Princeton Reverb Help

[Paul-in-KC]

So, something in jack #1 is either dumping signal to ground, or has some resistance. I need to investigate that more closely to see exactly what's going on.

The #1 jack (has the 1MΩ resistor) is considerably louder than the #2 jack by design.

I thought the 1M resistor was a "grid leak" tie to ground - and that it is effectively connected to pin 2 of V1.

[please pardon me if I'm using the term "grid-leak" incorrectly]

Is it just the "extra" 68K added to the grid leak resistor path that makes that much difference in the vol?

That is surprising (to me) since it's less than 7% of the 1M.

NOTE: Working from Mojotone's layout, input #1 is the one with the 1M directly to ground. So, #2 has the extra 68K in the grid-leak path; and it is the louder of the two; which makes sense.

-Paul

[xtian]

Fender's high/low input scheme uses two 68K resistors that form a voltage divider for the LOW input, giving you 1/2 the voltage with that input.

Yes, the 1M is a grid reference to ground.

[Paul-in-KC]

Fender's high/low input scheme uses two 68K resistors that form a voltage divider for the LOW input, giving you 1/2 the voltage with that input.

Yes, the 1M is a grid reference to ground.

Doh! Now I see it.

When plugged in to one of the inputs the "other" 68K goes to ground ( and hence divides voltage). Whereas plugging in to the other input lifts the ground on the other 68K resistor.

I missed the detail about the "opposite" grid resistor having a path directly to ground when using one of the input jacks, but not the other.

I'm slow, but I'm trainable.

-Paul

[Stevem]

The number two input should be 3 DB down, or half the volume of the first input as posted.
I test with a input signal voltage of 150mv as that is the lowest output you will get from a single coil pickup like in a vintage Strat when you are beating on the strings,but it is also enough to drive any amp to full clipping and peak wattage also long as the tubes are not toast.

This input test voltage is equal to - 10 Db pretty much just for reference!

Also note that with these amps if you do not have JJ brand output tubes that biasing them at more than 18 ma each can make for pretty farty distortion tones when driving the amp hard and is dependant on the low end output of your pickups, humbuckers can something's be done up to 21ma, but let your ears be the guide!

[Paul-in-KC]

...Also note that with these amps if you do not have JJ brand output tubes that biasing them at more than 18 ma each can make for pretty farty distortion tones when driving the amp hard and is dependant on the low end output of your pickups, humbuckers can something's be done up to 21ma, but let your ears be the guide!

Stevem - Thanks

Your post actually addresses something I was wondering about.

i.e. the "farty" distortion tones when driving it hard.

I was "testing" the amp with one of my LP's and indeed noticed that. My sense was that maybe the amp needed more negative feedback - but, as I have said, I'm still fairly "inexperienced".

I have some JJ's that I can pop back in to compare. I think I have Tugsol in there right now.

You speak of biasing the power tubes - but I built this as the original circuit . Which is fixed bias.

All of the other amps that I have built have bias adjustment. Is this something that really should be added to the AA1164?

NOTE: given the fixed bias - I didn't even measure the idle current draw on the power tubes.

-Paul

[xtian]

Implementing Fender's vintage schematics almost always makes modern amps run too hot (higher wall voltage today, for starters). You can easily make your fixed bias adjustable with a pot, and I recommend adding 1R resistors on power tube cathodes to ground to help bias measurements.

[Stevem]

Fender amps of a ton of years of production where made for fat clean sounds at volume levels before the amp started to clip, as that was Leo`s main goa, he hated distortionl and the CBS guys took it over the edge to the extreme !
What this all means is that when played above 4 or 6 on the volume control and when your on most guitars neck pickup
your gonna have more Bass out put then you need and the amp will fart out and loose too much control on the low end.
There are many ways of dealing with this and just this week on a buddys set up (strat, Highwatt 50 / 4-12 cabinet) I tried something different, I ran a frequency sweep for him and we found that for is liking and the volume level he runs the amp at that when I placed a cap in series with his neck pick up he got the needed low end roll off while still having a nice fat bridge pickup sound when on that one!
Like I posted, those large caps the Fender uses on the out bound side of the PI make the output tubes over drive too much at cranked up volumes.

[TimmyP1955]

One of my bosses had a BFPR with a dead OT and speaker.

We installed a DR OT. Moved the PS tap for the PI up one or two voltages (I forget which). Changed the .1 PI output couplers to .01. Installed a Jensen Alnico.

What an amp! Overdrives very nicely, doesn't fart with a Les Paul.

[Paul-in-KC]

All,

I finally got around to measuring the clean output with everything as originally assembled and got 13.1 W with a clean sine wave at the speaker output.

41 V p-p = 14.4935 RMS

14.4935 RMS ^2 = 210.0615

210.0615 / 16 Ohm load = 13.1 (clean) Watts


I want to make the bias adjustable, but other than that, it really sounds great as-is.

I may have to build myself one now.

Thank you everyone for sharing your knowledge so generously.

-Paul