Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: W5FH on November 29, 2024, 03:02:12 pm
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Hello-
I am wanting to try a push-pull reverb driver using center tapped transformer in a build to replace the parallel 12AT7 reverb driver. I use tanks of 8 ohms input. The intent is to have a clean tank driver and by using push-pull core saturation is of less problems. In past builds I used an Edcor 3.5 watt SE 22.5K to 8 ohm reverb driver transformer, gapped core, with parallel 12AT7, this is a good quality gapped SE transformer but a little more expensive.
My plans are to use one of the Musical Power Supplies transformers, the OT5PP+USA. This is a 5 watt with 22.5K primary center tapped. Secondary is 4,8,16 ohm. I want to try this transformer with the Merlin 12AT7 phase inverter reverb driver circuit. Attached is the circuit. Sorry but could not attach the OT5PP+USA data sheet.
I will have about 2.6 VRMS at 1 Khz drive to this circuit when volume control is near 50% as a guide to my drive level. Plan on about 400 VDC to transformer center tap. I plan to put a 1 meg dwell pot ahead of it. My reverb drive signal from preamp is coming off of a 500 pf coupling cap so this will be going to top end of 1 meg dwell pot. Should I leave C1 at 470 pf. or enlarge it some?
Can someone offer their suggestions or experience regarding using this type circuit? Should I expect about the same secondary output going to tank with either circuit? I am wanting to try push pull for first time.
Thanks for help.
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I think this is the OT you referenced.
Looks like a fun hack.
This (https://edcorusa.com/products/xpp1-15k-1w-15k-ohms-push-pull-tube-output-transformer?_pos=2&_fid=476d4e91a&_ss=c) one should work as well, however, you'll need a 600Ω input Z tank. Type 4E will have a 600Ω input Z. Note that comparable Types 8E and 9E will have a 800Ω input Z - Shouldn't be an issue.
BTW, Hammond Organs and some (higher end?) Sears Silvertone/Valco models used a PP tank driver. Check those out for some more sources to plagiarize.
Sears 1485 (https://el34world.com/Forum/chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://el34world.com/charts/Schematics/files/Silvertone/Silvertone1485.pdf)
Hammond (https://el34world.com/Forum/index.php?action=dlattach;topic=23056.0;attach=69775;image)
--Pete
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Should I leave C1 at 470 pf. or enlarge it some?
Leave it, it is only to protect the transformer from flyback. You can use 100pf to 470pF
Should I expect about the same secondary output going to tank with either circuit?
It should work fine with the transformer you mentioned. You have plenty of drive level available, so just tweak the actual level on test to get the sound you want.
https://www.musicalpowersupplies.com/app/download/7116845704/OT5PP_STANDARD.vsd.pdf (https://www.musicalpowersupplies.com/app/download/7116845704/OT5PP_STANDARD.vsd.pdf)
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Thank you Merlin -
Am getting ready to start build using the push-pull 12AT7 reverb driver. Excited to see how it does.
I appreciate your great books and teachings.
Byron