Voltage help!!

Wrongdog · Post by **Wrongdog** » Tue 05/20/03 5:47 pm

Thanks to everyone that has helped directly or indirectly on getting my Grommes Marshall 18 watt Lite conversion up and running.

I need some help on voltages. Be gentle since I am new.

The donator amp was a Grommes mono amp circa 1962. 2x EL84, 2x12ax7 and an EZ80. I used the old iron, can cap, heater wires, tubes and sockets. New pots, resistors, caps (except can) etc. It had limited real estate at about 8x13. its one input, running parallel.

Sounds pretty darn good!! Runs hot though. I was redplating but went to a 150 bias resistor. I was seeing 365 vdc at the EZ80 - yikes! I installed a 420ohm 5 watt at the EZ80 knocking the voltage down to 310/295/271 and ended up so far at a 270 ohm 5 watt there now seeing 328/311/287.

Heres the voltages now:

V2 pin-v p1-129 p3-1.2 p6-128.4 p8-1.2
V3 p1-207 p2-35.5 p3-74.5 p6-209 p7-35.7 p8-74.5
V4 p2-.84 p3-11.3 p7-322 p9-311
V4 p2-.02 p3-11.3 p7-323 p9-311
I am seeing 328 at 1st cap stepping down to 311 to 287

Should I up the 270R at the EZ80 to a ??? the 420 ohm seemed like it wasn't pushing the EL84's hard yet the pre-amp numbers came in to line better in theory. Bias resistor is a 150 now and I am thinking trying a 370 at the EZ 80 and trying a 130 at the bias.

Thanks for any help. I am kinda over my head with this thing. I just so stoked it even works forget sounds good. No buzz, no humm just growl pushed and chime when tickled!!!

Post by **zaphod_phil** » Wed 05/21/03 5:41 am

If you liked the sound with the 270 ohm and it wasn't red-plating, then you could stick with that value. Or maybe just try a 330 ohm, and leave in the 150 ohm bias. That should probably see you just about right. You're nearly there...

Wrongdog · Post by **Wrongdog** » Wed 05/21/03 12:04 pm

Okay. I feel better now! I am like lost in a forest. Its easy enough to chuck the 330 ohm in it and see what I have technically and tonally.

The pre amp numbers seem high but tone is what counts.

I "won" and old hi-fi amp on eBay yesterday. Here we go again. EZ81,
2x EL84, 1x 12ax7 and 1x 6AU6. What shall I make that into? $37 w/ shipping, working tubes and all. I figure the tubes used are worth $20 and the iron????

The bug has bit me good. Or is that bad?

Wrongdog · Post by **Wrongdog** » Wed 05/21/03 12:33 pm

HA! So it's you I have to blame!!

"What in $^% is UPS delivering you now??" Sez the lovely wife...

"Umm, its for my pal, Paul." Yeah, sure! Thats the ticket!

Post by **markh** » Thu 05/22/03 4:27 am

I know what you mean - in my case it's the west end of the basement.

And I have to correct you - I'm sure you can see your garage just fine, it's the garage floor (and perhaps back wall) you can't find!

--mark

Post by **Gabi** » Thu 05/22/03 9:49 am

What's a "garage"?

Is it a recto tube?

gab.

Post by **zaphod_phil** » Thu 05/22/03 12:10 pm

Wrongdog wrote: I "won" and old hi-fi amp on eBay yesterday. Here we go again. EZ81,
2x EL84, 1x 12ax7 and 1x 6AU6. What shall I make that into? $37 w/ shipping, working tubes and all. I figure the tubes used are worth $20 and the iron???? The bug has bit me good. Or is that bad?

Whether good or bad, maybe your wife should judge.

That's one heck of a great bargain.

OK you could build that into an 18W Lite II, with the pentode used for an extra Vox-like input channel - Vox would use an EF86, but what the heck - try the 6AU6. Look at the 'EF86 Variations' in the Downloads area, or http://65.104.178.73/NUKE/html/Storage/ ... s/EF86.gif Have fun!

Wrongdog · Post by **Wrongdog** » Tue 06/03/03 11:09 am

PaulRuby wrote:I almost bid on that amp and choose not to when I saw you were the high bidder.

Paul - As it turns out this strange little eBay amp rocks out of the box! I mean to a tin eared geek like me it does.

Pardon my rant but this thing is cool!!

It was from a Voice of Music 1958 phonograph and I have no clue about the circuit. I put a 3 way cord on it and some in/ot jacks and rigged a "gain" knob. It had a volume and tone set up before the preamps like old Silvertone 1481's Plus a "room size" acoustic contour in between the pre-amps. To tell the truth its so crammed in the tiny chassis I don't even want to try to figure it out until I am done fooling with it as is. Basically the room coutour thing is my gain knob and the amp seems to run flat out all the time. It must ad gain/volume and cut or add some bass and treble with the original three notch switch. I stuck a 1 meg log pot in there and I can dial it as needed.

Its very responsive to my Rio Grande single coil volume and tone controls. I can basically bring it to level of grind I want and use the guitar volume for everything. Very jangley clean to a Marshall roar. Extremely responsive too.

AH! Another one for the junk pile. I''ll fool with it, get bored, shelve it. Then I'll find it cleaning out crap, wonder why the hell I thought it sounded good, strip it and make what ever is the flavor of the month next spring!!

Wrongdog · Post by **Wrongdog** » Tue 06/03/03 12:10 pm

BTW Paul et al, The tube line-up is ez81, 2x el84, 12ax7 and a 6au6.

Okay, tons of questions:

The 6au6 is the mystery to me. Can anyone point to data on guitar amp use of this tube or its cousin 12au6? It is a triode/pentode right? Why does it sound good yet nobody ever used the lineup - or am I tone deaf?

If I have an extra 6.3 for a pilot light, should I use those for the extra heater and wire in a 9 pin and go 12ax7 when I re-vamp?

Re my other 18 watt homebrew: What loss do you get by adding a pilot light to the heaters of an amp that didn't have one originally? Should you use a 110vac neon instead?

Thanks for any help - Scott

Post by **markh** » Wed 06/04/03 4:43 am

Wrongdog wrote:BTW Paul et al, The tube line-up is ez81, 2x el84, 12ax7 and a 6au6.

The 6au6 is the mystery to me. Can anyone point to data on guitar amp use of this tube or its cousin 12au6? It is a triode/pentode right? Why does it sound good yet nobody ever used the lineup - or am I tone deaf?

U.S. tube guitar amp makers generally stayed away from pentodes in the preamp. Some Vox amps used them, and there's an 18watt varient that has a pentode input stage - it could probably be adapted to use the AU6 without much trouble.

With the notable exception of Ampeg, most manufacturers here stuck with the 12AX7 and 12AT7, I think more because of cost, availability, and tradition more than any other reason.

If I have an extra 6.3 for a pilot light, should I use those for the extra heater and wire in a 9 pin and go 12ax7 when I re-vamp?

I'd try the pentode varient with the tube you have and see how it sounds, particularly if you like the way it sounds unmodded.

Re my other 18 watt homebrew: What loss do you get by adding a pilot light to the heaters of an amp that didn't have one originally? Should you use a 110vac neon instead?

It depends on the power transformer - if it stays cool to the touch when idling, it's probably OK to add a light to the heater string.

--mark

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