Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TacosSupreme on October 21, 2024, 11:01:05 am
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Hi!
After a successful build of a Bassman Micro, I'm about to tackle another of Rob Robinette's micro amp designs: the deluxe micro.
https://robrobinette.com/Deluxe_Micro.htm (https://robrobinette.com/Deluxe_Micro.htm)
To keep things ultra simple, I want to go with the solid state rectifier version, which will keep the tube count at just 2.
Rob mentions that the recommended power transformer for the solid state rectifier version would be a 250-0-250V but doesn't give the exact Hammond model reference.
The Hammond 269JX is rated at 500VCT but in the "electrical data" portion of the spec sheet it mentions 280.9V from red to red-yellow (center tap). I'm confused as I would have expected 250V...
https://www.hammfg.com/files/parts/pdf/269JX.pdf?v=1697661948 (https://www.hammfg.com/files/parts/pdf/269JX.pdf?v=1697661948)
I think my other option would be the 269GX listed at 450VCT but with 253.8V center tap mentioned in the electrical data sheet.
https://www.hammfg.com/files/parts/pdf/269GX.pdf?v=1697661948 (https://www.hammfg.com/files/parts/pdf/269GX.pdf?v=1697661948)
I'm a bit confused here. Why the discrepency? Is it because it varies depending on your actual AC current from mains and the electrical data is for 125V?
I'm in Eastern Canada so I assume I'm getting a fairly steady 120V from mains. Which Power Transformer should I order to get 250-0-250V?
Also, will those give me enough current? they are respectively rated at 69 and 75 mA whereas the PT Rob Uses are 100 mA or above.
thanks
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The Hammond 269JX is rated at 500VCT but in the "electrical data" portion of the spec sheet it mentions 280.9V from red to red-yellow (center tap). I'm confused as I would have expected 250V...
Those are the measured voltages with no load current. Transformers only fall to their 'rated' voltage when the current also reaches the rated value.
Tbh I would go with the 269GX. The Micro is running at painfully high voltage for a 12AU7 as it is.
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The Hammond 269JX is rated at 500VCT but in the "electrical data" portion of the spec sheet it mentions 280.9V from red to red-yellow (center tap). I'm confused as I would have expected 250V...
Those are the measured voltages with no load current. Transformers only fall to their 'rated' voltage when the current also reaches the rated value.
Tbh I would go with the 269GX. The Micro is running at painfully high voltage for a 12AU7 as it is.
Thanks, that makes sense! What is the consequence of having a high voltage in the power tube? Shorter life maybe?
Couldn't I go with either PT and just adjust the values of the filter resistor and voltage dropping resistor to reach the target voltages in B+1 and B+2?
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The anode can swing to over a kilovolt which that little tube wasn't built for. Even 9 pin sockets can develop leakage problems (you see this in EL84 amps sometimes).
Yes you could increase the dropping resistor, so I guess use whichever option is cheaper. There may be even cheaper transformers available, since you really don't need a lot of current (30mA ish).
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Thanks. They're about the same price (100 CAD) and it doesn't look like I'll find much cheaper transformers that fit the bill unfortunately. I think I'll just go with your recommendation for the 269GX then. Sounds like you know your transformers 😉
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Was just looking at the that micro and think I'd rather use the EF80 for outout tubes. I do have a DSL-1 and the 12AU7s are ok but a pentode output would probably be better.. I'll let you know as I'm in process of gathering the parts up to build a JCM800 micro EF80 version.
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Robinette recommends the Hammond 269EX. Is there some reason for not using one? if using a SS recto maybe a 269AX would work well. I
He recommends that 269EX used with an EZ81 recto.. what kind of plate v are you looking for on the power tubes?
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It's his recommendation for a tube rectifier but not for solid state. Here's the quote where that's explained, and those are the voltages I was going to aim for:
For a solid state rectifier I believe the sweet spot comes from 250-0-250V of high voltage which gave me a B+1 of 390V, B+2 of 304, 150 and 146V on the V1 plates, 2.3 and 2.4V on the V1 cathodes; 375V on the V2 plate and 11.8V on the V2 cathode.
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I just checked and the 269AX is 250VCT. Wouldn't that be 125-0-125VAC?
In which case it's not enough according to Rob.
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It's his recommendation for a tube rectifier but not for solid state. Here's the quote where that's explained, and those are the voltages I was going to aim for:
For a solid state rectifier I believe the sweet spot comes from 250-0-250V of high voltage which gave me a B+1 of 390V, B+2 of 304, 150 and 146V on the V1 plates, 2.3 and 2.4V on the V1 cathodes; 375V on the V2 plate and 11.8V on the V2 cathode.
I don't see anything that says to use a tube rectifier and not a SS rectifier.
He's telling you to use a 250-0-250V PT for B+ with a SS rectifier. He says "For a solid state rectifier I believe the sweet spot comes from 250-0-250V of high voltage......"
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It's his recommendation for a tube rectifier but not for solid state. Here's the quote where that's explained, and those are the voltages I was going to aim for:
For a solid state rectifier I believe the sweet spot comes from 250-0-250V of high voltage which gave me a B+1 of 390V, B+2 of 304, 150 and 146V on the V1 plates, 2.3 and 2.4V on the V1 cathodes; 375V on the V2 plate and 11.8V on the V2 cathode.
Yes sorry the 290AX is not suitable my mistake.
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It's his recommendation for a tube rectifier but not for solid state. Here's the quote where that's explained, and those are the voltages I was going to aim for:
For a solid state rectifier I believe the sweet spot comes from 250-0-250V of high voltage which gave me a B+1 of 390V, B+2 of 304, 150 and 146V on the V1 plates, 2.3 and 2.4V on the V1 cathodes; 375V on the V2 plate and 11.8V on the V2 cathode.
I don't see anything that says to use a tube rectifier and not a SS rectifier.
He's telling you to use a 250-0-250V PT for B+ with a SS rectifier. He says "For a solid state rectifier I believe the sweet spot comes from 250-0-250V of high voltage......"
yes I meant that for a SS rectifier he recommends 250-0-250V, sorry if I wasn't clear enough. English isn't my first language.
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yes I meant that for a SS rectifier he recommends 250-0-250V, sorry if I wasn't clear enough. English isn't my first language.
Thanks for making it clear.