The Amp Garage

It looks like changing the rectifier is a relatively painless way to lower the B+ in this amp. I took the following measurements without changing bias. Line=120VAC GZ34 B+=462V Icathode 24/28mA 5U4GB B+=450V Icathode 22/26mA I may try the MOSFET B+ reduction circuit described in this current thread ...

Thanks Phil,
It looks like I may be able to use a 5U4 also. My power transformer should be able to provide the extra amp of filament current.

Update: I have been unable to sign in to this forum for a while but have been reading. I took the amp out to the gig last night and absolutely love how it sounds. My band mates were amused that I kept looking in the back of it (to be sure the plates weren't glowing). Perhaps a read through of this s...

This thing wants to run hot. with 120VAC line, the plate V is 455, when tubes are idling 26/30 mA with bias V -50.8V.

With the price of tubes throught the roof these days, I'd really like to make them last. Is there a relatively painless way to lower B+ down some?

Thanks Sluckey, Makes sense. I'll do that when I get to my workshop tomorrow. My B+ dropping resistors have increased to 11K and 11.5K. Is it worth replacing them at this time?

Thanks Stevem, I did not see a part number for the Xfrmr on my schematics. I'm tuning this amp up after some repair work. My initial concern was the B+ voltage(s) seem high. At 120VAC, B+ is 478VDC/plates are 476VDC with 17mA bias current. This is with the grid voltage dialed back as far possible -5...

My second tube noob question. I'm putting it in the Fender amp section as it seems (to me) to be more historical than it is technical. Does anyone know if the PT turns ratio changed when Fender Deluxe Reverb changed from AB763 to AB868 circuit? I am aware of the heater winding center tap removal and...

It's fixed! There was a broken wire at the connection to pin 7 of the PI tube. The wire insulation was covering the break inside which was touching intermittently, especially with probe pressure. Most of my measurements/experiments were conducted on the circuit board which likely did not disturb thi...

PI grid voltages are unchanged with both .1 shunt and .001 coupling caps removed. Additionally removed 820 ohm neg FB resistor with no change. Very strange....

Thanks for the ideas and suggestions. I didn't find any broken wires or connections (yet) but will double-check again today. I made some DCV measurements and it does look (to me) like the PI grids are the cause of the problem. I haven't quite wrapped my mind around how the LTPI works but from what I...

What does it do when you disconnect the NFB? That reminds me... when I first starting checking resistors, I discovered the NFB was 82K instead of 820 Ohms. I thought I had found the problem, but replacing it didn't change the imbalance. Disconnected it today and no change in imbalance. I also put i...

Yes, the resistors were replaced in an attempt to fix the imbalance. All of the old ones had drifted way out of spec. I started with the 5% tolerance ones then incrementally changed the rest. Some of the DC voltages changed slightly but the output imbalance remained (IMHO) large. For the purists: Th...

xtian wrote: ↑Thu Feb 22, 2024 3:17 am If the imbalance follows the tube, we know the tubes are mismatched, right?

Yes. In this case the imbalance stays with the amp. Even the 4mA difference in bias current.

I only have one spare 6V6 right now but will have a new matched pair tomorrow to experiment with.

Duplicate post... Doh!

I will check the OT using ACV. I had ohmed the input side previously with each leg showing 220 ohms to the center tap. I know that doesn't tell the whole story. It seemed to me that the power tubes and beyond were acting as expected given that their grid signal(s) are being driven with such unequal ...