5F6A in a Baldwin Chassis

[mresistor]

Guys I am progressing on the conversion of my Baldwin Orgasonic organ amp that I made some years ago. I have the chassis basically gutted and am looking at tube placement. There is one octal socket near the edge and a hole there next to it that could be enlarged with around 5/8 between the edges of octal tube sockets if I did that. The 6L6-G that were in the amp were in octal sockets mounted nearer the center of the chassis.  I would like to use those for the bassman 6L6GCs, and for space issues if I use KT66 in the future,  but if I do the power tube sockets will be underneath the turret board, but sort of to one side of it.  I've been thinking about it and two things come to mind. Testing and repair if necessary in the future. Since I am using a Hoffman board I see that I could check the screen, grid and cathode voltages on the board end. I would have to install a test terminal to check the plates and filaments. I suppose I could install a test terminal strip out in the open somewhere with the power tube pins connected to dead end terminals. Another thought was to check from the outside/topside of the chassis somehow. For repair the pots and inputs could be undone from the chassis and the board unbolted and swung up out of the way. The chassis is 2 7/8 deep so mounting the turret board well above the power tube sockets is easy to do, and in fact I have planned on elevating it some, say and inch or so.


I wanted to see what you guys think of this before proceeding to drill anything. A pic is below.


Another question I have is the choke. I intend to use this big original choke. The schematic says 8 W or 8 H I cannot tell for sure, and 30 ohms which I can make out clearly. I am sure it is probably overkill for a 5F6A but don't know for sure. I do have a 22699 bassman choke that I could use, which would lighten the weight of the chassis considerably. If I don't use the original choke HPB has in the past already spoken for it.


 

[sluckey]

I would use the bassman choke.

I would not put any sockets under the board. That's tempting some possible regret. I would punch two new socket holes and move the OT. Maybe even put a cover plate over those socket holes...

[mresistor]

Thanks Sluckey - I suppose I could cut the two new holes easy enough with the drill press. Why would you move the OT? 

[sluckey]

To get it away from the sensitive preamp end of the board, input jacks, volume tone controls, etc., and down close to the output sockets. The Hoffman 5F6A board was designed to work with this layout.

[mresistor]

OK, I'm going to use the 22699 Bassman choke, and I am going to cut an 8 x 4 inch piece of stainless sheet metal like the piece already on the chassis to cover the area where the OT was and the two octal holes in the middle as well as extraneous other little holes in that area. Then I will drill another octal hole on the edge near the one octal hole.  The Baldwin choke weighs 5 lbs 15 oz.  and the 22699 choke weighs 15 oz - so that is a weight savings of 5 lbs! The cabinet will appreciate that..  I can then relocate the OT to the new piece of stainless and all I have to do is drill 4 mounting holes and two wire feed-thru holes. 

[mresistor]

In my picture (above)  that black thing on the bottom left is the Baldwin choke. This pic (below)  is what it looks like now, and you can see I have more room, with that big choke gone, to drill another octal hole to the left of the one that is along the bottom edge..  I can mount the 22699 Bassman choke in the center line, leaving room for the new octal hole.  HPB  this Baldwin choke is yours if you still want it, just pay shipping in a flat rate box. 

[mresistor]

I might cut a new piece of the stainless sheet metal that is longer, like 10 inches in length,  10 x 4 , and take that piece that is on there off, and replace with one piece. It would look better.

[HotBluePlates]

In my picture (above)  that black thing on the bottom left is the Baldwin choke. ... HPB  this Baldwin choke is yours if you still want it, just pay shipping in a flat rate box.

Oh, jeez! Now I'm gonna have to build a choke input class A amp around that choke! Sure, if you have no other plans for it, I'll take it. I'll PM with details.

Thanks!

[eleventeen]

Want another one? Nobody should have to go through life with mismatched chokes.


Organ chassis can be problematical. I think we've had this discussion before.


I strongly suggest you place your tubes where they will go; get a piece of cardboard the size of the parts board you are going to use, and just be sure you have the room for the controls. As I believe we've had this discussion before, I'll stop.

[mresistor]

I share your concern eleventeen - I think I'll have enough room in this chassis for the hoffman board and the controls, tube wiring, etc.  I wanted to re-purpose some Sprague Atom 20uf 600v caps for the first filter node but they are too large to fit nicely in the PS end of this chassis, and still have room to fit the other caps. Still looking at a vertical mount for the other caps...  maybe I can squeeze them in there..... It's got almost 3 inches vertical but as you can see not a lot of horizontal area...

[mresistor]

I'm also thinking I might not have a sufficient OT. This Baldwin OT was for two 6L6-G tubes and it has 8 and 16 ohm taps.  I might use this one that has 2,4,8 ohm taps.. http://www.classictone.net/40-18000.html
and a decent price.

[eleventeen]

I'd think your existing OT would be perfectly fine if you are making a dual-6L6 amp. I forgot whether you started with a 2x 6L6 Baldwin chassis or a 4x. Admittedly you may be lacking the 2 ohm tap on the secondary, but I would just re-wire the speakers to a 4 or 8 ohm configuration. Will that affect tone? I can't tell you. I know we want to do all mechanical work (eg drilling holes & mounting components) before lashing up the wiring but the fact of the matter is that even though it is physically big, changing out a standup OT (after the rest of the amp is assembled) with appx the same footprint is not the biggest of big deals....if you have a drill press or can otherwise control drill depth. Yes, the steel chassis does not make things easy.

You may be forced to consider some type of doghouse for your cap collection. Sometimes I make a board type deal and silicone a row of PC-mount caps to it. That would be under the chassis, if you can find room. On top of the chassis, perhaps under a plexiglas insulating / protective square mounted on standoffs. At least those PC mount e-caps are cheap.

As you can see and which I hate to rub your nose in, the tight dimensions of that particular chassis create certain problems that can't be easily escaped and which force you to do non-trivial work to get around. I feel your pain on this one, I have that exact same chassis and I have stared at it for a long time without starting anything on it because I run into issues I am not eager to deal with.

[sluckey]

I agree. That OT is fine.

That big choke was there for a reason. Baldwin probably didn't use a 5lb choke if a 1lb choke would work just as well. The size of that choke indicates that the entire B+ current flowed through it. And that means there's a chance that the power supply used a choke input filter. Did you draw the schematic of the power supply before you gutted the chassis? And here's a question for you that may make you change your mind about giving away that huge choke. How much AC voltage do you measure between the HT secondary and HT center tap?

Most times it's advisable to keep PT, choke, and OT together because they are proven to work well together. If the original amp had a choke input filter then you will need to use the same choke input filter. Otherwise, your B+ will be very much higher than you will probably want.

[mresistor]

  Here's the schematic for it. The choke is on the ground side,  so  I don't think this is a choke input PS . 

[mresistor]

Sluckey - I'm getting 442VAC from one lead to center tap on this PT unloaded ..

[sluckey]

That's the classic choke input filter. Choke can either be on the positive side of the rectifier or the negative side. Makes no difference. Without that choke your unloaded B+ will be 625VDC! Better hang on to it.

The attached is from RDH4...

[mresistor]

Well guess I better tell someone that I am keeping this choke .. 

[mresistor]

OK   so  what would you do if you are going to make a 5F6A  bassman circuit with this iron setup. Would you use the choke on the PT secondary center tap to ground as in the Baldwin, and omit the Bassman choke, or would you use this Baldwin choke as it is used in the 5F6A circuit? I'm thinking that if I use this choke on the center tap to ground, then another choke would not be necessary in the PS.

[eleventeen]

Excellent point brought up by Sluckey and you owe him a beer. That PT is a monster that puts out essentially 900 volts CT no load. (I remind you, I have that same chassis and those trannies and I have measured it/them; both volts and inches. That chassis sits on my junkpile with the OT removed) The choke being in the CT doesn't much matter, ALL B+ current flows through it and with or without it, your B+ is gonna be really high, figure 440 * 1.4 = 616 volts minus 50 or so for a 5U4 and minus 10-15 volts for choke input = 550 volts, 520 under load. Very high, almost 100 volts high.

[sluckey]

OK   so  what would you do if you are going to make a 5F6A  bassman circuit with this iron setup.

The only reason I'd ever use that PT to build any guitar amp would be just to see if I could. The last project I did was a Hammond AO-63 conversion. It used a (swinging) choke input filter so I had to do the same, otherwise, my B+ would have been sky-high.

But I'm a tinkerer and I understand wanting to use what you have and convert it to something you can use. I'd put the choke between the cathodes of the rectifier and the first filter cap. (Look at the power section of this schematic) And the center tap straight to ground. And tape off the center tap on the 5 volt winding.

You need to use this choke with this PT.

[PRR]

> classic choke input filter

IMHO-- this is neither fish nor fowl.

Cap (26) is clearly in the "Cap Input" position. No-load it will charge to Peak of AC, 618V.

BUT a "good" cap-input cap for a 200mA amp would be 200uFd. Here we have just 4uFd, 1/50th of "good". We suspect it may sag.

Duncan PSD shows C(26) voltage going to 575V and to 300V on every half-wave. WAY too much ripple! Then Choke and C(2) filter that to about 435V (averaging the 575V-300V swings) with 20V p-p ripple, an acceptable value.

As you say, true cap-in tends to 618V, true choke-in tends to 398V, and this is something else.

_I_ think Baldwin were not as dumb as they look. The 4uFd 8H 4uFd makes a very reasonable B+ with a minimum of capacitance.

Why minimize caps? E-caps are cheap? I bet this is OIL Caps. E-caps fail. When you sell to churches which have stood for a hundred years, they don't want to find dead caps every 5 10 20 years. Oil caps will last longer than the rubber on the line-cord. This is a lifetime power supply. Assuming you KEPT that oil-cap bank, didn't chuck it out.

[mresistor]

Sluckey  thanks for the assistance and the info you have provided. I have certainly learned something. So it looks like I am at a fork in the road.  I could use this Monster PT with high voltage and current capability along with its Monster Choke and try to finagle a 5F6A sort of circuit. 
Or, I could source another more appropriate pt, in stand up configuration that I could use by covering the existing pt hole in the chassis with a bit of my stainless sheet metal. Going this route may cost some, but I would also gain better more suitable voltages for the 5F6A, more area in the chassis where I could use my 600V caps, and not use that boat anchor choke.
I already have a cabinet made but not covered for this chassis.


Would this be a suitable PT for a 5F6A in your opinion? http://www.ebay.com/itm/PT330M-USA-POWER-TRANSFORMER-TUBE-AUDIO-330-0-55-330-x140mA-/15214569578

[mresistor]

Thanks PRR    yes I still have the big 4uf oil caps..  which appear to be sound. The problem with the 4 uf oil caps in doing this , is they were mounted down on the opposite end of the chassis, which would now be the preamp end of a turret board.

[HotBluePlates]

OK   so  what would you do if you are going to make a 5F6A  bassman circuit with this iron setup.

... I'd put the choke between the cathodes of the rectifier and the first filter cap. ...

I wonder about how much insulation rating the choke has since it was positioned from center-tap to ground. Sometimes that is a reason for not using the choke between rectifier cathode & cap (lower voltage from coil to case). Or maybe the arrangement is just a clever way to get something between cap-input and choke-input voltage from the power supply, as PRR points out.

OK   so  what would you do if you are going to make a 5F6A  bassman circuit with this iron setup. Would you use the choke on the PT secondary center tap to ground as in the Baldwin, and omit the Bassman choke, or would you use this Baldwin choke as it is used in the 5F6A circuit? ...

OR use the Baldwin plan of choke from CT to ground and add the normal Bassman choke between plate and screen nodes. I doubt this route will be well-received, but I did just build a 1/3w amp with two chokes on board (though they are both physically small).

But my overall inclination would be (as I've often said, maybe even about this project before) to keep the entire PT, power supply, OT and output tube setup as it was in the original circuit, then change the preamp/phase inverter to the desired amp circuit.

... The problem with the 4 uf oil caps in doing this , is they were mounted down on the opposite end of the chassis, which would now be the preamp end of a turret board.

That's probably a non-issue, except for any consideration of space.

Have you already bought the 5F6-A board? If not, and you keep the power supply & output stage, then you don't need the room for the bias circuit and can mount screen resistors and grid resistors on the sockets. You could likely get a custom board which just has the preamp and phase inverter...

[mresistor]

Hi HPB - no, I make my own boards, and I haven't drilled any G10 yet (thank goodness) .  Interesting idea about the custom board.
Wow, I see Doug has a Heyboer PT for $70 with super specs..   Nice..    Mojo 762...   

[mresistor]

I think if I am to proceed with this PT and Choke, I would use Sluckeys AO-63  power supply as a guide.. He used a GZ34  so a 5U4 would drop the voltage some... maybe like 435v instead of 475v.
Then I would have to use a tap off the secondary for the bias voltage..

[HotBluePlates]

I wouldn't bank on such a large drop... Maybe more like 450vdc (or a bit more).

[mresistor]

I'm going to sleep on this whole project, part of me says like eleventeen to put it on the junkpile, and another part says  give it a go.. but I have a Super Reverb project in the wings   and as much as I would like to hear own and feel a 5F6A bassman  it may have to wait.  Many thanks to all who have provided input and ideas... 

[sluckey]

One more thing to consider... This chassis was a power amp only. The layout is fine for a power amp that deals with power amp level signals. Trying to shoehorn a guitar preamp into a power amp ONLY chassis can sometimes be very challenging. You need a spot on the chassis that is out of the way from all the big signal, high current stuff, reservoir caps, OTs, etc. And you usually need a well thought out layout that may not look much like a production guitar amp layout. It's usually easier to just restore the power amp and build the preamp on a separate chassis, maybe even bolted to the end of the power amp chassis. You can still power the preamp from the power amp supply.

[PRR]

> Maybe more like 450vdc (or a bit more).

Duncan PSD says 482V, with 5U4 and default winding resistances. (2K DC load, which approximates full output in a 4KCT OT such as 5F6a.)

Rising to 533V with 5K load (roughly 500V at the ~~100mA a Bassman usually idles at).

In retrospect, this was probably a fine Power Amp (most older organs are), and would have been a fine guitar amp (maybe without the rude growl of some g-amps). Of course it's been torn up to try to fit a preamp where none was intended.

Hmmmm ... seeing it was 6.6K load, and cathode bias, I wonder if the current rating truly matches a 4K load fix-bias. (It would be alright because it was over-built for long-term church use, and a fix-bias amp idles some time and then runs much lower current.)

[eleventeen]

I think the "separate preamp" notion was where we arrived the last time one of these particular Baldwin projects was being tossed around.


This PT is not going to produce a 5F6-sounding 5F6, IMHO. I think it will be a lot cleaner with 80 more volts of B+ unless you use zener voltage reduction = yet another physical assembly you have to create in an already cramped space because you will need high wattage zeners on heatsinks, several of them.


To make what I said earlier clear: This thing is not on my junkpile because it's junk, it is there because after buying it and shipping it here and staring at it at reasonable length, the sum total of the rigamarole I could see that would be required to 1: reduce the B+ to the output stage and 2: super-reduce the B+ to the preamp stages, 3: cram the parts in in a way that let's face it, there is always a risk that the thing will turn into an incurable hum monster with an improvised layout, 4: end up with a kind of awkward 3" tall chassis 5: have to work thick steel (vs alum) added up to an amount of time+money+effort with which I could buy a Mojo chassis....that would fit everything in on a known basis....and be able to accept a faceplate to make the final product look nice....and be done with it. Yes, there was an easy $70-$100 extra work getting it in shape as I saw it then and see it now. 


I don't mean to discourage. I hope you understand that.

[mresistor]

Hey eleventeen - I understood what you meant, no problem bro, I have a pile too..and not all of it is amplifier stuff.   :-).  This thing has sat in the "junk pile" room for nearly 2 years now being totally neglected. Ah well I have many original Fenders that don't get the attention they deserve either.  But I do drag them out and turn then on from time to time, maybe the intervals are too long though. Anyway, it was just time for me to make a decision on this one and get it done. I am trying to reduce the pile some.


I had already installed a rack of diodes on the center tap to lower the B+, the terminals fit nicely beside the PT end bell on the side of the chassis. The steel doesn't present that much more of a problem for me. Have drill press, grinder and carborundum discs will travel, LOL. What I could really use is a metal brake and some hole punches. I actually have a faceplate fashioned but it's not drilled, stained and finished yet. It's thin hardwood and with a wood burning tool the controls and such can be labeled, ala Ken Fischer. I wasn't considering building it to sell it, so as long as I like how it looks it's good enough.


So, this PT is not suitable for what I want to do with it. Having a PT with a bias tap would be nice too. Now if I used a stand-up Bassman PT on this chassis, the goal  of making a Hoffman 5F6A could be achieved. suitable PTs are not cheap, but it seems the only viable solution for this project. So I'll look for one for as little coin as possible.


Who in the world would want this Baldwin PT?  What use would it be to someone? Or is it really a boat anchor destined for the scrap heap? I really wouldn't want my relatives burdened with it after I pass on.


Sluckey, PRR, eleventeen, HPB - many thanks for all your input, it is much appreciated and respected.

[mresistor]

Have you folks seen this about back biasing? Looks like I could drop some PT secondary voltage this way and also get my fixed bias voltage. 


http://www.aikenamps.com/index.php/what-is-back-biasing

[eleventeen]

Very interesting set of circuits there.

[mresistor]

Ok guys,  lend me your judgement here..   which one of these would be best for a 5F6A build? (these are two reasonably priced stand-up PTs)


http://www.edcorusa.com/xpwr178
or
http://www.edcorusa.com/xpwr172


And am I safe to assume that all that would be required for using one leg of the PT HV secondary (330v leg) is to adjust the value of the bias range resistor on the Hoffman 5F6A turret board?

[eleventeen]

Either one of those, prefer the 330-0-330 volt one as it is closer to 5F6 specs....Believe it or not, earlier 5F6 used an 83 rectifier which is a mercury vapor tube....with VERY low voltage drop. Play too loud, not only the cops but the EPA shows up!


I think I would go with the higher voltage one, but let's wait til some more tweed-oriented (than I am) guys chime in. 
 



You could certainly use a 150 ma model if they make one. 200 ma is getting on to Twin (4x 6L6) range.


"And am I safe to assume that all that would be required for using one leg of the PT HV secondary (330v leg) is to adjust the value of the bias range resistor on the Hoffman 5F6A turret board?


Absolutely.

[sluckey]

I like the 172 better.

And am I safe to assume that all that would be required for using one leg of the PT HV secondary (330v leg) is to adjust the value of the bias range resistor on the Hoffman 5F6A turret board?

Yes. New value will be approx. 100K.

[HotBluePlates]

I think you already know my vote: Use this chassis almost exactly like it was to start with.

Everything works well together in the original plan, and you can adjust the 3.9kΩ feedback resistor if it's too polite. The paraphase inverter probably adds some welcome color to the sound, and you already have a built-in master volume.

I've built an amp with a split preamp/poweramp chassis, and it's still running along beautifully. It would surely be less money to build only the preamp chassis and use a cable between the two...

Despite the other recent thread showing Gibson using a cathode follower output from the preamp, you probably don't need it for any reasonably-short cable run (I sure didn't use one).

[mresistor]

Sluckey -that was what I was thinking concerning the bias range resistor. I was going to figure it out using 10-15ma current as a guide.  Thanks.


HPB - I have to say sorry, there is no going back to the old circuit. I look at tweed bassman amps and wonder why everyone is worried about hum. Some of those tweedies were pretty tight chassis and they didn't exhibit a lot of hum. So I am determined to make a Bassman 5F6A in this chassis. If it doesn't work, I can always yank the turret board, tubes and sockets and OT and use whats left for target practice.
I do hope the big choke winding insulation can take the higher voltage, you did bring up a valid point. So I guess I'll see.


Plus - I have a set of EL34s that might be good in this amp. Up the screen resistors to 1.5K 3 watt and make sure the screen voltage isn't over 425v and all should be good. So, I guess it won't be a stock 5F6A but
might be a nice amp anyway.

[eleventeen]

Incidentally, this is pretty kewl...


This is a mercury vapor rectifier powered 1000 watt amplifier and the guy is feeding a low frequency signal thru it. He is apparently in Europe where the power line freq is 50 Hz. Check out the behavior of the rectifier tubes when he drops the input freq 1 Hz at a time and gets to 25 Hz. Near the 4:00 minute mark.


https://www.youtube.com/watch?v=M_rdZsZasVk


 

[sluckey]

I just noticed that I could also use dual 5U4G rectifiers instead of a GZ34.

I don't think so. Isn't the attached pic the same as your chassis before you gutted it?

One 5U4 is capable of providing all the current that a pair of 6L6s will want. Now the Baldwin 45 looks almost like your Baldwin 40 amp except it has 2x5U4s, 4x6L6s, and bigger PY and OT. Are you maybe thinking of the larger amp?

[eleventeen]

Imagine if his amp was the 4x 6L6 and dual 5U4 version....if only the PT had dual 5v windings he could place 2 qty 5U4 in series (second one half wave, plates connected in parallel) for his voltage drop! Good room heater, too.

[mresistor]

I could say you are "quick draw Sluckey"  LOL ..   I didn't get that one deleted quick enough. The reason I deleted it was I remembered that voltage drop might be the same, output current level doubled, but the filament current would also be twice that of one tube, and since my PT only had one I don't know if it would handle it.
The reason I was asking is that upon looking at the 5E6a I see it has twin 5u4s and it is the predecessor to the 5F6a. I read that Leo switched to a GZ34 in the 5F6a for better current handling. So I thought, maybe I could use dual recto, but no.


Also it occured to me I could simply install an additional capacitor filter node before the B+ node to knock some voltage down and reduce hum and noise.  A bleeder resistor would also drop a little.

[sluckey]

There are many options to drop B+, including the way it was originally done using the big choke. 

[sluckey]

It is possible that this PT and 6.6K OT would be better suited for a 5E5A PRO build.  Or maybe a 5F4 Tweed Super.

IMO... The only way this PT is suitable for ANY Fender guitar amp is to use it with that big choke wired as a swinging choke, ie, choke input filter circuit.

You need to use this choke with this PT.

IMO... Without using the choke as it was intended that PT will put out 442VAC which equals 442 x 1.414 = 625VDC unloaded. It's simply ridiculous to try to reduce that amount of B+ for use in the Fender amps.

IMO... Use the PT and choke as a paired set or get an appropriate PT.

You need to use this choke with this PT.

[sluckey]

Arrrgh! I see you're posting and deleting again.

[mresistor]

Sorry ....       was entertaining the idea of the other Fender circuits because of the Output Transformer being 6.6K input inpedance .   I am going to use the big choke , I wasn't thinking of not using it .  But I squashed the idea and deleted that post, again   sorry.. 


Steve did you think I was talking about the PT?

[sluckey]

Steve did you think I was talking about the PT?

Of course. Based on your deleted question, how could I think otherwise?

I've run out of things to say about this PT, choke, and OT. Maybe someone with some fresh ideas can add more.

[mresistor]

This is what I deleted (paraphrased) "Maybe this amp is more suitable for a 5E5A or a 5F4 build  based on the 6.6k OT?"  I don' t know why you thought it was a question about the PT and choke. Again I apologize.
I appreciate your help and input. I have another little problem to deal with, which is making two holes for the power tubes so that one of them will clear that big choke. As previously planned it is very very close to it.
So I think I am off to Home Depot or Lowes to see what aluminum they have in stock. Cutting stainless with a grinder isn't bad, but drilling 1" holes in it isn't too much fun. 


There won't be much posting on this thread for a while  on my end..

[sluckey]

This is what I deleted (paraphrased) "Maybe this amp is more suitable for a 5E5A or a 5F4 build   based on the 6.6k OT?"  I don' t know why you thought it was a question about the PT and choke.

No it's not. This is EXACTLY what you said...

It is possible that this PT and 6.6K OT would be better suited for a 5E5A PRO build.  Or maybe a 5F4 Tweed Super