Need Some Guidance For Testing Process On Vacuum Tube Amp

[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.

 

[Planobilly]

OK...that test was really instructive. Sometimes a 2 dollar tool is better than a high dollar scope...lol There is distortion in the pre amp section. There is no signal at all to the grid of the 6l6's in the normal channel and a really distorted signal at the grid of the 6l6's when the channel select in pushed in. The channel select is a overdrive channel in general but should not be very distorted at a low volume.

So... I guess...find the pre amp distortion first and find out why no normal channel signal is getting to the 6l6's.

Looks like I got a lot of work to do to make this monster play

I am beginning to think this Normal/Bright switch issue and the relay circuit is perhaps the cause of a lot of the problem.

Tracing this on the schematic is not so hard but tracing this stuff on this bloody PCB is a ...lol

 

[jim hardy]

Looks like I got a lot of work to do to make this monster play

just think about how good it's going to feel when you've out-persevered it

i like your $2 tool

simpler is better

 

[Planobilly]

And on top of that I have a 12 gauge with buck shot and a boat to take this monster offshore when all else fails...lol

Actually, I need to learn a ton more about using a scope.

They do not normally issue a scope to pilots...lol...so I did not get any training in that area...lol

 

[jim hardy]

Actually you've made quite a bit of progress

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??

yet somehing gets through sometimes else you'd hear nothing...

well TP5 is plate pin6 of V1B not V1A ? Same tube opposite halves. Clockwise viewed from bottom...

TP4 ,grid of same tube, is pin 7 ,
so you have signal at grid but not plate ?
Voltage between cathode pin8 and grid 7 controls the tube's conduction

check there.

High DC can confuse some true RMS dmm's
see if you have signal on other side of C5 or C7 ?

unlikely but one half of heater could be open ... do both sides glow? Continuity between tube pin 9 and 4, 9 and 5 ?

 

[jim hardy]

do you read this to say

if ON DRIVE is not energized
and nothing is plugged into J4
signal should get to 6L6 by K2A 6-4, J4 4-5, R40 C24 ?
clean that J4 4-5 contact
inject a volt of audio at TP13

 

[Planobilly]

Hi Jim,

Last night I pulled the PCB out again and replaced both the relays that control the the drive and channel select. There are actually only two relays but are shown on the schematic as K1A and K1B and K2A and K2B. those were the problem stopping the signal from getting to the grid of the 6l6's. The power amp jack in always worked without issue and I cleaned the contact.

I plugged in the cd player and I can hear the signal with the probe and a small amp at all the grids in the pre amp tubes and also I now have a signal to the grid of the 6l6's...a bit distorted but there.

There is a audio signal at the plate of the 6l6's. I have amplification in all the stages.
I am playing the cd through my high quality speaker system and is is slightly distorted and not very loud.
I am out of new 6l6's tubes and have some on order from Eddie at Eurotubes. I ordered 20 12AX7's and 20 6l6's matched to a couple of different values. Some amps like "hotter" tubes and some don't. There are all JJ's. Eddie is a good guy to deal with and it is rare I ever get a bad tube from those guys
I also ordered 10 new input jacks and two complete sets of pots to have on hand.

There is still the issue of the bright switch not working..I removed the switch and cleaned it and the switch works so I assume there is some issue with the low pass circuit.
The B+ and the Z voltages at the 6l6's are close to the values on the schematics. The filter caps used in this amp are in general real crap to begin with. I also have some new F&T German filter caps on order which I will change out when they get here.

I own three of these Fender Hot Rod Deville amps and as soon as I get them working correctly I am going to sell them...never to darken my door again...lol

So..I started writing this post early this morning but got busy. Bottom line I think I need to get the parts I ordered before going forward. I will study your last post Jim.

Thanks so much for all the help. I will post something as things progress.

Cheers,

Billy

PS...Come visit your kids here in Florida and I will take you fishing!

 

[davenn]

There are actually only two relays but are shown on the schematic as K1A and K1B and K2A and K2B.

That's correct, 2 relays
K1A and K1B are 2 sets of contacts on relay K1 and
K2A and K2B are 2 sets of contacts on relay K2 Dave

 

[Planobilly]

Yep...thanks Dave

 

[jim hardy]

I am playing the cd through my high quality speaker system and is is slightly distorted and not very loud.

hmmm

V3a and B are a phase splitter, i never saw one cathode coupled before though.
Their purpose is to make the 6L6's push-me pull-you reciprocate -
looks like turning "presence" full cw should lock V3B's grid to DC
measure AC between TP24 and TP25 , that number should be twice what either reads to common.

Same for AC between plates of 6L6's

unbalanced phase splitter will give low distorted output

>>>>i'm sorry to say so can shorted turns in output transformer...

bright switch... should only work when K1B shorts out the 100K R43 .
Can you tack in a ~.068 around C4 to test?

 

[Planobilly]

Hi Jim,

I have been thinking output transformer from the start...After ten zillion hours of messing with this amp, I guess a hundred bucks for a transformer would be a blessing if that solved the issue...lol

I had to put this on the back burner for a bit. I have two QUAD ii 1957 high end tube amps a friend came by with today I am setting up for him. I will post a photo of the QUAD amp. The way it is put together is just as well done as anything I have ever seen. 1957 was a good year...57 Chevy comes to mind...lol...then there was that red headed girl about that time...better not go there...lol

On the Hot Rod Deville...I have seen plenty that don't work, and the Hot Rod Deluxe also. Not very good amps as Fender stuff goes. Fender has made some really great sounding amps over the years...most pre 1965...lol

I have a big mess on my bench ATM...Got to clean things up before I kill myself...lol...I will do the test you said in a bit.Thanks,

Billy

 

[jim hardy]

hmm searching on that transformer number

i found several references to 4200 ohm centertap
√(4200/8) = √525 = 22.9 to 1 turns ratio
that one's at http://www.oldguitarparts.com/fender-transformers.html

Do you have a variac? MIght be informative compare two transformers by driving from speaker jack with a few volts, measure between primary centertap and each plate. Plot low side current vs volts to get excitation curve. Should be linear up to point transformer saturates.

interesting writeup here
http://www.victoriaamp.com/understanding-your-output-transformer-part-one-by-mark-baier/

Leo Fender’s problem was obvious: How to get the maximum from the available Jensen speakers without risking catastrophic failure. Because of the basic physics of output transformers, he was able to solve the problem and create many of the classic tones we now treasure at the same time. Fender knew that the low frequencies were the factor most responsible for speaker overheating. It’s elementary; the lower you go, the more difficult it is for the electronics to swallow. And here’s where the transformer comes into the equation. In an output transformer, the low frequency response is largely governed by the size or mass of the stack. The bigger the stack, the lower it will go without saturating. (That’s why an SVT weighs so much, its OT is the size of a Oldsmobile!).

What Leo did, his genius, was to deliberately inhibit the low frequency response of the amplifier by diminishing the actual size of the output transformer. Now the 40 watts of low end that the tubes are delivering will never get to the speaker because it’s saturating in a smaller OT. Doing this had the unintended benefit of creating the creamy compression that’s so sonically satisfying. There is a specific, sweet distortion to be found here, and it’s the type that’s only revealed when the amplifier is driving the output transformer into saturation. The sense of a tube amp “opening up” and producing a singing overdriven note is certainly a function of the transformer compressing and putting it’s signature on things.

 

[Planobilly]

Hi Jim,

Thanks for the info on the transformer.

This whole transformer issue in general is a issue I need to really get smarter on. Big repair shops generally have a lot of transformers in stock and can pull one off the shelf to use as a test. I need to come up with a good method to be able to test transformers of all types. The power transformers ( for tube guitar amps) never give me much problem to test even if I don't what it came out of.

The output transformers are another world...half science and half "black magic". Data sheets that would tell what you really need to know are really hard to find if they exist at all. The design of output transformers is actually very complex and a ton of R&D goes into the design. The companies that make output transformers guard their designs super well as to keep other companies from copying the design.

It is pretty depressing to think you have a bad transformer and spend one to two hundred dollars only to find there was nothing wrong with the first one.

Thanks,

Billy

 

[jim hardy]

well you have several amplifiers.

A transformer is just an inductor with more than one winding.

It will have an impedance that's fairly constant until it saturates

Volts/amps will be fairly constant until it saturates.

So if you graph volts versus amps you''ll get a straight line until it saturates.

If it has an air gap it'll take a LOT of current to saturate, and more amps per volt in the linear part of the curve. An output transformer likely has a small air gap to avoid saturation, and note that article i linked describes Fender's genius at employing saturation to get the "sound" he wanted... maybe that's your 'Black Magic'

If it has shorted turns it'll draw more amps per volt.

If you have one good transformer and a Variac you can measure its K at 60 cycles

dont take it to saturation, just up to rated current or a comfortable number of volts.

Then you know K for a good transformer.
And you can write it on the transformer for reference.
Shorted turns will drastically raise LOWER K. A similar transformer with twice oops, HALF ! the K has something wrong with it.

That's the basics.

 

[Planobilly]

Hi Jim,

This morning I got some new parts, one of which was a new volume pot which I installed. I went back through all the test points and everything is within the stated values per the schematic. I also retested All power supply voltages both at the supply and at where the voltages go to. I found no issues at all. I did not change the filter caps with better quality F&T caps but the ones in the amp seem to be working well enough.
The Normal/Bright switch provides high frequency boost to the normal channel and working or not would not cause low volume or distortion...at least I don't think it could. Phase splitter is not unbalanced and the 6l6's are balanced also.

Still have low volume and some distortion and overheating on the 6l6's
I guess there are several things in general that could cause low volume and distortion...tubes for sure could cause that condition and unbalanced conditions ect.
I have checked every voltage on every pin of every tube and find nothing not normal.

All this points to the output transformer, at least in my understanding. One reason I say that is if all the voltages/bias is correct what else could cause a tube to overheat.

Am I correct in assuming only two things can cause a tube to over heat, one high current flowing through the tube or a impedance mismatch?

Thanks,

Billy

 

[Planobilly]

Thanks for the transformer info

 

[Planobilly]

Actually I guess the only thing that can cause a tube to over heat in current flow, what ever the cause of the current flow. Correct??

 

[Planobilly]

What actually happens for example when the load/speaker is disconnected from a amp while it is running? I assume the current is still flowing to the plate and overheats the plate until it melts and then could short out to other parts of the tube. That condition could send the current to other circuits in the amp and cause a cascade effect taking out other components.

Am I on track here with my thinking?

 

[Planobilly]

FIXED! Loud enough that my wife yelled at me when I plugged in my guitar and played super loud rock and roll...lol

Installed new output transformer which fixed the issue.

Learned a few new things in the process and a million thanks to Reallllly OLD Jim for all the kind help...I say he is old because he is one year older than me..!...lol

I have a spare bed room with a new bed so come on down to Florida Jim. The boat is tuggin at the dock lines..lolThanks to all the guys that helped.

Cheers,

Billy

 

[jim hardy]

Actually I guess the only thing that can cause a tube to over heat in current flow, what ever the cause of the current flow. Correct??

yep

 

[jim hardy]

What actually happens for example when the load/speaker is disconnected from a amp while it is running?

When that happens voltage in the transformer can run amuck and pierce the insulation
it happens often when the speaker wire falls off (or a thermally protected speaker disconnects itself wrecking a hundred dollar transformer to save a twenty dollar speaker)

you might solder a 50 or 100 ohm ten watt resistor across transformer secondary's highest tap to absorb voltage run amuck

Glad to hear you feel more confident now. Troubleshooting is a process of drawing a circle around the trouble and tightening it up.
Your intuition was right in the first place,
you're a natural !

I hope you'll measure K for a good transformer
that is a test you could do without disconnecting it.

and see my post 48 i made a mistake in it, fixed now

old jim

 

[Planobilly]

Thanks Jim...you have been a super nice teacher. Electronics is big fun for me but it is even more so when I have someone to talk to.

Lookin at post 48 in a few. I also read the Leo Fender post/link, cool stuff. I am going to dig up a link for you about transformers that twisted my head up good and proper if I can find it.

Billy

Looked at 48...think I will print it out an post it on the wall. I have CRS...lol...I will be nice and say that means "Can't Remember Stuff" lol

 

[Planobilly]

Number Next...lol

 

[Planobilly]

This was a pretty cool link on transformers http://education.lenardaudio.com/en/14_valve_amps_5.html

And here is everything you never wanted to know about the subject...lol http://www.dissident-audio.com/Transfos/Papers/Wolpert_Audio_Xfmr_Design_Manual.pdf

Time for Friday night TV wit the wife!

 

[jim hardy]

i REALLY like that Leonard article

Valve amps were part of the vinyl era where deep bass caused the needle to jump off the record. Most HiFi valve amps had a rumble filter to reduce sub-bass. The majority of speakers were approx 6dB to 10dB more efficient and larger than most speakers boxes today. A few Watts was very loud.

In those days speakers were likely vented because you get that 6db more sound per watt, and with tube amps watts were scarce.
Now a vented enclosure doesn't oppose low frequency over-excursion like a sealed one does
and I've watched the speaker try to follow the warp in a vinyl record . Switching in the rumble filter stops that.

Extreme high Voltages are generated across the primary winding if the speaker is disconnected while music is playing. Flashover between windings was a major problem with valve guitar amps.

Think about that - when the speaker wire falls off, the considerable Inductance of the transformer sees an immediate current drop , that is it experiences a large di/dt. e=Ldi/dt, so a huge voltage is produced. A snubber resistor in parallel with the speaker jack will help.

 

[jim hardy]

i re-read post 48 again

for the guy who made that chart, K is the knee.

I was thinking along the lines
volts = K X amps, K being the slope of that curve's straight part.
100 volts at 10 ma , K would be 10,000 and measuring from high side it might be that good

A good inductor opposes AC current stoutly, ie lots of volts to push amps through = large K
l
one with shorted turns is not stout , ie doesn't resist amps nearly so well, smaller K

that's why i hope you measure a good transformer so you'll know a real number