Good idea, I'll try that today.
With no EQ or tone controls, the amp seems a bit muddy and midrangey.
-Gnobuddy
I like the two-knob version of Fender's FMV tone control better than the three-knob version, with its bizarrely ill-behaved mid control. So I spent a little time on my lunch-break tweaking the two-knob tone control of the Fender AA764 Champ, until I got the curves you see in the attached image.
Interestingly, in the simulation, the tone control behaves better with a log pot for bass, and a linear pot for treble.
I will try inserting this between triode and pentode. I'll probably need a MOSFET gain stage there as well, to make up the gain lost in the tone control.
I don't have a 68 pF cap handy, which is why you see 100 pF in series with 220 pF in the schematic. They combine to produce 68 pF.
Yesterday I tinkered with the amp a little more, sticking an external preamp in between guitar and 6JW8 input. A gain of 20 dB in that external preamp seems about right for (mostly) clean tones from the 6JW8, though that might change when I add the tone-stack between triode and pentode.
I'm thinking of trying out this topology: MOSFET input, gain control, 6JW8 triode, tone control, MOSFET gain stage + phase inversion, 6JW8 pentode.
-Gnobuddy
Interestingly, in the simulation, the tone control behaves better with a log pot for bass, and a linear pot for treble.
I will try inserting this between triode and pentode. I'll probably need a MOSFET gain stage there as well, to make up the gain lost in the tone control.
I don't have a 68 pF cap handy, which is why you see 100 pF in series with 220 pF in the schematic. They combine to produce 68 pF.
Yesterday I tinkered with the amp a little more, sticking an external preamp in between guitar and 6JW8 input. A gain of 20 dB in that external preamp seems about right for (mostly) clean tones from the 6JW8, though that might change when I add the tone-stack between triode and pentode.
I'm thinking of trying out this topology: MOSFET input, gain control, 6JW8 triode, tone control, MOSFET gain stage + phase inversion, 6JW8 pentode.
-Gnobuddy
Attachments
It's been a busy week (and a difficult one due to a family friend's very poor health), but I just finished soldering together the tone control from post #43 during my lunch break today. I will try and plug it into my mini-amp tonight after work.
Tinkering with LTSpice a little more, I realized I could add one more capacitor (C5 in the attached image) to roll off harsh high treble starting above 5 kHz or so. C5 forms a low-pass filter working with the output impedance of the triode gain stage driving the tone control, which I've estimated at around 25 kilo ohms.
The idea is for this treble roll off to help out the speaker-emulation filter which I will eventually add to the output of this amp, so it can plug into a P.A. system.
I played through the little amp for a couple of minutes before leaving for work this morning. I think it produces about the right volume level for daytime apartment use, at least with the ancient thrift-store speaker I'm currently using.
Even without the tone control, it definitely needs another gain stage. With the additional tone control insertion loss, it might need two. I have a few 6AQ6s, but I don't think I can get them to work on the meagre 135-volt B+ rail. So I need to consider other alternatives.
A second 6JW8 is one possibility. But that requires me to make a second breadboard adapter. A MOSFET or two would be simpler, and the two valve stages in this amp might still provide all the "valvey" sound signature we want. Some more tinkering is in order.
-Gnobuddy
Tinkering with LTSpice a little more, I realized I could add one more capacitor (C5 in the attached image) to roll off harsh high treble starting above 5 kHz or so. C5 forms a low-pass filter working with the output impedance of the triode gain stage driving the tone control, which I've estimated at around 25 kilo ohms.
The idea is for this treble roll off to help out the speaker-emulation filter which I will eventually add to the output of this amp, so it can plug into a P.A. system.
I played through the little amp for a couple of minutes before leaving for work this morning. I think it produces about the right volume level for daytime apartment use, at least with the ancient thrift-store speaker I'm currently using.
Even without the tone control, it definitely needs another gain stage. With the additional tone control insertion loss, it might need two. I have a few 6AQ6s, but I don't think I can get them to work on the meagre 135-volt B+ rail. So I need to consider other alternatives.
A second 6JW8 is one possibility. But that requires me to make a second breadboard adapter. A MOSFET or two would be simpler, and the two valve stages in this amp might still provide all the "valvey" sound signature we want. Some more tinkering is in order.
-Gnobuddy
Attachments
Well, I got the tone control inserted between the triode and pentode. An ART Tube MP/C ( https://www.sweetwater.com/store/detail/TubeMPC--art-tube-mp-c ) acts as an external preamp, between the guitar and the 6JW8 triode. I hooked up a home-made cab with an 8" public address speaker mounted in it.
Some quick observations:
1) The tone control works, and quite well.
2) With this nominally 92 dB/W@1m speaker, the treble end of the spectrum is too loud for night-time apartment use. Bass, on the other hand, is too quiet to hear properly. We're probably bumping into ye olde Fletcher-Munson loudness contours here - our ears are very insensitive to bass at low volumes.
3) There is sufficient nonlinearity in both the 6JW8 triode, and the 6JW8 pentode, that there is no 100% clean tone. It sounds clean enough for single-note picking, but strum a chord lightly, and there is a little "hair" on the sound. Jazz players would not be happy, but pop and country players probably would not object.
If the speaker is replaced by a speaker emulation filter, and the output fed to a P.A., the signal level to the 6JW8 can be reduced while still making enough SPL to be clearly audible. This might allow a purer clean tone to be accessed.
4) With my external preamp replaced with one more gain stage, this might be a fun little amp for someone who wants to make a little noise with a guitar, and who doesn't want Twin-Reverb levels of truly clean tone.
For my purposes, I may have taken this as far as it can go while driving a speaker - it will never be loud enough to properly develop the bass tone, so it may be time to ditch the output transformer, replace it with a load resistor, and start developing what I actually wanted, something to sweeten the tone of a guitar before it goes into the P.A.
I will try and post a short sound clip in the next few days. Life is hitting me with a lot of stuff at the moment, so it may not happen immediately.
-Gnobuddy
Some quick observations:
1) The tone control works, and quite well.
2) With this nominally 92 dB/W@1m speaker, the treble end of the spectrum is too loud for night-time apartment use. Bass, on the other hand, is too quiet to hear properly. We're probably bumping into ye olde Fletcher-Munson loudness contours here - our ears are very insensitive to bass at low volumes.
3) There is sufficient nonlinearity in both the 6JW8 triode, and the 6JW8 pentode, that there is no 100% clean tone. It sounds clean enough for single-note picking, but strum a chord lightly, and there is a little "hair" on the sound. Jazz players would not be happy, but pop and country players probably would not object.
If the speaker is replaced by a speaker emulation filter, and the output fed to a P.A., the signal level to the 6JW8 can be reduced while still making enough SPL to be clearly audible. This might allow a purer clean tone to be accessed.
4) With my external preamp replaced with one more gain stage, this might be a fun little amp for someone who wants to make a little noise with a guitar, and who doesn't want Twin-Reverb levels of truly clean tone.
For my purposes, I may have taken this as far as it can go while driving a speaker - it will never be loud enough to properly develop the bass tone, so it may be time to ditch the output transformer, replace it with a load resistor, and start developing what I actually wanted, something to sweeten the tone of a guitar before it goes into the P.A.
I will try and post a short sound clip in the next few days. Life is hitting me with a lot of stuff at the moment, so it may not happen immediately.
-Gnobuddy
And now for a rather unexpected twist to the tale.
Today I made up an extension cable so I could plug the 6JW8 mini-amp output into the stock speaker in my Princeton Reverb.
While I was plugging the extension cable into the output jack on the mini amp, I noticed something odd about the 4.7 uF cathode bypass cap on the 6JW8 triode. The positive lead was plugged into the wrong hole on the breadboard, one tenth of an inch away from making a connection to the cathode. The triode had been operating with an unbypassed 2.7k cathode resistor all this time.
I fixed the cap, hooked up the Princeton speaker (still in its stock combo cab), and gave the amp another quick test with my beater Epiphone guitar.
Using a real guitar speaker made a dramatic difference. The mini-amp is now much louder. So much louder that I can turn it down well below max power, and so access a quite nice clean tone. Not a squeaky-clean Hi-Fi clean by any means, but a lovely "valvey" clean guitar tone that clearly has some additional harmonic content in it beyond what the guitar is actually putting out.
Plugged into the Princeton's speaker, I also found I had to use far less treble boost from the tone control, because the speaker in the Princeton has the usual strong treble emphasis that most guitar speakers have, and that the P.A. speaker I'd been using before does not.
The volume is now so much louder that I can also dial in bass boost without clipping the amp, making it possible to make it sound big and beefy at close range. It doesn't have the thin tinny sound typical of many small toy guitar amps.
In fact the volume is now so much louder that only thing I can't test is how the overdrive sounds through the Princeton speaker - because this little tiddly 1/4 watt amp is too loud to use at full bellow in my apartment!
The original intention with this amp was to feed its output to a P.A. system. I never expected this amp to be much more than a toy in stand-alone mode (with an output transformer driving a speaker directly).
To my surprise, it turns out that, as long as you have a proper guitar speaker and cab hooked up, it's much more capable than I expected. It's more than loud enough for an apartment, and I suspect it would even work for a typical coffee-shop gig - it's probably about as loud as strumming an unplugged acoustic guitar, maybe a bit more.
The economic equation is odd, though. The mini-amp is built around a $1 valve, a $55 output transformer, and a 10" guitar speaker whose equivalent probably can't be purchased for much less than $50 - $100 CAD ( Guitar Speakers | 10" Guitar Speakers ).
On the other hand, judging by the dozens and dozens of Internet posts and threads I've read on the subject, some of them going back ten or twenty years, it seems a lot of guitar players have been looking for a real valve guitar amp that they can use in their den or living room. I find this little prototype surprisingly good-sounding, even in it's present unfinished state. Maybe some other guitarists might like it.
I know, I know, I have to make a little recording and post it...
Anyone know the maximum file-size for a diyAudio attachment?
-Gnobuddy
Today I made up an extension cable so I could plug the 6JW8 mini-amp output into the stock speaker in my Princeton Reverb.
While I was plugging the extension cable into the output jack on the mini amp, I noticed something odd about the 4.7 uF cathode bypass cap on the 6JW8 triode. The positive lead was plugged into the wrong hole on the breadboard, one tenth of an inch away from making a connection to the cathode. The triode had been operating with an unbypassed 2.7k cathode resistor all this time.
I fixed the cap, hooked up the Princeton speaker (still in its stock combo cab), and gave the amp another quick test with my beater Epiphone guitar.
Using a real guitar speaker made a dramatic difference. The mini-amp is now much louder. So much louder that I can turn it down well below max power, and so access a quite nice clean tone. Not a squeaky-clean Hi-Fi clean by any means, but a lovely "valvey" clean guitar tone that clearly has some additional harmonic content in it beyond what the guitar is actually putting out.
Plugged into the Princeton's speaker, I also found I had to use far less treble boost from the tone control, because the speaker in the Princeton has the usual strong treble emphasis that most guitar speakers have, and that the P.A. speaker I'd been using before does not.
The volume is now so much louder that I can also dial in bass boost without clipping the amp, making it possible to make it sound big and beefy at close range. It doesn't have the thin tinny sound typical of many small toy guitar amps.
In fact the volume is now so much louder that only thing I can't test is how the overdrive sounds through the Princeton speaker - because this little tiddly 1/4 watt amp is too loud to use at full bellow in my apartment!
The original intention with this amp was to feed its output to a P.A. system. I never expected this amp to be much more than a toy in stand-alone mode (with an output transformer driving a speaker directly).
To my surprise, it turns out that, as long as you have a proper guitar speaker and cab hooked up, it's much more capable than I expected. It's more than loud enough for an apartment, and I suspect it would even work for a typical coffee-shop gig - it's probably about as loud as strumming an unplugged acoustic guitar, maybe a bit more.
The economic equation is odd, though. The mini-amp is built around a $1 valve, a $55 output transformer, and a 10" guitar speaker whose equivalent probably can't be purchased for much less than $50 - $100 CAD ( Guitar Speakers | 10" Guitar Speakers ).
On the other hand, judging by the dozens and dozens of Internet posts and threads I've read on the subject, some of them going back ten or twenty years, it seems a lot of guitar players have been looking for a real valve guitar amp that they can use in their den or living room. I find this little prototype surprisingly good-sounding, even in it's present unfinished state. Maybe some other guitarists might like it.
I know, I know, I have to make a little recording and post it...
Anyone know the maximum file-size for a diyAudio attachment?
-Gnobuddy
I stumbled across this EL34 World thread, and it featured a tiny little two-pentode amp (the "Tiny Giant") that might be suitable for this thread.
Here's the thread: Fun weekend project - Tiny Giant
I'm attaching the schematic. All copyrights belong to the original owners, of course. This is not my work in any way, shape, or form.
Speaking of my work, I will return to my own amp build soon. Just dealing with some difficult life stuff at the moment and needed a few days to get through it and recover my equilibrium.
-Gnobuddy
Here's the thread: Fun weekend project - Tiny Giant
I'm attaching the schematic. All copyrights belong to the original owners, of course. This is not my work in any way, shape, or form.
Speaking of my work, I will return to my own amp build soon. Just dealing with some difficult life stuff at the moment and needed a few days to get through it and recover my equilibrium.
-Gnobuddy
Attachments
I did a little tinkering with the curves from a 6AQ6 datasheet, and I think maybe I can make a 6AQ6 gain stage work with only 135V B+ volts. I won't be able to bias it to Vgk=-1.5 V, though, so it may overload more easily than a typical half-12AX7 stage. We'll see.
Tonight I ripped out all the parts on my breadboard and moved the 6JW8 circuitry over to the right end of the board, to make room for a 6AQ6 at the left end. In the next day or two I'll see if I can make the 6AQ6 work as an input stage.
-Gnobuddy
Tonight I ripped out all the parts on my breadboard and moved the 6JW8 circuitry over to the right end of the board, to make room for a 6AQ6 at the left end. In the next day or two I'll see if I can make the 6AQ6 work as an input stage.
-Gnobuddy
Life threw another curve ball to me today. I picked up a wallwart at work and it said 4V. I know it will be frequency challenged but I was wondering what it would do as a output transformer for a SE tube like the 6AK6. Sadly taking it apart I managed to pull the primary strands off the lead wires. I still have it but I thought if I find another one at one of the thrift stores I go to I'll pick it up rather than mess around with the one I have.
Well the store had this nifty looking console radio. Looking in behind the back was missing, so was the speaker. It had a power transformer and a decent sized output transformer. Popped out the big tubes and one was a 6F6G and a 5Y4G. Now do I need another SE OT, I have half a dozen or more. But It was a cool looking cabinet, a bit scratched but I could make it more presentable.
Normally I just grab the chassis and knock down the cabinet and if the wood is worth while use it in other woodworking projects. I already have a console by my door that I want to add shelves in and use for holding things like my cycling gear as well as stuff I usually haul back and forth to work. But the new one is a bit funky looking, I might just make it into a stationary guitar amp, not sure for where though. So I looked for a model number but could not find one. It did have a tube layout diagram, I looked up the schematic for a Viking radio.
Looked through the schematics at Pacifictv and found a match to the layout.
http://pacifictv.ca/schematics/viking48-74manual.pdf
Wait a second, mine is a SE rather than a P-P? I went back to the console and pulled out the metal tube in the 6F6G position. Yup, a metal 6F6. So I got myself a 10W P-P amp. Glad I didn't pass it up. And to think, it was tagged at $25. I guess they had it for a while as they had a 50% off sticker on it. So sometimes life smiles down on you. Not a mini amp though.
Well the store had this nifty looking console radio. Looking in behind the back was missing, so was the speaker. It had a power transformer and a decent sized output transformer. Popped out the big tubes and one was a 6F6G and a 5Y4G. Now do I need another SE OT, I have half a dozen or more. But It was a cool looking cabinet, a bit scratched but I could make it more presentable.
Normally I just grab the chassis and knock down the cabinet and if the wood is worth while use it in other woodworking projects. I already have a console by my door that I want to add shelves in and use for holding things like my cycling gear as well as stuff I usually haul back and forth to work. But the new one is a bit funky looking, I might just make it into a stationary guitar amp, not sure for where though. So I looked for a model number but could not find one. It did have a tube layout diagram, I looked up the schematic for a Viking radio.
Looked through the schematics at Pacifictv and found a match to the layout.
http://pacifictv.ca/schematics/viking48-74manual.pdf
Wait a second, mine is a SE rather than a P-P? I went back to the console and pulled out the metal tube in the 6F6G position. Yup, a metal 6F6. So I got myself a 10W P-P amp. Glad I didn't pass it up. And to think, it was tagged at $25. I guess they had it for a while as they had a 50% off sticker on it. So sometimes life smiles down on you. Not a mini amp though.
Great find, and what a steal! Congratulations!
It absolutely amazes me, the stuff you find in thrift stores in your area. Here in BC's Lower Mainland, there is too much turnover, too much real estate craziness, too much instability in people's lives. So everybody sells their house or gets evicted from their apartment or gets divorced or goes bankrupt, throws out their stuff which is still only a few years old, and moves on. Nothing that is five or six decades old remains intact in homes. Nobody has that settled a life.
So nothing from the tube era ever shows up in thrift stores now - that probably happened decades ago, long before I moved to Canada.
Meantime, stuff happened in my tiny apartment too...the mini-amp grew one more gain stage, a 6AQ6 in front of the 6JW8. There is now enough all-tube gain so you can plug in a guitar and drive the amp into clipping, and I've removed the external solid-state preamp as a result. The mini-amp is now self contained. All I need to do is wire in a gain control in place of the current jumper from 6AQ6 anode to 6JW8 triode G1, so I can lower the gain.
I will post the schematic as it currently stands soon. Until then, here is a pic of the whole thing on the breadboard.
-Gnobuddy
Attachments
Your breadboard looks great. Glad you found your gain, I have done the missed hole thing myself. On the find of the day, a lot of people moved from these parts to the west coast, these are not the kind of thing they are going to stick in the back of a truck and haul a thousand miles. We have the people that grew old and are now leaving their house for a old folks home, they hung on to the stuff. But even with that it has taken a lot of looking for me to acquire what I get. But this one was a surprise with finding out it was P-P. Some pictures.
The amp is running a low 260V on the plates, the datasheet for the 6F6 doesn't even list P-P at that low a voltage. Maybe run it with a SS rectifier. The 6F6 does list it at running Class AB2. http://www.r-type.org/pdfs/6f6-1.pdf
And while I do have six 6F6's now with this acquire I do have a number of 6K6's which would fit nicely with the 10k OT that seems to be the impedance of choice for the 6F6 on the datasheets. The 6K6 also has positive grid voltages on its graph, something to play with. Either way I think I might have some fun with this console. Shame it did not have the speaker. But then again for $14.http://www.r-type.org/pdfs/6k6gt.pdf
The amp is running a low 260V on the plates, the datasheet for the 6F6 doesn't even list P-P at that low a voltage. Maybe run it with a SS rectifier. The 6F6 does list it at running Class AB2. http://www.r-type.org/pdfs/6f6-1.pdf
And while I do have six 6F6's now with this acquire I do have a number of 6K6's which would fit nicely with the 10k OT that seems to be the impedance of choice for the 6F6 on the datasheets. The 6K6 also has positive grid voltages on its graph, something to play with. Either way I think I might have some fun with this console. Shame it did not have the speaker. But then again for $14.http://www.r-type.org/pdfs/6k6gt.pdf
Lovely woodwork! That must have been a thing of beauty before it got scratched and dented and dinged up.
The 6F6 has such a soft "knee" to the pentode curves. I wonder if that would translate to the same sort of softer bluesy distortion the 6V6 is famous for? The low voltage might help with that too, at such a low B+, the curves don't even begin to straighten out.
I know this amp wasn't designed to distort, I'm just wondering if it might actually turn out to do that well for guitar.
And now for the current schematic for my mini-amp. I had a long fight with Kicad last night. I lost and the software won, but I managed to make a somewhat readable schematic nevertheless.
There is surely tweaking left to do, but the thing does work fairly well as it is. It hums and buzzes a bit, not too surprising as it's on a breadboard and there is no shielding around it.
-Gnobuddy
Attachments
> a low 260V on the plates, the datasheet for the 6F6 doesn't even list P-P at that low a voltage. ...The 6F6 does list it at running Class AB2.
This is not running AB2. (R-C coupled grids.)
You can run any tube at a lower voltage, get less power. Probably 7W or 8W as worked. Pushing it up to say 13W is not going to get you higher-paying gigs. Leave it be.
As-is (assuming it works), go into the Phono jack (or right into the volume pot). That will "play". It will be HARD work to play loud. Now clean-up a 6SK7 socket, re-wire as audio triode, put that between input jack and volume control.
This is not running AB2. (R-C coupled grids.)
You can run any tube at a lower voltage, get less power. Probably 7W or 8W as worked. Pushing it up to say 13W is not going to get you higher-paying gigs. Leave it be.
As-is (assuming it works), go into the Phono jack (or right into the volume pot). That will "play". It will be HARD work to play loud. Now clean-up a 6SK7 socket, re-wire as audio triode, put that between input jack and volume control.
I have been thinking the same thing, a footswitchable extra input gain stage for the mini-amp to take it from mostly clean to definitely rock territory.
I'm not sure how noisy an LND150 is compared to a typical vacuum triode. Usually MOS devices seem to be very noisy compared to their junction counterparts. I don't think Supertex published any sort of noise data for the LND150, since the datasheet mostly discusses switching applications. A couple of analogue applications are mentioned (ex. constant current source), but once again, noise performance is not discussed.
I suppose there is always the JFET/MOSFET cascode, which has the good noise performance of the lower device (JFET), and the high-voltage capability of the upper device (MOSFET). It's a wee bit more complex, though, and I don't think I can actually fit it in the remaining space on my breadboard!
As for AB2 output stages, I see one in my future. But not until I'm finished with this mini-amp project!
-Gnobuddy
I' definitely prefer to restore this beautiful radio instead of scrapping it! Look at it's guts: Triple variable capacitor, PP output stage, beautiful case - this has to be considered a luxurious one!
Best regards!
Sadly, this is very true! And FM is on it's decline also.
But he's located in Canada, I think?
Best regards!
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