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I recently saw schematics for some small guitar amps that used a 6X4 rectifier. They use a single 6.3v filament winding to heat all the tubes - 6X4, a pair of 12AX7s and a 7189A. A couple of them had one side of the winding grounded and no center tap, another had a center tap.
Somehow I was under the impression that rectifiers needed to have heater supplies that were separate from the other tubes. Perhaps that's because so many use 5v rectifiers while the other tubes use 6.3v.
Is this a proper implementation of the 6X4? Can other rectifiers be used like this too? Specifically, can all tubes be powered by the same supply, assuming the voltages are the same and the current rating is sufficient? What's the downside of such an arrangement, if any?
And what about having one side grounded? Most rectifier windings seem to be floating with no center tap and most of the other tubes are heated with a CT winding.
Every Hammond B3/C3/A100 organ out there uses a 6X4 in its preamp, providing 375V (frequently up to 425V in practice) @ 50mA with a 40uF reservoir capacitor. Presumably, the PT winding resistance helps limit peak charging pulses.
Used this way, the 6X4s last quite a long time. It's not that unusual to find a preamp with its original Hammond-labeled RCA 6X4 rectifier.
Their failure mode, as mentioned elsewhere, is often a heater-cathode short. I test all 6X4s in Hammond preamps for heater-cathode leakage and replace them if I find it. The same goes for 6X5s in Hammond 2-series organs. The good part is that you can usually spot the leakage before there's a serious short.
Thanks for the clarification guys!
My query stems from a thread on another site: http://www.diyaudio.com/forums/instruments-amps/5246-el84-substitute-7189a-2.html
The poster is trying to rebuild a Japanese Teisco Express 5 guitar amp. After viewing a schematic he posted of a Teisco 10 (which seems to be the same amp) and other pics of his amp, I looked at some schematics of similar amps (Kalamazoo Models 1 and 2 and a Silvertone 1421) and noticed a similar use of the 6X4. While I don't own any of these amps, I hadn't ever seen the 6X4 - or any other rectifier tube for that matter - used like this.
If you read the thread you'll see that my main concerns were the use of a 40uF first cap in the PS (the data sheet calls for 10uF) and the owner's seeming reluctance to replace the infamous grey oil caps that the Japanese were so fond of using.
Abuse of vacuum rectifiers was, and remains, a problem. Look at some of Cary's PSUs. :> ((
The cheapskates would not use proper CLC filters that don't abuse the vacuum rectifiers.
40mfd is going to kill the 6X4. But, prolly the 6X4 was less than a couple bucks, when those Japanese amps were being made. So, the manufacturers didn't care.
Still, the mil spec NOS Tungsol 6X4W --- my favorite --- is not that $$$.
The stock configuration on the Teisco was 40-10-10. Fortunately, I was able to convince him to switch the first cap with the last one so now he has it 10-10-40, all 450v parts.
The others I mentioned used 20uF as the first cap, not quite as extreme but still too high.
That's the value in the old Akai decks that I mentioned. They followed the 20uF with a small choke, then another 20uF. The 6X4s in those never fail.
Buy Chinese. Bury freedom.
Does the ability to, apparently safely, exceed the data sheet specs by 100% have anything to do with current draw or other operating conditions? Or is this just an especially tough rectifier tube or ??? Do you think rectifier data sheets are deliberately ultra conservative in this regard?
Another rectifier, the 5AR4, has a max first cap rating of 60uF. Are there designs that safely use 120uF with this? The highest I've seen was with a Pilot 500 console amp that I restored (kind of mini version of the 232) which came with 80uF stock. I used a smaller cap when I rebuilt it.
I've always figured that going, maybe, 25% higher than a data sheet recommends would be about as far as any given rectifier should be pushed. That's just based on a vague philosophical belief not on analysis of technical data, though.
6X4 data sheets show 10uF as the typical value for a cap-input filter. To determine the actual maximum for a given application, it's necessary to use additional published data. The 6X4 is rated for a maximum transient peak plate current of 1.1A per plate. Charts published by the manufacturer(s) use this peak rating to show the required plate supply resistance in ohms VS the AC RMS voltage per plate, relative to a 10uF cap. Additional capacitance can be used as long as the peak plate current rating isn't exceeded. Simulation software is ideal for determining this.
Buy Chinese. Bury freedom.
Boy, the cost of these things... I would not design an amp with more than 40mfd in the first stage. 120mfd is crazy! At that point, I'd seriously considered solid-state rectification.
Which guitar amps were these? Some 1960s era Japanese ones used 6 volt rectifiers and were known to be unreliable. I've seen posts of rebuilds that involved rewiring & using separate trannies for the rectifiers.
These late 1950-1960s era low-end guitars and amps are considered somewhat collectible, to many today. Esp the "Jetsons" style 1960s mod-deco ones.
So, if you have one of these amps, it maybe worth restoration. Just wire it for safety. BTW... many cheap Sears, White Front, and Realistic guitar amps of the 1960s were Japanese imports.
TK gave you a warning, specifically about the 6X4. You can use the schematics with ZERO signal parts changes and a Loctal 7Y4 as the rectifier.
Don't mix apples and oranges. 5 V. types, like the 5AR4/GZ34 and 5V4/GZ32, have the cathode sleeve tied to the heater at pin 8. The construction is known as 1/2 indirectly heated and, like the directly heated 5U4 and 5R4, require a dedicated, separate, filament winding. Types like the EZ81/6CA4 and 6X4 electrically isolate the cathode from the heater and, as TK pointed out, can share the filament winding with signal tubes.
BTW, an advantage of 1/2 indirectly heated construction is that heater to cathode potential issues can't be present.
It's OK to power an indirectly-heated rectifier with a common filament winding that's grounded, as long as the cathode/heater voltage rating isn't exceeded. However, the 6X4 is a special case. Despite its 450V rating (cathode positive WRT the heater), there have been frequent reports of cathode-filament shorts when operated with the filament at DC ground. The '60s era Akai/Roberts tape decks used this tube and were inherently reliable, but they all employed a separate, ungrounded winding just for the 6X4.
Buy Chinese. Bury freedom.
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