Fender 75 amp issues again and again.....

Dreadneck

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Measure the AC coming out of the rectifier (at the 500V and 250V points) with no tubes installed. If you are only getting a few volts of AC ripple at those points, the chances are that the rectifier diodes and caps are doing what they are supposed to. If you see a bunch of ripple voltage (AC) on top of the DC, then you may have a partial short somewhere in the rectifier system which would continuously draw a lot of current.
I'm trying to measure AC on those points now and find it kind of difficult to read. My meter blinks at everything from 170VAC to 30VAC for about a millisecond and then imidiately settles at eighter 1.xVAC or nothing at all.
I took the measurements with no tubes installed and between point and ground. I also tried between the two points for a similar result.
 

Dreadneck

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Update again!

I managed to start the amp without the limiter and in standby.
Waited a while and took it out of standby.
Working!
Progress indeed!

I played guitar through it for a little while and everything seemed to hold up.

I doesn't sound good though. Like there is no headroom at all and the amp starts to break up at volume 1.. Now to be fair I do need to check that the bias is correct and put the amp back together in the cab.
Also the speaker I use i suspect is total crap.. So I can not give it a fair evaluation at this moment.
Given the ludicrous amounts of wire in the amp, there are loads of room for oscillation and noise.
I am using the old OT right now. I hope that it's not that that is causing the weird breakup/distortion. I examined my replacement OT and realized it is not a valid replacement since it isn't ultra linear. Which the original OT is. So I guess I have to hope against hope for a healthy original OT.
 

andrewRneumann

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This is a really good idea! Do you have an opinion of what values and type to pick? I've found them on elfa and I bet they're on mouser as well. But what ohms would you recommend? (And how would you know what ohms would work? Curious learner here ;))

The one guy I remember always using them was @NSB_Chris . Maybe he can enlighten us both. I see he has a 10Ω / 5A on one of his designs. Maybe he can chime in with some ideas on designing for them.
 

Dreadneck

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Oh I hope he comes swooping in! =) I don't know if it matters but we have 230VAC in the wall here in Sweden. If that would alter the value.

I'm trying out the amp hooked up to a nice Marshall 2x12 cab now. It sounds a lot better, though there are a hum present and if I play hard on the strings the sound "farts out". Which makes the low e string unusable at the moment.
I'm tapping around in the amp and the wires to the plates of the preamp tubes acts like a microphone when tapping on them.
That is the biggest thing concerning noise I can find by tapping.

Furthermore the mid boost seems not functioning as well as the bright switch. They are doing no difference what so ever.
The bass and treble boost on the other hand works fine.

It is very prone to distort the played notes even though I'm still at volume 1..
 

W.L.Weller

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I'm no expert but that OT is definitely a (if not the) suspect for "farty" low notes and low volume distortion. Presuming that Marshall 2x12 doesn't sound that way with a different amp.
 

Dreadneck

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I'm no expert but that OT is definitely a (if not the) suspect for "farty" low notes and low volume distortion. Presuming that Marshall 2x12 doesn't sound that way with a different amp.
You think so? That's not what I wanted to hear............
Problem is I do not think I have a suitable OT to replace it with. I have a Hammond 1750M that kind of fits, except it is not ultra linear. Could I just not care about that and use it anyways?
 

W.L.Weller

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You could use that Hammond 1750M, but as you correctly noted it is not a drop-in replacement, and also if I am reading the specifications correctly, it's only rated for 60W. Here's an overview of the differences between pentode, triode and UL push-pull configurations:


And here's FM @robrob's helpful OT diagnostic (I believe, but am not certain, that failing these simple DC resistance tests is proof that the OT is dead; but that passing these DC resistance tests is not proof that the OT is good):


Output Transformer

If the output transformer shorts between windings it can lead to no output, weak output or funky sounding output. You can test the transformer by measuring the resistance between its leads. Drain the filter caps and remove the rectifier and power tubes before making the following resistance measurements:

For push-pull transformers you should see approximately the same resistance between each secondary wire at each power tube and the center tap--typically somewhere between 10 to 200 ohms.

A shorted primary winding will have much lower resistance compared to the other winding and typically measure at less than 10 ohms.

The secondary windings will often measure less than 1 ohm between all the secondary leads so it is difficult to detect a shorted secondary.

If a transformer winding is open (break) it will show a very high resistance between the primary leads or between the secondary (speaker) leads (typically 500k or higher).

A short between the primary and secondary windings will typically show less than 10 ohms resistance between the primary and secondary leads.

Measure the resistance between all the leads and chassis ground--low resistance of less than 10 ohms indicates a short to the transformer's iron core.

For comparison my healthy Hammond 125C push-pull output transformer measured: red center tap to brown 150 ohms, red center tap to blue 116 ohms, brown to blue 268 ohms. All secondaries measured .2 to .6 ohms between them. A Hammond 125GSE single-ended output transformer measured 53 ohms between the two primary wires. All of the secondaries measured .2 to .3 ohms. Both transformers measured an open circuit (no connection) between their primary and secondary wires and between all wires to the transformer outer metal shell.

If any of the above faults are present your best bet is to just replace the output transformer. For valuable vintage transformers it is possible to have them rewound.
 

NSB_Chris

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The one guy I remember always using them was @NSB_Chris . Maybe he can enlighten us both. I see he has a 10Ω / 5A on one of his designs. Maybe he can chime in with some ideas on designing for them.

I noticed several modern amps used inrush current thermistors to provide a smooth start. If you decide to use them, verify before you buy, but the datasheets of all that I have seen say they are good up to say 260V. I don't remember what reference made me stick with 5A 10 ohm. I have only used them on 25watt or smaller amps.

Rob's site has a couple of references.

His section on "Soft start inrush current limiters" is here:
This talks about using 2A 220 ohm thermistor.

In his section on "protection mods":
https://robrobinette.com/Generic_Tube_Amp_Mods.htm#Protection_Mods
He provides a link to a 5A 10 ohm used on the '57 Deluxe:

I have seen some Marshall schematics use 15A 2.5 ohm on bigger bore amps.
 

andrewRneumann

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I'm trying to measure AC on those points now and find it kind of difficult to read. My meter blinks at everything from 170VAC to 30VAC for about a millisecond and then imidiately settles at eighter 1.xVAC or nothing at all.
I took the measurements with no tubes installed and between point and ground. I also tried between the two points for a similar result.

Bah... it's probably your meter not capable of distinguishing between DC and AC. If the fuses aren't blowing and the PT isn't getting hot, you are probably in good shape as far as PT/rectifier/first filter go. You have 500VDC and 250VDC at the proper place, so that's probably a good enough indication.
 

Lowerleftcoast

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I have a Hammond 1750M that kind of fits, except it is not ultra linear. Could I just not care about that and use it anyways?
You could use the 1750M. You would just need an alternate way to supply power to the screens.

From the symptoms, I was wondering if only half of the waveform is being amplified. Are both power tubes heaters glowing. Pop test on each power tube? (Pop test all the way to the input?) Maybe try a different phase inverter tube. Bad connection of the signal wires between PI and the power tubes? What happens when each power tube is removed (one at a time)? What happens when the power tubes are switched to the other socket?
Of course these thoughts do not rule out the OT...
 

Dreadneck

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If any of the above faults are present your best bet is to just replace the output transformer.
The ultra linear taps (brown/yellow and blue/yellow) had sub 10ohms resistance to center tap, around 6-7ohms. I don't know if this is normal in an ultra linear OT.
Other than that it checked out fine. Not as high resistances as in the example but in the range. The resistances on the primary ranged between ca 50ohms to ca 90ohms.

Secondary was all around 1 - 0.9 ohms.

You could use that Hammond 1750M, but as you correctly noted it is not a drop-in replacement, and also if I am reading the specifications correctly, it's only rated for 60W
I actually have an OT from an old 75w amp (Studiomaster Valve Leadmaster). It's not UL but the watts should be fine. I'll check it and see if it's ok and do the calculations to see if it would fit a 2x6L6GC amp. Now I believe the amp it came from used the same configuration.

You could use the 1750M. You would just need an alternate way to supply power to the screens.
Any good ideas how? I read the link the @W.L.Weller provided about OTs and it seems the easiest way would be to connect the screens to the plates. But how would that work in this amp?
Adding a choke and caps would be a major task since the amp has lack of space.

From the symptoms, I was wondering if only half of the waveform is being amplified. Are both power tubes heaters glowing. Pop test on each power tube? (Pop test all the way to the input?) Maybe try a different phase inverter tube. Bad connection of the signal wires between PI and the power tubes? What happens when each power tube is removed (one at a time)? What happens when the power tubes are switched to the other socket?
Of course these thoughts do not rule out the OT...
I'll look in to it! I must ask though, what is a pop test? It is apparently something that has escaped me.


I noticed several modern amps used inrush current thermistors to provide a smooth start. If you decide to use them, verify before you buy, but the datasheets of all that I have seen say they are good up to say 260V. I don't remember what reference made me stick with 5A 10 ohm. I have only used them on 25watt or smaller amps.

Rob's site has a couple of references.

His section on "Soft start inrush current limiters" is here:
This talks about using 2A 220 ohm thermistor.
There were a lot of good ideas in that link. The pop elimination on the power and standby switches is definately work looking into.
My amp is 75w so perhaps a 220 ohm is what I should go with as well? But is 2A not to small? The primary fuse is at 2.5A. Should I go with 5A as you use?
 

Dreadneck

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Bah... it's probably your meter not capable of distinguishing between DC and AC. If the fuses aren't blowing and the PT isn't getting hot, you are probably in good shape as far as PT/rectifier/first filter go. You have 500VDC and 250VDC at the proper place, so that's probably a good enough indication.
Yeh, I think so too. It's a midprice meter, so not the best but not to bad.
 

Lowerleftcoast

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what is a pop test?
It is an old test. When you use your meter to check for voltages, you will hear a *pop* when probing the plate. Generally you start with the power tubes and work back to V1. In gain stages the *pop* should get louder as you progress to V1 because of the gain stages amplifying the signal.

On the power tubes the screen and control grids will *pop* as well when metered.
 

Dreadneck

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So the "pop test" gave puzzling results.
The were noticeable pops on the powertubes, very loud "pops" (more like bangs) from the PI and just little tiny sparklings from V4-1..
So I'm going to take a closer look at the PI circuit and socket.
 

Dreadneck

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Another thing I noticed before but completely forgot about.
On some of the valves in the pre amp there should be small 500pF caps (some sort of snubber caps?) according to the schematic.
Two of them at V1 and V4 it seems. One on V2 and one on V3.

My amp does not have these. Also there seems to be something not right about the bright swtich wiring. I'm trying to work it out from the schematic.
 

Dreadneck

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Question:

Would it be possible to rewire the tone stack according to AB763 specs and ignore how it is wired according to Fender 75 schematic? Besides the pull-boost functions ofcourse.

The reason is that I have the hardest time figuring out how it is SUPPOSED to be wired from the schematic alone.. For some reason I find it hard to look at schematics when it comes to tone stacks.
 

Dreadneck

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Last update!

THE AMP NOW WORKS!!!!!!!!

What I did:

1. Replace PT (the old one was shorted)
2. Rewire/rebuild the power section that was less than optimal. All components now have more space and the large 10+ watt resistors rests against the chassis to use it as heat sink.
3. Replaced caps and diodes in rectifier.
4. Replaced one power tube socket were the lugs apparently were broken internally.
5. Rewired heaters correctly and elevated.
6. Replaced ALL plate wires, wires to/from bias pot. (This is what eventually made it work witout noise and distortion and actually made it stable, so that it doesn't burn fuses every other startup.)

I am beyond happy!!!

The only issue now is the tone stack that doesn't work unless the pull-boosts is activated (which robs the amp of volume... hmmm..)
As I said in the post above, I have a hard time figuring out how the tonestack is supposed to be wired from the schematic alone.. That's why I was thinking about wireing it according to AB763 schematic and layout.
If anyone know if that is possible, please let me know.


Last but not least:

THANK YOU ALL SOOOOOOOOOOOOOOOOOOOOOOOO MUCH!!
I am truly greatful to all of you who took time to look in to this. You are truly good people. <3
 

andrewRneumann

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Last update!

THE AMP NOW WORKS!!!!!!!!

What I did:

1. Replace PT (the old one was shorted)
2. Rewire/rebuild the power section that was less than optimal. All components now have more space and the large 10+ watt resistors rests against the chassis to use it as heat sink.
3. Replaced caps and diodes in rectifier.
4. Replaced one power tube socket were the lugs apparently were broken internally.
5. Rewired heaters correctly and elevated.
6. Replaced ALL plate wires, wires to/from bias pot. (This is what eventually made it work witout noise and distortion and actually made it stable, so that it doesn't burn fuses every other startup.)

I am beyond happy!!!

The only issue now is the tone stack that doesn't work unless the pull-boosts is activated (which robs the amp of volume... hmmm..)
As I said in the post above, I have a hard time figuring out how the tonestack is supposed to be wired from the schematic alone.. That's why I was thinking about wireing it according to AB763 schematic and layout.
If anyone know if that is possible, please let me know.


Last but not least:

THANK YOU ALL SOOOOOOOOOOOOOOOOOOOOOOOO MUCH!!
I am truly greatful to all of you who took time to look in to this. You are truly good people. <3

Thanks for all the good vibes. 😎 I have to admit I’m a little lost on the issue you are having with the tone stack. Might I suggest a new thread focused just on that one issue itself? Maybe start with a clear description of the problem and add in the appropriate schematics, layouts, and photos of your build. It just gets too cumbersome when these threads get long to go back and dig up the relevant information.

Great job getting that beast back in shape!
 

Wally

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Last update!

THE AMP NOW WORKS!!!!!!!!

What I did:

1. Replace PT (the old one was shorted)
2. Rewire/rebuild the power section that was less than optimal. All components now have more space and the large 10+ watt resistors rests against the chassis to use it as heat sink.
3. Replaced caps and diodes in rectifier.
4. Replaced one power tube socket were the lugs apparently were broken internally.
5. Rewired heaters correctly and elevated.
6. Replaced ALL plate wires, wires to/from bias pot. (This is what eventually made it work witout noise and distortion and actually made it stable, so that it doesn't burn fuses every other startup.)

I am beyond happy!!!

The only issue now is the tone stack that doesn't work unless the pull-boosts is activated (which robs the amp of volume... hmmm..)
As I said in the post above, I have a hard time figuring out how the tonestack is supposed to be wired from the schematic alone.. That's why I was thinking about wireing it according to AB763 schematic and layout.
If anyone know if that is possible, please let me know.


Last but not least:

THANK YOU ALL SOOOOOOOOOOOOOOOOOOOOOOOO MUCH!!
I am truly greatful to all of you who took time to look in to this. You are truly good people. <3

Dread, what pins were suspect in that power tube socket? I am guessing that there Was an intermittent problem in voltage supply to that power tube….whether that was plate or bias voltage and that that problem took out a PT…or two PTs????
and…just so I understand, when you say you replaced the PT, is this in reference to the new PT you mentioned in your first post, or have you replaced that replacement PT?
 

Dreadneck

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Dread, what pins were suspect in that power tube socket? I am guessing that there Was an intermittent problem in voltage supply to that power tube….whether that was plate or bias voltage and that that problem took out a PT…or two PTs????
and…just so I understand, when you say you replaced the PT, is this in reference to the new PT you mentioned in your first post, or have you replaced that replacement PT?
It was pin 6 that broke off when I tapped it with my chop stick. So one side of the heaters. But I don't know in what shape the other pins was since I just replaced the whole socket. Maybe I should've checked it for clues..

The PT that got replaced was the original. The replacement PT has survived through all testings =)
I think what took out the original was the 30W resistor that was pressed up against the bell of the PT.
I took that PT apart and the insulation around the core was completely melted and the core was touching the bell.
That resistor is now relocated to where it sits tight against the chassi and away from ther components.
 




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