Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mresistor on July 06, 2020, 06:22:33 pm

Title: '66 Single Showman 8 ohm OT
Post by: mresistor on July 06, 2020, 06:22:33 pm

I've got a '66 Showman on the bench.  I installed a brand spanking new Power transformer 3 years ago along with a new 6L6 matched quad. I also installed all new F&T filter caps and replaced all the other electrolytic caps inside the amp. checked the plate resistors and checked for dc on the pots. Cleaned and lubed all the pots.  I noticed back then that I had to set the bias less negative than expected to get the proper idle current.  If I remember right... around -42 volts. It has been in a music store since then on consignment.  A person bought it and played it on vol 5 -6 for around 15 minutes, 3 songs and the fuse blew. He says he was using a 4 ohm cab, but that shouldn't be a problem. One pair of power tubes looked like they took more heat than the other duo..  lettering discolored some.
 I just replaced all the screen resistors, grid stoppers and added 1 ohm resistors on the Ks to aid in checking and setting bias. I just biased it and it is 420v at 47ma on one side and 49ma on the other, so the OT secondaries aren't perfectly balanced but I don't see this as unusual. The bias voltage is -42v, again. The schematic shows -52v so it's 10v under that value. I am running it into a 1x12 cab with a near new 8 ohm Texas Heat in it. The amp does not seem to be as loud as one would expect an 80-100W amp to be.  That's what I see as a problem. My Super Reverb 4 ohm output head amp seems to be louder.
I have tested the OT using the following procedures. http://www.geofex.com/ampdbug/outtrans.htm (http://www.geofex.com/ampdbug/outtrans.htm)  It passed all these tests.   I don't have an isulation breakdown tester and I don't have a spare OT.
I would like to hear other opinions on this.  Would you let it go back out to be played?  Do you think the OT might be going bad under operating conditions? An insulation breakdown?
The amp sounds pretty good but I don't know if another would sound better. Also has a brand new JJ ECC81 PI. Both channels seem to be low on power but the Normal chan is louder which is normal for this amp.
https://el34world.com/charts/Schematics/files/Fender/Fender_showman_ab763_schem.pdf (https://el34world.com/charts/Schematics/files/Fender/Fender_showman_ab763_schem.pdf)

Title: Re: '66 Single Showman 8 ohm OT
Post by: shooter on July 06, 2020, 06:37:46 pm

420*.047, so ~20W

run the VDC's
chase a signal through
should find a couple roads to travel  :icon_biggrin: