Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: J Rindt on October 30, 2017, 05:26:27 pm

Title: tweed pro re-cap
Post by: J Rindt on October 30, 2017, 05:26:27 pm

http://el34world.com/charts/Schematics/files/fender/Fender_PRO_5C5.pdf (http://el34world.com/charts/Schematics/files/fender/Fender_PRO_5C5.pdf)

Friend of a friend has had one of these sitting for a long time...it never worked.
I installed new filter caps, fuse holder, Power and SB Switch, coupling caps at the preamp tubes (they had broken leads), and also replaced those same plate resistors (220k) because  they had drifted way up.

This is a later amp with AY7 pre tubes and the 1 Meg feedback resistor.
I did not change the heater wiring to a "Modern Scheme".....it still has one side to ground...i guess that is OK.?

I have not checked any of the coupling caps yet to see if they "leak".
He plugged in and the amp seems to sound and work fine

I have never worked on one of these, and cannot find any info about what kind of B+ it might have. There is 320V B+ going to the PI and Preamp Tubes.
The plate resistors at the PI drop that to about 150V.
But the preamp plates drop to about 70V.
My question is...Does that sound "Normal" to you guys.?

Thank You

Title: Re: tweed pro re-cap
Post by: tubeswell on October 30, 2017, 05:48:21 pm

Quote from: J Rindt on October 30, 2017, 05:26:27 pm

... There is 320V B+ going to the PI and Preamp Tubes.
The plate resistors at the PI drop that to about 150V.
But the preamp plates drop to about 70V...

Grid leak bias does not give as-consistent-a-result as cathode biasing does. With grid leak bias, the bias point of the tube is affected by the age of the tube. See if the voltages come up with fresh 12AY7s in each channel.

Title: Re: tweed pro re-cap
Post by: shooter on October 30, 2017, 05:50:20 pm

Quote

He plugged in and the amp seems to sound and work fine

write down your readings, run her hard for a day and re-check, if she's about the same, shipit :icon_biggrin:

Title: Re: tweed pro re-cap
Post by: J Rindt on October 30, 2017, 06:21:13 pm

Yeah, i forgot, this is also my first experience with Grid-Leak Bias.
All the tubes are new.
After i made sure there were no Problems/Shorts/Etc etc.......i did leave the amp at idle for about 5-6 hours. But as far as playing it hard, only for about 10 minutes at this point.

Title: Re: tweed pro re-cap
Post by: sluckey on October 30, 2017, 06:30:53 pm

Do your preamp tubes' cathodes connect directly to ground? Or do the cathodes connect to a 470Ω/25µF like the 5D5?

The 5D5 layout has voltages listed. It may be of some use to you...

     http://el34world.com/charts/Schematics/files/fender/Fender_pro_5d5_schem.pdf (http://el34world.com/charts/Schematics/files/fender/Fender_pro_5d5_schem.pdf)

I think those low plate voltages are probably normal.

Title: Re: tweed pro re-cap
Post by: J Rindt on October 30, 2017, 06:57:52 pm

Yes, the cats are straight to ground. I really have not worked on amps much for the last year...took me a while to figure out what i was looking at.....no cathode resistors.  :BangHead:

Anyway, Thanks For That Suggestion. :smiley:

Title: Re: tweed pro re-cap
Post by: PRR on October 31, 2017, 10:17:00 pm

The hard-grounded cathode grid-leak bias depends on a large drop in the plate resistor to stabilize it. (Plate voltage falls, grid leaks less, bias is less, plate voltage rises.... it finds its own balance but generally low on the B+ compared to later Fender designs.)

I would not worry about it.

Being an old and dysfunctional amplifier it needs a lot of beating-on to expose the lurking faults.