skal1
Well-known member
Dave p
what do you me by this statement {what's your set-up?} i am confussed
skal1
what do you me by this statement {what's your set-up?} i am confussed
skal1
GroupDIY Bridge compressor.
- Thread starter DaveP
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skal1
Well-known member
oh i get you now . i would test at my house and my friends studio, he records indie rock/electronic music lots of guitar/synth bits....
skal1
skal1
tommypiper
Well-known member
DaveP said:
Sorry, my dislexic mistake. There are also 6SN7s available, for $10 each, free international shipping. I haven't heard those, but we were amazed how good the 6SL7s were... http://www.ebay.com/itm/NEW-Shuguang-Chinese-6N8P-6SN7-6H8C-Vacuum-Tube-2PCS-/120852479451?pt=Vintage_Electronics_R2&hash=item1c235e7ddb#ht_500wt_755
They also have another version in "matched pairs" for twice as much... Still cheap.
FWIW...
I've made a welcome discovery with the two interstages which I can't understand, but maybe someone else will explain it...CJ? PRR?
I have been struggling to get a decent frequency response with various combinations of zobels or even just termination resistors. (each interstage has 4 x 3.75k windings). In desperation I tried various alternative wiring set-ups but then I found they still worked without having one end of the primary earthed!!!
Not only do they work without the input primary winding earthed but the frequency response is better and distortion of the waveform disappears. I am just using one winding for input and ignoring the other, the other two windings are connected (as the schematic) to the other interstage. The second interstage output winding is earthed and it has a termination resistor. So to be clear, I am just using 3 windings on each TX.
Any ideas?
best
DaveP
I have been struggling to get a decent frequency response with various combinations of zobels or even just termination resistors. (each interstage has 4 x 3.75k windings). In desperation I tried various alternative wiring set-ups but then I found they still worked without having one end of the primary earthed!!!
Not only do they work without the input primary winding earthed but the frequency response is better and distortion of the waveform disappears. I am just using one winding for input and ignoring the other, the other two windings are connected (as the schematic) to the other interstage. The second interstage output winding is earthed and it has a termination resistor. So to be clear, I am just using 3 windings on each TX.
Any ideas?
best
DaveP
I don't think I've wired it according to the TX recommendations (see attachment).
For best frequency response it should be interleaved
Doh, when all else fails read the instructions!
I shall rewire it on sunday afternoon.
best
Davep
For best frequency response it should be interleaved
Doh, when all else fails read the instructions!
I shall rewire it on sunday afternoon.
best
Davep
Time for an update and a request for ideas.
Rewired the transformers with the suggested interleaving of windings and sure enough much better response. I wired the input with two windings in series and put 15k across them. The remaining pair cross link to the similar pair on the second TX. I wired the output (on the 2nd TX)in parallel instead of series and it seemed to like 4.7k across that.
I am supplying the Transformers from a White CF which drives both ways and has a lower output resistance. The frequency response is now fairly flat, but the ringing on square waves is still there and not doing much other than extending the response to 30kHz.
Now the final snag is that turning up the compression past a certain point causes oscillation at about 2Hz, there is obviously some feedback or charging and discharging going on, but I'm not sure how its happening. I need some fresh insight as I'm almost brain dead tonight, I've been on this for months!
I am getting my CV from the attached circuit, apart from the afore mentioned WCF the schematic is much like the original. I've got 1M and 0.1uf on the control tube grids.
Help!
best
DaveP
Rewired the transformers with the suggested interleaving of windings and sure enough much better response. I wired the input with two windings in series and put 15k across them. The remaining pair cross link to the similar pair on the second TX. I wired the output (on the 2nd TX)in parallel instead of series and it seemed to like 4.7k across that.
I am supplying the Transformers from a White CF which drives both ways and has a lower output resistance. The frequency response is now fairly flat, but the ringing on square waves is still there and not doing much other than extending the response to 30kHz.
Now the final snag is that turning up the compression past a certain point causes oscillation at about 2Hz, there is obviously some feedback or charging and discharging going on, but I'm not sure how its happening. I need some fresh insight as I'm almost brain dead tonight, I've been on this for months!
I am getting my CV from the attached circuit, apart from the afore mentioned WCF the schematic is much like the original. I've got 1M and 0.1uf on the control tube grids.
Help!
best
DaveP
Attachments
ruairioflaherty
Well-known member
The insight you have provided by opening up your design process has been fantastic. Thank you.
Good luck with the next hurdle
Good luck with the next hurdle
Thanks Ruairi,
In fact I gave myself a clue by posting! When I looked at the circuit again it was 1m and 0.25uF and it was this pair charging and discharging because the CV circuit couldn't drive it hard enough.
I put in a 3.3k as an attack resistor and hey presto it stopped! This corresponds to an attack of 825uS or just under 1mS. The voltage drop across this was 17.3mV which is 5.2uA, not much really.
I can now say that the project works in principle and I can go ahead and build a prototype. I will look into ways of beefing up the CV but if I can't then I will have to live with the attack I've got. I might try some lower voltage drop diodes first. They are only 1N4007's after all.
best
DaveP
In fact I gave myself a clue by posting! When I looked at the circuit again it was 1m and 0.25uF and it was this pair charging and discharging because the CV circuit couldn't drive it hard enough.
I put in a 3.3k as an attack resistor and hey presto it stopped! This corresponds to an attack of 825uS or just under 1mS. The voltage drop across this was 17.3mV which is 5.2uA, not much really.
I can now say that the project works in principle and I can go ahead and build a prototype. I will look into ways of beefing up the CV but if I can't then I will have to live with the attack I've got. I might try some lower voltage drop diodes first. They are only 1N4007's after all.
best
DaveP
I've made some progress and sorted out a few things but the OPT has still not arrived as customs have not yet notified me of the duty to pay, it was ordered 2 months ago but only shipped the first week of march.
Looking at the published spec of the 26C it states Attack 2mS Release 2.5 Sec. I'm not going to get the 0.1mS of the 26W on the 26C because that is driven from a big output stage. What I can get is maybe just under 1mS and this will have to do for the time being. I have beefed up the rectifier arrangement used on the AM-864U and it works fine fed from half a 6DT8. The 26C had 4 interstages plus the IPT & OPT. I have managed to replace 2 of the "Interstages" with low cost tubes so it will be much cheaper to make.
The published spec was 13dB max compression and my version can make 16dB max, this is probably down to using a bridge tube that was not available back in '38.
The output with triode wired 6CM6's is 2.4W at the start of clipping, I've kept them triodes to keep the original sound (sine waves round off on overload) so there will be lots of 2nd harmonics if you want. But they can be wired as pentodes and you can get 10 or 12W if you wish.
I decided to elevate the heater supply to give me greater freedom linking to the preceeding tubes and the last 43V zener in the regulator makes a handy reference point. This means I can run cathodes at 100v without risking an insulation breakdown.
Thats all for now, I hope to start posting the schematics next week. I'll start the metalwork after that, hopefully with the OPT!
best
DaveP
Looking at the published spec of the 26C it states Attack 2mS Release 2.5 Sec. I'm not going to get the 0.1mS of the 26W on the 26C because that is driven from a big output stage. What I can get is maybe just under 1mS and this will have to do for the time being. I have beefed up the rectifier arrangement used on the AM-864U and it works fine fed from half a 6DT8. The 26C had 4 interstages plus the IPT & OPT. I have managed to replace 2 of the "Interstages" with low cost tubes so it will be much cheaper to make.
The published spec was 13dB max compression and my version can make 16dB max, this is probably down to using a bridge tube that was not available back in '38.
The output with triode wired 6CM6's is 2.4W at the start of clipping, I've kept them triodes to keep the original sound (sine waves round off on overload) so there will be lots of 2nd harmonics if you want. But they can be wired as pentodes and you can get 10 or 12W if you wish.
I decided to elevate the heater supply to give me greater freedom linking to the preceeding tubes and the last 43V zener in the regulator makes a handy reference point. This means I can run cathodes at 100v without risking an insulation breakdown.
Thats all for now, I hope to start posting the schematics next week. I'll start the metalwork after that, hopefully with the OPT!
best
DaveP
The first stage is attached.
It's as close to the original as I can get it. I've basically added a WCF to provide a low impedance drive to the Interstages. It does not need a large coupling cap as the low end is boosted by a small resonance with the interstage.
The dots denote the phase relationship of the windings. The input is two windings in series (15k) and the output is two in parallel (3.75k). The 4.7k is shunted down by the pots which follow in stage 2&3. The top end is tamed by the 1nF cap.
The 6BK7-B is a beefy TV tube at $4.50 and the 6DT8 is a 6V 12AT7 at $4.00. The 6SJ7 is the metal version triode wired.
I think I've covered everything, I'll post the 2nd stage tomorrow.
best
Davep
It's as close to the original as I can get it. I've basically added a WCF to provide a low impedance drive to the Interstages. It does not need a large coupling cap as the low end is boosted by a small resonance with the interstage.
The dots denote the phase relationship of the windings. The input is two windings in series (15k) and the output is two in parallel (3.75k). The 4.7k is shunted down by the pots which follow in stage 2&3. The top end is tamed by the 1nF cap.
The 6BK7-B is a beefy TV tube at $4.50 and the 6DT8 is a 6V 12AT7 at $4.00. The 6SJ7 is the metal version triode wired.
I think I've covered everything, I'll post the 2nd stage tomorrow.
best
Davep
Attachments
ruairioflaherty
Well-known member
Go Dave!
Here's stage 2.
Again, this uses the 6SJ7 input tube like the original 26C, but instead of another interstage there is a conventional LTP and output stage. I don't have the Edcor OPT yet, but with an 8k to 15 ohm OPT its giving 2.75W.
The 6CM6 tubes are 9 pin 6V6 and are a cheaper alternative. There needs to be a balance pot to equalise the DC, even with "matched tubes". For full output before clipping the input to each tube is ~18Vrms and this gives ~79Vrms on the anodes so gain is 4.4 triode -wired.
I'll post the power supply next as I'm still working on the final stage 3.
best
DaveP
Again, this uses the 6SJ7 input tube like the original 26C, but instead of another interstage there is a conventional LTP and output stage. I don't have the Edcor OPT yet, but with an 8k to 15 ohm OPT its giving 2.75W.
The 6CM6 tubes are 9 pin 6V6 and are a cheaper alternative. There needs to be a balance pot to equalise the DC, even with "matched tubes". For full output before clipping the input to each tube is ~18Vrms and this gives ~79Vrms on the anodes so gain is 4.4 triode -wired.
I'll post the power supply next as I'm still working on the final stage 3.
best
DaveP
Attachments
Kingston
Well-known member
That's one seriously overkill line driver you got there! Or should we say line melter. But I see how price still does soar though the roof. Only build complexity.
Looking very much forward to next steps.
Looking very much forward to next steps.
Hi Doug & Kingston,
I appreciate the compliment!
But seriously, this is supposed to be a 26C and if it didn't have an output stage comparable with the original, I would have got some stick for that.
The customs have now asked for their cut, so I guess the OPT is ready to be delivered!
I've attached the power supply so we are almost there now. The 270k plus 47uF decide the time delay for the HT and it works out about 12 seconds which is the spec in the tube data sheets when run from a tube rectifier. This means there is only one power switch to worry about.
I have been running this power supply for months now and it seems reliable. It is obviously not necessary to copy this exactly, if you have a single PT to hand then use it. The advantage to me of using toroids is that they are cheap, available and very compact when designing layout; they don't produce as much exterior field as IE transformers.
The diodes can be to choice, small rated zeners are more accurate than powerful ones, and schottky diodes are best for the heater supply, I'll list the ones I used when I post the parts list or BOM.
best
DaveP
I appreciate the compliment!
But seriously, this is supposed to be a 26C and if it didn't have an output stage comparable with the original, I would have got some stick for that.
The customs have now asked for their cut, so I guess the OPT is ready to be delivered!
I've attached the power supply so we are almost there now. The 270k plus 47uF decide the time delay for the HT and it works out about 12 seconds which is the spec in the tube data sheets when run from a tube rectifier. This means there is only one power switch to worry about.
I have been running this power supply for months now and it seems reliable. It is obviously not necessary to copy this exactly, if you have a single PT to hand then use it. The advantage to me of using toroids is that they are cheap, available and very compact when designing layout; they don't produce as much exterior field as IE transformers.
The diodes can be to choice, small rated zeners are more accurate than powerful ones, and schottky diodes are best for the heater supply, I'll list the ones I used when I post the parts list or BOM.
best
DaveP
Attachments
Kingston
Well-known member
I like the series secondaries trick, but the calculations don't quite add up. 4X 55VAC is only 220VAC, which in a very best case scenario (1.4X) will only add up to about 308VDC with the rectifier bridge. And it's nowhere near clean at that point. There's not much work for the regulator which you have set up at about 300VDC. Setting the regulator closer to 280VDC would be an easy fix since voltages are probably not critical at that point.
But if they are, something like 250VAC is a better starting point, but certainly finding that transformer (combo) would be more difficult/expensive.
But if they are, something like 250VAC is a better starting point, but certainly finding that transformer (combo) would be more difficult/expensive.
The regulator appears more of a timing trick than about regulation, really regulation in of itself isn't much needed here. In terms of filtering, there's more than 25x as much as an original 26-C. The output transformer combined with that 1st 47mfd filter is more than adequate for a PP power stage, and everything else down the line is well covered.
Doug's right,
It's so I can dispense with a tube rectifier, a choke and a separate heater switch and waiting for the tubes to warm up safely. Solid state is good for power supplies, audio is another matter.
I want to get the best signal to noise I can, hence the extra filtering. I found that on the BA-6A I made, the choke overloaded and imbalance on the output tubes allowed some hum to get through. It did not make it unuseable but it was better to design hum out on this project.
Kingston,
Don't forget I've had this working for months and its not theoretical, there is a measured 346V at the input to that regulator.
The toroids are rated at 50VA each so they don't work up a sweat in this amp. If the transformer has a regulation of 10% (which is good) you get 10% more voltage than the stated output. I did not set out to buy oversized transformers, its just that these are the smallest available in the UK from Farnell or RS.
best
DaveP
It's so I can dispense with a tube rectifier, a choke and a separate heater switch and waiting for the tubes to warm up safely. Solid state is good for power supplies, audio is another matter.
I want to get the best signal to noise I can, hence the extra filtering. I found that on the BA-6A I made, the choke overloaded and imbalance on the output tubes allowed some hum to get through. It did not make it unuseable but it was better to design hum out on this project.
Kingston,
Don't forget I've had this working for months and its not theoretical, there is a measured 346V at the input to that regulator.
The toroids are rated at 50VA each so they don't work up a sweat in this amp. If the transformer has a regulation of 10% (which is good) you get 10% more voltage than the stated output. I did not set out to buy oversized transformers, its just that these are the smallest available in the UK from Farnell or RS.
best
DaveP
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