Meet Dolly, the Olson clone!
- Thread starter Redboy
- Start date
PakProtector
AK member
hey-Hey!!!,
If swapping polarity on the OPT primary doesn't help( or makes it worse ), try reducing the amount of NFB. This will require a larger resistor from the secondary back to the cathode of the input tube.
Also, it wasn't just a gassy tube in the rectifier slot, it was another power tube...
that was no EZ81/6CA4!
cheers,
Douglas
you can also swap secondary leads, instead of the primary, and feed the FB loop from the other end of the winding( what's probably ground now )
If swapping polarity on the OPT primary doesn't help( or makes it worse ), try reducing the amount of NFB. This will require a larger resistor from the secondary back to the cathode of the input tube.
Also, it wasn't just a gassy tube in the rectifier slot, it was another power tube...
that was no EZ81/6CA4!cheers,
Douglas
you can also swap secondary leads, instead of the primary, and feed the FB loop from the other end of the winding( what's probably ground now )
A few random thoughts:
Playing with the windings is a good idea, as is playing with the level of NFB. I've seen and build designs that use a pot instead of a resistor there, so that you can more easily experiment.
If you wanted to do an experiment, and rule in or out the differences in iron conclusively, there is a way to do it: Carefully remove one of the outputs from the original circuit (not the chassis, just the circuit) and clip lead it in to your new amp. It'd be a lot of work, but I know that personally sometimes just knowing a certain fact can make all the difference in my thinking about a problem, and can be worth some time/hassle.
Good luck, I know you'll track it down!
Playing with the windings is a good idea, as is playing with the level of NFB. I've seen and build designs that use a pot instead of a resistor there, so that you can more easily experiment.
If you wanted to do an experiment, and rule in or out the differences in iron conclusively, there is a way to do it: Carefully remove one of the outputs from the original circuit (not the chassis, just the circuit) and clip lead it in to your new amp. It'd be a lot of work, but I know that personally sometimes just knowing a certain fact can make all the difference in my thinking about a problem, and can be worth some time/hassle.
Good luck, I know you'll track it down!
Redboy
a few good watts
Good ideas here, some of which I understand and a few that I really don't! I'm going to try the easy ones first since they're... well, easy!
I swapped the OPT primary leads this morning but didn't have time to test before heading out the door.
Squid, the idea of using a pot instead of a resistor in the NFB was suggested to me earlier (but just after I'd placed the order for parts, of course!), and I might have to do that if initial attempts fail.
Thanks for the help, everyone!
I swapped the OPT primary leads this morning but didn't have time to test before heading out the door.
Squid, the idea of using a pot instead of a resistor in the NFB was suggested to me earlier (but just after I'd placed the order for parts, of course!), and I might have to do that if initial attempts fail.
Thanks for the help, everyone!
Mike Stehr
Poverty Audio
Can a guy actually swap B+ to plate and plate to B+ with a single-ended output transformer?
Yikes! So much for being a smart ass....
5K primary seems more the proper plate load for a 6V6 I thought. Though one go higher in primary impedance for a little more damping factor I guess.
I figured around 7K was the 6BQ5 plate load spec?
The idea of lashing up a Olson OPT might be a option. Is the feedback values you used with your project amp the values used in the little Olson 6BQ5 SEP amplifier?
What value pot does one use in a NFB loop from the sec to driver cathode? Do you just input one side and tap off the center wiper?
I have a breadboard circuit set-up for single-ended 6L6's with a 12AX7 driver. I'm using the James 6123HS OPT's, with the screen taps connected.
I have NFB coming from the secondary to the driver cathode.
The pot would help quite a bit for optimizing feedback. Though using a scope with the pot would be best.....
A change of cathode resistors and primary taps, and the circuit converts to 6V6.
Mike
Redboy
a few good watts
Apparently you can! I did that yesterday and I'm listening to it right now as I type. Cool thing is, it seems to have eliminated my oscillation problem!
Things sound pretty good through my sacrificial Minimus 7s right now. Next thing is to wire up the Forte's so I can really hear what's coming through.
Happy day! I'm pretty excited about this! :thmbsp:
Redboy
a few good watts
Yeah, I'm looking forward to hearing more from this amp. I don't know why I keep buying those Minimus 7s - I only use them for testing, and they sure don't do much for me but I can't keep from picking up every pair I come across! What's wrong with me?!
The 5K primaries are my main concern now. These OPTs were a stab in the dark and they might be my undoing.
Mike Stehr
Poverty Audio
I dunno....I just checked the GE data for 6BQ5. And they show the load resistances from 5200-7000 ohms. Depending how the tube is set-up with respect to voltage, bias, etc...
PakProtector
AK member
hey-Hey!!!,
While it's a legitamate way of reversing polarity, it might be best to swap the secondary polarity. The voltage swing on the plate end of the primary is a lot bigger than the secondary's. In order to minimize effects of the winding capacitance to the secondary, its polarity should probably be maintained. If there isn't much AC voltage on a cap, it isn't going to matter how big it is, there won't be any current flow.
The difference might be subtle, but probably worth investigating.
The size of the NFB resistor is depends on how much FB you want, and on the size of the cathode R that forms part of the voltage divider.
cheers,
Douglas
Redboy
a few good watts
Alright Douglas, I'm going to chew on what you said for awhile to see if I can swallow it... I think I'm understanding about 80 percent of it. Put the primary leads back and swap the secondaries, right? I assume the feedback gets moved too, to stay on the positive side of the output. Right?
I just had things hooked up to the Forte's, and...
...not bad! There's a slight buzz at the top, like the sound of a fluorescent light fixture on the fritz. It's affected by the volume pot - not that it gets louder with increased volume, but the buzz is most intense at the low and high ends of the pot's sweep, and it quiets at midpoint. Should I suspect a grounding issue somewhere? The input jacks I used are not insulated, and they're the only "other" grounds to chassis aside from the main ground that everything else stars to.
How does it sound? Pretty good! I hear slightly better bass response than I get with my 2A3 amp, and it's got great soundstage, like the Olson original. I do think the sound leans toward sibilant - that is, there's slightly more edge to vocals and brass than I'd like. Could this be the my potential difference in primary impedance? It can't be far off if there is a difference - things are darn close to right!
I just had things hooked up to the Forte's, and...
...not bad! There's a slight buzz at the top, like the sound of a fluorescent light fixture on the fritz. It's affected by the volume pot - not that it gets louder with increased volume, but the buzz is most intense at the low and high ends of the pot's sweep, and it quiets at midpoint. Should I suspect a grounding issue somewhere? The input jacks I used are not insulated, and they're the only "other" grounds to chassis aside from the main ground that everything else stars to.
How does it sound? Pretty good! I hear slightly better bass response than I get with my 2A3 amp, and it's got great soundstage, like the Olson original. I do think the sound leans toward sibilant - that is, there's slightly more edge to vocals and brass than I'd like. Could this be the my potential difference in primary impedance? It can't be far off if there is a difference - things are darn close to right!
Redboy
a few good watts
Anyone care to weigh in on the buzz issue? I noticed today that it's affected slightly by other things on the same circuit - I've got a halogen torch lamp with a dimmer switch that affects the buzz slightly when I turn it on or off. This makes me think the issue is somewhere in the amp's power supply. Where should I start with this?
Mike Stehr
Poverty Audio
The easiest thing to do would be to get rid of that lamp.
I'll take a dumb geuss and say your amp is picking up the hash from that lamp.
I've read a lot of posts over the years of folks having noise/hash through their system. And sometimes they have a light circuit with a dimmer switch in the same circuit as the stereo.
I dunno.....
I've had luck beating down AC heater hum by taking a 100 ohm resistor off each heater winding to ground.
This is with a heater winding with no center tap, however.
Though I would guess it wouldn't matter much with a center-tapped winding either.
You can easily do it with the amp flipped over and going, just clip-lead a 100 ohm R from a winding leg to ground.
You should be able to hear the difference in hum.
Sumthin' to try if you want....
I've yet to get better bass response from a single-ended 6bQ5 amp/circuits than my 2A3 SET amp.
Close in tone of bass I suppose, but not in level, or depth. The little SEP 6BQ5 Magnavox has a tone not much unlike the 2A3 SET, just not as loud.
With a very slight sharpness on the mid and top which to me is just a character trait of the 6BQ5/EL-84.
Redboy
a few good watts
Well, I unplugged the lamp and it made no difference. I'm sure there are a lot of other things in the house on that same circuit though, so it's probably worth exploring further.
If what I'm hearing is coming from my house though, how is it that my 2A3 amp - which is plugged into the same socket - can be dead quiet? Could it be that the 2A3 circuit provides better filtering because it has a choke in the PS? This circuit does not.
I guess I'm just thinking out loud and starting to ramble.
That little buzz aside, this project is a complete success! It's starting to sound really good, and I've got no further complaints about the sound.
Mike, I'm definitely getting more bass out of this circuit than my 2A3 amp. Not tons more, but enough that it stands out in my mind. When I initially started listening to my 2A3 amp, my main complaint was that it was bass shy and I know that's not supposed to be the case. Over time I got over that, but when I got this little thing up and running I heard the difference right away. I've got to figure out how to get more out of that 2A3!
If what I'm hearing is coming from my house though, how is it that my 2A3 amp - which is plugged into the same socket - can be dead quiet? Could it be that the 2A3 circuit provides better filtering because it has a choke in the PS? This circuit does not.
I guess I'm just thinking out loud and starting to ramble.
That little buzz aside, this project is a complete success! It's starting to sound really good, and I've got no further complaints about the sound.
Mike, I'm definitely getting more bass out of this circuit than my 2A3 amp. Not tons more, but enough that it stands out in my mind. When I initially started listening to my 2A3 amp, my main complaint was that it was bass shy and I know that's not supposed to be the case. Over time I got over that, but when I got this little thing up and running I heard the difference right away. I've got to figure out how to get more out of that 2A3!
Mike Stehr
Poverty Audio
Try a couple 100 ohm resistors to ground off each heater winding. One should even work off one leg.
I've use the common CRC supplies with respect to SE 6BQ5and haven't had many issues with PS noise.
AC heater growl from a open non CT winding has usually been barely acceptable from my experiences, and a 100-300 ohm R to ground off a winding always seems to help quiet it down.
Try it....if it doesn't work, then you know it may other problems such as inadequate PS filtering.
Redboy
a few good watts
A couple new ideas on the noisy amp issue...
I picked up some insulated input jacks on the off-chance that the cheap ones that currently ground to the chassis are a source of some of my trouble.
If that doesn't help, I have another idea. The original design (the original Olson amp) does not have a grounded power cord, whereas my clone has a three-pronged cord that is grounded to the chassis. Could that be causing trouble? I've read some of the two-prong vs. three-prong threads and I guess I'd rather have three unless someone can convince me otherwise.
I'm stabbing blindly into the dark here.
I picked up some insulated input jacks on the off-chance that the cheap ones that currently ground to the chassis are a source of some of my trouble.
If that doesn't help, I have another idea. The original design (the original Olson amp) does not have a grounded power cord, whereas my clone has a three-pronged cord that is grounded to the chassis. Could that be causing trouble? I've read some of the two-prong vs. three-prong threads and I guess I'd rather have three unless someone can convince me otherwise.
I'm stabbing blindly into the dark here.
Mike Stehr
Poverty Audio
Lift the green ground wire off the chassis and try it...
I need to pony up and finish my SEP Maggotbox.....got that far and got broke.
I need to pony up and finish my SEP Maggotbox.....got that far and got broke.
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