help needed with pt

[Bugman3183]

Hello all, this is my first time posting and had a couple of questions for you guru's out there.  I recently aquired a hammond ao29 amp that used a field coil speaker.  I really haven't been able to get a whole lot of information on how to use this with a permanent speaker.  From what I've gathered I could use a power resistor as a replacement in the ps for the field coil speaker.  I have it narrowed down to either a 700 or 1000 ohm 20 watt resistor. (not sure which one) but I was wondering if a choke could be used instead, and if so what size would be required?  Also I wanted to know if this transformer would be suitable for a tweed bluesmeister.  pt rating is 355-0-355 @ 150ma 5.0v@ 3a and 6.3v@4a the amp ran a pair 6v6gt's and roughly six preamp tubes.  oh yeah, it also has a 5u4 tube rectifier. If anyone has any experience with this type of situation any guidance would greatly appreciated.  Thanks in  advance.

[HotBluePlates]

Hello all, this is my first time posting and had a couple of questions for you guru's out there.  I recently aquired a hammond ao29 amp that used a field coil speaker.  I really haven't been able to get a whole lot of information on how to use this with a permanent speaker.  From what I've gathered I could use a power resistor as a replacement in the ps for the field coil speaker.  I have it narrowed down to either a 700 or 1000 ohm 20 watt resistor.

The original speaker field coil is used in the power supply of such amps, for one of a couple of purposes. Most often, they take the place of a filter choke.

In the AO-29 schematic I have, the speaker field is used in a back-bias arrangement to:
 1. Create a source of negative voltage to fixed-bias the output tubes, and
 2. Lower the B+ voltage by 100v.

The schematic indicates the speaker field is 700Ω, and has 100v across it. 100v/700Ω = 143mA (the entire amp's B+ current). 100v * 143mA = 14.3w. You want to use a resistor rated for double the dissipated power, so you'd need a 30w or larger part. Probably a 50w aluminum-bodied resistor, bolted to the chassis as a heatsink.

But that seems like a pain. Your B+ will also be limited to 310v (per the schematic). You probably won't use as many tubes as the original circuit, so the total current through the new resistor will be less, and voltage drop will be less. It is probably better to build a new bias circuit without the back-bias arrangement, perhaps modeled on a Princeton Reverb.

By the way "back-bias arrangement" means inserting a resistor or choke between the input filter cap's negative terminal and ground, which makes that negative terminal appear to be at a negative voltage, rather than at ground.

... I was wondering if a choke could be used instead, and if so what size would be required?  ...

I might be wrong, but my understanding was the 700Ω indicated is the d.c. resistance of the choke. That's because you need it to have 700O of resistance at d.c., not just at a ripple frequency, so that the bias voltage will be developed. You'll be hard-pressed to find a modern choke with 700O d.c. resistance. So maybe it's best not to go down this road.

...  pt rating is 355-0-355 @ 150ma 5.0v@ 3a and 6.3v@4a the amp ran a pair 6v6gt's and roughly six preamp tubes.  oh yeah, it also has a 5u4 tube rectifier. ...

Are you sure about that B+ voltage rating? The schematic I was looking at with a field-coil version AO-29 has a 380-0-380v B+ winding. If you use it for the Tweed Bluezmeister, you'll probably want a choke-input power supply to drop the voltage to ~340vdc. That's a bit lower than the indicated TBM B+ of 374v, but it's close enough.

[Bugman3183]

Thanks for the reply, that was very informative.  I didn't realize compensating for the em speaker would be so complicated.  As far as the b+ of the pt I may have been mistaken, I just googled the part number stamped on the pt because every schematic I've seen of that amp illegible.  I'm not really sure how to proceed at this point because I've only ever worked on two amps up until now and both had full bridge ss rectifiers, and after looking at the power stage for the princeton reverb I'm left feeling like I'm down to only a few options.  Either buy a new pt, buy some more books,or both. It"s funny how I read all the time about people converting these amps for guitar but can never find a schematic on how the power stage is done for use with a pm speaker.  The amps I did build, I built using proven schematics and layouts, I'm certainly not at the point where I can design my own power stage so guess I better get started on changing that fact.  Thanks for all the information, I really appreciate it.

[HotBluePlates]

I didn't realize compensating for the em speaker would be so complicated.  ... It"s funny how I read all the time about people converting these amps for guitar but can never find a schematic on how the power stage is done for use with a pm speaker.  ...

You're welcome!

Do you remember high school chemistry class and empirical formulas? They're the ones where you are given the relative amounts of each element, like 2 hydrogens for every oxygen in water. You don't care how many total hydrogens are present, just as long as the proper ratio is there.

Same thing with guitar amps. The total AO-29 power supply circuit was designed to provide the right balance for the tubes and operating conditions used, and with the presence of a field-coil speaker. There was also some ingenuity expended to derive a bias supply without using a rectifier tube or solid-state rectifier and bias tap.

If you yank out the field coil, the balance is upset, and you have to figure a way to re-balance everything. Depending on how clever the original designer was, it may be easy or hard to re-balance. Some hi-fi circuits in particular have a given component doing double- or triple-duty, and simple mods make things get weird.

You can convert the AO-29 to do what you want, but you may have to have parts that act as a space-heater (i.e., waste off some power) to duplicate the field coil and extra preamp tubes.

[Bugman3183]

Thanks for the advice, I never really considered it in such a manner.  Looking at it from that point of view clears things up quite a bit and after studying the schematic I feel I have a pretty good understanding of how to approach this dilemma now.  I really appreciate all of your help, I think this project will be a great learning experience for me and hope that someday I can become knowledgeable enough to help out some new guy down the road.

[stingray_65]

Hey Bugman,

That PT will work with a lot of different amps. In my experience, it's often much easier and I have better results when I build a guitar amp from the main parts.

You have a PT, OT, chassis w/ sockets, some tubes i'm guessing.

The rest is cheap stuff, a turret board, resistors, handful of pots, and caps.

You could build a pretty nice 5E3 or maybe a Princeton Reverb.

If this is one of your first builds, I HIGHLY recommend building the 5E3. It is a simple enough circuit to troubleshoot (you will, WE ALL DO)  It is easy to mod after your done with it, it has a classic sound that suits MANY playing styles.

[Bugman3183]

That's my plan, to build a 5e3 because of it's simplicity while focusing on figuring out how to compensate for the lack of a field coil speaker.  Once I have that part squared away I figure I could always build it into something else, or who knows I might just want to leave it as a 5e3.  This definitely seems to be the most logical plan of action.  Thanks.

[stingray_65]

That's my plan, to build a 5e3 because of it's simplicity while focusing on figuring out how to compensate for the lack of a field coil speaker.  Once I have that part squared away I figure I could always build it into something else, or who knows I might just want to leave it as a 5e3.  This definitely seems to be the most logical plan of action.  Thanks.

compensating is simple. just don't use the FC speaker!
http://www.el34world.com/charts/Schematics/files/fender/DELUXE_5E3.pdf

5E3 has no choke, use a PM MI speaker,(Permanent Magnet Musical Instrument), build as designed.

The FC speaker was used way back in the day because permanent magnet technology wasn't all that. so they used an electromagnet, a field coil, as it happened it could do double duty and act as a choke. Chokes were used because they were cheaper and more reliable than big electrolytic caps. These days, big caps are cheap and very reliable, and the price of metals and assembly have gone sky high, so big inductors are expensive.

I've found on the average the PT,OT and choke accounts for about 1/3 -1/2 of the costs of building an amp, with the Iron you got from the old amp, plus a few of the reusable parts like sockets, the chassis and maybe jacks and switches. you're on your way to a pretty good amp.

[Bugman3183]

Thanks, that's great news.  I was under the assumption that I had to use a choke or power resistor with equal ratings as a replacement.  I guess it's time to order some parts and get this project underway.  Thanks.

[stingray_65]

BTW,
Your 355-0-355 PT is spot on for a 5E3.
Here is the specs for the tranny I used in my first 5E3.
http://www.classictone.net/40-18021.pdf
with the extra mA you have in your PT and the extra sockets, you might end up adding a gain stage, or maybe a reverb, or tremolo.

It may be enough to run 6L6's, which may be a better match to the more than likely under rated OT you got with the original amp.

Any way you go, you can pretty much just ignore that old FC speaker. Heck you might even get a few bux from it on flea bay to pay for some of your parts.

[Bugman3183]

It didn't come with the speaker which was my whole dilemma, but I do have a couple of old rola alnico  pm 12's that came from a hammond organ.  Hadn't really considered the reverb but definitely sounds like an awesome idea.  I think I'll just stick with the 6v6's though since I already have them on hand.  Can't wait to get started!  Thanks for the advice.

[sluckey]

Here's a cleaned up schematic for that AO-29 that may be of some use...

[Bugman3183]

finally one that I can read!  Thanks, I'll put it to good use.

[Willabe]

Point 4 in the PSU says -100dcv, so if you took out the field coil and the R's/C's that are in parallel with it, shown just below the field coil, you would end up with 410dcv at the 6V6's plate supply node?

If so, that'll work.

A 5E3 uses a 5Y3 rec. tube which will drop a few more volts, 20/30 volts?


                        Brad      

[Bugman3183]

That is the way I interpreted it, but at my skill level I can only know for sure by building it and testing it.

[Willabe]

Now thats also with the 30 ohm droping R in the PSU before the 6V6 plate node.

Which may add too much sag/compression for your taste and/or playing style?


                      Brad     

[HotBluePlates]

Point 4 in the PSU says -100dcv, so if you took out the field coil and the R's/C's that are in parallel with it, shown just below the field coil, you would end up with 410dcv at the 6V6's plate supply node?

Exactly. 410vdc and no fixed-bias supply.

It will end up a bit higher than 410vdc, because there's no as much current being drawn from the PT with the fewer tubes, but it'll work just fine.