#165756  by Jblue76
 Tue May 28, 2019 3:33 am
Hey all,
When tapping a Twin to an MC50, do any of you pull the power tubes? The way mine is set up, if I’ve got a plug in the “external” speaker jack then the power section of the twin is off, unplug it and the twin’s power is engaged. I never use the twin’s power really.
 #165768  by Jblue76
 Wed May 29, 2019 2:00 pm
Does anyone pull the power tubes? Any reason why to or not to do it?
 #165777  by Blacklodgebob
 Fri May 31, 2019 6:46 am
for one youll damage your amplifier. Google is your friend. You need to have your amp modified if its vintage and your trying to just run the preamp tubes. Seems like a huge pain in the ass considering the weight of that amp to begin with. Aka why haul the iron around and not use it? Unless you have roadies... Anyway, your amp will still need to see a load or youll fry your output transformer. One way to do it - may be to get some solid state power tubes, and a reactive load box. Plug the speaker output into the load box using a regular patch cable, not a speaker to amp cable, and swap out your real glass to for the SS tubes. Using the preamp tap out, you should be able to run the thing without breaking anything. I'd make sure my amp was biased properly before trying this out. As you can see - its much easier just pairing a tube preamp rack or similar to a power amp - for the weight. You can also just modify the twin - send it to Mike Wald and tell him to 'do everything'.
 #165807  by Pointclear
 Wed Jun 05, 2019 6:54 am
I have gone down a long road modding and tapping the preamp of my twin reverb. I had played it with the power tubes sitting idle and the speakers plugged in until recently. I finally upgraded the big filter caps to 150uf 450v caps, and pulled the tubes and unplugged the speakers. The difference in tone, volume, and fidelity is awesome - it’s Jerry. Apparently if you pull the power tubes without upgrading the filter caps there is a risk of overloading the normal caps that are in there. Interestingly, I noticed a frequency hump that has shifted to more bass with the upgraded filter caps. I’ve had to turn my meds down a bit on my amp and play with the tone knobs and caps on my guitar more.
I’ve tapped the preamp on my twin with a version of the Alembic tap, which grounds the preamp output into the power section.
Mike Wald pretty much walked me through the final version of what I’m using. He is a very generous fellow and full of knowledge.
 #165808  by Jblue76
 Wed Jun 05, 2019 7:15 am
Thanks for that. Your posts were really helpful to me when I was first starting to get the tantalum caps and everything going with the preamp tap. I don’t know the details of how my tech did the tap unfortunately. All I know if that when I plug into the “External Speaker” jack the signal goes to the mc50, and with no plug the signal goes to the twin’s power section. I wonder if it’s okay to just pull the tubes now the way it’s set up. Maybe I should talk to my amp tech... though he’s not an easy guy to get a hold of. We definitely didn’t change any filter caps in the power section, and I would be afraid to increase the bass. Actually I’ve been having tone problems since all the mods were done. I’ve had to play with both bass and mid on zero, and with most pickup positions use a treble boost too. I have discovered the 51 year old preamp tubes have been failing; changed v2 and that made a big difference but there’s still not enough treble. I have some rca greyplates coming so hopefully they will fix the isssue, but if not I’ll have to take it in. As for speakers, I use the k120s I’ve got in the twin. I have been considering disconnecting one of them and switching over to the 8ohm terminal on the Mac (Sarno’s suggestion), but haven’t gotten around to it yet. What do you think, just pull the tubes now or change those filter caps first? If you think I need to make some changes, please let me know if there’s anything else I out to get done while it’s in with the tech.
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 #165811  by Pointclear
 Wed Jun 05, 2019 10:15 am
Hey jblue! Thanks! I’m glad my posts were helpful. It’s been quite a journey getting my rig together! Sounds like you need to talk with your tech before you do anything. There are many ways to do a line out on your amp. Jerry used the Alembic-style preamp taps that grounded the preamp into the power section. The second channel is tapped right after the reverb return and bypasses the vibrato section (which Jerry took out of the circuit anyway).
You need to know what’s going on in your amp, whether you pull the tubes or not.
I have an 82 fender concert amp, as well. It has a factory line out that uses the power section of the amp and is totally different than the Alembic tap. Your tech may have done a line out like similar to the concert’s. You need to know what is going on in there.
I don’t have any diagrams or schematics on hand. Although, mike posted some hand -sketched Alembic tap schematics on one of the threads discussing these things. It is worth your time to find them and show them to your tech. It’s not a complex mod at all. But, you can mess your amp up if you don’t get it right. I’ve been there - and messed stuff up big time.

Dozin has a simple, ungrounded, line out mod on its site. It works but doesn’t have the fidelity that Jerry’s has.