SCA 35. What update mods should be done?

I heard that I can skip putting in the 12AX7 since I'm not using the phono section. If this is true, is it so the voltage is better used elsewhere?

To my surprise, all three boards have orange drops and on the power boards, there are Solens. Funny thing is that the wrapper has detached somewhat. I'm guessing the heat this thing makes. But hopefully with the EFB, it'll cool things down. I'm going to take some pics of the amp.

The orange drops and Solens are not original. The orange drops are a well made cap.The Solens were considered to be good low cost capacitors but lately I have heard of several that have failed do to heat. They are probably OK for now but should eventually be replaced.
I have run mine both ways with and without the 12AX7s I don't hear any difference and really don't see a reason why they should not be pulled.
The new EFB power supply will not change the temperature around the tubes. All SCA35's run hot.

sailor wrote:The orange drops and Solens are not original. The orange drops are a well made cap.The Solens were considered to be good low cost capacitors but lately I have heard of several that have failed do to heat. They are probably OK for now but should eventually be replaced.
I have run mine both ways with and without the 12AX7s I don't hear any difference and really don't see a reason why they should not be pulled.
The new EFB power supply will not change the temperature around the tubes. All SCA35's run hot.

Thanks Sailor. What type of cap and value you suggest to replace the Solens?

BTW, I heard that these amps hum without the cage. Is there a way, mod, where the amp won't hum without the cage?

poconoman wrote:

sailor wrote:The orange drops and Solens are not original. The orange drops are a well made cap.The Solens were considered to be good low cost capacitors but lately  I have heard of several that have failed do to heat. They are probably OK for now but should eventually be replaced.
I have  run mine both ways with and without the 12AX7s I don't hear any difference and really don't see a reason why they should not be pulled.
The new EFB power supply will not change the temperature around the tubes. All SCA35's run hot.

Thanks Sailor. What type of cap and value you suggest to replace the Solens?

BTW, I heard that these amps hum without the cage. Is there a way, mod, where the amp won't hum without the cage?

Yes, if you turn it off :-)

Seriously, the covers are part of the screening, without them the amp will pick up whatever fields you have in the room.

peterh wrote:

poconoman wrote:

sailor wrote:The orange drops and Solens are not original. The orange drops are a well made cap.The Solens were considered to be good low cost capacitors but lately  I have heard of several that have failed do to heat. They are probably OK for now but should eventually be replaced.
I have  run mine both ways with and without the 12AX7s I don't hear any difference and really don't see a reason why they should not be pulled.
The new EFB power supply will not change the temperature around the tubes. All SCA35's run hot.

Thanks Sailor. What type of cap and value you suggest to replace the Solens?

BTW, I heard that these amps hum without the cage. Is there a way, mod, where the amp won't hum without the cage?

Yes, if you turn it off :-)

Seriously, the covers are part of the screening, without them the amp will pick up whatever fields you have in the room.

That was funny!

Any advice on the caps?

If you do not have a copy of the original manual you can make a copy by clicking on the last Sticky.
On the amp. boards I would remove C17 and replace it with a jumper wire. C18 should be a 1uF/250 volt, C19 and C20 .1uF/ at least 400volts you can also use .22uF. In the manual there is a misprint C18 is listed as a .1 but it must be a 1. You will probably need to use metallized polypropylene or paper in oil. A film and metal version will probably be to big. Brand choice is up to you. There are a lot of good caps under a $. But fancy caps will cost more. But watch the size.

sailor wrote:If you do not have a copy of the original manual you can make a copy by clicking on the last Sticky.
On the amp. boards I would remove C17 and replace it with a jumper wire. C18 should be a 1uF/250 volt, C19 and C20 .1uF/ at least 400volts you can also use .22uF. In the manual there is a misprint C18 is listed as a .1 but it must be a 1. You will probably need to use metallized polypropylene or paper in oil. A film and metal version will probably be to big. Brand choice is up to you. There are a lot of good caps under a $. But fancy caps will cost more. But watch the size.

Thank you for the info. I'll download the manual.

When you say jumper, you mean remove the cap and replace it with a piece of wire soldered in the eyelets where the previous cap was?

Yes replace with a wire.

poconoman wrote:

sailor wrote:If you do not have a copy of the original manual you can make a copy by clicking on the last Sticky.
On the amp. boards  I would remove C17 and replace it with a jumper wire. C18 should be a 1uF/250 volt, C19 and C20 .1uF/ at least 400volts you can also use .22uF. In the manual there is a misprint C18 is listed as a .1 but it must be a 1. You will probably need to use metallized polypropylene or paper in oil. A film and metal version will probably be to big. Brand choice is up to you. There are a lot of good caps under a $. But fancy caps will cost more. But watch the size.

When you say jumper, you mean remove the cap and replace it with a piece of wire soldered in the eyelets where the previous cap was?

I offer one small note of caution here.  C17 is a DC blocking cap.  It's removal allows DC offset of unknown polarity and voltage from other attached equipment to hit the input grids of V3 & V4.    I would check all sound sources for stray DC voltage before bypassing that cap permanently.   Any replacement cap should be the lowest voltage, physically smallest polystyrene/teflon/polypropylene .1uF (or slightly greater) cap you can find.  Even a polyester ("Mylar") cap is an improvement over the original, but you should be able to do better.

DarthBubba, I'm not disagreeing with You or want to get into a debate about blocking capacitors. But just to show both sides, although the SCA and ST amps have blocking caps the Stereo 70 and Mark III do not and in fact I have had a lot of tube amps. in the last 50 years, the SCA is the only amp. that had a blocking cap on the input.
But in the end it is up to the owner of each SCA and ST amp to decide if he wants to remove it or not.
Again, not disagreeing with you. The cap is like having a fuse on a speaker. It protects the speaker but does it affect the quality of the sound? Almost everything in life is tradeoff.

sailor wrote:DarthBubba, I'm not disagreeing with You or want to get into a debate about blocking capacitors. But just to show both sides, although the SCA and ST amps have blocking caps the Stereo 70 and Mark III do not and in fact I have had a lot of tube amps. in the last 50 years, the SCA is the only amp. that had a blocking cap on the input.
But in the end it is up to the owner of each SCA and ST amp to decide if he wants to remove it or not.
Again, not disagreeing with you. The cap is like having a fuse on a speaker. It protects the speaker but does it affect the quality of the sound? Almost everything in life is tradeoff.

Look at the timeline/history of (separate) amps.  Note that input caps became ubiquitous starting with the ST-35 and afterward.  Around that time the PAS-3X (1966) marked the end of the tube pre-amp era with their output blocking caps, at least in large production.  I have the schematics (over 400MB .zipped); MOST amps designed after the early 1960s, tube and solid state, have input blocking caps.  The Dynaco MkVI (the last Dynaco tube amp) has straight (DC), AC (Cap coupled), and balanced inputs.  The solid state Dynaco amps ST-80; ST-120; ST-400/410/416 had cap coupled inputs.  Dynaco's progeny, Hafler's amps, had cap coupled inputs - at least until the XL-280 (which may not be a "true" Hafler).  The Marantz 8, 8b, & 9 tube amps all had input caps; so did most of the Heathkit and McIntosh tube amps.  Since 1963 (the last 50 years), virtually all amp sections and separate amps of significant (not boutique) production levels have had input blocking caps unless they were solid state amps with DC-compensating servos or designed for unity-gain at DC.  Prior to then the output cap of the tube preamp was, in effect, the input blocking caps for the amplifiers they fed.

That said, most of my amps do not have input blocking caps because nothing that I connect to them has significant DC offset.  My demo/loaner amps do have input blocking caps because I don't know what will be connected to them when they're out'n'about.

I stand by my recommendation as written.

I guess you missed reading the part where I said I agree with you. I also knew why the newer Dynaco amps added the cap. They were afraid of the terrible transistor and its DC.
The cap affects the sound so you can leave it in and feel warm and fuzzy knowing you are protected from those terrible transistors or you can remove it and have a little better sound with a very slight risk. None of my amps either transistor or tube have a blocking cap. and I have never had a problem. 2 were removed 6 tube units all new designs came that way.
As I said before its the owners choice to make.

Nice read gentlemen. So, the blocking cap is a 'fuse' for the speakers and not the amp?

No it was an example. The cap is there to protect the amp. A fuse is added to a speaker to protect the speaker. Not all speakers have fuses not all amps have a blocking cap that was my point. It's a matter of choice.
Ignore the post about the 12AU7 I will try to remove it . It was for another string.

sailor wrote:No it was an example. The cap is there to protect the amp. A fuse is added to a speaker to protect the speaker. Not all speakers have fuses not all amps  have a blocking cap that was my point. It's a matter of choice.
Ignore the post about the 12AU7 I will try to remove it . It was for another string.

Thanks for clarifying that.

I think I'll remove the cap and get new caps where the Solens are.

sailor wrote:I guess you missed reading the part where I said I agree with you. I also knew why the newer Dynaco amps added the cap. They were afraid of the terrible transistor and its DC.

Correct. I agree with them when it comes to transistor power amps. Many of them, presented with DC on the input, will happily amplify that DC and send it through your voice coils. The audible effect of a DC-biased amplifier and voice coil is going to be worse than that of the capacitor at the input. If the DC on the input becomes more than just slight, the amplified DC could be hazardous to your woofer's voice coil. The supposedly-greater sonic purity of direct-coupling the input of a transistor amplifier is, to my mind, not worth the risk.

Ok, I've come to a road block. It doesn't turn on. At first, there was no fuse. I put in a 3 amp 125v and turn the switch on. Nothing. Power supply cans shot? What other testing should I check for? Or just go ahead and install the EFB?

Time to get out the multimeter.

Have you ever had the unit operating?
First, I hope you know that caps can store a charge even unplugged from the wall so always test to see if there is any voltage in the caps. discharge if necessary. The next test leave amp unplugged and test each cap section to see if any section is shorted to ground. If any is shorted don't start up until power supply is replaced. If there is a chance it's the cans I would not turn it on with the tubes in it. Be aware that the voltages will be higher than published in the manual if there is no tubes installed if it is OK. The best thing to do is replace the power supply with the new power supply before turning it on. Shorted can caps can do a lot of damage. Again you need a copy of the manual to get the test voltages.

lol. The fuse was too short. Have some 15a 125v fuses and they're longer. Amps are high, but just to test it. So, I turn it on, all the tubes lit and about a minute later, smoke is coming from the cans. I think that's the main culprit. I don't see smoke anywhere else. Also, at what setting should I set the multimeter to see if the cans discharged? Yes, I'm new at this but I'm a good learner.

I must repeat an advice : change one thing at a time, listen in between and make shure you have a stable situation before additional changes. Don't use a precious dynaco as "learning tool" if you want a learning tool / labbench there is lots of cheaper stuff to use.

peterh wrote:I must repeat an advice : change one thing at a time, listen in between and make shure you have a stable situation before additional changes. Don't use a precious dynaco as "learning tool" if you want a learning tool / labbench  there is lots of cheaper stuff to use.

Hi Peter,

I haven't done anything. I simply started it up and smoke is coming from the cans. The only thing I have is the EFB and caps to replace the Solens. Like you said, one thing at a time.

Read very carefully. Do not turn on the unit again until you install the new power supply. Never use to big a fuse. It needs to pop to save your power transformer and tubes. After installing the new power supply recheck your work to make sure you did everything correctly, remove all tubes and put in the correct fuse. Turn on and test voltages they will be higher than in the manual because without the tubes there is no load to pull the voltages down. If the power voltages are OK and you didn't pop a fuse then turn off the unit put in the tubes and turn on if you don't pop a fuse turn off, hook up to speakers and see if it's working.

peterh is giving good advise however in this case the whole power supply should be changed out as a unit not one part at a time. You have the new power supply board install it and follow the steps I gave in the above post. The solen replacement can wait. Please read these posts very carefully.

sailor wrote:Read very carefully. Do not turn on the unit again until you install the new power supply. Never use to big a fuse. It needs to pop to save your power transformer and tubes. After installing the new power supply recheck your work to make sure you did everything correctly,  remove all tubes and put in the correct fuse. Turn on and test voltages they will be higher than in the manual because without the tubes there is no load to pull the voltages down. If the power voltages are OK and you didn't pop a fuse then turn off the unit put in the tubes and turn on if you don't pop a fuse turn off, hook up to speakers and see if it's working.

Yes, you're correct about the fuse. The one I'm using is a temp. Just to start it. Not staying at all. I will do what you advised. It makes total sense. First and foremost, I'll discharge the caps and double check with the multimeter.

BTW, the fuse holder says to either use 120v 2amp or 240v 1amp. Does it matter which one?