Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dpm309 on July 27, 2022, 11:41:52 am
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Does anyone know where to get a schematic for a ENGL Retro 50? I don't see this model on Doug's amp schematic page. All I could find on the web is the owner's manual.Thanks.
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Ask Engl
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I did send them a message yesterday, along with the serial number etc but they have not got back to me. The amp is experiencing horrible distortion on both clean and overdrive channels. Swapped out the preamp tubes one at a time, and put in new power tubes with no luck. Also tried a different speaker with the same results. Will pull chassis and check voltages. Don't know if this amp has a bias adjustment but will check that out also.
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Still no response from ENGL. Pulled the chassis and checked voltages which were all in line. The bias was set at 47mA and I lowered it to 40mA (plate voltage is 428VDC). Still no change in sound. What would be my next step? Still need to find a schematic.
Thanks
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Maybe the problem area can be narrowed down by process of elimination.
Are all of the controls and switches doing what they're supposed to do, even in the presence of the offending distortion?
Are the volume, treble, middle, bass, gain, fx loop, bright switch, channel switching, and everything else responding how they should?
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Also the amp has an fx-loop you could plug in directly. And you could measure DC at the tube pins.
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Engl finally got back to me and said that they are on vacation until late August. Went ahead and cleaned the power tube sockets and installed the new set of JJ El34s and powered it up. Amp is now working like it should. Cleaning the sockets did the trick. These are great sounding amps and are built like a tank.
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The amp is still experiencing loss of power after running for about 45 minutes. I let it cool for about 20 minutes and it works fine for about another 45 minutes. I am planning on measuring voltages after it warms up a bit and then let it run until is loses power and recheck the voltages. Hopefully this will help narrow down the problem. Did not see and signs of overheating on the board and will also check for cold solder joints. Is there anything else I should be looking at?
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Here is the schmatic.
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> check for cold solder joints. Is there anything else I should be looking at?
Bad solder joints. (Yes, again.)
Then signal tracer tells what you really need to know quicker than DC checks.
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I checked voltages on the power tubes when the amp was distorting and noticed that the plate voltage for V4 was 400VDC and on V3 it was 429VDC. Pulled both tubes and reinserted them and the amp was working normally and the plate voltages were the same. Would this 29VDC difference between the plates on the power tubes cause the distortion? I am in the process of re-flowing the solder joints on the power tubes and will also clean the sockets. The power tube sockets are mounted directly to the thin PCB which could be part of the problem. I read that ENGL has issues with their PCBs such as small traces and solder pads becoming loose.
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> the plate voltage for V4 was 400VDC and on V3 it was 429VDC.
That can't be good. What is the difference V3 or V4? A couple hundred feet of copper wire (the OT winding). Might be a couple volts different.
> Pulled both tubes and reinserted them and the amp was working normally and the plate voltages were the same. Would this 29VDC difference
29V different might be one side "dead". Tube crap-out. But since it "fixed itself" when you touched/moved things, I think you know what you should be looking at.
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After touching up the solder joints on the power tubes and resistors and cleaning the sockets, the amp seems to be working like it should. I played it hard for 50+ minutes on the distortion channel and using a Big Muff on the clean channel without it cutting out. These amps are a PIA to work on. Engl discontinued the Retro 50 sometime ago and was wondering if they did this because of reliability issues.Thanks for all of your input!
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Just curious - is the schematic accurate?
Most official looking Engl schematics I’ve found online are highly inaccurate, with obvious errors like missing coupling caps. I’ve seen one that was corrected by someone who actually had the amp and it was dozens of corrections.
I can see one probable error in the Retrotube schematic - the tone switch for the second tone stack switches between two identical treble caps.
Would be interesting to know if the rest is OK.