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Dynaco SCA-35 Heat with Wood Case

tlann

tlann

Member
Hey everyone,

I’d love to hear from anyone who has run a Dynaco SCA-35 with the Dynaco wood case that has the routed slots on top — that is, the Dynaco cab 1D. I’ve read some people speculating that the SCA-35 would get too hot in a wood case, but I would be curious to hear from anyone who has actually tested this out. Some people who have pointed this problem out seem to be referring to the cab 2D, which was made to house two Dynaco units on top of each other (preamp atop tuner, say). My main question is whether the SCA-35 would get too hot on its own, not whether it would get too hot in one of those double-stacked cases.

My SCA runs hot, like all of them, but perhaps a bit less so because it has Dave Gillespie’s EFB board. I have a line on the Dynaco cab 1D, but don’t want to spend the money if using this case might overheat my amp and harm it.

Here are some pictures of the case in question, all pulled from the internet.

Any insight at all would be greatly appreciated!
 

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I wouldn't.

I ran an SCA-35 for a long time mostly as an office amp. I wouldn't even run it with the original metal cover. I made a replacement from expanded metal (basically thick screen) in order to have full ventilation without the guts being left exposed. Still ran hotter than hell.
 
Thanks for these replies! I could use my variac to run the Dynaco exactly at 117, but it sounds like that probably wouldn’t even keep it cool enough to use the wooden case. Plus, then I’d have to figure out where to put the variac…out of curiosity, is there a noticeable change in sound when running tube amps at 117 instead of ever so slightly higher? Or is it just about extending component life? I measured what comes out of my wall back when I first got this amp and determined at the time that it wasn’t high enough to make too much of a difference in terms of the amp’s health.
 
My ST-35 gets hot with the factory cage on it. Perforated steel front, back, and top. But the sides just have two slots near the bottom of the cage. Those sides get really hot. If the sides had slots all the way up that would really help. Ya, I could run it with the cover off, but that amp is just too ugly without the cage. :biggrin:
 
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I have an old Dynaco catalog that may have that wood case listed, and if so it will say if it recommended for the SCA-35 or not. I know it has the dual wooden case listed, with a caution not to use it with the SCA-35. I'm away from home until the weekend. I will look when I'm back.
 
JonL said:
I have an old Dynaco catalog that may have that wood case listed, and if so it will say if it recommended for the SCA-35 or not. I know it has the dual wooden case listed, with a caution not to use it with the SCA-35. I'm away from home until the weekend. I will look when I'm back.
Click to expand...
Thanks, that would be very helpful to know!
 
This might be a silly question... I'd have any of you spent time with an ST35 in the SCA35 chassis?

I'm wondering if removing the phono preamp board would have any appreciable reduction in PT current draw, or maybe it doesn't matter
 
I havnt spent any time with a converted sca but I have both pieces. The st35 is a little less hot but I think that's because it is better ventilated. A couple less tubes helps too.
 
Last edited:
tlann said:
I have a line on the Dynaco cab 1D, but don’t want to spend the money if using this case might overheat my amp and harm it.
Click to expand...
I personally like the look of wood cases on hifi gear. I'd buy the case, and run it that way, but first I'd make sure of a few things.
All electrolytic caps should be rated for 105c
Any resistors which drift with temperature should be replaced with film (which don't drift)
Bias should be checked and adjusted regularly.
Dust should be blown out regularly.
Nothing at all should be stacked on top of the SCA35.
Wall voltage should be corrected to 110v by whatever means necessary, which was the voltage available from most outlets when the sca was produced.
If all of these things are done, the unit will probably be OK living inside the cabinet that the manufacturer built for it...
 
I was worried that my SCA-35 power transformer was running much too hot, so I built a high quality outboard supply with a nice choke for the B+ and use the onboard power transformer for the filaments. Bass improved dramatically. Anyway, I really like the look of those wooden enclosures, but wouldn't recommend them with the stock power transformer.
 
JonL said:
I have an old Dynaco catalog that may have that wood case listed, and if so it will say if it recommended for the SCA-35 or not. I know it has the dual wooden case listed, with a caution not to use it with the SCA-35. I'm away from home until the weekend. I will look when I'm back.
Click to expand...
I've seen the Dynaco SS tuners and SS preamps, or SS integrated amps in the dual cabinets for sale on ebay before, but never a SCA-35.
 
tlann said:
Hey everyone,

I’d love to hear from anyone who has run a Dynaco SCA-35 with the Dynaco wood case that has the routed slots on top — that is, the Dynaco cab 1D. I’ve read some people speculating that the SCA-35 would get too hot in a wood case, but I would be curious to hear from anyone who has actually tested this out. Some people who have pointed this problem out seem to be referring to the cab 2D, which was made to house two Dynaco units on top of each other (preamp atop tuner, say). My main question is whether the SCA-35 would get too hot on its own, not whether it would get too hot in one of those double-stacked cases.

My SCA runs hot, like all of them, but perhaps a bit less so because it has Dave Gillespie’s EFB board. I have a line on the Dynaco cab 1D, but don’t want to spend the money if using this case might overheat my amp and harm it.

Here are some pictures of the case in question, all pulled from the internet.

Any insight at all would be greatly appreciated!
Click to expand...
Nice! :thumbsup:
 
My understanding is that installing a CL-80 thermistor (maybe CL-90 would be more appropriate for an SCA-35/ST-35) will introduce basic resistance on the power load and lower the input power by about 3 volts. The thermistor itself will get hot, but in general the rest of the circuit will put out considerably less heat. I install a thermistor on my Dynaco amps instead of bothering with a bucking power supply, I have done measurements and it's a 3 volt reduction with a CL-80 on a Mark 3, and 3-4 volts on an SCA-35.

A bucking power supply is the best option, but the least convenient. I have also seen Mark 3 amps with a CL-80 on the hot and neutral wires, theoretically slowing the startup quite a bit and lowering the voltage 5-6 volts. If your Dynaco SCA-35 is running too hot and you want to reduce stress on the power transformer, a thermistor can help by soft-starting the amp and reducing incoming voltage.

Thermistor for the SCA-35:​

The SCA-35 draws about 1A to 1.5A under normal operation. Based on this current, the best inrush current limiter would be:

Option 1: CL-90 (Best for Moderate Reduction)​

  • Cold Resistance: 120Ω
  • Hot Resistance at 1A: ~4Ω to 5Ω
  • Steady-State Current Rating: 2A
  • Voltage Drop at 1A: 4V to 5V
  • Voltage Drop at 1.5A: ~3V to 4V
  • Effect: Drops mains voltage by ~3V to 5V, reducing transformer heat and tube stress.

Option 2: CL-80 (More Aggressive Drop)​

  • Cold Resistance: 47Ω
  • Hot Resistance at 1A: ~3Ω
  • Hot Resistance at 1.5A: ~2Ω
  • Steady-State Current Rating: 3A
  • Voltage Drop at 1A: ~3V
  • Voltage Drop at 1.5A: ~3V to 4V
  • Effect: Slightly reduces stress but less than CL-90.

Installation:​

  • Install the thermistor on the hot wire leading to the power transformer.
  • Avoid placing it on both hot and neutral unless you need a more significant voltage drop.

Expected Benefits:​

  • Reduced inrush current, prolonging tube, power filter capacitor section and transformer life.
  • Lower steady-state voltage, slightly reducing filament and B+ voltages.
  • Reduced heat output from the power transformer.

Conclusion:​

  • Use a CL-90 if your amp is running excessively hot and you want a stronger reduction.
  • Use a CL-80 if you want a milder effect.
  • If your mains voltage is high (~125V), a bucking transformer may be a better long-term solution.
 
Last edited:
Bicyclist said:
I havnt spent any time with a converted sca but I have both pieces. The st35 is a little less hot but I think that's because it is better ventilated. A couple less tubes helps too.
Click to expand...
Signal tubes don't put out a lot of heat, but it's still added heat in a box.
A lot of tube preamps don't really have much ventilation at all.
I haven't looked, but does the SCA-35 just have a cover that just has a perforated vent on top like my FM3 tuner?
 
Lavane said:
Signal tubes don't put out a lot of heat, but it's still added heat in a box.
A lot of tube preamps don't really have much ventilation at all.
I haven't looked, but does the SCA-35 just have a cover that just has a perforated vent on top like my FM3 tuner?
Click to expand...
Just the top of the sca35 is perforated.
 
albusark said:
My understanding is that installing a CL-80 thermistor (maybe CL-90 would be more appropriate for an SCA-35/ST-35) will introduce basic resistance on the power load and lower the input power by about 3 volts. The thermistor itself will get hot, but in general the rest of the circuit will put out considerably less heat. I install a thermistor on my Dynaco amps instead of bothering with a bucking power supply, I have done measurements and it's a 3 volt reduction with a CL-80 on a Mark 3, and 3-4 volts on an SCA-35.

A bucking power supply is the best option, but the least convenient. I have also seen Mark 3 amps with a CL-80 on the hot and neutral wires, theoretically slowing the startup quite a bit and lowering the voltage 5-6 volts. If your Dynaco SCA-35 is running too hot and you want to reduce stress on the power transformer, a thermistor can help by soft-starting the amp and reducing incoming voltage.

Thermistor for the SCA-35:​

The SCA-35 draws about 1A to 1.5A under normal operation. Based on this current, the best inrush current limiter would be:

Option 1: CL-90 (Best for Moderate Reduction)​

  • Cold Resistance: 120Ω
  • Hot Resistance at 1A: ~4Ω to 5Ω
  • Steady-State Current Rating: 2A
  • Voltage Drop at 1A: 4V to 5V
  • Voltage Drop at 1.5A: ~3V to 4V
  • Effect: Drops mains voltage by ~3V to 5V, reducing transformer heat and tube stress.

Option 2: CL-80 (More Aggressive Drop)​

  • Cold Resistance: 47Ω
  • Hot Resistance at 1A: ~3Ω
  • Hot Resistance at 1.5A: ~2Ω
  • Steady-State Current Rating: 3A
  • Voltage Drop at 1A: ~3V
  • Voltage Drop at 1.5A: ~3V to 4V
  • Effect: Slightly reduces stress but less than CL-90.

Installation:​

  • Install the thermistor on the hot wire leading to the power transformer.
  • Avoid placing it on both hot and neutral unless you need a more significant voltage drop.

Expected Benefits:​

  • Reduced inrush current, prolonging tube and transformer life.
  • Lower steady-state voltage, slightly reducing filament and B+ voltages.
  • Reduced heat output from the power transformer.

Conclusion:​

  • Use a CL-90 if your amp is running excessively hot and you want a stronger reduction.
  • Use a CL-80 if you want a milder effect.
  • If your mains voltage is high (~125V), a bucking transformer may be a better long-term solution.
Click to expand...
I love thermistors.
 
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