I'm working on a friends Fender BASSMAN 50 (Schematic here : http://ampwares.com/schematics/bassman_aa165.pdf) and it is running what I believe is a bit hot.
B+ is 465V
Grid bias is -45V
There is no bias adjustment, but there is a balance pot.
I can adjust it so both tubes are just slightly red at the seam of the fin (best seen with the lights out).
I'm considering increasing the value of the 15K resistor from the wiper of the balance pot to ground. This will increase the bias and reduce the cathode current.
So my basic question is "Is this how guitar amps run (which contributes to short tube life)? Or is the amp running too hot.?"
B+ is 465V
Grid bias is -45V
There is no bias adjustment, but there is a balance pot.
I can adjust it so both tubes are just slightly red at the seam of the fin (best seen with the lights out).
I'm considering increasing the value of the 15K resistor from the wiper of the balance pot to ground. This will increase the bias and reduce the cathode current.
So my basic question is "Is this how guitar amps run (which contributes to short tube life)? Or is the amp running too hot.?"
Gimp,
Are you sure the schematic is for that amp? If it is then the bias is adjustable at that pot. It's not a "balance" pot. Are you getting the full -45v? Can you not adjust it higher than that? May have a bad filter.
Is the bias rectifier a diode or is it a selenium type?
Are you sure the schematic is for that amp? If it is then the bias is adjustable at that pot. It's not a "balance" pot. Are you getting the full -45v? Can you not adjust it higher than that? May have a bad filter.
Is the bias rectifier a diode or is it a selenium type?
Well Mr. Gimp, you're throwing us a curve. If the schematic provided is correct, there is a bias adjustment but no balance pot. And the 15K resistor is not from the center of the pot, but from the low end.
With the plate voltage stated, and the negative grid voltage given, a 6L6GC should not red plate. If they do, I would guess that they are out of specification. What kind of tubes are they and how do they test? With today's poor quality tubes one can expect "OOT". So yes, raise the 15K to 20K or so and see if that helps. And no, I don't think amplifiers are designed to run their tubes excessively hot.
With the plate voltage stated, and the negative grid voltage given, a 6L6GC should not red plate. If they do, I would guess that they are out of specification. What kind of tubes are they and how do they test? With today's poor quality tubes one can expect "OOT". So yes, raise the 15K to 20K or so and see if that helps. And no, I don't think amplifiers are designed to run their tubes excessively hot.
Amplifiers and aging capacitors.
Hi
I studied the schematic, and if it appears as if the adjustment potentiometer in the grid circuit is a "balance pot" there is something wrong in the circuit. I suggest that one or both coupling capacitors to the output stage are leaking so that the grid voltages of the output stage are not what you think they are, but something much more positive.
In aging amplifiers the first thing to check are those old coupling capacitors if they leak, which they often do, replace them with new and more modern caps. I suggest a 630V rating, same value, maybe a 1000V rating to be on the safe side.
Hans J Weedon
Hi
I studied the schematic, and if it appears as if the adjustment potentiometer in the grid circuit is a "balance pot" there is something wrong in the circuit. I suggest that one or both coupling capacitors to the output stage are leaking so that the grid voltages of the output stage are not what you think they are, but something much more positive.
In aging amplifiers the first thing to check are those old coupling capacitors if they leak, which they often do, replace them with new and more modern caps. I suggest a 630V rating, same value, maybe a 1000V rating to be on the safe side.
Hans J Weedon
All guitar amp threads belong in Instruments and Amplifiers so I'll move it there. Plenty of troubleshooting horse power over there too.. 😀
Oops, sorry. Wrong version of the schematic (amp is from 1962). Try this one:
http://ampwares.com/schematics/bassman_aa270.pdf
The 15K resistor goes from the wiper on the pot to the pot body, so it is to gnd.
Rectifier appears to be silicon not selenium (given that all selenium I've e ver seen were stack type).
I guess one option is to change to the other bias adjustment circuit, although I hate to make more changes than necessary.
Tubes are 6P3S (multiple tubes trying to select a pair), 6P3S-EB and TAD 6L6GC. That the 6P3S red-plated is not surprising. I was surprised that the 6P3S-EB and TAD tubes did the same.
All tubes are new as the tubes that were in the amp were damaged (broken bases) and I did not trust using them. When the amp was purchased, it was turned on to demo it and smoke came out.
I suspect the smoke was the AL electrolytic caps venting, as three had done so. I replaced all caps before powering up the unit (light bulb test).
http://ampwares.com/schematics/bassman_aa270.pdf
The 15K resistor goes from the wiper on the pot to the pot body, so it is to gnd.
Rectifier appears to be silicon not selenium (given that all selenium I've e ver seen were stack type).
I guess one option is to change to the other bias adjustment circuit, although I hate to make more changes than necessary.
Tubes are 6P3S (multiple tubes trying to select a pair), 6P3S-EB and TAD 6L6GC. That the 6P3S red-plated is not surprising. I was surprised that the 6P3S-EB and TAD tubes did the same.
All tubes are new as the tubes that were in the amp were damaged (broken bases) and I did not trust using them. When the amp was purchased, it was turned on to demo it and smoke came out.
I suspect the smoke was the AL electrolytic caps venting, as three had done so. I replaced all caps before powering up the unit (light bulb test).
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Man, is this amp overbiased!
I added a 10K pot in series with the 15K resistor to gnd.
I dug out my bias adjustment socket and even at the minimum setting (-45V) it is still over 50mA per tube!
I'll crank it down below 40mA/Tube and let him try it and see how he likes it. (40mA is as low as I can go without changing something else.)
I added a 10K pot in series with the 15K resistor to gnd.
I dug out my bias adjustment socket and even at the minimum setting (-45V) it is still over 50mA per tube!
I'll crank it down below 40mA/Tube and let him try it and see how he likes it. (40mA is as low as I can go without changing something else.)
Hi with the new and corrected schematic, some parts must be missing there is over 100% negative feedback because there is not an attenuator to gnd in the feedback path. If the amplifier is really built like the schematic shows, I believe the reason for the "over-biased" condition is that the output stage is oscillating at supersonic frequencies. An oscillating amplifier does what it wants to do relatively independently of adjustments. Please either investigate what is wrong with the schematic, or correct the circuit so that the amplifier stops oscillating, and then adjust the balance or the bias condition.
I will be glad to estimate a bias-point based on circuit parameters for you. The proper way to bias a high quality amplifier is to set the bias at about 75% of maximum spec-sheet dissipation in the anode when the amplifier is idling. This is easily determined from the manufactures data sheet. Overbiasing the output stage of Push-Pull amplifiers does not buy you anything except repeated trips to the tube-store for new tubes. Personally I like to bias tubes like the 6L6, EL34, KT66, KT88, or 6550 at 1/2 the spec sheet maximum allowable dissipation. The bias point has very little to do with the amplifier performance in a push-pull amplifier. In a single ended output stage, however, you gain significant dynamic range by biasing the tubes close to maximum.
Hans J Weedon
I will be glad to estimate a bias-point based on circuit parameters for you. The proper way to bias a high quality amplifier is to set the bias at about 75% of maximum spec-sheet dissipation in the anode when the amplifier is idling. This is easily determined from the manufactures data sheet. Overbiasing the output stage of Push-Pull amplifiers does not buy you anything except repeated trips to the tube-store for new tubes. Personally I like to bias tubes like the 6L6, EL34, KT66, KT88, or 6550 at 1/2 the spec sheet maximum allowable dissipation. The bias point has very little to do with the amplifier performance in a push-pull amplifier. In a single ended output stage, however, you gain significant dynamic range by biasing the tubes close to maximum.
Hans J Weedon
Thanks Hans,
I'll check it out.
I was trying to adjust to 38mA which should be about 75% of where it was with the plates starting to glow.
The RCA datasheet shows 50W out in class AB1 at 450V B+ (close) with 95mA per pair (also close), but at -30V bias. So the bias certainly does look like it is off.
Ill check for the potential oscillation.
I'll check it out.
I was trying to adjust to 38mA which should be about 75% of where it was with the plates starting to glow.
The RCA datasheet shows 50W out in class AB1 at 450V B+ (close) with 95mA per pair (also close), but at -30V bias. So the bias certainly does look like it is off.
Ill check for the potential oscillation.
Hi
While you are checking: The plate current in a pentode is more a function of the potentials on the two grids rather than the plate voltage. The mu of the screen-grid to the control grid is about 8. That means that to a first approximation if you divide the screen grid voltage by 8 and apply that negative voltage to the control grid that should be the voltage that should cut the anode current to zero. Since the tube is somewhat non linear the anode current will only drop to like 10% of max, about 5 to 10mA or so.
Good luck.
Hans J Weedon
While you are checking: The plate current in a pentode is more a function of the potentials on the two grids rather than the plate voltage. The mu of the screen-grid to the control grid is about 8. That means that to a first approximation if you divide the screen grid voltage by 8 and apply that negative voltage to the control grid that should be the voltage that should cut the anode current to zero. Since the tube is somewhat non linear the anode current will only drop to like 10% of max, about 5 to 10mA or so.
Good luck.
Hans J Weedon
No oscillation.
I suspect this is where it is designed to operate.
The schematic shows Va=446V, and Vg2=430V at the 470R resistor so with the increase in line voltage G2 is at 450V.
Looking at the RCA datasheet with the anode at 450V, they have Vg2 at 350 with Ia=95ma(2 tubes) at Vg1=-30V and Vg2=400 with Ia=116mA (2 tubes) at Vg1=-37V.
Extrapolation for Vg2=450 is certainly going to require g1 at -45V or more to get Ia down below 45mA.
I suspect this is where it is designed to operate.
The schematic shows Va=446V, and Vg2=430V at the 470R resistor so with the increase in line voltage G2 is at 450V.
Looking at the RCA datasheet with the anode at 450V, they have Vg2 at 350 with Ia=95ma(2 tubes) at Vg1=-30V and Vg2=400 with Ia=116mA (2 tubes) at Vg1=-37V.
Extrapolation for Vg2=450 is certainly going to require g1 at -45V or more to get Ia down below 45mA.
Hi "Gimp"
It just occurred to me, something my father told me way back when, probably 1950 or so: pentodes and beam pentodes tend to burn out and melt the screen grids if they are overloaded without a proper load in the plate circuit. Melted and destroyed screen grid wires would not control the plate current properly and you would not be able to turn the bias on the output power-tubes off enough to stop the plate-circuit from overloading.
If you ever got the plate to glow red, the screen-grid would have been white-hot and would have been melting. The grids were made of nickel wire which melts somewhere in the 1600 deg C region. The beam pentodes were carefully assembled so that the screen grid was in the electrostatic shadow of the control grid. If that alignment was ever off the screen would catch the beam and dissipate like "the crazy" and most certainly melt.
I suggest that because you have had the plate hot enough to glow red, the screen grid has already been damaged. With a damaged screen grid the control grid can no longer control the tube current very well. You may actually already have output power-tubes that are damaged and do not work correctly.
My suggestion is to turn the tube bias "way low" remove the power tubes and measure the voltages of the empty sockets from those measurements you should be able to tell if the bias conditions of the amp are such that the amp even has a chance of working correctly.
I have been doing this sort of amplifier stuff since about 1948, so I date myself.
Good Luck
Hans J Weedon
It just occurred to me, something my father told me way back when, probably 1950 or so: pentodes and beam pentodes tend to burn out and melt the screen grids if they are overloaded without a proper load in the plate circuit. Melted and destroyed screen grid wires would not control the plate current properly and you would not be able to turn the bias on the output power-tubes off enough to stop the plate-circuit from overloading.
If you ever got the plate to glow red, the screen-grid would have been white-hot and would have been melting. The grids were made of nickel wire which melts somewhere in the 1600 deg C region. The beam pentodes were carefully assembled so that the screen grid was in the electrostatic shadow of the control grid. If that alignment was ever off the screen would catch the beam and dissipate like "the crazy" and most certainly melt.
I suggest that because you have had the plate hot enough to glow red, the screen grid has already been damaged. With a damaged screen grid the control grid can no longer control the tube current very well. You may actually already have output power-tubes that are damaged and do not work correctly.
My suggestion is to turn the tube bias "way low" remove the power tubes and measure the voltages of the empty sockets from those measurements you should be able to tell if the bias conditions of the amp are such that the amp even has a chance of working correctly.
I have been doing this sort of amplifier stuff since about 1948, so I date myself.
Good Luck
Hans J Weedon
Hans,
Yes, without a proper plate load the screen acts as a plate. However, this is not the case for the amp in question.
I have abused many tetrodes and pentodes without warping the screen grid sufficiently to change bias conditions. Given that the tube is operated with a proper anode load, short term over-dissipation is not extremely detrimental to tube operation in my experience.
What I experienced is very difficult to see in ambient light conditions and requires turning off the lights to see it well. It is confined to the junction of the radiating fin and body of the plate. The attached picture is of a 6P1P with worst over-dissipation than I was seeing with the 6L6GCs.
Should a tube be driven to the levels required to warp the screen, screen current would increase dramatically. This is not the case as under bias conditions Vk=470V, Vg2=470V) current through the screen resistor is calculated at 4.9mA with a cathode current of 40mA.
This is very close to the values for 450V operation in class AB1, and screen dissipation is only half max spec.
I did find one new 12AX7 that was quite noisy (Groove Tube branded). However this does not seem to be a contributing factor.
I am convinced that operating with the plate and screen at 470V is the most likely cause of the over-dissipation, and increasing the magnitude of the bias is the correct fix.
Yes, without a proper plate load the screen acts as a plate. However, this is not the case for the amp in question.
I have abused many tetrodes and pentodes without warping the screen grid sufficiently to change bias conditions. Given that the tube is operated with a proper anode load, short term over-dissipation is not extremely detrimental to tube operation in my experience.
What I experienced is very difficult to see in ambient light conditions and requires turning off the lights to see it well. It is confined to the junction of the radiating fin and body of the plate. The attached picture is of a 6P1P with worst over-dissipation than I was seeing with the 6L6GCs.
Should a tube be driven to the levels required to warp the screen, screen current would increase dramatically. This is not the case as under bias conditions Vk=470V, Vg2=470V) current through the screen resistor is calculated at 4.9mA with a cathode current of 40mA.
This is very close to the values for 450V operation in class AB1, and screen dissipation is only half max spec.
I did find one new 12AX7 that was quite noisy (Groove Tube branded). However this does not seem to be a contributing factor.
I am convinced that operating with the plate and screen at 470V is the most likely cause of the over-dissipation, and increasing the magnitude of the bias is the correct fix.
Attachments
Hi Gimp.
I really appreciate the picture of the glowing tubes. It does however remind me of something my dad showed me in 1945. In Norway the Germans had confiscated all Radios in 1942 and put them in storage. My dad had an RCA143 type Radio before WWII that we got back when the war was over in 1945. When we plugged it in all seemed to work OK, except the the 2 #42 output pentodes were glowing red with a blue shine to it, just like your output tubes.
My dad explained that the tubes were probably OK but they had for some reason become gassy. I seem to see that the internal screen grid supports and the screen grid wires are glowing red with a blue shimmer. are you sure that your output tubes are not gassy? I have worked a lot with power tubes and in my experience the only anodes that are allowed to glow dark red are transmitter output power tubes with large distances to the glass bulb. With a red glowing plate there is a greater risk that the heat from the plate may overheat the glass and the tube will implode.
There are specs on the maximum allowable glass bulb temperature, usually around 150 to 200 degrees C. With an infrared surface thermometer measure the maximum bulb temperature to avoid bulb implosion. If the bulb temperature is not exceeded with a dull red plate, I believe you are OK.
Also I may remind you that in a beam pentode the screen-grid and the control grid are carefully aligned to minimize screen-grid dissipation. if this alignment for some reason or other got disturbed due to overheating, the screen-grid current may be much higher than the specs state. Just a thought that is all.
Hans J Weedon
I really appreciate the picture of the glowing tubes. It does however remind me of something my dad showed me in 1945. In Norway the Germans had confiscated all Radios in 1942 and put them in storage. My dad had an RCA143 type Radio before WWII that we got back when the war was over in 1945. When we plugged it in all seemed to work OK, except the the 2 #42 output pentodes were glowing red with a blue shine to it, just like your output tubes.
My dad explained that the tubes were probably OK but they had for some reason become gassy. I seem to see that the internal screen grid supports and the screen grid wires are glowing red with a blue shimmer. are you sure that your output tubes are not gassy? I have worked a lot with power tubes and in my experience the only anodes that are allowed to glow dark red are transmitter output power tubes with large distances to the glass bulb. With a red glowing plate there is a greater risk that the heat from the plate may overheat the glass and the tube will implode.
There are specs on the maximum allowable glass bulb temperature, usually around 150 to 200 degrees C. With an infrared surface thermometer measure the maximum bulb temperature to avoid bulb implosion. If the bulb temperature is not exceeded with a dull red plate, I believe you are OK.
Also I may remind you that in a beam pentode the screen-grid and the control grid are carefully aligned to minimize screen-grid dissipation. if this alignment for some reason or other got disturbed due to overheating, the screen-grid current may be much higher than the specs state. Just a thought that is all.
Hans J Weedon
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What is glowing in the right tube is the cathode due to the angle of the picture. The tube on the right in particular is showing signs of being gassy, and the condition is probably not helped by the red plate which could evolve trapped gas.
Back to the 6L6GCs, I measured the screen resistor values, then measured the voltage drop across each and calculated the screen current. As stated above, it was only 4.9mA so no excessive screen current is indicated.
Back to the 6L6GCs, I measured the screen resistor values, then measured the voltage drop across each and calculated the screen current. As stated above, it was only 4.9mA so no excessive screen current is indicated.
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