RC filter stage, and add some ripple reduction benefit, or alternatively use a 5Y3GT with additional winding resistors and again reap some benefit with softer ripple waveform.

[DRH1958]

I do have one quick question though. When I hook up the negative feedback to the secondary of the output transformer which tap do I connect it to, 4-8-16ohm tap?

You can hook it up to any tap you want to. Each tap will give you different levels of FB, the 4 ohm being the lowest. Play around with it and see what tap and resistor values sound to your liking. I suppose try the 8 ohm first as it's in the middle but there are no rules in this department. The Bassman uses a different tap than the original Marshall JTM45 which was based on the Bassman. Marshall used the 16 ohm tap, so they got more FB . IIRC, each higher tap will give you 1.4 times the voltage of the lower one.