Why overvoltage on HT?

studeb · 23rd March 2011, 06:36 PM

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.

Vinylsavor · 23rd March 2011, 06:49 PM

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

studeb · 23rd March 2011, 07:00 PM

Dang that was a fast reply.

Part of the untold story is i have replaced three dead 7581A tubes, two cathode caps and one cathode resistor over the last five or six years in both amps.

So i now have a new quad of KT66 tubes in there and thought to check out if something is amiss.

ArtG · 24th March 2011, 03:02 AM

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"!

DigitalJunkie · 24th March 2011, 04:34 AM

Quote:

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.

I don't understand this?
The AC in the US is 120Vac(ish)..where'd you get 240V from!?

studeb · 24th March 2011, 05:04 AM

i bought the amps when i lived in europe, they are 220-240v units.
when i moved to the US, i bought a step up to run them. The FM3 goes straight into the wall.

i was puzzled why there is spec AC after the tansformer, yet over spec DC after a tube rectifier

DigitalJunkie · 24th March 2011, 06:49 AM

Ahh..Got'cha.

cerrem · 24th March 2011, 02:59 PM

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

studeb · 24th March 2011, 04:38 PM

You moved to downtown from Clairemont, where all the B+ is too high?

My heaters are ~spot on, 5V and 6.2V

cerrem · 24th March 2011, 06:39 PM

Quote:

Originally Posted by studeb

You moved to downtown from Clairemont, where all the B+ is too high?

My heaters are ~spot on, 5V and 6.2V

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....

23rd March 2011, 06:36 PM	#1
studeb diyAudio Member Join Date: Feb 2008 Location: San Diego	Why overvoltage on HT? 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.

23rd March 2011, 06:49 PM	#2
Vinylsavor diyAudio Member Join Date: Feb 2002 Location: Lindau	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 __________________ http://vinylsavor.blogspot.com/

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23rd March 2011, 07:00 PM	#3
studeb diyAudio Member Join Date: Feb 2008 Location: San Diego	Dang that was a fast reply. Part of the untold story is i have replaced three dead 7581A tubes, two cathode caps and one cathode resistor over the last five or six years in both amps. So i now have a new quad of KT66 tubes in there and thought to check out if something is amiss.

24th March 2011, 03:02 AM	#4
ArtG diyAudio Member Join Date: Nov 2006 Location: Central PA	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"!

24th March 2011, 05:04 AM	#6
studeb diyAudio Member Join Date: Feb 2008 Location: San Diego	i bought the amps when i lived in europe, they are 220-240v units. when i moved to the US, i bought a step up to run them. The FM3 goes straight into the wall. i was puzzled why there is spec AC after the tansformer, yet over spec DC after a tube rectifier

24th March 2011, 06:49 AM	#7
DigitalJunkie diyAudio Member Join Date: Apr 2003 Location: Portland,Oregon Blog Entries: 4	Ahh..Got'cha.

24th March 2011, 02:59 PM	#8
cerrem diyAudio Member Join Date: Jun 2005 Location: San Diego, CA	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

24th March 2011, 04:38 PM	#9
studeb diyAudio Member Join Date: Feb 2008 Location: San Diego	You moved to downtown from Clairemont, where all the B+ is too high? My heaters are ~spot on, 5V and 6.2V

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