SF Princeton Reverb - too bright!

[Bob M.]

The last few years I've been playing SF Princeton Reverbs (I have three). My #2 backup is alot brighter than the other two and sometimes (in certain rooms) I'll actually have the treble control on zero, which bugs me because there is no way to adjust less treble EQ, just more treble, which I don't want. I'm much happier when the treble control is on about on 4~5 and it sounds how I like it.

I have the amp on my bench now and I'm going thru it thinking of ways to get this amp to sound more like my other two (to get it to sound less bright than the other two). The speakers and transformers are all the same in all these PR amps. What should I check out or change out to get control of the highs in this amp? I know I can put a small cap from plate to cathode on V1 but I'd like to save that as a last resort. Tone stack is normal - 270pf SM, .022, .022, slope is normal, no bright cap across volume control. Any things you might think of to try out, measure or mod, I'm all ears. Thanks for you input.

Bob M.

[g1]

Side by side with the others are you getting similar volume levels, or do you have to set this one a bit higher?
Rather than being too bright, could it be that it is lacking in bass?

[Chuck H]

Amps will sound a little different. But if all things are created equal-ish the difference shouldn't be as you describe. Are these actual PR's or builds? Are these amps using the same speaker? Have they been tweaked differently? I think we need to discover what's different about the amps in order to make your #2 more like your #1.

[überfuzz]

Try determine what's not causing the wonky tone. I'd start by swap speakers around and see if some speaker is to very bright. If no. Scratch nr. 1 from the potential issue list.

[Tone Meister]

Adding to the speaker possibility, in my experience as a player, not a tech, I do know that internal voltages can affect the tone significantly. Unless you have an early model that uses a 5AR4, it should have come with a 5U4GB rectifier. I'd include comparing the important internal voltages to your checklist.

[Bob M.]

I'm using the same output transformer and same speaker model in all three of my PR amps, as I stated in my original post. I have very good bass response, more than enough, with all three and that is not an issue at all. All three of my PR amps were built by Fender in Fullerton, CA. My #2 backup, the one with the too much treble issue, is the earliest one of the three, a late 60s~early70s manufacture. My favorite is a mid 70s and 3# is a 'Pull for Boost' model.

My experience tells me all amps of the same model are a little different from one another. I like this; Viva La Difference I say. However, this treble situation with my #2 PR is of another degree entirely. The volume levels of all three amp are similar and I play them at about the same numeric setting on gigs.

Chuck H, your methodology is correct but these amps are have either been tweaked all the same, in certain places, and/or otherwise left stock. Usually, all caps have been replaced. All plate load resistors have been replaced. I've added grid stoppers and screen grid resistors in the power amp (yes, to all three and, yes, all the same values). I have been over all three quite carefully and thoroughly and there is no schematic differences but maybe parts from different manufacturers are seen. So, this is a complex issue. I was hoping for some advice or guidance on what parts or circuit areas might affect the treble response, besides the really obvious ones like the treble cap in the tone stack, the bright cap (not present in this amp). I have chosen my 'darkest sounding' 12AX7A to use in position V1 in this amp.

As I said earlier, I might use a small cap on V1 from plate to cathode, as seen in several Rivera-era models, but I'd prefer to do that as a last resort.

Thanks for your responses,

Bob M.

P.S. All three of my PR amps use a 5AR4A/GZ34 rectifier tube as that is my choice and I like the sound best.

[g1]

uberfuzz has a very good point, eliminate the speaker as a suspect first. Just because they are the same model does not mean one isn't a little off. If you haven't tried the swap yet, it's the first thing to do.

[Bob M.]

Yes, I've already done the speaker swap and you're right, it was the first thing I tried, without any improvement with my problem.

Bob M.

[g1]

Do you have a signal generator and scope? You could inject a signal at various frequencies and check with the scope at the coupling cap into the phase splitter. Compare the relative response between this one and #1 amp to see if the issue is preamp or power amp.

[Enzo]

I find it hard to see tone on my scope. To me this is a job for music as a test signal and a signal tracer as the search tool. Get out a CD player or tape deck or even a stereo receiver - I use one of those - and just play some music through the amps. You can instantly hear if the tone is skewed. And a signal tracer is nothing more than another amp with a probe plugged into it that allows you to listen to points in the circuit.

[überfuzz]

I find it hard to see tone on my scope. To me this is a job for music as a test signal and a signal tracer as the search tool. Get out a CD player or tape deck or even a stereo receiver - I use one of those - and just play some music through the amps. You can instantly hear if the tone is skewed. And a signal tracer is nothing more than another amp with a probe plugged into it that allows you to listen to points in the circuit.

Get a tone generator and do a frequency sweep. Voula there's your response.

[Bob M.]

Yes, I have a fully outfitted shop here. I'll take you advice and try some other things.

Thanks to all for contributing,

Bob M.

[Bob M.]

Hi all,

Just as a follow-up, I did end up putting small caps on both V1a and b, between plate and cathode, in a similar fashion to the 80s Rivera-era Fender amps. I fine-tuned the cap selection values until it was just right without attenuating too many highs. I now play my treble control around 4 or 4.5 with this amp and it sounds like a reasonable facsimile of my other two SF PR amps and the amp is back into gig-rotation with the others. I've run into a couple of other Fender amps (usually silverface models) over the years that had just too darn much treble no matter what you did. This plate/cathode cap mod is a good one to have in your bag of tricks for such situations. Again, thanks to all who contributed with their excellent insights and good ideas.

Bob M.

[Chuck H]

I've been using the small pf plate to grid thing for years on some amps. My understanding from here on the forum is that knee frequency decreases with higher voltage and gain. So the plate to grid cap sort of acts like a compressive low pass filter taking the spikes and peaks off. In moderation this can be a good thing for certain amps. Especially those that tend to be high-ish in voltage application. Values for a circuit like this tend to be very small. Like 4.7pf to 12pf. I use it on input circuits sometimes, but I can't recommend that because of the safety issue. If the cap ever shorts you'll have HV on your guitar! But IMHE it's also worth experimenting with this circuit because it can be dialed in for it's HF peaking mitigation effect as well as, or instead of a blunt HF roll off.

[TimmyP1955]

What's the advantage of a cap plate-to-grid as opposed to plate-to-ground - no impedance 'drag' on the plate?