Hammond L-100 conversion

[Rafael]

I'm working on converting an old L-100 organ amp into a guitar amp,and it seems like a no-brainer. BTW here is a link to the relevant schematic [sorry I can't find a better quality scan on the inet]

http://www.urbanmusic.com/L-100scheR.jpg

One thing I like about this particular chassis is that it can be used Marshall-style chassis down, and that puts the xformer on the right, there's room for a couple of switches near it on the rear, and the signal flow goes then from left to right across the other side which would be the front. The only slight argh moment is the 5U4 projects over the front of the chassis, hence my question 3 below.

1] you'll note C320: I figure this is as good a place as any to start the signal chain but am wondering if anyone sees an easy way to use the other 12ax7a and bh7 for an additional gain stage? I'm not too concerned about tone control, though any ideas would be appreciated. Is there a consensus on tone control to rob that could use the ax7a? Is there a no-brainer circuit to utilize that BH7 or AX7 for additional say pre-master gain stage?

2] I initially figured to ditch at least some of the pre-circuitry so I removed the choke that feeds the B+ to the BH7 and was wondering if it would be possible/desireable to use it in place of one of the PS resistors?

3] any bad ramifications if I sub a a5U4GB for the 5U4?

4] if I wanted to throw a standby in, would it go right before the very first 50uf cap? Since this is way over 300v here, can a normal 125/250v switch be used?

[Rafael]

Here's my first attempt at a mod:

OK so I snipped out the verb send/return circuit which is what is floating at the top of the schematic and replaced it with what was the verb return amp which is the circuitry surrounding the second 12AX7. I threw in the 1M volume control where you see it. The amp sounds great,but is a little unstable at full volume [should I add a fixed resistor in series with the volume wiper?] and it could use a tone control. Ideas?

[Rockn]

Hammond amp

I have exactly the same chassis I just purchased along with 5 other Hammond bits for $85. I was wondering what sort of voltages you are getting with that PT and the 5U4. I was thinking along the lines of a Carmen Ghia which is almost the same amp sans a 5Y3 rectiifer. If you want the schemo let me know. If the 5U4GB is almost the same I think it would be a go.

[Rafael]

I have exactly the same chassis I just purchased along with 5 other Hammond bits for $85. I was wondering what sort of voltages you are getting with that PT and the 5U4. I was thinking along the lines of a Carmen Ghia which is almost the same amp sans a 5Y3 rectiifer. If you want the schemo let me know. If the 5U4GB is almost the same I think it would be a go.

I'm getting 340v at the first b+ [the output xformer centertap]. I believe the voltage drop w/ the 5Y3 is different from the 5U4GB. I'd fire it up w/ rectifier by w/o output tubes and check voltages. The EL84s are rated to 300v so me personally I don't run 'em more than ~10% more so the 340v seems fine.

[Rockn]

Standby switches

I have seen different applications for the standby switch as far as it's placement, and the majority of them have been right before the first filter cap. As far as the rating goes you can use the 125/250V AC switch and as a rule you can multiply the AC rating by 2 to get the DC voltage the switch can handle.

[hoksila]

newbie

hi gang!
hope i'm not hijacking.i've got an l100 rev. pwr. and both alnico 12's.i'm thinkin i can sub a 5ar4(same saddle)use the gain on the chassie as vol.and put a pot on the outside connection to the reverb switch for rev amount. then put a 1/4 in.on the ex pdl in.and use the guit pot.am i crazy or is that pretty much plug & play? thx everybody this site is cooler than $%*#
hoksila

[Rafael]

hi gang!
hope i'm not hijacking.i've got an l100 rev. pwr. and both alnico 12's.i'm thinkin i can sub a 5ar4(same saddle)use the gain on the chassie as vol.and put a pot on the outside connection to the reverb switch for rev amount. then put a 1/4 in.on the ex pdl in.and use the guit pot.am i crazy or is that pretty much plug & play? thx everybody this site is cooler than $%*#
hoksila

THe 5ar4 has a higher rectifier resistance than the orig so expect slightly lower B+. Don't know but makes sense about the verb level, and exactly right: input to expression pedal jack and you're gold...however I would add a series resistor/shunt to the input jack say inline 68k w/ 1meg to ground. What's not to love about this amp?!

[hoksila]

good onya

thanks Rafael!
i have the orig 5u4.i just gathered from reading(matchless spit specs)that the 5ar4 made for hotter output.also..ok..keep the chuckling down..is there a simple explanation of B+(i'm a newbie,be gentle)I'm smarter than I look?
Is there a rudimentary wiring how to out there somewhere that you could point me at,although it would spoil allot of joke ammo.
thanks again.

[Rafael]

thanks Rafael!
i have the orig 5u4.i just gathered from reading(matchless spit specs)that the 5ar4 made for hotter output.also..ok..keep the chuckling down..is there a simple explanation of B+(i'm a newbie,be gentle)I'm smarter than I look?
Is there a rudimentary wiring how to out there somewhere that you could point me at,although it would spoil allot of joke ammo.
thanks again.

Check Tube Data Sheet Locator and look them both up, you'll see some differences...pinout similar, but the data sheets don't list each other as replacements...

In tube designs there were originally showing two DC voltage sources, represented in schematics by batteries, hence the abreviation 'B' for DC voltage source...B+ refers to the high voltage DC provided by the power supply. I'd stick with the original 5u4 as even the 5u4GB puts out more DC than the old 5u4s which could strain the output tubes unless you throw in a voltage divider on the high tension at the output transformer/EL84 plates supplies. Good luck and keep one hand in your pocket! ;-)

[olddawg]

Check Tube Data Sheet Locator and look them both up, you'll see some differences...pinout similar, but the data sheets don't list each other as replacements...

In tube designs there were originally showing two DC voltage sources, represented in schematics by batteries, hence the abreviation 'B' for DC voltage source...B+ refers to the high voltage DC provided by the power supply. I'd stick with the original 5u4 as even the 5u4GB puts out more DC than the old 5u4s which could strain the output tubes unless you throw in a voltage divider on the high tension at the output transformer/EL84 plates supplies. Good luck and keep one hand in your pocket! ;-)

You can always put a couple of 10v 5w zener diodes in the B+ line after the rectifier. Most 18 watt Marshal clones use a 290-0-290 or 300-0-300 secondary with around 350vdc after the EZ81 rectifier tube and filter cap.

[hoksila]

fly on the wall

thanks fella's
i'm gonna get this thing off the lot before i try puttin fat tires on it.
more reading-less writing...thanks again cats!I'll be around the campus.

[Rafael]

You can always put a couple of 10v 5w zener diodes in the B+ line after the rectifier.

No way, really? Why didn't I think of that?! I have a magnatone amp I converted to run EL84s but the 7189A B+ is waaay too high so I tried using a voltage divider which runs fairly hot....I'll have to give this a try...

[olddawg]

No way, really? Why didn't I think of that?! I have a magnatone amp I converted to run EL84s but the 7189A B+ is waaay too high so I tried using a voltage divider which runs fairly hot....I'll have to give this a try...

Yeah, put them in series. They get a little warm too but it is a nuch better fix. Just mount them where they can breath. It is very common to do tis in conversions. 3 of them will drop the B+ 30v etc.

[hoksila]

off the wall

oops... fell off the wall.
i'm thinkin that the alnico 12's(one for high's and one for lows...in the organ)will be cool for guitar,and suited for the amp.am i mistaken (once again)?
i've got a great 1-12 combo cab but the alnico's are 4 ohm(i think)so i cant use 1 unless i can adjust the imped to the speaker (1)also,i dont know if i should use the high spkr or the low,or just go w an 8ohm guit spkr. like a greenback? any thoughts???
found alot of very cool reading info on building,modding and basic amp theory.i have'nt actually tried to electrocute myself yet,but i'll get to it...
thanks alot (really) for the feedback and the patience. i really am reading and learning. ok going back to my wall now.(fly)

[olddawg]

oops... fell off the wall.
i'm thinkin that the alnico 12's(one for high's and one for lows...in the organ)will be cool for guitar,and suited for the amp.am i mistaken (once again)?
i've got a great 1-12 combo cab but the alnico's are 4 ohm(i think)so i cant use 1 unless i can adjust the imped to the speaker (1)also,i dont know if i should use the high spkr or the low,or just go w an 8ohm guit spkr. like a greenback? any thoughts???
found alot of very cool reading info on building,modding and basic amp theory.i have'nt actually tried to electrocute myself yet,but i'll get to it...
thanks alot (really) for the feedback and the patience. i really am reading and learning. ok going back to my wall now.(fly)

Your schematic is blurry but it looks like two 16 ohm speakers in parallel for an 8 ohm load. It will not damage your amp to run a 16 ohm load on it. If you want to run just the one speaker you should be fine. You have the speaker, why not try it. I run a 16 ohm cab on the 8 ohm tap for my 18 watt Marshall clone dimed all of the time. The only thing I would worry about is killing the speaker if you crank it. What brand of speaker? Model number? Also, there is no crossover so you could run both. My guess is that you won't be able to push them much. If the "highs" speaker is getting hit too hard you could always put a 22UFnp (or two standard electrolytics back to back) cap on that speaker's positive terminal so it won't get the low end at all. There are some organ guys on the board. Maybe they know what the stock speakers were off the cuff.