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#1
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Old amp reconditioning and mods questions
I got a Hoffman Stereo Hi-Fi console at the Goodwill because it had a tube
amp in it and I wanted to pirate the amp for a fun project. Well I pulled it out of the console and fired it up and it hummed badly. It has two multi part can style caps on the chassis. I found caps at the local supply house and wired them in below the chassis to the terminals on the original caps. The amp sounds fine now but I figured I'd ask if there are any issues with not having removed the original caps from the circuit? I hooked the caps up with what I think is about 18ga speaker wire, with the copper conductor on one side and silver conductor on the other. This shouldn't cause any issues should it? How hot might one expect the power transformer to get? I can touch it but it's hot enough you don't want to hold you finger on it long. My guess is around 110 - 120 degrees. I have seen reference to adding a modern power socket like a computer would use and using the 3 prong grounded cord, grounding the chassis to help reduce hum or noise. Is this usually OK or might I somehow get myself into trouble doing this? I want to make a wooden enclosure for this with a metal screen top or something like that. It seems that 17" is about the width of most components? Reading the comments along with new tubes it looks like JJ/tesla might be the winner of not being the cheapest or the most expensive, just inexpensive yet good sound. I would need 2 x 12AX7 and 2 x 6BQ5. Should these work well or might I consider others? The amp has a switch for off, stereo 1, stereo 2, mono 1, and mono 2. There are only one set of speaker outputs. Is this maybe a setting for 4 ohm and 8 ohm? I think the volume goes up a bit in the 1 positions. Are there voltages I should be checking or stuff like that? There is a little pot on the back marked HUM what should I do with this adjustment? Well it's late and I am sure there may be more questions but that's all I can think of for now. More than enough for one post anyways. TIA, Bryan |
#2
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Bryan McGivney wrote:
I got a Hoffman Stereo Hi-Fi console at the Goodwill because it had a tube amp in it and I wanted to pirate the amp for a fun project. Well I pulled it out of the console and fired it up and it hummed badly. It has two multi part can style caps on the chassis. I found caps at the local supply house and wired them in below the chassis to the terminals on the original caps. The amp sounds fine now but I figured I'd ask if there are any issues with not having removed the original caps from the circuit? I hooked the caps up with what I think is about 18ga speaker wire, with the copper conductor on one side and silver conductor on the other. This shouldn't cause any issues should it? If the original capacitor decides to short, you may have a problem. I would disconnect the original capacitor. How hot might one expect the power transformer to get? I can touch it but it's hot enough you don't want to hold you finger on it long. My guess is around 110 - 120 degrees. Hot is normal. What type of degrees are you talking about? I have seen reference to adding a modern power socket like a computer would use and using the 3 prong grounded cord, grounding the chassis to help reduce hum or noise. Is this usually OK or might I somehow get myself into trouble doing this? I don't think it will reduce noise or hum, but it will increase safety... I want to make a wooden enclosure for this with a metal screen top or something like that. It seems that 17" is about the width of most components? Most equipment is in the 42-43.5cm range. Reading the comments along with new tubes it looks like JJ/tesla might be the winner of not being the cheapest or the most expensive, just inexpensive yet good sound. I would need 2 x 12AX7 and 2 x 6BQ5. Should these work well or might I consider others? Any new tubes should work fine. But the 12AX7 tubes will most likely work fine for the next 50 years, so I would just leave them. The 6BQ5 may be worn out - just buy a new pair and see if you sound gets better or worse. The amp has a switch for off, stereo 1, stereo 2, mono 1, and mono 2. There are only one set of speaker outputs. Is this maybe a setting for 4 ohm and 8 ohm? I think the volume goes up a bit in the 1 positions. Are there voltages I should be checking or stuff like that? Check the voltage on grid 1 of the 6BQ5 tubes. It should be either 0V or negative. If it's positive you have bad coupling caps that should be replaced. Do you have a schematic? There is a little pot on the back marked HUM what should I do with this adjustment? Turn up the volume with no signal and adjust for minimum hum. Best regards, Mikkel C. Simonsen |
#3
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"Bryan McGivney" wrote in message ...
I got a Hoffman Stereo Hi-Fi console at the Goodwill because it had a tube amp in it and I wanted to pirate the amp for a fun project. Well I pulled it out of the console and fired it up and it hummed badly. It has two multi part can style caps on the chassis. I found caps at the local supply house and wired them in below the chassis to the terminals on the original caps. The amp sounds fine now but I figured I'd ask if there are any issues with not having removed the original caps from the circuit? I hooked the caps up with what I think is about 18ga speaker wire, with the copper conductor on one side and silver conductor on the other. This shouldn't cause any issues should it? How hot might one expect the power transformer to get? I can touch it but it's hot enough you don't want to hold you finger on it long. My guess is around 110 - 120 degrees. I have seen reference to adding a modern power socket like a computer would use and using the 3 prong grounded cord, grounding the chassis to help reduce hum or noise. Is this usually OK or might I somehow get myself into trouble doing this? I want to make a wooden enclosure for this with a metal screen top or something like that. It seems that 17" is about the width of most components? Reading the comments along with new tubes it looks like JJ/tesla might be the winner of not being the cheapest or the most expensive, just inexpensive yet good sound. I would need 2 x 12AX7 and 2 x 6BQ5. Should these work well or might I consider others? The amp has a switch for off, stereo 1, stereo 2, mono 1, and mono 2. There are only one set of speaker outputs. Is this maybe a setting for 4 ohm and 8 ohm? I think the volume goes up a bit in the 1 positions. Are there voltages I should be checking or stuff like that? There is a little pot on the back marked HUM what should I do with this adjustment? Well it's late and I am sure there may be more questions but that's all I can think of for now. More than enough for one post anyways. TIA, Bryan I would remove the original caps,and maybe mount the new ones in thier place,under the chassis is fine also.The old caps may be leaky (electricaly) and drawing excess current from the power tranny.(making it run warmer.) Make sure your new caps are close to the same values,and rated for the voltage,or higher. You might want to use wire rated at 600V..speaker wire might not cut it at B+ voltages. Those JJ tubes should work fine for ya. I think i have a pair in some SE 6BQ5 amps myself. ;-) The HUM control is just that.:-) Adjust it for the least amount of hum. (I believe you'll want to unplug the input first.) Good luck! |
#4
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As for the tubes in it now, they sound pretty good but are mismatched, two
different brands or 6BQ5. The 12AX7's have all the writing rubbed off so I figured I might as well replace them while I was at it since they don't cost all that much. Sort of a continuity of brands sort of thing more than anything else. Another question though, the phono input worked but it was very quiet and sort of thin sounding. I don't have a schematic for the amp. I am guessing that one 12AX7 is the phono stage and the other is the preamp stage. What parts should I check first to get the phono stage working? Do the caps I replaced need to run in for a bit before the sound will be optimal? TIA, Bryan "Fred Nachbaur" wrote in message news:jI6_a.22$zE1.13@edtnps84... Bryan McGivney wrote: I got a Hoffman Stereo Hi-Fi console at the Goodwill because it had a tube amp in it and I wanted to pirate the amp for a fun project. Well I pulled it out of the console and fired it up and it hummed badly. It has two multi part can style caps on the chassis. I found caps at the local supply house and wired them in below the chassis to the terminals on the original caps. The amp sounds fine now but I figured I'd ask if there are any issues with not having removed the original caps from the circuit? There might be down the road, if the old cap decides to short. That being said, this used to be a pretty common service practise, and I've never heard of a problem because of it. In my old age I'm getting marginally more careful about stuff like this, so I think I'd physically disconnect the old caps and put in a couple small terminal strips to which to wire the new ones. I hooked the caps up with what I think is about 18ga speaker wire, with the copper conductor on one side and silver conductor on the other. This shouldn't cause any issues should it? Not other than being kind of ugly. ;-) How hot might one expect the power transformer to get? I can touch it but it's hot enough you don't want to hold you finger on it long. My guess is around 110 - 120 degrees. That sounds about right. I have seen reference to adding a modern power socket like a computer would use and using the 3 prong grounded cord, grounding the chassis to help reduce hum or noise. Is this usually OK or might I somehow get myself into trouble doing this? You'll only get yourself in trouble if you wire it improperly. ;-) I want to make a wooden enclosure for this with a metal screen top or something like that. It seems that 17" is about the width of most components? Yes. Reading the comments along with new tubes it looks like JJ/tesla might be the winner of not being the cheapest or the most expensive, just inexpensive yet good sound. I would need 2 x 12AX7 and 2 x 6BQ5. Should these work well or might I consider others? What's wrong with the ones that are in there? The amp has a switch for off, stereo 1, stereo 2, mono 1, and mono 2. There are only one set of speaker outputs. Is this maybe a setting for 4 ohm and 8 ohm? I think the volume goes up a bit in the 1 positions. It's *probably* meant for 8-ohm speakers. You can measure the DC resistance of the speakers that came in the console to get a pretty good idea. 8-ohm speakers will usually measure around 6 ohms DC resistance or thereabouts. Are there voltages I should be checking or stuff like that? Wouldn't be a bad idea (and would be educational also) if you have the schematic showing the typical voltages at various points. There is a little pot on the back marked HUM what should I do with this adjustment? Adjust for minimum hum. Well it's late and I am sure there may be more questions but that's all I can think of for now. More than enough for one post anyways. Have fun. Be careful. Enjoy the music! Cheers, Fred -- +--------------------------------------------+ | Music: http://www3.telus.net/dogstarmusic/ | | Projects, Vacuum Tubes & other stuff: | | http://www.dogstar.dantimax.dk | +--------------------------------------------+ |
#5
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Bryan McGivney wrote: As for the tubes in it now, they sound pretty good but are mismatched, two different brands or 6BQ5. The 12AX7's have all the writing rubbed off so I figured I might as well replace them while I was at it since they don't cost all that much. Sort of a continuity of brands sort of thing more than anything else. Gotcha. One thing to check for - on the 12AX7's, look at the base, in the center of the ring of pins. If you see a little diamond shape hang on to them, they're Telefunkens. (These are considered "premium", and are notorious for having the lettering rub off.) Another question though, the phono input worked but it was very quiet and sort of thin sounding. I don't have a schematic for the amp. I am guessing that one 12AX7 is the phono stage and the other is the preamp stage. What parts should I check first to get the phono stage working? Your phono stage is *probably* working fine, but more info is needed to be sure. Are you using the turntable and cartridge that came with the console? If so, and the sound is weak and thin, the cartridge is probably no good. * If you're using a turntable with a magnetic cartridge, you've got a different problem entirely. * The phonographs provided with those old consoles were of the "crystal" or "ceramic" type. The crystal type (usually you can tell those because they're fairly large and have an aluminum "pan" under them) dry out over the years and therefore sound weak and tinny. The ceramic type was less prone to that problem, but can crack due to abuse - with the same result. Both of these "piezo-electric" types of cartridges had low compliance (i.e. they chew up records), high output (requiring little preamplification), and needed no de-emphasis (cheap). That's why when you play a magnetic cartridge into those inputs, it sounds quiet (more gain needed) and thin/treble-y (needs de-emphasis). You need an outboard "Magnetic phono preamplifier" to use with this amp, which will accomplish both the gain and de-emphasis. Do the caps I replaced need to run in for a bit before the sound will be optimal? I don't think so, but some people claim to be able to hear a difference between new capacitors and ones that have "broken in." In either case, I wouldn't worry about it. Enjoy the music if the sound is good, fix it if it's not. Cheers, Fred -- +--------------------------------------------+ | Music: http://www3.telus.net/dogstarmusic/ | | Projects: http://dogstar.dantimax.dk | +--------------------------------------------+ |
#6
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"Bryan McGivney" wrote in message ... As for the tubes in it now, they sound pretty good but are mismatched, two different brands or 6BQ5. The 12AX7's have all the writing rubbed off so I figured I might as well replace them while I was at it since they don't cost all that much. Sort of a continuity of brands sort of thing more than anything else. Another question though, the phono input worked but it was very quiet and sort of thin sounding. I don't have a schematic for the amp. I am guessing that one 12AX7 is the phono stage and the other is the preamp stage. What parts should I check first to get the phono stage working? Do the caps I replaced need to run in for a bit before the sound will be optimal? I bet you either have the wrong catridge or a bad cartridge in your phonograph. You probably should have a crystal cartridge. Either it has been replaced with a magnetic one or it is just plain bad. TIA, Bryan "Fred Nachbaur" wrote in message news:jI6_a.22$zE1.13@edtnps84... Bryan McGivney wrote: I got a Hoffman Stereo Hi-Fi console at the Goodwill because it had a tube amp in it and I wanted to pirate the amp for a fun project. Well I pulled it out of the console and fired it up and it hummed badly. It has two multi part can style caps on the chassis. I found caps at the local supply house and wired them in below the chassis to the terminals on the original caps. The amp sounds fine now but I figured I'd ask if there are any issues with not having removed the original caps from the circuit? There might be down the road, if the old cap decides to short. That being said, this used to be a pretty common service practise, and I've never heard of a problem because of it. In my old age I'm getting marginally more careful about stuff like this, so I think I'd physically disconnect the old caps and put in a couple small terminal strips to which to wire the new ones. I hooked the caps up with what I think is about 18ga speaker wire, with the copper conductor on one side and silver conductor on the other. This shouldn't cause any issues should it? Not other than being kind of ugly. ;-) How hot might one expect the power transformer to get? I can touch it but it's hot enough you don't want to hold you finger on it long. My guess is around 110 - 120 degrees. That sounds about right. I have seen reference to adding a modern power socket like a computer would use and using the 3 prong grounded cord, grounding the chassis to help reduce hum or noise. Is this usually OK or might I somehow get myself into trouble doing this? You'll only get yourself in trouble if you wire it improperly. ;-) I want to make a wooden enclosure for this with a metal screen top or something like that. It seems that 17" is about the width of most components? Yes. Reading the comments along with new tubes it looks like JJ/tesla might be the winner of not being the cheapest or the most expensive, just inexpensive yet good sound. I would need 2 x 12AX7 and 2 x 6BQ5. Should these work well or might I consider others? What's wrong with the ones that are in there? The amp has a switch for off, stereo 1, stereo 2, mono 1, and mono 2. There are only one set of speaker outputs. Is this maybe a setting for 4 ohm and 8 ohm? I think the volume goes up a bit in the 1 positions. It's *probably* meant for 8-ohm speakers. You can measure the DC resistance of the speakers that came in the console to get a pretty good idea. 8-ohm speakers will usually measure around 6 ohms DC resistance or thereabouts. Are there voltages I should be checking or stuff like that? Wouldn't be a bad idea (and would be educational also) if you have the schematic showing the typical voltages at various points. There is a little pot on the back marked HUM what should I do with this adjustment? Adjust for minimum hum. Well it's late and I am sure there may be more questions but that's all I can think of for now. More than enough for one post anyways. Have fun. Be careful. Enjoy the music! Cheers, Fred -- +--------------------------------------------+ | Music: http://www3.telus.net/dogstarmusic/ | | Projects, Vacuum Tubes & other stuff: | | http://www.dogstar.dantimax.dk | +--------------------------------------------+ |
#7
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I didn't try the turntable in the console. I had hooked up my Dual with an
Ortofon cartridge. Taking a look at the cartridge in the console it looks like a small cylinder with the needle going into it, no metal plate or anything. It's got a plastic tone arm. I will tinker a bit more and see what I find out. Thanks, Bryan "Jimmy" wrote in message om... "Bryan McGivney" wrote in message ... As for the tubes in it now, they sound pretty good but are mismatched, two different brands or 6BQ5. The 12AX7's have all the writing rubbed off so I figured I might as well replace them while I was at it since they don't cost all that much. Sort of a continuity of brands sort of thing more than anything else. Another question though, the phono input worked but it was very quiet and sort of thin sounding. I don't have a schematic for the amp. I am guessing that one 12AX7 is the phono stage and the other is the preamp stage. What parts should I check first to get the phono stage working? Do the caps I replaced need to run in for a bit before the sound will be optimal? I bet you either have the wrong catridge or a bad cartridge in your phonograph. You probably should have a crystal cartridge. Either it has been replaced with a magnetic one or it is just plain bad. |
#8
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Bryan McGivney wrote:
I didn't try the turntable in the console. I had hooked up my Dual with an Ortofon cartridge. So the amp is made for a crystal cartridge, which means the phono input is basically a line input. You will need a RIAA preamp to use your Dual/Ortofon. Best regards, Mikkel C. Simonsen |
#9
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So I still want to replace the 6BQ5/EL84 since they are not even the same
brands but I looked at some web pages and tried to identify the 12AX7's I have. On one tube I can make out a bit of the writing. What I can make out seems to say in white paint ECC83/12AX7 Made In Holland 915 and in gold paint really faint mCo 49B that's all I can see. I looked on the web for 12AX7 made in Holland and about all I come up with is Amperex. I found this page with pictures http://www.raretubes.com/XEECC83.htm if you go there scroll down to the first three entries for Amperex they say sq. getter, the insides of mine look like those with the very top piece being a D shape with the little flat plate as shown. I am guessing that is the square getter, I am not up on all the tube terms. So going by the insides I would say that is what I have but what's left of the paint doesn't look like it could have ever been that logo. Did any other tubes use guts that looked like that? This guys got them listed at $150 - $175 a pair so I guess I'll keep those. Someone mentioned that they last almost indefinitely? Having fun, Bryan Durham, Oregon. "Bryan McGivney" wrote in message ... As for the tubes in it now, they sound pretty good but are mismatched, two different brands or 6BQ5. The 12AX7's have all the writing rubbed off so I figured I might as well replace them while I was at it since they don't cost all that much. Sort of a continuity of brands sort of thing more than anything else. Another question though, the phono input worked but it was very quiet and sort of thin sounding. I don't have a schematic for the amp. I am guessing that one 12AX7 is the phono stage and the other is the preamp stage. What parts should I check first to get the phono stage working? Do the caps I replaced need to run in for a bit before the sound will be optimal? TIA, Bryan "Fred Nachbaur" wrote in message news:jI6_a.22$zE1.13@edtnps84... Bryan McGivney wrote: I got a Hoffman Stereo Hi-Fi console at the Goodwill because it had a tube amp in it and I wanted to pirate the amp for a fun project. Well I pulled it out of the console and fired it up and it hummed badly. It has two multi part can style caps on the chassis. I found caps at the local supply house and wired them in below the chassis to the terminals on the original caps. The amp sounds fine now but I figured I'd ask if there are any issues with not having removed the original caps from the circuit? There might be down the road, if the old cap decides to short. That being said, this used to be a pretty common service practise, and I've never heard of a problem because of it. In my old age I'm getting marginally more careful about stuff like this, so I think I'd physically disconnect the old caps and put in a couple small terminal strips to which to wire the new ones. I hooked the caps up with what I think is about 18ga speaker wire, with the copper conductor on one side and silver conductor on the other. This shouldn't cause any issues should it? Not other than being kind of ugly. ;-) How hot might one expect the power transformer to get? I can touch it but it's hot enough you don't want to hold you finger on it long. My guess is around 110 - 120 degrees. That sounds about right. I have seen reference to adding a modern power socket like a computer would use and using the 3 prong grounded cord, grounding the chassis to help reduce hum or noise. Is this usually OK or might I somehow get myself into trouble doing this? You'll only get yourself in trouble if you wire it improperly. ;-) I want to make a wooden enclosure for this with a metal screen top or something like that. It seems that 17" is about the width of most components? Yes. Reading the comments along with new tubes it looks like JJ/tesla might be the winner of not being the cheapest or the most expensive, just inexpensive yet good sound. I would need 2 x 12AX7 and 2 x 6BQ5. Should these work well or might I consider others? What's wrong with the ones that are in there? The amp has a switch for off, stereo 1, stereo 2, mono 1, and mono 2. There are only one set of speaker outputs. Is this maybe a setting for 4 ohm and 8 ohm? I think the volume goes up a bit in the 1 positions. It's *probably* meant for 8-ohm speakers. You can measure the DC resistance of the speakers that came in the console to get a pretty good idea. 8-ohm speakers will usually measure around 6 ohms DC resistance or thereabouts. Are there voltages I should be checking or stuff like that? Wouldn't be a bad idea (and would be educational also) if you have the schematic showing the typical voltages at various points. There is a little pot on the back marked HUM what should I do with this adjustment? Adjust for minimum hum. Well it's late and I am sure there may be more questions but that's all I can think of for now. More than enough for one post anyways. Have fun. Be careful. Enjoy the music! Cheers, Fred -- +--------------------------------------------+ | Music: http://www3.telus.net/dogstarmusic/ | | Projects, Vacuum Tubes & other stuff: | | http://www.dogstar.dantimax.dk | +--------------------------------------------+ |
#10
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Bryan McGivney wrote:
So I still want to replace the 6BQ5/EL84 since they are not even the same brands but I looked at some web pages and tried to identify the 12AX7's I have. On one tube I can make out a bit of the writing. What I can make out seems to say in white paint ECC83/12AX7 Made In Holland So you have Philips tubes - aka Mullard, Valvo, Miniwatt, Adzam, Mazda, Amperex. So going by the insides I would say that is what I have but what's left of the paint doesn't look like it could have ever been that logo. Did any other tubes use guts that looked like that? Just look at the brand list I wrote above - all those company names/brands were used by Philips, and they all had different logos. The tubes are the same... This guys got them listed at $150 - $175 a pair so I guess I'll keep those. Someone mentioned that they last almost indefinitely? I have never replaced one so far. The ones I have in equipment from the late 50's still work fine... Best regards, Mikkel C. Simonsen |