Super Twin Reverb blowing fuse

[yann]

Dear MEF members,

I'm currently working on a Super Twin Reverb which came to me because of blowing fuses.
The first thing I noticed is that instead of having 6x 6L6, there were 5 x 6L6 and 1 EL34

Diode rectifiers and main power caps checked good despite being stock.
All screen resistors checked ok, I have noticed one has been replaced in the past.

I have also measured the DC resistance between the OT primaries and I found nearly 25 ohms on each side.
It sounds a bit low for me but considering the amp is supposed to run with 6 x 6L6 maybe I don't have to worry.

Are these values sounding ok for you as well ?

If so, then I would be ok to slowly power on the amp (without tubes) on Variac + Bulb limiter while checking voltages.

[Leo_Gnardo]

Get that EL34 out of there, it clearly does not belong. 25 ohms on each leg of a large OT is not unusual. Some 100W Marshall OT's typically measure 15 ohms more or less. All it means is the primary winding is made with larger wire than usual, should be able to handle more current.

With the EL34 out of the lineup, yes slowly power up with variac & bulb limiter.

[yann]

Thanks Leo, I will proceed this way and keep the thread updated with next findings :-)

[nsubulysses]

Most of the time when an amp blows a fuse it's because of a shorted power tube.
If it is still blowing fuses (lighting the lamp of the bulb limiter brightly) when you turn it back on, you can remove one power tube at a time to see if the light dims and that is your shorted tube.

[fries]

Replace all original electrolytic caps even if they test OK (unless you have a proper HV leakage tester), capacitance testers don't test for leakage.

[Jazz P Bass]

It may be less stressful to the amp if you remove all of the power tubes & insert them one at a time to find the bad one.

[yann]

Thanks for your suggestions. I think I will tell the customer to replace all 6 6L6 anyway as one is missing (which has been swapped with an EL34...). So the sextet will not be matched anymore.
I have also noticed the 5 remaining 6L6s are all the same but one do have the getter completely disappeared. Tubes are stock, as well as caps. I will suggest him to replace at least the tubes.
Big caps tested well on ESR meter, did not had the time to tests other electrolytic caps.

[The Dude]

......capacitance testers don't test for leakage.

Mine does.

[fries]

ESR Meters do not test leakage... The easiest way to test leakage would be to let the amp sit on (with HV/Standby switch on) for 10/20 minutes and if the caps start to warm up then they are toast.

[fries]

OK, most capacitance meters don't test leakage

[yann]

Some update :

Variac + 46W bulb light limiter :
* without any tube in the chassis, got 340V on B+ (expecting around 500V), the bulb is lighting but not at full brightness.
* with preamp tubes added in chassis, got 280V on B+, the bulb is lighting more than previously.
* current draw on primary side in previous testcase is 140mA.

So I decided to switch to a 150W bulb light limiter :
* with preamp tubes, got 450V on B+, 270mA current drawing on primary side, bulb light with low brightness.
* added 2 brand new 6L6s, 407V on B+.

Then removed the bulb limiter and the last testcase is showing 525V with the 2 6L6s which sound good to me.

I have tested at low volume (because I only have 2 power tubes, and a load of 8 ohms instead of 4 ohms). The amp sounds pretty good in clean (distortion mode is horrible but it seems to be expected with a Super Twin Reverb).

As all caps are stock, I did not test directly from power line to avoid stressing caps when switching from standby mode.
I did not tests the old 6L6 because I want to test it with tube meter before plug in it back in the amp.

So I will tell the customer he needs to buy a new sextet of 6L6 and strongly recommand recap job to be done at the same time. Will do final bias check and adjustment if needed with the new set of tubes.
If you have more suggestion from check point of view, don't hesitate to let me know :-)

[yann]

Other question : for the recap jobs, I have some weird caps values I would like to replace as the following :

* 80uF / 75V ==> to be replaced by 100uF / 100V (this is for BIAS voltage filtering), I guess there is no issue with such a change.

* 750uF 3V ==> to be replaced by 1000uF / 63V, this one is V1 cathode bypass cap. I was wondering how much that would change the tone ? A bit more bass ?

[Leo_Gnardo]

Other question : for the recap jobs, I have some weird caps values I would like to replace as the following :

* 80uF / 75V ==> to be replaced by 100uF / 100V (this is for BIAS voltage filtering), I guess there is no issue with such a change.

* 750uF 3V ==> to be replaced by 1000uF / 63V, this one is V1 cathode bypass cap. I was wondering how much that would change the tone ? A bit more bass ?

The first one, sure no problem. I've used 47 uF there as well. Plenty of filtering. You just don't want to introduce hum by having insufficiently filtered bias voltage.

Second: why they bothered to design in this huge cap is beyond understanding. Any "extra bass" is incremental, iow not such a difference that anyone could hear it. The old standard 25 (or 22) uF cap is plenty. If you opt for overkill cap value here, 100 uF is way more than enough.

[yann]

Fair enough, let's use a 22uF cap. As you said, 22uF is pretty much common value here, I was also surprised when I read the schematic, but when I saw the big cap, they really did it :-)

[Leo_Gnardo]

Fair enough, let's use a 22uF cap. As you said, 22uF is pretty much common value here, I was also surprised when I read the schematic, but when I saw the big cap, they really did it :-)

Designed & built in the age of excess. What's the point when the time constant is measured in minutes? Clearly overdone.