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300B SET amp measurements without tubes installed
300B SET amp measurements without tubes installed
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Old 2nd December 2017, 06:16 PM   #1
MEPER is offline MEPER  Denmark
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Default 300B SET amp measurements without tubes installed

Is it ok to have power on a SET amp without tubes inserted?
Is it also ok in a 300B SET direct coupled amp with ECC83 as the driver only to have the ECC83 or only the 300B inserted?
I want to make some tests to try to find out where hum is comming from. E.g. if there is hum on the output transformer without any tubes inserted this will tell something etc........
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Old 2nd December 2017, 06:36 PM   #2
artosalo is offline artosalo  Finland
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Usually yes, but it is better that you post the schematic to be sure.
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Old 2nd December 2017, 06:50 PM   #3
6A3sUMMER is offline 6A3sUMMER  United States
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We need a schematic to more fully answer as to how to troubleshoot the hum.
Not just simply a schematic, but with component values too.

The 300B will not get its proper bias with the ECC83 removed (direct coupled, right?). It may blow the 300B, the output transformer, etc.

Can your filter caps stand the higher B+ voltage when the 300B and ECC83 are removed?

By the way, did you use a bleeder resistor on your B+ that will discharge the caps when you power down, and go in and get your hands in there?
Safety first!
And always use a DMM to check that the B+ is discharged before working on the amp.

If you remove the 300B, the hum could be caused by either 'the power transformer or the power supply choke (assuming you still load the power supply)', which then couples magnetically to the output transformer.
Are the output transformer windings oriented 90 degrees versus the power transformer and choke windings?
How far is the output transformer to the power transformer and the choke?

Did you use AC or DC filaments on the 300B?
If it is AC, did you use a hum balance pot?
Even with that, some hum may be present.
It depends on the exact 300B, some are worse than others, even from the same manufacturer.
I can give an explanation for that (mostly unknown effect) if that is the problem (ask
me).

How much filtering is in the B+ power supply, for the 300B and for the ECC83?

By the way, direct coupled is much harder to troubleshoot hum, because you can not just short the 300B grid to ground (you can for most RC coupled and most interstage designs).
Which brings us back to the need we have to see your schematic.
I have some tricks that might work, depending on your schematic.

A picture of the top of the amp and transformers and choke might help too, but not if they are potted
(can't see the winding orientations).

Last edited by 6A3sUMMER; 2nd December 2017 at 06:53 PM.
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Old 2nd December 2017, 07:21 PM   #4
Alllensoncanon is offline Alllensoncanon  United States
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300B SET amp measurements without tubes installed
Short answer. Unless you know your power distribution and regulation circuits well, power on without all the tubes installed are not good.

Last edited by Alllensoncanon; 2nd December 2017 at 07:24 PM.
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Old 2nd December 2017, 07:29 PM   #5
6A3sUMMER is offline 6A3sUMMER  United States
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How much hum do you have?
300uV?
8mV, etc.?

Sometimes even schematics may not help find some causes of hum, unless they
show the grounding techniques. Ground loops do not usually become apparent
on most schematics.

But we still need the schematics as a start.
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Old 2nd December 2017, 07:39 PM   #6
MEPER is offline MEPER  Denmark
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Thank you for the answer. I have some hand drawn schematics of each tube (made yesterday and today). I have both 50 Hz hum and also some higher frequencies (probably from PSU). The power transformer is oriented 90 degree to the output transformers. There is a star ground. 300B uses 5V DC (a hum pot is there anyway....or whatever the purpose is). The 6.3 V AC for the ECC83 has two 56R to ground. The PSU is not filtered very good. From the bridge a 220 uF cap and from there a 220R filter resistor and then again a 220 uF cap. This is all. B+ is approx. 535 V and caps 550/595. It is direct coupled......no caps in the signal path. Hum level at the 8 ohm ouput is about 4 mV for the one channel and 2.5 mV for the other. I can hear the 50 Hz on my 95 dB speakers. Not loud.....but still. Even with hum they play amazing.

Does this give more information?
I have re-wired the 6.3 V AC for the ECC83 a little bit. There was a loop around the sockets. Now it goes straight over.
There is a loop also on the 300B socket for the DC.....but I assume this is ok.

A picture from the bottom attached.......then the *-ground etc. is visible.

Updated schematics for the ECC83.....found some errors on component values......
Attached Images
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Old 3rd December 2017, 03:58 PM   #7
audiowize is offline audiowize  United States
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Your schematic makes it look like there are some mechanisms to get power supply noise injected into the amplifier to quiet it down. There is also some global feedback which should quiet the noise down a bit.

Have you tried adjusting the hum pot?

4mV and 2.5mV seem very high for a DC heated 300B amp. This is especially true since you have a 7805 regulator on each tube; this should be very quiet. For comparison, the 300B stereo amp I use has regulated DC on its filaments but a bit more power supply filtering, and I see something like 0.6mV of hum on the outputs.

You can buy a pair of 4K/50W resistors and solder one from pin 2 on each 4 pin socket to the wiper of the hum pot, lift each feedback resistor, then run the amp. This will let you listen to the power supply noise component of what you're hearing.

You can put your scope on the output of the filament supply to see how quiet it is (with a 300B connected so it's loaded).

Have you contacted the manufacturer?
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Old 3rd December 2017, 05:02 PM   #8
MEPER is offline MEPER  Denmark
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I have tried to adjust the pots but it has no real influence on the hum. And that was expected when it is driven by DC? .....what I was able to was to adjust the cathode voltage a little bit (2 volts) so each tube is approx. identical (180 V which gives 60 mA of bias current).
The idea with the 4k/50 W resistor was instead of having the 300b tube installed?
Sounds like a good idea.
I have talked with the manufacturer and it is possible to get some help. I have also contacted a small company from the Netherland called "Tentlabs" and it seems I can get some good advices from there and make it as a small DIY project.....instead of shipping the amp back. Then I will learn something about tubes :-)
The "Tentlab" person seems to know a lot about tubes, noise etc. both theoretical and practical. I am most concerned about the 50 Hz hum......so I am interested in knowing the source for this 50 Hz......
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Old 3rd December 2017, 05:23 PM   #9
audiowize is offline audiowize  United States
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Yes, the hum pot with DC can be a bit overkill, especially since you have a few DB of feedback.

Tent knows what's up, I wouldn't hesitate to take a drive and visit him if he's up for it.

50Hz is your mains frequency. In an amp like yours, we would expect you to get 50Hz noise from AC heating of the 300Bs. Your power supply noise will be 100Hz, so one other place you might look is the driver tube heaters.

If you short the 47K resistors at the inputs, do things change?
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Old 4th December 2017, 06:51 PM   #10
MEPER is offline MEPER  Denmark
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That was the next step I would try.....to short the 100k (first diagram was wrong) resistor at the input.
I did it at the RCA inputs. Doing this I got these results:
AC left channel: 4.2 mA to 3.7 mA (has the longest shielded wire to the input tube)
AC right channel: 2.6 mA to 2.4 mA.
I made the measurements several times with and with out input grounded so I am sure that there was a significant change.
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