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Old 26th August 2012, 05:55 PM   #1
heyraz is offline heyraz  United States
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Default Console Amplifier Pull:Voltages over Spec

Greetings all,
I've been checking over a Fisher 481A amp pulled from a Custom Electra console and am hoping someone can provide some insight. I attached the schematic for your reference.
Initially when I checked the power supply and tube voltages, I found they were slightly elevated. I assumed this was due to higher modern mains voltages (117VAC) and that the unit was designed for 110V. However, something about that assumption bothered me so I decided to look at the power supply from a different perspective and found something I can't explain.
This time, rather than setting the variac to 110VAC and then checking the voltages, I adjusted the variac till B+ was the specified 400VDC, which required 114VAC from the Variac. Next I measured voltages along the power supply and found they were still elevated above specs. Although I understand that resisters can vary with age/use/heating, from what I've read they usually increase in value rather than decrease. There may seem to be an exception that applies to my situation at the first power supply capacitor and dropping resister junction where the resister decreases in value resulting in overall elevated voltages.
I've only taken six measurements so far:
Between R32 and CR1 to chassis ground: 400VDC (as I set using the Variac)
Between R32 and R25 to chassis ground: I measured 369 instead of the specified 345
Between R25 and R37 to chassis ground: I measured 353 instead of the specified 297.
Between R37 and chassis ground : I measured 353 instead of 275 (implying to me that R37 is shorted)
At Socket Pin 1 of V201: I measured 316 instead of 260
At Socket Pin 1 of V204: I measured 326 instead of 260

These measurements don't make sense to me and I'm wondering if someone has insight. I am hesitant to "replace everything" until I'm sure of the cause. For example, I'm using a Fluke DMM and am wondering if wondering if it's too sensitive. Could there be AC riding on the DC? Should I be using an analog meter instead?
This amp is running "stand alone" without the tuner/preamp so there isn't as much draw as the schematic would imply. The only modification I've done is to add a 250ohm 10W resistor to ground between C14C and R31 at the output tube bias adjustment to make up for the three 12ax7 heaters connected in series from the disconnected tuner section.
The amp doesn't sound bad but I noticed that one of the 12ax7 tubes seemed hotter to the touch than the other. I don't want to run it again until I'm sure it's safe. This is my first "console pull" amplifier and would appreciate an experienced opinion.

Rich
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Old 26th August 2012, 06:28 PM   #2
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Are you testing with all of the tubes installed and an appropriate load resistor on the speaker terminals?
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Old 26th August 2012, 07:21 PM   #3
20to20 is offline 20to20  United States
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Interesting. This amp uses the filaments from 3 12ax7's to bias the outputs? I've seen an amp that does that with one but not 3.

Are the 33v and 21v biasing the EL 84's close?

I think it's entirely possible that lack of loading from the missing console sections is mostly to blame for the high B+ tertiary voltages. Those resistors seem small like they are expecting a fair amount of current to pass through. It's tough to know without the other diagram.

G'Luck
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Old 27th August 2012, 03:42 AM   #4
heyraz is offline heyraz  United States
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The only thing I did to the amp was add the 250ohm 10W resister to make up for the missing three filaments in series. The grids can be biased to 21v easily. I didn't take notes the first time around, but I recall the cathodes were a little higher than 33v.
I think I need to keep searching and see what that tuner was drawing.
Thanks
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Old 27th August 2012, 04:17 AM   #5
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Hi,
I did some calculation and the tuner should have load a .021 mlliamp. Do another calculation and you can connect a 13K 1 watt resistor to the resistor side that supply the voltage to the tuner.
here it is How I did the calculation.

I added all the resistors 1K8,3K3 and 1K8 = 6900 400\6900 = .059 amp
then 1K8 + 3K3 less 1K8 = 5100 then 400\5100 = .079 amp then .079 -.059 = .020 then 275/.020 =
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Old 27th August 2012, 04:19 AM   #6
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Hi,
Sorry I hit the wrong button the last calculation should be 275/.020 = 13K
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Old 27th August 2012, 04:36 AM   #7
heyraz is offline heyraz  United States
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Thanks! I'm going to spend some alone time with the amp in the next couple of days and continue to look for the tuner's schematic. I'll let you know what I find.

BTW-I had no idea using a console pulled amp would be anything other than "plug and play". I'm glad I took the time to check it over.

Thanks again. Rich
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Old 27th August 2012, 05:17 AM   #8
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Hi,
Sorry but the calculation was for maximum current at the close loop circuit. In tube it is different. Need to use the voltage drops in each resistors to calculate the tube current. So by calculating the voltage drop of the 1800 resistor feeding the tube 297 volts - 275volt = 22 volts The current it is 22 volts /1800 = .012 amps. Then the load resistor it is 275 /.012 = 23K The 23K resistor will simulate the tuner load to the power supply.
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Old 27th August 2012, 05:34 AM   #9
kevinkr is offline kevinkr  United States
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Note that the voltages specified in the schematic were measured with an analog voltmeter, look for the sensitivity of the meter used for these measurements which is probably 20K per volt.. (Or less) Modern digital volt meters are generally much better than this, and draw much less current which becomes relevant when the circuit resistances are more than a few K ohms.

Actual measured values in some cases can differ by as much as 20% from those given in the schematic depending on point measured, component tolerances, and the loading effect of the meter.

Idle current in the output stage should be in the vicinity of 140mA which equates to 235 ohms so you are reasonably close. The supply current furnished to the tuner will certainly have an effect, but I'd expect it to be nearly 12 mA based on a quick calculation which is not really in line with what I would expect, but OK... Closest standard value is 22K, and I would use a 10W resistor be conservative.

Do plan on replacing the supply and bypass electrolytics soon, (and coupling caps as necessary) as their failure could wipe out a hard to replace part like the power transformer.
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Old 27th August 2012, 02:42 PM   #10
TheGimp is online now TheGimp  United States
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Kevinkr, they specify a VTVM for measurements. The input impedance would have been greater than 1MEG ohm, so readings are accurate.

Measurements would have been taken with a line voltage of 115VAC. Modern lline voltage of 125VAC would result in an increase in voltages of 1+((125-115)/115)=1.08 or 8% increase in all readings.

Last edited by TheGimp; 27th August 2012 at 02:45 PM. Reason: add note about lline voltage
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