I know R.G. Keen. Very smart guy. The concept is sound. Of course, it will not protect a rectifier tube like a 6X4 from a heater-cathode short, which is how they tend to fail. Guitar amps tend to get banged around more than organs, and this results in more frequent rectifier tube failures, especially 5U4s where the filament cathode can break and touch the anode.
Hi Arne.
In accordance with R.G. Keen's article I have wired up the series diodes on the power transformer secondaries of my two friend's AO29 amplifiers, and I intend to wire up these series diodes in the amplifiers of my own Hammond organs and also on my Marshall JTM-45 amplifiier.
This is a relatively simple but very worthwhile protective modification which in my opinion should be a standard wiring set up in valve amplifiers which have valve rectifiers.
The series diodes produce a miniscule 0.7 volts voltage drop which would have no effect on the tonality or on the rectifier sag effect when the amplifier is played at full volume so therefore I would assume that even the most fundamentalist valve purists who despise the concept of adding any solid state devices in a valve amplifier, would not be able to hear any actual difference in the sound with this series diodes modification.
All the best.
Kon.
Cool Kon, I'm planning to do that myself. Do you use any of his other protective measures? I'm thinking about fuses on the transformers secondaries, here I will go just on the high voltages ones and not for the heaters, and varistors (MOV) acorss the output transformer (this I wil do for the PA and reverb A100 amps)? And for the fuses. I see the TREK kit uses Slow blow and secondaries, howevere I read in various forums that fast blow is prefered on the transformer secondaries.
Arne
Cool Kon, I'm planning to do that myself. Do you use any of his other protective measures? I'm thinking about fuses on the transformers secondaries, here I will go just on the high voltages ones and not for the heaters, and varistors (MOV) acorss the output transformer (this I wil do for the PA and reverb A100 amps)? And for the fuses. I see the TREK kit uses Slow blow and secondaries, howevere I read in various forums that fast blow is prefered on the transformer secondaries. Arne
Hi Arne.
Even though R.G. Keen's article says that you can use 1N4007 diodes, I used 6A10 6 amp / 1000 volt rated diodes on the two AO29 amplifiers that I modified. The 6A10 diodes are cheaply priced and they are readily available at my local electronics shop so therefore I used the 6A10 diodes instead of the 1N4007's because the 6A10's are more rugged heavy duty diodes.
I have also wired up fuses on my friend's AO29 amplifiers and my own two AO28 preamps.
I am not sure what the ideal fuse values to use are, but I wired up 1 amp slow blow fuses for the primary winding and for the high voltage secondary windings, and I used 5 amp slow blow fuses for the heater filaments.
A 1N4007 diode is rated at 1A continuous rectified current with an instantaneous 30A surge limit. It's more than enough for an amp like the AO-29, which only draws 140mA at idle. A 5U4GB is only rated for 275-300mA in typical operation. By the time you get 1A B+ current in an AO-29, you'd already be in catastrophic failure, so a 6A rating is not going to help you.
Some new-production guitar amps have fused heater filaments due to EU regulations. They were never fused in tube amps made at the peak of tube amp design expertise back in the 1950s and 1960s, and professional guitar amp techs are now running into problems created by these fuses in new amps, including burnt and melted connectors and fuses that blow when they're not supposed to. The thing about heater filaments is that when you turn them on, they momentarily look like a complete short with high surge currents. These surges damage the fuse contacts and result in spurious fuse failures. You can certainly do it if you want, but I tend to think that it's a bad idea. In almost 30 years of working on tube amps, I don't think I've ever seen a situation where fused heater winding would have helped, unless, perhaps, you were to fuse the center tap to ground where there is normally no current.
Why not simply fuse the power transformer secondary outputs, then fuses can be selected for the expected load, in fact Trek II makes a fuse kit specifically for the AO 28 amp ,stick with the KISS principal, no need to over think it with diodes.
The idea with the diodes is to present only the positive swing to the following electronics. So following caps, the electrolytics where never be "reversed biased". The fuses I guess will not be fast enough to prevent this. So that is idea behind Keens article. The best is to do both.
The failure mode of 6X4 rectifiers in AO28s is almost always heater-cathode leakage turning into a short, so routine testing of 6X4s for heater-cathode leakage will reveal a 6X4 that's headed for failure long before it fails. It's rare to have the electrolytic B+ filters in AO28s short. They usually fail open.
With 5U4s, they have a long, flexible combined heater/cathode that can occasionally break and touch the anode, and 5AR4s can arc and create a short due to the narrow spacing of cathode and anode.
The silicon diode concept is sound. Some guitar amp manufacturers are actually including it in new production amps.
The heater windings is 3.15-CT-3.15V not 0-6.3V which should require 2 fuses.
However at Ampeg on the V4BH they only use one 10A Normal fuse on the heater and a 5A SlowBlow on the mains. But they add an 'Inrush current limiter' (NTC Ametherm SL22 10008) serial with the mains.
Going back to the hammond (L100),
Adding Ground to this device which does not have one is an obligation.
Adding a fuse in addition as there is none is not luxury.
All this is incorporated into an IEC base which allows at the same time to get rid of the attached wire.
Confined JP
Yes, i'm surprised there are no fuse on the A100 power amp (AO-39). I'm adding one for the power amp, one for the preamp, and one for the whole organ (for motor etc.). I think for the Hammond amps (except the Leslie amp), there is allready a kind of inrush limiter since they are using a tube rectifier. There quite a delay before that is operational. I notice that on my VOX guitar amp.
With regard to fuses on Hammond products, there is an old story that at an early demonstration of a Hammond Organ, one stopped working due to a fuse that failed for no reason. (I suppose the making of fuses in the 1930s was not as well-controlled as it is today.) Laurens Hammond was obsessed with the perceived reliability of his products, so, furious at the failure of the demonstration, he decided that Hammond would not use fuses in their products, a decision that stuck for decades.
That's why there's no PT primary fuse on the AO39.
The inrush current on the filament winding that David refers to is always there on power-up. Having a tube rectifier does not limit filament inrush current. The tube rectifier does, however, limit the current on the B+ voltage, just by the fact that it takes a while to warm up and begin conducting.
Using a thermistor in series is certainly not a bad idea if you wish to prevent unintended filament fuse blows, but I can't speak for what particular kind of thermistor would be appropriate in Hammond preamps.
Rectifier tubes do limit the inrush current somewhat, but a power supply with a 5U4 comes up to full voltage surprisingly quickly. If you had test gear, I'm sure you'd see a time difference between a 5U4 and silicon diodes, but it's not a large as you might think. GZ34s and 6X4s do have a controlled warm-up.
A 1N4007 diode is rated at 1A continuous rectified current with an instantaneous 30A surge limit. It's more than enough for an amp like the AO-29, which only draws 140mA at idle. A 5U4GB is only rated for 275-300mA in typical operation. By the time you get 1A B+ current in an AO-29, you'd already be in catastrophic failure, so a 6A rating is not going to help you.
Hi David.
Thank you for your clarification and explanation about the 1 amp diodes being more than enough for this application.
I was not sure what the minimum safe amperage rating for the diodes in the AO29 B+ voltage application is so therefore I chose to use the rugged heavy duty 6A10 6 amp/ 1000 volt rated diodes in order to reduce or avoid the possibility of the diodes failing and going open circuit.
As well as that, the 6A10 diodes have stiffer, thicker wire leads than the 1N4007 diodes so therefore the thicker wire leads of the 6A10 diodes allow a more rigid physical connection that is less likely to slightly flop around and thus potentially cause metal fatigue on the diode wire leads if the organ is moved around or if it is rocked around whilst being transported or if it is crudely dropped after being lifted.
The mass of 1N4007 diodes is so small that lead fatigue is never an issue I've seen, even in guitar amps used by touring musicians. Lead fatigue and failure is most often seen in electrolytic filter capacitors that are not locked down adequately. I've even seen a couple of orange drop capacitors where the leads broke off right at the capacitor body due to the 90 degree bend necessary to use them in place of axials.
even the most fundamentalist valve purists who despise the concept of adding any solid state devices in a valve amplifier, would not be able to hear any actual difference in the sound with this series diodes modification.
The problem is, they often hear a difference even when there isn't one. :-)
So this is what I did with the AO-39 Power amp in my A100.
1. Added MOVs across the centre tap and the sides on the output transformer (red round components in the pictures)
2. Added Fuses on the PSU secondary high voltages lines
3. Added MOVs across the N and L, N and GND, L and GND at the power in from the wall. Black components in the picture. I connected them at the power console socket and used the free not used tab as a GND
4. Added a main fuse for the whole organ
5. Added a main fuse for the AO-39
6. Changed the Power connector to a Neutrik one
7. Added series diodes to the rectifier tube, used the spare tap on the tube socket
8. Changed all the electrolytes
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