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 Post subject: 6336 SET amp - Project
PostPosted: 31 Oct 2014, 09:22 
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Joined: 15 Nov 2013, 10:16
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Location: MB, Canada
I thought I'd post a thread to show the latest project I've put together.

A while back Matt gave me a schematic for a 12ax7 / 6336 based amp he thought might make a nice amp.
I have been accumulating parts over the past while to build this, and along the way decided I really wanted to make use of the 6SN7 as I have a number of 6SN7 tubes and haven't used them before.

At one point I read through Mark's 300B amp build project and noticed that the 300B as configured in that project had almost identical requirements for voltage swing as the 6336 in my configuration, so I stole that design from his 300B project and kind of made a mash of the two schematics. In the end I settled on the schematic attached (measured voltages included in red). As with the 300B project an active preamp is required because the 6SN7 doesn't have enough amplification to provide enough swing with a standard line level input. I built a 4S Universal preamp that will provide more than enough amplification with a 12AU7 (unbypassed cathode).

The amp uses Edcor transformers in the power supply, and Transcendar 15W output transformers.

As for tubes, the amp uses a GZ34 rectifier, which is partially because of the long warm up time. The 6336 datasheet specifies that it should have heater voltage applied for 30 seconds before applying B+. The GZ34 doesn't take a full 30 seconds to get B+ to operating voltage, but I think it's probably long enough to keep the 6336 happy.
I'm actually using 6336A tubes, as those are primarily what I have, I believe they're a slightly ruggedized version of the 6336 with graphite plates.

So the amp is built now. I used some Motor run capacitors for the 100uF ones in the power supply, and all the rest (power supply & bypass) are film. A few Solen's, as well as some Axon's, and a big Arcotronics that Gio gave me :).

Initially my voltages were a bit high, so I settled on a 60 ohm resistor before the choke to bring the voltage down a little bit. Also, the first 6336A I used seemed to have a problem, in that one triode was drawing WAYYYY more current than the other, so much so that one of the 330 ohm resistors in the power supply feeding that triode got extremely hot.. I think it's still okay, as I noticed that pretty quickly and powered the amp off. I have heard that it can be a little hard to find a good 6336. I have about 6 of them though, and did end up finding one that has almost perfectly matched triodes, all measurements within 1 volt of eachother.

Anyways, I know it's not the prettiest amp, (I still have to get the labels off of the output transformers and motor run caps, and yes the holes I cut for the motor run caps are a bit crude) I seem to be unable to build something that looks really pretty, but I think it looks good in it's own way :). And I know that all of the construction techniques are solid. I put a piece of angle aluminum running the width of the amp where the majority of weight from transformers is, and it keeps the chassis very stable. I haven't had the chance to really wind it up and see how it sounds at loud volumes, but so far I'm very happy with the way it sounds. One thing I have noticed compared to my 6AS7 SET build is that this amp seems to be a little stronger in the low frequency department. I'm not sure if that's partially due to the big output transformers I'm using, or just the way the amp is configured, but I like it.

I let it run for about 2 hours last night, and took final measurements. The voltages listed on the schematic are the voltages from that measurement. As for heat, the power transformer got warm after 2 hours, but not nearly as warm as the Hammond in my 6AS7 amp. The output transformers and choke were cool to the touch after 2 hours. The 6336 is BLAZINGLY hot after 2 hours.. Like still too hot to touch 10 minutes after being powered down. I used a ceramic socket for that tube as I had expected it to be a very hot tube. I also didn't use any sort of attenuation on the front end of the amp as it requires an active preamp anyways, and that just simplifies the power amp.

Anyways, further sound testing to come, but so far so good. This thing is a BEAST though, I think it's up near 30 lbs.


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PostPosted: 31 Oct 2014, 14:05 
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I had a chance to crank this baby up at lunch today. I am extremely impressed with the bass response, volume, and clarity out of this thing using my Universal 4S preamp, and Fostex 206e bass reflex speakers. I'm actually concerned for the safety of the drivers when it's cranked up.. how much excursion is okay on Fostex 206e drivers? I'll put a video up of how much they're moving now.

I don't think anyone will EVER guess that this thing maxes out at 7-8W per channel.


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PostPosted: 31 Oct 2014, 14:14 
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http://youtu.be/6P4BidT2soo Video of the amp in action, and driver excursion.
I can still bump the volume up significantly but was concerned to.. I remember reading about max excursion on the 206e of a few mm..


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PostPosted: 31 Oct 2014, 17:34 
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Location: Bulgaria - EU
Why 20 kohm in grid of 6336?

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PostPosted: 31 Oct 2014, 19:29 
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azazello wrote:
Why 20 kohm in grid of 6336?


You know what.. That's a good question. I adapted it from the 300B project that Matt and Mark worked on on this forum. I could probably lower that though hey?

Also, someone else just pointed out to me that the B+ should be taken directly from the cathode of the GZ34 since it's indirectly heated.. I should probably change that.


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PostPosted: 31 Oct 2014, 20:00 
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Don't let people talk you into changing things without good reason.

The input capacitance of the 6336 stage is 68.3µµf. With the 6SN7 output impedance, the 270kΩ grid leak and the 20kΩ grid stopper, this places the high frequency rolloff at 80kHz. However it also tames the bias excursions by taking the excursion time constant from <1mS to 2.8mS. This is a reasonable solution to control the excursions. In fact, I might even be tempted to push the grid-stopper higher. But if the amp is working well, don't change it.

Also, the 5AR4/GZ34 family is cathode/heater tied. There is nothing wrong with using the 5V center tap for the B+ and it will preserve symmetry if you do decide to go to a filamentary cathode rectifier later (say like a 5V3 if you wanted to drop the B+ a little).

Some people just aren't happy if there not picking nits! Just enjoy your amplifier.

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PostPosted: 31 Oct 2014, 20:15 
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Suncalc wrote:
Don't let people talk you into changing things without good reason.

The input capacitance of the 6336 stage is 68.3µµf. With the 6SN7 output impedance, the 270kΩ grid leak and the 20kΩ grid stopper, this places the high frequency rolloff at 80kHz. However it also tames the bias excursions by taking the excursion time constant from <1mS to 2.8mS. This is a reasonable solution to control the excursions. In fact, I might even be tempted to push the grid-stopper higher. But if the amp is working well, don't change it.

Also, the 5AR4/GZ34 family is cathode/heater tied. There is nothing wrong with using the 5V center tap for the B+ and it will preserve symmetry if you do decide to go to a filamentary cathode rectifier later (say like a 5V3 if you wanted to drop the B+ a little).

Some people just aren't happy if there not picking nits! Just enjoy your amplifier.


:D Well said. Since I'm in a tinkery mood though, I did Swap B+ feed directly the cathode.. It's easy enough to change if I try a different rectifier.. I actually am a little bit high on the B+ though and had to use that 60 ohm resistor to bring things down. I tried a 5U4GB without the 60 ohm resistor and it still dropped the B+ too low though. I hadn't considered the 5V3 rectifier though. Ideally it would be nice to do away with the 60 ohm resistor, I should look into that rectifier. I'm about 10V too high with the GZ34.

The datasheet for the 5V3 says it drops 42V @ 350mA draw.. So I guess about 24V drop @ 200mA. With the GZ34 I'm at about 15V drop @ 200mA.. So that actually might put me right in the ballpark I need. I have a lot of rectifiers.. But not a 5V3 unfortunately.


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PostPosted: 31 Oct 2014, 20:31 
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Don't sweat 10V. There's margin in the design and that won't make enough difference about which to worry!

As for the 6336 getting hot, consider this. The filiment is is 31.5W and each plate is disipating about 24W. This means that you're putting out 79.5W in heat. Think about how hot a 75W incandecent light bulb gets. But don't worry, the 6336 is designed to take it. Maximum rated bulb temperature is 250˚C (462˚F). That's HOT :hot:

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PostPosted: 31 Oct 2014, 21:24 
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Suncalc wrote:
Don't sweat 10V. There's margin in the design and that won't make enough difference about which to worry!

As for the 6336 getting hot, consider this. The filiment is is 31.5W and each plate is disipating about 24W. This means that you're putting out 79.5W in heat. Think about how hot a 75W incandecent light bulb gets. But don't worry, the 6336 is designed to take it. Maximum rated bulb temperature is 250˚C (462˚F). That's HOT :hot:


Well I'm actually more like 20V high.. I just used a 60 ohm resistor to bring it down by 12 volts.. So if the 5V3 did the same thing then that might be nicer.. Although I don't think there's really any issue with keeping the resistor in there... Or is there? Would it be an issue operationally having my B+ 20V higher than expected the way this amp is configured?

As for heat, I kind of expected it.. But didn't realize just how hot that actually was! lol


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PostPosted: 01 Nov 2014, 17:21 
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Okay.. I need some advice on something.

I pulled the 60 ohm resistor out before the choke (shown in the schematic). With the 6336A that I'm currently running, If I stick with the GZ34 I'm about 25-30V higher than I had expected from the original schematic. About 330V on the 6336A and the 6SN7.

If I stick a 5U4GB in, I actually am pretty much right where I had originally planned. 300V on the 6336A and 6SN7.

I'd just stick with the 5U4GB, except my first filter cap is actually a 22uF and 30uF cap in parallel.. So 52uF. From what I understand this is a bit high for the 5U4GB.
Is this unacceptable for the 5U4GB?

So my options as I see them:
1) I could run with the 5U4GB and 52uF capacitance after the rectifier. If that is indeed safe enough for the 5U4GB.
2) I could run with the 5U4GB, take out the 22uF cap and just have 30uF after the rectifier if that would still provide enough filtering.
3) I could run with the GZ34 and have about 30V higher than I had expected originally. I'm fine with this if my operating conditions will still be fine like this.

What's my best option here?
I'd rather not order any new capacitors, and since I went to the work of getting film caps everywhere, I'd really rather not get another capacitor. I can change resistance values if there are some more appropriate for a given case.

I've listened to options 1 and 3 now.. To be totally honest they sounded very similar. The idea I like best is going with option 3. Keep the gz34 in there, and run with the higher voltage. If I do that though I guess my follow up question is, would it be advantageous to change any of the values on either the 6SN7 or 6336 to better facilitate that higher voltage of about 330V. The original values were chosen based on about 300V.


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