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 #156801  by gratefulredhead
 Wed May 31, 2017 9:43 pm
Hi learned friends,
I just had the preamp tapped in my twin (finally) and I'm wondering how many of you deal with the power stage? I know Jerry used it to run his monitor at some point, but I won't be doing that. I was thinking either using my Weber Mass200 attenuator as a power soak or pulling all 4 power tubes. Any opinions on either of these solutions? Does pulling the power tubes send too much voltage to the preamp and kill all your headroom?

Any advice or insight would be a miracle. Thanks!
 #156825  by lovetoboogie
 Fri Jun 02, 2017 8:46 am
The power tubes should be pulled...

Your B+ will increase slightly but its negligible the effect it will have the pre's headroom...

Even if the preamp tap was done with the correct shorting jack which shunts the signal to the phase inverter to ground. The OT is never completely isolated. Even with no signal driving the power section all tube amps have slight parasitics that can load the OT enough to fry it gradually. A power soak or dummy load is sensible if you intend to go back and forth between the dedicated pre tap and the tube power section with enough frequency that it makes pulling the power tubes a pain in the ass. The Dead early on just pulled the power tubes on the Fender amps when they went to the supplemental Dynaco's, Mac 3500's & 2300's power amps. There were so many surplus power amps on stage in case of a failure.

In the case of one of Jerry's Boogie Mark I's, the primary on the plate leads of the OT is lifted and heat shrinked. I believe he kept anothe Boogie fully tubed and wired for original use. There are some cool pics floating around here of the Blues For Allah sessions. Jerry is plugged straight into the Mark I with no supplemental power amp.
 #156876  by waldo041
 Tue Jun 06, 2017 8:28 pm
lovetoboogie wrote:The power tubes should be pulled...

Your B+ will increase slightly but its negligible the effect it will have the pre's headroom...

Even if the preamp tap was done with the correct shorting jack which shunts the signal to the phase inverter to ground. The OT is never completely isolated. Even with no signal driving the power section all tube amps have slight parasitics that can load the OT enough to fry it gradually. A power soak or dummy load is sensible if you intend to go back and forth between the dedicated pre tap and the tube power section with enough frequency that it makes pulling the power tubes a pain in the ass. The Dead early on just pulled the power tubes on the Fender amps when they went to the supplemental Dynaco's, Mac 3500's & 2300's power amps. There were so many surplus power amps on stage in case of a failure.

In the case of one of Jerry's Boogie Mark I's, the primary on the plate leads of the OT is lifted and heat shrinked. I believe he kept anothe Boogie fully tubed and wired for original use. There are some cool pics floating around here of the Blues For Allah sessions. Jerry is plugged straight into the Mark I with no supplemental power amp.
Have you seen the inside of Jerry's Mark 1? Are the lifted OT Plate Leads the only modification?

As for the Twin preamp tap with the power tubes in, the output is shorted as well when you remove the speaker jack which is recommended by Healy. Also, I have found that the B+ is raised enough to add an audible difference when the power tubes are removed. The increase adds the need for the filter caps voltages to be attended to if they are stock or else the risk of them blowing is a real possibility.

~waldo
 #156919  by gratefulredhead
 Fri Jun 09, 2017 9:47 am
Thanks Waldo! I've used my new set-up for 3 shows so far, with a Weber Mass 200 attenuator plugged into the amp's internal speaker out. I'm using the Mosvalve 962 for my power amp. It think it sounds fantastic! I haven't smelled anything burning yet, so I guess that's good. I'm sending your suggestions onto my tech to see if he wants to tweak anything. Thanks to everyone on these boards for all the great information. Because of you all my tone is so much closer to what I've been searching for!