Echo Amb Bias Confusion - diyAudio
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Old 19th April 2013, 11:54 PM   #1
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Default Echo Amb Bias Confusion

I just got an early '70s Japanese Echo Custom Amp 30 head, and I'm very confused about the bias system. All the amps I've worked on have had cathode bias, with a cap and resister from the cathode to ground. This amp seems to the same, except the cathodes go straight to ground -- no resistor or cap.

FYI the power tubes are 7189s, the original Matsushitas. Actually all the tubes are original Matsushitas. I started looking at the bias circuit because the plates glow red. I thought maybe the bias resistor had drifted. Also FYI there's no bias pot.

What next? I have no idea what could be causing the glow. Certainly there are some bad caps & resisitors in there, but I don't even know where to start looking.

Thanks in advance guys...
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Old 20th April 2013, 12:20 AM   #2
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I finally found some info -- am I safe to conclude the Echo is a fixed-bias design? I still don't get the red plates though. Aren't fixed bias designs supposed to be forgiving to different tubes? Besides, it's still running on its original tubes.
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Old 20th April 2013, 12:45 AM   #3
Enzo is offline Enzo  United States
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Just because a circuit is forgiving, doesn;t mean it isn't defective.

Yes, it is fixed bias.

Look lower right corner, see that little single diode negative supply? The line from it goes up to the two 220k grid resistors of your output tubes. Make sure that is at a reasonable level and free of ripple. What negative DC voltage is at the anode of that diode? Is the voltage about the same at the grid pins of the two power tubes?

See the two 0.05uf caps connected to the power tube grids? They conect back to the phase inverter tubes. If either of those gets leaky, the high voltage from the phase inverter tubes will get on the power tube grids and increase current there.
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Old 20th April 2013, 02:07 AM   #4
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I figured it out. It was what you suggested Enzo, those two .05 caps. Put in new ones and it's purring like a kitten. Even the bias modulation vibrato sounds better. I also subbed out the crappy little reverb tank for a full size Hammond I had laying around. The original one is very very "boingy". The Hammond was less deep but way better sounding. Unfortunately they used a small tank for a reason. The big tank won't fit in the cabinet. Argh.

Sorry for the stupid post. I panicked.
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Old 20th April 2013, 05:57 AM   #5
Enzo is offline Enzo  United States
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Before you ship the amp of to its future, check the other caps. If those two phase inverter caps were leaky, there is a good chance the others are as well. Look at the brand and type of the ones you replaced. LOok at the others in the amp. If they are the same brand and style, I'd be suspicious.

But don't take my word for it, get out a meter, Look at all the cases where a plate signal is carried to the next stage grid through a cap. The 0.05uf leaving the plate of the input 12AX7 for example. Go to the other end of that cap - at the 100k resistor in the tone circuit. Is ther any DC voltage there? Ought to be zero, but if you have a few volts, that cap is leaky. Same deal on the 0.02uf caps leaving the plates of the other 12AX7 triodes. Look at the other end of each cap looking for unwanted DC. The trem tube, upper right has a 0.02 in its feedback loop, plus the 0.1uf feed to the signal path.

Does your trem work well? If not, or if it starts slow, or won;t work through the full speed range, it is probably leaky caps - the three between plate and grid, as well as the cathode cap, and that 0.1 to the intensity control.


Hey, if they are healthy, great. But it is worth it to check. If they sound OK now but are failing, well, they won;t get any better with more age. My experience is that when I find one leaky cap, it might just be a bad cap, but when I find more than one, I probably need to swap out all of them.
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Old 22nd April 2013, 08:07 PM   #6
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It's not going anywhere, at least for a while. There are a whole bunch of oil caps in there that have pretty poor reputations for leaking, but I'm out of work, so I'll be stretching the purchases out a bit.

I've used all the caps I have lying around that match and it's working pretty well, even with the original filter caps and mostly original film caps, so I'm enjoying it until I can replace more.

I have another question for you though: can you look at the schematic and tell me what "real" amp circuit it's most similar to? I'm curious to know if the guys that designed the Echo based on a more well known amp.
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Old 22nd April 2013, 11:49 PM   #7
Enzo is offline Enzo  United States
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I don't know that it is similar to anything, on the other hand most guitar tube amps are prety similar to the others. I don't think in those terms anyway, to me it looks generic

You have a push/pull power amp, a conventional phase inverter. The reverb is common tube drive, and the 3meg/20pf reverb mixing resistor is in a zillion amps. The tone control is not stock fender, but not off the wall either.

I imagine fixed all up, it would sound pretty good.
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Old 24th April 2013, 05:52 AM   #8
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Yep,
If those caps were labelled Suzuki Oil they are infamous for going leaky.

I used to test any cap labelled "Suzuki Oil" in any Japanese HIFi or Guitar Tube Amp I saw. So many of them tested leaky that I don't even bother to test them now, just rip them all out and replace them.

Cheers,
Ian
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