Need Some Guidance For Testing Process On Vacuum Tube Amp

[Planobilly]

Hi Guys,
I am having issues trying to repair this amp.
http://www.tangible-technology.com/schematics/fender/HR-Deville/Hot%20Rod%20DeVille.pdf

A little history is in order I guess. I have not been working on guitar amps for too long, several months now. I have repaired perhaps fifty or so. Most were tube but some were solid state. Some were easy to fix and some were not so easy but I got them fixed in the end.

I am having real issues with Fender Hot Rod Deville and Fender Hot Rod Deluex amps for some reason. I assume this is true because I do not have a process in place that will get to the answer to the problem.

Is there anyone here who can take the time to go through the process with me?
Here is a little video to kinda give you an idea of what I have to work with.



Thanks,

Billy

 

[tech99]

I suggest that you try injecting a tone of a few volts into the grid of the output stage and then gradually work back a stage at a time until the faulty stage is located.
Having located the area involved with the fault, start checking voltages around this stage.
The usual fault is going to be leaking coupling capacitors or burnt out resistors, both of which will give incorrect voltages. The amplifier has quite a lot of solid state circuitry, which may be vulnerable to mains spikes and overload, so is worth looking at. Generally, do not assume tube faults in the first instance because they are robust items and more often go down gradually.

 

[Planobilly]

Hi tech99,
So, if I understand you I should inject say a 1000 hz sign wave at say 500mV ( you said " a few volts", not sure what you really mean by a few volts) and look for clipping on the scope after the plate and also after the out put transformer.

I have to run out for about 30 min. Be back in a few.

Thanks,

Billy

 

[tech99]

Well of course, I don't know what fault you are seeking. But assuming it is no output, the power amplifier is designed to have a few volts applied to its grids, so you should hear the loud tone in a speaker. Of course, it is a push pull amplifier, so don't expect a symmetrical waveform out, but test on both sides of the push pull amplifier.

 

[Planobilly]

Give me a bit and I will set the amp up and describe the issues.

 

[Planobilly]

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 settings
The Normal / Bright switch does not seem to work
Low volume ie very little amplification on channel 1 more on channel 2
Out put tubes are over heating when a signal is injected but heat range is more or less normal at idle

One thing I do not understand from the schematic...what acts as a plate resistor for the two 6l6's?

 

[jim hardy]

.what acts as a plate resistor for the two 6l6's?

they don't really have one like you're used to
the output transformer let's DC flow with no power loss
it presents "resistance" only to ac - it's a transformer. For AC it looks like (speaker impedance X turns ratio)^2.
That's why you see output transformers specified in kilo-ohms.
It's important that secondary never be open circuited when signal is present.

https://en.wikipedia.org/wiki/6L6
In guitar amplifiers, this flashover problem sometimes occurs if the amplifier is operated without the speakers connected, causing the self inductance of the output transformer primary winding to generate high voltages when the current changes due to the applied signal. For this reason the speaker terminals of 6L6 tube amplifiers are sometimes short-circuited by a switching 6.3 mm jack when the speakers are disconnected. (it can wreck the transformer -jh)

Back to topic...
So you have B+ on output tube plates
working backward from that end of amp
what are voltages at both screen grids ? Junctions Pins 4 & screen resistors R61 , R62 ...
expect 80% to 90 % of plate voltage

and on control grids pin 5 - negative 40 or so?
http://www.r-type.org/pdfs/6l6.pdf

 

[Planobilly]

Hi Jim,

I will make those measurements. It is a bit late here so I need to drink some coffee before I go poking my best right hand into some 400 V stuff...lol...plus my left hand is stuck in my back pocket at the moment...lol

Actually...I think I will get into this tomorrow.Thanks,Billy

 

[Planobilly]

Hi Jim,

R61 and R62 are 470 ohms within 3%

the 6l6's have the following voltages
plate 479v
grid2 pin 4 478v
grid1 pin 5 -51v
cathode pin 8 0v

plates on the 3 12AX7 250v, 245v. and 280v

110 ohms on output transformer primary and 58 ohms center tap (both sides)and .3 ohms on secondary

Unless I am really forgetting something, only thing that could cause the 6l6 overheating and the low volume is the output transformer.

Just as a side note I assume the two 100uf 350V filter caps in series must have been cheaper that one 600V 47uf which is what I would have assumed they would have used...

If I have lost my mind...lol...yell!

Thanks,

Billy

BTW..I have a bottle of scotch with the cap rusted shut I think I need to open...lol...this amp is making me a little crazy..lol

 

[davenn]

tech99 gave you good advice about doing some signal injection to the input of each stage and find out where the signal dies

Also, I notice that there is a preamp out socket ( labelled on the top right of the schematic)
so do you get a good clean signal out of this point ?
If not, then the problem(s) obviously lie either in the earlier stages or the power supply
and you are wasting your time looking at the driver or final stagesDave

 

[jim hardy]

Hi Jim,

R61 and R62 are 470 ohms within 3%

the 6l6's have the following voltages
plate 479v
grid2 pin 4 478v
grid1 pin 5 -51v
cathode pin 8 0v

plates on the 3 12AX7 250v, 245v. and 280v

okay you have good screen and plate voltage on 6L6's
so those stages have no obvious excuse to not work
that much negative on control grid says both tubes should be at 10 to 20 ma ooops 20 to 30ma (lost my bifocals) which i think you verified,
sounds to me okay for class AB
https://www.tubesandmore.com/sites/default/files/associated_files/t-6l6gc-tad.pdf

dave and tech have the right idea , inject signal and listen

The approach i use is
check in this sequence
speaker
power supply voltages
output stage
then work backward
control grids of 6L6's is where i'd start, you'll need a few volts there, then to tp22 and see what it sounds like,
etc
but you can work other direction, starting at front and tracing with o'scope
jumping to someplace near the middle like j4 might tell you which half isn't working

i see from your video you have an oscilloscope
i think i'd set the function generator for an audio signal , i like 400 hz it's pleasant
and start injecting that through a capacitor to inputs of each stage, ~a .1 to 1uf ought to do

 

[jim hardy]

plates on the 3 12AX7 250v, 245v. and 280v

each 12ax7 has two halves
i assume you checked all 6 plates ?

 

[jim hardy]

Aha ! Just found their input signal, 4mv 1khz at TP1
those unsigned voltages at TP's are AC with that signal present, i presume...
they were thoughtful enough to show us stage by stage gain for the 12ax7's

what a nice drawing

 

[Planobilly]

Hi guys,

Thanks for all the help.

The issue I don't know how to deal with is that the output tubes are really overheating when I inject any signal into the amp. They do not over heat at idle. The temp at idle is around 300F and moves up to past 400F with a signal at which point I turn the amp off to keep from red plateing the tubes. The max per the data sheet is 450F.

I assume that because the idle current is normal, bias is normal, plate voltage is normal, and there is nothing wrong with the tubes themselves there are only two other things that could cause the output tubes to overheat. One, the output transformer is bad or two the screen voltage is somehow out of control. Check me if I a saying something stupid...but I assume the only cause of overheating in power tubes is too much current or a impedance mismatch between the tubes and the speaker.

I measured every wire on the output transformer for shorts...Primary 110.92 ohms...red center tap to blue 53.92 ohms red center tap to brown 57.92. On the secondary the green/ yellow to black .30 ohms.. There no shorts to the case are between any wires. I have no idea if these ohms reading are normal.

I have removed many of the caps to test them and found nothing.

One question I do not understand is a measurement on the first filter cap. There are two 350V 100uf caps directly after the bridge diodes and the reading from case ground to the + side of the second cap is around 470V...or close to that as I did not write it down. The first cap reads about half of that.?

I am putting the amp back together and starting over with the troubleshooting this morning to see if I have just missed something or done something really stupid!

This amp is becoming a PITB..!...lolCheers,

Billy

 

[Planobilly]

Hey JIm,

I am putting everything back together now and will post everything I am doing in the next few.

I saw the 4 mV and I assume that is pp but I guess it could be RMS..?

 

[jim hardy]

I saw the 4 mV and I assume that is pp but I guess it could be RMS..?

here's a snip from schematic

looks to me per 4.3 tp1 is rms ,

read dc at control grids when you apply signal
does it change much?
is R60 known to be 1 ohm by measurement?
6L6's run frighteningly hot. Still i laud your caution with them.

 

[jim hardy]

One question I do not understand is a measurement on the first filter cap. There are two 350V 100uf caps directly after the bridge diodes and the reading from case ground to the + side of the second cap is around 470V...or close to that as I did not write it down. The first cap reads about half of that.?

Do you notice R70 and R71?
Their purpose, in addition to bleed down, is to force DC to divide equally between C31 and C32. Without them DC would divide according to the caps's individual insulation resistances which ought to be in megohms but can vary a lot.
So if you got about 235, 470 DC to comon they're doing their job.
Read the AC across each cap for a similar sanity check on their capacitance ratio.

 

[jim hardy]

I measured every wire on the output transformer for shorts...Primary 110.92 ohms...red center tap to blue 53.92 ohms red center tap to brown 57.92. On the secondary the green/ yellow to black .30 ohms.. There no shorts to the case are between any wires. I have no idea if these ohms reading are normal.

A check of the transformer would be to put a small number of volts across speaker terminals (amp powered down of course) and measure voltages on plate side R-Blue and R-Brown. Hopefully they're well matched.
We don't know turns ratio
so let's assume it's similar to this Hammond 40 watt( God Bless Hammond for staying in the business!)
http://www.hammondmfg.com/1608.htm

sqrt(6600/8) = around 29 to 1, 15 at centertap

shorted turns will show as low turns ratio

hopefully you get close to that result and it's balanced

when nothing will show itself wrong - make 'em show themselves right.

If you're overdriving those tubes they'll get hot
how much signal are you injecting? How much signal shows at tp24 ?

 

[jim hardy]

Aha ! look at schematic I'm still learning my way around it

they show signal voltages on transformer
94primary to 7.8secondary = 12::1 per half
24::1 X 8 = 4647 ohm centertap , maybe it's closer to 30 watt transformer
look for that ballpark impedance and balance

 

[Planobilly]

ok Jim...let me read all your last post...be back in a bit

 

[Planobilly]

here's a snip from schematic
View attachment 96384
looks to me per 4.3 tp1 is rms ,

read dc at control grids when you apply signal
does it change much?
is R60 known to be 1 ohm by measurement?
6L6's run frighteningly hot. Still i laud your caution with them.

OK..I had it set to pp and will change to rms
R60 reads 220K per the schematic..?
Did you mean R60? that measures 1 ohm and is new as I changed it the other day..this is TP 30 used to measure the bias

read dc at grid 2 pin 4 of the 6l6?
I will read the grid now.

ok 5 volt drop when signal is applied at pin4 6l6 475 down to 470

tp1=3.5mv

tp2=222mv
tp3=
tp4=10mv
tp5=.493V
tp7= 55.1
tp8= 1.2v

I am not sure the relay is working because I am getting low voltage on down the line...think I will change the relay as they area normal failure point in these amps.

Also the is no real AC leakage on the filter caps..checked that

 

[Planobilly]

Also Jim I remover the op trans and these are the reading.

primary
blue to brown 110.92 ohms
blue to red 53.28
brown to red 57.92

Secondary
green to black .60 ohms
green to white .49
green to green/yellow .53
green/yellow to black .30

no shorts to the trans case any where

r70 and r71 are not open but I would have to remove one end to read them because of the cap

 

[Planobilly]

I need to take a break for a bit...lol

 

[jim hardy]

r70 and r71 are not open but I would have to remove one end to read them because of the cap

if you read equal DC across them they're not open
equal AC across those two filter caps will tell you they're equal capacitance.. AC reading should be less than ~10% of the DC reading , that's a quick check on your power supply filter

primary
blue to brown 110.92 ohms
blue to red 53.28
brown to red 57.92

Secondary
green to black .60 ohms
green to white .49
green to green/yellow .53
green/yellow to black .30

no shorts to the trans case any where

Transformer is probably okay
Shorted turns does not change the ohm reading very much but it changes the inductance a LOT .
you have to detect that by either inductance meter (and i have no idea how much to expect) or voltage measurement - shorted turns i think ought to change the apparent turns ratio by quite a bit.
Industrial transformers we check by measuring milliamps versus applied volts.

In my younger days i'd connect a Simpson 260 to a primary, set it for RX1 which puts tens of milliamps through the winding,
then remove the meter lead. If the transformer's "inductive kick" did not "bite" my fingers still on the transformer leads i knew it had shorted turns.
Ahh the wonders of analog meters...
If your DMM has a "peak capture" function you could arrange a similar test with a low DC source - AA or 6V lantern battery

As the guys said earlier we may be wasting time poking around output stage -- but on the other hand we're in this to learn and polish skills.

I'll look at those TP voltages later today or tonight

small change in grid voltage is consistent with more plate current and is expected

thanks for the 1 ohm check, yeah we both meant R66 i think , the cathode current sampler resistor

 

[jim hardy]

okay tp7 and tp8 show you're getting through V2A
have you hung a scope there to see if it's a ~faithful reproduction of input? You complained of distortion...

lots of old hands here on pf , i look forward to learning .
In my power plant i had to go very slow and deliberate in troubleshooting because the machinery was big...
it spilt over, now I'm way slower than i should be on small equipment
if you guys get frustrated with me plow right ahead and i'll watch
thanks
old jim

 

[Averagesupernova]

okay tp7 and tp8 show you're getting through V2A
have you hung a scope there to see if it's a ~faithful reproduction of input? You complained of distortion...

lots of old hands here on pf , i look forward to learning .
In my power plant i had to go very slow and deliberate in troubleshooting because the machinery was big...
it spilt over, now I'm way slower than i should be on small equipment
if you guys get frustrated with me plow right ahead and i'll watch
thanks
old jim

It's refreshing to see someone that 'enjoys the ride'. As long as the OP doesn't get frustrated and move on there is nothing wrong with a little sidetracking on something that may not have anything to do with the problem. I know in my case if there is something I am troubleshooting and I don't fully understand how it is supposed to work it often helps to poke around in other parts that are working. It tends to clear my head. I used to troubleshoot oscilloscopes and test equipment for a living. I can recall many times going back and forth between a working unit and a non-working unit if I didn't fully understand what was going on. It was not uncommon to get on a roll and suddenly forget which was which.

 

[jim hardy]

I used to troubleshoot oscilloscopes and test equipment for a living.

well now ! i have a Tektronix 2211 in the "fix me" pile - looking forward to learning from you !

i wonder if PF might enertain a hobbyist forum for such less-than-academic pursuits.

 

[Averagesupernova]

well now ! i have a Tektronix 2211 in the "fix me" pile - looking forward to learning from you !

i wonder if PF might enertain a hobbyist forum for such less-than-academic pursuits.

A dandy idea. Certain parts of scopes are a dream to fix and other parts not so much. The signal chain from the input to the deflection plates can be easy since if it is a 2 channel scope and only one channel is bad you have the other as a guide. I will have to look up the 2211.

 

[dlgoff]

i wonder if PF might enertain a hobbyist forum for such less-than-academic pursuits.

That IS a cool idea; but me, as an old retired guy, all threads in this forum interest me in the hobby sense. And trouble-shooting seems academic to me.

... looking forward to learning from you !

 

[Planobilly]

Well..I took off and drove down to Key Largo. Had to put money in my mad money account to deal with all this electronic stuff...lol

I also have a "fix me" pile and the next big job is to put my Navy TV-10 tube tester in good shape. I took one look inside and saw 10,000 wires and a zillion pots and decided to go play my guitar...lol

The Fender Hot Rod Deville has made me a bit crazy so a break was in order!

I really want to thank everyone for helping me along in my learning process. All the ideas Jim has given me have been a big help.

I am now going back to square one to go through all the test points. I will report the findings one by one.

Everything is normal until TP5. I will retest at that point an report in a few min.

Cheers,

Billy

 

[Planobilly]

 

[jim hardy]

I am now going back to square one to go through all the test points. I will report the findings one by one.

I now have no AC signal at TP5 pin 6 plate 1 of V1A There is DC 250 or so.
There is the 15mV Ac at TP4 which is grid1...so what could the signal from crossing to the plate? Cathode issue? R11??
I am gona put the scope on the grid and then on the plate...perhaps all that salt are in Key Largo has addled my brain...lol

okay
250 v at plate infers around 1.3 milliamp of plate current, agreeing with their 1.9v at tp12 (which you'll verify)

so what could the signal from crossing to the plate? Cathode issue? R11??
I am gona put the scope on the grid and then on the plate..

Put it on the cathode, too .

i don't understand what is the signal comes from U1B through Q2 and C9, looks almost like positive feedback?
but it's controlled by the "more drv" line...

I always preferred "no sound no picture" troubles, they're easier...

 

[Planobilly]

There a couple of things...the Normal/Bright is not working...More Drive only seams to work when channel select is pushed in.

I see the signal on all the pre amp tubes but V2B grid is on only when the channel select is pushed in, which I assume is normal.

The signal is there on all the pre amp tubes and is amplified but I don't read the correct AC voltages at TP5 .

I think I will remove the board again and change the other relay...just to be sure and look at the Normal Bright switch issue...I don't have another switch of that type but perhaps I can rig up something and also check the resistors around the switch.

Actually I am beginning to think the amp needs a new muffler bearing...lol...lol

 

[jim hardy]

swap 12ax7's see if trouble moves? maybe one cathode is losing emissivity. Both sides aglow? Pull tube and check for heater to cathode short.

 

[Planobilly]

I have swap all the tube one at a time. I will check the hearers and cathode short idea.

I also think I will go back to a old school idea and listen to the signal with a small amp and a probe with a .001 cap.

I am gussing the distortion is not in the pre amp section because I don't see anything on the scope, smooth sign wave on the scope. the distortion is not very bad...just not much volume. Just a small increase in volume as I turn up the volume control.

I also think we are dealing with more than one or two issues.
I also want to listen to the signal at the grid of the first 6l6.Be back in a bit.