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FDP Forum / Amp Mods, Repairs, and Projects / 74 deluxe reverb and DRRI problems


United States

Michael Landstrom
Nov 12th, 2018 06:34 PM   Edit   Profile  

Where do I start? My 74 Deluxe Reverb blew up a few years ago, it had a burnt out screen grid resistor. So, I got an overhaul kit from Vintage Fender Amp Repair also had to replace the rectifier tube and it worked for 3 or 4 hours. Then I had a phase inverter tube go bad and ground resistor on the heater circuit burnt, replaced and was good for another few hours. then it went bad again and it looked like some bad wire caused pins 2 and 3 to short on v7 power tube. So I replaced the tube socket, luckily the power tube and power transformer was still good. worked for a few more hours and then had a power tube, a preamp tube and one other tube go bad reverb driver I think. So I decided to get a DRRI thinking at least it would get me through a oldies tribute gig I'm doing this weekend. I've played the amp for maybe 6 to 8 hours and it did the same thing. Every time something goes bad it starts making crackling noises and drops way down in volume. I also have a Bassman 50 that I don't have that many hours on since I last had it serviced and lately it isn't sound very good. I have a 90's "The Twin" that just went bad. What the hell? Could there be some kind of grounding issue in my basement where I think most if not all the ampicide has occurred? The only amp I haven't had any problems with are my Ampeg V4B RI and my Orange dual terror. The dual terror is my main gigging amp since the deluxe died and haven't had it plugged in in the basement much. I don't know what the deal is. Bad luck? I exchanged the DRRI for a new one but I'm afraid to use it. I was thinking the 74 Deluxe Reverb maybe had a short or something in it somewhere and was going to send it to Fender Vintage Amp Repair to have checked over, but since the DRRI died, I don't know. Anybody have any suggestions.



Why do I keep fixing things that work?
Nov 12th, 2018 07:07 PM   Edit   Profile  

Man, I have to wonder if you have a bad moisture problem in your basement?!? This would cause moisture to accumulate on the chassis and maybe create a short across the surface of the eyelet board and on the RI Deluxe. The voltage would run across the surface of the circuit board and allow all of your amp grounds to allow all the current to run through the entire amp. You're lucky not to have gotten a deadly shock from the voltage present.
This is just a guess on my part, but think that this may be what is happening.
More educated people may chime in on this matter, but this may be a valid reason. Be safe!!!


United States

Michael Landstrom
Nov 12th, 2018 07:38 PM   Edit   Profile  

Thanks for the suggestion. I do keep it dehumidified in the basement. I keep it at 40% or less in the summer and right now it is reading 30% on the dehumidifier and is very dry during the winter months (I'm in norther Illinois). It may be something I need to look into more though.



Nov 13th, 2018 06:33 AM   Edit   Profile  

I think that your 74 DR may have / have had, a power tube with an intermittent short. That could have caused all the issues you mention. These things happen, that's why tubes are mounted in sockets rather than being soldered in place - they are the most likely thing to fail.

If an amp chassis and the wall outlet it's plugged into are properly grounded, there is almost zero likelihood of any conceivable fault creating a shock hazard. That's why mains equipment with external / accessible metalwork must have it connected to ground - the grounding protects the user.

(This message was last edited by pdf64 at 03:55 AM, Nov 14th, 2018)


United States

Michael Landstrom
Nov 13th, 2018 12:50 PM   Edit   Profile  

@pdf64, so besides being a shock hazard, improper grounding on the electrical outlets wouldn't cause any damage to the amp? The only other thing I can think of is maybe the short that occurred between pins 2 and 3 could have possibly damaged the power transformer? I had someone check it out and they seemed to think it was ok. Should I just go ahead and replace any bad tubes again and see what happens? It's just weird the same thing happened with the DRRI and I just got it like 6 or 7 days ago.



Nov 14th, 2018 02:02 AM   Edit   Profile  

I can't think how a bad ground connection to a wall outlet could cause damage to an amp.

Note that any arcing is likely to leave a carbon trace which will cause problems until it's completely removed; replace any blackened tube sockets or wiring.


United States

Michael Landstrom
Nov 14th, 2018 06:40 AM   Edit   Profile  

Well, I got good news and bad news. The bad news is I'm a dumb ass. The good news is there wasn't anything wrong with the DRRI, must have wire coming loose on the output jack on my strat. Hopefully I'll get a chance this weekend to take look at the 74 DR and have the tubes tested out.

pdf64 last time I had a few tubes replaced and got it working again but the wasn't any carbon tracing that I could see. There was the time before that when pins 2 and 3 shorted on one of the power tubes. Is it possible it could be shorting underneath the terminal board and is it possible to detach the board and lift it to check underneath, without having to take a bunch of parts or wires off the board?



Nov 15th, 2018 03:31 AM   Edit   Profile  

Terminal 2 is for the heater, 3 is for the plate; neither of those should have their wiring routed under the main circuit board.
If there's no arcing / compromised insulation, and the tubes are good and other faulty parts replaced, then the amp should be fine.
Don't trust tube testers, they don't subject the tube to the voltages found in your amps.


United States

Michael Landstrom
Nov 15th, 2018 06:02 AM   Edit   Profile  

Thank you pdf64. Would checking the voltages coming off all the tubes be a better indicator of a bad tube, or do I risk further damage to the amp by turning it on knowing something is already going bad? Also, after I did the blackface mod on the amp, I switched to a 5AR4 rec tube and it had a 5U4 before. Could that be part of the problem?



Nov 15th, 2018 11:52 AM   Edit   Profile  

I suggest that anytime a screen grid resistor or heater balancing resistor blows, there's a strong likelihood that a power tube has shorted, either momentarily or permanently; so mark them both as suspect, bad until proven otherwise, or just dispose of them.
I wouldn't risk a suspect power tube in an amp that doesn't have a HT fuse.
I don't think that measuring voltages of a suspect tube is beneficial, eg an intermittent short may only manifest when cranking out a signal, or they may have a hard short that stresses the amp as soon as it's powered up.

A 5AR4 puts out a higher Vdc than a 5U4, so I think it's a bad idea to fit one into an amp that's intended for a 5U4; it maybe ok if the 'blackfacing' includes fitting a blackface spec PT. But a silverface PT may have a higher HT winding Vac, to compensate for the lossier 5U4 rectifier.

(This message was last edited by pdf64 at 11:40 AM, Nov 16th, 2018)


United States

Michael Landstrom
Nov 17th, 2018 08:23 AM   Edit   Profile  

Thanks again for your help pdf64. Havent had time to open it up yet and take a look inside, but gonna go ahead and get a 5u4 and 2 6v6's ordered. I have noticed it seems to be when i crank the amp up is when it seems to be blowing. There was some worn off insulation on the wires for pin 2 and 3 on power tube when it shorted. Is there anything i should look for besides worn wires or solder bridging?



Nov 23rd, 2018 07:09 AM   Edit   Profile  

Check there aren't any black sooty deposits anywhere; if there are then clean them up and remove/replace any affected insulating materials.

I suggest to replace the resistors that are mounted on the power tube sockets, ie the 470 ohm 1 watt for the screen grid, and the 1k5 1/2 watt for the control grid.
They are subject to thermal stress from the heat rising from the tube below, and also mechanical stress as the socket's female contacts are necessarily loose in the base, so any vibration / movement of the tube is transmitted to those resistors.
Use a flameproof / retardant metal oxide type for the 470ohm, and a regular carbon or metal film for the 1k5.
Allow some space around the body of the 470 1 watt, as it will get hot in use and may fuse if its tube shorts.

(This message was last edited by pdf64 at 09:10 AM, Nov 23rd, 2018)

FDP Forum / Amp Mods, Repairs, and Projects / 74 deluxe reverb and DRRI problems

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