1947 Fender Dual Professional

[p4gblue]

I joined the forum years ago, but haven't posted a question before. I'm planning a Dual Pro, 6SJ7 front end, 6N7 PI and probably 6V6 outputs, rather than the original 6L6s.


The circuit schematic shows a choke input filter. I'd like to duplicate this, and the Hammond 193M looks like a possible candidate. 10H, 300mA, 63 ohms.


By my calculations, the DC current load would be less than 150mA, and with a rectifier output of 400V the AC load would be around 30mA. So the 193M should do the trick. I'm aware of the 'critical current' issue, and the potential to load the following cap if the current isn't high enough.


The chokes that were used in this situation in the past were also known as reactors, and I've read they were constructed specifically for this job. I've looked around, but apart from custom builds, can't find references to buying this style of choke.
 
Can anyone assist me? Would the 193M work? Are there available chokes that would be better?


Thanks in anticipation.

[sluckey]

Can you post the schematic?

[brewdude]

is this the schematic you are working with?
http://el34world.com/charts/Schematics/files/fender/Fender_1946_pro.pdf


I may be mistaken, but it seems like 300mA would be overkill for the choke and the Hammond 193J (10H@200mA, 82R) is almost $50 less expensive.https://www.tubesandmore.com/products/filter-choke-hammond-enclosed-10-h-200-ma



Though I've not seen many builds that utilize true choke input power supplies, I have an unusual fondness for choke input amps and have built a few of my own design....  I also love 6SJ7's.
I'm very interested in following this thread and wish you great success in this build.

[sluckey]

is this the schematic you are working with?

I don't see a choke in that schematic?

[Toxophilite]

Didn't the field coil speaker in those old designs sometimes also function as a choke?

[p4gblue]

Can you post the schematic?


I'm struggling with getting the schematics loaded. Is it because I'm using a Mac?

[p4gblue]

is this the schematic you are working with?
http://el34world.com/charts/Schematics/files/fender/Fender_1946_pro.pdf

Though I've not seen many builds that utilize true choke input power supplies, I have an unusual fondness for choke input amps and have built a few of my own design....  I also love 6SJ7's.


No its not, but I can't get an upload going. I've never heard anyone praise choke input power supplies before. Can you elaborate please?

[p4gblue]

Can you post the schematic?

The first schematic was from Aspen Pittman's old book, the little one with all the amps on the covers. He calls the circuit an 'Old Bassman', but its exactly the same setup and tube complement as the 1947 Dual Pro. Its on p380. I'm pretty sure its a Dual Pro, but happy to be corrected.


The second schematic was sent in by Mats to this forum on 13th August, 2013. If you put 'Fender Dual Professional 1947' into the search (above) the thread comes up under Brad's name, and the schematic is about 4 replies down the page. It's a different style of drawing, but the circuit is the same as the one above, as far as I can tell.


There's also mentions in various places that the chokes on these were very big. Hence my thoughts about leaving lots of room for overkill, and using the 193M.

[mresistor]

The one in AP's book does have a choke input   It's in the schematic library as a Bassman 5A6.  It also has either two OTs or one with two output windings.


http://el34world.com/charts/Schematics/files/fender/Fender_Bassman_5A6.png


http://el34world.com/charts/Schematics/files/fender/Fender_Bassman_5A6_Schematic.pdf

[sluckey]

Calling an inductor a choke or reactor is like calling a capacitor a condenser. Just another word describing the same thing. A rose is a rose no matter what you call it. Any 4H to 20H choke that can handle at least 200mA would work just fine in this circuit.

Here are a couple more schematics of that amp, including the copy from the Pittman book...

[mresistor]

I would think a Hammond 193J would be sufficient. Or even a 193H.

[p4gblue]

"Any 4H to 20H choke that can handle at least 200mA would work just fine in this circuit"

Ok that's what I needed to know. Thanks very much. The figures suggested 200mA would cover it, and that 10H would work fine, but nobody (as far as I can tell) uses choke inputs anymore, so there wasn't much available, and I needed to be sure.

[sluckey]

Didn't the field coil speaker in those old designs sometimes also function as a choke?

Yes, but not always. There are plenty of old field coils that were not used as a choke. They were simply connected as a parallel load to the B+ supply. This old juke box amp is one example...

     http://sluckeyamps.com/rocky/rocky.pdf

[sluckey]

but nobody (as far as I can tell) uses choke inputs anymore, so there wasn't much available, and I needed to be sure.

Yep, choke input is not very popular. I've only run across a couple musical instrument amps that used a choke input. Hammond AO-63 is one...

     http://sluckeyamps.com/hammond_2/AO-63.pdf

[brewdude]

Choke input PS’s are not economical.  They add considerable cost and weight and generally though to be unnecessary for a push/pull design especially with today’s inexpensive high value (uF) capacitors.


I don’t know how to describe the tonal effect of the choke input, and have never tried to build two similar amps except for the inclusion of the choke to compare.  However, I wonder if the choke input might make the amp less noisy and perhaps tighten and extend the bass response—I built an amp with a cathode biased 6V6 power section with a true choke input PS It has a parallel 12AX7 into a “band master” style TB stack feeding a 6SJ7 and a parallel 12AU7 cathodyne PI.  If you turn the bass up and the treble down a bit, it makes a great bass amp for a small studio (garage).


I recently finished an amp inspired by the early GA-40 built as a simple single channel using only seven pin tubes.  It has a capacitor input rectifier, but a choke that feeds the PS before the OT CT and a choke fed screen supply (directly from the OT CT node. It uses a 5725 pentode into a big muff style tilt tone control feeding another 5725 pentode, a 6C4 cathodyne PI and cathode biased 6AQ5’s.  Though I’ve not been able to properly test this build, due to not having a suitable speaker (8 ohm) cab yet, it seems to have a very low noise floor but does not seem to have the depth of bass response.


I also have a 4x6V6 or 2x6L6 amp that I’ve been working on (currently rebuilding the preamp to include a buffered parallel effects loop) but also a true choke input PS, cathode biased and cathodyne PI (using a pentode/triode tube. It uses Merlin’s medium gain preamp.  This thing is VERY loud and seems to have great bass depth. 


Sorry if this is a derail.

[sluckey]

Choke input PS’s are not economical.  They add considerable cost and weight

Not necessarily. This Hammond AO-63 amp uses a choke input power supply. Actually two small chokes in series. These would have been pretty cheap.

     http://sluckeyamps.com/hammond_2/big_guts.jpg

The old Sunn amps had a cap input filter but the entire B+ current (including plates) flowed through a physically small high current choke located under the chassis.

     http://sluckeyamps.com/sunn/sceptre1.jpg

[mresistor]

The Baldwin orgasonic power supply that I acquired had a choke between the PS and ground. Now it is a 5F6A with a choke input PS and I love the sound. Now running it with 12AX7 input and EL34s. It has a GZ34 rectifier.  I would mention that the choke does drop some voltage.

[brewdude]

True choke input PS drops a lot of voltage... probably the biggest reason they are almost never used.

[p4gblue]

True choke input PS drops a lot of voltage... probably the biggest reason they are almost never used.

Ok guys, that's all the information I hoped for and more. Thanks very much for your help with this. When I get the amp started, I'll post pictures etc.

[92Volts]

True choke input PS drops a lot of voltage... probably the biggest reason they are almost never used.

On the other hand, power isn't wasted. The higher-voltage transformer needed can be rated for lower current. So I wouldn't say voltage drop is always bad, but may be in specific cases, when available/cheap transformers have ratings suited to capacitor input supplies.

I think low/no load regulation and the cost of chokes are the main problems, at least they were for me when I recently considered using a choke in a build. A choke input would have been needed for best performance when tube rectifiers were the only option, and they couldn't handle a large capacitor input, even if those capacitors were available/affordable (they weren't). With solid state rectifiers and giant, cheap caps the need just isn't there and the many disadvantages of choke-input supplies are noticed.

[p4gblue]

"I think low/no load regulation and the cost of chokes are the main problems, at least they were for me when I recently considered using a choke in a build. A choke input would have been needed for best performance when tube rectifiers were the only option, and they couldn't handle a large capacitor input, even if those capacitors were available/affordable (they weren't). With solid state rectifiers and giant, cheap caps the need just isn't there and the many disadvantages of choke-input supplies are noticed".





Yes I agree with your thinking. Money dictates our new direction, and then we focus on why the old way wasn't any good anyway.. and the possible benefits of the old way get lost.

[p4gblue]

Now running it with 12AX7 input and EL34s. It has a GZ34 rectifier.  I would mention that the choke does drop some voltage.

So you think the sound is different due to the choke input, or not possible to tell why it sounds so good??

[mresistor]

The reason I went with choke input was because the Baldwin PT was putting out around 750 volts which needed to be reduced to be usable. Thanks go to S. Luckey for the recommendation and example. The amp is very quiet. Of course it is not a "real" 59 bassman but I think it has many of the characteristics. The Baldwin OT only has 8 and 16 ohm taps. The normal channel does breakup starting around 4 on the dial and progressing as it goes higher with some pretty nice pleasant distortion. The distortion gets hairier up around 7 to 8.  It has a different sound than what I am used to and is hard to describe but is very pleasing. I should say it is slightly darker and maybe more attack sensitive. I think it sounds very much like a 50 watt Marshall.  The bright channel IS crystal clear and bright. It has very solid and tight bass response. The amp has bucket-loads of sustain in both channels. All the tone controls work exactly like they should. Of course it is very LOUD when cranked and I seldom get over 4-5 on the volumes. I am playing it through an EVM15L speaker so I know the speaker isn't coloring the sound. I've have been playing clean and the amp pairs very well with a Strat and I assume a Tele would be great too. My Les Paul sounds great through the amp as well.  I should try it with my 2x12 cab and with some effects, but I have been very happy with this amp with just a guitar and cord.
Does it sound different than a cap input amp? It may be a tad quieter, a tad tighter bass and better sustain. But I'm not sure how much of that is caused by the choke on the input.

All in all I would not hesitate to build another choke input amp. Hope this helps.

[Ed_Chambley]

The 5F4 Fender Super is an inductor input.  Sure it sounds different, even if the tonestack is bypassed.  The tonestack is a bit funky on the 5f4.  I have a few Inductor input builds.  Certainly the voltage will load at about 8.8 to 9 as a factor instead of 1.41 for voltage, but it is insignificant unless you need to reduce for tubes.


I will tell you this.  I have a Fender Deluxe Stock which is Cap input and this the 6V6 tubes with a lot of voltage to get about 22 watts.  Hopefully you know a Deluxe tone.  If so, the same amp with an inductor input, even thought the plate voltage is lower (and since we generally refer to voltages instead of current) you may think less headroom, but actually it has a bit more.


Just makes DC with less ripple.  It is almost impossible to say what someone will like, but moving a choke and adding a cap is fairly easy.  Some swear the famous 5F6a sounds like it does because of poor filtration, which could be argued if compared to the original JTM45 since it is basically the same circuit with more filtration.


Some like the 5e3 which had no choke at all.  Build it and it you don't like it, mod it!

[sluckey]

The 5F4 has a cap input filter.

[p4gblue]

 
All in all I would not hesitate to build another choke input amp. Hope this helps.

Very interesting comments, I'm looking forward to hearing what it will sound like.

[Ed_Chambley]

The 5F4 has a cap input filter.

Yep, sorry it just filters the plates.