Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: worth on August 19, 2011, 05:51:26 pm
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I powered this one up today through my current limiter... NO light. I've built many Champs , but this is the only one where I used a salvaged PT... from an old Roberts reel to reel. The AC voltages are fine , 260-0-260, and the heaters are 6.3V. BUT.. when I check the voltage on the DC side of the rectifier ,( solid state , since there is no 5V winding ) I get 215V DC. Replaced the rectifier with the same results. Any thoughts on the low DC ?
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Sounds to me like maybe half the HV winding of your salvaged power trans is bad or kinda bad. So, you think you have a full wave rectifier which should put out 1.4 * 260 = 364 but you are being fooled and it's a de facto half wave putting out .7 * 260 = 182.
Plausible? Have you measured the AC volts of the two HV output leads under load? Key word: Under load. Same? Different?
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Thanks for the reply... yes .. I just measured AC volts of each leg of the HV primarys UNDER LOAD... both are the same.. 260V AC. I'm REALLY puzzled as to why I'm getting 260V AC on one side of the rectifier, and 215V DC on the other.
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Thanks for the reply... yes .. I just measured AC volts of each leg of the HV primarys UNDER LOAD... both are the same.. 260V AC. I'm REALLY puzzled as to why I'm getting 260V AC on one side of the rectifier, and 215V DC on the other.
If both sides of the HT winding measure the same VAC, that's okay. But how have you got it rectified? If you have 260-0-260 rectified with Full wave SS, you should get about 365 or so unloaded.
So methinks that maybe either one of the filter caps is partially shorted or you have something else (like an incorrect value bleeder resistor or a short in one of the tubes, or that the preamp supply is shorted to ground) which is dragging the B+ voltage down. What are the DC voltages at each of the B+ supply (filter cap) nodes?
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Either partial short, heavy load on the B+, OR...
A filter cap is a *very low* value. If the first filter stage is much lower than it should be (1uF or 2), the voltage will sag down below what you'd expect.
Final answer is check for the presence of output tube bias, but then unplug the current limiter for your voltage checks. The limited current has some strange side effects that you don't want to chase after needlessly.
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Well... woke up this morning , went straight to the amp, bypassed the standby switch with a jumper from the rectifier to the first filter cap , and the DC voltage shot up to around 360VDC .... brand new Carling switch was bad. It's odd , but I've never had a bad spst Carling switch.
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Yeesh. That's pretty wild.
I recall when I was working at Audiotronics in North Hollywood, fixing just-assembled and flow-soldered little CRTs of the type that used to be used in cash registers and early ATM machines. OEM 5", 7", and 9" CRT displays, basically. The factory got in a whole batch of reverse-marked diodes. Those kept us busy for a while, and they continued to pop up over many months. So you could never tell when one would find its way into something would stubbornly not work. I'll bet the factory saved .002 each on a lot of 10,000 = $20 and spent $20,000 in lost tech time over the next 6 months. Pretty good payoff, eh?
By the way, your last post sounds like the first verse and chorus of a blues tune:
Well... woke up this morning ,
(I) went straight to the amp,
I bypassed the standby switch
with a jumper [from the rectifier] to the [first] filter cap ,
the DC voltage...
shot up to [around] 360DC ....
brand new Carling switch was bad.
never had a bad spst [Carling switch] go tits-up on me!
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