SCA-35 Dynaco - Page 10

mr2racer · 13th January 2012, 03:47 AM

Hey All.
I've seen several ways of installing volume controls. In this case the SCA and the PAS have a 250k volume pot. At this point the phono preamp output is connected to the first position on the selector switch. The other four line level inputs are connected to the selector switch also. It seems the output from the selector switch should attach to one end of the pot. The wiper to the output to the 12DW7 on the ST35 board. And the opposite end to ground. The question I have is do I need anything between the shunt end of the pot and ground? There isn't anything in this position on the SCA. But the signal runs from the wiper volume through the PEC 555002. Can I go straight from the wiper to the .1uf cap at the input of the output section?
Kevin

jgf · 13th January 2012, 03:29 PM

That sounds correct, if you're removing the tone and balance controls.

mr2racer · 13th January 2012, 11:37 PM

Thanks again.

tofsound · 14th January 2012, 11:59 PM

Quote:

Originally Posted by SY

Replace the electrolytic capacitors and the tubes. The coupling caps would also be something I'd go after. Don't use fashion parts, normal polypropylene caps (e.g., Wima MKP or 715P Orange Drops) will be fine in this circuit. Clean the switches and potentiometers with spray contact cleaner and some rotation. Then sit back and enjoy.

After you've got it refurbished and used it for a while, we can talk about upgrades.

Do you have any info. for this capacitor? I bought them from Ebay several years ago but I don't know about them.

mr2racer · 15th January 2012, 05:46 AM

tofsound,
CDE is Cornell Dublier Electronics. Probably not a bad cap but I don't think its anything special either. Personally, I like Solen polypropylene caps. Which in that value cost about $3 a pair. So why would you want to risk using a cap of unknown quality and unknown age to save $3? Once you get past the real expense of transformers and to some extent tubes, the little bits are cheap. I agree with SY. You don't need to buy 'boutique caps.'

For example, I buy my caps from Tube Depot in Memphis. A .1uf @ 630 volt Solen metalized polypropylene is $1.45. That's peanuts even for me. An Auricap .1uf 450 volt is $11.85 from Tube Depot. In your average push pull power amp you'll need six coupling caps. That's $8.70 for Solens but $71.10 for Auricaps for an amp. Wima, Panasonic, and Orange Drops are about the same price as the Solen's.

My point is those caps could be twenty years old. As A.J Foyt says buy something "where you know where it come from". And how long its been on the road.

tofsound · 15th January 2012, 07:53 AM

Quote:

Originally Posted by mr2racer

tofsound,
CDE is Cornell Dublier Electronics. Probably not a bad cap but I don't think its anything special either. Personally, I like Solen polypropylene caps. Which in that value cost about $3 a pair. So why would you want to risk using a cap of unknown quality and unknown age to save $3? Once you get past the real expense of transformers and to some extent tubes, the little bits are cheap. I agree with SY. You don't need to buy 'boutique caps.'

For example, I buy my caps from Tube Depot in Memphis. A .1uf @ 630 volt Solen metalized polypropylene is $1.45. That's peanuts even for me. An Auricap .1uf 450 volt is $11.85 from Tube Depot. In your average push pull power amp you'll need six coupling caps. That's $8.70 for Solens but $71.10 for Auricaps for an amp. Wima, Panasonic, and Orange Drops are about the same price as the Solen's.

My point is those caps could be twenty years old. As A.J Foyt says buy something "where you know where it come from". And how long its been on the road.

Thanks for your advice.
I bought this capacitor at a value little it more than Solen. I can't exactly remember but the ebay seller told me that this is a good cap. Anyway I have 4 0.47uf/600vdc solen caps so I can use them later. I did not feel much differences with changing caps but did with bypassing tone control.

mr2racer · 15th January 2012, 09:12 AM

JGF,
Do you remember what value resistors you ended up with in the power supply voltage divider network? I replaced the first 50 ohm resister with a 50 ohm 240ma triad choke.
Kevin

mr2racer · 15th January 2012, 04:49 PM

Anyone,
Both the SCA and the ST use a 50 ohm 5w first resistor. On the ST the second resistor is a 6.8k to get the right voltage for the 7247. But the SCA uses 2.2k for the 7199. In the SCA it is followed by another 2.2k and a 22k that supplies the phono board. Since resistors are linear, if I use the 6.8k for R40 on the SCA instead of 2.2k, followed by the second 2.2k would it make sense to subtract the difference for the last resistor? I.E. 22,000 - (6800 - 2200) = 17,400. So, 17.4k for the last resistor before the phono board? Is it that easy?
Kevin

tofsound · 15th January 2012, 10:59 PM

After some modification(replacing Cap board, adopting EFB) I had square wave test with signal generator and oscilloscope.
The test wasn't precise but maximum output was around 16 watt with 7 ohm dummy load (it starts clipping from the output of 30v peak to peak). The output bias current was 27mA each. The interest thing is that outputs of phase spliter are not same. While the upper output(in the schematic) is something relatively modest the lower one is somewhat ugly. Here are the screen shots. I don't know why both are different. Maybe it's because the lower output related to feedback loop ??

dcgillespie · 16th January 2012, 03:30 AM

tofsound -- having a different signal appear at each output tube grid in a feedback amplifier with it driven to full power output is not uncommon. In fact, it is more common than not, depending on the frequency and output level used, as well as the overall design of the amplifier. The technique used to achieve proper HF stability also has a hand in this effect. Since the composite waveform achieved from the OP transformer secondary is the main goal, the individual drive waveforms are of little concern unless they are unequal in amplitude, or show parasitic oscillations.

With the stock power transformer operating from a capable 120 vac power source and fresh output tubes, the unit is capable of 17.5 watts RMS from each channel with both channels driven into 8 ohms when the EFB modification is installed. In your case, with the lower load impedance you were using for your measurements, 16 watts would seem about right. Use of the EFB modification has no bearing on the waveforms you observed at the output tube grids, as these waveforms will have the same appearance even with the original bias scheme in place under the same measurement conditions.

Dave

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13th January 2012, 03:47 AM	#91
mr2racer diyAudio Member Join Date: Jul 2009	Hey All. I've seen several ways of installing volume controls. In this case the SCA and the PAS have a 250k volume pot. At this point the phono preamp output is connected to the first position on the selector switch. The other four line level inputs are connected to the selector switch also. It seems the output from the selector switch should attach to one end of the pot. The wiper to the output to the 12DW7 on the ST35 board. And the opposite end to ground. The question I have is do I need anything between the shunt end of the pot and ground? There isn't anything in this position on the SCA. But the signal runs from the wiper volume through the PEC 555002. Can I go straight from the wiper to the .1uf cap at the input of the output section? Kevin

13th January 2012, 03:29 PM	#92
jgf diyAudio Member Join Date: Jan 2010 Location: PA	That sounds correct, if you're removing the tone and balance controls.

13th January 2012, 11:37 PM	#93
mr2racer diyAudio Member Join Date: Jul 2009	Thanks again.

15th January 2012, 05:46 AM	#95
mr2racer diyAudio Member Join Date: Jul 2009	tofsound, CDE is Cornell Dublier Electronics. Probably not a bad cap but I don't think its anything special either. Personally, I like Solen polypropylene caps. Which in that value cost about $3 a pair. So why would you want to risk using a cap of unknown quality and unknown age to save $3? Once you get past the real expense of transformers and to some extent tubes, the little bits are cheap. I agree with SY. You don't need to buy 'boutique caps.' For example, I buy my caps from Tube Depot in Memphis. A .1uf @ 630 volt Solen metalized polypropylene is $1.45. That's peanuts even for me. An Auricap .1uf 450 volt is $11.85 from Tube Depot. In your average push pull power amp you'll need six coupling caps. That's $8.70 for Solens but $71.10 for Auricaps for an amp. Wima, Panasonic, and Orange Drops are about the same price as the Solen's. My point is those caps could be twenty years old. As A.J Foyt says buy something "where you know where it come from". And how long its been on the road.

15th January 2012, 09:12 AM	#97
mr2racer diyAudio Member Join Date: Jul 2009	JGF, Do you remember what value resistors you ended up with in the power supply voltage divider network? I replaced the first 50 ohm resister with a 50 ohm 240ma triad choke. Kevin

15th January 2012, 04:49 PM	#98
mr2racer diyAudio Member Join Date: Jul 2009	Anyone, Both the SCA and the ST use a 50 ohm 5w first resistor. On the ST the second resistor is a 6.8k to get the right voltage for the 7247. But the SCA uses 2.2k for the 7199. In the SCA it is followed by another 2.2k and a 22k that supplies the phono board. Since resistors are linear, if I use the 6.8k for R40 on the SCA instead of 2.2k, followed by the second 2.2k would it make sense to subtract the difference for the last resistor? I.E. 22,000 - (6800 - 2200) = 17,400. So, 17.4k for the last resistor before the phono board? Is it that easy? Kevin

16th January 2012, 03:30 AM	#100
dcgillespie diyAudio Member Join Date: May 2010 Location: Ball Ground, GA	tofsound -- having a different signal appear at each output tube grid in a feedback amplifier with it driven to full power output is not uncommon. In fact, it is more common than not, depending on the frequency and output level used, as well as the overall design of the amplifier. The technique used to achieve proper HF stability also has a hand in this effect. Since the composite waveform achieved from the OP transformer secondary is the main goal, the individual drive waveforms are of little concern unless they are unequal in amplitude, or show parasitic oscillations. With the stock power transformer operating from a capable 120 vac power source and fresh output tubes, the unit is capable of 17.5 watts RMS from each channel with both channels driven into 8 ohms when the EFB modification is installed. In your case, with the lower load impedance you were using for your measurements, 16 watts would seem about right. Use of the EFB modification has no bearing on the waveforms you observed at the output tube grids, as these waveforms will have the same appearance even with the original bias scheme in place under the same measurement conditions. Dave