Here is a pic of the bottom of the chassis. All original except for the caps. I just powered it up without the rectifier tube and the fuse DID NOT blow. I also saw a trouble shooting vid about this amp and they said DO NOT put a three-prong power plug on this amp. Why not, I wonder? Still haven't taken any voltage measurements yet.
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I've circled two areas (there are some other scary bits but I don't think they're causing the problem). One is the orientation of the two power supply caps. It is correct? Second is the ancient diode (for bias?) which might need to be replaced.
GroverGardner: The cap arrangement "looks funny, but it's correct, polarity wise. Selenium Diode is original, and it has been suggested that that be replaced. What about a 3 pronged plug NOT being recommended?
Okay, just checking. Yes, I'd replace that old diode. 3-prong plug--advice varies, if you do so, then AC ground to chassis only, not the main circuit is what's usually recommended, otherwise ground loops can sneak in.
For the GZ34, could be that's the only problem. People say the TAD 5AR4 is sturdy enough, I like Gold Lion GZ34 but I don't use it in a stereo amp, only about 150mA. But they seem reliable. The "yellow sheet" mod is a safe bet and will help any 5AR4 hold up better.
For the GZ34, could be that's the only problem. People say the TAD 5AR4 is sturdy enough, I like Gold Lion GZ34 but I don't use it in a stereo amp, only about 150mA. But they seem reliable. The "yellow sheet" mod is a safe bet and will help any 5AR4 hold up better.
Could it be possible you replaced a slow blow fuse with a fast blow one? What is the fuse rating?
It could be the capacitors, or one of them failing. The originals were rated 525V and this amp needs it. The replacements you have in the picture look to be only 500V rated. I've heard somewhere that the MarkIII ran its voltage a bit high for use with 500V caps. You might have to pull the caps and test them at working voltage to verify that they are good... Not blowing the fuse when the 5AR4 was removed says its something downstream of it.
These old amplifiers used the chassis as a signal ground. It's not the best idea to do that because the chassis is not as good of a conductor as copper.
Anyway, grounding the chassis to earth could cause a ground loop and unwanted hum in your audio stream.
The best thing to do is to polarize the plug so that the "hot" prong goes directly to the switch, and then the fuse. (This is not how it was wired from the factory.)
Also, your capacitors are suspect. And others have noted that their voltage ratings may not be sufficient.
If you want to eliminate capacitors as a source of trouble I'd recommend the "SDS LABS" cap board, which replaces the four cap sections with 8 series pairs of 400V rated capacitors, which doubles the pair rating to 800V, well within the tolerance of the amplifier.
I think that if you have a new rectifier and it is the cause of the problem (after you've eliminated everything else) most likely due to internal arcing, then you should return it under warranty for another. Or buy a new production Mullard 5AR4. They seem to be quite well constructed and I haven't experienced any failures with them.
Some have suggested the "yellow sheet" diode mod, but this is just putting a band-aid on the problem.
And replace the selenium rectifier (they tend to explode at their age with a stink to high heaven) with a single IN4007 diode (which cost pennies).
Just curious - at around the 90 volt mark, just before the fuse blows, what is the bias current on the power tubes? I've seen fuses blow on amps that had tubes replaced without resetting their bias.
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Grunding the MkIII chassies with a 3-prong cable does not cause hum. I have done it with several. In fact
i have all my equip grounded together for security reasons and they are dead silent when used.
Regarding to can cap, i recommend installing a proper cap and removing those used. Voltage comes often up
over 500Volt in these amps.
And the "yellow sheet mod" is easily done and will protect the GZ34 with no side effects.
i have all my equip grounded together for security reasons and they are dead silent when used.
Regarding to can cap, i recommend installing a proper cap and removing those used. Voltage comes often up
over 500Volt in these amps.
And the "yellow sheet mod" is easily done and will protect the GZ34 with no side effects.
Yes - but the highest out there is a CE Manufacturing 4-section cap that is rated at 525V Still too low.
This is why I recommended the SDS Labs Cap-board option.
Actually, Authenticaps have a 550vdc rating with 600vdc surge capability. I've used them in rebuilds and they're very good. You can usually find something close enough in their line, in this case a 30-20-20-20. Also, Hayseed Hamfest will make replacement cans. Both will preserve the original look but neither is cheap. I have found the CE cans to not be very reliable at or near their rating. The SDS board is a good option.
AuthentiCaps are $65 each.
The whole SDS board costs about that much including all the parts and I think the end result is superior.
The whole SDS board costs about that much including all the parts and I think the end result is superior.
Thanks to everyone for their input. I now have several things to take care of. I'm in Prescott, AZ and get my supplies from "Tubes and More" in Mesa Arizona. I'm now thinking of scratch building a 12AX7 preamp to go with the Dyna. Does anybody have a favorite design for one with volume, bass and treble controls? I haven't been able to find a complete schematic online. I've got a 5MK9 power supply I'm thinking of using. Thanks again to all. I'll update everyone when I get my Dyna running.
The DYNA PAS circuit fits the bill exactly.
But I think I prefer the sound of the "AIKIDO" preamp from tubecad. Adding bass and treble to it is a bit of a task, but not impossible.
But I think I prefer the sound of the "AIKIDO" preamp from tubecad. Adding bass and treble to it is a bit of a task, but not impossible.
