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My First Amplifier Build!! 6L6/6P3s Push Pull Stereo

zastin17

zastin17

Active Member
This is my very first tube build ever. And I can't believe I did it!!

I'm only 22 years old, so I'm very young and new to this Tube design thing.

I have learned so mutch and now I'm addicted....

Now I can say without a doubt in my mind that my scratch built 6L6 Push Pull Stereo Tube Amplifier is finally complete with proper voltages and all!! All recycled parts. Stuff I found in parts boxes, ect. I paid nothing for the sound this amp puts out!

White steel Chassis was from head crashed 1980s hard drive;) I drilled out by hand using just a hand drill and lots of time. As I have no punches...... yet:) but It kind of ended up with a "Utilitarian Vibe" which I really like.

Wiring is never final. Odds are I will never stop fiddling with this amp.

Running perfectly now at a proper 450VDC on B+. 6L6/6P3s at 45Ma! And don't you go and say I'm exceeded the 6P3s tube specs. Most of you well know the 6P3s are touph as nails and perform well beyond listed specs. These ones I ran on my ST-70 for many months nonstop biased as if they where El34s.

Theoretically I could use the entirety of all my unused taps. And get B+ past 700V or even more if I ever needed too.

EL34s bias up and run great!! Look great!!
But I feel they don't sound quiet right with these little output trannies. Ile stick with 6L6 variants on the 6.8Kohm OPTs. And leave the EL34s too my Dynaco.

I can always swap OPT later I guess. They are very undersized anyhow and have 1x 8ohm tap. Bass could go a little deeper I think too. A slight LF rolloff for sure. But I'm also partially blaming my selection I used for those tiny little .1uf coupling caps. I probably could go up too .47uf as I'm at .1uf.

Everything else is glorious!!! Low mids/Mids are liquid gold!! The highs are actually mutch better than my early and stock Dynaco ST-70.

Very extensively detailed high end. Mabye due to 12AT7 variant Drivers, I'm not sure. But theirs a slight rolloff on my Dynaco in comparison using 6U8/7199 variants Compared to my DIY amp.

I also tried 6SN7 with an adapter. I have 2x entire Nos RCA Sleeves of short 6SN7 and Long 6SN7 I've never heared before. But it turns out 6SN7 sounds mutch better than 12AU7s in my opinion in this postion.

Something else ove noticed is how changing the driver tubes changes Feedback intensity on this amp as the resistor does 2x different things. A good way to "tune" the sound by changing tubes I guess.

I'm not upgrading that Dynaco to the VTA so don't even mention it. It's a relic of time! Even with the slight high Fr rolloff. I enjoy it so mutch as is anyhow.

Anyway...
Their are 3x Total power transformers in it to get what I needed for current and voltages. Haha. But it's dead silent, I can't believe it! And

I have so many unused taps I could implement later on! I could get over 700V if I ever needed too. Many unused Fillament and HV taps.

Mabye a power light. Or tube VU meter. Idk yet.... any ideas? Also, I have no idea what I'm going to use as a bottom/side cover....

I have no way to accurately measure the output power. But if I had to guess. 20-25W per channel.
Lots of tools I will deffinitly be needing which will make my tube journey mutch easier!!

Song is "Chasing Cars By Snow Patrol". And driver tubes are RCA 12AT7s Black Plates Military.

I based my drivers and output stage on this schematic. But everything but the driver stage has been heavily changed for my own needs.

the Scheme for the drivers, feedback/bias is super weird. It's like that feedback resistor is doing double duty!! Intresing, but it works!!! http://diyaudioprojects.com/Schemat...-6V6-6AQ5-Dynaco-A-410-Tube-Amp-Schematic.htm
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Sounding good, I like the look. A piece of wood on each end and some expanded metal on the bottom?
 
Nice!
I love to see the Can Do spirit.
If you can build a tube amp from scratch as you have done, there's not much in this world that you cant do. Very nice workmanship, and your next one will be even better!
Bravo!
 
Nice job! The best amps are the ones that were built from nothing and sound good.
I'm also a fan of the 6p3s tubes. I have quite and few and they sound really nice for the money.
Keep on building.
 
Actually, if 45ma is the cathode current- you are within dissipation spec, as listed in the tube data for that tube.

45ma cathode current, will break down to something like 40ma on the plate and 5ma on the screen.

450V B+ will break down to something like 25V on the cathode resistor, and 425V between plate and cathode.

Multiply 40ma times 425V- and you get 17 watts. No problem!

Regards,
Gordon.
 
GordonW said:
Actually, if 45ma is the cathode current- you are within dissipation spec, as listed in the tube data for that tube.

45ma cathode current, will break down to something like 40ma on the plate and 5ma on the screen.

450V B+ will break down to something like 25V on the cathode resistor, and 425V between plate and cathode.

Multiply 40ma times 425V- and you get 17 watts. No problem!

Regards,
Gordon.
Click to expand...

Well thank you;) That's some new stuff I learned today!
 
gadget73 said:
@Plimpington2 has an amp built to this same schematic I believe.

https://audiokarma.org/forums/index.php?threads/troubleshooting-my-6v6-build.944229/
Click to expand...

That’s right. The base is the Dynaco 410 schematic he shows adapted to run 6SL7’s for the paraphase front end. Since then it’s evolved to model DG’s Maggie 8800 improved circuit on the front (basically includes an AC balance control and a few tweaks to the cathode array). Also, my salvaged OPT’s don’t have UL taps, so I’m “pentode” connected.

This is a nice, functional circuit.
 
zastin17 said:
Their are 3x Total power transformers in it to get what I needed for current and voltages. Haha. But it's dead silent, I can't believe it! And

I have so many unused taps I could implement later on! I could get over 700V if I ever needed too. Many unused Fillament and HV taps.
Click to expand...

Congratulations! You might just insulate those spare taps that look as if they are just cut off on one of the transformers under the chassis. Just for safety.
 
Dandy said:
Congratulations! You might just insulate those spare taps that look as if they are just cut off on one of the transformers under the chassis. Just for safety.
Click to expand...
Plimpington2 said:
That’s right. The base is the Dynaco 410 schematic he shows adapted to run 6SL7’s for the paraphase front end. Since then it’s evolved to model DG’s Maggie 8800 improved circuit on the front (basically includes an AC balance control and a few tweaks to the cathode array). Also, my salvaged OPT’s don’t have UL taps, so I’m “pentode” connected.

This is a nice, functional circuit.
Click to expand...
Yes, I have no UL either so I did the same on these repurposed OPT.

And yes, I took those pics as I was super exited that everything was working, so it was a mess!

I am continuing to clean things up. I snipped and heat shrunk all the unused taps and tied them away. I am waiting for a choke, which I may or may not even use. Some new solder boards to redo the entire power supply with Newer Capacitors with properly high voltage ratings that don't push my luck.

This driver setup works. But I don't like how difficult it makes it to change things. And the shared feedback Resistor is weird to wrap my head around.

Changing to anything but a 12AX7 or 12AT7 causes a small buzz and changes feedback intensity, and probably bias.

What components corresponds to the noise floor on the Driver stage on this schematic if anybody knows? I would like to experiment with 12AU7s and 6SN7s, but besides the 100k resistors for the gain, i'm not sure what else to change to get rid of the tiny buzzing with different tubes.

I notice when the input tubes heat up, they start to buzz, and as they heat up, the buzz drops too far below audible levels as emission comes up is how it kind of works "audably". They all buzz at first, and come up to be silent.

6SL7 I did try with an adapter and they worked fine with no buzz.

I am running Mullard Blackburn El34s right now to hear on my Amp to see how 6L6 compares to El34 on this amp and "smallish" OPTs. El34s are truly lovely on it!

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Messing with the bias vs the feedback is a balancing act between the 1k resistor off the speaker terminal and the 100 ohm that runs to the cap. That effectively forms a 10:1 AC voltage divider. If you change the 1k to a 1.5k and the 100 to a 150, the amount of voltage coming back to the cathode will remain the same but the tube will be operating at a different bias point. If you need to mess with the feedback level without changing the DC bias on the tube, mess with the 100 ohm resistor only. That will tweak the AC voltage but not the DC.

One trick with changing tube types, global feedback level is tied to open loop gain. If you change the 12ax7 for something with lower gain, you'll get less global feedback. Input sensitivity will remain about the same, but with less global feedback the distortion level is probably going to go up. Thats one of the reasons you commonly see high gain driver circuits in global feedback amps. it allows for a considerable level of feedback while still having reasonable input sensitivity. The other side of that coin is that the greater the feedback level, the greater chances of instability.


The other thing to know here, if you aren't using the same transformers this was designed around, that 0.001uf cap across the 1k feedback resistor is probably not going to be the right value. Really ought to run some square waves through it to see what sort of response its actually doing. You may get better results with a larger or smaller cap, or it may want some tweaks elsewhere in the circuit.

Also would be worth measuring the feedback level so you know what it is. To do that you'd need to unhook the 1k resistor from the speaker terminal and ground it. Set your signal generator up so you get say 3 volts output at the speaker terminals Note the exact level as V1. Connect the feedback resistor back to the speaker terminal and record that as V2. 20log(v1/v2) = db of feedback. It should be a negative number since the level will drop when the feedback is hooked up.
 
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