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Tim Mellow 25W OTL amp.

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Posted on June 30, 2018 at 17:48:08
Cougar
Audiophile

Posts: 4593
Location: SoCal
Joined: June 25, 2001
 Today as I was looking over some possible DIY OTL projects, I ran across the Tim Mellow 25W OTL that uses 6C33C output tubes. I found it in a Audio Express article from a few years back.

Has anyone here built or even heard this tube amp? It looks like something I can do.

 

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RE: Tim Mellow 25W OTL amp., posted on July 2, 2018 at 13:33:44
Ralph
Manufacturer

Posts: 4779
Location: Minnesota
Joined: April 24, 2002
 There is a thread regarding this amp on DIYAudio.com

http://www.diyaudio.com/forums/tubes-valves/204960-tim-mellows-otl-project.html

There is also a thread there that has a lot of information about building the M-60 amplifier, entitled 'what tubes for a tube amp'.

http://www.diyaudio.com/forums/tubes-valves/161112-tubes-otl-tube-amp.html

 

Nice!, posted on July 2, 2018 at 16:25:38
Cougar
Audiophile

Posts: 4593
Location: SoCal
Joined: June 25, 2001
 Ralph,Thank You for the links. I will start reading these.

 

RE: Tim Mellow 25W OTL amp., posted on July 3, 2018 at 01:00:35
cpotl
Audiophile

Posts: 1002
Location: Texas
Joined: December 6, 2009
 "Has anyone here built or even heard this tube amp? It looks like something I can do."

Yes, I've built it. It's been my main amplifier for about six years now, and it has behaved more or less flawlessly. It performs exactly as described in the article, with low distortion, superb bandwidth, low output impedance, and delivers almost exactly 25W into 8 ohms before clipping.

 

RE: Tim Mellow 25W OTL amp., posted on July 5, 2018 at 19:35:40
Cougar
Audiophile

Posts: 4593
Location: SoCal
Joined: June 25, 2001
 cpotl, Thank You for the info. I would really like to try building this, seems like a nice power amp. That with some nice high Efficiency speakers.

 

RE: Tim Mellow 25W OTL amp., posted on August 13, 2018 at 07:04:41
rogerh113
Audiophile

Posts: 508
Location: San Francisco South Bay
Joined: October 20, 2014
 Would you post a pic or two of your amp??

thanks -- Roger

 

RE: Tim Mellow 25W OTL amp., posted on August 13, 2018 at 07:17:57
cpotl
Audiophile

Posts: 1002
Location: Texas
Joined: December 6, 2009



I'm traveling at the moment, and I think I only have one rather poor picture accessible. I can post some better ones when I return later this month.

 

Nice! (nt), posted on August 16, 2018 at 14:38:07
Crazy Dave
Audiophile

Posts: 14371
Location: East Coast
Joined: October 4, 2001

 

RE: Tim Mellow 25W OTL amp., posted on June 9, 2022 at 05:51:15
Klarskov
Audiophile

Posts: 4
Joined: June 20, 2012
 I have built this amplifier and in general I am very pleased with it, but perhaps there is room for improvement?

Tim Mellow mentions in his article that one can increase the input impedance by grounding C1 and connecting the input to C2. However, this will, as he says, "tend to unbalance the input stage slightly unless you use an "ideal" solid-state current source in place of R7". I tested the arrangement briefly and quite superficially (with a LM317 wired as a current source - the "cold" end of the current source connected to ground not to -430V). It seemed to work - but I did not test it very thoroughly.

One of the reasons I am interested in this modification is that this would make the amp non-inverting (as it is published it inverts the absolute phase - which, in most cases, in not problem since the remedy is to invert the loudspeaker connections, but for aesthetic reasons...). Yet, there would be a couple of other small advantages: The neon bulb N1 would not be needed and the burden on RV2 during warm-up would also be reduced.

Has anybody experimented with this approach.....?

In my version the DC at the output does not really stay as low as Mellow says - would a DC-servo be a good idea?

 

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