Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: alerich on November 20, 2013, 11:19:42 am
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A few months ago I sold my old Goodsell Super 17 and bought a Fender 65 Princeton Reverb Reissue. The Goodsell was a great amp but it just doesn't do that great Fender clean tone. I love the PR (creamsicle white and orange). I primarily play it clean with a Strat or a Tele and use a pedal (Jetter Red²) for dirt. I have been trying things to coax as much clean headroom out of the amp as possible while retaining the PR flavor. I did the Strokes mod and that was an immediate and positive improvement. Before, the amp used to start to get just a bit of hair in the tone about on 5. Now it's clean on five and doesn't really start to distort until about 6. I have a few caps enroute and I plan to add a little bit of capacitance to the power supply as this gets general good reviews for what I am trying to accomplish. I'm looking at the Allen/BillM TO20 OT as a possibility. It gets high marks. Changing the speaker is an absolute last resort. I happen to love the tone of the stock Jensen C10R.
I have been reading a bit about the PaulC mod. I understand basically what it does and how it works. By all reports it is designed to provide a better balance when you overdrive the cathodyne phase splitter whilst running the amp hard. Since I primarily play clean (hence the measures to increase clean headroom) and use a pedal for dirt would I gain any benefit from this? Would it still improve the performance of the cathodyne while clean or worse yet... do you sacrifice anything on the clean tone end to improve the overdrive end with this mod? Just looking for insights from anyone who has been there or knows it better than I.
Lastly, my apologies to PaulC. I understand that he just tossed this idea out in a discussion forum one day (it's an old textbook circuit that was also employed in Ampeg amps) during a discussion about balancing cathodyne phase inverters and his name got attached to it - something he is not terribly enamored with. I only use his name here as a convenient point of reference.
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Never on a Re-Issue, but I read they are the same. The mod will change the saturated tone only. The clean, I have never noticed any change. I have tried it many different ways. Here is what I found to give the most headroom and still be a Princeton. Instead of moving the PI plate up one node, I reduced the rail resistor and increased the next. Sort of in-between. Ended up with a 4k7 to bias the tube. I have made this switchable.
It will become a little boomy especially if you do the true stokes and increase the plate that much. Easy to take care of tho. Simply reduce bypass caps till you get what you want.
The Allen OT is nice. I have this one and also a Deluxe OT from Mercury multitap in 2 builds. Both will remove the princeton saturation and the amp will become much louder, but will not sound like a stock princeton anymore especially if you increase the first filter to 40uf.
Hope this helps some.
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I am NOT saying you should try these mods. This is what I personally would do to get more of a clean tone. And I would use a 5751 in V1
Totally just a personal preference thing.
With respect, Tubenit
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Exactly what I needed to know. I suspected I would not gain much for my needs from the PaulC mod. I believe I will pass on it. The Allen OT is still on the table. Since I play mostly clean I think it will give me more of what I like without sacrificing too much of the basic tonality. Thanks, Ed.
tubenit - I think you read my mind. With the exception of the tone stack mods I have also been considering tweaks of all the caps you outlined. Good call.
Thanks, gentlemen.
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... bought a Fender 65 Princeton Reverb Reissue. ... coax as much clean headroom out of the amp as possible ... Just looking for insights from anyone who has been there or knows it better than I.
I have owned 3 or 4 original Princeton Reverb's through the years. The fastest (probably cheapest) way to "more clean headroom" is to get a 2x 6L6 Fender, or at least trade up to the Deluxe Reverb.
... the amp used to start to get just a bit of hair in the tone about on 5. Now it's clean on five and doesn't really start to distort until about 6. ...
Be aware that if you're not measuring power output at the (subjective) onset of distortion, or using an SPL meter, you may be kidding yourself about how significant the change is.
For instance, if you use a volume pot which has a 30% taper (https://taweber.powweb.com/store/potsord.htm), then when it is on "5" it is the same as a 10% (true log pot) on ~7.5 (the link is provided to show 10, 20, 30% log pots exist and are commonly sold). See the graph below, and look for where 0.3 on the Y-axis falls on the audio taper curve.
(http://www.geofex.com/article_folders/potsecrets/pottaper.gif)
By all means, try the mods to see how they impact the amp, but it is already running 400v+ on the plates of the 6V6's and you're unlikely to get any more clean output unless you switch to the Deluxe's 6.6kΩ OT and/or use more efficient (or just more) speakers. I don't know, but the cost of that approach might equal/exceed just finding the bigger amp on the used market.
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This is why I was careful and specifically said "coax as much clean headroom out of the amp as possible" instead of "I need more clean headroom". I don't really need it, per sé. I only play at home in my living room. The amp stock is plenty loud and had decent headroom - more than other PR owners claim. I don't have to compete with a drummer - only occasionally with my girlfriend's electric piano. That being said, I do enjoy playing loud at times. I had a SF Super Reverb several years ago. Great amp until I carried it up the steps to the apartment I moved into at the time. I swore I would never schlep it up or down stairs ever again. I didn't. I sold it on Craigslist. I also prefer the cathodyne phase splitter and especially the bias vary trem so anything Deluxe on up is out of the question.. I'm really pretty happy with the amp right now. I'm just trying to wring out whatever little bit is left on the table.
With the Stokes mod you provide the cathodyne a larger supply voltage to work with. That translates into a larger voltage swing on the output before it clips. That in turn delivers a larger clean signal to the grids of the finals which translates to a more clean signal into the OT... which couples to the speaker. I don't really think there's a whole lot of smoke and mirrors going on here. Assuming the OT is up to the task (it probably isn't - that's why I'm considering the Allen OT) this all gives you more volume for a given volume control setting since that control is before the cathodyne.
My filter caps arrived in today's mail (thanks, Doug!) so I will install them tomorrow and play it through the weekend. I'm doing these things one at a time and letting them marinate before I try something else. The caps should be a noticeable bump. so I expect I will order an OT next week. I really like this speaker. I know a more efficient speaker or one designed for more wattage may help the cause... maybe if someone I know has a 10" speaker sitting around that I can use for audition purposes I may try a different one.
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Sir, I always avoid getting into very technical discussions,,,because I don't belong there
BUT, I would like to report some findings in hopes that it may help you in some way......
While experimenting recently with the AB763 circuit (I realize this is a different animal), I found that I was very surprised at the amount of clipping that was occurring at the LTPI, when I was trying to keep the circuit clean
So, it is my understanding that this cathodyne PI in the PRRI has about twice the gain as my LTPI, and therefore in my mind, would cause even more clipping....
The solution that worked the best for me was using a 12AU7 and adjusting the bias to a point where I could see the most gain without clipping (with a dual trace scope)....and that made a huge improvement....so maybe I'm agreeing with the concept on the Stokes mod
If you have a scope, I would highly recommend using it for this purpose, because there was a direct correlation between what I was seeing, and what I was hearing.....and I was able to visually "tune" the amp cleaner.....priceless
I also wound up with a boost limiting resistor on the first stage bypass cap,,,,and just the combination of those 2 steps was enough to see a noticeable improvement in cleans through the entire upper range of the volume control....
Only thing is,,,I was using 6L6s and didn't care at all about any inherent loss of output volume.....but if you're considering switching out the OT anyway, this might be a good time to go to 6L6'ville :dontknow:
Good luck and I hope you find what you're looking for........if not, just build what you want :icon_biggrin:
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maybe if someone I know has a 10" speaker sitting around that I can use for audition purposes I may try a different one.
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I think you are in Marietta and I am in Fayetteville and while most of my "parts" are at my shop in Lake City, I do have at home a few 10" speakers. If you want to make the drive through Atlanta sometime, PM me and we can schedule a time.
All 10"
Vintage 30
Celestion Gold
Weber Blue Pup (15 watt) Alnico
Weber Silver bell (15 watt) Alnico
A couple of horseshoe Alnicos I reconed for 30 watts. Not sure of the brand but look like old CTS baskets.
2, 30 watt cermaic webers and 2, 20 watt Alnicos.
I have more, but with the Jensen you already have these would be the best to compare. If you get to the point where you want to test some speakers, PM me. I know how difficult it is to select speakers. No better way than install a few different ones and listen.
If I am thinking correctly, 4 of them are in cabinets and 4 are not.
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SG - No 6L6 tubes. That's not what I'm going for. The OT swap is simply wimpier 6V6 OT for more efficient 6V6 OT. We're not going to change the fundamental topology of the amp here. I am mulling over the idea of building the Weber high power Princeton clone with 6L6 tubes but that's a horse of a different color.
Ed - Thank you for the kind offer. Yep, I am in Mayretta. After I experiment with new iron we may do that.
Here's a quick question. This is a recent reissue amp with Illinois 22uf 500VDC filter caps. I ordered Illinois 22uf 500VDC filter caps from Doug. Why are they two different sizes?
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Illinois caps used to make a 32mm and a 40mm in 500v/22uf. Their website only shows a a 32mm. In the 450v they still make a 31mm and a 40mm. The caps are slightly different in spec.
Here is the link
http://www.illinoiscapacitor.com/ic_search/_lytics_products.aspx (http://www.illinoiscapacitor.com/ic_search/_lytics_products.aspx)
Looks like fender planned for the larger looking at the PCB.
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Doug offers more for your money. :icon_biggrin:
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I strapped in an additional 22uf 500VDC filter cap on C28 (B+). Another very noticeable improvement. I know numbers on a volume knob don't translate from amp to amp but in this case I think it's a valid observation. I am now clean on 6 (as clean as I was on 5 before the cap addition) and just a hint of hair on 7 if I dig in (less hair than I had on 6 before the cap addition). Increased the touch responsiveness once the hair sets in. This was straight in with my Strat. I like it. I don't know how much more this little amp can give me. I'll play it this weekend and ruminate on pulling the trigger on the Allen OT. Push comes to shove it should give me a little more tone and I can use the PR OT in my old Pine/Pepco Riviera 725 head. That OT looks like it was wound by some kid in a junior high basic electronics class.
I also changed R14 at the output of the trem oscillator from 1M to 500K. In concert with this I lowered the bias a tad to about 19ma with 435VDC on the plates. It doesn't impact the tone but it makes life easier on the bias vary trem. Fender recommends 23ma. That's a little on the warm side and causes the trem to struggle. Still not quite as nice as the trem in a Vibro Champ (but what is?) but it's much nicer.
After some tube rolling I ended up with this:
V1: Sovtek 12AX7 LPS
V2: NOS Philips 12AT7
V3: Tung Sol 12AX7 reissue
V4: Sovtek 12AX7 LPS
V5/V6: The stock Groove Tubes re-branded EH 6V6 tubes. I actually like them a lot.
I stuck an old 1958 Mullard Blackburn GZ34 that I have been saving for a special occasion in the rectifier spot.
I'm pretty much there. If I do go for the Allen OT I'll post my thoughts here in an update.
Thank you guys for your input. It's always appreciated.
(Edited because I cannot count.)
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Ordered the Allen/BillM TO20 OT from BillM on Thursday night. It shipped Friday and arrived Monday. Great service.
Just finished installing it. First impressions: I don't like it. I'm sure it's a great OT but it's not the right OT for me in a PRRI. With the first notes and chords I noticed that the Fender shimmer was gone. Cranking the treble to 10 (I normally play it on 7) didn't help much. I honestly didn't really notice much improvement in the headroom beyond what I got from the Stokes mod and the additional filter cap. The only discernible difference I can hear is a loss of highs and that Fender "chime" (for lack of a better overused term). I think this OT may be a good choice for the player who likes to use the natural overdrive of the amp. It's not a great choice for the player trying to maximize clean headroom while retaining the basic tone of the stock amp.
The original OT will go back in tomorrow or Thursday. I can use the TO20 in another project.
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I don't know so I thought I'd ask-
Has a prior mod changed the characteristics of the amp so that the Allen OT is now a mismatch?
Silverfox.
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Not really. I only did the Stokes mod and the added filter cap. They increased the headroom but kept the basic tonality intact. It's still a 2 x 6V6 power amp. The TO20 OT effect was immediately noticeable to my ears. Bear in mind that I am not faulting the OT per sé - it just isn't the right iron for my particular application.