Dear all, I recently acquired an old Pamphonic 1003 hifi tube amp from 1955, which I converted succesfully to a nice sounding bass amp. For its output power of only 10 watts, it has a huge power and ultralinear output transformer. AC rates 270, DC 323 volt. I think the amp can deliver much more, at least 25 watt. Currently it runs 2 6AQ5 tubes. My guess is that the primary of the OT is roughly 8k, and it has only a 16 and 4 ohm connection, while I need 8 ohms.
My first thoughts are to put 6L6GC or KT77 in, and change the GZ32 for a solid state rectifier. I looked around for other type of tubes, but these typically run above 400. Will these also run well on, say 300-350 Dc? Any suggestions and or schematics? Attached are the schematic and a pic of the transformers.
Thanks and regards, Paul
My first thoughts are to put 6L6GC or KT77 in, and change the GZ32 for a solid state rectifier. I looked around for other type of tubes, but these typically run above 400. Will these also run well on, say 300-350 Dc? Any suggestions and or schematics? Attached are the schematic and a pic of the transformers.
Thanks and regards, Paul
Attachments
That surely is a LOT of iron for two small bottles.
Where you gonna fit a pair of big bottles?
I would be mildly fretful about heater current. This leads to 6L6 instead of the hungrier EL34.
With double-fat bottles, putting 8 Ohms on the "15 Ohm" tap tends to double the power. If the power transformer does not overheat.
That hollow-state rectifier is lovely, but for racing it has to go. Taking that heater off the transformer leaves some leeway for fatter audio bottle heaters. Silicon rectifier makes higher voltage and 20%-30% more power.
Use self-bias whenever possible; but if you can contrive a negative bias supply you can gain another 10%-20% audio power and lower idle power.
Personally _I_ would not risk damage to such a fine old machine. I do agree that Mono is awkward in this day and age.
Where you gonna fit a pair of big bottles?
I would be mildly fretful about heater current. This leads to 6L6 instead of the hungrier EL34.
With double-fat bottles, putting 8 Ohms on the "15 Ohm" tap tends to double the power. If the power transformer does not overheat.
That hollow-state rectifier is lovely, but for racing it has to go. Taking that heater off the transformer leaves some leeway for fatter audio bottle heaters. Silicon rectifier makes higher voltage and 20%-30% more power.
Use self-bias whenever possible; but if you can contrive a negative bias supply you can gain another 10%-20% audio power and lower idle power.
Personally _I_ would not risk damage to such a fine old machine. I do agree that Mono is awkward in this day and age.
Thanks for the quick replies. Much appreciated, and I am already thinking on how to best move forward. I would like to get as much as reasonable and with little risk as possible out of my amp. I agree that this oldie needs proper treatment, and probably best follow a gradualist approach. So changing sockets, and putting in EL84s is likely to be safest. But the big rectifier probably does not allow this. I am out of country now, but will check on Friday. If it does, I could go for solid state, but then maybe the EL84 will not hold. Would KT77 be an option? Thanks again for the quick reply. And in due time I will post a complete overview of all the other changes I mad to the amp, including some pictures. Cheers, Paul
Thanks for this suggestion Mozz. It is probably easier with EL90s or El84 as the current sockets are 7 pin and 6AQ5 quartets will be difficult to get. Still worthwhile to consider, although my preference is to have a pair of tubes. Anyone suggestion on other tubes, like KT77 that may run well on indicated voltages? Thanks and regards, Paul
My first thought would be to run 6CW5 / EL86 tubes with the 8 ohm speaker on the 16 ohm tap. The screen voltage must be kept to about 170 volts via a mosfet regulator, but you can get 20 to 25 watts into the 4,000 ohm load created with the mismatched speaker.
The 6CW5 will eat 300 to 350 volts on the plate provided the screen is run at no more than 170 volts. I have a DIY guitar amp that runs 170 screen and 340 volts plate on 45B5 / UL84's (a 45 volt heater version of the 6CW5)
The 6CW5 will eat 300 to 350 volts on the plate provided the screen is run at no more than 170 volts. I have a DIY guitar amp that runs 170 screen and 340 volts plate on 45B5 / UL84's (a 45 volt heater version of the 6CW5)
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Dear Tubelab, thanks for the suggestion.
Meanwhile I returned home from abroad and had some looks and thoughts. I think I follow a gradual approach.
1. Change to solid state rectifier. This will increase the B+ to 375 volt, and save on heater power. (maybe later work on delay for B+) With the current schematic, would the 6AQ5s fry or die? And would it raise the output?
2.a. If not change to a pair of EL84s - What output to expect with B+ =375? (JonSnell suggestion)
2.b. If not change to a pair of 6L6s, while watching temperature of power transformer. - What value for cathode resistors would be appropriate? What output to expect?
Hope anyone can shed some light on this, as I am still in doubt.
Thanks and regards, Paul
Meanwhile I returned home from abroad and had some looks and thoughts. I think I follow a gradual approach.
1. Change to solid state rectifier. This will increase the B+ to 375 volt, and save on heater power. (maybe later work on delay for B+) With the current schematic, would the 6AQ5s fry or die? And would it raise the output?
2.a. If not change to a pair of EL84s - What output to expect with B+ =375? (JonSnell suggestion)
2.b. If not change to a pair of 6L6s, while watching temperature of power transformer. - What value for cathode resistors would be appropriate? What output to expect?
Hope anyone can shed some light on this, as I am still in doubt.
Thanks and regards, Paul
I'm only happy when it rains......
The most effective way would be to put about 0.5 ohm in series with your speaker (in the ground leg) and blend the voltage generated thereon with 5% of the amplifier output voltage. Then feed that into a great, steaming pile of Class D amplification and get 15 odd db of headroom.
Which is a whole 4db above the nominal power of 10W. Changing your speaker would be a more effective to get 4db. (If much less fun).
The most effective way would be to put about 0.5 ohm in series with your speaker (in the ground leg) and blend the voltage generated thereon with 5% of the amplifier output voltage. Then feed that into a great, steaming pile of Class D amplification and get 15 odd db of headroom.
I have plenty of experience melting tubes......the easiest tube to melt is the 6AQ5. You can't squeeze these guys at all beyond their specs. The glass is just too close to the plate, and the plate tends to develop a red spot in one small place when pushed. There is a very fine line, just a few milliamps between pale red glow, and hole in the glass. I have melted enough of these guys to know......of course I had about 1000 used ones to play with, so I "tested" some of the ugly ones, but they all have the same issue, no margin for error.
I have seen 10 watts, and even 12, but don't run them at 12 watts output for long. You won't even hear the sucking sound......just silence!
Still exploring. Assuming power transformer and output transformer would hold, putting ss rectifier in and thus have B+ of above 350, what would two 6l6s roughly deliver? And what cathode resistors value would be appropriate? Seems that putting El84s in would not give much gain compared with current 6AQ5s.
Before you do any of this, be sure the power transformer can handle the higher filament current and the higher plate current, or you will burn it out. I said to take out the dial lamps because that gains you 300ma filament current right there, also any unused tubes. I doubt very much you can swap to a pair of 6l6gc without damaging the power transformer.
The output power of an amp is based on the PT capabilities and the OT primary impedance. In the amp as it is, these parameters are set and there is nothing you can do about it.
The only way you might get a little more power is if the amp is cathode-biased and you change it to fixed-bias. This will make the amp more dynamic BUT may not increase useful output, since full-load is still the same.
You would be more successful changing the speaker complement and cabinet positions. Use two cabinets separated by a space and let psychoacoustics work for you.
The only way you might get a little more power is if the amp is cathode-biased and you change it to fixed-bias. This will make the amp more dynamic BUT may not increase useful output, since full-load is still the same.
You would be more successful changing the speaker complement and cabinet positions. Use two cabinets separated by a space and let psychoacoustics work for you.
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