Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 8wattjack on December 11, 2018, 07:25:00 pm
-
1
-
However that brings me to question why what appears to be the two 10k resistors that I think are the balancing resistors for the first filter. But on my amp, and in the schematic, one goes to ground, and the other to the opt.
That's not what I see on the schematic you linked. I see two 100K balancing resistors and they are connected correctly across the 220µF caps.
There is also a massive 20W 30K resistor on the second stage to ground. It gets HOTT!!! I never understood this Power supply, but it work fine, but why so much voltage shunted to ground
That 30K works with a 2700Ω as a voltage divider to get that high B+ down to 410V for all the little tubes. Both of those resistors will be hot. Don't touch! :icon_biggrin:
-
no
-
I understand the interest in poking inside and maybe tidying a few things up. However, other than sounding a little thin you seem to be very happy with the amp and how it is, and how it has performed. A main gig amp with lots of hours is going to need some new tubes. Especially if it's been played as described and just sounding thin - the sign of tired tubes. Yes the tubes will like 1k on the screens and you could change some things out. However, you have a working car that drives just fine. You just need to put good gas in it! If it aint broke? I would hate to mess with the mojo! Maybe I'm lazy, but there are things I should do to several of my amps to make them easier on the tubes/components/transformers. However, they sound great and have never been a problem. And they are true to the original design. If there were some identified true design flaws, Id be on it. Otherwise, meh..!
JMveryHO.
Jim
-
That's fantastic! Not a stupid question at all. You identified a problem and you fixed it - obviously a worse problem than my stupid comment about tubes! It's always a good feeling to know what to do with these amps, when things are not right, congrats!
Jim
-
Fender amps like this got a bad reputation since players were used to Twins from the blackface era. If you like the UL trans and the higher voltages on preamp tubes which are typically about 190 vdc which lowers headroom.
And a variety of caps,which doesn't bother me, but some of the electrolytic have different .maximum voltages or appear to have been replaced.
I would move the standby switch so it doesn't throw 510 vdc on your filter caps all at once. THE LATER Twins have a part that slows inrush so those get to receive current slower.
I agree if it ain't broke idea, but this needs a service. Replace all electrolytic caps. The series 220 uf I would get higher voltage caps. Disconnect the NFB and you will notice more pick dynamics and high end shimmer.
The master volume is not a gold design and WIll thin the amps tone. I have serviced one of these a couple times. I replaced all carbon comp with metal film using nothing less than 1 watt. All get a voltage in reaseasI have measured 530vdc at first node. The 20uf next I used 32uf as the amp sounded better to me.
The first 135 watt twin I converted to the AB763 circuit. Removed the roach Tremolo and added a gIn stage. Some new pedals have it covered.