hi there,
I'm currently building a little power amp using an ecc83 and two 6v6's in puhs pull mode. The output trannies came out of an old lafayette 224 and unfortunately i'm not quite sure about the winding layout. I know this trannie has a secondary of 0-8-16 ohms layout but don't know which is which. Right now the unit gives music but is very limited in power output before distorting and worse, when i connect the secondary "ground" to the signal ground of the system i get hum like crazy.
I've measured resistances between the windings and they are as follows: 1-2 / 0.4 ohms 2-3 / 0.9 ohms 1-3 / 1.5 ohms. Right now the speaker is connected between 1-2 and feedback is taken from 3. Also hum is only there when earth is connected to winding one not with winding 2.
Is there someone wise enough to shed some light to this and help me out?
thanks in advance
Joris
I'm currently building a little power amp using an ecc83 and two 6v6's in puhs pull mode. The output trannies came out of an old lafayette 224 and unfortunately i'm not quite sure about the winding layout. I know this trannie has a secondary of 0-8-16 ohms layout but don't know which is which. Right now the unit gives music but is very limited in power output before distorting and worse, when i connect the secondary "ground" to the signal ground of the system i get hum like crazy.
I've measured resistances between the windings and they are as follows: 1-2 / 0.4 ohms 2-3 / 0.9 ohms 1-3 / 1.5 ohms. Right now the speaker is connected between 1-2 and feedback is taken from 3. Also hum is only there when earth is connected to winding one not with winding 2.
Is there someone wise enough to shed some light to this and help me out?
thanks in advance
Joris
First of all, disconnect feedback, that can cause problems.
Second, with that disconnected, there should be absolutely no change (except perhaps a slight difference in HF peaks, for you pedants out there ) between grounded and floating output.
What is the resistance of the primary side, and it to the secondary?
Tim
Second, with that disconnected, there should be absolutely no change (except perhaps a slight difference in HF peaks, for you pedants out there
) between grounded and floating output.What is the resistance of the primary side, and it to the secondary?
Tim
hi,
I'm actually a bit reluctant to disconnect the feedback given possible stability problems. It looks as if there is a frequency/ffilter circuit in there as well, 100pf in series with 180k from the first (common cathode) stage anode to the hv. I'll give it a try tough.
What about the gain tough? the amp is quite sensitive as is, ain't i gonna oversteer the output stage?
The impedance of the primary is 8k anode-anode. I'll have to look up the resistance but seem to remember it was higher. the resistance from primary to secondary is at least more than two megs.
Does it look like i'm doing something wrong or have i got the windings right?
thanks
Joris
I'm actually a bit reluctant to disconnect the feedback given possible stability problems. It looks as if there is a frequency/ffilter circuit in there as well, 100pf in series with 180k from the first (common cathode) stage anode to the hv. I'll give it a try tough.
What about the gain tough? the amp is quite sensitive as is, ain't i gonna oversteer the output stage?
The impedance of the primary is 8k anode-anode. I'll have to look up the resistance but seem to remember it was higher. the resistance from primary to secondary is at least more than two megs.
Does it look like i'm doing something wrong or have i got the windings right?
thanks
Joris
That's a good point. You want negative, not positive feedback.
The R+C (Zobel network) on the first stage is to reduce gain at HF, without causing phase shift (such as a straight capacitor would). This is generally above hearing (ultrasonic), and stabilizes the amplifier when NFB is applied. Similarly, there is often another network across the OPT primary, for the same effect. On the NFB resistor there might be a small capacitor, this is again for HF stability.
Testing without NFB is useful because it can cause many problems if it isn't hooked up right. Like the motorboating and whatnot that you were getting.
Think of it this way, NFB feeds back some of the output to the input; if the amp is weird, the weirdness is compounded throughout the entire circuit, making troubleshooting difficult. By opening the loop, the weirdness (which may appear at one point in the circuit) is easy to find, going stage by stage.
Tim
The R+C (Zobel network) on the first stage is to reduce gain at HF, without causing phase shift (such as a straight capacitor would). This is generally above hearing (ultrasonic), and stabilizes the amplifier when NFB is applied. Similarly, there is often another network across the OPT primary, for the same effect. On the NFB resistor there might be a small capacitor, this is again for HF stability.
Testing without NFB is useful because it can cause many problems if it isn't hooked up right. Like the motorboating and whatnot that you were getting.
Think of it this way, NFB feeds back some of the output to the input; if the amp is weird, the weirdness is compounded throughout the entire circuit, making troubleshooting difficult. By opening the loop, the weirdness (which may appear at one point in the circuit) is easy to find, going stage by stage.
Tim
Hi there,
Just to let you know, i've had the amp singing for a week now and i love it. I took it over to my dad and played along with an fvp5 on his AR3 loudspeakers. I didn't expect it to be able to drive these but it had both me and my dad totally blown out of our socks. Guess i'll be going along this path a little further...
Here's a pic.
Just to let you know, i've had the amp singing for a week now and i love it. I took it over to my dad and played along with an fvp5 on his AR3 loudspeakers. I didn't expect it to be able to drive these but it had both me and my dad totally blown out of our socks. Guess i'll be going along this path a little further...
Here's a pic.
Attachments
> AR3 loudspeakers. I didn't expect it to be able to drive these but....
That's nearly the same amp everybody had when the AR3 was born.
Your P-P 6V6 make 10-15 watts. Williamson P-P Triodes make 10-20 watts. Yes, a few people had tame P-P 6L6 for a whopping 20-40 watts, and this is really what AR was thinking about when they made such a low-efficiency speaker. (Today an AR3 is "average", but an AR3 is 1/2 or 1/3rd the efficiency of the old big-box speakers.) But they don't -have- to be force-fed 40 watts to make an impressive noise. There were a LOT of "high-end" 15 watt amplifiers in those days. A 40-watt was more of a theater amplifer. I annoyed 4-storey dorms with a Heath 25 watt and a trashy acoustic suspension speaker.
You have a hell of a lot more power supply capacitance than we did in those old days. 40-40-40µFd was considered "big". What do you have there, a couple of 470µFd?
That's nearly the same amp everybody had when the AR3 was born.
Your P-P 6V6 make 10-15 watts. Williamson P-P Triodes make 10-20 watts. Yes, a few people had tame P-P 6L6 for a whopping 20-40 watts, and this is really what AR was thinking about when they made such a low-efficiency speaker. (Today an AR3 is "average", but an AR3 is 1/2 or 1/3rd the efficiency of the old big-box speakers.) But they don't -have- to be force-fed 40 watts to make an impressive noise. There were a LOT of "high-end" 15 watt amplifiers in those days. A 40-watt was more of a theater amplifer. I annoyed 4-storey dorms with a Heath 25 watt and a trashy acoustic suspension speaker.
You have a hell of a lot more power supply capacitance than we did in those old days. 40-40-40µFd was considered "big". What do you have there, a couple of 470µFd?
hi,
I realize now that the ar3's are indeed a lot more sensitive then i expected. my dad used to play with a quad 33-303 so that was quite a lot more powerful, currently he uses a marantz pm78 mainly in class a mode ie. some 25 watts.
The psu is actually even bigger than you're estimate. the first stage is 150uf the second stage are slce caps of 680uf with additional 150uf decoupling directly underneath the opt. The driver stage has it's own 150uf second stage. Per channel that is.
well what can i say, i like big psu's and i had the caps lying around anyway. It is a bit crazy / overkill i admit.
Joris
I realize now that the ar3's are indeed a lot more sensitive then i expected. my dad used to play with a quad 33-303 so that was quite a lot more powerful, currently he uses a marantz pm78 mainly in class a mode ie. some 25 watts.
The psu is actually even bigger than you're estimate. the first stage is 150uf the second stage are slce caps of 680uf with additional 150uf decoupling directly underneath the opt. The driver stage has it's own 150uf second stage. Per channel that is.
well what can i say, i like big psu's and i had the caps lying around anyway. It is a bit crazy / overkill i admit.
Joris
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