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

[Dreadneck]

This is a complex amp on which to learn, imho. Cross your fingers that the OT Has not suffered damage during all of this. Yes, an OT can put sound out and be rendered useless due to certain damage.

I agree. It truly is. My first and main "learning" amp was my Dual Showman Reverb which was a heck easier to decipher and understand. But I'm thinking of building a champ or something similar just to learn the how's and whys of amp electronics.
I know my way around an amp in terms of parts and sections and what they are doing. But on component level I'm screwed.

Gently tighten the speaker and anything else that can vibrate. It can also be a tube issue - try substituting other tubes if you have them.

I'll try it. Might dig up another speaker as well.

Kind of a difficult and expensive OT to use the substitution method of troubleshooting on, but output transformers don't love dead shorts across the secondaries. (At least it wasn't open)

It looks like Hammond sells an OT that would work (https://www.hammfg.com/part/1650P ), more or less (4-8-16 secondaries instead of 2-4-8). Not sure if there are Swedish manufacturers of transformers that might save you a few bucks on shipping/customs.

I actually have that Hammond transformer new and unpacked because it was talk about the OT being shot some year ago.
Only thing that worries me with that is that it is specced at 60w while the amp in high power is 75w.

However I have not made serious tests on the stock OT.
Do you have any prefered methods for evaluating?

 

[W.L.Weller]

I don't have any good advice regarding transformer testing beyond the simplest "open circuit/short circuit" measurements. Which isn't your issue. Which is why I brought up substitution.

I will say that trying to get 75W from a pair of 6L6GC was a design decision predicated on the easy availability of "fresh from the factory" GE tubes. 500V on the plates of modern tubes is going to decrease tube lifespan. I have heard from a bandmate that the Sovtek 6L6WXT+ holds up well to the plate voltage of his Traynor YBA-3 (over 500V) but have no first-hand advice on modern production tubes that'll stand up to the voltages these high power designs are built for.

Basically, if it were my amp, I'd forget that it has a "high power" mode. If you genuinely require more vacuum tube output power, get an amp that uses 4 output tubes to make 80-100 watts.

 

[Wally]

if you have a signal tracer with an audio generator or an oscilloscope, you could ascertain where in the circuit that signal is going bad. Those tools could tell you if you have an OT that is bad. A multimeter will not tell you if a transformer is working properly under a load. If you do not have those tools, then you can temporarily connect that new Hammond and see if your Bad signal goes away.

As for @W.L.Weller’s suggestion to forget that this amp has a high power mode, ime these Fender 75’s are anemic in the low power mode. Ommv.

 

[Dreadneck]

I have an oscilloscope and hooked it up to the amp today.
The signal is really distorted and weird and guess what?
The fuses blow AGAIN!
I think I should disconnect the OT and see if it still blows fuses.
I honestly don't get it..

 

[Dreadneck]

Ok.. So I have disconnected the OT primary side.
Fuses blow.......

What in the world could be drawing to much current at this time???

I know that the tubes are good. So it is not those. The PT is brand new, so surely not that. The OT is disconnected, so not that eighter. I have replaced all diodes in the rectifier bridge, so not those.

What is left? What am I missing?

 

[Wally]

I reviewed the thread. At this point, I would suggest reconnecting the OT primary and pulling all tubes. Fire the amp up and take your voltage measurements again.
Then, install the tubes one at a time starting at the V1 input stage. Check voltages after installing each tube. If and when the fuse blows or the voltage drops, you then have a clue.

 

[andrewRneumann]

Installed standby switch (non existing in EU model). 0.5AT fuses blow...

Can you draw or explain where you installed the standby switch?

Also, we are talking about the 2 HT fuses on the HT secondary right? Do they always both blow, or sometimes just one?

 

[andrewRneumann]

Have you isolated the power level switch and gone through it with an ohmmeter to make sure it’s not shorting?

 

[Dreadneck]

I installed the standby switch according to the schematic, so after the filter cap but before the power level switch.
Yes it's the 2 HT fuses that blow. Always both, never one. Funny thing is that those fuses are non existant on the original schematic. But they are there on revisions made by others and also in other 75 amps that I looked at.
The larger 2.5AT fuse on the primary never blows.

Yes I have checked the power level switch and it seems fine.

I stumbled across another thread yesterday where a Dual Showman had issues with blowing fuses and in that case it turned out to be a backwards bias cap. I've checked the caps in my amp, but they seem to be correct.

When I checked the amp with my oscilloscope the wires coming from the output tubes matching pot and the wires coming from the doghouse were sensitive and made the wave on the scope jump. I don't know it that could be an indication of bad wires or just that there are high voltages on them.

 

[Dreadneck]

I was checking the voltages now with no tubes installed and thought I sensed heat comming from the PT. I touched it and it was FLAMING HOT!! Waaaay waay to hot. More like a power tube in Angus Young's rigg.

I didn't get the chance to check all voltages before this, but the ones I did check was very pleasing.

502VDC After rectifier
250VDC on the low power side
-50VDC on the control grids on the 6L6 sockets.

One measurement that was to high was after the 2.7K 10W resistor on the B+ going to the filter caps. According to schematic it should drop the voltages to 410VDC but i got 459VDC. To much? Or is it to be expected when no tubes are in?

I did not have any 500mA fuses on hand so I used 630mA. I know, I know... ALWAYS USE CORRECT FUSE!!!
But I only used them so that I could take some measurements now. I have to pick up new fuses later. And I was constantly looking for trouble so that I could avoid damage if that would be the case.
I will not use the 630mA ones again I promise =)

 

[Wally]

First, get some 500ma fuses. I am not going to review the thread again, but Inhave to ask if you have a current limiter and are using it? I would check that 2.7k dropping resistor, but I don’t th8n’ that is your problem.
Considering that the PT is hot without any tubes, I would lift the B+ connecti9n to the center tap of the OT and see what happens. If the PT does not heat up and the 500ma fuses hold, one begins to suspect strongly that the OT is the problem…or at least one problem.

 

[Dreadneck]

First, get some 500ma fuses. I am not going to review the thread again, but Inhave to ask if you have a current limiter and are using it? I would check that 2.7k dropping resistor, but I don’t th8n’ that is your problem.
Considering that the PT is hot without any tubes, I would lift the B+ connecti9n to the center tap of the OT and see what happens. If the PT does not heat up and the 500ma fuses hold, one begins to suspect strongly that the OT is the problem…or at least one problem.

I'll check the resistor but I don't think that's it eighter. Though the power section of the amp is a mess with wires going everywhere.

I do have a current limiter but when I use it the amp only gets about 180VAC. The limiter sucks up 50VAC! I do not know why. I'm using a 100w bulb.

I've got some new fuses today.

Worth noting is that I reconnected the OT with alligator leads. Ive done this with good results before and I made sure nothing was shorting or loosing grip.

 

[Wally]

Yes, the limiter is not intended for playing. It is useful in finding problems such as you are having. It prevents the fuses from being blown and from damage to major components due to the problem. Does the limiter show a short…both with and without tubes installed?
And…..Try lifting that B+/OT CT connection.

 

[andrewRneumann]

We need to see some photos. I'm curious about the PT, rectifier, and first set of capacitors.

You mentioned earlier that you replaced all of those 0.002 caps across the diodes. Are they 1KV?

 

[Oxidao]

 

[Dreadneck]

Ok I've taken some photos.




I see that one of the caps has a burn mark. Problably from the soldering iron touching it. Hope it's fine, couldn't have been touching long.
They are 600V caps


Here is the 2.7K 10W resistor that seems to drop about 50V to little.


This resistor is the 30K 20W that goes to ground and connects to one side of the 2.7K 10W resistor and is in parallell with one of the 20uF 500V filter caps.
It got really HOT while powering the amp up now. More on that below.


So I unhooked the B+/OT center tap.
The current limiter flashed and then went dark. So no direct issues except a spike at startup i assume.
The PT now does not get hot. The 30K 20W resistor however does! Not warm, HOT!
Makes me wonder if it used the PT bell as heat sink before.. But I don't know. Large space for a resistor to heat up to that level.

Now, the question is, why does that 30K resistor get really hot? And not the PT this time..
Is it dissapating tons of watt to ground?

As you can see perhaps, the planning of this amp is less than uplifting.. There are soooo much extra wire lenghts that could've been avoided by a slightly larger chassi or perhaps better layout.

 

[Dreadneck]

Here's the schematics again.

 

[Dreadneck]

Fired the amp up again with current limiter, no tubes and OT center tap/B+ connected again.

The PT doen't get hot but the 30K 20w resistor does get very very hot. So I actually asume it was touching the PT before and used it as a heat sink.
Now I'm thinking of putting it flush against the chassi instead and run wires so that it doesn't fry anything close by, plus using the chassi as heat sink.

 

[andrewRneumann]

Now, the question is, why does that 30K resistor get really hot? And not the PT this time..
Is it dissapating tons of watt to ground?

Watts = Volts x Volts / Resistance = 459 x 459 / 30,000 = 7W. That resistor is designed to get hot and will get hot whenever the amp is on and out of standby. Definitely don’t have it touching the PT. What you are experiencing looks to be expected.

The 50V extra will disappear when you install the rest of the tubes.

Fired the amp up again with current limiter, no tubes and OT center tap/B+ connected again.

So the fuses hold on the current limiter.

The current limiter flashed and then went dark. So no direct issues except a spike at startup i assume.

I wonder if this is all due to inrush current. Like you said, the original schematics don’t have HT fuses. Maybe moving up to 630mAT fuses is all you need. I expect the HT secondary to draw over 1A per side momentarily on turn on. If that PT was getting hot because it was rubbing up against that 20W resistor, maybe there isn’t a short after all.

A couple of ideas for consideration:
1. 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.

2. Consider using the standby switch for start up. It may help reduce the inrush current. If you wired it up after the first set of 220uF caps (like the old schematic) that should help a little.

3. You can get an NTC thermistor and install it on the primary side of the PT. This is the typical way I have seen builders limit inrush current. It’s like having a light bulb limiter that turns itself off after a few moments.

4. Be careful getting in there to change fuses especially if you have a standby switch. This thing can have 250VDC on the HT secondary even after the amp is off, although it should drain through the 100K resistors. Leaving the Standby switch in the closed position when powering down the amp will drain the caps pretty quickly because of that big honking 30K. It‘s not a standard design to have DC on the HT secondary, so I thought I would point it out.

 

[Dreadneck]

Watts = Volts x Volts / Resistance = 459 x 459 / 30,000 = 7W. That resistor is designed to get hot and will get hot whenever the amp is on and out of standby. Definitely don’t have it touching the PT. What you are experiencing looks to be expected.

I'll move it so it lays against the chassi instead and away from the PT. If I solder it to the center tap ground of the PT it could work to get it out of the way.

So the fuses hold on the current limiter.

Yes they do. It seems like you said, the inrush current is what blows the fuses.

You can get an NTC thermistor and install it on the primary side of the PT. This is the typical way I have seen builders limit inrush current. It’s like having a light bulb limiter that turns itself off after a few moments.

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 )

I wonder if this is all due to inrush current. Like you said, the original schematics don’t have HT fuses. Maybe moving up to 630mAT fuses is all you need. I expect the HT secondary to draw over 1A per side momentarily on turn on. If that PT was getting hot because it was rubbing up against that 20W resistor, maybe there isn’t a short after all.

They have been puzzling me as well. I don't know why those fuses show up here. I've seen them in another Fender 75 as well so I wonder if it is something installed when imported. I know that amps imported to Sweden has had som extensive mods done to them before, usually by Hagström, I don't know who imported the 75.
They do seem to small. If the HT seconday would draw 1A on startup, would 630mAT manage that? I can try them again without the limiter and see what happens.

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'll get right on it!

Be careful getting in there to change fuses especially if you have a standby switch. This thing can have 250VDC on the HT secondary even after the amp is off, although it should drain through the 100K resistors. Leaving the Standby switch in the closed position when powering down the amp will drain the caps pretty quickly because of that big honking 30K. It‘s not a standard design to have DC on the HT secondary, so I thought I would point it out.

Thank you for that! I'll be careful!