Question about Metroamp Dual Tap PT

Ray · Post by **Ray** » Wed Jan 30, 2008 7:49 pm

Hello all,

My name's Ray and while I have been a member for quite a while and reading I don't get to post much. I would like to start by saying it's a real blessing to be a member here and my many thanks to everyone, and I mean EVRYONE A-Z for graduating me from a newbie tech to being able to completely tear my plexi reissue apart and do whatever I want to it. All of your talents as amp workers and as players is a real inspiration and I appreciate you all sharing your insights, finds and "tricks".

Now on to my question- I'm getting the dual tap stand up power trans former to install in my 93 plexi reissue and I have a concern. I want to use the lower voltage tap, but my question is, while this will lower the plate voltage to the power tubes, will it lower the plate voltages (and any others) to the preamp tubes? I ask because I get a great amount of drive, or overdrive, from the whole pre amp section and don't want to lose any of it because of the apparent lower voltage taps. Can anyone shed some light on this for me?

Many Thanks,

Ray- San Jose, CA

rockstah · Post by **rockstah** » Wed Jan 30, 2008 8:19 pm

of course using a lower voltage tap compared to yours at the moment will change everything in the amp but will it be noticeable in the preamp?
only u will be able to tell.
if after that you take some readings and then compare them to readings you took before the PT swap u can always change a couple resistors to get it back.

keeping all this in mind wait till u get it installed and if you decided your not getting enough in the preamp you can change a couple resistors that will yield more voltage or less where u want it.

i would be curious to see voltage readings( v1 through v7 ) of your amp before and after the PT swap.

Mark

Ray · Post by **Ray** » Wed Jan 30, 2008 10:58 pm

Mark,

Thanks a lot for your insight on that. You're right I can't forget to take some voltage readings before and after to get a comparison. Maybe after the swap, I'll like it as is and won't have to change anything (YEAH RIGHT- us guys not change anything!). Really though, I'm hoping that the drive remains the same. What did you notice when you first put yours in and used the low voltage taps?

I gotta say, you have some monstrous chops my man and just being able to get some ideas from all of the work you've done on those amps is really some food for thought. Keep up the good work!

Ray

rockstah · Post by **rockstah** » Fri Feb 01, 2008 4:22 pm

Thanks Ray. keep me posted on your findings.

Mark

Ray · Post by **Ray** » Mon Feb 04, 2008 5:52 pm

Mark,

Will do, gonna be taking some pre-install measurements tonight, but I'm not getting the new tranny till tomorrow. Hopefully I will get to it in the next 2 days or so.

Ray

rockstah · Post by **rockstah** » Mon Feb 04, 2008 6:13 pm

kewlness. i look forward to this.

Ray · Post by **Ray** » Fri Feb 08, 2008 11:26 am

Mark,

Sorry for the delay on my findings, but with work and all...you know. I took the stock measurements last night and finally got the tranny in and all wired up except for the high voltage taps. I can't make a descision on which to do first, the regular taps or the low voltage ones. You have a suggestion? I know you have at least one of these in one of your builds that I've listened to. I'm going to put quick connects on the taps so I can easily connect and disconnect them from the connection points on the standby switch. What a PITA putting the tranny in, as I had to switch end bells with the stock ones because the holes on the feet of the end bells that came with the tranny didn't line up at all. Friggin plexi reissues. As it was I had to blow two new holes in the chassis when I put in the MM 3H choke a couple of weeks back.

I'm still waiting on my quad of Shuguang EL34B's. I had heard from Tubetramp (Terry) that these are the only ones that can really stand up to the high voltage, but I'm gonna test with my stock of Electro Harmonix EL34's and 6CA7's. I love 'em both but the bias drift is absolutely horrendous, especially on the 6CA7's and these are current production.

Just a note, there's a 20 volt plate jump up on the power tubes with them removed when taking measurements, and then installed, they load the system down which in turn, drops the plate voltage back 20 volts (I'm sure you knew this, but I thought I would share).

Sorry for the long winded post, but wanted to give you the run down.

Regards,

Ray

Ray · Post by **Ray** » Mon Feb 11, 2008 3:36 pm

Mark,

Ok got George's new dual tap installed this weekend. Took voltage measurements before and after. I won't bore ya with a whole list of readings (but I can if you want =). I hooked up the low voltage tap first and needless to say, that was the last time. Although it has a decent tone, it wasn't what I was after. Just the fact of the heater voltage being up and yes, it dropped all voltages on the 3 preamp tubes way down, made it sound, well, congested and off.

The high taps were a diff story. GREAT sound! At full 120 wall, the voltages were about 2-7 volts higher on the preamp tubes, with 511 being on the plates on the power tubes. Too high for tubes nowadays. I found the best sound being at 110. Pins 123, 678 on the pre's are maybe 1-2 volts lower than normal but the heaters are right on at 6.2 total. Sounds fantastic. It made this reissue sound like a real vintage '68.