Hi,
I have two Quad II amps and a Dynaco FM3 and have over spec VDC.
On one of the amps and the FM3 the HT is higher than spec after the rectifier (GZ34 or 6V4). For one amp it is 356VDC and should be 340VDC. The other amp is 336VDC. I have a step up giving me 230VAC at the power plug last night, and the amps are wired to the 240V tap, so the voltages should be low. Both amps give me 310VAC at the transformer taps, spec is 310VAC
The FM3 was giving about 300VDC, should be 285VDC. I was getting 258VAC, spec is 255VAC.
Are these rectifiers going bad, and giving higher DC voltages?
Or are the PS caps going bad (CLC and CRCRCRC)? The good amp does have a factory serviced replacement cap, as well as pretty much all newer resistors and coupling caps.
I will swap the GZ34 to see if it is the tube, thought of that too late to try and measure last night. One amp does not seem to run hotter than the other, but there is a low level hum in the bad one.
Thanks for any help.
I have two Quad II amps and a Dynaco FM3 and have over spec VDC.
On one of the amps and the FM3 the HT is higher than spec after the rectifier (GZ34 or 6V4). For one amp it is 356VDC and should be 340VDC. The other amp is 336VDC. I have a step up giving me 230VAC at the power plug last night, and the amps are wired to the 240V tap, so the voltages should be low. Both amps give me 310VAC at the transformer taps, spec is 310VAC
The FM3 was giving about 300VDC, should be 285VDC. I was getting 258VAC, spec is 255VAC.
Are these rectifiers going bad, and giving higher DC voltages?
Or are the PS caps going bad (CLC and CRCRCRC)? The good amp does have a factory serviced replacement cap, as well as pretty much all newer resistors and coupling caps.
I will swap the GZ34 to see if it is the tube, thought of that too late to try and measure last night. One amp does not seem to run hotter than the other, but there is a low level hum in the bad one.
Thanks for any help.
Hi!
That's about 5% off. I wouldn't worry about that.
A reason for the higher voltage could be ageing of the output tubes. If they draw less current, voltage drop across the PSU will be less, resulting in higher voltage readings.
But again don't get wound up about 5%. Tube circuits are tolerant against much more variation.
If the differenc get's higher, check the output tubes
Best regards
Thomas
That's about 5% off. I wouldn't worry about that.
A reason for the higher voltage could be ageing of the output tubes. If they draw less current, voltage drop across the PSU will be less, resulting in higher voltage readings.
But again don't get wound up about 5%. Tube circuits are tolerant against much more variation.
If the differenc get's higher, check the output tubes
Best regards
Thomas
Another factor could be your line voltage. Most, if not all, vintage Dynaco equipment was "spec'd" at 117 VAC. Today, typical line voltage at many locations is at least 120VAC. I've often seen it much higher than that. Mine is typically in the range of 122-124 volts, and at my prior residence, I would have considered that range to be at the "low end"!
A couple things to look at....
The AC heater voltage is usually a good indicator to look at...this will tell you if you have the mains voltage correct...
As for this 117V main or 120V mains that is just an average number....there is NO equipment designed for a specific exact mains voltage... Mains voltage has a wide variation....in the old days the mains voltage had much worse variation in the US then it does now... As for voltages listed on a schematic...take that with a grain of salt in most cases...
There is a growing problem on the grid due to the number of power supplies without Power Factor correction....Here is a real example.... You have a amplifier with 450V DC on the plates and 6.3V AC on the heaters with exactly 120V AC on the mains...which was controlled on a variac... Now you take the same amp and go to another neighborhood and repeat the same measurements with the variac and same amp...and you come up with the exact same readings....now you go to a third neigboorhood and repeat the same thing but now you have 425V DC in the amplifier B+...Heaters are still at 6.3V AC and the mains are 120V AC....So where did the 25V DC go???
Before I explain what happened....is there any thoughts on this??
Chris
The AC heater voltage is usually a good indicator to look at...this will tell you if you have the mains voltage correct...
As for this 117V main or 120V mains that is just an average number....there is NO equipment designed for a specific exact mains voltage... Mains voltage has a wide variation....in the old days the mains voltage had much worse variation in the US then it does now... As for voltages listed on a schematic...take that with a grain of salt in most cases...
There is a growing problem on the grid due to the number of power supplies without Power Factor correction....Here is a real example.... You have a amplifier with 450V DC on the plates and 6.3V AC on the heaters with exactly 120V AC on the mains...which was controlled on a variac... Now you take the same amp and go to another neighborhood and repeat the same measurements with the variac and same amp...and you come up with the exact same readings....now you go to a third neigboorhood and repeat the same thing but now you have 425V DC in the amplifier B+...Heaters are still at 6.3V AC and the mains are 120V AC....So where did the 25V DC go???
Before I explain what happened....is there any thoughts on this??
Chris
HAHAhahaha...real funny...
I also have a pair of QUAD II amps that are from the UK.... They only run at 220 or 240V since they do not have 120V tap...
I also use a step up transformer.... I am in RB...the AC is good where I am at...
OK....the answer relates to PEAK measurement vs RMS measurement....
Very rarely frequent this forum (SS 
) but a couple of things come to mind...
As cerrem mentioned, don't rely on service manual voltages as absolute... have come across that so many times even in modern stuff.
Two amps, different HT's. I would imagine valve rectifiers are much "higher impedance" ! is that the right word, than SS so any small differences in amplifier current draw will alter the apparent measured output from the reccy. Could that explain the difference of 336 vs 356. And the reservoir cap, differences in values maybe ?
just a thought
) but a couple of things come to mind...As cerrem mentioned, don't rely on service manual voltages as absolute... have come across that so many times even in modern stuff.
Two amps, different HT's. I would imagine valve rectifiers are much "higher impedance" ! is that the right word, than SS so any small differences in amplifier current draw will alter the apparent measured output from the reccy. Could that explain the difference of 336 vs 356. And the reservoir cap, differences in values maybe ?
just a thought
Hi!
Yes, also variation in the cap value of the cap after the rectifier will have an impact
Thomas
Yes, also variation in the cap value of the cap after the rectifier will have an impact
Thomas
Yes... The place where the DC voltage was 25V lower had a nasty 3rd harmonic on the mains... This "fooled" the true RMS meters in the measurement showing 120V RMS on the mains... The waveform was not a real sine wave and was slightly clipped at the peak.... The PEAK measurement of the AC showed otherwise.... The problem was convincing the public utilities person on the phone that they need to come out and fix this
Chris
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