So I just picked up a Harmon Kardon 250 series integrated amp (Eflea) with a dead OT. I'm going to pull all the iron out of the transformer and see what it looks like at the inner windings. See if I can find an arc through the paper to the iron. I've been successful before in making repairs to the PRIMARY of OT, but never the secondary. Depending on what goes on the core first. Re-wax in a vacuum pot etc.
Anyway, I really like the user interface of the Epic amp, in other words it's got nice knobs and a nice faceplate. If I cant salvage the bad OT I'm going to use this thing anyway in my garage system after a complete strip and rebuild. It's got Baxendall tone controls and some other nice features. Some will change of course.
If it were you, would you go with the Triode Electronics A470 replacements (both of course) or the Hammond 1650H? I'm going to burn KT-66 instead of 6L6GB as in the original. The B+ is 380. I'm going to use Ultra Linear. Other transformers would be considered of course.
Thanks gang, fire away.
Dan
Anyway, I really like the user interface of the Epic amp, in other words it's got nice knobs and a nice faceplate. If I cant salvage the bad OT I'm going to use this thing anyway in my garage system after a complete strip and rebuild. It's got Baxendall tone controls and some other nice features. Some will change of course.
If it were you, would you go with the Triode Electronics A470 replacements (both of course) or the Hammond 1650H? I'm going to burn KT-66 instead of 6L6GB as in the original. The B+ is 380. I'm going to use Ultra Linear. Other transformers would be considered of course.
Thanks gang, fire away.
Dan
Hey Danny -- In general, the Hammond transformers can't hold a candle to the Dynaclone transformers. The only thing in this case that favors the Hammond is that they all use a 40% UL tap, which is a very favorable point for 6L6 class tubes. But their power bandwidth and phase shift is not even in the same ball park as the Dynaclone. If you go the A-470 approach, you will definitely want to use fixed bias, and possibly think about regulated screen pentode operation as opposed to UL. For UL operation, 6L6 class tubes really need no less than 6600 ohms, where as the A-470 is 4300 ohms with 33% taps.
Dave
Dave
I do like those tubes at 6600. Especially at 380 on the plates. So rock and a hard place. The question was asked knowing that Triode A470s are a superior transformer. I've quit using Hammond in my guitar amps and spec my own now.
So thanks, barring some radical shift that's what I'll do, regulate the screens, Dynaclones, and fixed bias. The HK is fixed, with it's current drive arrangement at -27.5 volts. They claimed 25 watts but it's really more like 18. Plenty for Altec VOTT drivers. I hope the transformer is a fixer. You got to pull all the Es and Is out of it, varnish, wax, etc to get at the interior of the coil. I'll bet it's an arc across the paper at the first layer. We'll see. I'm probably dreaming. It's unbelievably tedious. Forget it if the whole transformer has overheated and windings are shorted throughout. You'll never find out the interleave for rewind unless somebody finds a drawing from HK archives. This transformer has a primary to case short.
Don't tell anyone, but I've actually used PT with primary shorts with the case stuck up on a garolite board. Paint the top red and write "case hot!" on there. Not sure if an OT will cooperated with that scene.
Maybe somebody got a HK 250 OT eh? I'll bet a pair of Dynaclones would be cheaper than unearthing one of those. On the other hand, I could sell the iron off this project chassis to finance the Dynaclones. Maybe.
Last edited:
Danny,
Watch your step with KT66s, as you may overtax the power trafo filament winding(s). KT66s draw approx. 40 mA. more heater current, per tube, than 6L6s draw.
The SED (=C=) 6L6GC is a proven winner. I've been hearing good things about New Sensor's TungSol labeled, ST bottle, variant.
Edcor O/P "iron" is easily as good as Hammond and the price is lower. Edcor's model CXPP60-8-6.6K may be just what you need.
Tone controls come with their own set of problems. IMO, Max Robinson's "unity" gain implementation of the Baxendall setup is as good as it gets. To hold the bottle count down, change the AC coupled cathode followers to DC coupled ZVN0545A source followers. Be sure to have switches which totally remove the tone controls from the signal path, when they are not being used.
Watch your step with KT66s, as you may overtax the power trafo filament winding(s). KT66s draw approx. 40 mA. more heater current, per tube, than 6L6s draw.
The SED (=C=) 6L6GC is a proven winner. I've been hearing good things about New Sensor's TungSol labeled, ST bottle, variant.
Edcor O/P "iron" is easily as good as Hammond and the price is lower. Edcor's model CXPP60-8-6.6K may be just what you need.
Tone controls come with their own set of problems. IMO, Max Robinson's "unity" gain implementation of the Baxendall setup is as good as it gets. To hold the bottle count down, change the AC coupled cathode followers to DC coupled ZVN0545A source followers. Be sure to have switches which totally remove the tone controls from the signal path, when they are not being used.
Attachments
Well Gents, I've been wrestling with the pdf scheme to stick it up here so that we can all talk about what's available etc. My printer has decided it won't print pdf today. It did fine a few days ago, so I've messed up a driver default of some sort. And Adobe of course makes it so easy to change format on their prescious documents. I'm going to try a visio import/conversion or Word to change to .doc then go from their to another conversion to get to jpeg so I can post the dang thing.
I may have photoshop express around here somewhere, though I hate it, maybe I'll reload. Here's the url; pages 14-15 http://www.harmankardon.com/resourc...dDocuments/en-US/OwnersManual/EpicA250-om.pdf
Thanks
I may have photoshop express around here somewhere, though I hate it, maybe I'll reload. Here's the url; pages 14-15 http://www.harmankardon.com/resourc...dDocuments/en-US/OwnersManual/EpicA250-om.pdf
Thanks
Danny,
Install Irfan View and then install Ghost Script, both which are free. Converting PDFs to other formats ceases to be problematic.
The DC heater supply is "interesting". Are you planning a revision?
The RCA jacks are probably spaced too closely for modern interconnect cables. That can be dealt with using a nibbling tool to create an opening and installing a piece of FR4 based and Copper clad PCB material therein. While retaining Faraday shielding, new RCA females can be spaced out to fit today's requirements.
Replacing the complicated source selection switching with something straight forward is (IMO) a good idea. Mouser stocks Lorlin switches, which are good performing and modest in cost. Check this catalog page out.
Install Irfan View and then install Ghost Script, both which are free. Converting PDFs to other formats ceases to be problematic.
The DC heater supply is "interesting". Are you planning a revision?
The RCA jacks are probably spaced too closely for modern interconnect cables. That can be dealt with using a nibbling tool to create an opening and installing a piece of FR4 based and Copper clad PCB material therein. While retaining Faraday shielding, new RCA females can be spaced out to fit today's requirements.
Replacing the complicated source selection switching with something straight forward is (IMO) a good idea. Mouser stocks Lorlin switches, which are good performing and modest in cost. Check this catalog page out.
No way in the world that series DC supply stays in place but I don't want to waste the current. It will be regulated with ordinary of the shelf IC based regulator to 12VDC. I've been down that road before. With 123 VAC in the wall, that's a tube killer. One 12AX7 goes away. That's .33 amp at 6.3. I've seen these "scratch filter plates" and "Baxandall plate" before. It's a multi lead sealed unit. I don't know of anybody besides HK that used that stuff. OEM. So like I said, it won't be HK when I'm done, but it will be a handy size and look. I contacted a guy today that may have a transformer for me.
Yes to the switching changes. I dunno, it almost seems that by the end most of the stuff will be changed in some way. The wiring underneath has had many hands in there or HK had the worst QC in the world....something I find hard to believe.
Danny --
A couple of things you might consider, even if you can replace the original OPT that is damaged:
1. This amp is a cathode biased amp. Converting to a true fixed bias system would get you a few more watts and lower the distortion all at the same time. Based on your comment that the amp puts out around 18 watts in stock form, that means that the original OPTs are probably around 6250 ohms plate to plate, which will work very well with 6L6 class tubes with fixed bias and the voltages offered in this amp.
2. Consider helping out the screens. Either some active regulation, or some zeners and a small choke to help limit the drop during increased power demands. Converting to fixed bias and bolstering the screen supply would do complete wonders for this amp. How are the OEM OPTs in the power bandwidth department? Can they handle any increased power at the frequency extremes that these changes would produce?
3. This amp has almost no NFB at all in the power amp stage. It's almost there for show only. With only a 12AU7 gain stage through the loss of a phase splitter stage and the relatively high 6L6 drive requirements, there is precious little OLG for the NFB loop to work with. Looks like they're trying to use some positive FB from the splitter to help the first stage out, but still, there's hardly any gain there at all. With the size of that FB resistor, I be surprised if there is even 10 db NFB. At a minimum, I'd consider a 7247 for the input/splitter tube, with the AX7 section as the gain stage. With the increased power that fixed bias and regulation could produce, the transformers may need some help in the NFB department if they are already stretched. At least it would help the damping factor.
All of this really depends on how good the OEM OPTs are, or what you end up replacing them with. Looks like there's lots of potential in this one for sure!
Dave
A couple of things you might consider, even if you can replace the original OPT that is damaged:
1. This amp is a cathode biased amp. Converting to a true fixed bias system would get you a few more watts and lower the distortion all at the same time. Based on your comment that the amp puts out around 18 watts in stock form, that means that the original OPTs are probably around 6250 ohms plate to plate, which will work very well with 6L6 class tubes with fixed bias and the voltages offered in this amp.
2. Consider helping out the screens. Either some active regulation, or some zeners and a small choke to help limit the drop during increased power demands. Converting to fixed bias and bolstering the screen supply would do complete wonders for this amp. How are the OEM OPTs in the power bandwidth department? Can they handle any increased power at the frequency extremes that these changes would produce?
3. This amp has almost no NFB at all in the power amp stage. It's almost there for show only. With only a 12AU7 gain stage through the loss of a phase splitter stage and the relatively high 6L6 drive requirements, there is precious little OLG for the NFB loop to work with. Looks like they're trying to use some positive FB from the splitter to help the first stage out, but still, there's hardly any gain there at all. With the size of that FB resistor, I be surprised if there is even 10 db NFB. At a minimum, I'd consider a 7247 for the input/splitter tube, with the AX7 section as the gain stage. With the increased power that fixed bias and regulation could produce, the transformers may need some help in the NFB department if they are already stretched. At least it would help the damping factor.
All of this really depends on how good the OEM OPTs are, or what you end up replacing them with. Looks like there's lots of potential in this one for sure!
Dave
Hey Dave:
Thanks for the comments. I dunno about the OT. Seems the Citation was always right there at the head of the pack with McIntosh etc. In my mind anyway, over the top of Fisher/Scott etc. They were more expensive. Anyway, they're big.
I saw that 27.5 V at the cathodes and just passed it off as fixed bias. No. You're right. It's cathode biased from the balance pot. The cathode bypass cap is back there after the filament connection. Hmmmm. That 27.5 volts is "Borrowed" from the cathodes to stick the filaments above ground to curtail spurrious migration of hum electrons across from the filament to cathode. Clever but usually you want it a good deal higher than the power tube bias voltage. Hmmmmm.
I don't know from 7247s. Today is a travel day for me and I don't expect to be back until this evening. I'll check it out later.
Thanks guys. More to follow
Danny
Thanks for the comments. I dunno about the OT. Seems the Citation was always right there at the head of the pack with McIntosh etc. In my mind anyway, over the top of Fisher/Scott etc. They were more expensive. Anyway, they're big.
I saw that 27.5 V at the cathodes and just passed it off as fixed bias. No. You're right. It's cathode biased from the balance pot. The cathode bypass cap is back there after the filament connection. Hmmmm. That 27.5 volts is "Borrowed" from the cathodes to stick the filaments above ground to curtail spurrious migration of hum electrons across from the filament to cathode. Clever but usually you want it a good deal higher than the power tube bias voltage. Hmmmmm.
I don't know from 7247s. Today is a travel day for me and I don't expect to be back until this evening. I'll check it out later.
Thanks guys. More to follow
Danny
Ok. Amp arrived today. Funny thing, somebody has been in that transformer before. Imagine that. Anyway, no harm, whoever it was was looking for something obvious.
No continuity between primary outside wires. 194ohms between center tap and one end of the primary. Open the other way. 595ohms between common tap of secondary and primary center tap. Go figure. Somebody got hot.
Good news: With the caps pulled from the transformer the interleave is obvious. I can measure wire size, calculate turns from the good transformer and work a deal with Phil at Heyboer to tear apart the iron, let him duplicate the coil, and put it all back together. It will likely cost more than new iron but in the end it will still be HK. I intend to disconnect a lot of stuff but leave most of it intact with new components. I'll never run a turntable for instance, (who needs two kinds of vinyl equalization anyway?) but somebody might when I tire of it. Line stage only with CD, tuner and an Aux. All that RCA connector mess has to go. It's rusted out and as noted by ELI not fit for modern connectors.
I like that Baxandall circuit Eli, way to go. How much loss through it do you think? Does it need a recovery stage or is it a unity gain stage in it's own right?
Hey Dave: Why go to the trouble of a 12DW7 when a Cathodyne phase inverter is a unity gain or less device anyway? Can't I just use a 12AT7 or 12AX7 for a bit more push?
The problem with these HK 250s and why they often don't have two good output transformers is that the bias scheme is amongst the worst possible. Loose the wiper of the balance pot and away she goes. I'm sure that's what happened. The primary wires are toasted. The interleave is much more straight forward though than (say) a Hammond Organ transformer so should not pose much difficulty duplicating.
I hope Phil will work with me. If I place a $500 dollar order for some guitar amp OTs maybe he'll come along for the ride.
On another note, this amp has been underwater. I've rebuilt Dynaco 70s that have been underwater but this one wasn't dried out enough when somebody threw the juice to it. Been dead a long time is my guess. It's got smokey wires all over the place.
I'm gonna strip it to the bone. Use this process to make it shiney:
Cupreous Metal (Copper, Bronze, Brass) Conservation - Conservation Manual - Conservation Research Laboratory - Center for Maritime Archaeology and Conservation - Texas A&M University
Also the OT is not well waxed or varnished. When I get the OT repaired I'll vacuum wax fill all three transformers.
What do you guys think of 5881s in place of the 6L6GB? I've got a matched quad of 1960's vintage Tung Sol's (brown bakelite base). My experience in guitar amps is that they lack a little in the bass department but the mids to highs are a delight. Lot's of sweet harmonic content. Maybe with some help here, we can design around that; oversize a bypass cap or something. Might be as simple as tube rolling. I think Sylvania's older 12AX7 has a little richer bass than some. I haven't measured the primary impedance on the good one yet; KT66s are still a possibility with an added 6 amp filament transformer; there's room. If the existing OT are closer to 5K than 6.6K it's going to be 6L6/5881 of some type.
Thanks for the help,
More later.
No continuity between primary outside wires. 194ohms between center tap and one end of the primary. Open the other way. 595ohms between common tap of secondary and primary center tap. Go figure. Somebody got hot.
Good news: With the caps pulled from the transformer the interleave is obvious. I can measure wire size, calculate turns from the good transformer and work a deal with Phil at Heyboer to tear apart the iron, let him duplicate the coil, and put it all back together. It will likely cost more than new iron but in the end it will still be HK. I intend to disconnect a lot of stuff but leave most of it intact with new components. I'll never run a turntable for instance, (who needs two kinds of vinyl equalization anyway?) but somebody might when I tire of it. Line stage only with CD, tuner and an Aux. All that RCA connector mess has to go. It's rusted out and as noted by ELI not fit for modern connectors.
I like that Baxandall circuit Eli, way to go. How much loss through it do you think? Does it need a recovery stage or is it a unity gain stage in it's own right?
Hey Dave: Why go to the trouble of a 12DW7 when a Cathodyne phase inverter is a unity gain or less device anyway? Can't I just use a 12AT7 or 12AX7 for a bit more push?
The problem with these HK 250s and why they often don't have two good output transformers is that the bias scheme is amongst the worst possible. Loose the wiper of the balance pot and away she goes. I'm sure that's what happened. The primary wires are toasted. The interleave is much more straight forward though than (say) a Hammond Organ transformer so should not pose much difficulty duplicating.
I hope Phil will work with me. If I place a $500 dollar order for some guitar amp OTs maybe he'll come along for the ride.
On another note, this amp has been underwater. I've rebuilt Dynaco 70s that have been underwater but this one wasn't dried out enough when somebody threw the juice to it. Been dead a long time is my guess. It's got smokey wires all over the place.
I'm gonna strip it to the bone. Use this process to make it shiney:
Cupreous Metal (Copper, Bronze, Brass) Conservation - Conservation Manual - Conservation Research Laboratory - Center for Maritime Archaeology and Conservation - Texas A&M University
Also the OT is not well waxed or varnished. When I get the OT repaired I'll vacuum wax fill all three transformers.
What do you guys think of 5881s in place of the 6L6GB? I've got a matched quad of 1960's vintage Tung Sol's (brown bakelite base). My experience in guitar amps is that they lack a little in the bass department but the mids to highs are a delight. Lot's of sweet harmonic content. Maybe with some help here, we can design around that; oversize a bypass cap or something. Might be as simple as tube rolling. I think Sylvania's older 12AX7 has a little richer bass than some. I haven't measured the primary impedance on the good one yet; KT66s are still a possibility with an added 6 amp filament transformer; there's room. If the existing OT are closer to 5K than 6.6K it's going to be 6L6/5881 of some type.
Thanks for the help,
More later.
Last edited:
"Hey Dave: Why go to the trouble of a 12DW7 when a Cathodyne phase inverter is a unity gain or less device anyway? Can't I just use a 12AT7 or 12AX7 for a bit more push?"
For all practical purposes, any of the common dual triodes are unity gain in cathodyne service. However, the lower mu tubes like the 12AU7 have more current capability, and therefore can provide greater output at lower impedances. The problem is that both triodes in a 12AU7 have the same characteristics, so it's use in the AF amp portion provides little gain. You could switch to a 12AX7 for more gain in the AF amp section, but then the inverter section's drive capability would suffer.
The idea behind the 12DW7 is that it is made up of one half 12AU7, and one half 12AX7. If you use the 12AU7 half for the inverter and the 12AX7 portion for the AF amplifer, you get the best of both worlds.
Sounds like this project will keep you busy for a while! Good luck!
Dave
For all practical purposes, any of the common dual triodes are unity gain in cathodyne service. However, the lower mu tubes like the 12AU7 have more current capability, and therefore can provide greater output at lower impedances. The problem is that both triodes in a 12AU7 have the same characteristics, so it's use in the AF amp portion provides little gain. You could switch to a 12AX7 for more gain in the AF amp section, but then the inverter section's drive capability would suffer.
The idea behind the 12DW7 is that it is made up of one half 12AU7, and one half 12AX7. If you use the 12AU7 half for the inverter and the 12AX7 portion for the AF amplifer, you get the best of both worlds.
Sounds like this project will keep you busy for a while! Good luck!
Dave
Ok, I didn't know that. The 12AU7 is often used in Flea power amps as a stereo output tube. Like two 6C4 in one bottle. But if the power tubes operate in AB1 the 12AU7 section will never pass power, only voltage. So I need to study up on lower mu vs output impedance. Just intuitively R=E/I means that R is equal to voltage divided by nothing, regardless of tube. The answer has got to be the internal R of the tube. Gotta open the resistive coupled amps section again.
Anyway, I bid on two lots of Jan 12DW7 over the weekend and missed them both.
Thanks
Dan
Dan,
Stop fretting about 12DW7s. JJ is currently producing them.
Pay attention to gm. High gm is protection against slew limiting.
Max Robinson's implementation of the Baxendall tone control setup is "unity" gain. The issue, as I see it, is bottle count. IMO, a pair of 'X7s/channel, just for tone controls, is a bit much. Replace the cap. coupled cathode followers with DC coupled ZVN0545A source followers and you're down to 1 bottle/channel.
Stop fretting about 12DW7s. JJ is currently producing them.
Pay attention to gm. High gm is protection against slew limiting.
Max Robinson's implementation of the Baxendall tone control setup is "unity" gain. The issue, as I see it, is bottle count. IMO, a pair of 'X7s/channel, just for tone controls, is a bit much. Replace the cap. coupled cathode followers with DC coupled ZVN0545A source followers and you're down to 1 bottle/channel.
Eli: I had to go to Wikipedia twice just to understand you in your last post. Slew rate is actually thrown around in guitar amp design because Ken Fisher (Trainwreck designer as well as Komet amps. Died 2003?) eschewed Teflon wire because he felt that polypro insulation had a better slew rate at guitar harmonic frequencies.
I shall get out my Radiotron Designer's Handbook (3rd) and take another look but what I remember is that gm= change of I out/change of E in = transconductance. So I'll get out the tube manual and try to figure out what part of that I missed. I gather from your comment that the AU7 has higher transconductance and results in a higher slew rate; which echos Dave's advice to get the 12DW7 stupid and quit arguing. I'm OK with that. Sounds good.
I must say, that on a general level, you hi fi guys are way ahead theory wise of most of us old hams and guitar holders.
Now, with regard the transistor source follower. Seeing as how this amp will loose it's phono section the bottle count will remain the same and (I'm afraid even to say this) I don't want any silicon between my source (said CD player being a box full of sand) and my speakers. I would just as soon just put in the extra pair of tubes. I'll try to splice together and re-draw the HK tone control circuit for comparision. It's more like this http://www.duncanamps.com/technical/baxandall.html But having said that, if you would be so kind, (I can't wire a transistor other than a darlington, which I took the time to understand) could you draw in said ZVN0545A source followers so that I can learn what in the world you are talking about? I would be indebted to you. I'm not being obsequious here, I just have never fooled with silicon outside the power supply (I use Fairchild stealth diodes and diode packages. Have you tried this one: http://www.mouser.com/catalog/specsheets/ISISL9K8120P3.pdf Lovely, tough, and quiet)
Anyway, I use JJ most of the time so I will go that route if I can't find a pair of Jan Philips for $25.
Now, both of you have avoided the 5881 question. No comment?
And by the way, I think after your answers, we'll let this one die. The thread title is misleading. We'll call it Resurrection of the HK Epic 250 integrated amp. We might not attract quite the same class of people but it's worth the chance. ;o)
I shall get out my Radiotron Designer's Handbook (3rd) and take another look but what I remember is that gm= change of I out/change of E in = transconductance. So I'll get out the tube manual and try to figure out what part of that I missed. I gather from your comment that the AU7 has higher transconductance and results in a higher slew rate; which echos Dave's advice to get the 12DW7 stupid and quit arguing. I'm OK with that. Sounds good.
I must say, that on a general level, you hi fi guys are way ahead theory wise of most of us old hams and guitar holders.
Now, with regard the transistor source follower. Seeing as how this amp will loose it's phono section the bottle count will remain the same and (I'm afraid even to say this) I don't want any silicon between my source (said CD player being a box full of sand) and my speakers. I would just as soon just put in the extra pair of tubes. I'll try to splice together and re-draw the HK tone control circuit for comparision. It's more like this http://www.duncanamps.com/technical/baxandall.html But having said that, if you would be so kind, (I can't wire a transistor other than a darlington, which I took the time to understand) could you draw in said ZVN0545A source followers so that I can learn what in the world you are talking about? I would be indebted to you. I'm not being obsequious here, I just have never fooled with silicon outside the power supply (I use Fairchild stealth diodes and diode packages. Have you tried this one: http://www.mouser.com/catalog/specsheets/ISISL9K8120P3.pdf Lovely, tough, and quiet)
Anyway, I use JJ most of the time so I will go that route if I can't find a pair of Jan Philips for $25.
Now, both of you have avoided the 5881 question. No comment?
And by the way, I think after your answers, we'll let this one die. The thread title is misleading. We'll call it Resurrection of the HK Epic 250 integrated amp. We might not attract quite the same class of people but it's worth the chance. ;o)
Last edited:
No phono? HISS, BOO!
Since you have nice 5881s, use them.
FETs, like tubes, are voltage controlled, high impedance, devices. The drain curves of FETs strongly resemble the plate curves of pentodes. The ZVN0545A in an enhancement mode device and will not conduct unless the gate is forward biased. That's very convenient for DC coupling to tube anodes. I've uploaded a tweaked version of the "classic" passive EQ RCA phono stage that employs a ZVN0545A source follower at its O/P. Notice the gate stopper. Once again, like tubes, parasitic oscillation requires suppression.
BTW, when working with MOSFETs, wear an anti-static wrist strap. Metal oxide substrate (MOS) semiconductors are very sensitive to static discharge. Once wired in place, they are OK. Careful handling until wired up is essential.
Since you have nice 5881s, use them.
FETs, like tubes, are voltage controlled, high impedance, devices. The drain curves of FETs strongly resemble the plate curves of pentodes. The ZVN0545A in an enhancement mode device and will not conduct unless the gate is forward biased. That's very convenient for DC coupling to tube anodes. I've uploaded a tweaked version of the "classic" passive EQ RCA phono stage that employs a ZVN0545A source follower at its O/P. Notice the gate stopper. Once again, like tubes, parasitic oscillation requires suppression.
BTW, when working with MOSFETs, wear an anti-static wrist strap. Metal oxide substrate (MOS) semiconductors are very sensitive to static discharge. Once wired in place, they are OK. Careful handling until wired up is essential.
Attachments
Last edited:
I moved on a boat for the first time in about '73? Something like that. I went to a very nice Nakamichi tape system and transferred all my vinyl. Then as life moved along, CDs came out, and the guy who stored all my truely priceless vinyl collection (mostly Baroque, mostly Vivaldi, and some early rock) died and his estate was long gone by the time I found out. Etc.
I'm uninterested in starting to chase vinyl again. Besides, this thing is going in the garage. If I build an indoor setup (based ,as Dave knows, on Model 19 Altec Lansing speakers.) then maybe I'll add a turntable, moving coil cartridge, $2K worth of input transformers etc.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.