Fender 75 wierd power tube issue

[bnwitt]

Well I started working on this 75 yesterday and have run into a weird power section problem.  I replaced the octal socket that was burnt along with the 1k5 ohm and 470 ohm resistors, set the bias voltage at full (-51) and balanced it the same on both tubes. verified the plate and screen voltages on both power sockets and installed a new matched set of winged C 6L6GC's.  I turned it on and came off of standby and the socket which was burnt before was pulling 61ma at 490vdc while the other was pulling 26ma at the same 490vdc.  Of course I didn't want to leave it on long for fear of ruining the one power tube but I was able to verify all voltages at each socket were identical.

I measured the OT resistances and came up with 95.5 ohms from plate to plate, 51.5 from hot socket plate to center tap, 43.7 from cold socket plate to center tap, 12.1ohms from screen tap to screen tap and 6.3 from each screen tap to center tap.  Now it seems odd that the resistance between the center tap and the plate wire of the hot socket is higher and that tube is pulling more current.  It's not the tubes, as the overcurrent stays with the socket when tubes are swapped and I retested them as matched in my maximatcher.

Someone has modded this amp and of course it is a spaghetti bowl of wires so this is making trouble shooting laborious.  Another issue is when the high low power switch is in low power I get no plate voltage at the power tubes at all instead of the 250 volts that is supposed to be there.  Unfortunately I have to draw photovoltaic systems today so amping is out but I thought I'd throw this issue out there for you fellows to ponder along with me.


http://www.schematicheaven.com/fenderamps/fender_75_schem.pdf

[Tiny_Daddy]

OK just thinking out loud here but if there is a problem with even one of the output tube sockets in a vintage Fender, I punch out all of the output tube sockets to 30mm using a Deltron punch and install new Belton sockets. I suppose the electrolytics have been replaced? Also the nasty burned terminal strip?

[bnwitt]

Thanks for the response TD.  The other power tube socket has already been replaced by whomever modded the amp and it is in good shape.  This amp is a very interesting beast with the PT center tap half power feed and the opto coupler -6.2V control circuit hanging off the bias circuit.  I'm sure I've got more than one problem going on here.

[Tiny_Daddy]

How about a under-chassis photo of all the wonderful mods?

[RicharD]

Measure the DC resistance from each plate tap to each respective ultra linear tap (obviously with the tubes remove).  You're looking for a possible short, which = bummer.  Double check for a cold solder joint or loose socket pin at the grid of the hot tube.  Hopefully that tube isn't getting correct bias voltage and that's why it's drawing a lot of current.

[bnwitt]

The resistance between plate and screen taps on the hot tube side is 46 ohms while the resistance on the plate and screen taps on the cold tube side is 36 ohms.  So there is higher resistance on the hot tube side which is counter intuitive to higher current.  I did check the two dropping resistors between the balance control and the grids and instead of 82k/100k they are 95k/120k.  Typical CC upward drift.  Now I did check the bias voltage on both sockets prior to coming off standby and they were both equal at -51 volts which I achieved with the balance control.  When I got the amp, the bias was lower on the hot tube side than the cold tube side, and the modded bias pot was set to a lower output overall.  I believe it was -36 and -46.  I have that control set to full output now and the balance tweaked to get equal settings.  I have not checked the bias voltage under load so that may be the issue.  Can someone tell me why Fender used two different values for the dropping resistors in the bias circuit 82,/100k?

[bnwitt]

How about a under-chassis photo of all the wonderful mods?

I've undone all of the mods except the bias pot mod.  I still have a 6.8 ohm 10 watt cement resistor I can't account for on the schematic and I'll need some time to trace it out to determine to what it is connected to.  I'll try to get a photo of the guts today just so you can see what I'm dealing with.

[bnwitt]

Ok some gut shots

[bnwitt]

one more

[bnwitt]

I think I'll put in a fairly matched set of old pulls and try to get a bias voltage reading on both tubes under power.

[bigdaddy]

Not being a repairman and more of an amateur hack I would follow some things I have learned about this.

I would replace those wires and I would put the amp back to original after making notes of the mods. Once everything is back to the schematic and every questionable part is replaced then you can start finding out what's wrong with the amp. It could be a bad solder joint from the previous guys handy work or in a lot of cases with those amps the quality of the work from the factory inside is poor especially lead dress. Those wires look burnt from a soldering iron, I would replace them.

[Rich]

the socket which was burnt before was pulling 61ma at 490vdc while the other was pulling 26ma at the same 490vdc.

I measured the OT resistances and came up with 95.5 ohms from plate to plate, 51.5 from hot socket plate to center tap, 43.7 from cold socket plate to center tap,

In order to achieve similar ma readings, I'd expect the OT resistances at each socket to be about half the plate to plate reading or about 47.7 ohms.

I'd call the OT defective.

[RicharD]

The minor difference in DC resistance is nothing to be alarmed about.  It's actually to be expected where as when you wind a transformer, the coils gradually get longer which will result in a higher DC resistance, but transformers are all about AC and turns ratio.  AC impedance and DC resistance are 2 entirely different thangs.  A drastic difference or a shorted DC reading is cause for alarm.

The next step is to fire it up with tubes and grab neg bias readings right at the output tube grids.  Break out your "beater" tubes as planned and work swiftly.  Actually you should be able to read neg grid voltage while in standby, but you really need to see it under load.

[PRR]

The OT resistances do NOT matter for getting proper DC current.

The resistances are small: 95 ohms at high 60mA is only 6V, which is "nothing" compared to the 490V it comes from.

Likewise an internal short, while murdering the audio power, will not affect the DC conditions enuff to matter.

> it is a spaghetti bowl of wires

Yes indeedy. Combined with symptoms, it seems likely that a 6L6 plate lead's electric field is sneaking back into the driver-stuff and inciting oscillation, probably hypersonic.

Fix that mess. Special attention to short plate-OT leads laid away from the driver.

Add 6L6 grid resistors, at socket, at least 5K6, maybe 22K.

The half-power missing voltage is wrong but an incredible maze. Perhaps you should build a 18W DeLuxe to use on half-power gigs?

[eleventeen]

New poster:

Here's a partial goofball theory that's worth checking out:

I am coming in on this conv late, so if I've missed something, pls forgive. If the screen grid, nominally connected to the cathode either internally in the tube or externally via components or, it should be noted, via the UL winding taps were somehow NOT connected, then its (the screen grid's) influence would not be there.

This tube (not really this "tube", this primary branch of the OT circuit) is acting and measuring like it is running too hot. We would scarcely believe that a tube would lack this internal "jumper" from the factory, and you say the problem lives in the socket, not the tube. So I would tend to think that whatever cooked the socket (that made you have to replace it) might have overheated and partially fried something in the OT. Or, could it be that you have an amp that was started on the way from being populated w/6L6's and was intended to be made into a EL34 amp, the completion never occurred, and then someone put it away for ten years, looked at the tube chart, and threw 6L6's in there??

OR, an empty pin is being used as a tie point for the screen resistors and there is a hidden short you're overlooking..?

Goofy, I know, but cheaper to fry a few brain cells thinking about it than either tubes or trannys.

Keep us informed!

[bnwitt]

Ok Folks here it is.  On the socket with the low current flow whoever worked on this puppy before installed a 47k screen resistor instead of a 470 ohm screen resistor.  Also the 2k7 10 watt dropping resistor on the B+ rail was that 6k8 10 watt resistor I couldn't account for on the schematic.  That had the voltages down stream all low.  I replaced it with 2@ 5kohm 5 watt resistors in parallel since I didn't have a 10 watt 2k7  That brought voltages very close.  The high low power switch had some whiskers shorting the B+ to the bias circuit across the common terminals of the DPDT switch so when it was in low power mode it was doing wacky things.  Fixed that.  Now I have good even current flow through both power tubes when the bias is the same (-50)on both tubes.  Plate and screen voltages match but I am pulling 57ma at 490VDC plate which is way too much current for 6L6GCs. 

The schematic says I should have -56VDC to the output matching pot but I've only got -52.  So I'm losing -2 volts in the pot.  -52 at the pot is pretty darn close and it seems to me that the power tubes shouldn't be running this dad gum high.

Any body have any ideas?

[bnwitt]

Ok, I just answered my bias voltage question.  I took my heathkit power supply and put -57VDC on the balance pot, got -56 on the power tube grids and got 41ma at 490VDC on both tubes.  Not 70% but close.  So, it seems my bias supply is under producing.  I'm getting 55VAC out of the PT bias winding and -65VDC at of the diode but only -52 out of the bias capacitors.  Maybe new capacitors??

[bnwitt]

Allright, replaced the two 70uf/100V bias caps with 100uf/100v new ones and when I did I checked the 220 ohm resistor between them and found it had drifted up to 347 ohms.  So I changed that too.  Now I have -55 at the balance pot and -54 at the power tube sockets with 48ma on the tubes.  There is no doubt this amp needs -56 volts bias to get to 70% at idle.  So now I guess I'll try a 180 ohm 1 watt at the bias caps.

[bnwitt]

Ok, I ended up putting in a 150ohm 1 watt resistor between the bias caps to get the idle down to 41ma at 495V.  That was the best I could do without building a blackface style bias circuit and the customer doesn't want to spend the money.  Had everything working except the clean signal was very weak.  Drove me nuts until I saw the effects in/out jack in the path on the schematic.  Cleaned that up and rebent the contacts and all is good.  See the other posters thread on the mystery board for a post with the Fender 75 cap board and footswitch layouts

[Custom_Amp_30]

So did my layout help, I'm still checking the values to see if I posted the value correctly.

[bnwitt]

No layout would have helped on this amp because the wiring is such a mess and somone screwed with it and put in several incorrect value components on top of that. Following the schematic was the only way to figure what had been goofed up.  As much as I hate working on this model fender, I must say that it has a great clean channel sound and reverb.  The lead channel would be ok if you used some grease on the effects "in/out" like delay or something.  Otherwise it's too dry.  Not as bad an amp as it's internet reputation makes it out to be.  If it were mine I'd do like PRR said and cut out all the extra wire lengths.  Good luck with yours CA30.