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I like these Welborne300b amps with Elektraprint transformers.

107.213.0.240
Posted on April 9, 2014 at 23:15:17
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005


This is one of the ribbons the amps drove with no sweat..Even the five watt single ended Magnavox amp drove these ribbons but not quite as loud or full but similar sound.






These are nice little amps that came into me for service and upgrades..The diodes in both amps were shorted from a power surge so I put Schottkys and so far they are sweet and energetic amps. I put these on my big ribbons and it drives them just to 98db all day long..Minimum recommended power on them is 50 to 450 watts.There is excellent detail,beautiful balance and deep rich bass.I honestly they would drive the big Martin Logan ESLs,I just have a feeling because they have absolutely no issues with the big ribbons..The 300b reminds me a lot of the 7189 black plate Sylvania sonically.It also drove the big IMF SACM transmission line speakers I have which are only 86db speakers...I can assure you if I was biamped on the ribbons,I could play then to well over 100db without breaking a sweat.Right now they are playing 98db to 100db according to the SPL meter.That Elektraprint iron is on its game and I love the 2 ohm tap as well..I really think I can drive the CLXs because I biamp the CLXs with the Martin Logans now..
I'm not saying that these sound better that what's on my system because I love all kinds of tube amps but I can say they are a very capable set of amps sonically.I may replace the series connected 560uf electrolytics with a pair 100uf at 600v EC polys.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

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RE: I like these Welborne300b amps with Elektraprint transformers., posted on April 10, 2014 at 04:36:47
Chip647
Audiophile

Posts: 2652
Location: The South
Joined: December 24, 2012
 What are the two small transformers? A plate choke on the driver and a grid choke on the 300B?

 

RE: I like these Welborne300b amps with Elektraprint transformers., posted on April 10, 2014 at 09:48:52
GSH
 I think one is the "DRD" choke that loads the driver plate from the output's cathode, and the other is a filament choke for the 300B.

 

RE: I like these Welborne300b amps with Elektraprint transformers., posted on April 10, 2014 at 10:13:12
Palustris
Audiophile

Posts: 2408
Location: Cape Cod
Joined: September 12, 2008
 I bought a pair of these amps about five years ago on epay. Mine have the rectifier tube and were a kit assembled by a very competent tech. They sounded fine, but this is a DIY hobby and my least favorite part is building the chassis so I bought these as a platform upon which to build my own version.

I replaced the 300B with a 2A3 and added a 2.5V filament transformer. I added a volume control and moved the RCA jack next to the volume pot. Since the 2A3 tubes were making expensive sounding noises on power up I added a switch for the filament transformer. The existing power switch powers on the B+ and the JJ caps were replaced with a Mundorf. This required making a new panel on the back to replace the Welbourne panel. I moved the speaker posts to a spot near the output transformer and added a hum pot on the panel along with a fuse holder.

I changed the input tube to a 6N23P-EV and biased it with a diode. In the process of these mods I removed about a foot of wire and tightened up the layout. The filament supply for the 2A3 is AC so the discrete diode bridge is not used and the filament choke was removed. Note the extra resistors added to the 2A3 cathode resistor to dial in the operating points. The upper left transformer is the plate choke and the bottom left is the power supply choke. There are only three capacitors in the amp.






 

RE: I like these Welborne300b amps with Elektraprint transformers., posted on April 10, 2014 at 13:23:18
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 Exactly.Those are two chokes on the top side..I'm taking out the series connected 560uf JJs and I put in a 100uf@600v EC concepts poly.I'm going from 280uf in electrolytic to 100uf in a poly,but at 550v, I would think that 100uf would more than enough filtering there..I did parallel in a 2nd 100uf poly and couldn't hear any difference in hum as it was dead quiet.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

careful now, posted on April 10, 2014 at 13:47:24
GSH
 Mike, if that's a DRD circuit which it looks like, remember that one of those caps, likely the big motor run is the "ultrapath" cap (which has to be rated at the B+ rail PLUS the AC signal). It needs low ripple at the PS node, otherwise it'll get into the cathode circuit. By lowering the final C's value in the PS, you might affect this issue.

 

RE: careful now, posted on April 10, 2014 at 20:18:08
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 That's good to know..The series connected 560s feed the plate of the 300b thru the output transformer but what if I add pair of 30uf polys in parallel,giving me 160uf total? These caps are ultra low esr and they really store and deliver nice bursts of energy.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

Is it a DRD or not?, posted on April 10, 2014 at 20:48:24
GSH
 Is one of those chokes connected between the driver plate and a voltage divider in the 300B cathode circuit?





If, yes then this what you have. I don't know how low of ripple is "low enough", but figure that probably the designer already made that call.
I not advising, just saying pay attention to what is important when tweeking this circuit, you know what you're doing. Since you claim to have good recipes for signal cap combinations, the cap marked "oil" above, is one.

 

RE: Is it a DRD or not?, posted on April 11, 2014 at 01:28:35
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 It is DRD..The 100uf poly is dead quiet tho but I can parallel in another 100uf poly or a pair of 30uf giving me 160uf total.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

PS cap, posted on April 11, 2014 at 01:55:11
smart845
Audiophile

Posts: 668
Location: East london
Joined: May 24, 2000
 Hi Michael,

Add a 0.1uF Teflon cap after the last power supply cap. It made a big difference on my Starchiefs.
You could also parallel a 0.1uF Cu cap (Audyn true copper or Vcap CuTf) across the Ultrapath cap.

You get lower noise and greater transparency with the Teflon and body and soundstage with the Cu.

Other than that not worth spending any more on the DRD as I have found out.

Cheers
Smart845

 

another thought, posted on April 11, 2014 at 08:08:33
GSH
 I don't know how the DC filament supply is done there, but if it's CLCRC then that could be improved by going LCRC or LCLCL or LCRCL. If a C is last then a pair of resistors to the cathode node instead of off one side might help get the final PS C out of the equation. I think it's Thomas Mayer who
suggests an L for the last component having good AC rejection.

 

RE: PS cap, posted on April 11, 2014 at 21:21:38
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 Thanks Smart
I have both Teflon and paper in oil caps..I have Audyn caps and SonicCap plats.I love the Russian FT-2 and FT-3..Both are excellent as are the KBG pio and the K40Y caps but they all need break in.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

Too bad Welborne lost control,and many customer were cheated. , posted on May 16, 2014 at 12:04:03
Balle Clorin
Audiophile

Posts: 781
Joined: July 30, 2003
 He never delivered my DRD kit after payement. My cc compay refunded me...

 

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