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 Post subject: Class A OPT
PostPosted: August 2nd, 2018, 9:47 am 
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Joined: December 14th, 2013, 2:19 pm
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Planning on using OPTs from a Class AB amp in a Class A PP amp. Current will be up to 180mA total.XFMR is ~200 ohms PP. So each half should have under 100mA across 100 ohms. 2 watts idle dissipation per transformer.

The transformer was originally designed for 30mA per plate. Can I safely use it in this way, or am I missing something?

B+ is very close to original, power output wilm be less than the original design.

Stuart


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 Post subject: Re: Class A OPT
PostPosted: August 2nd, 2018, 11:39 am 
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Joined: July 24th, 2015, 4:17 pm
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Location: Parkville, Maryland
Stuart Polansky wrote:
Planning on using OPTs from a Class AB amp in a Class A PP amp. Current will be up to 180mA total.XFMR is ~200 ohms PP. So each half should have under 100mA across 100 ohms. 2 watts idle dissipation per transformer.

The transformer was originally designed for 30mA per plate. Can I safely use it in this way, or am I missing something?

B+ is very close to original, power output wilm be less than the original design.

Stuart



There is no way to know if the designer that specified the output transformer, used with the original amp., had enough "head room" to keep it out of trouble. I've learned the hard way that some designers don't think these issues through. Also, a lot depends on the coating used for the winding's. Even epoxy coatings have their limits. Since the primary is typically buried inside the secondary it has limits in terms of heat rejection. My advice? Don't use it unless you can keep it within its limitations.

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 Post subject: Re: Class A OPT
PostPosted: August 2nd, 2018, 5:03 pm 
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Joined: March 2nd, 2013, 2:43 pm
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Location: Potomac, MD
Stuart,

The higher the current the harder it is to keep the two halves balanced, so the further in saturation the core is likely to be. Expect a hit on the bass response.

David


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 Post subject: Re: Class A OPT
PostPosted: August 3rd, 2018, 7:39 am 
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dberning wrote:
Stuart,

The higher the current the harder it is to keep the two halves balanced, so the further in saturation the core is likely to be. Expect a hit on the bass response.

David


David,

Do you mean AC or DC balance? The stage is a common current sink self inverting push-pull stage. There is an adjustment pot between the cathodes and current sink. So, DC balance should be readily achievable. AC will depend on tube curves matching, I guess.

It's the Oddwatt Octal amp. See attached schematic.

Thoughts?


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 Post subject: Re: Class A OPT
PostPosted: August 3rd, 2018, 3:27 pm 
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Joined: March 2nd, 2013, 2:43 pm
Posts: 141
Location: Potomac, MD
Man, who comes up with these crazy circuits? Never seen this one. I see why it has to be class A. The LM317HV has a spec 60-V limit, so that would probably be exceeded on musical transients, especially with UL as grid to cathode peak voltages are higher. Anyway, I was thinking of dc balance. I have found that it takes as little as 1/4 mA to begin to unbalance the typical E-I core. The lower the bias current, the easier it is to keep the imbalance within reason.

David


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