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looking for help diagnosing volume loss on tube amplified reel to reel player
looking for help diagnosing volume loss on tube amplified reel to reel player
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Old 7th June 2010, 09:20 PM   #21
Wavebourn is offline Wavebourn  United States
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looking for help diagnosing volume loss on tube amplified reel to reel player
Quote:
Originally Posted by lassoharp View Post
Using a higher voltage in these apps is fine. You may very well need a bit higher than 150V on the PS caps anyway with the addition of the diode bridge rectifier.
Don't forget about an isolation transformer in addition to a diode bridge rectifier!!!



http://www.diyaudio.com/forums/tubes...h-voltage.html
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Old 7th June 2010, 10:58 PM   #22
lassoharp is offline lassoharp  United States
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Quote:
Originally Posted by Wavebourn View Post
Don't forget about an isolation transformer in addition to a diode bridge rectifier!!!



http://www.diyaudio.com/forums/tubes...h-voltage.html

Waveborn, That was already covered earlier in the thread. I'm well aware of the need for an iso transformer and I'm sure diblet is too. It was assumed this was going to be in use with the bridge.

The SS bridge is what is contributing to the higher output voltages - the iso is simply 1:1 from the mains. So maybe you misinterpreted what I said or maybe I'm missing something here?


Diblet,

After looking at the tube specs and the PS changes you proposed make sure and take a quick reading of the main B+ supply voltage ( right at the 35W4 output) and a reading on the 30A5 plates and make sure dissipation values and max ratings aren't going to be exceeded.

http://frank.pocnet.net/sheets/133/3/30A5.pdf

http://frank.pocnet.net/sheets/127/3/35W4.pdf
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Old 8th June 2010, 03:41 PM   #23
diblet is offline diblet  United States
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@wavebourn yes i am in the process of adding the isolation transformer - thanks to the heads up earlier from other members here - just waiting for the rest of the parts and still researching/learning the specifics of the modification

@lassoharp thanks so much for the good advice. as soon as i have an isolation transformer inline i'll make sure to take those readings. if the ratings are exceeded, what changes would i theoretically have to make to bring the voltage down to a level that is within spec?

i am still new to amplifiers and electronics in general, but am really enjoying learning all of this stuff (just fixed my mixer yesterday that had a resister that went open circuit - knowing how to diagnose and deal with it saved me a chunk of change and had me back up and running in no time)

again i can't thank everyone enough for all the input - there is a really great and helpful community here!
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Old 18th June 2010, 05:52 AM   #24
diblet is offline diblet  United States
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isolation transformer is in place! and it works!

initial readings seem ok - the solid-state rectified voltage coming into the 35w4 is 114v - once it leaves the 35w4 it is at 146v

the plate on the 30a5 is seeing 130v, the plate on the 12av6's are 98v and 54v

everything is under the maximum values - however i am not sure how i determine if their dissipation values are in spec...?

_

also, i noticed there is a small amount of ac on the chassis (anywhere from .7 - 2v ac) when it is plugged in - i am not sure if this is cause for alarm though, as it remains regardless if the switch is on or off, if i disconnect the motor, b+ supply voltage or the switch from the circuit (seems to be related to the ground connection somehow? as it is the only thing that stayed hooked up to the chassis during testing).

thanks again for everyone's help - good to know the unit is a little safer these days...

Last edited by diblet; 18th June 2010 at 05:56 AM.
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Old 18th June 2010, 07:18 PM   #25
lassoharp is offline lassoharp  United States
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Quote:
Originally Posted by diblet View Post
isolation transformer is in place! and it works!

initial readings seem ok - the solid-state rectified voltage coming into the 35w4 is 114v - once it leaves the 35w4 it is at 146v

the plate on the 30a5 is seeing 130v, the plate on the 12av6's are 98v and 54v

everything is under the maximum values - however i am not sure how i determine if their dissipation values are in spec...?

_

also, i noticed there is a small amount of ac on the chassis (anywhere from .7 - 2v ac) when it is plugged in - i am not sure if this is cause for alarm though, as it remains regardless if the switch is on or off, if i disconnect the motor, b+ supply voltage or the switch from the circuit (seems to be related to the ground connection somehow? as it is the only thing that stayed hooked up to the chassis during testing).

thanks again for everyone's help - good to know the unit is a little safer these days...

Reading the voltage at the top of the 30a5 cathode resistor(and using ohms law) will give you the current through the entire tube(plate + screen current). Screen current can be obtained by measuring the voltage drop across the 300ohm screen resistor. Subtract this from the total current to get plate current. From there: P = IV gives plate dissipation.

Now, using the plate current value you just calculated, go to the data sheet and compare the calculated results with the dissipation curve (the single curve dissecting the Eb/Ei curves).

From your calculated results, any answer below 7.5W is safe. From the dissipation curve, anything below/left of the curve is a safe operating region.

There shouldn't be any AC on the chassis when the AC main switch is OFF. Try and carefully trace this down. Even a few volts can give you an unnerving tingle should you have one hand on the chassis and one hand on another equipment's chassis and this is not good. That amount of stray voltage can certainly cause hum to flow when the unit is interfaced to another piece of gear.

You can test this safely beforehand by hooking the machine up to another piece of gear(power off). Then power up and measure AC voltage between the two chassis. If their grounds are equal there should be hardly any(millivolts) AC.
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Old 18th June 2010, 07:31 PM   #26
Wavebourn is offline Wavebourn  United States
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looking for help diagnosing volume loss on tube amplified reel to reel player
Quote:
Originally Posted by diblet View Post
wow, how could something that is so dangerous ever be made then?
Yes, it was dangerous, but cheap, and could work from both AC and DC. Some people did not want to pay for safety, now we are forced by Law.
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Old 18th June 2010, 09:24 PM   #27
diblet is offline diblet  United States
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Quote:
Originally Posted by lassoharp View Post
You can test this safely beforehand by hooking the machine up to another piece of gear(power off). Then power up and measure AC voltage between the two chassis. If their grounds are equal there should be hardly any(millivolts) AC.
i actually did that last night after i had made my post - i get no reading from chassis to chassis - so i must have been doing the ac reading in error somehow. i'll keep an eye on it, but everything seems well.

just in case, i followed a tutorial for troubleshooting an ac leak issue and found no such leaking components/connections.

-

i'll do the calculations you kindly gave me for calculating values for the tubes - i am optimistic things are well, as the unit is running the same as it did before both in sound and temperature. thanks so much for the help.
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Old 19th June 2010, 02:56 PM   #28
diblet is offline diblet  United States
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alright, plate dissipation is at 6.6 watts - within spec! sucess

thanks again to everyone for all the help - and potentially saving my life!
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Old 23rd August 2010, 09:40 PM   #29
jrbinc is offline jrbinc
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would a 1A rectifier be good for the isolation transformer mod?
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Old 24th August 2010, 02:16 AM   #30
smeezekitty is offline smeezekitty  United States
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^ rectifiers do not isolate.
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