Yet another amp blowing fuses post... Friedman Dirty Shirley

[dbishopbliss]

Related to my earlier post about the cost of fuses, my friend's Friedman Dirty Shirley keeps blowing fuses. We have tried swapping the rectifier tube and the power tubes but it is still happening. On another forum I read the following:

Replace fuse, remove all tubes and see if the fuse blows when you turn it on and take it out of standby. If the fuse holds, install preamp tubes and retest. If the fuse holds, install power tubes and retest. Even though you had new tubes installed, one may have gone bad. If the fuse blows without any tubes installed, it's time to see a tech.

I have never heard of the "remove all tubes" method. Is this something safe to try?


Another thing I read (I think on a post here) is that the tubes could be biased to hot. There is a bias adjustment inside the chassis but no bias points. Would it be safe to just turn down the pot (counter clockwise) and see what happens?


Thanks.

[sluckey]

If the amp seems to be working properly but the fuse blows occasionally for no apparent reason, then inspect the fuse. If it's a fast blow then switch to a slow blow.

I have never heard of the "remove all tubes" method. Is this something safe to try?

Yes, it's safe. It's unlikely that a preamp tube is causing the fuse to blow. I'd first just pull the rectifier and see if the fuse holds.

Another thing I read (I think on a post here) is that the tubes could be biased to hot. There is a bias adjustment inside the chassis but no bias points. Would it be safe to just turn down the pot (counter clockwise) and see what happens?

I suggest having a meter connected to something any time you will be turning the bias pot. If no current monitor points are available just connect the meter to the control grid of an output tube. This is pin 5 for most big bottles. Note what the voltage is just in case you want to reset the bias pot to the same. Now turn the pot to increase the negative voltage a bit. For example, if you measure -40V at pin 5 then turn the pot so you have about -45V. This will cool the current.

[acheld]

the "remove all tubes" method. Is this something safe to try?

If you're not sure where the short is causing the fuse to blow, then this method is the best way to trouble shoot.  I take it one step further and use a light bulb limiter before turning the power back on.

There is a bias adjustment inside the chassis but no bias points. Would it be safe to just turn down the pot (counter clockwise) and see what happens?

No.  Two things here.   a) Hot biasing itself should not cause a fuse to blow.  Hot biasing CAN damage a tube such that it shorts, and causes a blown fuse.  b) You need to know your bias setting so as to avoid "a."

Robrob has a good section on biasing amps.   There are several methods, and you don't need "bias points" to accomplish the task.   See:https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm  Most of this is pretty basic and stuff you know already, but it is a good review and is followed by his very useful Bias Calculator.

[AlNewman]

I would think if you're blowing fuses, you have a short somewhere.  First thing I would do is try to figure out what the fuse is protecting.
Have you tried testing values and resistances of resistors and capacitors, especially filter capacitors?
Have you checked the diodes, (if applicable), to make sure they aren't shorted?  Likely if a diode is shorted, it was introduced somewhere else.
If it were me, before I even turned the amp on, I would find a schematic if I could, and test every component i could to find the source of the problem.
I'm not as bright as some here, but I've normally found that I can't hurt anything if the power switch is off.

[Latole]

The Friedman Dirty Shirley is a modern take on the iconic JTM 45 circuit from the 1960's. Friedman has headed towards a more high gain take on the circuit with a high and low output at 40 watts with 1

https://www.russomusic.com/products/friedman-2020-dirty-shirley-40-watt-tube-head-amplifier-used

[Latole]

" I have never heard of the "remove all tubes" method. Is this something safe to try?  "

It is the first thing to do , and well know by who often repair amps.

[pdf64]

It’s a tiny exception, but it’s not safe on some old 100W Marshalls, as the increase in HT voltage all down the chain can exceed rating of caps for the preamp nodes.

[PRR]

Probably only the power tubes matter. All the rest have >50K series resistors so can't suck more than 10mA @ 500V, or 5 Watts, or <0.1Amps at 120V.

Yes, the big-tubes-out voltage will rise, maybe too much for steady running. I never saw a large Aluminum e-cap blow-up in less than a minute, so for instant-blow faults it should be safe for many seconds.

[Latole]

It’s a tiny exception, but it’s not safe on some old 100W Marshalls, as the increase in HT voltage all down the chain can exceed rating of caps for the preamp nodes.

You can do that safely to see if fuse blow or not, you need 2 minutes to see.

[tubeswell]

… Friedman Dirty Shirley keeps blowing fuses. We have tried swapping the rectifier tube and the power tubes but it is still happening…

Then the short-circuit isn’t likely to be coming from the tubes. Use a limiter and multimeter to test the power supply to see where the short is coming from, starting at the reservoir cap. Do the voltage testing without tubes installed. Suspects may include the power transformer, filter caps (or bias supply filter caps) or SS diodes, or arcing across the output tube socket pins (if the original failure mode was a bad output tube) - I.e., anywhere where there is an electrical short between any current source and the ground return.

[sluckey]

Related to my earlier post about the cost of fuses, my friend's Friedman Dirty Shirley keeps blowing fuses.

Tell us more about blowing fuses. Does the fuse blow everytime you flip the power switch? Or only when you flip the standby switch? Or is this more of a nuisance issue, ie, amp plays fine for hours or days then just blows the fuse.

[pdf64]

It’s a tiny exception, but it’s not safe on some old 100W Marshalls, as the increase in HT voltage all down the chain can exceed rating of caps for the preamp nodes.

I was thinking about this guy, where the HT might rise to 600V unloaded, with the 15uF preamp node cap being a 450 or 500V part. Charging via 18k resistance.
https://el34world.com/charts/Schematics/files/Marshall/Marshall_jtm100_lead_trem_100w_1959t.pdf

[dbishopbliss]

I had my friend bring the amp over today. I used a light bulb limiter to bring it up with both a 60 watt bulb and a 150 watt bulb and the fuse held fine with both. I measured voltages and did not see anything out of the ordinary. Around 340 on pin 5 of the power tubes. Around 200 +- 20 volts on the plates of the preamp tubes. Bias voltage was around -40. I didn't write them all down but nothing stood out.


I took the amp off the limiter and turned on... fine with power switch. As soon as I turn on the standby switch the fuse blows.


So then on a whim, I swapped in a NOS Mullard 5AR4 rectifier instead of the no-name Chinese rectifier that came with the amp. Everything worked fine. Turned it on/off multiple times never blew a fuse.


My friend has several amps that use 5AR4 tubes and the rectifier works fine in all those amps. He has tried swapping in a JJ 5AR4 but that tube blew fuses as well.


Any reason why a NOS Mullard would work but a newer rectifier doesn't?

[sluckey]

My friend has several amps that use 5AR4 tubes and the rectifier works fine in all those amps.

But do all his other 5AR4 tubes work in Shirley?

[dbishopbliss]

But do all his other 5AR4 tubes work in Shirley?

Sorry I wasn't more clear. None of the other 5AR4 work in Shirley. The only one that is working is the Mullard. However, the China and JJ 5AR4's work in his other amps.

[pdf64]

…
I took the amp off the limiter and turned on... fine with power switch. As soon as I turn on the standby switch the fuse blows.
…

I assume it’s a stupid hot switching standby? If so try a different rectifier again, but just ignore standby.

[dbishopbliss]

I assume it’s a stupid hot switching standby? If so try a different rectifier again, but just ignore standby.

That seemed to work... installed Chinese 5AR4, left standby in ON position THEN turned ON the power. Fuse did not blow.


So, seems to be a a inrush current issue. Any way to fix this other than "don't use the standby"?

[pdf64]

It’d be best to move standby to after the reservoir cap.
And / or fit back up silicon diodes in series with the valve rectifier anodes.

[dbishopbliss]

And / or fit back up silicon diodes in series with the valve rectifier anodes.

Will diodes help with inrush? I have them on my builds but I thought they were just backup for tube failure.

[pdf64]

They stop arcs from lasting more than a 1/2 cycle, thereby preventing the valve being eaten up by it, and stopping fuses from being blown.
So yeah, the stress on the valve is still there, but the symptoms disappear.

Hence as a preventative action I think it would be better to move the standby switch, rather than just relying on the diodes. As my feeling is that on their own, they just put a sticking plaster on the issue.

[dbishopbliss]

Hence as a preventative action I think it would be better to move the standby switch, rather than just relying on the diodes. As my feeling is that on their own, they just put a sticking plaster on the issue.

What do you think of installing a 47K 2W resistor across the standby switch per Merlin Blencoe/Valve Wizards recommendation?

[sluckey]

What do you think of installing a 47K 2W resistor across the standby switch per Merlin Blencoe/Valve Wizards recommendation?

I rather just go all the way with a 0Ω across the switch.

[pdf64]

Hence as a preventative action I think it would be better to move the standby switch, rather than just relying on the diodes. As my feeling is that on their own, they just put a sticking plaster on the issue.

What do you think of installing a 47K 2W resistor across the standby switch per Merlin Blencoe/Valve Wizards recommendation?

That’s good, but still move the switch to after the reservoir cap.