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Old 12th October 2010, 04:14 PM   #1
dwhitf is offline dwhitf  United States
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Default OTL designed by Tim Mellow with 4 6C33C?

I have decided to build the OTL designed by Tim Mellow, which appeared in the February issue of Audio Express. I am considering using 4 6C33C's instead of 2 outputs for more power.
Do you think the driver section will be able to push 4 output tubes? I Would like to hear opinions.
I am planning 2 mono blocks.
Thanks in advance!
dwhitf
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Old 14th October 2010, 02:13 AM   #2
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Quote:
Originally Posted by dwhitf View Post
I have decided to build the OTL designed by Tim Mellow, which appeared in the February issue of Audio Express. I am considering using 4 6C33C's instead of 2 outputs for more power.
Do you think the driver section will be able to push 4 output tubes? I Would like to hear opinions.
I am planning 2 mono blocks.
Thanks in advance!
dwhitf
I am doing the same thing right now. Did you finish yours?
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Old 14th October 2010, 04:24 AM   #3
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I'm considering the same, using all Russian tubes (6N2P-EV in place of the ECC83; 6J32P for the EF86), and am also wondering about adding a second pair of 6C33C-B's. I was thinking about designing my own, but I don't have the EE chops - and Mellow's schematics look fairly straightforward.
I don't know that I'd use that small a chassis, though - those 6C33s are REALLY close to each other... I'll probably go with monoblocks, also.

PDF of article from audioXpress:
http://diyaudioprojects.com/Forum/do...le.php?id=2101
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Old 14th October 2010, 04:34 AM   #4
djn is offline djn  United States
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I am collecting parts for that build as well. I am very happy to see this thread start. Maybe we can be support to eachother.

Are you saying you want to use 4 tubes per side?

Yes, the chassis Tim used is way too small. I have an artsy looking chassis that I am using. It is two levels and I will put the tubes on top and the power on the bottom. Those tubes are known for getting pretty hot.

I was going to follow Tim's scheem to the T, but I would like to do tube rectification. Has anyone thought about that or worked it out?
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Old 14th October 2010, 04:53 AM   #5
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I was thinking of a hybrid bridge, using a pair of 6D22S - I like the delay and power ramp-up they provide. I was trying to design a PS using PSUII, but that doesn't allow hybrid bridges (or, more than likely, I don't know how to set the program up for them...). I've got the tubes, sockets, and caps, so I'd like to use them.

djn - yes, 4 6c33s per side. I'm hoping for a little more power, and less stress on the tubes. 6C33s may be fairly cheap, but I don't have a lot of cash to spend on this - I would like the tubes to last a while...

Last edited by Guiseppe; 14th October 2010 at 05:07 AM. Reason: added response
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Old 14th October 2010, 06:44 AM   #6
djn is offline djn  United States
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I've got an extra pair of the 6d22s as well. I am using that tube (2 of them) in my GM70 amp and yes, the warm up time is great for protecting the tubes. I get into the PSU thing tomorrow to see if the hybrid can be done, but I've never seen it.

My speakers are high eff so I don't need the extra power, but the longer life is something to think about assuming they are not run as hard as just two of them.

I have not really looked at the PS in Tim's article that hard but enough to know it is not that high powered so there should be room to play. My GM70 is running 850v B+ and that is just a big pain in the *** to ensure there is addiquite room to avoid shorting and corrona effect.
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Old 14th October 2010, 01:34 PM   #7
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Quote:
Originally Posted by desperateaudio View Post
I am doing the same thing right now. Did you finish yours?
I am in the parts collection phases... glad to see others here and the thread take off.
dwhitf
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Old 14th October 2010, 05:15 PM   #8
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Quote:
Originally Posted by djn View Post
...My speakers are high eff so I don't need the extra power, but the longer life is something to think about assuming they are not run as hard as just two of them.
Mine aren't. 90dB AR M5s. Can't afford to replace them, and I really like the way they work with the room's acoustics. They can be bi-wired/bi-amped, though - I could just build 2 full amps and use 2 channels per speaker.
Hmmm...

Quote:
I have not really looked at the PS in Tim's article that hard but enough to know it is not that high powered so there should be room to play. My GM70 is running 850v B+ and that is just a big pain in the *** to ensure there is addiquite room to avoid shorting and corrona effect.
I have. HT3 is at 430v - not huge. Schematic may need proofing, though - Mellow states in the text that R29 limits the current through the output stage and speakers in the event of a fault that forces the output stage to latch either up or down; looking at the schematic, I don't see how R29 could do that. Also, the schem shows -50v to the grid of one 6C33 (V4), and -200v to the grid of the other 6C33 (V5) - is this correct?

What am I not seeing or understanding here?
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Old 14th October 2010, 05:30 PM   #9
djn is offline djn  United States
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Having the same issue with not understanding the scheem, but I've never delt with OTL so I just don't know.

90db is pretty loud if you look at the charts. I think you could drive yourself out of the room with 25wpc. Yes, bi-amping the passive crossover is easy to do.
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Old 14th October 2010, 06:39 PM   #10
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Yup. All my DIY exp is in SS; this will be my first glass project. From what I've read here and elsewhere, I get the impression that OTL is generally not something for a noobe to tackle - but this amp circuit schem is less complex than the last ss headphone amp I built. I've dealt with HT in the past, and respect it.

Hopefully some of the more knowledgeable members will chime in...

Re bi-amping: Yeah, the M5 have seperate connectors for the woofer and mid/tweeter sections - easy.
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