Fender Princeton Reverb Reissue...

[Bob M.]

I've been looking at a Princeton Reverb Reissue amp lately. It's not an exactly copy of the mid 60s icon but has some added stuff, mostly good: bias pot, grid stoppers, screen grid resistors, diodes on the rectifier. But one thing I wondered about is an added 10 ohm/1 watt resistor (R60) added from ground to ground between filter points 'Z' and 'Y'. Essentially, the ground of two (of the 4 filter caps) are on one side of this resistor (B+ and screen) and the ground of the PI and Preamp are on the other side of this resistor. What do you think was Fender's rationale for adding this resistor? As the original version of this amp used a multi-cap can, this was not possible with that capacitor array. The reissue uses discreet caps. Do you think this is an improvement or disimprovement and why?

Bob M.

[loudthud]

Is one side or both sides of R60 connected to the chassis?

Do you have a schematic you can post or a link?

[JoeM]

Here:

http://support.fender.com/schematics..._schematic.pdf

[Bob M.]

Neither side of the 10ohm/1 watt resistor (R60) is directly connected to the chassis but, of course, indirectly it is.

Filter Supply - PR-RI.rtfd.zip


Bob M.

P.S. I hope this works - attachments aren't really my thing.

[Enzo]

In the old days, "they all hum a little" was an accepted fact of life. These days, not. Fender has carefully engineered the grounds in the amp for hum reduction. Look lower left, under the reverb drive. The reverb ground is connected to the 400 ground. Under the trem tube, the 400 ground connects to the V1B ground. Ther are different distinct grounds all over this schematic. Follow the ground trace to the left from your 10 ohm and see it goes to chassis over by the center tap of the OT. Now look upper left at the input jacks. They are grounded to the chassis, and there is a specific ground line from the jacks over to the ground of the first stage of th preamp. In fact, look close at the jacks. See the little pointy thing just under the bushing of each? That is a little metal point on the front face of the jack. it pokes into the metal chassis when the jack is installed. And in fact, if you replace those jacks with ordinary ones that otherwise are identical, they will not have that grounding function, and the amp will likely hum more.


So if it were mine, I;d leave the newer ground scheme the way it is, as any change I make to it is likely to result in more hum.

[J M Fahey]

In the old days, "they all hum a little" was an accepted fact of life.

Amen Brother.
I started in '69, where everything was tubed, turned to SS in '72 and slow but sure started selling SS amps to Pro Musicians which until then were Tube equipped (unbelievable today, they sold Tube amps to buy my SS stuff) .

One selling point was that they abandoned the pesky, heavy, underpowered, humming, buzzy tube equipment to get clean as a whistle, loud, silent as the night SS amps.
Heavy intermodulation (which I now guess was caused by leaky caps shiting horribly the bias points) was practically a built in feature.
Go figure.

[The Dude]

Rock and Roll isn't meant to be pretty. A little noise just lets you know it's on.

[loudthud]

It's not crystal clear to me on the schematic, but I think the ground is connected as described below.

Usually the inverted pitchfork symbol indicates a connection to the chassis. There are other triangle grounds, some with names like GND_400V. Most grounds in the preamp are un-named triangle grounds like the one on the ground side of the preamp filter cap C29.

On page 2 of the schematic you see on the right a wire with a ring lug that says it goes to the chassis and to WJ15. The schematic shows WJ15 grounds to the chassis near the center tap from the HV transformer winding. There are additional pitchfork grounds at the output jack, the footswitch jack, the reverb pan jacks and the input jacks. I must assume these are the connections to the chassis.

In the old seemingly random ground scheme used in Fender amps, the preamp filter ground had to go to a separate chassis connection to keep hum out of the preamp. This amp roughly follows that scheme except R60 is between the two gounds on the PCB. When everything is buttoned up, R60 is shorted out through the chassis but the hum pulses won't go through R60 and only DC current for the preamp flows through the chassis. When the board is being tested before assembly into the chassis, R60 provides a path for ground so everything works but some hum is expected.

[Bob M.]

Enzo is right - Fender is taking grounding alot more seriously than Leo did back then. The FEIC Champs, and their variations, Princetons and Princeton Reverbs - the student amps - are fraught with technical issues. I'm not changing anything, just making an observation and wondering why - the technical why - of that 10 ohm resistor. Remember, we're Theory and Design here.

For your information, the PR-RI has two ground points from the main board. One is the 'inverted pitchfork' marked as chassis ground. On the left side of this 10 ohm resistor in question (connected to grounds for B+ and screen supply filter caps), there is zero ohmage between this resistor and the 2nd chassis ground. On the right side of this resistor (connected to grounds for the PI and preamp filter cap/s), there is, of course, approx. 10 ohms resistance and +.34 Vdc. using my Fluke meter.

Of course, anyone building better amplifiers these days pays special attention to grounding for each and every stage. My amps are dead quiet and usually one has to put an ear to the speaker to hear anything. Of course, once you've plugged in that '59 Strat, all that goes out the window.

Thanks for the thoughts,

Bob M.

[tubeswell]

And if you check out the RI '63 reverb tank, its the same sort of concept (albeit ostensibly more purposed to counteract ground loops occurring because of the 3-cord mains that the RI tanks have)

[Dave H]

But one thing I wondered about is an added 10 ohm/1 watt resistor (R60) added from ground to ground between filter points 'Z' and 'Y'.Bob M.

It looks like the 10R is an attempt to counter the huge ground loop they have introduced by having the input jacks 0V and the PT CT/first filter cap 0V both connected to chassis. A star ground scheme it isn’t.

[Alex R]

I had one of these in, only a few months old, the complaint was too much hum. Warranty work had found no faults. It did hum a little, could be dialled out with the volume control. Probing in a 22uF cap to ground on the plate of the first stage stopped it, which isolated the problem. Turned out to be pickup hum from the ribbon that runs from the input jacks to the board, and probably, I suspected, also the trace that runs across the board to the grid wire. I ran a piece of screened cable from the input jack board to the grid direct, with the screen grounded at the jack end. Separate new wire from the jack board ground to the main board ground where the ribbon was removed. This stopped the hum.

I tried shorting the 10R resistor (R60) and doing without the wire connecting the preamp ground to the jack board but a little hum returned that way, not sure I can see why, but you know how it is with hum. So yes it does seem that Fender might have had a little wrestle with hum in this amp, with R60 as a solution that wasn't fully working on this particular amp (only a few months old) until I put in the screened lead from the jacks to the stage 1 grid.

[Malcolm Irving]

It looks like the 10R is an attempt to counter the huge ground loop they have introduced by having the input jacks 0V and the PT CT/first filter cap 0V both connected to chassis.....

Sounds like some serious 'trial and error' design going on there!
I think we recognize (these days) that signal ground should only connect to the chassis at a single point.
Yes, there are various ways that multiple connections to the chassis can be made to work (as in various classic amps), but by only connecting at one point you are off to a good start on your grounding scheme.

[Malcolm Irving]

The mains electrical supply ground has to be very solidly ‘bolted’ to the chassis for safety reasons.

When folks consider the connection from the PT centre tap to the 0V of the reservoir capacitor, they appreciate that this connection caries the heaviest (and dirtiest) power supply ripple current and somehow think this must also be solidly bolted to the chassis – big mistake (IMHO).

That point can get its ground from the next filter cap in the chain. The only (signal) ground connection to the chassis can be right at the beginning of the pre-amp. Then a series of local star grounds can be daisy-chained back to the reservoir cap 0V. Works every time for me (no hum or buzz).