1959 SuperLead problems

[cos000]

Hello
I have a little problem with my amp.
It sounded ok since yesterday...
I let it worm 5-10 minutes then, when I engaged the standby switch I saw a flash from one of the power tubes and a "boom" from the speakers that could throw down the house... After that a buzzing.
I switched it immediately off...
Why? It sounded ok till the day before when I switched it off...
So today I took it out from the cab and powered it on without power tubes.
I began to measure the voltages: they are about all a little higher than the instruction's ones. Only few are a little lower...
The one that captured my attention was the voltage of pin 6 on PI...
It should be like on pin 1: 229-230V...
Instead it's 330V... Too high...
I thought it could be because there aren't the EL34s but by thinking a little bit that wouldn't matter...
What could it be?

I hope you can help me

[neikeel]

Hello
I have a little problem with my amp.
It sounded ok since yesterday...
I let it worm 5-10 minutes then, when I engaged the standby switch I saw a flash from one of the power tubes and a "boom" from the speakers that could throw down the house... After that a buzzing.
I switched it immediately off...
Why? It sounded ok till the day before when I switched it off...
So today I took it out from the cab and powered it on without power tubes.
I began to measure the voltages: they are about all a little higher than the instruction's ones. Only few are a little lower...
The one that captured my attention was the voltage of pin 6 on PI...
It should be like on pin 1: 229-230V...
Instead it's 330V... Too high...
I thought it could be because there aren't the EL34s but by thinking a little bit that wouldn't matter...
What could it be?

Doubt that is the issue. What are the EL34 socket voltages like? Did you get a receipt with the tubes?

I hope you can help me

[cos000]

Hello
I have a little problem with my amp.
It sounded ok since yesterday...
I let it worm 5-10 minutes then, when I engaged the standby switch I saw a flash from one of the power tubes and a "boom" from the speakers that could throw down the house... After that a buzzing.
I switched it immediately off...
Why? It sounded ok till the day before when I switched it off...
So today I took it out from the cab and powered it on without power tubes.
I began to measure the voltages: they are about all a little higher than the instruction's ones. Only few are a little lower...
The one that captured my attention was the voltage of pin 6 on PI...
It should be like on pin 1: 229-230V...
Instead it's 330V... Too high...
I thought it could be because there aren't the EL34s but by thinking a little bit that wouldn't matter...
What could it be?

Doubt that is the issue. What are the EL34 socket voltages like? Did you get a receipt with the tubes?

I hope you can help me

Hello
Tubes are a matched quad JJ.
The voltages on EL34s are around 494-495V without tubes. Bias goes from -50 to -39V. With tubes they went down to 475-480 if I remember correct.
I have 230VAC and I use it on the 240V winding so I think the voltages would be a little bit lower...

[cos000]

No idea?
Could be simply bad EL34s?
Is there a test I can make?

[neikeel]

No idea?
Could be simply bad EL34s?
Is there a test I can make?

So you have 490v or so on pin3 and same or a little lower on pin 6 of all of your output valves?

None of the valves look toasted?

Usual turn of events is an output tube goes pop, it shorts transiently and then either takes the HT fuse out or takes out one of the screen resistors that then goes open circuit and then the HT fuse goes (assuming that it is the correct value ).
You replace the HT fuse, check all the screen resistors are working to correct spec and you replace any dead ones (the dead tube will be the one on that socket. If you are flush with cash you buy a new set of matched and burned in tubes and are good to go. If you are feeling tight you replace the duff tube with one form the same source.

If that does not work then let us know!

[cos000]

So you have 490v or so on pin3 and same or a little lower on pin 6 of all of your output valves?

None of the valves look toasted?

Usual turn of events is an output tube goes pop, it shorts transiently and then either takes the HT fuse out or takes out one of the screen resistors that then goes open circuit and then the HT fuse goes (assuming that it is the correct value ).
You replace the HT fuse, check all the screen resistors are working to correct spec and you replace any dead ones (the dead tube will be the one on that socket. If you are flush with cash you buy a new set of matched and burned in tubes and are good to go. If you are feeling tight you replace the duff tube with one form the same source.

If that does not work then let us know!

It's 494V on pin 3 and 6 with no EL34 inserted.
They looks absolutely like out of the box...

HT fuse didn't blow (it's a T1A slow blow) I measured screen grid resistors and they are 1K, control grid ones are 5,6K...

All seems ok except for that 330V on pin 6 of v3...

I'm gonna build a dummy load so I don't throw down the house...

[Carbia]

Try to play with only 2 tubes.

Discard the suspicious tube and the opposite one (if the tube is the nº1, pull the tube nº4, if it's the 2, pull the 3)

Set the impedance to half (if your speaker is 16ohm, run the amp at 8ohm to match the impedance for two tubes)

Test the amp. If it runs fine, change the tubes to another socket. (if you're using sockets 1 and 4, plug into 2 and 3).
If it runs fine again, is a bad tube and the rest of the amp is ok.

[cos000]

Try to play with only 2 tubes.

Discard the suspicious tube and the opposite one (if the tube is the nº1, pull the tube nº4, if it's the 2, pull the 3)

Set the impedance to half (if your speaker is 16ohm, run the amp at 8ohm to match the impedance for two tubes)

Test the amp. If it runs fine, change the tubes to another socket. (if you're using sockets 1 and 4, plug into 2 and 3).
If it runs fine again, is a bad tube and the rest of the amp is ok.

I'll try. If I use a 8ohm cab then I set the amp on 4ohm. Right?

[Carbia]

yes

[cos000]

Ok, today I ended to build a 200W 8Ohm resistive dummy load so I connected the amp on it.
I checked all the tubes and they looked perfect.
So I put them on, let them warm up and switched on the amp. No flashes or explosions. Well.
I begin to measure:
B+ 474V... Oh, yes I'm in the kitchen that has lower voltage...
BIAS voltage on the 1Ohm resistors 6.2, 5.9, 8.1mV, 34V... 34V!!!! Switch all off and swipe tubes: 34V on same position. So tubes aren't the problem.
So I tried with only 2 tubes: on outside positions ok, o inside ones 34V on one tube.
I Switched all off and discharged the caps.
I measured the resistors: 0.9, 1.0, 0.9, infinite...
Aaahhh so that's the problem: one 1Ohm resistor is interrupted...
But why? It doesn't look cooked outside... It's clean like the others...

[Carbia]

who knows... change it and try again.

MAybe it's only a wrong production one.

[neikeel]

Ah so is it a build (with1 ohm resistors for bias) or an original with them added?

These little details all add up whe trying to solve problems 100s of miles away .

The 1 ohm resistors act as a fuse when the current goes too high (careful of that tube!)

[cos000]

Ah so is it a build (with1 ohm resistors for bias) or an original with them added?

These little details all add up whe trying to solve problems 100s of miles away .

The 1 ohm resistors act as a fuse when the current goes too high (careful of that tube!)

It's built from the metroamp instructions.
With the dummy load I tried all the tubes (2 at a time) and throw the amp to full power (the resistors of the load sounded) and no problem of spark, red plating, pops, crackling....

[cos000]

I changed the 1ohm resistor of V5 and the old one had a little hole on it... I checked for shorts around but I didn't find anything.
I put on tubes and connected to the load (maybe was only a bad resistor...).
I let it warm few minutes and set the bias voltage to -50V.
When I engaged the standby switch another big flash from V5 and smell of burned component came from the socket, probably from the resistor...
I shutted down everything and drained the caps (the T1A HV fuse was blown), checked the OT primary winding resistance (32ohm seems good, no burned OT), and I measured the 1ohm resistos. The one on V5 was burnt. So the problem is V5.
I put on another 1ohm resistor (I bought 20 of them) and today I'll check all tubes except V5 on all the sockets (obviously 2 per time). I think V6 and V7 is the best choice because they have the same voltage drop of 6mV with bias to -50V. V4 has 8mV...

[cos000]

All seems to work... I run it on 2 tubes and I like it very much... Nice sound with the Lar/Mar PPIMV also at low volume: it's not like any MV system...
I wanted to do some measures on it so I swithced it on with two tubes and connected to the 8ohm resistive load.
On input I connected a 1kHz sine generator with 1VPP signal and I have seen that, with PPIMV disabled, with volume at 9 hours it began the distortion that in few degrees of turning the pot goes to max value with an output signal of 70VPP...
Is it normal?
Someone who did measures can confirm it?