What to build with this> Ideas?

[mresistor]

Hi all, I'm contemplating what to build with the case amplifier out of the Model 40 Baldwin organ. It has twin 6L6 with a 6SL7 PI.
I was looking at schema's and noticed that it wouldn't be too difficult to build a 5C3P Proluxe in this chassis. As well one could build a 5F4 or a 5C5 amp. I'd like to hear what you guys think would be a good circuit to build into it. Any non-Fender circuits too. I would really like to use the choke, its a big one, its 8 henries. There is no bias tap off the PT. The OT has a 6600 ohm primary to 8 and 16 ohm taps. Has a 5U4 recto and the PT puts out 410v on the first node.

I have a schematic as well.

Here's the amp

[LooseChange]

It would make a great power amp as it. Need another tube to add a preamp.

[RicharD]

I agree with Loosey, leave the power section be.  Add a grounded cord, switch, fuse ect.  Devise a cable or 2 to connect B+, filaments, and signal return to a smaller preamp only chassis.

[mresistor]

Interesting, there is a ton of room to add a preamp in it. Might be a good idea.

[sluckey]

The PA is very similar to some of the old blue diamond Ampegs, B12, B15N, R15R, etc.

[mresistor]

Would you use a 12AY7, 12AX7 , 6SL7, or ? in the preamp section. The Proluxe uses a 6SL7 I think. this organ had a 6SN7 and a 5879 in the preamp section. All those Ampegs use a 6SL7. What about using a 6SN7?

[mresistor]

OK, I pretty much feel that 6SN7 is out. So I am going to try an octal front end. A 6SL7. If I don't like it I can always change to a 12A_7 type tube.

[sluckey]

Here's a simple preamp I put in one of my conversions. It would plug in nicely to the top side of your volume control.

[mresistor]

Thanks Steve - Looking it over, it looks great. What did you do with your heater on the preamp 6SL7? I've read some people say they can get a little noisy, so they elevate them.

[sluckey]

I just connected the heater to the existing 6.3vac heater string. Actually, I already had the 6SL7, but it was originally wired for a jukebox preamp.

[sluckey]

... the Bassman 5D6 did not have a bias tap, and the bias source was generated using resistor splitter.  Maybe some one on this forum knows how to pull this trick out of the hat.

Nothing tricky about it. Look at the schematic for Princeton Reverb or some of the very popular Marshall amps. Ampeg too.

[PRR]

> leave the power section be

Yes; except consider increasing the value of part 6 the 3.9K resistor for a less tight/tame sound.

Combined with Steve's 6SL7 preamp, you might omit one preamp cathode capacitor to get gain to a reasonable value. (You always want to try various caps there; 10-25uFd, 0.68uFd, open.)

[mresistor]

> leave the power section be

Yes; except consider increasing the value of part 6 the 3.9K resistor for a less tight/tame sound.

Combined with Steve's 6SL7 preamp, you might omit one preamp cathode capacitor to get gain to a reasonable value. (You always want to try various caps there; 10-25uFd, 0.68uFd, open.)

Thanks PRR  - I will experiment when I get it running. I was just looking at layout and noticed (and recalled) that the 6SL7 has separate cathodes, right. Hence the two cathode circuits. Which cathode cap would you omit/experiment with? the first stage or the second?

I was going to leave the PA cathode biased and select a good resistance for bias, if need be.

[jeff]

What's the third transformer on the left? I assume the two on the right are the PT, OT. Is that the choke??? Looks big.

[mresistor]

Jeff - The far right is the PT, the far left is the OT, and the big thing just to the left of the PT is the 8H Choke, it's a monster choke.

[jeff]

Holy cow. The choke's bigger that the OT. Well I guess all the current is going through the choke not just the screens like on most amps I've seen. Pretty cool. My vote's for keep it and build a preamp too.

[balcorn80]

Thanks Steve - Looking it over, it looks great. What did you do with your heater on the preamp 6SL7? I've read some people say they can get a little noisy, so they elevate them.

I'm no expert, but I LOVE 6sl7's. I even like the cheap Russian NOS versions that are around. They can be a little noisy. I have found that putting my heater's center tap(whether its one artificially created with resistors  or a real one) to one of the power tubes' cathodes(before the resistor and bypass cap. I have never used them in a fixed bias amp...) and this has really helped cut down noise in my builds with these tubes.

[mresistor]

Here's the inside of the PA and as far as I have gotten ...  I have ordered the parts. Going to mount the pots and input on one side and output jacks on the other side. Also there's a pic of the capacitors that were in this organ. Micamold Tropicap and Tiny Chief caps. Are these good caps? going to point to point wire in the Preamp and mount the 6SL7 in one of the empty holes you see in the pic. Remaining cap can is coming out in favor of more modern e-lytics mounted above the choke.

[mresistor]

Another question I have is should I use the old 4uf 600V capacitors on this unit? They are pretty large and modern electrolytics are quite a bit smaller. although the 20-20-20 cap can isn't leaking, the other one in the organ was and has been discarded. I think I should plan on new elytics. Right?
Notice also that lead dress didn't seem to be a top concern.

[mresistor]

I would replace the old 4uf 600v electrolytics.  I would even replace the 20-20-20.  Cheap insurance.  

What (brand/type/etc) would you replace them with? Actually I think they are non-polarized caps, don't know for sure.

[mresistor]

One of the two big 4uf caps is non polarized and permanently sealed (it appears), and I've read should they last a lifetime.  It is an Aerovox, The other one is an NBD, and I'm not sure about polarization on that one but it too is sealed. Maybe one of the old timers would look at it in the pics and see what they think of the NBD (one post terminal on that one).  I'll use them anyway. The 20/20/20 cap can is an FT, and I'm looking at replacing it with 22uf Illinois elytics I have on hand.

[sluckey]

Has any noticed that the circuit provided is a hot chassis?  A secondary center tap feeds the choke

That doesn't make the chassis hot. It just means that the HT center tap has a small negative voltage on it. Chassis is still at ground if the power cord green is connected to chassis. No different than putting a resistor or zener in the ct to reduce B+.

The choke works just as well in the B- lead of the power supply as it does in the B+ llead.

[sluckey]

This circuit is worthy of discussion.  The approaches the manufacturer took is different from the common guitar amplifiers most of us have been looking at.

That's true, but this isn't a guitar amp. There's nothing odd about this amp schematic when you consider the time period. The only things I see that may seem unusual to someone who hasn't looked beyond guitar amps is...

 1) The choke is in the B- lead of the power supply rather than the B+ lead.  So, it works the same in either lead. And since this choke is not actually connected to a high dc voltage, you don't have to worry about the voltage rating of the choke.

2) The B+ rail is not connected directly to the cathode of the rectifier tube. Rather, the 5VAC winding has a center tap and B+ is taken from that center tap. This was quite common in higher end amps in the '50s. I believe the center tap was an attempt to equalize the amplitudes of the positive half wave pulses produced by the unified heater/cathode of the 5U4. The amplitude difference is not much but it is there and it produces a 60Hz component on the B+ rail that's hard for a 4µF to smooth out. (Todays bigger caps don't have any problem.) Regardless of whether or not I'm on target with the theory, it was a common practice that has faded away just like the choke in the B- lead.

What clued me in to look at the set up was, B which feeds the center tap on the output transformer usually has the highest voltage and least number of caps smoothing the DC signal.

The OT center tap is connected to the highest voltage node in this amp and this node has a cap tied to the PT center tap and another tied to ground.

Also look at the way the tube cathodes are wired. In this case B is farthest down the line upstream of a bunch of caps and resistors, again indicating highest voltage is being fed to the center tap.

What do you mean? All the tube cathodes are tied thru a simple resistor to ground just like most cathode biased circuits. Are you talking about the cathode of the rectifier? Please explain.

Also note the other power source from the secondary is a center tap.

I don't understand this either.
   

[Tom_Hull]

heres a lowery organ schematic

[sluckey]

If you look at how the power is provided from the p/t to amplifier, power is being supplied from the center taps on the secondaries.  The center tap from 5 volt transformer, while the other leg is being powered from the center tap of the "high tension" secondary.

I thought I explained this. The center tap of the 5VAC filament winding is the equivalent of the battery positive terminal. The center tap of the HT winding is equivalent to the Battery negative terminal.

That particular circuit has been surpassed like so many other circuits. You may need to look at some '50s books to get it. It's not complicated. But Leo and Jim didn't do it in their lo-fi guitar amps.

And don't get me wrong. This is not just a hi-fi thing. It's just part of the technology of the '50s, based on the components of the period. That's just how they did it.

[Willabe]

The 5v rec. heater wind with a CT is a coil of wire and if you take the B+ from the CT it acts like a choke.

Sluckey corrected me on this one time, IIRC it only works if the rec. tube has an indirectly heated cathode? 

[Shrapnel]

I forget where I heard this, but...

I've heard that the CT on a 5V power winding (used to power the recto tube) was also a way of hum reduction induced from the 5VAC...

If this is preposterous, let me know so I can forget that bit of knowledge.

[sluckey]

Is there continuity between the center taps?

No.

Just curious, how many guitar amps were designed using this approach.

I've never seen a name brand amp using a 5VAC center tap. 

Regarding chokes, We know from the schematic, the value of the installed choke.  What choke value would be expected from this design.

No different from a choke in the B+ side of the supply.

Regarding hum, it the hum 120 Hz or 60 Hz.

The power supply ripple is 120Hz, just like any typical full wave rectifier. The type rectifier used will determine the ripple freq.

[mresistor]

Very interesting dialog about this amplifiers design.  Learning more every day here. 

The amp itself is so tall I had to build a cradle for it so I can work on it safely. 

[mresistor]

Hey guys, could use a little help deciphering what the wire is at point 27 on the schematic. It looks like a wire bypassing the 1250 ohm resistor between the 4uf and one of the cap can 20uf caps. ?? What is it for?

[sluckey]

It's a resistor. The dashed lines suggest it's optional. Note 27 probably explains.

[mresistor]

thanks Steve, dummy me sold the manual with some of the parts so I don't know what note 27 says...   

[DummyLoad]

i suspect that 27 is reference designation and that it is optional. used to "tune" g2 voltage or compensate for different PT type(s) or organ preamp load(s) perhaps?

i'd keep the current configuration and roll with what's been built and document that.

--DL

[mresistor]

i was able to get a .pdf scan of the notes, and note 27 is indeed an optional 15K 1w resistor. Not going to add it unless necessary.

[Willabe]

note 27 is indeed an optional 15K 1w resistor.

Optional as/for what?   


                     Brad       

[mresistor]

Making a little headway on this octal amp, I got the octal tube socket installed and made an aluminum piece to mount the preamp on, and got it populated and installed, the cathodes and plates are hooked up. I had to remove the old cap can and I also made a stainless steel plate to cover the two unused holes where the cap can and a connector were....



moved the filter caps over to near the power trans, seemed like a logical spot for them.



I'm waiting on some 1/16 " thick maple I'm going to use for the faceplate, ala trainwreck style..  Going to build a head cab for it..

it's not perfect but I think it will work just fine....

[mresistor]

I have the amp wired up today and fired it up. Sounds great !! but I have no volume or tone controls. Going to have to backtrack and take a look at the pots and preamp circuit. Something wrong there for sure, but the preamp 6SL7 is working fantastic. This amp sounds really nice, and should be better when I get the volume and tone controls working. I was working off of the normal channel of a Fender amp as a guide. But, it's all physically backwards. Probably got something backwards. If I have to I'll draw it out.

[mresistor]

Whelp-  first thing I found is I had pins 2 and 5 interchanged on the 6SL7 preamp tube.  That isn't good. Swapped them and I have volume control, and some raw adjustment , but no treble or bass tone control..   

[Tyrannocaster]

Wow, what a neat old amp! Congrats on that find. Nice work with the aluminum, too.

[mresistor]

Thanks man. It is up and running. I have no tone controls yet, I have to t-shoot that out. Here's a pic and a vid so you can hear it. HPB it is very clean and I suppose you're right about the feedback. I'll try it without feedback and try all the things you guys have mentioned to try as soon as I get the tone controls ironed out. I have to say the bell-like sound is pretty nice as it is, and it has a funky kind of nasal sound to it.  (pardon the terrible playing, I'm a hack and was not warmed up at all) (the guitar is a Music Man Sub 1 with tremolo and fitted with two Brian Gunsher Pure 90's. )



what it sounds like now

[Tyrannocaster]

It sounds good although I have to make allowance for the compression your video cam is adding, which is pretty heavy. But it does seem like it makes a great clean amp. Let us know how it sound without the NFB. That should make it somewhat more raw sounding; if there's much difference you could put it on a switch.

[tubeswell]

Sounds beyootiful! As far as the TS controls go, maybe the stack isn't grounded properly?

[mresistor]

Well, the ground for the stack is on the raw pot. I now have the tone controls working as they should, but at a big drop in volume. I jumped around the raw pot, so the stack would be normal with a ground going through the 6.8K resistor to one side of the bass pot. I did have a chassis ground wire on the raw pot ground, but I also took that off and am getting ground through the pot shell to chassis. I'll try putting that star ground wire back on the end of the raw pot.
This is a little perplexing.

[Tyrannocaster]

I now have the tone controls working as they should, but at a big drop in volume.

You may find that's just the price of having the tone circuit in there. They do eat up a lot of dB. Unless you're talking about a loss on the order of 40 or 50, I mean.

[mresistor]

In the vid I made the vol was at 25% now I have to turn it up to around 65% to get the same loudness. I'd say quite a drop.

[HotBluePlates]

If you have a blackface-type tone circuit in it, that's about normal. It's why folks remark that a 5E3 Deluxe tone circuit has "low loss" compared to the blackface circuit. Or said another way, why a blackface VibroChamp is clean but can have the gain pumped up by disconnecting that tone circuit from ground.

It's also why the raw control actually does something when you turn it.