Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Platefire on August 18, 2015, 12:06:28 am

Title: Does the Marshall Class 5 Combo have NFB?
Post by: Platefire on August 18, 2015, 12:06:28 am

I was helping a harp player that got one of these who wanted to disconnect his NFB. I've been looking at the schematic and I can't find a NFB loop but it's not the easiest schematic to read! Can someone take a look and see if you find one?

Title: Re: Does the Marshall Class 5 Combo have NFB?
Post by: sluckey on August 18, 2015, 04:21:51 am

I don't see a NFB loop.

Title: Re: Does the Marshall Class 5 Combo have NFB?
Post by: Platefire on August 18, 2015, 07:19:51 am

OK, thanks for looking! Platefire

Title: Re: Does the Marshall Class 5 Combo have NFB?
Post by: Platefire on August 18, 2015, 01:06:52 pm

The schematic doesn't have any voltages listed. To loosen it up I know a lot of harp amps like to get the preamp plate voltages low to about 100VDC or even lower. I don't know about lowering the cathode follower?  If you upped R4 to about 20K/3 watt that would lower all pre-amps stages in front of the power tube---or the 100K plate resistors(R1 & R3) could be upped to about 150 or 165K. Which would be best? Platefire

Title: Re: Does the Marshall Class 5 Combo have NFB?
Post by: sluckey on August 18, 2015, 02:07:06 pm

Measure voltages as found before changing anything. R2 affects V1 only. R4 affects V1 and V2. Don't mess with plate resistors (R1 and R2).

I'm guessing these resistors may be on a pcb? If so, I'd just crush the body of R2 and/or R4, leaving the leads soldered to the board. Now just temporarily tack solder a larger resistor across the old leads. This will make it easy to experiment with different values. Once you discover the magic values, remove your temporary work and lift the board to install the resistors correctly. Leave the leads long so the resistors stand at least 1/2" above the board.

Title: Re: Does the Marshall Class 5 Combo have NFB?
Post by: PRR on August 19, 2015, 06:58:57 pm

> disconnect his NFB.

Find a scrap of blue wire behind your bench, hand it to him.

> may be on a pcb?

Why would you think that?? Just because there are more "LK" links than actual parts?

R2 is very dubiously small. I doubt R4 and R2 together are dropping even 30V; also not doing much ripple-filtering. Possible sneak-back through power rail and lurking instability.

IMHO R2 ought to be 10K minimum just for cleanliness.

If the genre wants V1 starved, take R2 to 100K or 200K 2W.

I suspect you really want to match the harp-mike output to the first-stage squash/bent level? A close-micced harmonica may be 0.1V-0.2V. Stage gain around 50. So 5V-10V signal at first plate. At 300V supply the tube can do 50V, real-clean at 5V-10V. At 100V supply the tube can do 10V-14V before going bent, which may be how a harmonicaist gets musical emphasis. So you want to vary the supply to suit the specific player, harp, mike, and situation. This suggests a back-panel knob to set the amp for tonight's gig. It is just in reach of a simple HI-power pot, the mil-spec series.

Title: Re: Does the Marshall Class 5 Combo have NFB?
Post by: Platefire on August 21, 2015, 04:44:12 pm

Thanks for the info and ideas sluckey and PRR. I think now that he knows he has no NFB, he apparently is not interested in doing anything else. If he does I'll re-activate  :happy1: Thanks for the help! Platefire