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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Replace the field coil spk with Electromagnetic dynamic spk

Hi erybody in 4RUM,

I have just got the vintage power Amp using 6V6, the schematic is in here: http://www.nostalgiaair.org/PagesByModel/570/M0002570.pdf (page 4).

But the problem is now, the Power Amp was designed to drive field coil speaker not for electromanegtic one. Right now, I decide to mod this Amp in order to be able to operate with electromanegtic psk. But I am not sure what I have to implement now. (Step by step).
I tried to connect my speaker to the output connector but it genertes a large hum. I think there are some troubles relating to impedance matching and the source filter. Should I replace all caps with compatible modern onces

My speaker is: 6Ohm, 30W.

Please give me your advices.
Looking for your help and thanks in advance!!!


Paid Member
2003-06-12 7:04 pm
Maine USA
> I am not sure what I have to implement now. (Step by step).


Have you ever worked with tube amps before? Trying to bring a 70 year old amp back from the grave is actually pretty hard. Every component has to be examined, many of them replaced. This is less easy than working on a kit or from-scratch chassis because all the old parts (and dust and rust) are in the way.


> I tried to connect my speaker to the output connector but it genertes a large hum

Power transformer isn't completely dead. Until you go through the rest of the amp, you have not proved anything more.

> I think there are some troubles relating to impedance matching

That would not cause hum.

> and the source filter. Should I replace all caps with compatible modern onces

Put it this way. I just found a 1937 Packard automobile that has not run since 1944. The tires are flat. Should I replace them?

You can tell a flat tire by looking. You can't always tell a "flat capacitor" by looking. But 1930s electrolytics in a mass-market radio are surely dead by now.

> I think in the future all the Caps should be replace and the better sound will be achieved.

To get ANY sound, all the power supply caps have to be replaced now. It won't work on un-filtered AC (which is what you get when the power supply caps have died).

Yes, 8 micro farad. That was as big as they could get in 1936. Under today's numbering systems, use 10uFd 500V electrolytics for an "equivalent", use 22uFd or 47uFd 500V as a "better" replacement for all the 450V, 475V, and 500V caps. Yes, 500V electrolytic caps are hard to find. Panasonic EB Series 450V will probably last a long time in this chassis. Panasonic TSUP is available in 47uFd 500V for about $3 each. Or use two identical 47uFd 400V caps in series.

Then use a power resistor to replace the speaker field coil and pass power to the 6v6s.
Hello PRR and every body,

This is the first time I working with tube-amp. I have only read the related document about them before. So I do have no experience on it. Anyway, I still need to mod this mummy as the first practical study:D.
I have connect 2 port at the speaker socket (near by the filter) with a power resistor, fed the input with signal from output of my CD player, fortunately, she sang but the sound contained much noise (hum, I mean). The next step, I checked all the components, one by one. The first result is:
- All trans and choke are still in good condition.

- The voltage on the filter cap is around 400V (8MFD, 500V)

Now I want to continue the next step.

Please give me your instruction. Thank you in advance!!!

hope to see you soon.
All the electrolytic capacitors in the power supply have to be replaced. Replacing Carbon resistors that drift in value and go noisy over time is a good idea. Waxed paper coupling capacitors are candidates for replacement with film parts. 716P series "Orange Drops" are good as replacement coupling capacitors.

Using a modern permanent magnet speaker is fine. However, you do need to make up for the absence of a field coil. Many OLD units used the field coil as a 2nd inductor in the power supply. You replace the field coil with a series combination of a choke and a resistor. You want the total resistance of the combination to be the same as that of the field coil. That way, the power supply voltage does not change.


Paid Member
2003-06-12 7:04 pm
Maine USA

Replace the filter caps.


You are correct that we should emulate the Field Coil's inductance. But I bet it makes very little real difference. Field coils were crummy chokes: the air-gap (voice-coil gap) is too large for an efficient choke. And commercial pressures forced wire size down until the increased HV costs to cover IR drop exceeded the savings of the next smaller wire-size. So they typically have only a few Henries and many pure-R ohms. By neglecting the I we may get a few dB more ripple, but if pioneervn takes the hint to use 22-47uFd caps we will get many more dB ripple rejection than the original designer could get with the biggest Spragues he could buy.

Also: field-coil current had to be reasonably clean. Speaker output is a function of field current. If there is significant ripple, you get intermodulation between the audio and the ripple, a very unpleasant effect. So we may assume (or try to pretend) that the voltage after the C-L-C section is "pretty clean" and we don't need much more clean-up to feed the push-pull stage.

A C-L-C-R-C filter, even with small-uFd caps, "should" be clean enough if the R is several K ohms. If hum gets low but non-zero, we can break the R into two 1K 10W parts and drop another C in the middle.

I have a feeling the "400V" is nearly unfiltered. A push-pull stage can "work" with remarkably choppy "DC", if you don't push the limits. Assuming nearly-no filtering, the "400V" is the Average or a wee bit more, so the AC must be 500V peak (also confirmed by the odd cap voltages).

Your analysis of the field coil as a choke makes sense to me. However, the DCR is the controlling factor in so far as PSU O/P voltage is concerned. A fairly hefty choke whose resistance is trimmed appropriately should have an impact on ripple. Space and cost considerations could easily make a simple resistance the "winner".


PRR is 100% correct about using 10 muF. as the 1st filter cap. That value is more than enough to keep the B+ voltage up without stressing either the rectifier or the power trafo. The 1st inductor in the PSU isolates the rectifier from the remainder of the filter. So, larger value caps. there are FINE.


2005-02-20 1:42 pm
No Brainer.......

Just add a choke with acceptable current capacity where the field coil speaker coil was in circuit. Tune proper voltage with a resistor if too high in series with the choke. This is performed in vintage tube radios when the old field coil speaker fails & a non-field coil replacement speaker is desired.