As I've said before, when the amp is unfixable it usually ends up at my place This DeVille had a few bad tubes that I've swapped and it really sounds good. However, tehere isa a volume issue going on. After a minute on the volume drops a whole lot, you can crank it and it's not unpleasantly loud like it's supposed to After a very long time on the volume usually come back. Has anyone experienced this before with this (or a similar) amp? I really hate these PCB-amps, but I really want to get it going again. There seems to be some mods done to it and it seems like it's been recapped (F+T filter capacitors) at some point. Any help or pointers are appreciated
Have you eliminated the fx loop switching jack as being the cause, eg by patching a cable between send and return?
If not that, then a failing solder joint on the pcb seems the next most likely.
I've eliminated the FX-loop and I've also fixed some solder bad looking joints, but there could be more bad solder joints... I guess I'll have to scan the entire the PCBs with a magnifying glass
When it's in fault state, run some music thru the amp so you have your hands free, and probe everything with a chopstick. If it is a bad solder joint, you should be able to find it.
I build and repair tube amps. http://amps.monkeymatic.com
Does the amp distort easier when the issue takes place, or just the loss of volume?
If you can pump audio into the amp you can trace audio level thru the amp with just a voltmeter, you don’t even need a scope if you do not have one.
If you set the meter for ac volts you can then read the audio signal level at the down stream end of any decoupling cap that comes off the plate of a preamp tube and make normal and F’ed up signal level comparison.
You should also note the before and after plate and cathode voltage readings also, since a drop in plate voltage of even 25 volts can make for a noticeable reduction in volume.
When the plate voltage drops so will the gain stages cathode voltage.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Yes, it distorts easier, or at least there's some not pleasant distortion when hitting it hard(er) on the input. That's some good advice @stevem, I'll get down to tracing when I get back from work in a week or so
All of these types of two 6L6 Fender amps have a fairly common issue of having one of the too dam small 1/4 watt plate load resistors on the PI tube go open.
If you have very low to no dc on pins 1 or 6 of that tube then you have found the issue!
I have seen this many times and I clip out the open resistor from the front of the board leaving its leads as long as possible, then I fold them over some, solder them into a ball and then tack on the new 1/2 watt resistor.
You need a very hot iron to do this fast so you don’t loose the connection on the rear of the board.
Even if these resistors are not bad I would replace them both.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
It seems like these resistors already are replaced with better ones (they seem to be 1W based on the size), but I did replace them anyway with 2w ones. I also started to replace other resistors around the power tubes and PI with 1 or 2w versions. Still not acting any better. The standby switch also had a bit loose terminal so I replaced that as well (better clean up as much as possible when I'm at it). I've started to measure some voltages and I think there's some errors on the schematic - it calls for 94.3V on pin3 (plate) on the power tubes, and 14.7v on the plates of V3 (PI). That seems "a bit" on the lower side I guess
Yes, I see that now that I've read the other test points as well. there is no voltages on the schematic, only signal levels. I'll have to build a dummy load then so I can do a full signal trace. However, there seems to be a really high voltage on one half of V1. There is about 405VDC om pin 1 on V1 (ca 5V on pin 3) - is it supposed to be that high? I don't think I've ever seen that high plate voltage no a pre amp before..
Voltages that high, that far back up stream in the power supply chain can only be that high if there is a main ground that’s not making contact.
I would also venture a guess that on pins 2 and 8 of the first two preamp tubes you have little to no voltage since the tubes can not conduct and pass current if they have no ground connection at the bottom of there cathode resistor and or resistor bypass cap combo.
When all is well and the preamp tubes are basically good you should read at least 1.2 VDC at these pins.
Here’s a shot of the main ground wire on a Fender HRD, and if I recall right the Deville has the same set up.
Is this wire there on yours?
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When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
goldenGeek wrote: ↑Sun Nov 14, 2021 12:23 pm
Yes, I see that now that I've read the other test points as well. there is no voltages on the schematic, only signal levels…
It’s a really good idea to include a link or image of the most accurate schematic.
Is this the schematic for the amp in question? https://el34world.com/charts/Schematics ... eville.pdf
If so please note the cathode VDC, the ‘X’ HT node VDC and anode load resistor values that have been provided.
If it’s more than a moment’s effort to derive the anode VDC from those, it would be very beneficial for you to get more conversant with Ohm’s Law
Are the preamp valves all getting hot?
@pdf64 - I finally got my head around the markings on the voltages at the schematic.. the oval ones are signal, the rectangle ones are DC volts. Thanks for the hints Anyway, doing some calculations and some common sense I figured what was wrong with the amp. It had been modded and fixed a long time ago and some of the broken PCB tracks that were fixed was not completely fixed. The 1k5 resistor at V1a was not properly connected to the cathode/pin3. Also the cathode capacitor for V1a was wired the wrong way. After flipping the cap and fixing the track it finally works as it should. Thank you to everyone who chipped in Now, on to the next amp with weird issues
This DeVille had a few bad tubes that I've swapped and it really sounds good. However, tehere isa a volume issue going on. After a minute on the volume drops a whole lot, you can crank it and it's not unpleasantly loud like it's supposed to 
