DCAudioDIY.com

DC Area Audio DIYer's Community
It is currently April 13th, 2021, 5:54 am

All times are UTC - 5 hours [ DST ]




Post new topic Reply to topic  [ 92 posts ]  Go to page 1, 2, 3, 4, 5 ... 10  Next
Author Message
 Post subject: GM-70 Amplifier
PostPosted: February 18th, 2016, 9:47 am 
Offline

Joined: February 28th, 2013, 1:19 pm
Posts: 599
Attachment:
20160215_141508_resized.jpg
20160215_141508_resized.jpg [ 723.21 KiB | Viewed 22488 times ]
Attachment:
20160215_210949_resized.jpg
20160215_210949_resized.jpg [ 883.57 KiB | Viewed 22488 times ]
Attachment:
20160215_141517_resized.jpg
20160215_141517_resized.jpg [ 752.33 KiB | Viewed 22488 times ]
It has been a long time since I started, but I finally completed the first of two GM70 monoblock SET amps. The circuit is a simple two stage, interstage transformer coupled design, using an SiC diode biased E280F (7722) triode connected pentode voltage amp/driver, through a Hammond 126B 5K/5K interstage, to a cathode biased (for now) GM70. The power supplies are all-tube rectified, using oil capacitors and over-specified iron, with a B+ of 1025V for the GM70, and 190V (separate supply) for the E280F. The GM70 filament is fed 20V/3ADC through a Rod Coleman regulator. Based on the operating point, I figure I am getting around 20W in Class A, and due to using an interstage transformer, and go into A2 to the limit of the driver stage.

The B+ power transformer is dead cold when operating. It is a big UTC 1500VCT/1900VCT/300mA continuous duty unit that Grover donated, and after running a half hour or so, is still cool to the touch. Also using a big 10H/500mA choke, another Grover donation. The big oil capacitors are 88uF/1200V pulse capacitors (either paper/oil or PP/oil).

Based on initial informal listening in mono, the amp is dead quite background, and is very nice tonally. I am not using very good speakers while it is down in my shop, though the best sound so far is with a classic Dynaco A25, which I probably keep set up to amuse me while building the twin brother.

David


Top
 Profile  
 
 Post subject: Re: GM-70 Amplifier
PostPosted: February 18th, 2016, 10:00 am 
Offline
User avatar

Joined: February 28th, 2013, 3:31 pm
Posts: 1499
Outstanding layout and construction! Great job David.

Tom


Top
 Profile  
 
 Post subject: Re: GM-70 Amplifier
PostPosted: February 18th, 2016, 10:01 am 
Offline

Joined: January 14th, 2015, 11:15 pm
Posts: 448
Lovely work. Is the chassis a drawer? How heavy is the amp?

ray


Top
 Profile  
 
 Post subject: Re: GM-70 Amplifier
PostPosted: February 18th, 2016, 10:05 am 
Offline
User avatar

Joined: April 20th, 2014, 5:21 pm
Posts: 233
Cool heavy-@ss amp there, Sir!! Well done!! :handgestures-thumbupleft: :handgestures-thumbupleft:

I am not confident that that puny wood handle can bear the strain, however!

Looks like a carbon plate GM-70. See if you can snag a comparison pair of the metal plate units which I thought were more open sounding, a somewhat different flavor, but the carbons do a nice fat "run you over" presentation in their own right.

I'd expect that this is a powerful sounding machine indeed!

Is that a fan on the Coleman reg? I believe these things get quite hot, no? Rod's instructions seem to prescribe lots of heatsink.


Top
 Profile  
 
 Post subject: Re: GM-70 Amplifier
PostPosted: February 18th, 2016, 10:27 am 
Offline

Joined: February 28th, 2013, 1:19 pm
Posts: 599
Joe,

I have a pair of copper plate GM70s on the way from Ukraine as we speak. I have a healthy heatsink on the Coleman regs, which I have tested and it keeps pretty cool, around 50C in operation according to my IR meter. No fan.

The sides of chassis is built out of oak stair tread (1-1/8" thick), the top is 3/4" oak plywood, poplar core. You can probably park your car on top of it. Figure the entire amp weighs around 70 lbs.

David


Top
 Profile  
 
 Post subject: Re: GM-70 Amplifier
PostPosted: February 18th, 2016, 11:13 am 
Offline
User avatar

Joined: July 24th, 2015, 4:17 pm
Posts: 1374
Location: Parkville, Maryland
Very nicely done! BRAVO!!
My question is -- where did you find sockets for those tubes?

_________________
Walt


Top
 Profile  
 
 Post subject: Re: GM-70 Amplifier
PostPosted: February 18th, 2016, 12:06 pm 
Offline

Joined: August 16th, 2013, 8:52 pm
Posts: 28
Location: Pikesville
I was always wonder: if one use interstage transformer, with obviously air gap, why not use it as a phase splitter so the O.T. former wont have to be air gap?

_________________
SamB


Top
 Profile  
 
 Post subject: Re: GM-70 Amplifier
PostPosted: February 18th, 2016, 12:49 pm 
Offline

Joined: February 28th, 2013, 1:19 pm
Posts: 599
Ceramic sockets for GM70s are available from a number of Ebay sellers, usually modified slightly (drilled out) for one of the heater pins. The fancy teflon sockets are harder to find and expensive. I just have the ceramic socket.

Regarding the air gapped (SE) vs centertapped interstage transformer, the circuit is single ended by design, including using a very fine 10K SE Tamura output transformer. A centertapped transformer for phasesplitting is applicable to a push-pull or differential amp topology. Two different approaches. One can debate the merits and disadvantages of either design. I chose SE due to having the output transformer for it (a big sunk expense), and I generally like the simplicity and sound of SE circuits. And just because. I have built compact PP EL84 amps with the same output power as this big amp.


Top
 Profile  
 
 Post subject: Re: GM-70 Amplifier
PostPosted: February 18th, 2016, 12:59 pm 
Offline

Joined: March 5th, 2013, 9:35 am
Posts: 232
Location: Highland, MD
David,

It's good to hear from you again - I hope your performing is going well!

I wouldn't expect anything else from you but a beautiful job, and you didn't disappoint. Maybe we'll be able to listen to the pair of amps in the future. :)

_________________
- Guy


Top
 Profile  
 
 Post subject: Re: GM-70 Amplifier
PostPosted: February 18th, 2016, 1:56 pm 
Offline

Joined: February 28th, 2013, 1:19 pm
Posts: 599
Guy,

I am hoping that once I get the other amp finished, I can hold a meet at my place like old times. It will be fun to reconnect to old friends as well as new.

David


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 92 posts ]  Go to page 1, 2, 3, 4, 5 ... 10  Next

All times are UTC - 5 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 6 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron
Powered by phpBB® Forum Software © phpBB Group