Subject amp was brought for next to nothing and it was gutted. All that was left was the chassis, the transformers, the tube sockets, and the heater wiring. I replaced the sockets and the heater wiring and I tried not-so-successfully to nickel-plate the bell ends of the transformers. From what I've been able to tell the amp was made circa 1960 and was part of a transistor-based organ (part of what was in this chassis was the power supply for the organ electronics; all of that's gone and the associated PT secondaries are blocked off and wrapped up).
Obligatory gut shot:
The schematic shows one component change (unobtainum; substituted 250 ohm 5W for 225 5W) and I've crossed off sections that were omitted. I crossed off the input/V1 stage because I changed enough to just redraw it; both are below:
I simplified how that first segment of coax is drawn; the actual layout doesn't look like that. Rather, ground "emanates" (if you will) from the end of the preamp-side ground busbar to the low leg of the volume pot and is carried via coax shields from there towards both the input jack shield contact and toward the V1 grid(s) but note that the segment of coax from the volume pot to V1 is grounded only at the pot end - this was a late addition to address some audio-range feedback that was creeping in. Both input and speaker jacks are isolated from the chassis. The big changes in the front end are the paralleling of the two V1 triodes instead of leaving one unused, the values of V1's cathode resistor and bypass cap (I had intended to use a 10uF but didn't have one on hand), and the value of the V1 plate resistor (started with 56K; wound up with 47K). I tested putting a 4.7K resistor in front of the V1 grid(s) to reduce how much the function generator input signal was getting dragged down when the volume pot was turned up but it didn't seem to help much at all so I removed it.
The power tubes are new TAD 6L6GCM STR Redbase. The two 12AU7s and the 5U4GB are vintage; they were collected from the same source and are branded "Beckman" (electronic medical equipment manufacturer). All were tested on an Eico 667.
The two ground busbars meet and turn together toward the preexisting solder blob. The ground wire from the power cord and the PT center tap are soldered together with the two busbar ends into a thick column. There was a now-unused connector near one of the 6L6es with a terminal placed ideally for the far end of the power stage ground busbar to be soldered to in order to stabilize it; I chose to leave it like that.
It amplifies.
Power - By connecting the amp to an 8-ohm resistive dummy load with a scope across it and feeding a 1000Hz sinewave into the input, before the output clips I measure/calculate 18.6W going into the load ((Vrms^2/8). That's not the ~50W I was expecting. I have read here that it is possible to have a dual-6L6 amp in that power range in Class A but I'm not savvy enough to know if that is in fact how this amp was designed or if I have something else going on that's out of whack. I can say, however, that when I adjust my variac to get the 500V B+ as indicated on the schematic, all the other indicated voltages are very close to what I measure. It may be the case that the 250-for-225 substitution of the 6L6 cathode resistors wasn't a great idea; I could parallel a 1/2-watt 2.2K there to bring it to 224.5.
My intent was to recreate the APS-5 as opposed to put something else into an APS-5 chassis (after all, the transformers are for the original APS-5) so if it is in fact an 18-19W amp then so be it.
I've been doing some reading and my understanding is that what characterizes Class A operation is that the power tube plates are carrying a current which tracks the input signal throughout its entire excursion. I suppose I could put a small sampling resistor of sufficient power rating in series with a plate and throw a scope across it (while the amp is on an isolation transformer, of course) to determine if it's running as Class A; does that seem worthwhile?
Frequency Response - my -3dB knees are at 130Hz and 25kHz (!). I would have thought I would see a lower bottom end but I don't know what's typical for this sort of amp. It is not audibly lacking for bass running ordinary music program through a three-way loudspeaker.
Phase Response - If my input signal is around 10kHz, I've got something like 30 degrees of phase shift. Is that normal?
I welcome your insights and I thank you in advance.
Obligatory gut shot:
The schematic shows one component change (unobtainum; substituted 250 ohm 5W for 225 5W) and I've crossed off sections that were omitted. I crossed off the input/V1 stage because I changed enough to just redraw it; both are below:
I simplified how that first segment of coax is drawn; the actual layout doesn't look like that. Rather, ground "emanates" (if you will) from the end of the preamp-side ground busbar to the low leg of the volume pot and is carried via coax shields from there towards both the input jack shield contact and toward the V1 grid(s) but note that the segment of coax from the volume pot to V1 is grounded only at the pot end - this was a late addition to address some audio-range feedback that was creeping in. Both input and speaker jacks are isolated from the chassis. The big changes in the front end are the paralleling of the two V1 triodes instead of leaving one unused, the values of V1's cathode resistor and bypass cap (I had intended to use a 10uF but didn't have one on hand), and the value of the V1 plate resistor (started with 56K; wound up with 47K). I tested putting a 4.7K resistor in front of the V1 grid(s) to reduce how much the function generator input signal was getting dragged down when the volume pot was turned up but it didn't seem to help much at all so I removed it.
The power tubes are new TAD 6L6GCM STR Redbase. The two 12AU7s and the 5U4GB are vintage; they were collected from the same source and are branded "Beckman" (electronic medical equipment manufacturer). All were tested on an Eico 667.
The two ground busbars meet and turn together toward the preexisting solder blob. The ground wire from the power cord and the PT center tap are soldered together with the two busbar ends into a thick column. There was a now-unused connector near one of the 6L6es with a terminal placed ideally for the far end of the power stage ground busbar to be soldered to in order to stabilize it; I chose to leave it like that.
It amplifies.
Power - By connecting the amp to an 8-ohm resistive dummy load with a scope across it and feeding a 1000Hz sinewave into the input, before the output clips I measure/calculate 18.6W going into the load ((Vrms^2/8). That's not the ~50W I was expecting. I have read here that it is possible to have a dual-6L6 amp in that power range in Class A but I'm not savvy enough to know if that is in fact how this amp was designed or if I have something else going on that's out of whack. I can say, however, that when I adjust my variac to get the 500V B+ as indicated on the schematic, all the other indicated voltages are very close to what I measure. It may be the case that the 250-for-225 substitution of the 6L6 cathode resistors wasn't a great idea; I could parallel a 1/2-watt 2.2K there to bring it to 224.5.
My intent was to recreate the APS-5 as opposed to put something else into an APS-5 chassis (after all, the transformers are for the original APS-5) so if it is in fact an 18-19W amp then so be it.
I've been doing some reading and my understanding is that what characterizes Class A operation is that the power tube plates are carrying a current which tracks the input signal throughout its entire excursion. I suppose I could put a small sampling resistor of sufficient power rating in series with a plate and throw a scope across it (while the amp is on an isolation transformer, of course) to determine if it's running as Class A; does that seem worthwhile?
Frequency Response - my -3dB knees are at 130Hz and 25kHz (!). I would have thought I would see a lower bottom end but I don't know what's typical for this sort of amp. It is not audibly lacking for bass running ordinary music program through a three-way loudspeaker.
Phase Response - If my input signal is around 10kHz, I've got something like 30 degrees of phase shift. Is that normal?
I welcome your insights and I thank you in advance.
A 500k Potentiometer Volume Control that is turned anywhere other than at Max Volume; that outputs to a coax cable makes two things:
1. A low pass filter
2. A lagging phase shift (- degrees).
A 6L6GC push pull pair: Class A output much much less than 50 Watts
A 6L6GC push pull pair, Class AB output might have 50 Watts, but it might be a struggle, or have low tube life (keep a small fire extinguisher near).
The 6L6GC is the most power capable of all the 6L6 tubes.
Start with 50 Watts Class AB, from a push pull pair of tubes going to the output transformer primary.
Use a less expensive output transformer that has primary DCR and secondary DCR, each causing 0.5dB loss, that totals 1 dB loss.
50 Watts - 1dB = 50 x 0.79 = 39.7 watts.
Does your car transmission loose any power from the engine?
engine horsepower; road horsepower.
50 Watts into 8 Ohms, is 28.28 Volts peak (56.6V peak to peak) for a sine wave.
A square wave that has 28.28 Volts peak (56.6V peak to peak) into 8 Ohms is 100 Watts (extremely severe clipping).
Please do not ask me what the percent (%) of harmonic distortion is for a square wave. Thanks! It is such a high number, no marketeer would ever publish a number that high.
If I had a THD meter, that would be a simple measurement.
But taking a spectrum analyzer or FFT, and re-exponentiating each dBc harmonic reading, and then doing a root sum squares, is not for a retired person like me.How many harmonics, where would you like to stop, at what harmonic number. I do not have the time or the patience. Thanks!
1. A low pass filter
2. A lagging phase shift (- degrees).
A 6L6GC push pull pair: Class A output much much less than 50 Watts
A 6L6GC push pull pair, Class AB output might have 50 Watts, but it might be a struggle, or have low tube life (keep a small fire extinguisher near).
The 6L6GC is the most power capable of all the 6L6 tubes.
Start with 50 Watts Class AB, from a push pull pair of tubes going to the output transformer primary.
Use a less expensive output transformer that has primary DCR and secondary DCR, each causing 0.5dB loss, that totals 1 dB loss.
50 Watts - 1dB = 50 x 0.79 = 39.7 watts.
Does your car transmission loose any power from the engine?
engine horsepower; road horsepower.
50 Watts into 8 Ohms, is 28.28 Volts peak (56.6V peak to peak) for a sine wave.
A square wave that has 28.28 Volts peak (56.6V peak to peak) into 8 Ohms is 100 Watts (extremely severe clipping).
Please do not ask me what the percent (%) of harmonic distortion is for a square wave. Thanks! It is such a high number, no marketeer would ever publish a number that high.
If I had a THD meter, that would be a simple measurement.
But taking a spectrum analyzer or FFT, and re-exponentiating each dBc harmonic reading, and then doing a root sum squares, is not for a retired person like me.How many harmonics, where would you like to stop, at what harmonic number. I do not have the time or the patience. Thanks!
Last edited:
The 270k between the first and second stages might cut highs and serves no useful purpose (probably there as part of the expression circuit which you've removed). The coupling capacitors (0.01uF and 0.022 uF) and the cathode bypass capacitors are small and will cut bass when you add up the combined effect.
For power output, is the B+ sagging at high output? Datasheet for 55W class AB1 PP (less losses as mentioned by 6A3sUMMER) is 450 plate and 400V screen into a 5.6k transformer. You have 465V plate and 330V screen into an unknown load, and those voltages may be dropping a lot under load.
For power output, is the B+ sagging at high output? Datasheet for 55W class AB1 PP (less losses as mentioned by 6A3sUMMER) is 450 plate and 400V screen into a 5.6k transformer. You have 465V plate and 330V screen into an unknown load, and those voltages may be dropping a lot under load.
I just noticed the speaker plug.
It has a jumper to activate B+.
That prevents powering the amplifier unless the speaker plug is inserted. But there is no guarantee the customer connected the speaker to that same plug.
Nothing is idiot proof.
0.022 and 270k is -3dB at 27Hz. But it is Inside the global negative feedback, which will make the bass rolloff lower (but does not account for the rolloff caused by the primary inductance of the output transformer). Those two factors dictate the low frequency rolloff.
It has a jumper to activate B+.
That prevents powering the amplifier unless the speaker plug is inserted. But there is no guarantee the customer connected the speaker to that same plug.
Nothing is idiot proof.
0.022 and 270k is -3dB at 27Hz. But it is Inside the global negative feedback, which will make the bass rolloff lower (but does not account for the rolloff caused by the primary inductance of the output transformer). Those two factors dictate the low frequency rolloff.
Some info I should add is that my output transformer primary leg measures 196.9 ohms from center tap and the other leg measures 147.2 ohms. Someone on another forum made the assumption of an OT primary resistance in the range of 3.6-4k and @tikiroo mentioned a 5.6k transformer - well, my OT is nowhere near that. Is this something that suggests that the amp is meant to run in Class A, not AB?
The original speaker plug arrangement is gone; conventional isolated 1/4" phone jack is connected to the OT secondaries.
With mains adjusted to give me 500V B+ at nil input, going to full tilt drops it by 4-5V. The 6L6 plate voltages drop pretty significantly with the same input change but I need to take a pause and go back and redo the coax business at the input. I'm getting an intermittent short due to insulation melt from soldering.
The original speaker plug arrangement is gone; conventional isolated 1/4" phone jack is connected to the OT secondaries.
With mains adjusted to give me 500V B+ at nil input, going to full tilt drops it by 4-5V. The 6L6 plate voltages drop pretty significantly with the same input change but I need to take a pause and go back and redo the coax business at the input. I'm getting an intermittent short due to insulation melt from soldering.
Don't confuse resistance and impedance - the DC resistance measurements of 147 and 197 ohms are fairly typical, although the halves are usually more equal than that. The higher resistance will be the half that is wound on the outside of the transformer as the wire length is longer, although it's not out of the question that the lower resistance of the other half is due to a short somewhere. The AC impedance that the output valve sees (3.6k or 5.6k or whatever it is) is a function of the square of the transformer turns ratio and the secondary load. You can measure it yourself with a low voltage AC source and a multimeter (look it up...).
DCR: 197 Ohms - 147 Ohms = 50 Ohms difference.
1. A 6L6GC Beam Power tube in native mode does not care. They have typical plate impedances of about 22.5k Ohms.
What is the significance of 22.5k Ohms in series with either 197 Ohms, versus 22.5k in series with 147 Ohms? . . . Extremely little.
(50 / 22,500) x 100 = 0.2%
Do you care? If so, put 50 Ohm Power resistor in series with the 147 Ohm lead, and those of you who are Insomniacs can sleep at night.
2. A 300B plate impedance of 700 Ohms in series with either 197 Ohms versus 147 Ohms is different by 50 Ohms.
The difference is: (50 / 700) x 100 = 7 %.
Now we care.
So, put a 50 Ohm power resistor in series with the 147 Ohm secondary lead.
Problem solved!
3. Know thy correct transformer applications.
Save an otherwise good transformer from mediocre performance.
1. A 6L6GC Beam Power tube in native mode does not care. They have typical plate impedances of about 22.5k Ohms.
What is the significance of 22.5k Ohms in series with either 197 Ohms, versus 22.5k in series with 147 Ohms? . . . Extremely little.
(50 / 22,500) x 100 = 0.2%
Do you care? If so, put 50 Ohm Power resistor in series with the 147 Ohm lead, and those of you who are Insomniacs can sleep at night.
2. A 300B plate impedance of 700 Ohms in series with either 197 Ohms versus 147 Ohms is different by 50 Ohms.
The difference is: (50 / 700) x 100 = 7 %.
Now we care.
So, put a 50 Ohm power resistor in series with the 147 Ohm secondary lead.
Problem solved!
3. Know thy correct transformer applications.
Save an otherwise good transformer from mediocre performance.
???? No one said the resistance mismatch was necessarily a problem that needed to be addressed. The OP seemed to be confusing transformer resistance with impedance so I pointed out the difference. The resistance mismatch is quite large, so it might be a symptom of an internal short which should be checked out. Or it might be just how that transformer was constructed. If it's the latter then I agree that it's of no significant consequence in this application.
Here's the original schematic just to save folks from scrolling around:
Updates:
And now the lower 6V6:
I'm given to understand this is the normal way a cathodyne bias arrangement should act.
I'm getting to be at my wits' end as to why 18.9W is the best this amp can do. What's left other than to suspect the output transformer - as in, either it's defective or it's actually wound to give this result? My understanding is that this amp came from an organ with a single speaker of perhaps 12" mounted face-down above a Leslie-like rotating baffle. 19 watts full-tilt would have been pretty damn loud; 40+ would have been nuts city especially when considering the idea of jamming that much power into a single circa-1960 speaker.
Updates:
- With nil input the voltage at the 6L6 cathodes is 26.26V when the variac is adjusted until B+ is at the schematic-indicated 500V.
- I've unwired the second triode in V1 and reverted the entire preamp to stock except I've removed the 270K resistor between V1 and V2A.
- I think I mentioned before that my coax between the input jack and between the pot and V1 was problematic - apparently heat-damaged from soldering. I ripped all that out and replaced it with regular wire, but this left me with a feedback problem; the input pot is between the input jack and the speaker jack (that's just how it worked out and the OT secondary leads are short, limiting my options without splicing 4-5" of wire onto those leads). I redid the coax at the same time I backed out the parallel V1 triodes using and the feedback problem is nearly gone; I can probably eliminate it altogether by riveting in a steel shield.
- With the amp operating just below output distortion, the amplitude of the AC on the 6L6 grids is 26.0V (52V peak-to-peak - that's what they both tubes read). When that happens, I calculate power into the dummy load as 18.9 watts.
- I changed V2; no difference. I still need to try changing V1 (probably using the tube I took out of the V2 socket).
- I went through and checked every connection against the schematic after taking a couple days' break and I confirmed all the resistor values. I can't find anything amiss.
- Just to remind, I'm operating the preamp and power heater circuit biased to +52.5V via two 100-ohm resistors in the typical "false center tap" configuration but with their common node connected to a voltage divider set about a tenth of the way between B+ and ground.
And now the lower 6V6:
I'm given to understand this is the normal way a cathodyne bias arrangement should act.
I'm getting to be at my wits' end as to why 18.9W is the best this amp can do. What's left other than to suspect the output transformer - as in, either it's defective or it's actually wound to give this result? My understanding is that this amp came from an organ with a single speaker of perhaps 12" mounted face-down above a Leslie-like rotating baffle. 19 watts full-tilt would have been pretty damn loud; 40+ would have been nuts city especially when considering the idea of jamming that much power into a single circa-1960 speaker.
I modelled the output stage in LTspice, it seems you aren't too far away from what the simulation is saying. You might expect 26W for 6k plate-to-plate down to 16W for 3k plate-to-plate. Maybe try a higher value load resistor, say 12 ohm and see if the power increases.
Would also be worthwhile checking the output transformer - disconnect it from the amp, connect a low voltage AC supply to the secondary and measure the voltage across each half of the primary as well as from end to end.
Would also be worthwhile checking the output transformer - disconnect it from the amp, connect a low voltage AC supply to the secondary and measure the voltage across each half of the primary as well as from end to end.
- I tried increasing the load resistor to 12 ohms (I'm using the heating element out of a space heater - zig-zag resistance wire - with which I'm able to control the resistance in roughly-half-ohm increments - and all of the sudden I'm able to get 23 watts. Now, I got thrown off because I had not caught that it actually says "8<omega>Z" right up against the OT secondary and therefore started wondering if I'd been using the wrong load this whole time, but I'm not.
- Re checking the OT, should I (instead or additionally) try reverse-driving it with 1kHz, like at low level out of a power amp, so I can try to get an actual impedance value out of each primary leg?
- One thing I just remembered tonight (after it was suggested that I reduce the value on another forum) was that that first dropping resistor, 12k 10W on the schematic, is in fact two 25k 5W resistors in parallel for an actual 12.5k, not 12k. If I can get another 300K in parallel across that (and I'm pretty sure I readily can) that'll bring it smack down to 12k. I'll have to do the math to make sure it's okay but I think three 100k 1/2Ws in series across those two 25k 5W may work.
- Replaced V1 with the now-known-to-be-good-after-replacement tube that was in V2 and no difference. All of the 12AU7s I have on hand are just fine.
Reverse driving the OT will provide useful information, but try to follow a proper method (there are a few on youtube etc) so you are loading the transformer correctly and don't get excessive voltages on the primary.
The 12k screen resistor does contribute a little to power loss but probably <1W drop. Don't bother trying to get it exactly 12k, it will make no practical difference. You could monitor the screen voltage at idle and full power to see how much it's dropping, will probably drop ~20-30V at sustained high power.
Assuming the OT is OK I really don't think you can expect much with this amp. For cathode bias AB1 you need to run the tubes quite hot to get the most out of them, but the very high B+ you have means that you can't do that. See below for more typical operating points (this is for a 7027 which is the same internally as the 6l6GC but the datasheet has more useful operating points for cathode bias).
The 12k screen resistor does contribute a little to power loss but probably <1W drop. Don't bother trying to get it exactly 12k, it will make no practical difference. You could monitor the screen voltage at idle and full power to see how much it's dropping, will probably drop ~20-30V at sustained high power.
Assuming the OT is OK I really don't think you can expect much with this amp. For cathode bias AB1 you need to run the tubes quite hot to get the most out of them, but the very high B+ you have means that you can't do that. See below for more typical operating points (this is for a 7027 which is the same internally as the 6l6GC but the datasheet has more useful operating points for cathode bias).