183 with no sound and high preamp voltages

[tubey]

Hi all,

I have a #183 copy, it sounded so great until friday night. Suddenly the sound faded away while playing. I opened it up to search for the problem, switched the preamp tubes ( no change ). And when I measure the voltage on the plates of V1, it's 365 volts instead of the 200 volt range I think is normal. there are a few small ceramic caps used in the tone section ( 50v rating ) the 100pf, 220pf, 390pf, 500pf. Could that be the problem ?. Rest of the caps in the amp are high voltage types.
Can anyone give me some advice what to look for?

[ampgeek]

Seems like V1 is not conducting.

Triple check cathode to ground wiring and components.

With amp off and unplugged and V1 pulled, measure the resistance from pins 3 and 8 to ground. It should measure pretty darn close to the value of the cathode resistors at those points.

Good luck!

Dave O.

[glasman]

Check your cathode grounds and your filament voltage. You will get the same result if the heaters aren't lit.

Gary

[tubey]

Dave & Gary, thanks !

I checked the cathodes, everything measured OK ( switched of )
Then I switched the amp on, there was sound for a few seconds, just like yesterday. The power tubes are lit and have 6.4 volts on the heaters. Then I looked at the preamp tubes, no power on the heaters !! All three of them are off. So when I switch the amp on, it works for a few seconds and then the heater of the preamp tubes shuts of. I guess now the heater connection from the first power tube to PI tube is bad ? I did switch the preamp tubes with no change.

Thanks again guys !
Richard

[tubey]

Next problem: Hum

The heater connection at the PI tube was the problem, altough connections look good and shiny, one was loose. Sudden loss of heater is why my sound faded instead of being cut of.
Now I repaired the problem, but got a big fat hum back. Really terrible. When I switch the drive channel on with the switch on the back of the amp, the hum gets even worse. What have I done?
Again I switched preamp tubes, no difference. I did not change anything in the heater wiring, I made sure that it looked the same as when the amp worked perfect.
If you could help me out with some advice again, I would be very grateful.

Richard

[talbany]

Next problem: Hum

The heater connection at the PI tube was the problem, altough connections look good and shiny, one was loose. Sudden loss of heater is why my sound faded instead of being cut of.
Now I repaired the problem, but got a big fat hum back. Really terrible. When I switch the drive channel on with the switch on the back of the amp, the hum gets even worse. What have I done?
Again I switched preamp tubes, no difference. I did not change anything in the heater wiring, I made sure that it looked the same as when the amp worked perfect.
If you could help me out with some advice again, I would be very grateful.

Richard

Put the preamp tubes (that you used last fri) back in it..Did you change anything else while you were troubleshooting the amp?

Tony

[glasman]

Check your heater balance to ground. Should be 3.15 to 3.3Vac to ground on each of the heater wires. (4/5 and 9). Also check your balancing resistors for the heaters. You will have to disconnect the filament wires from the transformer and pull the tubes to check them.

Gary

Next problem: Hum

The heater connection at the PI tube was the problem, altough connections look good and shiny, one was loose. Sudden loss of heater is why my sound faded instead of being cut of.
Now I repaired the problem, but got a big fat hum back. Really terrible. When I switch the drive channel on with the switch on the back of the amp, the hum gets even worse. What have I done?
Again I switched preamp tubes, no difference. I did not change anything in the heater wiring, I made sure that it looked the same as when the amp worked perfect.
If you could help me out with some advice again, I would be very grateful.

Richard

[tubey]

Thanks Gary,
I just measured the voltage between heater and ground. 95 volts ac.
When I check the heaters, there is 6.4 volts on them.
I'm going to follow what you said and disconnect and check the balance resistors. Could those be broken and the reason of the hum and the high voltage to ground?

Hi Tony,
I did not change anything else, I will try to swap those preamp tubes. Thanks !

[glasman]

95VAC ti ground on the heater wires. Somehting is not right there, shouldn't be higher than 3.2Vac if everything is right, you might have a shorted set of windings in the power transformer.

Gary

Thanks Gary,
I just measured the voltage between heater and ground. 95 volts ac.
When I check the heaters, there is 6.4 volts on them.
I'm going to follow what you said and disconnect and check the balance resistors. Could those be broken and the reason of the hum and the high voltage to ground?

Hi Tony,
I did not change anything else, I will try to swap those preamp tubes. Thanks !

[ampgeek]

Is it possible that the PI cathode is touching the heater string?

Take a really close look at the area of most recent repair for a solder blob or wire strand creating a bridge at the PI tube socket tabs.

Good luck!
Dave O.

[tubey]

Gary, Dave & others, thanks !

My amp is working again, the hum is all gone. I'm very happy and grateful for the help I got from you.
It started with a loose heater connection between power tube and PI and this morning I found out that the two 150 ohm resistors to ground were both broke. Infinite resistance, one was even broken in half. I bought some new resistors and the hum was gone. I'm happy that it's not the transformer. I've learned a few things. Those power tubes light up with a crazy amount of heat, so I would not have looked at the heater supply quickly. Also with voltages so high on the preamp tube plates.

So I'm happy that it is all working again. My former amp was a JCM 900, so after I finished this one I was shocked by the great sound. I have to get some fancy power tubes, but I have GE shortplates 1958 that sound very nice. I'm trying to get the drive channel as good as possible, it sounded good with a certain combination of tubes that made the clean channel sound bad. Since I first switched the amp on, after being in playing mode, the amp gives a pop when switched in standby. Is there a trick to get rid of that, or does it just come with the great sound?

Thanks again guys, come over to have a beer, but the Netherlands might be a bit to far for you. Richard

[talbany]

Gary, Dave & others, thanks !

My amp is working again, the hum is all gone. I'm very happy and grateful for the help I got from you.
It started with a loose heater connection between power tube and PI and this morning I found out that the two 150 ohm resistors to ground were both broke. Infinite resistance, one was even broken in half. I bought some new resistors and the hum was gone. I'm happy that it's not the transformer. I've learned a few things. Those power tubes light up with a crazy amount of heat, so I would not have looked at the heater supply quickly. Also with voltages so high on the preamp tube plates.

So I'm happy that it is all working again. My former amp was a JCM 900, so after I finished this one I was shocked by the great sound. I have to get some fancy power tubes, but I have GE shortplates 1958 that sound very nice. I'm trying to get the drive channel as good as possible, it sounded good with a certain combination of tubes that made the clean channel sound bad. Since I first switched the amp on, after being in playing mode, the amp gives a pop when switched in standby. Is there a trick to get rid of that, or does it just come with the great sound?

Thanks again guys, come over to have a beer, but the Netherlands might be a bit to far for you. Richard

This seems rather strange to me that a loose connection on the PI socket would take out both balancing resistors,since there is little current draw from the preamp tubes..(stranger things have happened),but keep an eye on it and if it goes again you may have other problems (output tubes on the way out) ..Also keep an eye on the PT and see how hot it gets before you return it to the stage..Glad you figured it out..

BTW.. This is the main reason why it's a good idea to use a magnifying glass to solder each connection just to make sure the solder adhere's (flows)properly..

From Dumbles interview..

An extreme amount of attention is paid to every connection.

Tony

[CHIP]

To curtail the pop you can put a .001 uf ceramic cap, 1k rating, across the standby switch lugs. See if that does the trick.

[ampgeek]

Right on Richard! Glad you got it up and going.

I don't think that I helped at all...but...I was was rooting for your success all along!

Cheers,
Dave O.

[glasman]

The only time I have seen balancing resistor open up is where one of the output tubes decided to take a dump and short out. Usually takes the balancing resistors and sometimes the screen resistors.

Gary