Monkey Wards / Valco Hack Job

[RicharD]

About a year ago, I got this cool old Montgomery Wards amp made by Valco.  The thang was is pretty bad shape from sitting in a basement for a decade +.  Gave it the usual cap job and got it back to life.  Meh... nothing special.  Had to replace the speaker cuz the old one squeaked.   I ripped out the tremolo and added an EF86 as a 2nd input.  Now the amp sounded pretty good for a little lo watt "grunge" amp.  Well it lunched an OPT so I replaced it.  About a month later it lunched another one.  I had already raised the cathode resistor to tame the power dissipation but I guess it wasn't enough, or the failure coulda been caused a bad socket.  The 6V6 did fall out while moving the amp 1 day, also there's a lotta green contacts from being stored in a basement.  I had to clean the contacts on the reverb driver tube..... who knows.  Well it's been sitting ever since.  Visiting with Dummy Load, he decided it was time I fix the dang thang.  He scrounged up a 15W Edcor transformer.  We hacked that baby into the chassis using his new mill (maiden voyage).  The chassis looks like stainless at a glance but it is not.  None the less, it's a quite hard chrome plated steel.  Since I now have a larger OPT, I decided to upgrade it to a 6L6GC because it's mo powerful and a better match for a 5k load.  It sounds pretty good but I still need a better speaker.  Total dissipation is about 24 watts at idle which is right at 70% of max dissipation for a 6L6GC.  A lot of this is still original and I recycled as many parts as possible.  I stuck with the typical Valco "rat's nest" wiring scheme.  New stuff is the OPT, octal sockets, and a few caps -n- resistors.

Attachments:
Original Schematic
Modified Schematic
Front Picture
Mill Picture
Gut Shot

[tubeswell]

Cool.

Soundbyte(s)?

[LooseChange]

Nice work!!  Great job working out the electronics and the mechanical.

[DummyLoad]

cool!

hacked by monkeys.

(365V-22V) * 67mA = 23W

you should give an EL34 a spin. swing by and grab a beater if'n ya don't has one.

[jjasilli]

   Nice mill too!

[phsyconoodler]

I don't have the patience to get into re-working those old 'rats nest' amps. Nice job!

[jojokeo]

looks very cool but shouldn't you be able to get the pDiss up to about 90% w/out any issues if you wanted?

[DummyLoad]

looks very cool but shouldn't you be able to get the pDiss up to about 90% w/out any issues if you wanted?

loads the power supply too much. remember, it was a 6V6 A1 5 watt amp - on a good day, with a tail wind, and a variac...

  Nice mill too!

thanks! plans in the works to convert to CNC. 

[Iannone]

 :worthy1:I love that grill cloth!

[RicharD]

I was told do what ever I want to the amp but not to touch the grill cloth.  I agree.

I plan to put it though it's paces tomorrow.  I'm gonna play it full out for an hour straight.  Let's see if I can't kill the PT this time.   

[PRR]

> shouldn't you be able to get the pDiss up to about 90% w/out any issues if you wanted?

Good tubes with good parts, yeah.

I have seen instability with modern 6L6GC working very near their "30W" rating. At that heat, gas is liberated, grid leakage makes plate current drift, usually UP, which makes more heat. I have not had one run-away; I suspect it will wait until an Important Gig to turn into an incandescent glass-melt.

Also... he's got 365V and 5K load. Allowing 20-25V of cathode bias, 340V in tube. 340V swing across 5K needs 340V/5K = 68mA to cover the swing. True, he's "only at 67mA" now, including G2 current, he could probably go 75mA total (68 in P, 7 in G2). The gross difference is negligible, not "WOW! More Power!" At full blast, under-current versus over-current break-up different, he may want to explore this.

To really beat his cone, replace 5Y3 with 1N4007s. Save 10W filament load on PT right there. 450V raw supply. Now he should pull 90mA to suit a 5K load. 400V on tube, 90mA, 36 Watts.  Ah, but at 90mA the PT will sag below present voltage, say 420V, or 390 across tube. 390/5K= 78mA, 30W Pdiss. 6L6GC "may" work. 10W-15W.

I bet Leo didn't have a mill. (However in that time and place, there may have been a mill on every other block of 2nd street...)

[RicharD]

Never even thought about a SS rectifier.  That's a good idea. 

[Rev D]

 That's some far-out grill cloth on that bad boy!

Regards,

Don

[jojokeo]

That's some far-out grill cloth on that bad boy!

if you expand the pic & move your head around a bit while staring at it, it starts looking psychadelic like an optical illusion. Very 60's-ish

[jojokeo]

Also... he's got 365V and 5K load. Allowing 20-25V of cathode bias, 340V in tube. 340V swing across 5K needs 340V/5K = 68mA to cover the swing. True, he's "only at 67mA" now, including G2 current, he could probably go 75mA total (68 in P, 7 in G2). The gross difference is negligible, not "WOW! More Power!" At full blast, under-current versus over-current break-up different, he may want to explore this.

In a SE el34/kt77/6l6/5881 amp recently built w/ 250v on the plate & screen - I used a tranny rated right at 100mA Ip. It gets pretty hot as I'm running it close or at it's limit/rating. I have an amp or so extra reserve w/ the heater current, installed a VVR, and finally biased it a bit cooler to help w/ the heat. There wasn't much if any difference noticed here but thought it could only help a bit? As for sound and output power it was very negligable and in a blind test one may not even hear or tell any differences at all running any of the tubes before and after the bias change. So I said what the heck and left in the larger cathode resistor.

[RicharD]

I just ran it hard for about 40 minutes.  No problems.  PT gets warm but not hot.  I'm gonna build a SS octal plug recto and try that here in a little bit.  I REALLY need a better speaker.

I love the grill cloth.  This amp was given to my oldest brother for Christmas in 1965 or 66 judging from the old pictures.  His step-son abused it in the 80's and it was sitting in a basement after that.  He sent it to me when he found out about my vacuum tube fetish. Now it's a Franken-amp. 

DL's mill is flippin schweet.  Honestly we coulda cut the hole quicker with a dremel but where's the fun in that?

[DummyLoad]

I REALLY need a better speaker.

i have one. i bought a crappy TUSK DS100 to part out. it has 2 x 12in. alnico. it's a 100W hybrid - has 2 small bottles and 4x6L6GC. the PT is MASSIVE & the OT looks to about fender twin size. gimme a ring after 6:30 or thereabouts. you're welcome to one of the speakers.

--DL

[RicharD]

DINNER!

I'm a bachelor this week.   

[tubeswell]

I REALLY need a better speaker.

I just bought a bunch of various Weber 12" speakers. One which has quite pleasantly surprised me is the Alnico Sig12 (smooth cone) https://taweber.powweb.com/weber/asig12r.htm Incredible value for the money (US$45) FWIW. It has a charming papery early break-up (once broken in) that really screams in the mid-highs when pushed. At 25W I reckon it would be right at home with a Class A 6L6GC SE.

(BTW I have also tried the Sig 12S and the vintage series 12A125A (which is really terrific))

I also think a 10" speaker also sounds fab with 1 x 6L6 SE. Here are some soundbyte links from an amp I built earlier this year that had a modern-production Celestion G10 30W (made in Ipswich) being run with a SE 6L6 (with a plate voltage of about 300 IIRC).

http://www.nzguitars.com/forum/download/file.php?id=8232

http://www.nzguitars.com/forum/download/file.php?id=8258

[jojokeo]

I really like the 12a125a also. Keep one in a test cab to switch back & forth for testing w/ a Celestion greenback, good contrast btwn the two & out performs the greenie in many ways.

[alerich]

I can do without the Austin Powers grill cloth but I dig the way cool flood lamp current limiter.