picked this neat lil ChiFi headphone/speaker amp up at an antique faire, looking to get it working well for my parents so they can use the passive speakers in the walls in the place they just moved into!
nothin too fancy, but decent iron, and everything's there that needs to be. haven't started it up yet, testing through some 832A's and 6N2P-EV's first, and i'd like to understand some basic things about how to properly load this amp first, as well as how to pick a good set of headphones to pair with it.
i mostly work with guitar amps and heavy overdrive, so i am unused to circuits like this with significant power output, but where there seems to be a wide flexibility for what's an acceptable load. the impedance ratio for the output transformer indicates there would be a 1,420 ohm primary impedance with a 4 ohm speaker and a 2,840 ohm primary with an 8 ohm speaker; without seeing a triode mode plate characteristics graph or having suggested load resistances in the datasheet, i guess these seem like friendly numbers, maybe a bit low?? and the way i read the 100R resistor and switch network for the output is this: the shunt resistor is there to ensure the load is never greater than 100 ohms even if nothing is connected, and the series resistor with the headphone output ensures the load is always between 50 and 100 ohms when headphones are connected. my questions are:
first off, are secondary loads all the way from 4-100 ohms really okay? and is there an ideal range within that range? i'm used to much higher loads being untenable due to the potential for the signal swing to exceed screen dissipation maximum on the low plate voltage side, and exceed plate voltage maximum and/or OT or power supply voltage maximums on the high plate voltage side. i know we're at very low plate voltage considering the max for the 832A and what the filter caps are rated, but i figure with a few volts at the input like from a CD player or something, the 6N2P-EV could easily deliver a few tens of volts to overdrive the 832A's, and my parents aren't too picky, their hearing is going, and they like it LOUD haha! so i figure there's a possibility those overdrive conditions could happen.
second, (and these are total noobie questions) how can only a pair of headphones dissipate all the power on the OT secondary? do they rely on their higher impedance values, aided by both 100R resistors, to limit the current enough to not damage them? when picking out headphones, should i verify their power handling capability can handle the amp? or should i only worry about it with lower impedance headphones?
third, if loads from 4-100 ohms are really okay, i can't really see any reason to have this speaker/headphone switch... i don't see any detriment to having headphones and speakers connected at the same time. can i just disconnect it or use it for something else? i was thinking of attaching the other OT secondary for use with 2-4 ohm loads and turning the switch into a selector between the two windings.
lastly, i'd like to try 829B tubes with this amp, but i want to make sure i don't exceed component ratings. as far as the B+ current, based on the choke reading 125DCR, i'm not comfortable pushing it above about 80-100mA, so i may have to step up the cathode resistors to keep the current lower. for the filaments, i know i wrote "6.3V - 0 - 6.3V" and then "6A", but as you can see the transformer actually says "6.3V/3A - 0 - 6.3V/3A". so here's another noobie question - does that mean that both windings used together for 12V can handle 6A, or can they still only handle 3A?
one more lil question - if this is receiving a phonograph signal, is it still adviseable that you use a designated phono pre between the phonograph and the amp, for additional signal amplification and/or RIAA filtering?
cheers! happy holidays!
nothin too fancy, but decent iron, and everything's there that needs to be. haven't started it up yet, testing through some 832A's and 6N2P-EV's first, and i'd like to understand some basic things about how to properly load this amp first, as well as how to pick a good set of headphones to pair with it.
i mostly work with guitar amps and heavy overdrive, so i am unused to circuits like this with significant power output, but where there seems to be a wide flexibility for what's an acceptable load. the impedance ratio for the output transformer indicates there would be a 1,420 ohm primary impedance with a 4 ohm speaker and a 2,840 ohm primary with an 8 ohm speaker; without seeing a triode mode plate characteristics graph or having suggested load resistances in the datasheet, i guess these seem like friendly numbers, maybe a bit low?? and the way i read the 100R resistor and switch network for the output is this: the shunt resistor is there to ensure the load is never greater than 100 ohms even if nothing is connected, and the series resistor with the headphone output ensures the load is always between 50 and 100 ohms when headphones are connected. my questions are:
first off, are secondary loads all the way from 4-100 ohms really okay? and is there an ideal range within that range? i'm used to much higher loads being untenable due to the potential for the signal swing to exceed screen dissipation maximum on the low plate voltage side, and exceed plate voltage maximum and/or OT or power supply voltage maximums on the high plate voltage side. i know we're at very low plate voltage considering the max for the 832A and what the filter caps are rated, but i figure with a few volts at the input like from a CD player or something, the 6N2P-EV could easily deliver a few tens of volts to overdrive the 832A's, and my parents aren't too picky, their hearing is going, and they like it LOUD haha! so i figure there's a possibility those overdrive conditions could happen.
second, (and these are total noobie questions) how can only a pair of headphones dissipate all the power on the OT secondary? do they rely on their higher impedance values, aided by both 100R resistors, to limit the current enough to not damage them? when picking out headphones, should i verify their power handling capability can handle the amp? or should i only worry about it with lower impedance headphones?
third, if loads from 4-100 ohms are really okay, i can't really see any reason to have this speaker/headphone switch... i don't see any detriment to having headphones and speakers connected at the same time. can i just disconnect it or use it for something else? i was thinking of attaching the other OT secondary for use with 2-4 ohm loads and turning the switch into a selector between the two windings.
lastly, i'd like to try 829B tubes with this amp, but i want to make sure i don't exceed component ratings. as far as the B+ current, based on the choke reading 125DCR, i'm not comfortable pushing it above about 80-100mA, so i may have to step up the cathode resistors to keep the current lower. for the filaments, i know i wrote "6.3V - 0 - 6.3V" and then "6A", but as you can see the transformer actually says "6.3V/3A - 0 - 6.3V/3A". so here's another noobie question - does that mean that both windings used together for 12V can handle 6A, or can they still only handle 3A?
one more lil question - if this is receiving a phonograph signal, is it still adviseable that you use a designated phono pre between the phonograph and the amp, for additional signal amplification and/or RIAA filtering?
cheers! happy holidays!
Attachments
220k Ohms grid return resistor to return two output tube control grids, g1 and g1.
Wow!
If it does not go into thermal run-away, consider yourself lucky.
I think the maximum spec for each g1 is 25k for fixed bias; self bias 'might' work for audio application, with 100k.
The 832 is similar.
The 829 is so different. Lots more filament current, lots more power dissipation, lots more transconductance, etc.
Two 6.3V 3A filament windings in serial give you 12.6V at 3A.
Well, if you wire them out of phase, you get 0V.
Wow!
If it does not go into thermal run-away, consider yourself lucky.
I think the maximum spec for each g1 is 25k for fixed bias; self bias 'might' work for audio application, with 100k.
The 832 is similar.
The 829 is so different. Lots more filament current, lots more power dissipation, lots more transconductance, etc.
Two 6.3V 3A filament windings in serial give you 12.6V at 3A.
Well, if you wire them out of phase, you get 0V.
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wowwww good catch, you're super right, the max for both sections together is 25K, it's nearly 10x higher than that! any other adjustments you'd recommend when dropping it to 25K?
Because it is Self Biased by the cathode resistor, I can probably use much more than 25k.
The problem with changing that resistor to a lower resistance:
the driver tube is loaded more heavily, reducing its gain, and increasing its distortion.
Part of this has to do with the quiescent voltage and current of the tube.
Maxing out to 7.5 Watts per plate (15 Watts) makes everything hotter, more moderate quiescent conditions make it less liable to go into thermal run-away.
If it works reliability with the tubes supplied, that is good.
If it goes into thermal run-away, it may need re-design, or better output tubes.
Has it been working OK?
Be careful of high output load impedances. That is a separate problem.
If the volume is turned all the way up, there is a possibility of arcing.
Some switches and attenuators both load the output, and attenuate the signal to the headphones.
The problem with changing that resistor to a lower resistance:
the driver tube is loaded more heavily, reducing its gain, and increasing its distortion.
Part of this has to do with the quiescent voltage and current of the tube.
Maxing out to 7.5 Watts per plate (15 Watts) makes everything hotter, more moderate quiescent conditions make it less liable to go into thermal run-away.
If it works reliability with the tubes supplied, that is good.
If it goes into thermal run-away, it may need re-design, or better output tubes.
Has it been working OK?
Be careful of high output load impedances. That is a separate problem.
If the volume is turned all the way up, there is a possibility of arcing.
Some switches and attenuators both load the output, and attenuate the signal to the headphones.