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Dirty-South Blues Harp forum: wail on! > Another amp question for gurus, '65 PRRI
Another amp question for gurus, '65 PRRI
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John M G
419 posts
Mar 02, 2021
3:07 AM
I'm starting to get somewhere with my 65 PRRI
I've swapped out V1 and V4 with JJ 12AT7's and like the tone now. Also the change in the gain makes setting the volume much easier with a wider sweep.
My only question is what does V4 do? Does it influence the tone too?
I usually set the reverb at close to 2 and never use the tremolo.
6908 posts
Mar 02, 2021
4:30 AM
V4 is the phase inverter and tremolo recovery.
The past inverter part of this is important, or it can be, if you want it to be.
What a phase inverter circuit does is direct and divide your signal to the power tubes. There are various ways to achieve this. Some are more efficient than others but that isn't really the point here. To my mind, when considering a PR and the tube selection, v4 represents an opportunity to get a little more control.
I don't remember what the original tube is in v4 of the PR but I think it's a AX or AT.
If you drop from AX to 5751 or AT, or from AT to AY, you will reduce the voltage of the signal going to the power tubes. For a harp player with a hot mic, practically this means you can turn up the volume a bit more.
It will no doubt also affect the tone, because there is more to a pre-amp tube than the amplification factor. One effect of this is that if you plug a valve into a circuit designed for a different valve, not only the degree of amplification will change. There could be other impacts. For instance, there may be higher demand placed on some other components and this may result in overheating and perhaps component failure. We generally accept that we can plug any of the 12__7series of nine pin valves into a socket which had one of the others in it, and usually it is OK, but in some cases it is a problem.
I don't think you are going to have a problem with the PR but in a less robust amp such as my Hot Rod Deville, the use of 12AU7 and 12AT7 tubes can cause failure of particular resistors which are running fairly close to the limit under standard spec.
Again though, I don't think you need worry with the PR.
The other thing that can happen when swapping tubes is that the bias point is shifted. Normally circuit design considers the best bias point to amplify the signal for both high and low frequencies before distortion sets in. When you swapin a different tube you may find the point has shifted so now either the high or low frequency starts to distort at lower volume than before, whole the other end of the spectrum remains clear. This can be good or not, depending on what you like. This is the reason people find some tube swaps make the amp sound muddy or lifeless. It's not all about the change in the amplification factor. We often say a 12AU7 in some pre-amp circuits is going too low and has killed the tone. Not always though!
I have a 12AU7 in v2 of my silver face PR. This has really opened up the Reverb and made it more user friendly. I'm not recommending this! It works on my amp but Reverb is a quite variable aspect.

Anyway, I think I have an AT or AY in my V4 and I like it but I also have one or the other in V1 and I've forgotten which way it is. My amp is a 78 PR with the dreaded pull-boost circuit and rather different in some ways to yours, also I am a different person using different mics and probably playing different stuff so definitely wouldn't expect my size to fit yours but the principles are analogous I think. The amps are basically similar conceptually but 40 years apart
73 posts
Mar 02, 2021
2:59 PM
The PI in the Princeton is a ax7. If looking at swapping the pi, as Dave alluded to, the at7 has different current draw. You might be better to firstly try a 5751 and then go to a ay7 as their current draw is closer to that of the 12ax7. The same goes for V1, a 12ay7 will sound better than a 12at7 as the 12at7 was never meant to be used as an audio valve.
John M G
420 posts
Mar 02, 2021
3:19 PM
At one time I had an idea of building a valve amp!
I can see now it's an area that I'm never going to acquire enough knowledge to fully understand what I'm doing!
I like the tone of the amp much better now I've made the swaps to the JJ 12AT7's. Both the V1 and V4 had Russian GT 12AX7's. I do have a Tung sol 5751 that I tried in V1 initially that I can also try in now in V4 but I suspect that a JJ 5751 may give a creamier tone.
I'm going to give the Princeton a run at practice today and see what the guys think about it compared to my new love in the Laney Cub 10.
Thanks for the help and will let you know where I go from here.
Cheers John
74 posts
Mar 02, 2021
9:51 PM
John, building your own valve amp is not beyond your ability. Check out the Lamington amp from Grant Wills at valveheaven.com. He has a number of builds that can be done by sourcing materials and parts in Oz. There are loads of Lamington posts on AGGH (Aussie guitar gear heads). I bought an instruction book for the original Lamington amp and have learnt plenty from reading building, rebuilding and taking what I have learnt and applying it to other circuits. I knew nothing before hand and now I love reading about this stuff and I think I might be addicted to the smell of solder fumes.
John M G
421 posts
Mar 03, 2021
1:57 AM
I'll keep it in mind Prento, at the moment I'm still recovering from an operation I had in June last year that had to be revised in January and I'm still on crutches so getting out to the workshop is a good 6-8 weeks away yet (I hope!)
But today at our practice I ran the 65PRRI against the Laney Cub 10 and sad to say but we all thought the Laney has the better sound.
That's with the Princeton running the 2 JJ 12AT7's in V1 and V4 and the Li'l Buddy speaker. The Laney is bog stock, original speaker and valves.
I was thinking of dropping in the 5751, but the Laney just sounds so good there doesn't seem any point in pursuing it because I really prefer the Laney.
I think I'm going to have to let the Princeton go. The longer I keep it the less warranty there is to transfer to the next owner and its value will reduce. It's a tough decision to make but I just can't justify it after the back to back comparison with the Laney today.
The upside is I have an amp I'm really really happy with
75 posts
Mar 03, 2021
2:35 AM
Fair comment about the Laney and passing on the Princeton. I have read elsewhere about people liking the Cub 10 for harp. I think one of its benefits from a harp perspective is it’s relatively low plate voltages.

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