Hammond M3 to 5E3 conversion

[Pdavis68]

I got all the parts for my Bandmaster build and I detoured on that for a few days, but I'm going to go ahead and finish the 5E3 build first. My reasoning being that I care more about the Bandmaster and I'm still at the stage where every amp is significantly better than the one before it. So I should practice more on the 5E3, then do the Bandmaster...

So without further ado... I cut out some sheets of metal and riveted them to the front of the chassis to cover where the terminal boards were. It created a lot of headaches for drilling holes (not a problem with the next build as it's a pre-drilled chassis!!!!)

After doing that, I got the holes drilled out. I tore up one of my little metal sheets in front drilling holes, so I had to drill out the rivets, cut a new sheet, drill it, blah blah blah... Accidentally broke the plastic fitting that held one of the big caps, so I had to yank that out, cut a sheet to cover the whole it left, drill and then rivet it in... Used a wire brush drill bit to scrape off some rust spots. Tedium... Not the part I like doing...

Got everything drilled out for input jacks, speaker jack, power switch, pots, fuse. So I'm all set to go... I'll have to drill a hole to screw in a terminal strip, but I'm not 100% sure where I want that yet. I'll drill it when I figure that out.

So I should move forward quickly on this, now that the slow, boring stuff is done... I've decided I'm going to add Rob Robinette's Fixed Bias/Cathode Bias switch mod. I want to play with fixed bias and I have to drop 60V off the PT anyway, so I'm going to use a string of 5x 12V 5W zeners to create a -60V tap off the CT of the PT secondary and then I'll add a voltage divider with a pot for it. Now the fun stuff. I'm excited...

[Pdavis68]

I've got the parts for the Bandmaster/Concert on order. They'll be in this week.

So yesterday I had nothing to do and decided to start cleaning out the M3 chassis. Removed components from the chassis and the two board. Washed the board down in acetone and then rubbing alcohol. I removed solder from the sockets I'm going to reuse (2 octals, 2 novals. I left the rectifier and heater wiring in place, 'cause... "Done!").

So then I decide I'll go ahead and start populating one of the boards for the 5E3, with whatever components I had on hand. I had completely forgotten about a bunch of .1uf polyester caps I had on hand. So next thing you know, the board was done...

I need to put rip out one of the strips of terminal connectors on the front (already removed 2 others) and then I need to put a metal plate along the front and rivet it into place, to cover the holes left by the terminal strips. When I'll drill out the holes for everything.

Turns out the only components I didn't have on hand to complete the build are filter caps, a power switch (not doing a standby switch) and a power indicator lamp...

So this thing is going to be done in no time...

You'll notice I didn't leave room on the board for the filter caps. There are some terminal strips on the inside of the chassis that are well placed for them, so I'm going to put them there instead.

[Pdavis68]

Doh! Of course. As I pointed out, doesn't need a lot of current, so a voltage divider ought to do the trick. Thanks, Justin! Sometimes I overthink it...

Yes, the 6G12-A/6G7-A are the target. think they're pretty much the same. In fact, if you overlay the schematics and layouts, they're from the same hand drawings, just with a different label.

I'm trying to decide on the caps. My original plan was 7 F&Ts, but I'm starting to wonder if maybe a JJ 40/20/20/20 can and a JJ 16/16 can on top might not be be a better choice. The first node is 2x20uf in parallel, so 40uF, so that works and keeps my e-cap costs to under $25. I was going to buy all nice coupling and tone caps too, but now I'll just use whatever I have handy, which includes some no-names, and buy what I don't have.

[Justin Thomas]

Rather than make a quadrupler, you can just tap off the HVAC winding like Marshall and some Princetons... just might be a simpler route with less parts. Besides, if youo use a switchable rectifier, you'll need that 5V winding after all, right?

Ir you're doing a Concert, may I recommend the 6G12-A? And yes, the Normal channel <IS> worth having, too. Go all out!

Cap brands didn't matter in MY Concert - somebody swapped ALL of my caps out, and it still sounds damn good. If that helps you save any money...

Justin

[Pdavis68]

I spent a few hours thinking about all this last night and I think I've decided what I'm going to do... I'm going to build a Concert-ish kind of amp, but it's going to be a bit of a departure from the real thing. I've been really wanting to build the Concert, but it's looking like it's going to be at least a year, if not 2 before I'm going to be able to build it with the parts I want, so....

1> I discovered that I have 335-0-335 PT. It's a monster from a Baldwin organ. It weighs in at around 8lbs...
2> I have the Trainwreck OT
3> I have a choke from a Hammond H100 organ that's big enough to handle 24W in power tubes and maybe 8-10 preamp tubes.
4> I have a Bandmaster/Concert chassis.

I'll have to cut out a bigger hole for the PT and drill some new screw holes, but that's not too bad.

There's no bias tap on this PT, but it has 12.6V and 5V taps I don't need. I was thinking I could put them in series for 17.6V, create a voltage quadrupler and then have a pot to make the bias adjustable from there. I'm thinking since it's grid bias it won't need to supply much current, so it shouldn't matter that its' a voltage multiplier as the source. Am I right about this?

The chassis I have has a 3rd octal hole, so I'm going to set it up for switchable diode & tube rectification. Between that and the adjustable bias, it should be able to handle a wide range of power tubes (6V6-EL34 and possibly KT88s. Not sure about the power handling of the OT. Sent a question to Heyboer about it) with a good impedance match on the OT.

I'll buy the parts that I don't have piece-meal over the next few months, but with the iron in hand and opting for some cheaper parts than I was originally planning, I should be able to keep the price tag within budget.

Anyone see a problem with the bias idea or anything else here?

[Pdavis68]

I would actually prefer a 6L6 or EL34 amp, but then I wouldn't be able to use the OT. I do have a Heyboer Trainwreck OT that I got when I was thinking I wanted to build a TW, but I'm not really that inclined to do a TW anymore, so I'm happy to use it in something else. But if I go that direction, then I probably just won't do a 5E3. I'd build something with an LTP PI.

I'm going to need to think on this a bit. Why do there have to be so many options?????

[mozz]

Chances are the M3 tranny has a 110v or 117v primary. So figure at least 5% higher on the secondary with todays 120-125v. Quick test is just wire up the filaments to the 5y3 and see how much higher that is than 6.3vac.

I use a old chassis as a mock up, AC in to subject transformer with alligator clips, choice of 5Y3ish wired tube socket or diode bridge rectifier for those trannys with no 5vac. A old 8mf 600v cap, then 2 8k parallel switchable load power resistors, 2 pin sockets for the meter. Almost anything i test, especially old organ transformers, put out more voltage than the days gone past. I have some i can't use for 6v6 pairs because they are too high. I'd post a picture but my homeowners and life insurance companies would drop me.

Any more when i pull parts unknown, i write down how much tube current it was used at. Deluxes do sound best at certain plate voltages for the output tubes and preamp tubes, i often tweak the node resistors and 5y3, 5v4 5r4 5ar4 for rectifiers. You could always pull a 5e3 Neil Young and use 6l6's.

[Pdavis68]

I'm just going to do the diode stacking and drop 50V off the transformer. I can get 5x10V 5W diodes and run them to the HV CT and I'll be right on (actually probably a bit hot anyway because it's a 110V PT). Then I don't have to worry about it.

[Pdavis68]

According to this, it would be 475V off the first node without a choke (I calculated with 375VAC and then added 7V to their result. Close enough.) So yeah, I think you're right. That's too high for 500V caps.

But here's what's got me confused. Classictone's transformer for a 5E3 is 355-0-355. But now that I'm looking around at other transformers, most of the others look like they're 330-0-330. That gives 413V on the first node.

What's adding to my confusion is that Rob Robinette's page has the voltages he got with his Weber 5E3 and it's only 362V on the plates and you know it didn't lose 50V in the transformer... And even more confusion: on Rob's page, at the top, if you scroll down to the first layout titled, "The Unmodified 5E3 Deluxe", it shows a 355-0-355 power transformer....

I think I trust that calculator the most...

[Justin Thomas]

The only thing that's throwing me here is this: I see your point with the math, and even including voltage drop for a 5Y3, I'm thinking you'd end up with about 440-450? But, would the M3 have been running that high? I know the M3 has about 6 more preamp tubes than a 5E3, but I don't see preamp tubes drawing enough current to drop that theoretical 440 down to the 305 on the schematic... unless they were using a straight-up choke input filter maybe? I don't have an M3 schematic in front of me...

My friend had an M3; I don't remember it having the 30 12AU7s that some of the others had...

Justin

Edit: is 380 a typo and supposed to be 280?

[mozz]

Your 380v will be over 400v, times 1.414 and you are over 500v easy. Even loaded with 4k you are gonna have very high plate voltage. Just build the mock up and you will see.

[Pdavis68]

If I build the PS like a normal 5E3, I don't see why my voltage would be much more than ~35V over that of a normal 5E3, given that the unrectified voltage is only 25V higher. 5E3 schematic calls for 450V caps. 500V caps will give me an extra 50V rating with only about 35V or so increased DC.

Am I missing something?

[mozz]

Your plate voltage is going to be a lot higher than you think. Also, when you turn it on, the voltage across the caps will be over 500v or more , (i have seen it many times but none blew up yet) until the rest of the tubes warm up and start conducting. Your best bet is to wire up the power transformer, basically build the power supply in a mock up, and see what voltages you will have.

[Enzo]

When they were being made, 5E3s didn't even sound like one another. If you went to buy one, you generally tried as many of them as you could to find a "good one." The wide variations in parts resulted in each amp having its own characteristics.

[Pdavis68]

Thanks, Justin. That was kinda my thinking, at least in terms of the, "You might end up with something that kicks ass..." I was thinking, it probably won't sound exactly like a regular 5E3, but that's not necessarily a bad thing. I mean, the one thing I would want from a 5E3 is more clean headroom and the extra voltage (and the NFB) ought to achieve that. So obviously a stock 5E3 isn't really my goal anyway.

I just wanted to make sure I'm not deviating too much. I'll probably use a JJ 16/16 can plus a 16uF axial. Those will be rated 500V so that should cover any voltage rating issues the extra 25V might cause.