Dropping string question

[scotto]

124 based 50 watt clone; I noticed that my clean headroom had gone away and checked the V1 pin voltage and it was 225 volts. I had previously adjusted my string such that it was 185 volts and everything sounded very good fo a long time. pin 1 on V2 was also quite high in this recent measure. What might cause this?

[trane34]

Did you change preamp tubes?

[scotto]

Nope. I should have mentioned that.

[Structo]

Tubes should always be the first suspect when any working amp with history suddenly has a problem.

Check tubes then report back.

[scotto]

Thanks Tom. Did that. it brought down the values, especially in V1. Do these voltages go up as tubes wear? Here's some history; String started life with 3k3, 22k and 2k2. I adjusted the string down by putting power pots in place of the resistors and dialing in to where Funk told me per my playing style (light touch,legato, not too concerned about loss of ping). Then I put in resitors to match the dialed in values and wound up with 6.7k,18.5k and 6.2. This gave me 185 and 192 at pins 1 for V1 and V2, and 268 at pin1 V3. Sounded,and played wonderful for quite a while (over a year, but I don't play this amp much). Then the strange distortion kicked in and i noticed the rise in voltages. put a 21k resistor in the last string position and it came down to about 209 at V1. This still seems high and I don't know/understand what changed. When I just swapped pre tubes V1 came down to 181, V2 is 206 and V3 is 300. This seems high to me given my string values.

[talbany]

124 based 50 watt clone; I noticed that my clean headroom had gone away and checked the V1 pin voltage and it was 225 volts. I had previously adjusted my string such that it was 185 volts and everything sounded very good fo a long time. pin 1 on V2 was also quite high in this recent measure. What might cause this?

This is strange because in theory a 12AX running at 225V will have more headroom than an AX running at 185 DCv.. IMO this should be barely noticeable depending on the tubes mu trans conductance rating..(12AX usually around 1600)..Where you notice it the most is in the tone and feel..Lower plate voltages won't be as bright as well as a loose low end and a bit more organic feeling//,but I don't really notice to much in the way of actual headroom..Could be that tube took a dump!!..If you want to take it a bit further check the voltage drop across the plate resistor..100k generally drops around 100v (this set's your rails) and also check your cathode voltage should be around 1-2V range..If you you replace it with another tube and still sounds the same (loss of headroom..Your plate resistor might be breaking down) and on the way out..

Pre amp tubes can have as much as a 10-12V swing..depending on how they bias up..

values and wound up with 6.7k,18.5k and 6.2.

Is that a 6.2K or ohms?..If it's a 6.2k that string you have (if were talking 450V B+) that string would make sense for you to get around 185 on V1 and 210 on V2 (your PI seems a little high though) I can see that being the case.. BUT not 225 there..What's you B+....When the whole amp shifts always check your AC line voltage first..


Good Luck

Tony

[scotto]

Thanks Tony. I'll check some more. Not that savy around electronics. I'm a mechanical engineer so not much knowledge beyond Ohm's law, F=MA and you can't push on a rope. I thought higher voltage was causing the strange distortion, even on clean.

[scotto]

6.2k. Sorry for the confusion. I have 1.1 and .95 volts on cathode pins 3 and 8. Still checking other stuff you mentioned...

[scotto]

B+ is 451 and line voltage is 123. BTW the distortion went away when I changed that last string resistor. No idea why. Still confused as to why the voltage would change that much before I touched anything. Maybe a bad tube as mentioned, but things still seem high to me with that last resistor change. Is 300 too high for a PI?

[talbany]

6.2k. Sorry for the confusion. I have 1.1 and .95 volts on cathode pins 3 and 8. Still checking other stuff you mentioned...

.95 seems low.. on a high plate (220k/150k) with respected cathodes and a plate voltage V1 around 200v... You should be getting around 1.7 to 2V..V1b should be around 2v....with a B+ feed to V1 around 330v you should be getting around 220v drop across both plate resistors (maybe a bit less across the 150k)..If memory serves me??

Tony

[talbany]

B+ is 451 and line voltage is 123. BTW the distortion went away when I changed that last string resistor. No idea why. Still confused as to why the voltage would change that much before I touched anything. Maybe a bad tube as mentioned, but things still seem high to me with that last resistor change. Is 300 too high for a PI?

300v on the PI is Dumble spec.. No telling really about the Tube and resistor..If that tube was drawing too much current it could have dragged down the entire preamp..The bad resistor could have done the same..no tellin from my screen!!

Tony

[scotto]

Thanks Tony! Really appreciate the help. I can ride with it as is so long as I'm not hurting anything. It sounds pretty good now. Just curious about cause and effect. On another note, I had found a set of 3 RCA blackplates in our abandoned electronics lab at work a couple of years back. 2 of them matched well so I put them in. Now one of them is runing at 45ma and the other around 35. Any issue with that? They still sound very good. Don't remember where they were originally, but they were within a couple of milliamps of eachother. The third tube was way higher, around 55.

[talbany]

Thanks Tony! Really appreciate the help. I can ride with it as is so long as I'm not hurting anything. It sounds pretty good now. Just curious about cause and effect. On another note, I had found a set of 3 RCA blackplates in our abandoned electronics lab at work a couple of years back. 2 of them matched well so I put them in. Now one of them is runing at 45ma and the other around 35. Any issue with that? They still sound very good. Don't remember where they were originally, but they were within a couple of milliamps of eachother. The third tube was way higher, around 55.

For me once they get 10ma apart I usually change them.. That's when I start to hear things I don't like.. It's not going to damage the amp and if you like the tone,keep an eye on them!!

Dumble style amps to me sound best when everything is balanced!!
Glad we got it going again...ROCK ON!

Tony

[scotto]

Thanks again!!