Blues Deville low distorted sound

[Randall]

Blues Deville PR247 is in for a dirty sounding, and seemingly not loud enough sound. First I replaced the 12AT7 he had in the PI to 12AX7, and things got a little better. Tried different cab, 6L6s, probed around to no avail. Then I went from a GK bass head preamp into the power amp in jack, still breaks up and doesn't sound loud enough. New tubes are idling at 41 and 39 mA with 492 plate volts. Screens are a few volts below that. I have -51v on the grids. Feeding a sine wave I only get 17vac max into 8 ohms on my NOT true RMS meter, but by then the wave is very spiked.

Output transformer perhaps? If so can I use a 4 ohm 35watt OT for testing?

https://www.thetubestore.com/lib/the...-Schematic.pdf

[nosaj]

Blues Deville PR247 is in for a dirty sounding, and seemingly not loud enough sound. First I replaced the 12AT7 he had in the PI to 12AX7, and things got a little better. Tried different cab, 6L6s, probed around to no avail. Then I went from a GK bass head preamp into the power amp in jack, still breaks up and doesn't sound loud enough. New tubes are idling at 41 and 39 mA with 492 plate volts. Screens are a few volts below that. I have -51v on the grids. Feeding a sine wave I only get 17vac max into 8 ohms on my NOT true RMS meter, but by then the wave is very spiked.

Output transformer perhaps? If so can I use a 4 ohm 35watt OT for testing?

https://www.thetubestore.com/lib/the...-Schematic.pdf

Check those resistors around the phase invertor for being open.

nosaj

[Tone Meister]

Check the PI plate resistors R51 (91k) and R52 (100k) on the tube PCB. Both are 1/2 watt.

[Tone Meister]

If not those, next check R55 and R56 (1.5K) also on the tube PCB.

[Randall]

Well, I'll be darned, that's exactly what it was, R52! Now that's some magic sh!t right there! Is this a common thing with these amps?

Thank you guys so much for that deadly aim, I really didn't want a long and drawn out battle with this thing.

[nosaj]

Well, I'll be darned, that's exactly what it was, R52! Now that's some magic sh!t right there! Is this a common thing with these amps?

Thank you guys so much for that deadly aim, I really didn't want a long and drawn out battle with this thing.

no, just been there before with a traynor amp and went round early in my training. Some things you just remember when they bite you real good.
For me the hints were, power amp section in low output and the power tubes looked ok. So right before the power tubes is the invertor which splits the signal = low power 1/2 signal

nosaj
nosaj

[The Dude]

I have seen a few of the earlier built Deville/Hot Rod/Deluxe/Whatever that had only 1/4W resistors installed there. Not sure if it was just wrong parts or a production change, but I replaced them with 1/2W when I found them open.

[Leo_Gnardo]

Is this a common thing with these amps?

Yes. Now you know a common trouble spot to check in future encounters with them. I usually replace with 1W parts.

[Tone Meister]

Well, I'll be darned, that's exactly what it was, R52! Now that's some magic sh!t right there! Is this a common thing with these amps?

Thank you guys so much for that deadly aim, I really didn't want a long and drawn out battle with this thing.

Change all four while you're in there.

[g1]

Yes, agree with Leo it is fairly common in this series. If complaint has to do with weak or distorted the PI resistors are the first thing I check with these.
Also, a bad power tube screen resistor will create a similar symptom, for any make or model.

[Tone Meister]

Yes, agree with Leo it is fairly common in this series. If complaint has to do with weak or distorted the PI resistors are the first thing I check with these.
Also, a bad power tube screen resistor will create a similar symptom, for any make or model.

Yes, R55 and R56

[fingerpicking]

every time I had this type of a problem, about 3 times in 15 years, it was the output transformer, but that is my experience

[fingerpicking]

''every time I had this type of a problem, about 3 times in 15 years, it was the output transformer, but that is my experience.''

sorry, I take that back. Now that I remember, although in all these cases the output transformer WAS bad, there was low output but the sound was NOT distorted.

It's been a while.

[g1]

Yes, R55 and R56

Those are the control grid resistors, the screen grid R's, R57 and R58 were the ones I meant.

Not to argue though, if you have seen multiple failures of the control grid R's, I'll take your word for it. Just have never seen it myself.

[Tone Meister]

Those are the control grid resistors, the screen grid R's, R57 and R58 were the ones I meant.

Not to argue though, if you have seen multiple failures of the control grid R's, I'll take your word for it. Just have never seen it myself.

Indeed I have seen it a few times, prevents the power tubes from drawing current, thus low, distorted output when one is open.