New amp project baldwin organ amp

[Sowndman]

I was gift the amp from an old tube repair guide. 1 12ax7, 2 5u4. And 4 6L6's. OP appears to have two outputs but one is only being used as NFB. The central iron must be a large choke. I plan to added a tone stake and hopefully another 12ax7. The main problem with the amp since I got it is it will work for about 10 minutes the loses a lot of volume. I am in the process of checking out the voltages to see what is going on. I had a tech do a quick check on it and he claimed it really needs all new sockets for the 6L6's.

There are some pics below as I do not know the model.

[trobbins]

http://www.diyaudio.com/forums/tubes-valves/237912-baldwin-45p-amp-project.html

[trobbins]

Plenty of room for another preamp tube or two, and a few taming stoppers would tame the overdrive nicely, especially if you ditch the feedback and add some bias senses and PT prim and sec fusing.  And dumb down the 0.1uF coupling caps!

[eleventeen]

I have the dual-6L6 version. Yes the middle guy is a big fat choke. HUGE B+ in that, IIRC the PT is 450-0-450.


You may wish to consider placing the tone controls on another chassis. I looked at mine long and hard and did not feel the chassis had enough width for a parts board, plus it is 3" high (awkward) and weighs a ton. You may come to a different conclusion.

[PRR]

> two outputs but one is only being used as NFB.

They are the same winding. The taps on a transformer are 99+% coupled to each other. NFB on one is NFB on all.

This IS a musical amplifier. I would not muck-around with the basic design.

NFB is small, but guitar amps often run less. Increase resistor "6" from 3.9K to 10K or 33K.

Do NOT change the power supply configuration until you UNDERSTAND what it is and what you are doing. Not choke-input but not full cap-input because the first cap is not "large". If you remove the choke and increase the caps to kill the buzz the raw B+ will rise an uncertain amount, possibly beyond what we can conveniently load with an available OT.

Likewise the big oil-caps should be retained until they burst or leak. They do not go bad like electrolytics do. This is a very deluxe church-worthy amplifier.

I would be very dubious about the "bumble-bee" caps. I've seen these just fall apart from age. And as mentioned, guitar does not want the heavy bass drive that an organ wants. 0.1u into 270K is near flat to 6Hz (dead-flat at 16Hz). Guitar voicing and typical guitar speakers would suggest 3 octaves less, say 0.01uFd. However the caps plus the OT roll-off with the NFB could get unstable. All things considered, 0.01u and a 10K-22K NFB resistor is probably a safe/happy setup for gitar.

The amp works between class A and AB. Very large idle current. Split among the four 6L6 it is 19 Watts each plate, so even older 6L6 will live long and prosper. 6L6GC or any modern guitar-market "6L6" will be quite happy.

You "could" re-tube with two 6550/KT88- the 38 Watts each is acceptable for good bottles (1960s Tung-Sol, 1990 GE, or 2005 Sovtek). I don't see any real point to this.

Power output is 30 Watts very clean, 50 Watts without strain.

So this is 50 Watts power in the length and weight of a 100 Watt guitar amp. The difference between fixed-install (church) and traveling musicians' gear.

With PT here and OT there and high-power wiring in the middle, I too would really think about external preamp. The sockets are FOR powering the organ console small-tube chassis, you got 280V DC which ought to be plenty, and a 6V AC CT winding for heaters, and the signal input is there. This convenience may justify buying a cheap _7_pin tube just to bust-off the base and use it for the plug.

117V wall power comes in at console 7-pin 1 and 7, must be jumpered at the speaker plug 1 and 7 (so the amp can't light-up without load). This means your main wall-power wiring is in the "console" box.

[dmp]

I did a remake of this same amp years ago. I found it sitting on the curb.  I gutted it and PTP wired a 5f6a / jtm45 amp. It sounded pretty good.
Added blackface reverb by squeezing in two more tubes on the far side.
Replaced the OT with a vintage Schumacher Twin Reverb transformer - huge improvement. Sorry to say, I didn't think the OT was very good for guitar. This made it a great guitar amp IMO.
But I've had some ground loop problems - and - like you - had reliability problems, that seemed to be bad octal sockets. It took me a long time to figure this out (if only I'd replaced the sockets right away).
Anyway, I decided to start over - replacing the sockets meant rewiring all the heaters and the ptp I had done was not very good. So I've just finished a turret board for it and am about to wire it all up.
The choke is 4H. I switched it to a standard pi filter (cap-choke-cap-resistor-cap etc... like the 5f6a schematic)
I'll post a pic of my front panel - I found there was room for the JTM45 arrangement. The reverb pot is on the back, since I added this after I drilled out the front. A master volume on the back to, since this amp is a beast and it is nice to drive the front a little harder.

[Sowndman]

I took two 6l6 out to test.

A voltage(b+) is 270 DC. Schematic says it should be 280+or - 5
6L6 plate voltage 310.9 DC but after 10 minutes was climbing to 313 DC

Working on adding another pre amp stage. All ready install a 7 pin socket for 6au6.

My amp has a 12ax7 instead of 6sl7.

The inverting circuit seems strange for a newbie.

[eleventeen]

I have one of these, the dual-6L6 version.


As I said, the B+ on these is very high, the PT is (from memory) 450-0-450 which should produce somewhat over 500 volts, loaded up. (1.4x = 630, 1.2x = 540)


I have not measured this on mine as I started dismantling it (because I did not see a way to build an amp in such a narrow chassis) Go look at the pix, the chassis is only about 1/2" wider than the PT, which is a big PT, but still. When I was pondering what to do with it, I did not think a preamp could be favorably inserted into the middle of the chassis given the layout plus I did not like the odd sized chassis.


But with a highest-high B+ of 500 volts strong, I very seriously doubt you have 270 volts on the 6L6 plates. Said differently, something isn't right if that's what you read on pin 3, 6L6. If that is what you are saying. Your B+ is running at half strength.

[sluckey]

The inverting circuit seems strange for a newbie.

It's called a paraphase inverter. It's an old design. Lot's of info on the web.