Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: wsscott on April 13, 2025, 01:47:30 pm
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A buddy has this 1947 amp and wants to replace the field coil speaker setup with a permanent magnet speaker. I'm attaching the schematic.
I've never done this, so I would appreciate some direction on how to wire it up. I think he probably would want to use an 8 ohm speaker. I assume using a choke would be the way to go, but I don't want to screw it up.
Thanks.
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I'll use a 1K ohms 5 watts resistor to replace the field coil in the circuit .
You can also use a 600 to 1K ohms choke but it is more expensive and no need for
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How about re-wiring the circuit to make the change using the choke?
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Or mod like this Oahu Tonemaster amp with no field coil
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Sorry, but I can't see all of the circuit layout in that photo. Can I download it online somewhere?
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Or this one , better picture ;
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I have this one, but this one is with the field coil in the circuit. Your other one has the resistor or choke I think for use with a permanent magnet speaker.
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Sorry, but I can't see all of the circuit layout in that photo. Can I download it online somewhere?
There is no link , it is a picture found with Google ; Oahu tonemaster schematic
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I have this one, but this one is with the field coil in the circuit. Your other one has the resistor or choke I think for use with a permanent magnet speaker.
The first one with no field coil is clear enough to make the mod.
The dark part is same wiring you find in most amp ; a simple negative feedback resistor and cap.
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I looked at Hoffman's Schematics but Oahu is not shown on the list of manufacturers. Would it be under some other company, like Valco?
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I looked at Hoffman's Schematics but Oahu is not shown on the list of manufacturers. Would it be under some other company, like Valco?
You no need more . New on a tube amp ?
Circuit is like this; Fender deluxe reverb
Where you see the 820 resistor here , Oahu use a 39 k and 015 cap; same wiring
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A 600R resistor in place of the field coil should work. Probably lesser resistance would even be better, as long as it worked with the caps.
Or probably just about any choke that could be used as a filter with enough current capacity.
Even with no resistor the tubes would still be happy voltage wise, except for maybe the bias at the power tubes. The resistor or choke would basically be used as a filter.
Why does he want to change the speaker?
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Why does he want to change the speaker?
a 1949 speaker..........
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He's a regular playing musician and is really into sounds, etc. Just wants a better speaker and flexibility to change.
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Would a change in the OT be justified? The existing one is attached to the speaker. I thought he should go with a choke and a new OT. The expense is not that much.
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The higher the resistance of the resistor used, the more sag it will create, would be the downfall of a resistor vs choke.
The OT should be fine.
You could theoretically leave the field coil in circuit and run an extension cab.
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Would a change in the OT be justified? The existing one is attached to the speaker. I thought he should go with a choke and a new OT. The expense is not that much.
No, why ? Keep the original one.
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Many small amplifiers are built without choke, and sag can be a desired effect as well.
I would only use a resistor.
And if afterwards the sag effect isn't appreciated by the musician (and if he hears it), there's always time to install a choke.
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So I take it that all I need to do is to remove the field coil from the circuit and install either a 600 ohm, or less, resistor, or a choke in its place. Then I would just disconnect the OT from the speaker frame and mount it somewhere else in the cabinet or on the chassis, leaving its wiring as is.
And that's it?
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Then I would just disconnect the OT from the speaker frame and mount it somewhere else in the cabinet or on the chassis, leaving its wiring as is.
The field coil OT won’t have an appropriate Pr:Sec winding ratio for a modern (non-field coil) speaker. You’ll want to replace the OT (as well as the speaker)
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Tubeswell, that's what I was wondering. I assumed a new modern speaker wouldn't work with the existing OT, I didn't know why I just assumed it wouldn't. So were those forum members who said I could use the old OT meant I could use it if I also used the existing FC speaker? I'm getting a little confused on options.
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I've used standard output transformers to replace the OT's while still using the field coil speaker. Your schematic shows a 6 Ohm speaker, so I assumed an 8 Ohm speaker would work with the same transformer, but I may be wrong.
Either way, it may be better to replace the OT with a current one, especially if it would cause extra grief or damage to the original speaker trying to retrofit the same transformer.
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Am I correct that in the schematic shown in Post #3 above, the choke would replace the 500 ohm resistor that is shown in the schematic?
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Am I correct that in the schematic shown in Post #3 above, the choke would replace the 500 ohm resistor that is shown in the schematic?
The 500 ohm resistor shown in post #3 and the field coil shown in post #5 are electrically in the same position in the circuit. A choke or a 500 to 1K ohm high wattage resistor can go in that same circuit location, and a permanent magnet speaker installed.
Can you use the same OT? Maybe - you will get sound, may or not be good sound :dontknow: When I have done this, I have replaced the OT.
But, I think you should be concerned. The info offered in the previous posts is pretty clear; but you are not getting it. No crime in that, but tube amps are dangerous and you are going to be working on the part that packs the highest voltage and current. I suggest you study up some before tackling your buddy's amp.
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The 500 ohm resistor shown in post #3 and the field coil shown in post #5 are electrically in the same position in the circuit.
Not so. In the circuit with the 500Ω, the plate supply is fed prior to the 500Ω. That means only the screens and small tubes draw current through the 500Ω. That small amount of current is not nearly enough to work for a field coil.
In the circuit with the field coil, the entire amp current (plates, screens, and little tubes) flows through the field coil. Big difference. This amount of current is enough to create a good magnetic field in the field coil.
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Thanks. Yes, he is getting a new OT and a choke.
Thanks for the warning. I'm aware of the high voltages in tube amps and I've built a couple of them from scratch, not kits. I acknowledge that I don't know all the electrical theory involved. I've just never worked with a field coil before, that's why all the questions.
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My buddy wants to use a choke, and so we're going to install a 125C1A that's 4H and 90mA. Its resistance is 104 ohms. The spec on the field coil shows 610 ohms. Do I need to add another resistor, like maybe a 500 ohm, in series before the choke connection to Node A on the power supply to get the resistance on the choke higher?
If so, is that the proper resistance value and what wattage should it be?
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My buddy wants to use a choke, and so we're going to install a 125C1A that's 4H and 90mA. Its resistance is 104 ohms. The spec on the field coil shows 610 ohms. Do I need to add another resistor, like maybe a 500 ohm, in series before the choke connection to Node A on the power supply to get the resistance on the choke higher?
If so, is that the proper resistance value and what wattage should it be?
If you're going to ditch the FC speaker and OT, then it doesn't matter. If you're using the same PT and rectifier, the B+ voltage will be a little higher after the choke. 6 of 1, half-a-dozen of the other.
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Thanks. Yes using the same PT and rectifier.
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Yesterday, finished with replacing necessary caps and resistors, and decided to turn on the amp before doing the final conversion to the permanent magnet speaker. I used a dim bulb limiter. Turned the amp on and just a low glow. The pots were a little scratchy, but the main thing was that the amp was picking up a radio station!
Now what could cause that?
The on/off/tone switch is right next to the pilot light. There are no 100 ohm resistors for a simulated center tap since the PT has a center tap for the 5Y3 rectifier tube. And as you can see from the schematic, the microphone and instrument input jacks have their volume pots connected to the 1st pre-amp tube before the tone pot. Also a lot of noise from that pot when it is rotated, increasing in intensity as the tone is increased from bass to treble. Very little noise at its lowest/bass position.
The microphone input jack has much less noise than the instrument jacks. Note that each of the instrument jacks has a 100K resistor, and the mic jack has no resistor before connecting to their respective 1M volume pots. Should the 100K's be reduced?
So after turning off the amp, we finished with the conversion to the new OT and choke and the new permanent magnet speaker. That seems to be working fine. We don't seem to be hearing the radio station, at least for the moment, but the noise associated with adjusting the tone pot is still there and reacting in the same manner.
Any suggestions on this?
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First, I'd try cleaning the jacks and pots. Make sure the contacts on the switching jacks are working properly.
Was the chassis shielded when you were experimenting?
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a lot of noise ... increasing in intensity as the tone is increased from bass to treble. Very little noise at its lowest/bass position.
What manner of 'noise'?
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It’s a little hum, but main issue is a higher pitched hissy type of noise, that disappears when the tone pot is turned down to all bass, and increases as pot increases the treble.
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AINewman, initially we did not have the bent metal shield that goes between the 2 volume pots installed, when we heard the radio station. We were also getting the noise I described. We later installed that shield, and don't seem to be picking up a radio station, but the noise is still there with the same symptoms with no improvement.
There is no other shielding. As I said, it seems to be related to adjustments on the Tone pot.
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It’s a little hum, but main issue is a higher pitched hissy type of noise, that disappears when the tone pot is turned down to all bass, and increases as pot increases the treble.
So, like white noise (or pink noise)?
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Probably more like white noise.
My buddy is out of town for the week, so I don't have access to it right now.
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The volume controls have no effect on the noise?
Is there DC voltage at the wiper of the tone pot?
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I don't want to send you guys on a wild goose chase, so I'll wait until my buddy gets back so I can respond to your questions when I have access to the amp.
However in the meantime, if you want, still send questions for me to check out.
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I checked with my buddy who is back, and he said the change in either of the volume pots has no effect on the noise. Only the on/off/tone pot affects it.
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Unless you have it in front of you, you won't be able to troubleshoot it.
But, by that info, it means the noise is originating somewhere between the volume pots and the tone pot on the schematic.
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We got back working on the amp, and decided to replace several resistors connecting with V2. This really cleaned up the hiss and hum, and its got really good volume level. My buddy is thinking of replacing the tone pot and installing a separate on/off switch. There doesn't seem to be much of a range in the bass/treble, and thinking is that maybe the pot's age is showing.
Otherwise, I think this is about it for now on this amp. Thanks for everyone's help. Best.
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Well I spoke to soon. We're really having a lot of problem with the Tone pot and Volume controls.
The schematic posted earlier in this topic, shows 100K resistors coming off the Instrument inputs. Only 1 instrument is going to be used, and my buddy wanted to take out the 1st input jack to use the hole for something else if needed. So he did that.
The Mic input doesn't have any resistor connected.
Both of these Input jacks are old and are not Switched jacks.
We replaced the Tone/On/Off switch with a 500K audio taper pot, and then installed a separate on/off switch.
Have had problems with the Mic jack picking up radio frequencies when a guitar is plugged into that jack. Is that because there's no resistor and an impedance issue is involved?
I guess a guitar shouldn't be used in the Mic input.
There is still a steady low background hum.
Thoughts are appreciated.
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Thoughts are appreciated.
Sorry, but it takes quite a bit of mental effort to follow a verbal description of the changes you are describing and go back through the entire thread in order to work out where things are going wrong. For my part, I've got a boatload of other stuff I juggle with in my day-to-day life and I just don't have time to crawl into other people's head spaces to help fix their issues in a forum where help is voluntary. If you want help, the most helpful thing you could do is update the schematic to show your changes and take some hi-res gutshots of the changes you've made so everyone else (me included) can more quickly jump to assistance. Apologies if I sound naggy.
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If you want help, the most helpful thing you could do is update the schematic to show your changes and take some hi-res gutshots of the changes you've made so everyone else (me included) can more quickly jump to assistance.
Agree.
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Solved it-turned out to be a cold solder joint on an old filter cap connection.