A Star is Born

[EWBrown]

Or at least a new G*S*G i s born....

I've had all the parts around, but not the time (and somewhat not the ambition) but this weekend I got my arse in gear and built up the scond G*S*G. No wild and crazy mods or major design changes, in spite of all my earlier ramblings, I stayed with about 90% of Shannon's original design.

I built it into a black powder-coated Hammond 12 X 10 X 2 chassis, which allowed room for the somewhat oversized Magnequest RH-60 OPTs and two chokes under the chassis, located at the front to avoid any possibility of magnetic coupling into and from the OPTs.

The first choke, I used a Hammond 158Q (5H, 150 mA, 105 ohm DCR) and in place of the two paralleled 180 ohms resistors, I used a Hammond 156R (1.5H, 200 mA, 56 ohms DCR). Everything else was per specifications.

For the two 6B4Gs, I used some WWII vintage mil surplus ceramic octal sockets, which mount with the "wavy washers" and thusly don't need the two extra mounting holes. These have full surround contacts and grip the pins like a starved pit-bull. Yellow_Light_Colorz_PDT_04

The MQ RH-60s have 5K primary, 30H inductance and 185 ohms DCR, secondary is 8 & 16 ohms. They do have exposed solder terminals, so there is live voltage present (use shrink tubing, as I explain further down...)

(yes, I did manage to tale a 412 VDC "hit" during the initial power-up test before the tubes were inserted) :o Yellow_Light_Colorz_PDT_09 Yellow_Light_Colorz_PDT_21 Yellow_Light_Colorz_PDT_09 :o


Stopped everything, applied heat shrink sleeving over the hot terminals, problem solved Yellow_Light_Colorz_PDT_08


I did all the metal mangling Saturday, and stuffed the board and final assembly on Sunday, and the "smoke test" and listening started Sunday evening.

I've always liked the sound of the James OPTs, but these RH-60s simply blow them away! With the 5K primary, the output power is a bit lower than with 3.5K, but the distortion is also a lot lower, and these trannies have sound quality that is just one more step "beyond"...

With the 158L and 156R chokes, the B+ at the OPT is 312VDC, (about 10VDC lower than with the original configuration) and the extra inductance virtually eliminates any residual hum or ripple.

I started out with a Westinghouse 6SL7, GE 6AX5A and Sovtek 6B4Gs.
Great sound, deep bass and very clear treble response.

After an hour, I tried out the vintage dual-plate Russian 6C4Cs. They were good but the Sovteks are noticeably better all the way around. Now, the 6B4Gs already have a few hundred hours run time on them, the 6C4Cs were NIB / NOS, and almost 50 years old , so perhaps that explains the difference.

I'll do a similar comparison with GSG # 1 with the James OPTs and slightly higher B+ and see how the 6C4Cs compare in that amp.

/ed B in NH

[RobertWStephens]

So where are the pics? Peace. Robert

[SteveH]

These have full surround contacts and grip the pins like a starved pit-bull. Yellow_Light_Colorz_PDT_04

That is why I enjoy reading each and every post by Ed Yellow_Light_Colorz_PDT_03

[Shannon Parks]

Yarg! I thought I was going to go back to megawatt push-pull amps in early '08 and you mention these 'Robin Hood 60's':
http://www.magnequest.com/robin_hood_series.htm

Very interesting! Question for you, Ed: the picture shows a bell covered transformer, is that just ad-art and not typical of the output?

[EWBrown]

The ad shows a bell covered trannie, but it does state accurately that these wre "open frame" with a brass channel frame, and they have "exposed" solder tabs for the primary and secondary (a high potential for shock hazard, pun intended) .

The trannie with Miss Robin Hood standing upon it looks like one of the more costly DS-025s or FS-030. The cute green RH outfit and archery gear are not included in the purchase price. Yellow_Light_Colorz_PDT_08 Yellow_Light_Colorz_PDT_03 Yellow_Light_Colorz_PDT_09

The RH series looks like larger versions of the better-known BCP-15 and EXO-45 series.

Kinda look like this:



Some of the more enterprising and courageous DIYers have re-worked these into bell ends, but the risk of messing up the laminations and air gap is an always-present danger.

The RH series also has a UL / SG tap so they are good for use with EL84s, EL34s, etc, as well as triodes.

As good as the MQ iron is, their website is really lagging behind the times, and hasn't been updated in years... Other than the active MQ forum, everything else on the site is aged somewhere between antique and antediluvean.

Pictures are coming, be patient Yellow_Light_Colorz_PDT_06

/ed B in NH

[EWBrown]

On my original G*S*G, I replaced the James 6113HS OPTs with a pair of 6112-HS (3.5 & 2.5 K primary, as I need the 5K UL tapped primary for another upcoming project. The DCR of the 6113's 3.5K tap is 238 ohms, and is 195 ohms with the 6112. I'll have to check the specs on Euphonia to see if there is a difference in the primary inductance values.

From a quick check, the primary inductance appears to be around 15 H for the 3.5K tap on either transformer.

I'm guessing that the 6112s may play a tad better, with the lower primary DCR and by not having an unused portion of the primary winding "flapping in the breeze".

WIth this amp, I'm using the (1958) vintage Russian 6C4C dual plate tubes., GE 6AX5 and Vintage NOS Tung-Sol 6SL7.

Now it's going to need some "burn-in" time before I Can give an honest evaluation of eh new iron and new-old Russian tubes.

/ed B in NH