Can someone help me troubleshoot a Fender Hot Rod Deville 212 tube amp - Page 1
Forum section: electronics / repair

author: Planobilly
author_rank: Regular Contributor
timestamp: February 22, 2016, 09:49:29 pm
content: Hi guys,A little history. I am pretty new to amp repair. I have only repaired 30 or 40 amps to date, mostly tube but some solid state.What I would like is for someone to walk me through the troubleshooting process for this type of amp.All voltages from the filter caps are within range. Very small amount of AC at filter caps, ie..no ripple. Plate voltage for both 6l6 463V . 61mV at TP 30 which has both out put tubes running at 27mA + or _ .3 mA.No resistors on the main board are open but I have not checked the values of all of them.Issues:Distortion at all settingsThe Normal / Bright switch does not seem to workLow volume ie very little amplification on channel 1 more on channel 2Out put tubes are over heating when a signal is injected but heat range is more or less normal at idle.Here is the stuff I have to work with.http://www.tangible-technology.com/schematics/fender/HR-Deville/HotRod DeVille.pdfToolsI can test most anything...I guess...lolThanks,BillyAlso I have high quality test speaker systems 16 ohm, 8 ohm, 4 ohm, and the same dummy loads up to 250 watts

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 22, 2016, 09:59:45 pm
content: Excellent start.Great explanation of what you have to work with, and great evaluation of the problem at hand.Next step: Do you have service information for the gadget you are trying to repair?At least a schematic diagram?With that, you could do some more specific voltage measurements at different points in the circuit.A general technique (which you probably already know) is to inject audio at various stages to see where it disappears (or goes wrong).And feeding a sine wave of various frequencies into the input and then looking at the output on the 'scope could yield some clues what is happening.Like what KIND of distortion? Is it simple clipping? At what level? At what stage through the amp does it start distorting?What is its history? The output tubes (assuming that is what that pair of octal firebottles are) look different.Do you have a matched pair that you can substitute? Assuming you tested all the tubes?

author: Andy Watson
author_rank: Super Contributor
timestamp: February 22, 2016, 10:35:34 pm
content: I echo Richard's advice, however,Quote from: Planobilly on February 22, 2016, 09:49:29 pmas an opening shot, I would check the output transformer for shorted turns - or there might be a short in the jack-sockets.

author: voltz
author_rank: Frequent Contributor
timestamp: February 22, 2016, 10:51:20 pm
content: +1 Andy Watson.But do check the 6l6's too.If the valve pin voltages are as expected (grid bias and screen bias in particular with anode good) then this does suggest o/p transformer short circuit. The overheating on signal gives it away. Could either be primary or secondary winding failure. But also check for speaker jack short circuit and its wiring.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: February 23, 2016, 01:14:14 am
content: Check for dead filaments (no glow) in a dark room. That's free and quick.

author: Planobilly
author_rank: Regular Contributor
timestamp: February 23, 2016, 02:25:22 am
content: Hi Andy Watson,Yes I have a schematic.http://www.tangible-technology.com/schematics/fender/HR-Deville/HotRod DeVille.pdfThe tubes are matched and checked with my Navy TV-10 and also I installed another set just to check...so I don't think this is a tube issue. Also changed several of the 12AX&'s just because. I have seen some truly strange stuff with tubes..lolThanks for all the feedback.1. My radar has been going off about the out put transformer...not sure why just a guess. The coil reads 111.47 ohms on the primary, from one side to the center tap is 58.45 ohms and from the other side to the center tap is 53.83. Also there is no short to the secondary from the primary on the op trans. I have no idea what the correct readings should be.The one major issue that I do not understand is the overheating issue and what could cause it. The 6l6 out put tubes run at normal temps, volts, milli amps, bias voltage at idle. As soon as I inject a 1000 hz signal at 100 mA into the first pre amp tube the out put tube start to heat up and I have shut down the amp when they get over 420 F as 450 F is the limit per the data sheet.Something else I do not understand...ON the schematic Test Point 24 says 14.7 V...does that mean AC voltage?? That point is the plate of V3A which connects to the Y DC voltage of 441 V...well there is a 82k resistor between the y voltage and the plateI think I will post some fishing photos...much more fun...lolhttps://www.eevblog.com/forum/Smileys/default/EatingPopcorn.jpgWhat I am really trying to figure out is what voltage is causing the 6l6 tubes to heat up...We know the plate voltage is ok. We know the idle current is OK. we know the bias OK. So...Gona check out put jack next.I been at this for a while so a cup of coffee is in order. I also quit smoking about a week ago. The good news is I have not kicked the dog or the wife ...yet...lol Well...the night is young...lol

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 23, 2016, 03:20:32 am
content: You could use you oscilloscope to confirm that you don't have ultrasonic oscillation going on somewhere in the circuit.

author: Anks
author_rank: Frequent Contributor
timestamp: February 23, 2016, 07:39:34 am
content: A simple way to check the power amp section would be to break the circuit before the coupling cap to the phase inverter and inject your sine wave at this cap. Also try to turn the bias as negative as possible at the moment until your are sure the problem is resolved (the crossover distortion can be ignored at this point) and observe the output and see if you get a sine wave out. A bad output transformer is still possible but the output tubes could also be bad. By doing this you will of ruled out oscillation in the preamp from the equation if the problem still exist and oscillation in the power section is much less likely and I would then pull the power tubes and measure resistance between plates and main b+ to the output transformer.

author: voltz
author_rank: Frequent Contributor
timestamp: February 23, 2016, 01:16:01 pm
content: "My radar has been going off about the out put transformer...not sure why just a guess. The coil reads 111.47 ohms on the primary, from one side to the center tap is 58.45 ohms and from the other side to the center tap is 53.83. Also there is no short to the secondary from the primary on the op trans. I have no idea what the correct readings should be."That sounds reasonable enough, center taps are rarely 'center'. The secondary is harder to test, much lower resistance. Remember you're measuring DC resistance and not the impedance. So an output tap for an 8ohm speaker for example would show a DC resistance lower than that. But if its near zero, then that would be suspect. The wiring to the speaker socket would need to be ruled out too. Some transformer manufactures specify the DC resistance of primary and secondary windings to aid simpler fault finding.

author: Andy Watson
author_rank: Super Contributor
timestamp: February 23, 2016, 02:27:41 pm
content: Quote from: Planobilly on February 23, 2016, 02:25:22 amI think the key is: Square/rectangular boxes indicate a DC measurement, round/ellipses indicate an AC signal measurement. At the input, J1, there is an ellipse that indicates 4mV and along side 1kHz sine - I assume this is the test input condition for all the AC signal measurements.

author: Planobilly
author_rank: Regular Contributor
timestamp: February 23, 2016, 02:57:52 pm
content: Quote from: Andy Watson on February 23, 2016, 02:27:41 pmThanks Andy...I will check that out...just now drinking coffee and getting back into this.Thanks,Billy

author: Planobilly
author_rank: Regular Contributor
timestamp: February 23, 2016, 10:52:36 pm
content: Hi guys,Did all the checks mentioned plusR61 and R62 are 470 ohms within 3%the 6l6's have the following voltagesplate 479vgrid2 pin 4 478vgrid1 pin 5 -51vcathode pin 8 0vplates on the 3 12AX7 250v, 245v. and 280v110 ohms on output transformer primary and 58 ohms center tap (both sides)and .3 ohms on secondaryUnless I am really forgetting something, only thing that could cause the 6l6 overheating and the low volume is the output transformer.No joy. Guess it is time to order an new op transformer. If that does not work I need a new boat anchor anyway...lolThanks,Billy

author: Seekonk
author_rank: Super Contributor
timestamp: February 23, 2016, 11:06:16 pm
content: That would be a pricey test. I would try this. With the amp not powered or speaker connected, feed 6VAC into the secondary of the output transformer through a 5 ohm resistor. I don't think an unloaded transformer will take that much magnetization current. Certainly prefer doing this at a higher frequency if you have a small amp and oscillator to drive it. Shorted turn would place a high load.I just tried this with two 12V transformers and 5 ohm. Input 12.3VAC , other side of resistor was 11.5VAC.

author: voltz
author_rank: Frequent Contributor
timestamp: February 24, 2016, 12:52:21 am
content: Yep, i echo Seekonk with testing the transformer better before ordering a new one. Not cheap!.But dont forget to check the two speaker jack sockets. They are internally switched in such a way that a fault could occur and short out at least one of the secondaries.Good luck!

author: Planobilly
author_rank: Regular Contributor
timestamp: February 24, 2016, 03:42:36 am
content: Hi Seekonk,I am not sure I understand just what you are recommending I do. I can produce 6V rms at 60hz with my frequency gen. Sorry to be so dumb tonight. Can you make a little drawing to show me how to do this? Or give me a call...not sure how to do a PM on this site.Thanks,Billy786-601-7010...planobillyfl@gmail.comSorry..I have no idea what part of the country you live in!!

author: bills
author_rank: Frequent Contributor
timestamp: February 24, 2016, 04:03:26 am
content: Billyto PM someone click on their profile and you will see the PM link.I think you are asking the wrong group for advice, most of the people here have no experience with vacuum tubes(valves) the 20-30 somethings may have never had any experience with tubes. I think you should go with the link I gave you in the PM.What brand are the output tubes? If the bias is out of wack you will have problems.Please before the flames begin I think some of the greatest electronic minds are on this site (for modern stuff)Not so much for the tube gear.BTY I do not claim to be an expert I have only been working with tube gear for 50 years.Please keep this thread updated.bill

author: Planobilly
author_rank: Regular Contributor
timestamp: February 24, 2016, 05:55:16 am
content: Thanks bills,I found the pm deal..lolThe younger guys never had any introduction to vacuum tubes in the schools now days for the most part...not their fault.I buy JJ's from Eurotubes and CE dist which is a wholesale place. I stock a good many so I have known good tubes. I also have a good tube tester...well..if any of them are any good..lol I also just pulled the output transformer out and check everything I could for shorts.One of the first things I did was check the bias...very easy on a deville...there is a test point...60 mV more or less. I also checked the plate current at idle and it was normal. 35 mA or so...nothing strangeThe output tubes don't over heat at idle...perhaps 300 F but as soon as you put a signal in the amp it starts to get out or control. I shut it down around 400F. 450F is the max per the data sheet.I guess there could be some excess voltage getting to the grid..something I need to look into next.All the voltages look pretty normal at idle 480V plate...478V grid 2..perhaps a bit high...negitive 51V on grid 1...I have spent more time messing with this stupid amp than the last 30 combined...lolActually the output transformer has no issues I can find...110 ohms across the primary and no shorts anywhere...center tap is 53 and 57 ohms...more or less in the middle. Across the secondary about.5 ohms...not sure if that is correct.The radio guys gave me a bit of advice on a 1934 RCA I rebuilt the other day..actually a fun project and simple to work on...just had to remind myself about how to line up a superhetrodyne radio...lolI have been working on Marine stuff for a while but I am too damn old to climb around on boats anymore..lol or too lazy...lolIn general I don't have too much issue with point to point old school tube amps. This new stuff is not really made to repair in the first place and getting worse every day. Repair guys job now days is find the bad PCB and replace it!! Just the way of the world I guess.The whole PCB with everything on it is only about $100 bucks but Fender is out of stock as usual. The filter caps cost around 30 bucks and god only knows what all the rest of the parts would cost.At least all this is cheaper than the boat...but that is another story...lolCheers,BIlly