Turning the TX has resulted in a noticeable reduction in hum as advised. Not gone all together but not audible at normal listening distances.
I was in too much of a hurry to try turning the TX that I didn't get round to bonding the TX chassis. Will do this the next time it's open.
If there is still a hum and I were to try and put a plate around the TX, shouldn't it be ferrous rather than Alu or stainless?
Might the hum be coming via the heater bias because I currently share the same reference with the EL34 heaters that are AC? I intend to try MJ's THINGY bias voltage arrangement in the future so will try separating the two.
Mike.
Rocky - The Quatroquel
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little eddy
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#106
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Mike H
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#107
Oh well looks like moving the TX is on the right track, not sure about the rest
Good question, I sometimes used to use tin plate for such things in the old days, but that was when food tins were actually plated steel. Which they're not now. (?) Yes I used to cut up food cans for pieces of tin.I were to try and put a plate around the TX, shouldn't it be ferrous rather than Alu or stainless?
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
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little eddy
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#108
Well a big thank you to Toppsy for being the first independant critic to have a listen to my efforts.
First up we compared Rocky to Colin's 6EM7. In my view the 6EM7 was a tad more detailed and refined but Rocky carried extra weight. I would suggest 'advantage' to the 6EM7 but there's plent of time left in the game.
So Rocky had a competent start but Colin suggested there was a bit of sibilance so we decided to play around with some 6SN7s he had to hand. The benchmark was set with a pair of RCAs that Colin had previously loaned me. Next up were a pair od Holland Mullards if there can be such a thing. A little bit more refined but altogether more rosy and not considered an improvement. Then came a pair of USA Sylvania JAN 6SN7GTs VT-231 and what an impact. From the off there was quite a bit of imbalance between the two valves but the stronger one added some definite bounce and pace to the music. The net of this is that I've come away with the stronger valve and now need to try and find a match. An extended loan to the RCAs was secured for the meantime.
We thought about changing the decoupling caps as Colin has a pair of 0.47uF Mundorf Silver Supremes. Unfortunately one of the leads meant that this was not straight forward so we decided to leave this for another day.
With time to spare, we decided to give my untried Mullard GZ34 a go in place of my 'test' Sylvania. Following Cressy's earlier comment regarding the limited audible differences in his opinion/set-up we weren't expecting much but when the music started we just looked at each other. Gone was some of the sharpness and the whole thing was just more musical. A definite improvement so the Mullard will now become a permanent feature.
Lastly Colin suggested we connect up Rocky to his ELS63s and although pleasant, the overall presentation was not involving. Colin thought that this might be down to my cheap pot volume control so he set-up his buffer/pre and we tried again. What a transformation - Rocky really held up well. Finally to knock me down a peg or two, Coling connected up his massive GM70 monoblocks and wow, what a sound. Evidently superion but I don't think Rocky was shamed.
So what are the learnings and next steps:
1. Try to get some other 6SN7s.
2. Try to get some NOS EL34s as an alternative to try against the fitted 6L6s.
3. Maybe then try some different caps.
4. However the main learning from tonight was that the amp is being limited by the cheap volume control so this should be upgraded or an active pre-amp is the next project.
First up we compared Rocky to Colin's 6EM7. In my view the 6EM7 was a tad more detailed and refined but Rocky carried extra weight. I would suggest 'advantage' to the 6EM7 but there's plent of time left in the game.
So Rocky had a competent start but Colin suggested there was a bit of sibilance so we decided to play around with some 6SN7s he had to hand. The benchmark was set with a pair of RCAs that Colin had previously loaned me. Next up were a pair od Holland Mullards if there can be such a thing. A little bit more refined but altogether more rosy and not considered an improvement. Then came a pair of USA Sylvania JAN 6SN7GTs VT-231 and what an impact. From the off there was quite a bit of imbalance between the two valves but the stronger one added some definite bounce and pace to the music. The net of this is that I've come away with the stronger valve and now need to try and find a match. An extended loan to the RCAs was secured for the meantime.
We thought about changing the decoupling caps as Colin has a pair of 0.47uF Mundorf Silver Supremes. Unfortunately one of the leads meant that this was not straight forward so we decided to leave this for another day.
With time to spare, we decided to give my untried Mullard GZ34 a go in place of my 'test' Sylvania. Following Cressy's earlier comment regarding the limited audible differences in his opinion/set-up we weren't expecting much but when the music started we just looked at each other. Gone was some of the sharpness and the whole thing was just more musical. A definite improvement so the Mullard will now become a permanent feature.
Lastly Colin suggested we connect up Rocky to his ELS63s and although pleasant, the overall presentation was not involving. Colin thought that this might be down to my cheap pot volume control so he set-up his buffer/pre and we tried again. What a transformation - Rocky really held up well. Finally to knock me down a peg or two, Coling connected up his massive GM70 monoblocks and wow, what a sound. Evidently superion but I don't think Rocky was shamed.
So what are the learnings and next steps:
1. Try to get some other 6SN7s.
2. Try to get some NOS EL34s as an alternative to try against the fitted 6L6s.
3. Maybe then try some different caps.
4. However the main learning from tonight was that the amp is being limited by the cheap volume control so this should be upgraded or an active pre-amp is the next project.
#109
Hi Eddy,
Yes it was an enjoyable evening and you can be justifiably pleased with yourself on the sound of the amp.
When comparing the Rocky to my little 6EM7 amp through the MLTL speakers I have to agree Rocky carried more weight to the music and especially Rock type music. Indeed the amp is aptly named. The 6EM7 gave a more refined presentation but this perhaps was down to the fact this amp only pushes put around 1.5 Watts with a good tail wind and this showed when we swapped over to the Rocky.
I wouldn't bother getting fired up about different coupling caps. Wait until you get your new preamp. It was this part of your build that is sadly holding back the full potential of the Rocky. This was clearly audible when we changed over to the Quad63 speakers with Fostex T90A supertweeters and my Unity gain D3A buffer and Silk TVA's through the Rocky amp. This was a setup I could happily live with.
Get rid of that cheapy volume pot and put in at least a Alps Blue until you get the new pre promised by your mate.
As I said at the beginning of this reply that for a first build you can be justifiably pleased with the results. Well done.
Yes it was an enjoyable evening and you can be justifiably pleased with yourself on the sound of the amp.
When comparing the Rocky to my little 6EM7 amp through the MLTL speakers I have to agree Rocky carried more weight to the music and especially Rock type music. Indeed the amp is aptly named. The 6EM7 gave a more refined presentation but this perhaps was down to the fact this amp only pushes put around 1.5 Watts with a good tail wind and this showed when we swapped over to the Rocky.
I wouldn't bother getting fired up about different coupling caps. Wait until you get your new preamp. It was this part of your build that is sadly holding back the full potential of the Rocky. This was clearly audible when we changed over to the Quad63 speakers with Fostex T90A supertweeters and my Unity gain D3A buffer and Silk TVA's through the Rocky amp. This was a setup I could happily live with.
Get rid of that cheapy volume pot and put in at least a Alps Blue until you get the new pre promised by your mate.
As I said at the beginning of this reply that for a first build you can be justifiably pleased with the results. Well done.
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Mike H
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#110
Or an Alps blue in 'attenuator' mode with a series resistor al la WD forum recommendation. See Philip's 'resistors' thread. 'Least I think it ws he that started it...
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
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little eddy
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#111
I noticed tonight that the remaining hum on my amp is there as soon as I switch it on, i,e, before the valves warm up. I then removed the pairs of valves one at a time and I don't know whether this is common but the hum remains with all of the valves removed. I therefore assume that this must be leaking or be induced on my HT and/or 0V rails.
Anyone any suggestions how I might track the source down and how reasonable it is to expect to remove it all?
One idea I had about eliminating my new DC supply as a possible source was to disconnect the TX. If there is not audible reduction then the culprit is elsewhere. If there is a reduction then I'll try the metal cage/screen as previously suggested.
The other option I have is to disconnect the 0V rail from the earth, (currently via a 100R resistor) or perhaps tie it down solidly to earth. For some reason I don't feel comfortable with letting the whole thing float.
Anyone any suggestions how I might track the source down and how reasonable it is to expect to remove it all?
One idea I had about eliminating my new DC supply as a possible source was to disconnect the TX. If there is not audible reduction then the culprit is elsewhere. If there is a reduction then I'll try the metal cage/screen as previously suggested.
The other option I have is to disconnect the 0V rail from the earth, (currently via a 100R resistor) or perhaps tie it down solidly to earth. For some reason I don't feel comfortable with letting the whole thing float.
#112
If ALL the valves are removed and it still hums, then the output transformer must be picking up hum from the mains transformers/chokes. With no valves there is no path through the transformer to the power supply (both sides)
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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little eddy
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#113
Not all of the valves were removed - the GZ34 was left in. I guess if I remove the recifier and there is still a hum then that rules out any of the rectification or HT so any induced hum must be from the heaters, mains wiring or the power TX. Logical?
So out comes the GZ34 (other valves fitted) and the hum.......... is still there so can I assume that it must be from the heater TX tapping or the mains cabling to the on/off switch that happens to be on the left hand side of the amp, or the TX. The TX is central and a few inches away from the output transformers and wiring so I'll rule this out for now. However am I right in recalling that field strength is proprotional to current rather than voltage? If so, the mains should be small and therefore unlikely to be the culprit.
Now I'm struggling to think of the cause. My ac heater cabling to the 6l6s is routed beneath the base but with them being open circuit, no current will be flowing and hence no induced hum from the wires should be possible. The hum is not reduced in magnitude so I would therfore rule this out as the source.
What other AC is near to the output transformers or their cabling? Well there is my newly fitted TX for the 6SN7 DC heater supply but the TX will be taking very little current with the valves removed, again resulting in very little field strength. It makes sense that this could be the culprit as it is physically closer to the left channel than right. I'll disconnect it next time it's open. If the hum stops I have found the reason and have perhaps 2 options:
Try to put a screen/sheild around it as previosly recommended or
Move the TX onto the top of the amp at the rear with the rest of the AC and route the AC cables to the bridge beneath the base as per the 6l6 heater wiring.
So out comes the GZ34 (other valves fitted) and the hum.......... is still there so can I assume that it must be from the heater TX tapping or the mains cabling to the on/off switch that happens to be on the left hand side of the amp, or the TX. The TX is central and a few inches away from the output transformers and wiring so I'll rule this out for now. However am I right in recalling that field strength is proprotional to current rather than voltage? If so, the mains should be small and therefore unlikely to be the culprit.
Now I'm struggling to think of the cause. My ac heater cabling to the 6l6s is routed beneath the base but with them being open circuit, no current will be flowing and hence no induced hum from the wires should be possible. The hum is not reduced in magnitude so I would therfore rule this out as the source.
What other AC is near to the output transformers or their cabling? Well there is my newly fitted TX for the 6SN7 DC heater supply but the TX will be taking very little current with the valves removed, again resulting in very little field strength. It makes sense that this could be the culprit as it is physically closer to the left channel than right. I'll disconnect it next time it's open. If the hum stops I have found the reason and have perhaps 2 options:
Try to put a screen/sheild around it as previosly recommended or
Move the TX onto the top of the amp at the rear with the rest of the AC and route the AC cables to the bridge beneath the base as per the 6l6 heater wiring.
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Mike H
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#114
OK but if there's still no output's plugged in there's no circuit for the o/p TX so still wot Nick said.
Think you might find only complete answer is completely remove mains TX(s) away from the o/p TX(s).
NB if you're using overall negative feedback it often does a lot to reduce such inter-TX picked up hum as well.
I've also found by experience bad idea to have them screwed in a steel box as well, the steel can 'transmit' the magnetic field along to the o/p TX's
Appendix
Helps if mains and o/p TX's cores are 90 degrees to each other
Think you might find only complete answer is completely remove mains TX(s) away from the o/p TX(s).
NB if you're using overall negative feedback it often does a lot to reduce such inter-TX picked up hum as well.
I've also found by experience bad idea to have them screwed in a steel box as well, the steel can 'transmit' the magnetic field along to the o/p TX's
Appendix
Helps if mains and o/p TX's cores are 90 degrees to each other
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
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little eddy
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#115
Well I live and learn. Resited the DC heater TX at the rear with other AC ironwork and the dominant hum from the left channel is now gone. There is now only a very slight hum and it is the same on both channels so I'll leave it there.
Won some Sylvania JAN VT-231s on slease bay tonight so can't wait to try.
Won some Sylvania JAN VT-231s on slease bay tonight so can't wait to try.
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little eddy
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#116
Well no photo just yet because the family are in Wales with the camera.
Anyhow having listened to the amp for some time with the 6SN7 SRPP input running at about 7.4mA, it was time to try something else and this was in the form of MJ's Beta-follower (page 126), supposedly a SRPP variant with the low distortion benefits of the mu-follower.
Anyway I got it working using some small pieces of stripboard for the passive components. It is running with a lower half grid bias of 3.6V (two red LEDs), an upper grid bias of 4.0V and current of 7.0mA using his proposed circuit and components for a 6J5 pair in his case. Schematic is attached.
So how does it sound? Well I haven't picked up on any downsides yet. The one thing I did notice when listening to Natasha Bedingfield's 'Soulmate' was the acoustic guitar, but this time it wasn't a generic guitar but one with a distinctive character and the whole thing just seems a tad more 'liquid'. So it'll stay for a while.
I guess the one caveat is that for convenience I'm using my CD amd AN DAC as the source, but I guess if this sounds OK the vinyl can only be better!
Anyhow having listened to the amp for some time with the 6SN7 SRPP input running at about 7.4mA, it was time to try something else and this was in the form of MJ's Beta-follower (page 126), supposedly a SRPP variant with the low distortion benefits of the mu-follower.
Anyway I got it working using some small pieces of stripboard for the passive components. It is running with a lower half grid bias of 3.6V (two red LEDs), an upper grid bias of 4.0V and current of 7.0mA using his proposed circuit and components for a 6J5 pair in his case. Schematic is attached.
So how does it sound? Well I haven't picked up on any downsides yet. The one thing I did notice when listening to Natasha Bedingfield's 'Soulmate' was the acoustic guitar, but this time it wasn't a generic guitar but one with a distinctive character and the whole thing just seems a tad more 'liquid'. So it'll stay for a while.
I guess the one caveat is that for convenience I'm using my CD amd AN DAC as the source, but I guess if this sounds OK the vinyl can only be better!
Last edited by little eddy on Sat Apr 18, 2009 11:28 pm, edited 1 time in total.
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little eddy
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#117
Hopefully with schematic this time but had to import it into PowerPoint.
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Last edited by little eddy on Sun Apr 19, 2009 6:19 pm, edited 2 times in total.
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colin.hepburn
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#118
Hi Eddy
That the one schematic with the BC558 Transistor added in ah ok youve just posted it
That the one schematic with the BC558 Transistor added in ah ok youve just posted it
Colin
The Blues man
The Blues man
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little eddy
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#120
I've been reading up about coupling circuits and have a few questions about the values I've nicked from previous designer's amp.
Currently my EL34 set up is a grid R of 220k and ac-coupling cap of 0.68uF. If I understand correctly this gives an f-3dB of just over 1Hz (also suggestedt by MJ as a good target).
I have also read however that it is best to make the grid resistor as large as possible and if I have read the datasheets correctly this is say 700k for the el34 and 500k for the 6l6.
If I target the 1Hz point my two combinations would apear to be 0.33uF/470k or 0.47uF/330k, the latter perhaps being more logical due to the greater choice of ac-coupling caps.
Why not aim for a lower f-3db point by using say 0.47uF and 470k giving 0.72Hz if the capacitor cost increase is small. I could use the current 0.68uF orange drop and 470k to give a f-3db point 0.5 Hz if goinf lower than 1Hz is not detrimental. Or does the sonic impact of the cap increase with size in which case the 0.33uF/470k would be the optimum combination?
Is it worth playing around with these in order to minimise this drain current and might these introduce an audible impact/benefit?
What other benefit might this give other than potentially reducing the cost of the ac-coupling capacitor? I guess the gain might increase marginally because the input resistance of the EL34 stage increases slightly.
Currently my EL34 set up is a grid R of 220k and ac-coupling cap of 0.68uF. If I understand correctly this gives an f-3dB of just over 1Hz (also suggestedt by MJ as a good target).
I have also read however that it is best to make the grid resistor as large as possible and if I have read the datasheets correctly this is say 700k for the el34 and 500k for the 6l6.
If I target the 1Hz point my two combinations would apear to be 0.33uF/470k or 0.47uF/330k, the latter perhaps being more logical due to the greater choice of ac-coupling caps.
Why not aim for a lower f-3db point by using say 0.47uF and 470k giving 0.72Hz if the capacitor cost increase is small. I could use the current 0.68uF orange drop and 470k to give a f-3db point 0.5 Hz if goinf lower than 1Hz is not detrimental. Or does the sonic impact of the cap increase with size in which case the 0.33uF/470k would be the optimum combination?
Is it worth playing around with these in order to minimise this drain current and might these introduce an audible impact/benefit?
What other benefit might this give other than potentially reducing the cost of the ac-coupling capacitor? I guess the gain might increase marginally because the input resistance of the EL34 stage increases slightly.