Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tommyzcat on November 09, 2022, 10:21:23 am
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Greetings,
This is my 1st post here and pertains to my very 1st tube amp build - I noticed that Travis8 had posted last year about a similar question but he was building a TubeTown kit, rather than Hoffman 5f6a.
The Robinette pentode/triode half power switch mod diagram does not match with Doug's kit - particularly how pins 6 of V4 & V5 do not just route from B+2, but 1st go through seemingly different value resistors and same value caps (assuming this has to do with the phase inverter feeds)...
Am I missing something? I could see how Travis8's build was slightly more straightforward given his diagrams...so I don't feel ashamed reaching out (other than I am a drummer/sound engineer and not an axe slinger!)
This is literally the last step for me before wiring the output jacks and beginning my pre-testing of all circuits. I am hopeful that this mod is possible but cannot find anyone else who has posted about this mod to the Hoffman kit.
My (noob) thought is that instead of both poles of the B+2 feed being tied together, that the separate B+2 legs will each feed 1 side individually (unlike the Robinette diagram) and the rest of the legs of the switch will match what is shown. Is this correct thinking?
Any help is much appreciated - my biggest fear is smoke/flame at power-up!
"But if I do not strive...who will?" - LaoTzu
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If you can understand this very simple schematic you can wire the pentode/triode switch to ANY amp, no matter what layout is used.
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Wow, awesome!
Thank you so much.
Yes, I can decode the schematic (even of I have to use some reference material).
I did find myself wondering why I couldn't use the legs tied to those 470Rs (I used 5W for my build) - but not being intimately familiar with pentode/beam tetrode/triode differences coupled with fear of death or destruction of gear - I wasn't going to experiment before asking more knowledgeable folk for help.
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I did find myself wondering why I couldn't use the legs tied to those 470Rs (I used 5W for my build) …
Could you clarify what legs you’re referring to? Maybe pin6 of the valve socket?
If so that would be fine.
Be aware that triode mode may be perceived to sound naff.
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perceived to sound naff
As a Scot, I love this use of the word naff.
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I can confirm it indeed sounds naff
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... pentode/triode half power switch ...
I'm not sure Triode mode is really "half" power.
Even if it is half-power, that does not make it half-loud (you need 1/10th-power for half-loud).
Triode mode sounds very clean & dark. I didn't like it when I tried it, and never re-tried it after that.