Convert Hoffman PR to a Hoffman 68 Custom PR?

[CordovaNate]

I built my first and only amp, a Hoffman PR, about a year ago. Fired up 2nd try, not had any problems.  As much as I like the amp, I'm beginning to think that I would like a 68 Custom PR better.

I've googled around a bit and it seems that swapping out a few component values might get me close:
NFB R value from 2.7K to 5.6K
Bass pot R value from 6.8K to 22K (edit: I think the 68'CPR value is 18K, though I might like 10K-12K better)
Cap value at C10(?, tied to one of the bass pot lugs) from .047 to .022

Anyone have any thoughts about doing a conversion?

[sluckey]

Go for it. Easy to undo if you don't like it. Here's a '65 PRRI schematic with custom '68 changes...

[MFowler]

I built a 68 custom PR and did not like it so converted it back to regular PR.
Started with Mojotone cab/chassis and added ebay faceplate. Components I had sitting around.


Photo show as 68 custom PR

[CordovaNate]

Thank you both, with an addition tip 'o the hat to Sluckey for that very helpful Sluck-matic.

I will need to order some components, so I'll report back in about 2 weeks.

[sluckey]

I didn't do that schematic. Google flushed that out in about 2 minutes.

[CordovaNate]

Your google is better than my google.

[sluckey]

I used this search phrase... "68 custom princeton reverb schematic"

Here's where I found the schematic. There may be some other info of interest...

https://guitarampblog.wordpress.com/2017/01/26/fender-65-princeton-reverb-schematic-vs-68/

[CordovaNate]

I did see that, and book marked it, but quickly moved on. I saw the Black Face/Silver Face & USA/Mexico references and my brain bounced off of it, but the value changes were similar enough to what I had seen elsewhere to notice and book mark should I want to come back to it.  I'm not gonna call it ignorance...nope, I'm gonna smile and call it presentiment.

I'm curious about the cap changes at C10 and C16.  What effect will they affect?

The C10 I mentioned in my original post is actually C23 in the tone stack.  The one in the tone stack was what I was thinking of.  Kind of a weird coincidence.

[Ed_Chambley]

C10 and 16 will trim bass at the power tubes. The bass pot change will let you have more travel. Princeton stock, generally the tone stack is kept very low on bass. 3 or less. The see mods will give you more adjustments.


The change  in NFB will ad more harmonics due to less cancellation. A touch raunchy, you can even put it on a 10k pot and roll it completely off if you desire. Ads more grind.


This is only one method. And these mods are not uncommon. A easier and cheaper way is to reduce the bypass cap values. What you are doing will change the amps tone In relation to eq positioning.


If I were wanting a Princeton that had attitude I would simply (and have) changed bypass cap on v1 to 4.7uf and the next one to 6.8uf.  Then there are not a lot of bass to shave further in the signal chain.


It is really short of the same, just shaving bass. Instead of shaving bass when you change bypass values you just do not amplify the lower tones and the Princetons OT doesn't saturate as easily so the result is what sounds like an amp with less bass because that is what it is happening.


It will work just fine, but you may consider a raw pot added somewhere.

[CordovaNate]

Thank you Ed.

Parts ordered.

[MFowler]

I have the actual 68 custom PR schematic in .pdf but too large to upload and not sure how to decrease to fit if you want it.

Mark

[SILVERGUN]

I have the actual 68 custom PR schematic in .pdf but too large to upload and not sure how to decrease to fit if you want it.

Mark

One way,
Open the pdf and get it as big as you can on screen.
Use the snipping tool to save the screenshot as a pic.
Reduce pic size as needed.

[CordovaNate]

Thank you MFowler. I appreciate the offer, but I think the schematic Mr. Sluckey pointed me to will get me there.  I've located the components on the Hoffman turret board I'm using for my PR, just waiting on UPS.

[MFowler]

I tried my Coral draw software again and this time it worked.

[sluckey]

Here's the schematic in pdf form...

[CordovaNate]

Completed the conversion about two weeks ago.  I'm happy with it.  I was happy with the stock PR version as well.

I think I just want to build something...