I have worked on some tube amps in the past. Basic stuff such as recapping and fixing wiring and bad components. I have decided I want to build a little small bookshelf amp that I can use with computer/mp3 player/streaming. I don't have speakers dedicated to this setup yet but will probably located a set of vintage bookshelfs for the project. I am not going for absolute most awesome sound in the universe with this, just a solid little amp as my first scratch build. I understand compromises will have to be made by using what I already have rather than buying everything new. I have several older 70 volt line transformers that I can convert to air gapped outputs pretty easily. Turns ratio appears to be usable when applying low voltage ac to the secondary and measuring the voltage at the primary. If these turn out poorly I will buy something.
I have kind of settled on either the rh84 (built with 6v6 tubes as I have those and I have found the mods to use the 6v6 instead of the EL84) or this little amp:
Five Tube Stereo Amplifier
I like the simplicity of the above amp. Does anyone have any insight as to how good of a first build this would be?
I have a few 6V6GT outputs, and a 5Y3GT rectifier. I have no power transformer, so I realize that I may end up needing to get a more suitable rectifier tube with less vdrop to match up to my power transformer.
I also don't have the 6SQ7 tubes but I think I have found those locally. I know I can replace these with a single 12ax7 since the circuit only uses the triode section of the 6SQ7. But I will probably source the 6SQ7.
The only other real mod I really want to do is incorporate a couple of em84 tubes up front for eye candy. Which is completely irrelevant to the rest of the design except to make sure I have enough overhead in the power transformer to run those.
Planning to build on an aluminum plate set in wood chassis. Transformers across the back, outputs under aluminum RF can enclosures, power exposed if I get a pretty enclosed one, other wise under a can too. Rectifier tube centered in front of the power transformer, 6V6s flanking it in front of the outputs. 6SQ7s and EM84s up front. Few inches between everything. I have been reading a lot about other people building chassis that have things too close, using twisted pair heater wiring and general layout so I think I have a good handle on keeping everything tidy and reducing issues there.
Am I out in left field? Any advice for a first time scratch builder.
I have kind of settled on either the rh84 (built with 6v6 tubes as I have those and I have found the mods to use the 6v6 instead of the EL84) or this little amp:
Five Tube Stereo Amplifier
I like the simplicity of the above amp. Does anyone have any insight as to how good of a first build this would be?
I have a few 6V6GT outputs, and a 5Y3GT rectifier. I have no power transformer, so I realize that I may end up needing to get a more suitable rectifier tube with less vdrop to match up to my power transformer.
I also don't have the 6SQ7 tubes but I think I have found those locally. I know I can replace these with a single 12ax7 since the circuit only uses the triode section of the 6SQ7. But I will probably source the 6SQ7.
The only other real mod I really want to do is incorporate a couple of em84 tubes up front for eye candy. Which is completely irrelevant to the rest of the design except to make sure I have enough overhead in the power transformer to run those.
Planning to build on an aluminum plate set in wood chassis. Transformers across the back, outputs under aluminum RF can enclosures, power exposed if I get a pretty enclosed one, other wise under a can too. Rectifier tube centered in front of the power transformer, 6V6s flanking it in front of the outputs. 6SQ7s and EM84s up front. Few inches between everything. I have been reading a lot about other people building chassis that have things too close, using twisted pair heater wiring and general layout so I think I have a good handle on keeping everything tidy and reducing issues there.
Am I out in left field? Any advice for a first time scratch builder.
Sounds good. Layout has a few different angles to it such as cooling, EMI, wiring access. You might want to do some sketches or some cardboard cutouts. Placing the OPT between the rectifiers and output valve, and the power tfmr and driver next at either end and close by, tends to give reasonable results. It is sometimes easier to do the heater wiring early.
I find the 6V6 to be capable of very good sound, biased properly and in triode mode. Power is moderate and plate resistance is not particularly low, so you might be looking for some efficient speakers and an OPT with a fair primary inductance (see how you go with gapping)... otherwise, you could always turn into a mid/treble amp where these issues are easier to accommodate.
I find the 6V6 to be capable of very good sound, biased properly and in triode mode. Power is moderate and plate resistance is not particularly low, so you might be looking for some efficient speakers and an OPT with a fair primary inductance (see how you go with gapping)... otherwise, you could always turn into a mid/treble amp where these issues are easier to accommodate.
6v6 can sound very good, I wholeheartedly agree with the triode mode suggestion.
i have a 6v6 amp coupled with a home grown speaker with quite low efficiency although with a very easy impedance graph sounding just great (if not capable of going veery loud).
You could try to fins some 12sn7 or likewise as these sound better than the 12ax7 imho.
i have a 6v6 amp coupled with a home grown speaker with quite low efficiency although with a very easy impedance graph sounding just great (if not capable of going veery loud).
You could try to fins some 12sn7 or likewise as these sound better than the 12ax7 imho.
Thats a great point. I was so focused on the output side even after I asked about the preamp portion. I got sniped over the weekend on ebay for a complete set of silvertone transformers so I am still looking for iron.
I am also starting to consider another build thats even less ambitious. Possibly a headphone amp. I have a largish 120-120 isolation transformer (perhaps its a 120-240...gotta find it again) in my junk box that could provide B+ to a single 6AS7G power stage with a E88CC or such on the front end. I would like a neat little headphone amp.
My search continues. I know I could fork out the cash to Edcore and have nice new transformers. But I enjoy the scrounge.
I am also starting to consider another build thats even less ambitious. Possibly a headphone amp. I have a largish 120-120 isolation transformer (perhaps its a 120-240...gotta find it again) in my junk box that could provide B+ to a single 6AS7G power stage with a E88CC or such on the front end. I would like a neat little headphone amp.
My search continues. I know I could fork out the cash to Edcore and have nice new transformers. But I enjoy the scrounge.
For a 1st build I would consider this one. You will have to share the power supply. The 6V6 needs a bit more drive, so I like the 6SL7 for even phono inputs. The 2nd link is a good little OPT & cheap, not much bass however, but effective. I have some hook up notes on this little bugger around here somewhere,,,,
http://www.notecannons.com/schematics/valco_amp_schematic_510-11_72dpi.jpg
OPT: 70V 10W Line Matching Transformer
http://www.notecannons.com/schematics/valco_amp_schematic_510-11_72dpi.jpg
OPT: 70V 10W Line Matching Transformer
I already have two of those little 70 volt line transformers. Mine have a common plus taps at 1, 2 ,4 and 8 watts on the primary as well as a common plus 4 and 8 ohms on the secondary. One neat thing about mine is the varnish/resin on the core doesn't seem too tough to disassemble and restack for a proper air gap as I have seen in other articles.
I like that design as well. I will try to drag up a few more details about it. Does the treble control act more as an overall tone control in that design?
I like that design as well. I will try to drag up a few more details about it. Does the treble control act more as an overall tone control in that design?
Yes, the tone control works well, but I would say mostly in the treble range, however I can detect more bass with the knob turned to the left. Disclaimer, my ears are not used to hearing bass, more like feeling it ~ It is very responsive.
The schematic is actually a factory schematic of a guitar amp from Valco, circa 1946.
The schematic is actually a factory schematic of a guitar amp from Valco, circa 1946.
The distortion that I see is from allowing the 6SL7 to run slightly negative in response to the typical sine wave, as this is what I normally do. However I also use a "pre volume control" which is normally a 1 meg pot to ground in the input before the first grid connection. Reducing this gain (making more of the input drive signal go to ground) will make the amp run clean. It's just an extra adjustment that you can eventually employ a switch with the typical resistor that you would select would would directly relate to how much sensitivity you need- I.E. "phone" or tape (mp3 player). You'll need more gain for phone, not to mention the RIAA playback curve- another subject for another time. But expect to start off with too much gain, and you can always swap out the 6SL7 for a lesser gain tube directly such as the 6SN7. The good thing about the 6V6 is it can take a bunch of plate voltage. Basically any power transformer out there that is small and has about or more than 370Vac up 440Vac will do fine in this schematic. Note: this link http://members.iinet.net.au/~jamesrr/Legacy/Valve tube data sheets/6v6gt-1h.gif and a 1 ohm resistor in series with your cathode (pin 8) on the 6V6 read across it and limit the current to 100 Ma Max. Use the 330 Ohm resistor to control the current by adding as 20 Ohms at a time. Having a decade resistor box helps out greatly, but you'll need this resistor to be about 10 watts or so.
I tore open a EICO signal tracer (with an eye tube!!!) and with a friend, I showed him how to build this amp for his guitar. It came with a 5Y3 wired up already. Also.....grab some 1 Ohm resistors, 20 watt for setting the filament voltage to 6.3Vac. Any old transformer (XMF) will have 110Vac primaries, which today will produce a filament burning 7Vac+ when hooked up the mains these days, typically 125+Vac.
I would recommend to have some test equipment, quality Fluke digital voltmeter (DVM) and a Variac to slowly add the mains voltage on start up. You can always find a decent name brand DVM at almost any pawn shop for $35 with probes. I use "mini clip" meter probes that fit on the wire and stay there.
So see this one for $39 Eico Signal Tracer Model 147 A Working | eBay
Doug
I tore open a EICO signal tracer (with an eye tube!!!) and with a friend, I showed him how to build this amp for his guitar. It came with a 5Y3 wired up already. Also.....grab some 1 Ohm resistors, 20 watt for setting the filament voltage to 6.3Vac. Any old transformer (XMF) will have 110Vac primaries, which today will produce a filament burning 7Vac+ when hooked up the mains these days, typically 125+Vac.
I would recommend to have some test equipment, quality Fluke digital voltmeter (DVM) and a Variac to slowly add the mains voltage on start up. You can always find a decent name brand DVM at almost any pawn shop for $35 with probes. I use "mini clip" meter probes that fit on the wire and stay there.
So see this one for $39 Eico Signal Tracer Model 147 A Working | eBay
Doug
raven007,
Please realize that my experience is limited.. I could be totally wrong about my general advice, but it works for me ~
Actually I'm sitting on a hoard of 6AN8 (I'll send you a few if you like) tubes and have been looking into putting some time into a design & bench proto. However I would be using a separate amp for the bass-subwolfer, so nice imaging and plenty of upper bass to 16Khz is where I'll want sensitivity to span. Hopefully with a $35- 25watt OPT. I'm still playing with coupling caps vs resistors for interstage connection. For some reason I'm convinced that the higher input Q into the output transformer, the better. But trying different configurations is what it's all about for me.
Doug
Please realize that my experience is limited.. I could be totally wrong about my general advice, but it works for me ~
Actually I'm sitting on a hoard of 6AN8 (I'll send you a few if you like) tubes and have been looking into putting some time into a design & bench proto. However I would be using a separate amp for the bass-subwolfer, so nice imaging and plenty of upper bass to 16Khz is where I'll want sensitivity to span. Hopefully with a $35- 25watt OPT. I'm still playing with coupling caps vs resistors for interstage connection. For some reason I'm convinced that the higher input Q into the output transformer, the better. But trying different configurations is what it's all about for me.
Doug
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I have a 99B fluke scopemeter and a decent DVM (its import but testing for accuracy against the fluke results in same readings so its more sacrificial than the fluke) for test gear. I am really spending a lot of time reading the old RCA manuals but the practical advice is extremely valuable.
As an aside, I now have all of the tubs (NOS RCA from ebay) to build a wheatfield HA-2. I basically need a transformer for that build. And better headphones.
its funny you should mention the 6AN8. While looking at various ways to use my line transformers i came across this:
6AN8 Project Amplifier
That design uses a 6AN8 to drive a push pull pair of 6BQ5s. Are you gonna try to use only the 6AN8s?
I still am not at a level where i feel i can speak intelligently about the nuances of tube amps. I know more than the people around me (but basically if you can describe how a triode works you know more than the average person...) but i feel like I am sailing around on top of a vast ocean filled with tube demons beneath me.
As an aside, I now have all of the tubs (NOS RCA from ebay) to build a wheatfield HA-2. I basically need a transformer for that build. And better headphones.
its funny you should mention the 6AN8. While looking at various ways to use my line transformers i came across this:
6AN8 Project Amplifier
That design uses a 6AN8 to drive a push pull pair of 6BQ5s. Are you gonna try to use only the 6AN8s?
I still am not at a level where i feel i can speak intelligently about the nuances of tube amps. I know more than the people around me (but basically if you can describe how a triode works you know more than the average person...) but i feel like I am sailing around on top of a vast ocean filled with tube demons beneath me.
Ah yes the 6AN8 little powerhouse. You know raven since you have posted this thread, many similar threads have just been started.
I will use these to drive my current fav, the 6V6. But I like the "cyclotron" or symmetrical type circuits. I just may snag that Eico chassis if you don't want it. Actually one of my best working circuits yet & I could use a exact pair of power XMFs anyway.
So my advice is this, get a variac to start with, mandatory in my book. And a couple of "decade boxes" of resistors and one with some small tone caps, say ranging from .005 up to say 25uf.
When you have these, the sky is the limit. What also helped me to realize how versatile my "Ad Hoc" tube creations were to be... was holding the 5 & 6 volt supplies back on the new higher line voltage with those 1 ohm 10 or 20 watt resistors (you can CA them down & never have to worry about them running warm at all) and sit back & enjoy a slightly higher B+ if you so select with your power supply B1, B2, B3 setpoints (resistors). I found a great 1 watt resistor kit on Ebay that had 10 of every value resistor under the sun for $25 shipped. This helped greatly.
The Wheatfield amp looks good at 10Khz, but I'm always skeptical of multi signal (mixed tones) or "plain music" with regard to reproduction accuracy. It's just what sounds nice at the time. I may be married to an angel, but my ears can still wander! Enjoy my man & wear some safety glasses and dive in.
With these little SE's (single ended single output tube) and anything I do with my design 6V6 amps, I try and have the highest possible input impedance into the audio output transformer (typ. 8Kohms or more) as to not load the output voltage swing too much, to have the largest difference between soft & loud signals as possible (dynamic range).
Enjoy, Doug
I will use these to drive my current fav, the 6V6. But I like the "cyclotron" or symmetrical type circuits. I just may snag that Eico chassis if you don't want it. Actually one of my best working circuits yet & I could use a exact pair of power XMFs anyway.
So my advice is this, get a variac to start with, mandatory in my book. And a couple of "decade boxes" of resistors and one with some small tone caps, say ranging from .005 up to say 25uf.
When you have these, the sky is the limit. What also helped me to realize how versatile my "Ad Hoc" tube creations were to be... was holding the 5 & 6 volt supplies back on the new higher line voltage with those 1 ohm 10 or 20 watt resistors (you can CA them down & never have to worry about them running warm at all) and sit back & enjoy a slightly higher B+ if you so select with your power supply B1, B2, B3 setpoints (resistors). I found a great 1 watt resistor kit on Ebay that had 10 of every value resistor under the sun for $25 shipped. This helped greatly.
The Wheatfield amp looks good at 10Khz, but I'm always skeptical of multi signal (mixed tones) or "plain music" with regard to reproduction accuracy. It's just what sounds nice at the time. I may be married to an angel, but my ears can still wander! Enjoy my man & wear some safety glasses and dive in.
With these little SE's (single ended single output tube) and anything I do with my design 6V6 amps, I try and have the highest possible input impedance into the audio output transformer (typ. 8Kohms or more) as to not load the output voltage swing too much, to have the largest difference between soft & loud signals as possible (dynamic range).
Enjoy, Doug
I scored a couple of transformers on ebay. A power with 540 ct and 5 volt and 6.3 volt filament windings. The other one is a 5 and 6.3 volt filament only transformer. I am guessing a bit large for the wheatfield design. But these may prove robust enough to get me back to building a 6v6 unit.
Thank you for all of the advice. I am still collecting bits
Thank you for all of the advice. I am still collecting bits
Excellent!
Soon you will have them coming out of your ears.....like most everyone else around here I presume. I have a gigantic pair. Still can't decide exactly which project they will be assigned to, but that's just the fun of it!
Perfect, 5, 6.3 & HV. Remember, it may indeed be an older xmf requiring a dropping resistor, problem is this, how to check the 5 with the rectifier tube hooked up filament only, (tape the high voltage wires off separately) that is "under load". A variac is very important to use as so you can stop the incoming line voltage at what appears to be the 5.0 Vac with the rect tube in place, going over if & when your line voltage is higher, the tube would have never been exposed to the higher output voltage of your line. Remember the old power transformers (XMFs) used a lower input voltage; 112Vac, these days we typically have 125Vac, so the filament & rectifier voltages coming out of your XMF will be higher, enough to harm the filaments, both 6.3 & 5.0 volts. This is only one of many reasons why using a Variac is a good idea. I find that a 0.8 ohm (radio shack special order) 10 watt, knocks down the voltage just enough (in most cases). As far as the HV goes,,,,he, he, I like to have the extra volts, so I tend to use the voltage dropping resistor on the secondary(s) where needed. Also test the 6.3 filament string with the tubes in place as the unloaded voltage will drop a volt (+/-) unloaded. Maybe you could simply hook 3each 1.0 ohm 5watt resistors in series and with the 5 volt rectifier tube installed & wired, then test the voltage at the tube to see what the filament voltage is, then jumper one resistor at a time to see it be as close to correct as possible. Do the same with the 6.3 volt load as well. I wish you were closer to me as I would fix you up with 1 each Variac.
Can't wait for your next post with some more good news, until then...
Cheers
Doug
Soon you will have them coming out of your ears.....like most everyone else around here I presume. I have a gigantic pair. Still can't decide exactly which project they will be assigned to, but that's just the fun of it!
Perfect, 5, 6.3 & HV. Remember, it may indeed be an older xmf requiring a dropping resistor, problem is this, how to check the 5 with the rectifier tube hooked up filament only, (tape the high voltage wires off separately) that is "under load". A variac is very important to use as so you can stop the incoming line voltage at what appears to be the 5.0 Vac with the rect tube in place, going over if & when your line voltage is higher, the tube would have never been exposed to the higher output voltage of your line. Remember the old power transformers (XMFs) used a lower input voltage; 112Vac, these days we typically have 125Vac, so the filament & rectifier voltages coming out of your XMF will be higher, enough to harm the filaments, both 6.3 & 5.0 volts. This is only one of many reasons why using a Variac is a good idea. I find that a 0.8 ohm (radio shack special order) 10 watt, knocks down the voltage just enough (in most cases). As far as the HV goes,,,,he, he, I like to have the extra volts, so I tend to use the voltage dropping resistor on the secondary(s) where needed. Also test the 6.3 filament string with the tubes in place as the unloaded voltage will drop a volt (+/-) unloaded. Maybe you could simply hook 3each 1.0 ohm 5watt resistors in series and with the 5 volt rectifier tube installed & wired, then test the voltage at the tube to see what the filament voltage is, then jumper one resistor at a time to see it be as close to correct as possible. Do the same with the 6.3 volt load as well. I wish you were closer to me as I would fix you up with 1 each Variac.
Can't wait for your next post with some more good news, until then...
Cheers
Doug
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I have not given up! Still slowly collecting parts and pieces.
Someone gave me a SG-24 sweep generator. It has these tubes in it: 6841, 5UP1, 2X2A, 0A2, 5R4GA, 6X4, 0B2, RTF4, 6005, 12AT7, 12AX7, 5814A, 5654, 6481,5UP1, 2X2A, 0A2, 5R4WGA, 6X4W, 0B2, 3TF4, 6005, 12AT7WA, 12AX7, 5814A, 5654. I am terribly torn about scrapping it versus attempting to repair it. I wish I could find a local ham who would be interested in trading tubes and such for it.
I have two stashes of tubes ot go investigate soon.
Someone gave me a SG-24 sweep generator. It has these tubes in it: 6841, 5UP1, 2X2A, 0A2, 5R4GA, 6X4, 0B2, RTF4, 6005, 12AT7, 12AX7, 5814A, 5654, 6481,5UP1, 2X2A, 0A2, 5R4WGA, 6X4W, 0B2, 3TF4, 6005, 12AT7WA, 12AX7, 5814A, 5654. I am terribly torn about scrapping it versus attempting to repair it. I wish I could find a local ham who would be interested in trading tubes and such for it.
I have two stashes of tubes ot go investigate soon.
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