On the SSE website George said the value for C1 was not critical and spec'd anything between 22uF and 47uf (47uF being the upper limit for the 5AR4 rectifier). After he added some components to "make life easier" for the rectifier, he mentioned doing some experimenting to see if a higher value like 50uF was acceptable.
I can't find a subsequent thread or message that confirms whether 50uF/500V is safe to use or not with the 5AR4. Anyone?
At the other end of the spectrum, what would be the effect (if any) of using 22uF/500V?
I can't find a subsequent thread or message that confirms whether 50uF/500V is safe to use or not with the 5AR4. Anyone?
At the other end of the spectrum, what would be the effect (if any) of using 22uF/500V?
Still wondering if it's safe to use a 50uF/500V cap in C1, with a 5AR4 rectifier and 750VCT (375-0-375) power transformer.
50uF would be about the upper limit. But that depends on the secondary DC resistance of the power transformer you're using. It's possible you may need to put small value resistors in series before the 5AR4 plates.
Check the 5AR4 data sheet - https://frank.pocnet.net/sheets/093/5/5AR4.pdf
Check the 5AR4 data sheet - https://frank.pocnet.net/sheets/093/5/5AR4.pdf
You should see a drop in B+ compared to a larger C1 value. Either way, Id definitely be using sic diodes in series with the rectifier plates.At the other end of the spectrum, what would be the effect (if any) of using 22uF/500V?
Still wondering if it's safe to use a 50uF/500V cap in C1, with a 5AR4 rectifier and 750VCT (375-0-375) power transformer.
You're gonna have a pretty high B+ with that trans. What's your target?
jeff
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I think if you use 22uF instead of 50uF, the drop in B+ voltage will be small. There's a certain critical value beyond which the voltage doesn't go up very much.
The suggestion to put silicon diodes in series before the rectifier tube plates is a very good one. It could save the whole amp if a rectifier were to fail short circuit (it can happen!).
The suggestion to put silicon diodes in series before the rectifier tube plates is a very good one. It could save the whole amp if a rectifier were to fail short circuit (it can happen!).
Thanks for this very useful information. I don't have a "design target". I'm a relative newbie, having built a few kits and now venturing a little farther out. I'm basically just following George's advice and instructions on the TubeLab website, supplemented by searching and reading a lot on this forum.
I would like to experiment with 3 families of output tubes: EL34/6CA7, 6L6GC and KT88. That's a major feature that drew me to George's design. But I'm experiencing some difficulty in interpreting George's simulations to try to decide on a starting value for the cathode resistors. I can then tweak/adjust from there when I am more settled about which tubes I will use most in this amp. Based on past experience that is likely to be a KT66.
Yes, I do plan to use both ss and tube rectification, probably without the switch installed.
I have not yet purchased the power transformer. I could change the spec to 700VCT (350-0-350) if a lower B+ is considered to be advisable. And/or use a lower value C1 (like a 22uF/500V).
Your further input/advice would be appreciated.
I would like to experiment with 3 families of output tubes: EL34/6CA7, 6L6GC and KT88. That's a major feature that drew me to George's design. But I'm experiencing some difficulty in interpreting George's simulations to try to decide on a starting value for the cathode resistors. I can then tweak/adjust from there when I am more settled about which tubes I will use most in this amp. Based on past experience that is likely to be a KT66.
Yes, I do plan to use both ss and tube rectification, probably without the switch installed.
I have not yet purchased the power transformer. I could change the spec to 700VCT (350-0-350) if a lower B+ is considered to be advisable. And/or use a lower value C1 (like a 22uF/500V).
Your further input/advice would be appreciated.
An in-the-ballpark estimate for the raw B+ you'll get from a particular transformer is the secondary RMS voltage * 1.3 if using silicon diodes.
That's based on the more correct (secondary voltage * 1.414) minus copper and iron losses in the transformer.
So if you take a 700VCT transformer (350V-0-350V) and use UF4007 rectifier diodes or similar, you should get something like 350 * 1.3 = 455V DC.
A 5AR4 will drop around 20V at 150mA, so that would be roughly 435V DC if using a 5AR4.
These are ballpark estimates only. The idea is to have a quick estimate to start from, then use the design formulae for more precise predictions, if needed.
That's based on the more correct (secondary voltage * 1.414) minus copper and iron losses in the transformer.
So if you take a 700VCT transformer (350V-0-350V) and use UF4007 rectifier diodes or similar, you should get something like 350 * 1.3 = 455V DC.
A 5AR4 will drop around 20V at 150mA, so that would be roughly 435V DC if using a 5AR4.
These are ballpark estimates only. The idea is to have a quick estimate to start from, then use the design formulae for more precise predictions, if needed.
Thank you very much for this. Referring to the Tubecad SEamp simulations George posted on his website, for each of the output tube families ~435V DC on the plate indicates a ~560R cathode resistor, which is exactly what George recommended to start with 🙂 which is what I will do.
I have a little meter gadget for my bass guitar amp that plugs into an octal tube socket and measures plate V and mA simultaneously. I can use that, if needed to adjust a particular output tube type into it's sweet spot by varying the cathode resistor (if needed).
My first batch of small components is ordered and on its way so I will start populating the PCB next week.
I have a little meter gadget for my bass guitar amp that plugs into an octal tube socket and measures plate V and mA simultaneously. I can use that, if needed to adjust a particular output tube type into it's sweet spot by varying the cathode resistor (if needed).
My first batch of small components is ordered and on its way so I will start populating the PCB next week.
I would also go with a 350-0-350 volt transformer today. The 750 volt Allied unit was a bargain at about $40 when I started making SSE's. Line voltages were also a bit lower and tubes that eat 500 volts for years without complaining were more common. The Hammond sourced Allied transformer has gone through at least one "cost reduction" redesign since I bought a bunch (2004) and they are known for putting out more than their rated voltage today. It will put the voltage across C1 real close to, or over 500 volts. Electrolytic capacitors have very wide tolerances. For all practical purposes a 47 uF and a 50 uF are the same thing. I would not go beyond 50 uF with a new production 5AR4 in an SSE.
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