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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Transcendent 300B SEOTL - Strange DC Offset

Robbie010

Member
2019-02-08 2:24 pm
Good evening all!

I’ve had a transformer hum issue with one of my monoblock amps and decided to test the DC offset.

Something rather strange seems to be happening!

Using my multimeter and testing at the speaker binding posts with no load (speakers) attached, I switch them off for a short while and then turned them on to test from cold. When I first switched them on, both amps had a very high DC offset - somewhere around 6volts! - this quickly came down in one amp and settled at 0.7 volts, however, the other amp was much slower to stabilise and is sitting at around 1.5 volts after 5 minutes.

After connecting the speakers and testing again with music playing I get a reading of zero on both........ so I decided to unplug the speakers from the amps and test again, I got a reading of almost zero on both amps........

Can anyone explain whats going on here?

Am I not testing correctly?
 
This is a bit of an awkward question to ask here. We might be able to answer the question if the schematic came with this post to demonstrate how DC offset is managed, but you shouldn't post the schematic because it would likely annoy the manufacturer.

Speaking of, have you contacted the manufacturer?
 

Robbie010

Member
2019-02-08 2:24 pm
This is a bit of an awkward question to ask here. We might be able to answer the question if the schematic came with this post to demonstrate how DC offset is managed, but you shouldn't post the schematic because it would likely annoy the manufacturer.

Speaking of, have you contacted the manufacturer?

Thanks.

Given that they are a DIY kitform amplifier, I would assume that posting the schematic would not be an issue??

I have the schematic, BOM and construction details.
 

Robbie010

Member
2019-02-08 2:24 pm
As it's described as SE OTL, it probably is capacitor- coupled to the speakers. I'm guessing you don't get the DC measurement with speakers attached (how could you, voltages like that would mess with your drivers)?

Thanks.

After taking further advice I knocked up a load using an 8ohm 50w resistor and re-tested across the speaker binding post, both amps read zero volts DC.

:up:
 

Sorento

Member
2008-03-12 9:59 pm
Is the output stage something like this ?
http://www.tubecad.com/2014/02/22/Rozenblit 300B SE OTL.png
A capacitor coupled output requires a DC path to ground. Otherwise the voltage on the output is simply undefined when left open. Your DVM is 10Mohm, the cap is huge, so it takes a long time to charge/discharge the cap through your meter. There should be a resistor connected from the live speaker terminal to ground inside the amp, say 1kohm. I suspect, it is missing - or way too big in terms of megohms.
 
It definitely looks like 4x300Bs wired up as paralleled cathode followers.
dPJSzQd.jpg

You can see the two coupling caps just right of center. I do not see any resistance across the speaker posts to help those caps charge. A 100 ohm resistor across the binding posts seems like a no brainer.

Those amps must have 25 or so dB of feedback to reach any kind of useful damping factor.