Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on December 14, 2011, 08:08:39 pm
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Hi, I have bought a kit for a super champ with a PT too hot. I had to use an EL34 insteaad of a 6V6. The voltage on the 6V6 plate was 454 V, 150V too much, or so, enough for 2 or 3 6V6's. With an EL34, the voltage droppedto 415V and I adjusted the bias at 48mA, a bit on the cold side, the tone is really good, but it is not a super champ. I thought using the VVR I have in one of my drawers for years, but I thought I might as well get a new tranny. How can we know in advance before buying a tranny if it has the good voltage for the purpose it was supposed to be manufactured for ? We just can't. One can sell you about anything , arguing it is the right thing , and after paying and receiving the merchandise, we realize it is not what we wanted. ( I guess I am just frustrated, here...hey hey ). If I buy a clone from Mercury Magnetics, by example, should I be 100 % sure that it'll do the job I want it to do ?
Colas
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JJ6V6S can handle that sort of voltage and are very reliable (unlike their 6L6GC) and sound primo
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While 454VDC is certainly on the high side for a 6V6 it's not 150 volts too high. My Fender Vibro Champ runs the plates slightly in excess of 400VDC as do many Deluxe Reverbs and Princetons. Those old Jim Kelley amps run 475-485VDC on the plates. Real 6V6 killers. The JJ6V6S would be a good choice. A really tough to kill tube that sounds good.
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Hi, I have bought a kit for a super champ with a PT too hot. I had to use an EL34 insteaad of a 6V6. The voltage on the 6V6 plate was 454 V, 150V too much, or so, enough for 2 or 3 6V6's. With an EL34, the voltage droppedto 415V and I adjusted the bias at 48mA, a bit on the cold side, the tone is really good, but it is not a super champ. I thought using the VVR I have in one of my drawers for years, but I thought I might as well get a new tranny. How can we know in advance before buying a tranny if it has the good voltage for the purpose it was supposed to be manufactured for ? We just can't. One can sell you about anything , arguing it is the right thing , and after paying and receiving the merchandise, we realize it is not what we wanted. ( I guess I am just frustrated, here...hey hey ). If I buy a clone from Mercury Magnetics, by example, should I be 100 % sure that it'll do the job I want it to do ?
Colas
Maybe it will be simplier and better to use a power source with the voltage of 300V?
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While 454VDC is certainly on the high side for a 6V6 it's not 150 volts too high. My Fender Vibro Champ runs the plates slightly in excess of 400VDC as do many Deluxe Reverbs and Princetons. Those old Jim Kelley amps run 475-485VDC on the plates. Real 6V6 killers. The JJ6V6S would be a good choice. A really tough to kill tube that sounds good.
On the one hand, a rather big amount of this voltage (I mean, out of these 475VDC) might have subsided on the cathode resistor. Thus, a part of the voltage didn't go to the tube, but just caused bias. But on the other hand, in a scheme with the voltage of 300V the tube must have worked under the fixed bias created another source of voltage.
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"My B+ is too high" is one of the more common complaints with amp builders. I try to avoid buying PTs that are 'made' for a particular amp unless they also list the voltage and current ratings. I really like Hammond PTs for this reason.
How can we know in advance before buying a tranny if it has the good voltage for the purpose it was supposed to be manufactured for ?
Measure the AC voltage on the secondary HT winding of your PT. Then buy a PT that has a lower voltage rating. For example, your PT measures 350-0-350 and that's hotter than you want. So replace it with a 325-0-325 or even a 300-0-300 PT. Then cross your fingers and hope you did good! :wink:
Hammond makes this easy because they list the voltage/current ratings for their PTs rather than just saying this PT is meant for a Fender Super Reverb, or that PT is for a Marshall 50 watt amp.
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I was under the impression that a power tube could handle higher voltages when run single ended. Is this a false assumption?
Dave
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I was under the impression that a power tube could handle higher voltages when run single ended. Is this a false assumption?
Dave
It is theoretically possible with a large bias. More than bias the lower tube plate current.
Although I am interested to look at this scheme
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Sounds like you're using a tube rectifier. If so is it a Sovtek 5Y3? The Sovtek 5Y3 doesn't drop as much as a real 5Y3.
I had an amp with high B+ and tried two different Sovteks with the same result, then finally got an old Jan Phillips 5Y3 and everything was fine.
What's your AC from the transformer? A real 5Y3 should give a DC of ~1.1 X AC. (under load)
If it's not a Sovtek another thing to consider is that, with a tube rect, the voltage will change on the plate as you bias it. Did you measure the plate voltage after you biased the amp?
If you measured the plate voltage first, when it was biased stone cold, then biased the tubes to allow more current, more current would pass through the rect tube causing the plate voltage to drop. Even though you measured 454V that may not be where it ended up after biasing. With a tube rect I usually check voltage, adjust bias, check voltage, adjust bias... Know what I mean?
Hope this helps.
Jeff
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> the impression that a power tube could handle higher voltages when run single ended.
I don't see why.
Anyway extra-high voltage is no benefit in class A, which is all SE amps.
> rather big amount of this voltage ...might have subsided on the cathode resistor.
Rarely over 10%.
> it is not a super champ
What is the OT impedance?
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454 V is after the JJ 6V6 comfortably sat in the socket with a GZ34.....definitely too hot. But, another ball game with an EL34 that is very happy with 420V and 48mA. It's not that the amp is a piece of crap, not at all, the tone is just great. But what if I had to build a particular amp for a client, with a tranny out of the blue ? That is my point.
regards
Colas
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454 V is after the JJ 6V6 comfortably sat in the socket with a GZ34.....
Did you try the JJ with a 5Y3GT?
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No I didn't because I would have had to change the reservoir cap from 60 uF to 20uF. You think it could have lowered the voltage significantly ?
Colas
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Yes. What was the kit you bought?
If you need to use a 60uF cap, what I would do is(and hopefully someone will confirm this) is break the connection from the rect tube to the first cap. Add a 20 uF cap from rect to ground and add a resistor between the 20uF cap and the 60uF cap. That resistor will limit the current to the 60uF when the amp is turned on. Also it can be adjusted to tune in the voltage. Use the 5Y3(not a Sovtek).
I think the whole reason you can't use a 60uF cap with a 5Y3 is because when the amp is first turned on the cap needs to charge. A bigger cap will draw more current which can be bad for the rect tube.
If you can find a small(physically) high wattage resistor it's easy to mount right on the rect tube socket.
move the 60uF cap to an unused pin, add the 20uF where the 60uF was, then connect the two pins with the resistor. My favorite little amp is set up this way and it is dead quiet.(I think I used a 250 ohm 5W, but ask the experts for what's best)
I'm not 100% on this so wait til someone more experienced says it's OK.